Opening…

Preface

The object of the present work is not so much to instruct as to suggest. It represents a sort of reconnaissance, and embodies the information collected by scouts and skirmishers regarding practicable lines of advance and accessible points of attack, with a view to annexing for the realm of knowledge some further strips and corners from the territory of ignorance. Its inspiring motive, in short, is the desire for a rectification of the frontier in the interests of science. Material resources for the furtherance of such encroachments are not lacking. The globe is studded with observatories, variously and admirably equipped. Yet innumerable objects in the sidereal heavens remain neglected, mainly through inadvertence to the extraordinary interest of the questions pending with respect to them. In the following pages it has been sought to indicate some of these individually, and in their relations to the larger meanings of cosmical research. But this could be done effectually only from the vantage-ground of our actual acquirements; hence the book, although primarily designed to stimulate the progress of astrophysics, necessarily includes an account of its present state. Before attempting to add to our store of learning, we must realise what is already possessed.

The unknown, it is true, is indefinitely vast, and the rays of light which we can project into its darkness penetrate but a short way. Our programme of inquiry must accordingly be limited to what is now practicable, or promises to become so in the immediate future. The unforeseen, too, will have something, perhaps much, to do with prescribing directions for fresh researches. Queries, in the coming years, will be put to the skies very different from those here propounded; and answers of a surprising kind will doubtless be afforded to our present interrogatory. The keen delight of such revelations will reward those who, loving truth for its own sake, have laboured for its promotion; and if these pages should, in any degree, help to quicken and guide their noble enthusiasm, they will have amply served their purpose.

They do not, however, cover the whole field of astrophysics. Planetary and cometary astronomy are deliberately, although for different reasons, excluded from treatment in them. The moon and planets still belong to the theoretical and descriptive departments of the elder celestial science. Nearly all that is known about their condition has been learned by direct telescopic observation. The items of information added through the aid of the camera and the spectroscope, though valuable, are very few. The orbital characteristics, on the other hand, of comets and meteors are too prominent to be set aside in any profitable discussion of their nature. They are of the very essence of the phenomena; yet they would be out of place in a book strictly limited to the consideration of the heavenly bodies under their physical aspect.

The writer has received much courteous help from various quarters in preparing the illustrations, and desires in particular to acknowledge her obligations to Sir William and Lady Huggins, to Sir David Gill, to Dr. Roberts, F.R.S., to the Rev. W. Sidgreaves, S.J., Mr. W. E. Wilson, F.R.S., M. Deslandres, Professor Hale, Professor Barnard, to Professors E. C. and W. H. Pickering, and to Professor Campbell.

LONDON, December 1902.

INTRODUCTION PAGE RISE AND SCOPE OF ASTROPHYSICS 1

PART I PROBLEMS IN SOLAR PHYSICS CHAP. 1. PROGRESS OF SOLAR PHYSICS 13 2. THE CHEMISTRY OF THE SUN 21 3. PECULIARITIES OF THE SOLAR SPECTRUM 36 4. THE REVERSING LAYER 44 5. HYDROGEN, HELIUM, AND CORONIUM 52 6. THE PHOTOSPHERE AND ITS DUSKY VEIL 62 7. STRUCTURE AND MOVEMENTS OF SUN-SPOTS 73 8. THE SPECTRUM OF SUN-SPOTS 88 9. FACULÆ AND PROMINENCES 98 10. THE CHROMOSPHERIC SPECTRUM 112 11. THE CORONA 123 12. THE SUN’S ROTATION 142 13. THE SOLAR CYCLE 150 14. THE SUN AS A WHOLE 161

PART II PROBLEMS IN SIDEREAL PHYSICS 1. PROGRESS OF SIDEREAL PHYSICS 171 2. THE CLASSIFICATION OF STELLAR SPECTRA 179 3. HELIUM STARS 189 4. HYDROGEN STARS 197 5. SOLAR STARS 203 6. STARS WITH FLUTED SPECTRA 209 7. CARBON STARS 215 8. STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES 222 9. HELIUM STARS WITH BRIGHT LINES 229 10. WOLF-RAYET STARS 237 11. THE GENERAL QUESTION OF BRIGHT LINES IN STELLAR SPECTRA 243 12. ANOMALOUS AND VARIABLE SPECTRA 246 13. COLOUR VARIABILITY 253 14. THE SPECTRA OF DOUBLE STARS 263 15. THE EVOLUTION OF THE STARS 271 16. ROTATION OF THE STARS 280 17. SPECTROSCOPIC BINARIES 286 18. ECLIPSING STARS 299 19. SHORT-PERIOD VARIABLES 319 20. SHORT-PERIOD VARIABLES—continued 328 21. THE PROBLEM OF BETA LYRÆ 337 22. STARS VARIABLE IN LONG PERIODS 347 23. PECULIAR AND IRREGULAR VARIABLES 363 24. TEMPORARY STARS 375 25. DARK STARS 399 26. THE GENERAL QUESTION OF STELLAR VARIABILITY 404 27. IRREGULAR STAR CLUSTERS 409 28. NEBULOUS CLUSTERS—THE PLEIADES 415 29. NEBULOUS CLUSTERS—continued 423 30. GLOBULAR CLUSTERS 428 31. WHITE NEBULÆ 439 32. DOUBLE NEBULÆ 452 33. NEBULOUS STARS 460 34. PLANETARY NEBULÆ 469 35. ANNULAR NEBULÆ 484 36. THE ORION NEBULA 495 37. OTHER IRREGULAR NEBULÆ 506 38. NONDESCRIPT NEBULÆ 516 39. VARIABLE NEBULÆ 522 40. THE NATURE OF NEBULÆ 531 41. THE PHYSICS OF THE MILKY WAY 538 APPENDIX 547 INDEX 553

LIST OF ILLUSTRATIONS

PRINTED SEPARATELY PLATE I. Photograph of the Argo Nebula Frontispiece II. Photograph of A-Band in Solar Spectrum (McClean) to face page 22 III. Flash and Cusp Spectra compared „ 46 IV. The Sun portrayed in Calcium Light (1); Photograph of the Chromosphere, 31st May 1894 (2) „ 100 V. Types of the Corona (Hansky) „ 128 VI. The Corona of 1900 (Jewell) (1); Reversal and Distortion of the C-line (Hale) (2) „ 96 VII. The Corona of 1896 (Morin) (1); the Corona of 1898 (Wesley) (2) „ 134 VIII. The Corona of 1900 (Wesley) „ 136 IX. Photographs of Typical Star-Spectra (Huggins) „ 180 X. Photographed Spectrum of ε Canis Majoris (Lunt) (1); Photographed Spectrum of α_{2} Centauri (Gill) (2) „ 182 XI. Spectrum of α Herculis (Espin) (1); Photographed Spectrum of 152 Schjellerup (Hale) (2) „ 184 XII. Photographed Spectra of Carbon Stars (Hale and Ellerman) „ 186 XIII. Photographed Spectra of Mira and of α Herculis (Sidgreaves) „ 188 XIV. Spectrum of γ Cassiopeiæ. Photographed by the Rev. W. Sidgreaves „ 190 XV. Photographed Spectra of Types II., III., and IV. compared „ 220 XVI. Sections of the Spectra of ß Lyræ and Nova Persei „ 344 XVII. Visible Spectrum of Nova Aurigæ (Campbell) „ 376 XVIII. Nebulosity round Nova Persei (Ritchey) (1); Spectrum of Nova Persei, 11th August 1901, with corresponding Intensity-Curve (2) „ 394 XIX. Photograph of Messier 11 (Roberts) „ 410 XX. Photograph of a Spiral Nebula in Cetus (Roberts) „ 442 XXI. Photograph and Drawings of a Spiral Nebula in Pegasus (Keeler) „ 444 XXII. Photograph of Whirlpool Nebula (W. E. Wilson) „ 446 XXIII. Fan Nebulæ (Barnard) (1); Planetary Nebulæ with their Spectra (Keeler) (2, 3, 4) „ 448 XXIV. Photograph of Owl Nebula (Roberts) (1); Photograph of the Orion Trapezium (Pickering) (2) „ 472 XXV. Photograph of the Ring Nebula in Lyra (W. E. Wilson) „ 484 XXVI. Photograph of the Orion Nebula (Pickering) „ 494 XXVII. Nebulous Formation in Orion. Photographed by W. H. Pickering „ 496 XXVIII. Photograph of the Trifid Nebula (Roberts) (1); Photograph of Messier 77 (Roberts) (2) „ 510 XXIX. Photograph of a Nebula in Perseus (Barnard) „ 514 XXX. Photograph of the Dumb-Bell Nebula (W. E. Wilson) „ 516 XXXI. Photograph of the Milky Way in Ophiuchus (Barnard) „ 540

PRINTED IN THE TEXT FIG. PAGE 1. General View of the Atmospheric Spectrum 24 2. The Infra-Red Spectrum (Langley) 24 3. Oxygen-Triplet in the Solar Spectrum 29 4. Curves representing the Distribution of Light in Fraunhofer and Arc Lines (Jewell) 41 5. Diagram of the Helium Spectrum 57 6. Sun-spot photographed by Janssen, 1st April 1894 73 7. Photograph of a Bridged Sun-spot (Janssen) 76 8. Sun-spot drawn by J. de M. Pereira, 18th June 1894 78 9. Group of Sun-spots drawn by Miss E. Brown 78 10. Portion of a Sun-spot Spectrum photographed by Professor Young 89 11. Reversal of the D-Lines in the Spectrum of a Sun-spot 94 12. Prominence observed at Kalocsa, 19th September 1893 104 13. Prominence observed at Kalocsa, 3rd October 1892 107 14. Forms of a Prominence in Hydrogen, Helium, and Pseudo-Coronium (Fényi) 117 15. Dark Markings in the Corona of 1871 (Wesley) 134 16. Dark Markings in the Corona of 1896 (Wesley) 134 17. Spoerer’s Curves of Sun-spot Latitude 152 18. Curves of Sun-spot Frequency and Magnetic Agitation (Ellis) 155 19. Intensity-Curves of Ηγ in Spectrum of Mira (Campbell) 225 20. Hydrogen-Envelope of a Wolf-Rayet Star (Campbell) 241 21. Photometric Curves of Algol Variables (Pickering) 309 22. Light-Curve of η Aquilæ (Schur) 322 23. Orbit of η Aquilæ (Wright) 323 24. Light-Curve of S Sagittæ (Yendell) 326 25. Light and Velocity Curves of ζ Geminorum (Campbell) 329 26. Orbit of ζ Geminorum (Campbell) 330 27. Photometric Curve of U Pegasi (Pickering) 332 28. Light-Curves of Cluster-Variables (Bailey) 336 29. Light-Curve of β Lyræ (Argelander) 337 30. System of β Lyræ (Myers) 341 31. Distribution of the Periods of 208 Variables 347 32. Light-Curve of U Orionis (Porro) 351 33. Light-Curve of S Ursæ Majoris in 1894 354 34. Mean Light-Curve of S Ursæ Majoris 355 35. Mean Light-Curve of T Ursæ Majoris 355 36. Mean Light-Curve of T Cassiopeiæ 355 37. Light-Curve of T Andromedæ 357 38. Light-Curve of S Cephei 359 39. Light-Curve of R Scuti 364 40. Long and Short Maxima of U Geminorum 365 41. Light-Curve of S^2 Cygni 367 42. Light-Curve of Nova Aurigæ 380 43. Photographs before and after the Apparition of Nova Persei 389 44. Light-Curve of Nova Persei, 22nd February to 6th March 391 45. Light-Curve of Nova Persei, 17th March to 3rd May 1901 391 46. Exterior Nebulosities of the Pleiades 419 47. Photograph of ω Centauri (Bailey) 430 48. Drawing of a Pair of Spindle Nebulæ (Spitaler) 457 49. Sketch of T Tauri and Hind’s Nebula (Barnard) 524 50. Drawing from Photographs of T Tauri and Hind’s Nebula (Keeler) 525

INTRODUCTION. RISE AND SCOPE OF ASTROPHYSICS.

The astronomy of the ancients was purely formal. It did not profess to look beyond appearances. Its aim was reached, provided that phenomena were—in the old Greek phrase—mathematically “saved,” at whatever cost of material impossibility. Kepler first speculated on the causes of celestial movements, and introduced the term “physical astronomy” with a full sense of what it implied. Its establishment as an effective branch of knowledge was a prime desideratum with Francis Bacon. While rejecting the Copernican system and ignoring Kepler’s laws, he aspired towards a science of the skies that should be no simulacrum, like the “stuffed ox of Prometheus,” but should have in it the breath of life and the instinct of progress.[1] Anticipating with imaginative insight the prosecution of researches which Comte, two centuries later, declared to be, in the nature of things, futile, he broadly laid down the lines of a new astronomy, indistinguishable from modern astrophysics. As the province of this “philosophy” of the heavenly bodies, he assigned inquiries into the nature of their substance, of their qualities, properties, and influences, as well as into the source of the motive power acting upon them. Vitalised, as it were, by contact with motherearth, it was destined in his prevision to a community of advance with terrestrial science, one imparting to the other novel truths tending to mutual profit and simultaneous development. Thus the long-divorced sublunary and translunary worlds were conjoined, and their material unity—the essential principle of astrophysics—was definitively proclaimed. This daring forecast transcended the scope even of Newton’s discovery, and left Kepler’s prescience far behind. For Kepler, as an astronomer ex professo, took full account of apparent impossibilities, while Bacon’s was “the golden guess”—

That’s morning star to the full round of truth.

Yet it might have been derided as that of an uninformed amateur. It rose indeed out of sight of ordinary minds.

The establishment of the law of gravity was the first step towards its realisation. Thereby the terrestrial regimen was, in part, extended to the skies. The moon assumed the aspect of a projectile in perpetual flight, tangentially launched ab initio, and hence keeping its distance from the earth, while the planets, including our own globe, proved to be similarly related to the sun. Thus celestial movements lost the mystical character long ignorantly attributed to them, and were found to own a common cause with movements at the surface of the earth. They became predictable, since the cause acts uniformly and simply; theoretical astronomy, with practical astronomy at hand to provide its raw materials and test its results, took rank as the most perfect of the sciences; the idea of a definable force put to the rout the old vague notions of “tendencies,” “appetites,” “passions,” or “potencies,” and a dynamical was substituted for a merely kinematical system.

Gravity, however, is a force of the utmost generality in the way it affects matter. It takes no notice of distinctions of kind or quality. The substances acted upon may be hot or cold, dense or rare, elementary or compound; they may be of any imaginable chemical or mineralogical constitution; they may be in any state of aggregation; they may be organic or inorganic; no difference is perceptible; gravity is concerned solely with mass, and is measured strictly by movement; and from gravitational inquiries, accordingly, mass and movement can alone be learned. So far, then, only one principle of unification was introduced. One fundamental property of matter was known from 1687 to belong equally to the earth and planets; and Herschel’s discovery in 1802 of mutually revolving stars virtually made the “writ to run” throughout the sidereal world as well. The universality of an apparent mass-attraction was a great fact, but seemed destined to remain isolated; for Olbers’s “electrical theory” of comets amounted to no more than a suggestive speculation. Then in 1852 the triple identification by Sabine, Wolf, and Gautier of the sun-spot and terrestrial magnetic periods showed the reality of solar influences exercised in a manner not easy to apprehend, but capable of being brought to the test of experimental investigation. Cosmical physics began to separate out and take recognisable shape. The spring of its most rapid growth, however, lay in another direction.

The discovery (in Professor Keeler’s words[2]) “that the light which reveals to us the existence of the heavenly bodies also bears the secret of their constitution and physical condition,” afforded a solid basis for a science of far-reaching import. “The spectroscope placed new and hitherto undreamtof powers in the hands of men. It is to the astrophysicist what the graduated circle and the telescope are to the astronomer.” Observations of the heavenly bodies by means of their analysed light came to the aid of observations through their integrated light. Their radiations, visible and invisible, were brought within the range of detailed study.

Of study, not only visual, but photographic. The sensitive plate has three leading prerogatives. It sees where the eye is blind; its impressions are cumulative to an indefinite extent; they are permanent; they constitute documentary evidence of incontestable validity, which can be produced or referred to at pleasure. Spectroscopic photography, or “spectrography,” dates from Sir William Huggins’s adoption of the dry gelatine process in 1876; and his discovery, three years later, of the ultra-violet series of hydrogen-lines in stellar spectra started it on its career amid acclamations. Nor has the promise been belied. The efficiency of the camera is of so high an order that direct visual observations of prismatic light are now only by exception made. This is perhaps unfortunate, since the two kinds of results are, to some extent, supplementary, and can often be most usefully compared and correlated. The superiority of the chemical method, however, is nowhere more conspicuous than in the motion department of the new astronomy. Its powers in this direction were tested with striking success by Vogel in 1888, and their development, rapid as it has been, does not seem to be near its term. The determination of radial velocities through changes in the refrangibility of light emanating from the bodies actuated by them, has made astrophysicists free of a territory which belongs equally to the domain of traditionally equipped astronomers. Here we get back to elementary facts of mass and motion, ascertained, however, not frontally by measures of position, but strategically by inference from radiative modifications. They are, indeed, of a nature unaffected by position, and hence undiscernible with the micrometer. A body shown spectroscopically to be in swift movement might be absolutely immobile telescopically; or the conditions might be inverted, each method taking cognisance of only one component of the total velocity. An immense extension was accordingly given to the field of research in sidereal dynamics by the application, through Sir William Huggins’s initiative, of “Doppler’s principle.” It supplied not alone the means of completing investigations which could otherwise be pursued only in a one-sided manner, but of setting on foot entirely new ones of incalculable significance. Thus the rate of the solar translation through space, valued little better than conjecturally from the proper, or thwartwise motions of the stars, can be derived securely and at once from their radial motions. Of the movements of nebulæ nothing is known—and very little is likely to become known for some centuries yet to come—except through spectroscopic measurements; for they are so remote that their positions change with extreme slowness, while the evidence of radial speed is tendered immediately, without regard either to time or distance. But the most curious discoveries afforded by it are of double stars revolving in such close contiguity as to be permanently inaccessible to telescopic observation. And these are precisely the systems of highest cosmogonic interest, as being, most likely, at the outset of their evolutionary careers. They are surprisingly numerous, and will doubtless prove to be linked on to telescopic binaries by an uninterrupted succession of couples farther and farther apart.

This common ground of the two astronomies, where motion in the line of sight is the object of research, has already proved fruitful of varied novelties, and its yield is not within view of being exhausted. It could, however, never have been worked to advantage but for the timely assistance of photography. The living retina is here conspicuously inferior to the chemical retina; for aerial disturbances are eminently baffling to eyeestimates of line-shiftings, while the sensitive plate, ignoring momentary fluctuations, records true mean positions. Visual measures are hence rarely trustworthy; advantageous occasions for securing them are few; so that they must always be either poor in quality or scant in quantity; and accumulated data are needed as the bases of systematic inquiries. The use of the camera is accordingly indispensable, and has become all but exclusive.

Astrophysics widens in scope year by year, and as it wins extension it gains intension, each advance carrying it deeper into the secrets of nature. Towards this result the alliance with photography has contributed with signal effectiveness. The impersonal method confers a certainty and power in dealing with obscure phenomena which can only in special cases be claimed for the eye. Moreover, it is of larger application. It can be employed on an expanded scale both of time and space. It is thus the fittest means for collecting statistics of the heavens; and statistics are urgently in demand for the ultimate purposes of celestial science. The whole future of astronomy has indeed come to depend upon the validity of photographic evidence, and specialties of manipulation and development, the idiosyncrasies of variously prepared plates, the shrinkage of gelatine films, the effects of graduated exposures, have to be studied no less diligently than the theories and errors of brass and glass instruments where immediate determinations of celestial situations are in question. Astrophotography is an art, and has a technique of its own needing labour for its mastery.

The ramifications of astrophysics are numerous and intricate. To trace them out in detail would be to unroll an elaborate chart of the sciences. Celestial chemistry is in itself an all but limitless department. It includes terrestrial chemistry, thermotics, thermo-electricity, and slides inevitably into the wonderland of molecular physics and ethereal powers and qualities. For the interpretation of spectra demands acquaintance with the nature of the vibrating systems originating them, with their relations to “imponderable” agencies, with their perturbations, modifications, and disruptions under the stress of circumstances at present scarcely definable. Here there are worlds to conquer. One phase of these inquiries is marked by the recognition of harmonic line-series in the spectra of the chemical elements. Another by the discovery that wave-length is a function of density, that an increase of pressure slightly shifts the rays emitted by a glowing vapour downward towards the red. A third, still more significantly, by the “Zeeman effect,” with its barely conjectured implications. It consists in the distension and subdivision of lines normally slender and single, when the radiation takes place in a strong magnetic field; and the specification of the laws of its production, whether close at hand under controlled conditions, or far out of reach at the surface of celestial spheres, allures the imagination with possibilities of far-reaching consequence. Above all, there seems to be a reasonable chance of learning from it something about the electrical state of the stars. The relative strength and brilliancy, moreover, of spectral lines afford criteria of temperature, density, and modes of electrical action, but not with satisfactory explicitness. There is much difficulty in duly apportioning the effects. Thermal and electrical conditions are rarely separable; degrees of density and of temperature again need very careful discrimination. Electricity is the indispensable agent for exciting luminosity; precisely, however, what part it reserves for itself in the matter—whether heat, as generally assumed, is its plenipotentiary, or merely a delegate with limited powers—is, so far, unknown. Nor is it easy to define what takes place in the path of the discharge, yet it is from the carrying molecules only that the light examined is derived, and it is their state only that is indicated by its peculiarities. Still, beginnings have been made in the experimental disentanglement of this web of interdependent operations, and specific inferences of value regarding the heavenly bodies have already been drawn from some preliminary ordering of the various classes of facts.

The rotation of the heavenly bodies is a department annexed, while their chemistry has been created by the new astronomy. No longer treated as a simple geometrical datum, it is studied as an index to their physical constitution. Spectroscopic observations of axial movements in the sun and planets are among the most delicate and curious that have been made. They may possibly be extended to stars, nebulæ, and comets, but the prospects here are dubious. Nor has the old direct mode of determining rotation been superseded by the novel method. Its employment, in some cases supplementary, is rendered in others, by the force of circumstances, exclusive.

Moreover, nearly the whole “descriptive” section of astronomy is embraced by astrophysics. It is now extensively yet not altogether worked by photographic means. The camera has so far succeeded very imperfectly in depicting planetary surfaces; but the required special conditions are being carefully studied, and will perhaps before long be realised. The difficulties attending lunar photography have of late been, in the main, overcome, as the magnificent Paris and Lick Atlases of the moon testify. They nevertheless record essentially what was known before; they elucidate no perplexity; selenology has been adorned and illustrated, but not greatly promoted by their compilation. The self-portrayal of recent comets, on the other hand, has been accompanied by remarkable disclosures. They need, however, skilled interpretation, and experts in this branch are to seek. The pictures are there, full of rapidly changing and significant detail; yet patience must be exercised before we can read in explicit terms what they implicitly convey regarding the constitution of the bodies they represent.

The photographic study of the Milky Way—pursued systematically by Professor Barnard—has been more definitely and distinctly communicative. For his plates not only bring clearly to view the mixed stellar and nebulous nature of that gigantic assemblage, but also afford grounds for inferences of great moment as to the general distribution of the stars. This indeed is a subject which might seem expressly reserved for treatment by the older astronomy. Yet the all-pervasive physics of the skies has a lien upon it. Spectroscopic considerations come into play. The modes of stellar scattering in space are different for the various stellar types, and the connection suggests queries, not readily answered, regarding the origin of those types, and the very genesis of the sidereal system itself. Abysses of speculation open before us as we contemplate the surging galactic cloud-forms depicted through the simple instrumentality of a portrait-lens and a sensitive-plate.

In the photometric branch of astronomy there is a similar concurrence of claims. The arrangement of the stars in light-ranks serves primarily as a test of their arrangement in space; the test, however, is illusory unless the nature of their spectra be taken into account. Again, while measurements of the brightness of individual stars are essentially physical in their import, they are also carried out for the geometrical purpose of determining occultation-phases. The photometric observation of the eclipses of Jupiter’s satellites is a corresponding example in the solar system. Otherwise, in its varied applications to the sun and moon, to planets, asteroids, and comets, photometry may be said to have purely physical aims. These have to do, not only with integral, but also with analysed light. The “spectrometric” division of photometry consists in the comparative estimation of ray-intensities, in balancing one against another the differently refrangible beams from a given source of luminosity, in constructing, that is to say, its spectral energy-curve. In both departments the camera proves an invaluable ally. Photographic photometry occupies, indeed, a place apart among the arts and crafts of astronomy. It has its own laws, its own problems, its own difficulties, and it furnishes data which can be interpreted on principles valid for them alone.

The specialties of solar physics are too numerous to be particularised. Among stars, perhaps an insignificant star, the sun is nevertheless by its comparative vicinity to ourselves brought within range of a whole series of observations impracticable elsewhere. In solar research, accordingly, novel devices abound; such as the “double-slit method,” so happily availed of by Hale and Deslandres for the spectrographic portrayal of “flames,” facular and chromospheric. The complex operations conducted under shelter of eclipse are equally peculiar in their objects and in their system; by them only is the unique problem of the corona at present accessible to attack; that of the “reversing layer” is even more elusive in its momentary presentations. Sun-spots, on the contrary, are open to leisurely daylight inspection; yet the perplexities connected with their structure and spectra grow rather more than less acute as facilities for their scrutiny are increased. But this is no uncommon experience in the arduous walks of science.

The pliancy and generality of astrophysics contrasts singularly with the austere exclusiveness of gravitational astronomy. The new mode of celestial inquiry follows every indication, lays hold of every clue; it promises much, it often performs more; yet its advance is at times hampered by the very circumstances which make it brilliant and surprising. For it “deals,” as Professor Mendenhall said in 1892,[3] “with a matter of many properties, some of which are but little understood. While its conclusions are of vital importance and of intense interest, they result from deductions in which the premisses are insufficient, and are proportionately uncertain. The new astronomy must for a long time abound in contradictions and controversies, until, and largely through its development, we shall possess a knowledge of the properties of matter when subjected to conditions differing enormously from those with which we are now quite familiar.”

Here indeed lies the fundamental peril, and at the same time the essential prerogative of astrophysics. Its concern is with phenomena falling partly within, partly without the range of ordinary experience. It has to do with matter in transcendental states. Hence the necessity for having recourse to the risky expedient of “extrapolation”—that is, of applying unrestrictedly to the unknown, rules gathered from observation over a comparatively narrow area. The indefinite continuity of natural laws is assumed by it, but certainly on no sufficient warrant. There is indeed no help; no other means are available; the line and plummet that have proved serviceable for sounding the estuary must be used likewise for the ocean. The upshot, however, is merely a “first approximation,” to be subsequently corrected and controlled. And it may be of immense importance as an index to consequences or possibilities which could not have been foretold, and defy even imaginative realisation. But just here resides the exploring faculty of astrophysics. It often acts as the pioneer of terrestrial science. “The discovery of unknown laws” (in Professor Keeler’s words), “as well as the explanation of phenomena by laws already known, is one of its most important objects.”

A great future is reserved for it. It postulates a law of order, the same always and everywhere, and its primary function is to verify that postulate, step by step, point by point, under continually widening horizons of knowledge. There is no such thing as chaos, it tacitly asserts, in the sidereal world or outside of it. For chaos is the negation of law, and law is the expression of the Will of God.

Spotted a typo or a problem in this chapter? Tell us

I. PROGRESS OF SOLAR PHYSICS

Solar Physics is the science of the sun as an individual body. It is not concerned with the sun as the ruler of the planetary system, or as a member of the sidereal system. The questions which it seeks to answer relate exclusively to the “thing in itself.” And these questions, through the effectiveness of modern methods, have become answerable. Few of them, it is true, have yet been answered, and all can never be set at rest, since each reply marks only the starting-point for a fresh interrogatory. This must be so; the prospect inevitably widens with the attainment of a higher point of view. Nor is it likely that the ascent will soon terminate. It is indeed towards a summit cloud-wrapt and self-withdrawn. The essential point, however, is that stagnation has given way to progress, surmise to inquiry, and barren wonder to stimulating curiosity.

Solar research made a threefold start about the middle of the last century. First came Schwabe’s discovery of a decennial sun-spot period, followed up by Sabine’s announcement of a coincident terrestrial-magnetic period. Then, in 1860, Kirchhoff published his momentous chemical interpretation of the Fraunhofer lines, showing the presence of familiar metals as glowing vapours in the sun’s atmosphere. Finally, on 18th July of the same year, the “red prominences” were photographically referred to their true location, and the whole marvellous eclipse-garniture was at once annexed to the domain of solar physics. The investigations corresponding to these three beginnings were pursued at very unequal rates of advance, and with considerable disparity of success. The chief triumphs were those of the prismatic method. From the spectroscope single-handed, the old order of ideas received its death-blow. Glaring incongruities notwithstanding, it might have survived a couple of decades longer had it not been for the reading of the strange Fraunhofer inscription. But the subversive effect of the attack delivered in 1860 was too palpable to be ignored. At last, unmistakably, the Herschelian theory of the sun was in ruins, and it only remained to clear away the rubbish of the structure preparatory to erecting a modern edifice on new foundations.

Its corner-stone was the principle of the conservation of energy. This obtained its first solar application from Helmholtz in 1853. His gravitational hypothesis explained the enormous outflow of heat from the focal hearth of the planetary household with a directness and simplicity that compelled conviction of its truth. Energy of position is, in this view, the store drawn upon by radiation; and it is a store so vast that millenniums of thermal expenditure will make no perceptible encroachment upon it. As the great globe cools, it contracts; and each one of its constituent particles falls, day by day, infinitesimally nearer to the centre, heat being thus mechanically evolved. Potential energy is in this way converted into actual energy, and we are warmed and lighted because the sun shrinks, and is raised by shrinkage to a surpassing pitch of incandescence. His constitution must then be such as to meet these requirements. For an ideal body, endowed at pleasure with fanciful properties, a machine has to be substituted, definitely adapted to the fulfilment of a recognised function. This change in the point of view is characteristic of astrophysical aims. It makes all the difference between antique and modern science. A great deal is involved in it. The demands of the novel situation are multitudinous, and can be met, not by speculative efforts, but only by toilsome experimental comparisons. These will need time and much patience, and can never be wholly satisfactory in view of the contrast between terrestrial and solar conditions. Yet the efforts towards their assimilation, unremittingly prompted by the new astronomy, lead to a continual growth of knowledge, and are unlikely to be relaxed until the torch has finally dropped from human hands.

The absence, then, of a satisfactory all-round theory of the sun need not be taken as an implication of failure. On the contrary, progress is necessarily attended by incompleteness. Facts, when research is most active and successful, accumulate too rapidly to be at once collocated. Finality means stagnation. Compare the map of the world drawn by Hecataeus with that by Herodotus. The earlier is by far the more finished production. Neat and trim, with its circumfluent Ocean Stream, it pictured the earth mainly from ideas of what it ought to be. The later chart, on the other hand, as the upshot of wider experience, admitted ignorance by abolishing limits and leaving room for the unknown. A true theory must always be somewhat expansible. It must be capable of accommodating new facts. Otherwise their intrusion will speedily rive it asunder.

Only the broad lines of solar theory can then at present be laid down. Details must be filled in gradually with the progress of research. The preliminary ideas, however, already acquired are unlikely to be subverted; we can represent to ourselves a sun which is a reality, and no figment of the brain.

Our luminary is neither solid nor liquid. It is mainly, perhaps entirely, gaseous; but its gaseity is of the “critical” kind, due to the combination of intense heat with enormous pressure. The thermal supplies needed to meet its vast emissive expenditure must be continually and rapidly brought from the central parts to the surface; and this can only be accomplished by the actual transport of the heated materials, conductive processes being much too slow to meet the exigencies of the situation. We thus recognise in the sun a globe riddled with convection-currents, of which the shining cloud-shell of the photosphere constitutes the limit. At the photospheric level the uprushing torrents deliver their cargo of radiative energy, and from the photospheric level the corresponding subsidence of cooled matter starts for the unimaginable furnace below. This course of exchange, however, must be greatly complicated by the rotation of the plastic mass in which it progresses. A true vertical circulation is rendered by it impossible; the ascending and descending currents must be variously and incalculably deflected. Incalculably, since the state of the sun’s interior lies, in some respects, beyond the range even of conjecture.

The very remarkable circumstance has been emphasised by recent inquiries that the photosphere fixes a boundary between two solar regions scarcely less strongly contrasted—to speak illustratively—than the terraqueous globe and its encompassing atmosphere. The sun has several distinct envelopes, but none, apparently, in the condition of atmospheric equilibrium. There is first a shallow, veil-like covering by which the disc is reddened and darkened. Next comes the “reversing layer,” a bed of mixed incandescent vapours, some hundreds of miles in thickness, the absorptive action of which mainly produces the dusky lines in the Fraunhofer spectrum. It is overlaid, to a depth of four or five thousand miles, by the chromosphere, a gaseous ocean incarnadined by the crimson blaze of hydrogen. The irregularities of its outline develop, locally and temporarily, into “prominences,” often of gigantic size, but belonging to the chromosphere as essentially as mounting waves and tossed spray do to the ocean. Finally, we reach the far-spreading corona, a mere lustrous phantom, approaching the absolute zero of density, yet of astounding decorative effect during total eclipses. Between the corona and the chromosphere there seems to be absolutely no material continuity, although structural relationships have been traced.

These appendages are distinguished by two peculiarities, rendered more obvious at each step forward in research. The first is that they contain an extremely small quantity of matter. The second, that the effect of the sun’s gravity upon them is, in some way, neutralised. We have only to consider that at an elevation of three and a half miles air is reduced to one-half its sea-level density, while the corresponding height at the surface of the sun—where gravity is twenty-eight times more powerful than it is on the earth—is but one-eighth of a mile. Into the compass of a shell just one furlong thick, accordingly, half the substance of the reversing layer, chromosphere, and corona should be compressed, if the sway of gravity over them were undisputed. Its comparative impotence is attested not only by their vast extent and excessively slow rate of luminous degradation, but still more emphatically by the almost total absence from them of spectroscopic symptoms of internal compression.

The various “claims” into which the wide field of solar physics has inevitably come to be divided are marked by curious differences of productiveness. Some are thickly sown with “pockets” of bright ore; others have hitherto yielded little beyond the “sparkle of golden splendour” on the surface. Thus the geometrical relations of sun-spots are not now more surely known than in the days of Derham and Cassini. A consensus of opinion that lasted a full century has given way to notorious disagreement. The elementary question as to whether spot-umbræ are elevations or excavations, is once more actively debated. True, the overthrow of an artificial unanimity often preludes a forward movement; yet it might have been expected that the immense mass of photographic records accumulated during thirty years would have amply sufficed to settle this matter once for all. It must indeed be admitted that direct sun-pictures, notwithstanding the exquisite perfection to which the art of taking them has been brought, and the striking nature of the details they often exhibit, have contributed only in a minor degree to the promotion of definite knowledge. Super-eminent among them are the long series due to M. Janssen’s skill; yet after twenty years the réseau photosphérique, a phenomenon of “blurring” manifested by their means, continues enigmatic as to its nature, and open to doubt even as to its solar origin.

The swiftest advances in solar physics have been along the various routes opened by light-analysis. Four of these are broadly separated by differences of aim and method. The inquiries they have made practicable relate to the chemistry of the sun itself, the daylight study of prominences and faculæ through a selected element of their emissions, to radial movements in the sun, and to spectroscopic disclosures during eclipses. The Fraunhofer spectrum has been studied year by year with minuter accuracy, and similar refinements in the treatment of the arc-spectra compared with it have assured real, and annulled fictitious correspondences, largely, as may readily be imagined, through photographic agency. Spectra, to be exactly collated, must be durably imprinted. The fine measurements now executed upon them would be an impossible task for the eye. Professor Rowland’s invention of concave gratings in 1883 has also contributed very notably to the development of solar chemistry. They simultaneously focus impinging rays and disperse by diffracting them. No lens needs to be interposed, and thus the disturbing effects of selective absorption and unequal deviation are avoided. Improvements in the technical processes of photography—the substitution of gelatine for collodion as the vehicle of the decomposable salt; the intensification of sensitiveness in plates; modifications in their colour-susceptibility—have been equally essential. Mechanical contrivance has not been behindhand. Without faultless screws, for instance, there could be no perfectly ruled gratings. The chemistry of the sun has indeed drawn upon many and unexpected resources for its promotion.

Spectroscopic work at the sun’s edge was carried on steadily for twenty-three years after its initiation in 1868. Its outcome was the collection of a mass of valuable information regarding the chromosphere and its jutting eminences. Their forms, movements, and duration were registered, the law of their distribution was ascertained, the mode of their conformity to the spot-cycle inferred. So rich a harvest was, in fact, gathered at once that the soil began to show signs of exhaustion; the prospect seemed dim of detecting any further essential novelties in this direction; the routine task of daily promenading the slit of the spectroscope round the limb lost its zest. Then in 1891 a novel commencement was made, and made in duplicate by Professor Hale at Chicago and M. Deslandres in Paris. They transferred the business from the eye to the sensitive plate, definitively and with splendid success. The photography of prominences, although tried on a correct principle by Professor Young in 1870, remained in an abortive experimental stage until recourse was had to the device of isolating the K-line of calcium, and depicting them in this single element of their light. It proved applicable to faculæ as well, and in one minute a complete picture of the disc and its appurtenances, as shown in the violet ray profusely emitted by them, can be secured with the spectroheliograph whenever the sun shines on either side of the Atlantic. The very name—“spectroheliograph”—of the instrument invented for the purpose comprises a history of changing methods—of the supersession by photography of eye-and-hand delineation, and of the replacement in turn of direct photographic portrayal by impressions of spectral images. And there has been a corresponding modification of ideas. New conceptions are gaining ground, not through the broaching of startling theories, but under the steady guidance of undeniable, and often surprising facts.

Doppler’s principle was applied by Sir Norman Lockyer about 1870 to meteorological investigations (as they might be called) in the sun. They disclosed the not infrequent occurrence there of portentous cyclonic agitations. Distortions and displacements of the hydrogen lines attested the rushing of incandescent whirlwinds at speeds up to 250 miles a second. Much has yet to be learned regarding these extraordinary phenomena, their relationships having scarcely received the detailed and particular attention that they deserve. Professor Young’s use of the same method in 1876 to measure the solar rate of rotation served both as a test of its validity, which it established beyond cavil, and as a prelude to important refinements in the treatment of that intricate subject. Notwithstanding its anomalous retardation north and south from the equator, M. Dunér obtained in 1887–9 spectroscopic evidence of axial movement up to fifteen degrees from either pole, and thereby brought a widened range of its complexities under observational control. Line-displacements, too, similarly produced, have become the standard criterion for discriminating between the genuine solar, and the merely telluric constituents of the Fraunhofer spectrum; and it need scarcely be pointed out that to set them decisively apart is a pre-requisite to solar chemical progress.

Each favourable eclipse since 1842 has furnished to science its quota of new facts and inspiring suggestions. In 1851, the solar status of the “sierra” and “red protuberances,” demonstrated in 1860, was recognised by all except a few obstinate sceptics. In 1868, the hydrogen and helium spectrum of these wonderful objects came into view; in 1869, the green coronal ray was detected. Then on 22nd December 1870, Young’s tangential slit was momentarily lit up by the “flash spectrum” of the reversing layer, which, after twenty-six years, was photographically captured by Shackleton, and so became the subject of definite and critical investigation. This was greatly promoted by the multiplied records of it obtained along the line of totality which crossed India, 22nd January 1898. The cyclical variation of coronal types, indicated by the substitution of luminous “wings” for the more familiar “glory,” during the Rocky Mountains eclipse of 29th July 1878, was verified by a splendid series of coronal photographs taken in Egypt in 1882, at the Caroline Islands and Grenada in 1883 and 1886, in California and at Cayenne during the January and December totalities of 1889, at Novaya Zemlya in 1896, in the Deccan in 1898, in Sumatra and Mauritius in 1901. Sir William Huggins’s device for photographing the corona in daylight, invented under the stimulus of the Nile-eclipse disclosures, unfortunately remains in abeyance. Its realisation is a prime desideratum in solar physics.

The progress of science in this branch might with substantial accuracy be described in the condensed statement that a fabulous luminary has made way for a working machine—a machine, it is true, of infinite complexity, yet in touch with, although transcending, the common order of things. Just here reside the extreme interest and value of such inquiries. They deal with what is concrete; they can be pushed on by experiment, but by experiment always straining to widen its resources. Limits are accordingly pushed back little by little—limits of temperature, of rarefaction, of ethereal stress as manifested by electric and magnetic intensity. The end of the process is not within view. The way, arduous though it be, lies open, and is securely travelled by those who, relying on the unity and continuity of nature, confidently hope to attain by it to the knowledge of higher truths.

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II. THE CHEMISTRY OF THE SUN

Knowledge of solar chemistry is based exclusively upon the analysis of solar light. It advances pari passu with the interpretation of the Fraunhofer lines. And by their interpretation is signified the process of identifying them, one by one, with the rays of known substances, made to glow artificially in the laboratory. They are the characters of a script in the main decipherable, and already, to a satisfactory extent, deciphered. Their reversal from bright to dark simply implies that the prismatic background upon which they are projected represents a hotter source of radiation than theirs. In other words, the temperature of the photosphere is above that of the ignited vapours through which its light is sifted, and by which it is selectively absorbed.

Fraunhofer’s survey of the solar spectrum was necessarily confined to its visible section, and was executed with very imperfect appliances. Yet the lines laid down in his map had the importance of permanent landmarks. The following is a list of the chief among them, their wave-lengths on Rowland’s scale, and the chemical origins ascertained for them, being added:—

Designation. Wave-length in ten-millionths of a Origin. millimetre. A 7594·059 (upper edge of a band) Terrestrial oxygen B 6867·461 „ „ „ „ C 6563·054 Hydrogen D_{1} 5896·156 Sodium D_{2} 5890·182 „ E_{1} 5270·495 Iron _b__{1} 5183·792 Magnesium F 4861·496 Hydrogen G 4308·034 Iron H 3968·620 Calcium K 3933·809 „

The first and last lines in this table approximately define the range of dispersed sunlight visible to ordinary eyes. It extends over nearly an octave; and a higher half-octave in the ultra-violet is disclosed photographically. The infra-red, however, offers a far vaster scope for exploration. Using a “bolographic” method, in which the camera registers what the bolometer[4] feels, Professor Langley has surveyed a stretch of dark radiations eight times longer than the bright strip mapped by Fraunhofer; nor was his advance downward in the spectrum checked by any insurmountable barrier. There is, indeed, much probability that long heat-waves and short “Hertzian” waves are really indistinguishable, and that the luminous spectrum passes without a break through the thermal into the electric spectrum.

In the ultra-violet region, on the other hand, a peremptory stop is put to research by the interposition of the air. It excludes by absorbing ether-waves shorter than about λ 2950. Cornu found sunlight to be arrested just at this point. Sir William Huggins fixed the limit for the photographic spectrum of Vega (α Lyræ) at λ 2970.[5] It does not fluctuate with meteorological conditions. Dampness and drought are equally ineffective in shifting the atmospheric barrier against the entry of quick vibrations. Cornu ascertained in 1881[6] that it is affected only by the height of the barometer. Nor is the reduction of impermeability through ascent above the earth’s surface nearly as great as it would be if aqueous vapour were the producing agent. That oxygen is chiefly concerned is rendered certain by converging proofs. Nitrogen seems to be, in this respect as in others, nearly inert.

The general enfeeblement, by transmission through our atmosphere, of the violet and blue sections of sunlight becomes obvious in the redness of the sinking sun. On the lower radiations telluric absorption acts more specifically. They are interrupted by a multitude of dark bands and lines certainly referable to it. The question as to the terrestrial or solar origin of such effects is evidently of fundamental importance to solar chemistry. It can be answered in two distinct ways. The earlier and simpler method is by comparing the spectra of the high and low sun. The groovings that gain strength with approach to the horizon stand self-declared as atmospheric, while lines unaffected by altitude tell plainly of exotic conditions. The latter class are much the more numerous. Of 3200 lines mapped by Thollon, 2090 are purely solar, 866 telluric, and 246 of compound production.[7] And the proportion is not very different in Dr. Becker’s catalogue of 3637 spectral lines, published in 1890.[8] Among them 928 came out blackened in “low-sun” observations, and proved in the main due to the selective absorption of water-vapour. A considerable proportion, indeed, belonged to the “rain-band,” and varied hygroscopically. Dry-air absorption is almost exclusively an oxygen product. It takes effect chiefly in three wide bands, Fraunhofer’s “A” and “B” and Ångström’s “a,” all relieved against a crimson background. They are characteristic of cool oxygen. The molecules, whose vibrations they in a manner reflect, are broken up at high temperatures. They survive, however, the liquefaction of the gas at −181° C. Professors Liveing and Dewar observed the atmospheric A and B in light that had been transmitted through three inches of this frigid fluid.[9] The rhythmical flutings composing the former are shown in Plate II., from a photograph by Mr. McClean. The work, of which it is a specimen, portrays the solar spectrum in seven sections, from D to below A (λ 5800 to λ 7700), the dispersion having been effected by means of a Rutherfurd grating of 17,296 lines to the inch.

Fig. 1 gives a general view of the atmospheric spectrum, so far as it can be seen, but it is largely invisible. Langley found the immense tract of the heat spectrum, down to wave-lengths of nearly six “microns,”[10] thronged with “cold” rays, 652 of which[11] were accurately determined from “bolographs,” but remain, with few exceptions, chemically unidentified.

Fig. 2 reproduces Langley’s drawing of part of the infra-red spectrum. The blank strip to the left shows the comparative brevity of the visible part of the scroll. The invisible part includes the distinctive signature of one other atmospheric constituent besides oxygen and water-vapour. Two strong bands in the infra-red are assigned by Knut Ångström to the absorption of carbon dioxide,[12] a substance of which four volumes are present in ten thousand of air at sea-level. The huge nitrogen envelope of our globe, together with its argon-ingredient, appears to be perfectly transparent to rays of all refrangibilities. The unexplained fact of its spectral nullity emphasises the inadmissibility of negative conclusions regarding the chemistry of the heavenly bodies.

The second peculiarity which distinguishes telluric lines is a negative one. They do not shift as the sun rotates. But lines genuinely emanating from the equatorial edges of the sun are displaced towards the blue by the advancing movement of the left or eastern limb, towards the red by the recession of the western limb. The juxtaposition, accordingly, of spectra from these two opposite sources serves as an unfailing test of the origin of their constituent markings, those claimed by the sun being perceptibly notched at the points of junction, while their telluric associates run on continuously.

Little has been added to knowledge of the sun’s constitution by researches in the infra-red part of the spectrum. They are as yet crippled by the lack of metallic comparison-lines. Sir William Abney obtained in 1879 a modification of bromide of silver sensitive to slow heat-vibrations, and thus succeeded in directly photographing the solar spectrum between the wave-lengths λ 7600 and λ 10,750. Of 590 absorption-lines measured by him in this region in 1886,[13] only an insignificant fraction have been identified. All these belong to metals with low melting-points.[14] Further, certain bands which Becquerel succeeded in rendering visible by phosphorescence proved assignable to magnesium, calcium, sodium, and potassium.[15] This confirmation of the presence in the sun of potassium was far from superfluous, as only one line due to it can ordinarily be seen.

The recent era in solar chemistry may be said to date from Rowland’s production of a perfect screw in 1882. This minor feat of ingenuity opened the way for vital improvements. Through its means, gratings ruled with almost ideal regularity became widely available, and the difficulties impeding the diffractive mode of light-analysis were removed or diminished. Now observations are mutually comparable only when the absolute wave-lengths of the observed rays are known; and they are derivable immediately from the diffraction spectrum, while in the refraction spectrum several complicating circumstances come into play. Hence the supreme value of gratings. For in the spectra afforded by them the positions of rays depend simply and solely upon the distance from crest to crest of the minute ethereal undulations they represent.

Rowland’s photographic map of the solar spectrum[16] was a document in advance of the time. The amount of detail shown in it may be exemplified by the statement that 150 lines of absorption could be separately reckoned between H and K, the great calcium pair in the violet. No comparable delineations of terrestrial spectra (apart from that of iron) were, however, then extant; coincidences between the rays in them and Fraunhofer lines might, accordingly, be often apparent only, and devoid of chemical significance. Dr. Scheiner gave expression to a general sense of discouragement when he wrote in 1890: “It is unfortunately the case that less is known to-day as to the meaning of the Fraunhofer lines than was supposed to be known ten years ago.”[17]

The need for fresh efforts was, however, promptly met. Photographic investigations of metallic spectra, fully coming up to the new standard of accuracy, were set on foot, among others, by Kayser and Runge at Hanover, by Hasselberg at Stockholm, above all, by Rowland and his coadjutors at the Johns Hopkins University. Here the spectra of nearly all the chemical elements have been photographed with high dispersion for the purpose of solar comparisons. And the end of the process is well within view. Measurements have already been carried far enough to give a multitude of identifications. Between 1895 and 1897 Professor Rowland published in the Astrophysical Journal a “Preliminary Table of Solar Wave-Lengths,” extending from λ 7331 to λ 2976—that is, from dusky crimson up to the highest ultra-violet ray capable of penetrating the aerial barrier. He unhappily did not live to make the list definitive; but it comprises, as he left it, nearly 20,000 lines, about a third of which, by a rough estimate, may be confidently referred to absorption by various terrestrial substances. These are enumerated below, according to the number of lines associated with them in the sun. The corresponding atomic weights are given in a second column.

ROWLAND’S TABLE OF SOLAR ELEMENTS. Element. Atomic Weight. Iron (about 2750 line-coincidences) 56 Nickel 58 Titanium 48 Manganese 55 Chromium 52 Cobalt 59 Carbon (about 240) 12 Vanadium 51 Zirconium 65 Cerium 140 Calcium (over 75) 40 Scandium 44 Neodymium 140 Lanthanum 139 Yttrium 89 Niobium 94 Molybdenum 96 Palladium 106 Magnesium (about 24 coincident lines) 24 Sodium (13) 23 Silicon 32 Hydrogen 1 Strontium 87 Barium 137 Aluminium 27 Cadmium 112 Rhodium 103 Erbium 166 Zinc 65 Copper (2) 63 Silver (2) 108 Glucinum (2) 9 Germanium 72 Tin 117 Lead (1) 207 Potassium (1) 39

Of the following substances no traces could be found the solar spectrum:—

Element. Atomic Weight. Antimony 120 Arsenic 75 Bismuth 208 Boron 11 Cæsium 133 Gold 197 Indium 113 Lithium 7 Mercury 200 Nitrogen (vacuum tube) 14 Phosphorus 31 Praseodymium 144 Rubidium 85 Selenium 79 Sulphur 32 Thallium 232

Argon may now be included among the non-apparent elements, and the presence in the sun of platinum and the metals cognate with it is still an open question. A metal belonging to a very different class was added to the number of solar ingredients by Messrs. Hartley and Ramage in 1897.[18] They convincingly identified two blue rays of gallium with Fraunhofer lines, pointing out that the proportion to iron of the new metal indicated as existing in the reversing stratum was by weight only one to thirty thousand. This accords well with its terrestrial relations. Gallium, discovered by Boisbaudrin in 1875, seems to be widely, but very minutely, diffused throughout the earth’s crust. It occurs also in meteorites. It has an atomic weight of 70, is singularly volatile, and melts almost as readily as butter.

No substance has been more eagerly looked for in the sun than oxygen. But the search was long in vain. Henry Draper’s recognition, in 1877, of bright lines of oxygen in the solar spectrum created a sensation, but proved illusory. J. C. Draper’s dark lines were a still less plausible personation. Eisig ascertained in 1894 that none of the eighty-one emission-lines measured by himself occur in the solar spectrum.[19] Janssen demonstrated, by observations from the summit of Mont Blanc, that the cool oxygen-absorption conspicuous in it is of purely telluric origin;[20] and his conclusion was ratified by Dunér’s application of the motion-displacement test.[21] Oxygen, however, is a substance of most complex, and perhaps unstable molecular structure. No less than six distinct spectra characterise it, two of them produced at low temperatures, and known through their absorptive effects alone; four derived from vacuum tubes, under varying degrees of electrical excitement. Moreover, one of these forms of emission is a series spectrum of the most intricate kind, comprising six different sets of harmonic vibrations, three made up of triple, three of single lines.[22] And here at last a significant coincidence was found. A triplet in the red part of the Fraunhofer spectrum, photographed by Higgs and McClean, was in 1897 clearly identified by Runge and Paschen as a fundamental oxygen group[23] (see Fig. 3). The representation of the element, although certain and authentic, is reduced to a minimum.

The spectrum of helium, which is analogous to the “series spectrum” of oxygen, makes no show in analysed sunlight, but appears bright above the limb. In the case of this substance, moreover, the usual order of detection was reversed. Its recognition as a chromospheric material preceded by nearly a quarter of a century the expulsion in Professor Ramsay’s laboratory of an identical gas from clevite. But about helium more will be said presently.

So far, then, thirty-nine of the chemical elements are known to be common to the earth and sun, and the remaining forty may very well be so likewise. The absence from among the solar ingredients of any single terrestrial species of matter is unproved, and perhaps unprovable. The Fraunhofer spectrum sums up the combined absorption of a heterogeneous mixture of vapours. But the aggregate is widely different from what would be obtained by simply adding together the separate effects. For it is the outcome, so to speak, of struggle and survival. In a medley of ignited substances, the rays of certain among them predominate, while those of others are effaced. Thus non-metals, as a rule, make a poor figure in the spectral competition with metals, and this is doubtless one reason for their inconspicuousness in the sun. Apart from hydrogen, the properties of which are exceptional, only three metalloids, silicon, carbon, and oxygen, contribute to produce the Fraunhofer lines, and their contributions are feeble and fragmentary. That other similar substances—selenium, sulphur, nitrogen, argon, and the rest—may be there, yet exercise no perceptible absorption, is amply possible.

The metals themselves, too, differ widely as regards conditions of visibility. Some are rich in strong lines, favourably situated for observation. Iron is an example. It emits thousands of rays, widely distributed over the spectrum, although most crowded in its higher sections; and they hold their own vigorously against the adverse influences of dilution. Again, the rare metal cerium is extraordinarily prolific of blue rays. No less than 400 were measured by O. Lohse in 1897 in the comparatively narrow region between λ 4000 and λ 4600.[24] Most of them, however, are quite feeble, and only twenty-nine have yet been identified as Fraunhofer lines. Those of bismuth, on the other hand, being all compound, are too diffuse (as Rowland observes) to be detected in sunlight. And most of the radiations of lithium are so highly refrangible as to fall under the ban of atmospheric exclusion. Their reversal in the solar spectrum can thus only be a matter of inference. That the inference should be negative is suggested by the absence of a strongly characteristic line in the carmine red. It ought to be readily seen, if lithium be a solar constituent. Its visibility should be promoted by the small atomic weight—only seven times that of hydrogen—and low fusing-point of the metal; and the persistence of the carmine beam is actually shown by its emergence in the spectrum of the Bessemer flame. Yet it is unlikely that lithium is, in fact, missing from the sun. The case deserves particular attention.

Heavy substances are obviously at a disadvantage as regards the production of absorptive effects. Their vapours must tend to lie low, like carbonic acid in the earth’s atmosphere. Hence the average lightness of the solar elements is only what we should expect. The mean atomic weight of the thirty-five metals represented in the Fraunhofer spectrum is, in fact, just 72, while that of the non-apparent metals is 159. Atomic weight, however, is only one of many conditions affecting this result. The inclusion of lead-absorption in the scroll, and the exclusion from it of lithium, sufficiently prove that vapour-density is far from being alone concerned.

The detection of carbon in the sun was difficult and noteworthy. Originating with Sir Norman Lockyer in 1878,[25] it was ratified by Trowbridge and Hutchins in 1887,[26] and still more decisively ten years later by Rowland. It was, and could only have been made, photographically. The visible carbon bands are barely discernible in the Fraunhofer spectrum. One, however, in the ultra-violet (beginning at λ 3883) comes out unmistakably on sensitive plates. It is due, according to the best authorities,[27] not to elementary carbon, but to cyanogen—that is, to carbon in its combination with nitrogen. The fact is not easy to explain. No other compound body is known to exist in the sun; and it might have been judged à priori impossible that any could prove capable of resisting the enormous temperature reigning near the photosphere. Sir Norman Lockyer attempted to get rid of the anomaly by locating the absorbing cyanogen in coronal regions, where relative coolness must prevail; but recent observations point rather to its presence as a shallow, deeplying stratum. It is certain, moreover, that the line-spectrum derived from free carbon through the exciting influence of a powerful electric spark has no counterpart in the sun. We are then bound to admit, at least provisionally, that the ultra-violet solar band genuinely indicates absorption by cyanogen. There is a further complication. The green fluting, a few shreds of which were measured by Rowland among the Fraunhofer lines, makes part of the typical hydro-carbon spectrum given primarily by acetylene. The whole subject is indeed thick-set with embarrassing considerations; they need careful sifting out. Carbon molecules are remarkably sensitive in their modes of vibration, four of which have been separately distinguished.[28] The conditions, however, prescribing the replacement of one by another are still in large measure obscure. Temperature is concerned, but it is not alone concerned; density, admixture with foreign substances, perhaps variations of electrical state, come into play. Yet the broad certainty has been gained that carbon, in one, if not in several of its many forms, exists in the photospheric neighbourhood. And this has an important bearing, not only upon theories of the solar constitution, but also upon questions of great interest regarding solar relationships with the stars.

Professor Rowland disbelieves in any fundamental difference between solar and terrestrial chemistry. The earth, heated to the solar pitch, would give, he affirms, a spectrum virtually identical with that of the sun. Yet we cannot well ignore evidences, apparently valid, of some real diversity. Even if all our “elements,” without exception, are found in the sun, they are unlikely to occur in the same proportions there as here; quantitative, if not qualitative dissimilarity must be recognised. Thus certain metals, so scarce that their ores rank as mineralogical curiosities, produce marked effects of absorption in the sun. Zirconium, yttrium, cerium, lanthanum may be instanced. Titanium and vanadium are multitudinously represented in the solar spectrum. Hasselberg ascertained for the former substance in 1896 no less than 562 coincidences with Fraunhofer lines out of a total of 718 photographed by him from the metal.[29] On the exclusion of the feeblest rays on his plates as being of quite uncertain origin, the percentage of agreement rose to 88 per cent. The Swedish spectroscopist might well claim that “the presence of titanium in the solar atmosphere is confirmed, with even superfluous evidence, by these investigations.”

Vanadium—first registered as a solar constituent by Sir Norman Lockyer—might be called the satellite of titanium. Where one is, the other is sure to be not far off. Hasselberg’s recent discovery of vanadium in the Scandinavian mineral rutile[30]—a form of titanic acid—accentuates the relationship. Both occur, too, although very scantily, in lead and iron ores, and just traceably in trap and basalt. This close association may be accounted for by inherent resemblance. The atomic weight of titanium is 48, that of vanadium 51. Both are eminently infusible. They share the unusual peculiarity of exhibiting a strong high-temperature affinity for nitrogen.[31] They exemplify, moreover, a transition-stage from metals to metalloids, titanium approximating to silicon, vanadium to phosphorus. Both, it may be added, have been detected in meteorites. A large proportion of the numerous rays emitted by vanadium are reversed in the sun, but somewhat faintly reversed, except in cases of special disturbance, and with these we are not at present concerned. It is worth remarking that titanium and vanadium, notwithstanding their near kinship, physical and chemical, show no coincident spectral lines. In these twin elements, if in any, a common material substratum might be looked for. There is not the slightest sign, however, that it exists.

The new metal germanium is an obvious solar constituent; yet it only lurks in one uncommon terrestrial product. Winkler recognised it in the mineral argyrodite in 1886. And the sun appears to be much richer than the earth in Sir William Crookes’s “meta-elements.”[32] The number of these is almost indefinite; their individualisation, resting upon the dubious principle, “one band, one element,” is often imperfect or misleading; and many of the evasive substances, ranked for a time as separate entities, have failed to make good their footing, and relapsed into the condition of “sub-aggregates of atoms.” The chemistry of “rare earths” has of late assumed a kind of departmental importance. It began in 1794 with the extraction by the Finnish chemist Gadolin of “yttria” from a jet-black material picked up at Ytterby, near Stockholm. “Ceria,” detected in 1803 in the “heavy stone” from Bastnäs, was named after the first asteroid; “lanthana,” obscurely associated with it, came to light in 1839; “didymia,” “terbia,” “erbia,” successively followed.[33] The opening, in 1878, of a fresh and fairly abundant source of supply in the American mineral samarskite started vigorous inquiries into the nature of these remarkable bodies; and Cleve enumerated in 1895 nine fully characterised metallic bases, most of them emitting, under electrical excitation, a brilliant array of spectral beams. The nine “rare” metals are scandium, yttrium, lanthanum, cerium, erbium, praseodymium, samarium, gadolinium, and ytterbium. The first four absorb strongly in the sun, where the presence of erbium and of neodymium, a constituent of the original didymium, is also evident. Twenty-two additional meta-elements swell Sir William Crookes’s “suspense account,” but only a minority are at all likely to obtain ultimate recognition as substantive forms of matter. Perhaps the surest test of their quality will be found in the appearance among the Fraunhofer lines of their characteristic emissions. Those, at any rate, of terbium, holmium, and thulium should be carefully looked for.

The light of glowing metallic vapours tends, as we have seen, to suppress or efface the rays of non-metals. Professor Trowbridge made some experiments in 1896 with a view to determining the conditions of obliteration. Photographing on a single plate the spectra of pure carbon and of an electric arc between carbons containing 28 per cent of iron, he found that the iron ingredient sufficed very nearly to wipe out the carbon bands in the arc.[34] “This proportion, therefore, of iron,” he remarked,[35] “in the atmosphere of the sun, were there no other vapours of metals present, would be sufficient to prevent our seeing the full spectrum of carbon.” An interesting illustration was thus afforded of a fundamental principle in solar interpretations. The principle, indeed, has scarcely yet begun to be applied. Hitherto the absorptive effects of each of the forty substances vaporised above the photosphere have been considered apart. But they are not independently produced. They are often profoundly modified by extraneous action. A systematic investigation of the various modes in which it comes into play is desirable, although likely to prove arduous. “There is at the present time,” Dr. Ames wrote in 1895,[36] “no more fruitful field open to research than that of the study of the influence of the presence of one substance upon the spectrum of another.” And Mr. Percival Lewis’s recent treatment of the subject has had the preliminary result of showing that “very small traces of an impurity in a gas may cause considerable changes in its spectrum, whether this impurity be chemically active or not.”[37] The changes, too, are in many ways perplexing. They are governed by no traceable rules, depending apparently, in each case, upon intimate, and to us unknown, molecular relations with electricity.[38] The explanation of their anomalies is evidently needed for the satisfactory future progress of solar chemistry.

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III. PECULIARITIES OF THE SOLAR SPECTRUM

The solar spectrum is densely thronged with unidentified lines. Of these upwards of twelve thousand have been measured and registered, but lack chemical interpretation. They are, however, on the way to receive it. Their recognition will doubtless attend the gradual progress of acquaintance with metallic spectra. Thus cerium, scandium, and other bases of “rare earths” may satisfactorily account for a considerable proportion of them, these substances emitting crowds of rays, as yet only in part recorded. But besides, say, twelve thousand catalogued “unknown” lines, an inestimable number remain unnoticed.[39] They are still, as it were, “in the street”; they have not been admitted even to the antechamber of science; the preliminary steps to their identification have not been taken. They will of course be taken in due time, little by little, as the photography of the Fraunhofer spectrum is brought nearer to perfection; and to many of them chemical meanings full of interest will certainly be assigned. Nevertheless, it can scarcely be expected that the significance of all can ever be made plain. To the very end, probably, a residuum will keep the secret of an origin due to forms or conditions of matter strange to terrestrial experience.

That the type of the sun’s spectrum becomes modified in the course of ages—that it has been, and will again be different from what it now is—may be admitted without hesitation. But this evolutionary change is effected imperceptibly at more than millennial leisure. It might, however, have been expected that transient alterations would manifest themselves—alterations caused by tumultuous movements in the “reversing layer,” or connected, possibly, with periodical outbreaks of spots and prominences. Yet almost none of this definite and obvious character have been noticed. Only a few lines may be set down as somewhat vaguely and indeterminately variable. To take a few examples. In 1891 Father Sidgreaves of Stonyhurst obtained several photographs of the group b (magnesium and iron) in the green part of the spectrum. They showed with excellent definition more faint lines than are contained in Rowland’s or Thollon’s maps; yet one relatively strong in them—“Winlock’s No. 17”—was barely discernible.[40] This indication of change does not seem to have been followed up. Again, Sir Norman Lockyer noted in 1873 the disappearance of a zinc line in the red (Ångström λ 6361·16), which, nevertheless, was seen as usual in 1878, and has not since been missed.[41] The most recent instance of the kind was vouched for from Baltimore. To a faint, slightly nebulous line of unknown origin in the ultra-violet (λ 3719·796) Rowland attached the note, “Variable, though not atmospheric.” Jewell[42] describes it as situated within the shading of a strong iron line, and as “quite distinct upon some plates, while not visible upon others showing lines closer to the iron line, and much weaker than the variable line,” as it originally appeared. It has also been photographed in an intermediate condition, so that its fluctuations of intensity may be said to be ascertained, although their law and cause remain wholly obscure. The only hope of learning anything about these is by continuous and minute observation, which should extend to other suspicious cases of the same kind. Certain interesting questions might thus be answered. For instance, are the alleged alterations connected effects of some general disturbance, or do they occur sporadically, each on its own account? Can they, in any way, be brought into relation with the spot cycle? Are they visible in light taken indiscriminately from all parts of the sun, or are they confined to special localities? These may serve as specimens of the inquiries suggested by phenomena, perhaps none the less significant for being inconspicuous. With the camera at hand the task of daily comparison becomes easy and simple. As Professor Hale wrote in 1896,[43] “Every photograph of the solar spectrum taken with high dispersion must now be regarded as a document of great value, which may ultimately reveal irregular or periodic changes in the condition of the gases and vapours of the solar atmosphere.”

The Fraunhofer lines have of late forfeited their early reputation as “constants of nature.” They are not really “fixed”; their positions in the spectrum are affected by several minutely modifying causes, and they cannot, accordingly, be depended upon as standards for the most refined measurements. It is true that only the extreme accuracy of modern methods has caused them to “step down” from the high level of invariability, for their deviations are very small, and might, superficially regarded, appear negligible. They are of two kinds, physical and kinematical, the former being produced in the very act of emission, the latter in the course of transmission. Pressure-displacements and motion-displacements are, in fact, respectively concerned.

Symptoms of a persistent shift of the Fraunhofer lines towards the red were first detected in 1890 by Professor Lewis E. Jewell of the Johns Hopkins University. Persistent, although unequal. It is not the same for the lines of different elements; it is not even the same for all the lines of the same element. Motion, then, is not its cause. Fortunately, a clue was supplied by laboratory-experiments. Attentive study of the behaviour of metallic lines under varying conditions showed that “with an increase in the amount of material in the arc there was increasing displacement towards the red.” “Considering the subject carefully,” Professor Jewell adds, “there seemed no reason to doubt that the wave-length of a line depended, to a certain extent, upon the conditions under which the material producing the line was present in the electric arc, the vacuum tube, or the solar atmosphere; or, in other words, the vibration period of an atom depends to some extent upon its environment. An increase of the density of the material, and presumably an increase of pressure, seemed to produce a damping effect upon the vibration period.”[44] Confirmatory results were obtained by Messrs. Humphreys and Mohler,[45] and the assumption of a constant vibration-frequency as an essential attribute of the ultimate particles of matter had to be finally abandoned.

The observed changes are clearly distinguishable from ordinary temperature effects. Lines are often broadened; under peculiar circumstances they may be unsymmetrically broadened by thermal influences; but simple displacements are never due to heat. Moreover, they can be produced artificially by condensing the air about an electric arc, so that their immediate cause is not doubtful. “It was often easy,” according to the Baltimore investigators, “to observe a line gradually change its position while the pressure was being let off without alteration in width or other appearance.”

The general upshot of their inquiries[46] was to show that the spectral shifts in question, far from being an isolated phenomenon, stand in close relationship to all the most intimate properties of matter. Their amount, cœteris paribus, is proportional to the pressure and to the wave-lengths of the shifted lines. It differs, however, for each series in a given spectrum. For different substances it is usually large or small in the inverse ratio of the absolute temperatures of their melting-points. Again, it is largest for those substances which expand most readily with heat. Finally, and most significantly, line displacements are, in the same group of elements, proportional to the cube roots of their atomic weights. Or, as Mr. Humphreys expresses it, “The shift of similar lines is a periodic function of atomic weight, and consequently may be compared with any other property of the elements which itself is a periodic function of their atomic weights”—that is to say, the measured displacements show recurring maxima and minima in passing from one to the next of Mendeléef’s elemental families. Their gradations thus correspond with those of other physical attributes of material species, and plainly imply that the retarded vibrations are executed by “ultimate” atoms. The confirmatory fact should be noted that band-spectra, universally associated with aggregates of atoms, display no sensitiveness to pressure. And by pressure in this connection is to be understood, not the separate density of the vapour emitting the damped rays, but the total pressure of all the substances promiscuously diffused throughout the stratum or enclosure.

Its complex effects add, in some respects, to the difficulty of interpreting spectral appearances; but they lend to them, on the other hand, new and unlooked-for significance. In solar inquiries more particularly, they have started a fresh lead, sure to be followed up. Thus indications may be gathered from them as to the relative altitudes in the sun’s atmosphere at which different Fraunhofer lines originate, no less than as to the absolute pressures to which they correspond. These are lower than might have been anticipated. They range, according to Professor Jewell, “from little more than zero to only two or three atmospheres, though the shading of the stronger lines may be produced at a greater pressure.”[47] The subject, however, has not got beyond the stage of inception. One important branch of it, the discrimination of lines belonging to the same series, is barely sketched. The phenomena of displacement through pressure evidently involve much more than is yet apparent. They must be present in stars and nebulæ, and may afford curious disclosures regarding their states of density and rarity.

The Fraunhofer lines are, as a rule, narrow and sharp; but minute photographic study reveals, in a certain proportion of them, singular complexities of structure. These are illustrated from Professor Jewell’s observations in Fig. 4, which shows graphically, by four typical examples, the comparative distribution of light in corresponding solar and arc lines.

No. i., a green ray of iron, is bordered in the sun (where it is of course reversed) by a filmy illumination, “the remains of an emission line, either produced at the photosphere, or lower down in the solar atmosphere than the absorption line.”[48] The notch at the summit of the same line gives evidence of radiation at a high level. It is, in fact, an abortive bright iron line, superposed upon a strong absorption line, itself superposed upon a faint effusion of light of identical quality from underlying vapour. Thus this single line is built in three stages, although the foundation and coping are barely discernible as traces of luminosity. In Nos. ii. and iii., “shaded lines” are depicted in the same manner as the “sharp line” in No. i. They belong respectively to iron and magnesium (λ 5183·8 = _b__{1}). Their characteristic feature is the outlying obscurity, which deepens from the edges towards the central shaft. Professor Jewell remarks that the gas producing these shadings “extends through a much greater range of pressure” than that giving rise to the green iron line (No. i.), while the clean-cut line in the middle must be due to absorption “much higher up in the solar atmosphere, where the pressure is very much less.” Similar appearances are conspicuous in the great calcium pair H and K (see Fig. 4, No. iv.). Here the abnormal breadth of their wing-like appendages proves that the “absorption must persist through an extreme range of pressure, or that the amount of calcium gas varies enormously in the solar atmosphere where this absorption is produced.” These lines are obviously twice reversed. A stratum of radiative calcium is apparently interposed, in the sun’s neighbourhood, between two absorptive strata of the same material. That their arrangement is, however, subject to some kind of disturbance is indicated by the irregularity of the diagram; nor is it always disturbed to the same extent. “Upon some plates,” Professor Jewell says, “the central absorption line is almost symmetrical with respect to the emission line, while upon other plates its unsymmetrical character is very marked, the central line being displaced considerably towards the red, and the part of the emission line on the violet side of the central line being much the strongest.” Motion-displacements due to ascending and descending currents are thought to be in question,[49] but there are obstacles to be removed before this explanation can be unreservedly accepted. The extreme difference of velocity suggested by the observed dissymmetry of the calcium lines amounts to no more than 75 miles a minute, but is notably variable. All the “shaded lines” in the spectrum appear to be similarly affected, though in a minor degree. Thus the descending motion corresponding to the narrow central components of the sodium “D” are at the rate of barely one-fifth of a mile per second. Most of the fainter lines, on the contrary, indicate ascending currents over the solar surface at an average speed of about a third of a mile a second.[50] Motion-displacements, besides, due to the earth’s rotation and the eccentricity of its orbit, can be detected, and have been allowed for. These latter minute corrections naturally apply to all the solar lines without distinction.

Enough has been said to give an idea of the manifold considerations which have to be taken into account in estimating the true wave-lengths of the solar absorption rays. They are changed, according to a special and complex law, by the sun’s rotation; they are changed by the movements of approach or recession of the earth as a whole, as well as of each particular spot on the earth; they undergo alteration through the solar atmospheric circulation; they are affected by pressure, perhaps by other undetected influences, and each of these modifying causes acts variably, either in time, or according to locality on the solar surface. Happily, most of them act only to an infinitesimal extent; but their unquestionable, although slight effectiveness illustrates very strikingly the subtlety which every increase in accuracy necessitates in the methods of science.

An embarrassing peculiarity of the Fraunhofer lines is their virtually uniform intensity all over the sun’s disc. Just as the telluric bands develop with the sinking of the sun, they ought to become strengthened near the limb; yet they remain sensibly the same, notwithstanding the greatly augmented depth of the absorbing strata traversed by the light before reaching the eye. This is really a glaring anomaly, and one almost forgotten through sheer hopelessness of getting rid of it.

In concluding this brief chapter we would once more draw attention to the curious individualities of the Fraunhofer lines. They are constructed, in many cases, at successive levels; they are modified by various influences. Some are of hair-like fineness; others, emanating from an identical substance, have nebulous edges. Moreover, the sharp and the diffuse lines respond differently to pressure, so that their characteristic aspects are significant of profound distinctions in their mode of origin. The more closely, in fact, these mysterious rulings are examined, the less trivial or casual their slightest diversities appear. They are charged with meaning, transcending, in part, our actual powers of interpretation, but challenging efforts towards that end, which cannot fail to breach, if they do not wholly raze, the ramparts of ignorance.

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IV. THE REVERSING LAYER

During the eclipse of 22nd December 1870 a new phenomenon came into view. Professor Young of Princeton, New Jersey, was the fortunate observer. With the slit of his spectroscope tangential to the sun’s limb and perpendicular to the moon’s advance, he awaited the moment of second contact. The thin solar crescent narrowed second by second; at last it vanished; then “all at once, as suddenly as a bursting rocket shoots out its stars,” the ordinary Fraunhofer spectrum previously visible was replaced by a serried array of bright lines on a dark background. They seemed a complete reversal of the familiar absorption-rays, and the impression was also conveyed to Mr. Pye, a member of the same party, of “all the dark lines being converted into bright ones.”[51] The “flash” at the edge of the eclipsed sun was not unlooked for. Something of the kind had been anticipated as the due accompaniment of the beginning and end of totality. For Kirchhoff’s explanation of the Fraunhofer lines implied the interposition, between the eye and the sun, of a screen of glowing vapours, which should be separately, if only instantaneously, visible on the withdrawal of daylight glare—visible, that is to say, spectroscopically; with the telescope nothing more distinctive than a silvery shimmer[52] corresponds to the dazzling variotinted fireworks disclosed by the prism.

But their disclosure was not enough; they demanded close investigation. The question is fundamental in solar physical theory whether the flash is the true reversal of the Fraunhofer spectrum, and no conclusive answer could be given to it except by photographic means, visual reports as to the details of so intricate and evanescent an apparition counting for very little. Twenty-six years, however, elapsed before a permanent record of it was secured. The result ensued from a skilfully-timed snapshot by Mr. Shackleton at Novaya Zemlya during the Arctic eclipse of 9th August 1896. He gave an exposure of half a second with a “prismatic camera”—a simple form of spectrograph, destitute both of slit and collimating lens, the employment of which in eclipse work has been vigorously promoted by Sir Norman Lockyer. An impression was thus caught of singular interest and value. We may quote Professor Young’s description of it. “The photograph,” he writes,[53] “shows a long range of several hundred bright curved images, of which there are nearly 250 in the blue portion of the spectrum between F and H. About 25 are much more extensive and conspicuous than the others, and are images of the chromosphere and prominences. They are due to hydrogen, calcium, helium, strontium, and one or two other elements which often appear in the chromosphere. The rest are simply reversals of the Fraunhofer lines, as Mr. Shackleton has shown by developing the flash spectrum into a bright-line spectrum of the usual form (which is easily done by a simple mechanical contrivance), and comparing it with an ordinary dark-line solar spectrum photographed with the same camera and prisms, but with the addition of a collimator and slit. The agreement is practically complete, although there are two or three somewhat conspicuous Fraunhofer lines which are missing in the flash spectrum, probably because they originate, not above the surface of the photosphere, but in its depths, as probably also do the wide, hazy shadings that accompany the H and K lines and some others; but this is a matter for further investigation.”

The solitary success of 1896 was manifolded a year and a half later. “Reversing-layer” photography stood in the forefront of the programme of work for the Indian eclipse of 22nd January 1898, and the documents collected during its hundred seconds of obscurity showed that a complete mastery of the art had been attained. Sir Norman Lockyer and Mr. Fowler, Captain Hills, Mr. Evershed, and Professor Naegamvala secured photographs of the flash, not only in its full development, but also when incipient and vanishing, so that the phenomenon could be traced at leisure throughout its brief phases. Two specimens are reproduced in Plate III., both taken instantaneously by Mr. Evershed with a prismatic camera of 2¼ inches aperture and 36 inches focus.

The conventional straight appearance of the lines results from the employment of a cylindrical lens to give breadth to narrow slices of the curvilinear originals. The upper section represents the spectrum of the last thread of sunlight just as the accustomed dark lines were fading out before the incoming of their bright correlatives. The range is from below H and K—the strong pair to the left—to λ 3350 in the ultra-violet.[54] The lower section corresponds to a moment twenty seconds later, when the continuous light was gone, and vivid rays dominated the field. Amid the throng, twenty-seven members of the hydrogen series are recognisable, and three titanium lines rival them in importance.

Ample materials were provided during the eclipses of 1898, 1900, and 1901 for at least a preliminary discussion as to the true character of the reversing layer, although the difficulties still remaining to be encountered are neither few nor trivial.

To begin with, the fact has been ascertained that a shell of mixed incandescent vapours, five or six hundred miles thick, encloses the photosphere on every side. We see it, however, in projection. The line of sight penetrates it tangentially at the edge of the sun, and to an extreme depth near the base of about forty thousand miles. Between the reversing layer and the chromosphere there is no solution of continuity; to some extent, undoubtedly, the lower merges into the upper formation; yet they are essentially distinct. Each has its own spectrum apart, notwithstanding a certain amount of community, apparent, casual, or partial. Thus since the reversing layer is visually accessible only through the enveloping chromosphere, the spectrographic prints taken at sun-and-moon contacts are inevitably composite. They include the chromospheric together with the “flash” rays. Discrimination is, however, facilitated by the notably greater length of the arcs representative of the former corresponding with the higher extension of the substances emitting them. The class of discrepancies between the flash and the Fraunhofer spectrum thus accounted for are discrepancies by excess. The flash includes helium rays; the Fraunhofer spectrum has none. The flash exhibits the complete hydrogen series up to its extreme limit in the ultra-violet; the Fraunhofer spectrum reverses only its visible members. Titanium lines strong in the flash are feeble in the Fraunhofer spectrum; besides other analogous dissimilarities. Yet they do not affect the claim of the reversing layer to be, speaking broadly, the locus of solar absorption. Rather they bring us face to face with the totally different question, Why do the chromospheric gases exercise no appreciable arresting effect upon the light transmitted through them? Later on we shall attempt to answer it; here we need only remark that the rays from the chromosphere cannot be excluded from photographs of the flash. They necessarily appear in them, and it was known beforehand that they had no counterparts in the Fraunhofer spectrum.

There are besides discrepancies by defect. Many solar absorption lines do not show bright at the first and last instants of totality. But this is easily understood. Some are doubtless too faint to assert their presence photographically. Others may be supposed, with the utmost probability, to originate out of sight at the base of the reversing stratum. The shadings of H and K certainly do, for the central lines start out clear, though strong, in the flash, and their hazy appendages are indubitable products of augmented pressure. Nor do the denser vapours rise high enough to make any perceptible display. Mr. Evershed tells us[55] that, while nearly all the metals with atomic weights less than 60 are represented in the sun’s marginal spectrum, none, of which the ultimate particles are heavier than 92, make any assured contributions to it. A formation at least five hundred miles in vertical extent must vary widely between top and bottom both in composition and density. So at least we should reasonably anticipate. In point of fact the changes indicated are surprisingly slight. One criterion is available by which chemical differences can be correlated with differences of depth. This consists in the various lengths of the curved lines representing the emissions of the sundry constituents of the “layer.”[56] Substances attaining high altitudes, like the chromospheric gases, give long arcs because their visibility extends over a wide section of the sun’s circumference, while low-lying materials, illuminating a narrow verge, are characterised by short arcs. They are, moreover, the most difficult to catch as the moon goes by. Now the great majority of the flash lines are of a nearly equal length, corresponding to an arc of about 40° on the solar limb, and this equality implies a considerable approach to uniformity of constitution throughout the greater part of the momentarily exposed layer. But its basal stratum, perhaps not more than a few miles in thickness, should be that most effective in absorption,[57] and it forms a crescent much too fine to be directly seen. Here below, then, down near the photosphere, missing Fraunhofer lines with no apparent corresponding radiations may be produced; nay, must, since there is absolutely no evidence of the corresponding light-stoppage taking place in, or above the chromosphere.[58] Mr. Evershed’s conclusion is indeed fully warranted, that “the flash spectrum as a whole appears to represent the upper more extensively diffused portion of a stratum of gas, which, by its absorption, gives the Fraunhofer spectrum.”[59] The appellation “reversing layer” would then seem to be no misnomer, but to indicate correctly the seat of the linear absorption which serves as our alphabet for spelling out the secrets of solar chemistry.

The density of this vaporous envelope is measurable by the “pressure shifts” of the Fraunhofer lines. It would seem to be nowhere less than that of our atmosphere at sea-level; otherwise displacements towards the blue end should occur, and none such are perceptible. Nor, on the other hand, is there proof of its exceeding, even in the lowest depths, three or four times the standard value. So that the increase of pressure downward is exceedingly slow—a fact to be carefully noted. The distinction (already adverted to) between the effects of total and of partial pressure is also most important. Through the former the positions of spectral rays are modified, through the latter their characters. In other words, the shifts of the Fraunhofer lines correspond to the sum of incumbent vapours, while the quantity of each separately present determines their width and diffuseness. Most are associated, by their fineness and sharpness, with individually tenuous substances. The hydrogen lines, for instance, represent, according to Mr. Maunder,[60] a pressure of only one-hundredth of an atmosphere. But the indications in this respect, as in others, vary greatly for the different vapours.

The most refractory substances, such as titanium and vanadium, are volatilised in the reversing layer. It is, then, enormously heated. But it is cooler than the photosphere, since its rays show dark against the vivid background they are projected upon. Now the temperature of the photosphere, by the most authentic recent determination, is about 6600° centigrade,[61] and this marks an upper limit for the temperature of the reversing layer. A lower limit is fixed by the temperature of the electric arc, estimated at 3500°. The much higher grade of the spark is almost certainly not attained. The inverse behaviour of two magnesium lines, first commented upon by Professors Liveing and Dewar,[62] led Dr. Schemer to this conclusion.[63] One at λ 4352 is prominent in the sun and strong in the arc, but fades out in the spark; the other, at λ 4482, of which a mere trace is perceptible in the sun, is a characteristic spark-product. It must, however, be borne in mind that comparative temperatures are subject to great uncertainty where electricity is the exciting agent. Dissentients are even to be found from the broad proposition that the spark is hotter than the arc; nor is it one capable of direct demonstration. Qualifying circumstances come in, and their separate effects are not easily unravelled.

M. Deslandres made some curious experiments at Paris in 1894 in photographing the sun by means of the dusky rays in its spectrum.[64] For their isolation he used a “double slit”; and since they are only comparatively dark, no difficulty was encountered through want of actinic power in the rays dealt with. He thus succeeded in obtaining with each a monochromatic picture of the sun delineated exclusively with emissions from some particular ingredient of the reversing layer. The uniformity of elemental distribution was, by this ingenious device, put to the test. Photographs of the disc, for instance, taken on an iron line might be expected to show different features from those taken on calcium or magnesium lines if local accumulations of those vapours were present; but no divergences of the kind became perceptible. The composition of the absorbing envelope did not seem to vary regionally. The investigation, however, was not carried far, and would be worth prosecuting.

The fact of the existence of a true reversing layer may now be looked upon as established; yet the mode of its existence remains in several ways perplexing. The slightness of its absorptive action needs explanation at the outset. One notes with amazement that the miniature atmosphere surrounding an electric arc is equally effective for light-stoppage with this ocean of vapours. Then there is the singular, and perhaps related circumstance that the spectrum from the limb is not more deeply grooved than the spectrum from the central parts of the disc. The results upon light of being sifted through six hundred and through twenty thousand miles of the mixed materials glowing near the sun are virtually the same. Their comparative tranquillity, too, is unexpected. The reversing layer lies between two agitated structures. Beneath are the photospheric clouds, rent and whirling under the stress of cyclonic disturbances; above, the chromospheric flames, driven hither and thither by influences of fantastic violence. Yet a region of peace seems to intervene. The Fraunhofer lines indicate a steady vertical circulation, but scarcely ever a temporary commotion. By a rare exception, Father Fényi observed at Kalocsa, 27th July 1887, the dark C in the neighbourhood of a spot-group, displaced alternately towards the blue and the red, indicating, he supposed, a powerful disturbance of the reversing stratum by an irruption of hot hydrogen.[65] Such invasions of its precincts, however, are under the ban of some prohibitive decree, or they encounter unknown difficulties. They occur, at any rate, with remarkable infrequency.

The reversing layer is heated from below, and gravitates downward. Thermal equilibrium is doubtless maintained by the convective transport of material, but the due effects of superincumbent weight are unapparent. Evidence is not indeed wanting of some increase of density with descent, but of an increase relatively insignificant. Gravity at the sun’s surface possesses nearly twenty-eight times its terrestrial power; hence a true solar atmosphere should double its density with each furlong of approach to the sun’s surface,[66] and the total increase of pressure in an envelope five or six hundred miles deep would be “inexpressible by numbers that have name.” Actually there is, at the most, a quintupling of pressure. This formidable discrepancy is altogether unexplained. We are debarred by it from considering the reversing layer to be in statical equilibrium. Its successive strata do not rest one upon the other under the sole dominion of gravity. Some counteracting influence is brought to bear. This problem of levity—so to call it—is one that perpetually recurs in studying the solar surroundings.

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V. HYDROGEN, HELIUM, AND CORONIUM

Three tenuous gases—hydrogen, helium, and coronium—are of essential importance in solar physics. The first plays also a leading part in terrestrial and vital economy. The second exists on the earth merely as a chemical curiosity. The third must for the present be classed as an exclusively solar product.

Solar hydrogen was discovered by Ångström in 1862. He recognised it by the identity of its three least refrangible rays with the Fraunhofer lines C, F, and G[67] (now designated Ηα, Hβ, and Hγ), to which, in 1865, he associated the indigo line h (Ηδ). A fifth line (Hε), photographed by H. W. Vogel in 1879, is situated quite close to the calcium H—so close that, like Teucer behind the shield of Ajax, it lies concealed, in the sun, under covert of its neighbour’s broad shadow, if indeed it be present in the Fraunhofer spectrum at all; for it is so effectually hidden that the point remains uncertain. Shortly afterwards, Sir William Huggins’s spectrographic investigations of Sirian stars gave the key to the true character of the hydrogen emissions. Nine ultra-violet lines came out on his plates, and their rhythmical arrangement at intervals continually lessening upward left no doubt of their forming a connected series. That this included the visible lines was manifest at sight. Its law was stated by Balmer in 1885.[68] The relations expressed by his formula are not those of wave-lengths, but of their reciprocals, wave-frequencies. These quantities obviously bear to each other an inverted proportion. Deep crimson light, for instance, consists of undulations about twice as long as those of violet light; only half as many of them, accordingly, enter the eye in a given time. A doubled length corresponds to a halved frequency, a tripled length to a frequency of one-third, and so on. Now oscillation-frequencies are, for several reasons, more important natural constants than wave-lengths; hence until they were made the basis of investigation, no real progress was effected in the detection of spectral series.

Balmer’s law has the following form: N = N_{0} − (4N_{0})/(m^2), where N is the wave-number ((1)/(λ)), N_{0} is a constant to be determined by trial, and m is any integer greater than 2. By assigning to N_{0} the empirical value 27418·75, the places in the spectrum of each individual ray emitted by hydrogen may be calculated with approximate accuracy. That of C (Hα) corresponds to m = 3, and the series has been photographed up to m = 34, its constituent lines growing fainter and more crowded as the scale is ascended. They approach, in fact, with the increase of m, indefinitely near to a definite limit, marked by the constant N_{0} minus 0 (the second term having disappeared). This limit, known as the “convergence frequency,” is a distinctive feature of spectral series.

Many have contributed to their elucidation. Johnstone Stoney,[69] Alexander Herschel,[70] Hartley,[71] and Cornu[72] prepared the ground, and the subject was treated, in its larger bearings, and with more definite results, by Liveing and Dewar,[73] Schuster,[74] Rydberg of Lund,[75] Runge and Paschen[76] of Hannover, Kayser[77] of Bonn, and Ames[78] of Baltimore. Their labours have been unexpectedly successful in educing partial order out of all but total emissive confusion. Harmonic series of identical type were marshalled from promiscuous throngs of rays, and their association into sets of three, or even into double sets of six, simultaneously given forth by a single element, proves the extraordinary complexity of the molecular systems through the movements of which they originate. That some of these movements are of an orbital nature is strongly indicated, and they not improbably show perturbative effects analogous to those manifested in lunar and planetary revolutions. “The final impression,” M. Balmer writes,[79] “which our mind involuntarily receives in contemplating these fundamental relations is that of a wonderful mechanism of nature, the functions of which are performed with never-failing certainty, though the mind can follow them only with difficulty, and with a humiliating sense of the incompleteness of its perception.”

Until 1897 the spectrum of hydrogen was thought to be of unique simplicity. It apparently consisted of one individual series resembling that formed by a musical note and its overtones. No outstanding lines interrupted the perfect regularity of the progression. All this, however, was changed by Professor Pickering’s discovery, in a few peculiar stars, of a second hydrogen series.[80] It is associated with the first in such a manner as to indicate that both are subordinate to a principal series, the three together forming a triple group on the normal pattern. Of the principal series, one member has been probably identified as a blue band in certain “bright-line” stars,[81] the rest being placed inaccessibly high up in the ultra-violet. They would be cut off by atmospheric absorption. None of the new hydrogen rays occur in the sun, and none have, so far, been rendered visible in the laboratory, possibly because the temperatures available are inadequate for their production. This indeed is a matter of conjecture; what is certain is that hydrogen affords the only known example of a spectral series capable of isolation from its fellows. Here evidently we have a clue to some specialty of intimate structure, the guidance of which may lead to surprising disclosures.

Hydrogen has other singularities. In some respects it is solitary among the elements. The “periodic law,” by which their properties are connected with their atomic weights, does not apply to it. Chemically and electrically it behaves as a metal; reduced to the liquid state, however, it definitely ranges itself with non-metals. Its condensation is effected with the utmost difficulty, physical and mechanical agencies being only just competent to vanquish the elasticity of this lightest of terrestrial substances. But what force can barely compel, affinity readily obtains. United to oxygen under the form of water, it can exist as a liquid up to a temperature of 100° C., and it is of all gases the most readily “occluded.” Imprisoned thus in metallic masses, it remains inert for unlimited periods, but recovers freedom and activity by heat. Meteoric irons bring to the earth no inconsiderable supply of occluded hydrogen, and palladium can take it up to the extent of six hundred times its own volume. In this quasi-combination it is, by a curious anomaly, six times denser than when liquefied by sheer cold.[82]

The volatility of hydrogen perhaps transcends the earth’s power of control. By a necessary consequence of the kinetic theory, adverted to by Dr. Johnstone Stoney in 1870,[83] light gases in a free state can be permanently retained only by massive globes. For atmospheric particles no sooner attain a speed just overbalancing the holding power of gravity than they irrevocably fly off into space, and the process being continued unintermittently, eventuates in the total dissipation of the envelope they once constituted. It is, however, a matter of some delicacy to discriminate between the gases that may escape from any individual planet and those that must remain. According to a recent calculation,[84] the earth could now maintain a hydrogen atmosphere virtually without waste; but in former ages, when the agility of the gaseous molecules was quickened by heat, the strength of its grasp upon them must have been insufficient for their lasting retention. This was nevertheless effected by their reduction to the liquid state in the form of water. The presence of an excess of oxygen hence saved terrestrial hydrogen.

Only the four lowest members of the hydrogen series show dark in the sun.[85] The absence of the higher rays is enigmatical. All are ablaze in the chromosphere; but the chromospheric gases emit sensibly as much light as they stop. In the reversing layer it would then seem that hydrogen glows so imperfectly as to emit vibrations of no more than four or five qualities, the upper “notes” being somehow quenched. It might be supposed that the temperature there is too low for their production, were it not that they have been photographed from vacuum tubes held, on good grounds, to be cool relatively to the electric arc. The true explanation is probably to be found in the heterogeneous composition of the stratum in question. Intermixed particles of different kinds of matter mutually check each other’s oscillations, and those of shortest periods are the most susceptible to this adverse influence. Its nature and the laws of its action remain obscure, but much may be learned about them by careful experimental inquiry.

A similar anomaly is more markedly visible in the case of helium. This gas exists near the sun in scarcely less profusion than hydrogen, yet the Fraunhofer spectrum includes no trace of its action. Absorptive nullity is not a quality inherent in the substance, as we shall see presently; hence it probably depends, like the partial inertness of hydrogen, upon conditions present in the reversing layer.

Until March 1895 helium was known only as a chromospheric element. A bright yellow ray at λ 5876, distinguished as “D_{3},” because it forms a trio with the sodium pair D_{1} and D_{2}, was noticed in the prominences uncovered during the eclipse of 18th August 1868, and can always be observed spectroscopically at the edge of the sun. But the substance emitting the yellow ray lay outside the range of our acquaintanceship, and seemed unlikely to be brought within it. That contingency, nevertheless, came to pass. In the course of a search for compounds of argon, Professor Ramsay, at the suggestion of Professor Miers, fortunately examined the reputed nitrogen occluded by the Scandinavian mineral “clevite.”[86] This velvety-black stone, remarked as peculiar by Nordenskiöld and analysed by Cleve, is a kind of pitch-blende, composed of uranate of lead mixed with rare earths. The gas evolved from it at University College gave a brilliant spectrum, in which the prominence-line D_{3} shone conspicuous. Helium was indeed captured! A beautiful confirmation of the identity was soon afterwards afforded. The golden line seen in the laboratory was perceived by Runge to have a faint close companion, and he declared that, unless the solar D_{3} were also double, clevite-gas should be regarded as different from helium.[87] The challenge was taken up on both sides of the Atlantic. Professor Hale on 20th June, and Sir William Huggins independently on 10th July, succeeded in resolving the prominence-ray into a delicate, unequal pair, and our possession of helium as a truly indigenous element was rendered incontrovertible.

Meantime sundry other leading chromospheric rays—four especially, coloured deep red, green, blue-green, and intense violet[88]—had been recognised in the complex spectrum of clevite gas.[89] The task, however, of reducing its tangled rays to harmonic order seemed desperate until it was performed. Without exception, they ranged themselves, at the bidding of MM. Runge and Paschen, into six related series (see Fig. 5). These form two sets, each consisting of a subordinate pair drawing together towards a common limit in the ultra-violet, with a principal series “leaping over the other two in large bounds,” and ending in the more refrangible part of the spectrum.[90]

Not only did Runge and Paschen’s formula (which may be regarded as a modification of Balmer’s law for hydrogen) include all the perceptible emissions of helium, but it intimated the presence of others beyond the reach of ordinary observation. Each of the principal series, it was inferred, should own a “leader line” far down among the heat rays, and with the aid of the bolometer the prediction was strictly verified. Thus “the actual spectra” (as Mr. Maunder remarked) “corresponded to the theoretical, and were complete from their rise far in the obscure regions of the infra-red till they died away in the darkness which lies on the other side of the visible spectrum.”

Their number, however, suggested a twofold origin, since there was then no precedent for assigning more than three series to a single substance. Clevite gas was accordingly regarded as a mixture of two solar elements, distinguished as “helium” and “parhelium,” the rays of the former, like D_{3}, being all double, those of the latter single. Each set of three series was, in fact, “analogous to the complete spectrum of a distinct element.” Yet parhelium has failed to make good its footing in either cosmical or terrestrial chemistry. Attempts to isolate it have entirely failed, and the spectroscopic argument for its existence collapsed with the discovery that oxygen, no less than clevite gas, claims six series, which are certainly inseparable, and represent in combination the vibrations of perfectly similar, highly intricate molecular systems. “Parhelium” may then safely be treated as fictitious. Clevite gas, or mineral helium, is the identical undiluted material of prominences. The hypothetical companion-stuff exists neither in the sun nor upon the earth.

The qualities of helium are most unusual. Like argon, it is monatomic; its ultimate chemical units are the same as its ultimate mass-units. This inference is based upon the heat-relations of the substance. Its vapour-density is hence only half its atomic weight; for the molecule of helium possesses nearly double the mass of the hydrogen molecule; and it is, by hypothesis, indivisible, while that of hydrogen includes two combining atoms. But helium has no “atoms,” or rather its molecules are its atoms. Its chemical equivalent is accordingly 3·96 on the hydrogen scale. A value indeed rather nominal than real, since helium is devoid of sensible affinities. It enters into no combinations. It again resembles argon in being a “rogue” element. To both equally, one of the ordinary properties of matter is wanting. They form with three other inert gases a class apart as “non-valent” substances.[91] In choosing its mineral cloisters, helium showed, nevertheless, some original preferences. The heavy metal uranium has a special attraction for it, and it is constantly associated with rare earths. Once released, however, it can scarcely be re-incarcerated. Mr. Tilden’s experiments led him to conclude that helium-yielding rocks must have been primitively charged under a pressure of several hundred atmospheres.[92] The earth may in those early days have possessed a vast helium-envelope, since dissipated in space. Opinions differ on the subject,[93] and data for precise calculations are wanting. But the probability is strong that the helium now sparsely lurking on our globe is a mere remnant of a far ampler store, which terrestrial potencies, whether gravitational or chemical, were incompetent to hold.

Helium scarcely retards the passage of light. Its refractive index, which is the smallest known, is expressed by the fraction 0·146, those of air and hydrogen being respectively 1·0 and 0·5. Still more unexpected than its low refractivity, is its high conductive power for electricity. Professor Ramsay ascertained that the “sparking distance” through helium at atmospheric pressure is nearly 300 millimetres, while the same current is stopped in hydrogen by a gap of 40, in oxygen by a gap of just 24 millimetres.[94] Another surprising property of this gas is its abnormal faculty of diffusion. It has a rate of self-dispersion ten times that of hydrogen, or fifteen times what, by Graham’s law, it ought to be. On the other hand, it has the lowest solubility on record;[95] water absorbs it in evanescent quantities. This led to the anticipation, amply justified by experience, that helium would prove to be one of the most obstinately gaseous bodies in existence. It has not indeed yet (October 1902) surrendered to the compulsion brought to bear by Professor Dewar in his memorable researches at low temperatures. Still, the cooling efficacy of liquid hydrogen evaporating under exhaustion may be expected finally to overcome its all but invincible recalcitrance, and the “salamander gas” of the chromosphere will assume the guise of a frigid fluid boiling five or six degrees above absolute zero.

The reversing layer—properly so-called—emits apparently no helium rays. A reason for their absence has been already suggested, and is tolerably obvious. They are extremely sensitive to damping influences. Foreign admixtures readily occasion their suppression. Thus 10 per cent of helium just shows spectroscopically in hydrogen, and that only if the pressure in the tube is very low; while one part of hydrogen in 100,000 of helium glows manifestly when the current is made to pass.[96] Nitrogen has a similar adverse effect upon helium-radiation, which would, however, gain relatively in strength with diminution of pressure in ascending through the reversing-layer into the chromosphere.

Of the chief coronal ingredient no terrestrial trace has yet been found. A bright green ray observed during total eclipses is its only assured badge, for eight or nine other more refrangible associated rays may quite possibly emanate from different substances. As the leading gaseous constituent of a structure on the borderland of nothingness, coronium must be an unimaginably subtle form of matter. It exists in prodigious volumes near the sun, rising to heights altogether inaccessible to hydrogen or helium, yet under conditions differing from those of an ordinary atmosphere. Successive coronal strata are not mutually superincumbent. There is no sign that their density increases downwards. The characteristic green line is no less fine and sharp given out by the inner than by the outer corona. No reversal of it has ever been detected. There is no corresponding Fraunhofer line. Thus the substance most plentifully present about the sun is, by a strange anomaly, absolutely passive as regards its light. This is most probably a result, not of any specific incapacity, but of the peculiar state in which it subsists. The real qualities of coronium, however, and its entire spectrum can only be ascertained by laboratory investigations. That these will ever become practicable it would be rash to assume, but it is permissible to hope. There seems, at any rate, no valid reason why coronium should not be added to the number of unearthed or frozen-out recondite gases. The former possession by our planet of a coronium-atmosphere may be plausibly surmised. But it most likely vanished still earlier than that of helium. Had its material been endowed with chemical affinities, some compound or compounds should have preserved it more or less abundantly. It would have been detained, as hydrogen was detained in water, and kept available for our late acquaintanceship. Since no compound of the kind appears to exist, coronium presumably resembles helium in being “non-valent.”

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VI. THE PHOTOSPHERE AND ITS DUSKY VEIL

The sun is virtually bounded by a spherical envelope of intense lustre. What lies outside is negligible in mass and function. What is hidden within has its energies concentrated, so to speak, on the maintenance of the “photosphere” at the highest point of radiative efficiency. This implies enormous internal activity, the slackening of which would be the prelude to speedy extinction. The materials of a self-renewing stratum of concentrated emission are necessarily in a state of flux. Each constituent particle, as it delivers up its store of light and heat, becomes instantaneously effete, and is replaced by another. Charging and discharging processes pursue a ceaseless round, ceaseless, that is to say, until the growth of viscosity fatally impedes them. When that time comes, convection-currents cease to flow, superficial cooling advances rapidly, and the sun-like stage terminates. The epoch of inertness must in fact arrive when, for a circulatory, a fixed surface is substituted. During some long antecedent period, again, the same body was presumably too rare to be definitely limited, and might fitly be designated a nebula. So that a “sun” is definable as a gaseous mass clothed with a pellicle of dazzling luminosity, and organised for long continuance in the capacity of a distributor of light and heat.

The shining pellicle of our sun is, to all appearance, of a cloud-like nature. It is a locus of condensation, where uprushing gases, chilled by expansion, momentarily change their state, and thus acquire the power of suddenly parting with their stored energy. The “mottlings” of the photosphere mark visibly, perhaps, this rapid course of interchange, brilliant floccules denoting regions of arterial ascent, dusky tracts those of corresponding venous descent. That it is accompanied by violent turmoil, the evidence of the camera shows conclusively. The reticulated areas are highly evanescent. Ridges and brilliant cumuli, some hundreds of miles in extent, form only to be swept away. “When we come to study the minute details of the granulations,” Professor Young writes, “we find movements at the rate of a thousand miles an hour to be the rule rather than the exception.”[97]

Since photospheric light is purely continuous, photospheric chemistry remains a terra incognita. Only conjectures are possible regarding the kind of matter present in the solar condensations. The idea that they may be formed of carbon, started by Dr. Johnstone Stoney in 1867,[98] is still very generally entertained. It is indeed hampered by difficulties at present insurmountable; but the same may be said of every other hypothesis on the subject. Carbon was recommended for the position assigned to it by its refractoriness to heat and by its great radiative power. Lampblack, we need hardly say, is, in this latter respect, the standard substance. An unfavourable peculiarity, on the other hand, is its inability to exist as a liquid under conditions at all likely to be realised in the sun. Carbon has no fusing-point in the ordinary sense. At a temperature of about 3500° C. it sublimes without melting. Preparatory to crystallising as diamond it perhaps liquefies through the incomparable stress of molecular forces, but the process is transitory and obscure. It has never been observed; it is only reasonably supposed to take place. Moreover, at or near the photosphere, pressure of the required intensity certainly does not exist. The cumuli forming it should then consist, not of carbon droplets, but of carbon dust, and the analogy with terrestrial clouds would disappear. A still more serious objection is that carbon volatilises at a temperature far below that of the photosphere. Nor are we acquainted with any kind of matter the condensation of which might be thought of as possible under the conditions there prevailing.

The question of temperature is fundamental in solar physics. Everything that regards the nature, structure, and innate activity of the solar globe depends upon the answer furnished to it. And of late the answers have become much more plausible than those discordant to the extent of some millions of degrees arrived at thirty years ago. The main cause of this wide uncertainty lay, not in the actual measurements, which can be made sufficiently precise, but in the failure to establish on secure grounds some definite relation between temperature and radiation. There is no doubt that thermal outflows increase far more rapidly than the accompanying thermometric rise—that the heat received at a distance corresponds, in an augmented proportion, to a gain of heat at the source; but the correspondence has, until lately, been expressed only by empirical rules, not implicitly or unconditionally to be trusted. Boltzmann,[99] however, supplied an ex post facto theoretical basis for a law published by Stefan of Vienna in 1879, according to which radiation grows as the fourth power of temperature. Its agreement with facts, so far as they are available, is besides tolerably close. Yet the security is precarious that it continues to match them in regions of cosmic heat, unattainable by experiment. It was, however, employed by Messrs. Wilson and Gray, with some modification and with excellent results, in their authoritative determination of the sun’s temperature.[100]

They adopted a method of direct observation, involving the fewest possible uncertainties of principle. Sun-heat, allowed to fall upon a “radio-micrometer”—an instrument of extreme sensitiveness invented by Professor Boys—was measured by the “balancing” of its effects against those of a strip of platinum heated to a known pitch. This gave the means, by the aid of Stefan’s law, of translating them into terms of temperature. Allowance had then to be made for a double absorption, first in the sun’s, again in the earth’s atmosphere. That only a fraction of the heat emanating from the solar condensations reached the apparatus in the West Meath observatory was unmistakable; how large a fraction was less easy to decide. Langley finds that the intensity of radiation at the centre of the disc is reduced near the limb by one-half,[101] and the total loss is estimated by Wilson and Rambaut at one-third of the whole.[102] Hence the sun’s thermal power would be one and a half times greater than it is if the emitting surface were stripped of its absorbent covering, and the correction of temperature demanded by its action amounts to at least 1000° C.

The despoiling effect of our own air has next to be considered. It is very large, and so are the discrepancies in its valuation. Rosetti of Padua, who in 1879 determined the temperature of the sun to be 20,000°,[103] concluded for a zenithal heat-stoppage of 29 per cent; Langley estimated it at 41; Knut Ångström[104] in 1890, laying stress for the first time upon the thermal opacity of the carbonic acid ingredient of the atmosphere, obtained 64 per cent as the ratio of absorption. This seemingly authentic result, namely, that only 36 per cent of the heat rays striking the earth vertically are transmitted to its surface, was provisionally admitted by Wilson and Gray, and after having made careful allowance for various kinds of possible error, they arrived in 1894 at an effective solar temperature of 8700° C. Substituting Langley’s value for terrestrial atmospheric absorption, and working up fresh experimental data, Mr. Wilson in 1901 reduced this figure to 6590°,[105] which probably underestimates the truth. At some such inconceivable degree of heat the undimmed photospheric clouds glow.

This is not all. The value just given belongs to an ideal stratum in the sun. It stands for the “effective,” not the actual temperature—the temperature, that is to say, which should be attributed to a surface of standard radiative capacity sending out the measured quantity of heat. Now it is certain that the photosphere falls very far short, in emissive power, of its imaginary substitute. There is no such thing in nature as a “perfectly black body,” or its correlative, a perfect radiator, the efficiency even of lampblack being only six-tenths of what it is assumed to be for purposes of calculation. And the sun is unlikely to be as good a radiator as lampblack. It must then be hotter in proportion to its inferiority, but to what extent it falls short of the ideal standard remains undetermined. It must also be very unequally hot. The brilliant granules giving its flocculent appearance to the photosphere radiate much more intensely than the gray interspaces. Hence computed temperatures represent an average higher than prevails in some formations, lower than is assignable to others. It is noticeable that several corrections based upon recent improvements in experimental data tend to enhance our conception of the tremendous energy of solar heat.

Le Chatelier’s method[106] of employing the intensities of selected rays in various light-sources as a criterion of temperature gave 7600° C. for that of the sun (uncorrected for solar absorption). It is, however, of doubtful validity. A parallel line of research was opened by Langley’s establishment of the principle that temperature is connected by a definite relation with the wave-length of maximum energy in the spectrum of a radiating body. Divergent views, nevertheless, prevail as to the form of the relation. Michelson[107] and Rubens[108] agree that the wave-length of most powerful emission varies in length inversely as the square root of the temperature, while Paschen and Wien[109] maintain that the simple inverse ratio tallies more closely with facts. The outcome in determinations of the sun’s heat differs of course vastly with the law chosen. From Michelson’s, H. Ebert deduced in 1894 a temperature of 40,000° C., but added the qualifying remark, “The parts of the sun to which this value applies belong to the more interior regions; they are at any rate deep under the reversing layer, and therefore probably below the photosphere.”[110] Now subphotospheric heat may be of almost any intensity; hence the result, although not very informing, is safe to be in some sense correct. Paschen, on the other hand, obtained the low value of 5130°. It might be added that the law upon which he relied is suspiciously simple, “in view of the known complexity in the radiation of a solid body, and the various rates of increment with temperature attaching to different rays.”[111] It seems to be one of those formulæ which cannot be trusted far out of sight. They are not true enough to bear extension into regions beyond experience. Useful over a moderate compass, they prove treacherous adjuncts to investigation. Difficulties, indeed, all but insuperable hamper attempts to infer the solar temperature from comparisons of spectral energy-curves. Unexpected peculiarities are found to characterise the modes of emission of solid bodies. Even continuous spectra are to some extent distinctive. Thus the same quantity of energy is very differently distributed in the rays sent out respectively by polished and lampblacked platinum, by carbon filaments, copper, and iron oxides; while with an equal increase of energy, the distribution becomes diversely modified for each substance. For each, that is to say, the maximum ordinate of the energy-curve creeps upward at a different rate. In the absence, then, of precise knowledge as to the composition and condition of the photosphere, inquiries as to its temperature, based on this principle, are futile. We should first need to be acquainted, in Professor Very’s words, with “the selective radiating power of the solar photosphere.”[112] Generalisations are here eminently unsafe, since laws of radiation derived from the experiments with one kind of material are by no means certain to prove applicable to others. Besides, the “absolute solar spectrum” (as Langley calls it) cannot be directly observed, and the shape of its representative curve is most materially altered by the effects of absorption in the solar atmosphere.

On the whole, the straightforward plan of attack on the problem of the sun’s heat seems the most promising. Messrs. Wilson and Gray’s practical operations left little room for improvement, and the uncertainties affecting their final result will gradually diminish with the progress of other kinds of research. As higher temperatures, for instance, are brought under command, the range allowed to perilous processes of “extrapolation” can be restricted. And improvements, sure ere long to be realised, in the value assignable to telluric atmospheric absorption, will effectually reduce the marginal errors attached to present estimates of the primitive heat-power of the sun.

Fluctuations in the sun’s heat-power must be regarded as possible, and they might be either irregular or periodical. Indeed, the superposition of both kinds of change would perhaps be more likely than the occurrence of either separately. Their detection would, in any case, be extremely difficult, although it is not, in Messrs. Wilson and Gray’s opinion, to be regarded as hopeless. The required measures would be simply differential; and differential measures escape many of the snares that hamper the execution of absolute measures. But comparisons in this matter are rendered almost nugatory by inconstancy of weather. Variations in the “solar constant,”[113] even if real, would probably be masked by local and temporary changes in the diathermancy of the air. Professor Very holds that “under these circumstances refinements in actinometry are of small avail,”[114] and he suggests “that the problem will have to be solved entirely by meteorological methods.”[115] “If temperature and humidity observations could be collated from the logs of vessels crossing the torrid zone, estimates of oceanic evaporation from day to day, combined with rainfall measures, might lead,” he believes, “to the detection of the variation of solar radiation.” But the chance of their doing so appears, all things considered, to be incalculably small. The elements of disturbance are too numerous and too strong to permit the emergence of the slight residual effects looked for. Far preferable appears Piazzi Smyth’s plan of earthing thermometers deeply enough to be inaccessible to superficial vicissitudes of temperature. And it can scarcely be without significance that the readings of those buried on the Calton Hill showed oscillations coincident in period with the sun-spot cycle.

Absorption in the sun’s atmosphere may also prove to be variable. And here again differential observations should suffice to test the question. They were undertaken by Wilson and Rambaut in 1892, but relinquished after one series had been made. The method employed was to pass an image of the sun across the radio-micrometer, while the motion of a spot of limelight, reflected from the mirror of the instrument, recorded the changing amounts of heat received from the different parts of the disc. The intention was to obtain such “curves of absorption” frequently throughout an eleven-year cycle, and thus determine the question of concurrent fluctuations in depth of the absorbing envelope. “If we find,” the authors wrote, “that such changes are taking place, as would be shown by the alteration in the ratio of the heat from the limb and centre of the disc, we think it will be quite possible, by an investigation of the co-ordinates of these curves, to determine the change in the value of the solar constant.”[116]

This theoretical possibility, nevertheless, is still a long way from realisation. Divers indications lead almost irresistibly to the conclusion that the sun is hotter at certain times than at others; and Professor Young counts it as “one of the most important and difficult problems of solar physics now pending to determine the actual amount of these variations and ascertain the laws that govern them.” But they are, as we have partly seen, disguised by manifold complications.

The one clear upshot of inquiries into the temperature of the sun is to show that it stands high above the boiling points of the most refractory among the chemical elements. The fact is embarrassing, but cannot be evaded. Apart from its consideration, no theory as to the nature of the photosphere is of the slightest value. And it is no easy task to frame one bringing it into harmony with other circumstances equally well assured, and equally rigid in their consequences. Three alternative hypothesis may be said to exhaust the possibilities of the subject. They are as follows:—

1. The photosphere is a surface of condensation for unknown materials capable of maintaining the liquid or solid state at a transcendent degree of heat.

2. It is a surface of condensation for known materials under unknown conditions.

3. It is no true surface of condensation, the substances composing it being, although viscous, still vaporous.

Now each of these explanations is largely an appeal to ignorance, and so far scarcely deserves to be ranked as an explanation at all. Yet one of them must be fundamentally true. The first may be dismissed as contradictory of a strong consensus of evidence. The third involves glaring incongruities, both with what can be seen and with what must be inferred. There remains only the second. We seem bound to adopt the view that the sun is veritably clothed in a kind of cocoon—a web of incandescent filaments. It is perhaps of mixed composition. The surface is irregular. It comprises “fleece-like floors” at apparently different levels. Possibly they represent the successive condensations of various substances—silicon, carbon, titanium, vanadium, platinum, to mention a few of those most resistant to heat. The diversity of their emissive powers might contribute to produce the tonings brought out in photographs of the disc, and the arrangement would be analogous to the surmised replacement in our upper air of aqueous by carbonic acid cloud-fields. But the postulated “unknown conditions” needed to enforce condensation at the enormous temperature of the photosphere may long continue to baffle the scientific imagination.

A darkening of the sun’s disc towards the limb is obvious telescopically, and conspicuous photographically. Its amount, measured by Bouguer in 1729, formed the basis of Laplace’s calculation that the arrest of light indicated was no less than eleven-twelfths of the entire. The data were correct, but the result, owing to certain mistaken assumptions, was greatly in error. Modern authorities, nevertheless, are far from being unanimous on the subject. Pickering finds that the intrinsic lustre of the sun exceeds its apparent lustre four and two-third times; the disparity, according to Vogel, is about twofold. There are, however, distinctions to be made. The absorption in the solar, as in the terrestrial atmosphere, is markedly selective. The brunt of its attack falls upon the most refrangible rays. Father Secchi noticed in 1870, and Professor Langley again in 1875, that the light from the limb is, in consequence, tinged with chocolate brown, while that from the central parts of the disc seems bluish by comparison. This general indication was, in 1877, analysed by Dr. Vogel,[117] who, by detailed measurements with a spectral photometer constructed on the polarising principle, ascertained that 30 per cent of the red, but only 13 per cent of the violet marginal rays penetrate the solar atmosphere. Hence an alteration in tint corresponding in its mode of origin to the ruddy suffusion of the setting sun. Now Seeliger has pointed out that selective absorption implies a medium of high refractive power; but equivalent conditions might, according to E. von Oppolzer,[118] be supplied by “a rare atmosphere in which flying particles are suspended.” It is, beyond doubt, an exceedingly shallow one. This was inferred by Vogel from the rapid degradation of light towards the edge of the disc, and it is rendered patent to sense by the brilliancy of facular summits, which, rising above the absorptive strata, shine unveiled against the dusky limb. Obviously, then, the darkening effect is produced in the immediate neighbourhood of the photosphere. It cannot be due to cool gases, and hot gases stop light distinctively in isolated beams. An alternative hypothesis was suggested some time ago by Professor Hastings of New Haven. The sun’s so-called “atmosphere” is, in this view, nothing more than a smoke-laden stratum.[119] Minute solid particles of carbon or silicon, carried upward from the photospheric clouds, are the agents of obscuration. The assumption of a solar analogue to a London fog is certainly a daring expedient, yet none more satisfactory is at present available.

The sun’s “veil” is indeed particularly difficult to fit in with the rest of its economy. It manifestly exists, and the position seemingly prescribed for it is between the photosphere and the reversing layer, although Dr. Scheiner prefers to place it in chromospheric regions.[120] In some torrid locality, at any rate, it exercises a kind of action characteristic of cool substances. Its composition out of refrigerated materials is strongly indicated. The refrigeration, however, may be excessively transient as regards each individual particle, although permanent in their aggregate. The general effect is to diminish the sun’s heat by one-third or one-half, and its light by fully two-thirds, with an attendant change to pale primrose of its original glacier-green tint.

Among the many enigmas of solar physics there is none more curious or more evasive than that which confronts us in this intimate appurtenance of the photosphere. Even the lines of approach to it are very few. Yet some are practicable, and almost untried. Researches of a special kind into the spectra of sun-spots should help towards its elucidation; still more, perhaps, careful spectroscopic comparisons with the obscurer interstitial spaces of the brilliant granules strewing the solar surface. If these are relatively dark through a mere lowering of temperature, then little can be learned from them in this connection. But if they are dark through increased absorption, it will be important to determine whether the absorption corresponds to that produced by the problematical “veil.” Are they, in other words, sinks for solar soot? Do they mark the lines of subsidence of the same refuse materials which by their interposition dim and tarnish the shining face of the sun? A definite reply to the question would bring us preceptibly nearer to the goal of our inquiries into the nature of the sun’s “smoke” envelope. What cannot be doubted is the importance of its function as a regulator of the sun’s output of energy. This has for the first time been adequately discussed by Dr. J. Halm in a paper incorporated with the Annals of the Royal Observatory, Edinburgh.[121] His views on the subject demand careful consideration.

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VII. STRUCTURE AND MOVEMENTS OF SUN-SPOTS

A normal sun-spot consists of a round black “umbra,” garnished with a circumferential “penumbra.” The chief member of the group shown in Fig. 6 is a good example. The ground should be almost white, with the granular texture delicately indicated. The details of such objects, however, are seen much better than they can be photographed even by the consummate art of M. Janssen.

One of their characteristic features is the definite separation of their parts. The umbra does not merge into the penumbra, nor the penumbra into the photosphere; the lines of demarcation are as sharp as the edge of a cascade. Their darkness is indeed accentuated by the enhanced brilliancy of the regions invaded by them. An immense area of disturbance usually surrounds an active spot, and this area of disturbance is also an area of actual elevation. A series of micrometrical measures carried out by M. Sykora of the Charkow observatory in 1895 showed that, as a very general rule, the sun’s diameter is lengthened in the direction of a spot on the limb.[122] The fact is most significant, for it indicates relief of pressure as perhaps the cause, and certainly as an accompaniment of solar outbreaks, and thereby associates them with volcanic explosions. Now a region lifted is, on the sun, a region brightened, the effects of absorption diminishing with the rise of level. Hence the exceptional vividness of the photosphere in a spotted neighbourhood.

Faculæ gain higher altitudes, and are consequently still more lustrous. They are like the summit-ranges of a tableland. Their connection with spots is intimate, but not inseparable. Every spot, it is true, claims a retinue of faculæ; but faculæ exist abundantly where there are no spots. It is a moot point whether spots can come into being apart from preceding facular disturbance. “Which is the forerunner of the other?”[123] is a question not to be answered off-hand. As a rule, the embryo spot has apparent priority. The rule, however, is not invariable, and the priority recorded may often be illusory, faculæ being extremely evasive of observation. Their survival, on the other hand, after the openings enwreathed by them have closed, is obvious and constant. The complex relations of the two kinds of phenomenon must be unravelled before the nature of either can be thoroughly understood.

Photospheric structure is very curiously modified in the penumbræ of spots. The roundish granules of the unbroken surface seem as if drawn out into threads, which lie side by side, pointing radially inward, and overhang the umbra with ragged edges, compared by Dawes to those of an untrimmed straw-thatch. And the eaves of this luminous thatch are its brightest part, possibly because of the crowding together of materials forced into a narrow circular space. The whole effect suggests the subjection of viscid masses to a pulling action emanating from the centre of disturbance, by which they are stretched and carded like wool-flocks.

The umbra of a sun-spot shows a cloudy texture, markedly unlike the streaky aspect of the penumbra. This can be perceived, however, only when the seeing is exceptionally good. The ordinary impression is of uniform and very profound darkness. Contrast, indeed, greatly heightens this effect. The obscurity is only comparative. Mr. Evershed estimates the light-emissions from umbræ as varying from about one-twentieth to one-hundredth those of the dazzling photosphere,[124] and when intersected by the black advancing moon during the progress of a total eclipse they seem dully bright.[125] Yet with differences. They are rarely of the same tint throughout. Dawes perceived in 1852 a “black opening” in the umbra to be a characteristic of all well-developed spots. It can only, however, be discerned visually, and that by the aid of special precautions; the sensitive plate takes no notice of this deeper depth of shadow, which has, accordingly, received somewhat less attention than it deserves. Fortunately eye-and-hand portrayals still continue to be made, and they not infrequently afford valuable records of the Dawes phenomenon. M. de Pereira, a Portuguese observer at the Azores, wrote as follows of a group which came into view 20th April 1895: “The sense of a cavity in this spot is unmistakable, as though the crust of the sun were torn and scratched, and the black, or rather dark, under-skin were visible beneath. Definition on this day was the best I have ever seen, enabling me to make the smallest detail reliable. On the 24th, this same spot showed a conspicuous black hole in what I may call the centre of gravity, a dazzling white bridge crossing it from south to north-east, and a smaller one lying on the northern edge of the abyss, the brims being full of curiously intertwined points of photospheric matter.”[126] The chief member of the splendid group visible in the sun’s southern hemisphere during the last half of February 1894 had also an inner nucleus,[127] and the same feature has been studied in numerous examples by Father Cortie, Mr. Maw, and others. It is commonly associated with the presence of “bridges,” and both belong characteristically to the final stages of active spots. Bridge-building is preliminary to the indraught of luminous matter by which photospheric breaches are closed; it might be compared to the trickle under the dyke that preludes the rush of inundating waters. The process is a remarkable one. From abutments (so to call them) at opposite sides of the umbra, segments of light protrude; then at a given moment they unite with a leap or a flash, and the arch stands complete. A beautiful photograph by Janssen of a spot doubly spanned is reproduced in Fig. 7. But the ground is altogether too dark. The surface near the spot was dazzling, likewise the facular masses crossing the nuclei.

“On the negative,” Mr. Ranyard wrote in describing it, “the brilliant bridge which stretches across the great spot is seen to break up into a number of distinct elongated masses,” and these are evidently the “rice-grains” of the photosphere laid end to end in single file for suspension above the abyss. This kind of structure is probably always present, although often imperceptible. Exceptional facilities are needed to bring out the finer details in spots. Thus Professor Young tells us that, on the rare occasions when powers of six hundred and upward could be profitably used with the twenty-three-inch Princeton refractor, he succeeded in resolving “the apparently club-like, almost bulbous ends of the penumbral filaments” into “fine sharp-pointed hooks, reminding one of the curling tips of flames, or grass-blades bending over. Ordinarily,” he adds, “they are seen as club-like simply because of their brightness and the irradiation and diffraction effects of moderate-sized object-glasses.”[128]

The connection of “bridges” with “black holes” was tentatively explained in a valuable paper presented by Father Cortie to the Royal Astronomical Society, 11th May 1900. He considers that the latter may be the portions of the umbra left uncovered by “faculous veils,” which, extending from the penumbra, not unfrequently lighten up certain regions of nuclear gloom, leaving others more profoundly dark by contrast. Now the relationship between “veils” and “bridges” is obviously quite close. Both represent luminous invasions, although differently organised and conditioned, and both are heralds of decay. Their kinship is on occasions emphasised by the development of one from the other. Twice at least, in 1865 and 1866, the transformation into “roseate veils” of brilliant arches spanning the umbræ of spots was observed by Father Secchi at Rome.[129] He was quite positive about the colour of these “veils,” which seems to intimate for them a gaseous nature, assimilating them to prominences rather than to faculæ.[130]

Sun-spots are rarely solitary. They ordinarily appear in clusters or processions, consisting of one or two dominating members and many satellites, down to mere umbral dots and penumbral scraps. Individual spots show endless varieties of conformation. The nuclei are often multiple; as many as nine umbræ have been seen within the compass of a single penumbra. Again, they become pear-shaped, or spiral, or caudate, as if through the action of stresses or twisting forces of an unknown character. The penumbra is equally subject to irregularities. It is sometimes a mere torn strip of fringe; half the umbra may be duly furnished with its valance, while the other half remains bare; or the umbra and penumbra may be disjoined by intruding photospheric matter. Fig. 8 shows a “fimbriated” spot from a drawing by M. de Pereira, the partially veiled and bridged umbra of which included two conspicuous black holes.

In Fig. 9 three “confluent” spots are represented, drawn by the late Miss E. Brown, 15th August 1894. She described the group as covering “a vast extent of surface with a mass of nuclei and penumbra partially connected and very variable in form.” A few hours later the largest nucleus had assumed a helical form, and seemed to be “throwing out feelers like a jelly-fish.”[131] Strong hydrogen incandescence was spectroscopically perceived to be an accompaniment of these rapid changes. The great spot of September 1898 had in its declining stage a nucleus divided by intersecting bridges into three lobes; it assumed on 3rd October the shape of an ace of clubs.[132] Mr. Maw perceived in the same object on 11th September delicate veins of comparative brightness, termed by him “submerged bridges.”[133] Indeed he believes this to be a constant feature of large spots, the umbræ of which, viewed with a suitable eye-piece, appear no longer uniformly dark, but marked with fine traceries in chiaroscuro. These cannot at present be photographed, and their visual detection accentuates the indispensable co-operative functions of the eye and the sensitive plate.

The level of sun-spots has once more become a subject of active debate. Yet it was believed to have been determined once for all in the eighteenth century by the geometrical reasonings of Dr. Wilson. The characteristic perspective effects of depression below the surface were noted by him in a well-developed spot as it circuited the sun’s globe in November 1769, and the saucer-like conformation of all such objects was universally admitted for a hundred years and upwards, notwithstanding frequent failures to verify the due optical consequences of their changing situations. At last, however, this “venerable theory” (as Professor Frost calls it) has been uprooted from the soil of conviction. It is denied by many on geometrical grounds alone; by some on physical grounds as well. Not that it has been finally discarded, but its credit is gravely impaired. Certainly all spots do not follow its prescriptions; probably very few strictly comply with them. Irregularities of form account for a good many of these deviations, but others cannot be so readily explained away. Mr. F. Howlett, in offering to the Royal Astronomical Society, 14th December 1894, three volumes of drawings representing the fruits of thirty-five years of solar scrutiny, declared uncompromisingly that the Wilsonian view must be abandoned.[134] Father Cortie’s examination of them convinced him too that “the phenomena presented by many spots are directly contrary” to the current hypothesis, a mountainous rather than a cavernous structure being, in certain cases, indicated for the umbra.[135] Nor does the umbra usually vanish near the sun’s edge, as it should if it were simply an excavation with sloping sides. It remains, on the contrary, persistently visible, although foreshortened into a black line. This, to be sure, might be a simple consequence of refraction by vapours congested within the cavity. The explanation is tempting, since it would avail to get rid of many anomalies; still it must not be adopted unreservedly. Originally suggested by Proctor,[136] it has been taken into fuller consideration by Mr. East[137] as a means of exit from the difficulties that hamper attempts to conceive rationally of the build of sun-spots.

There is, indeed, pressing need to conciliate opposing evidence. Thus M. Riccò,[138] at Catania, from eleven years’ study of spots in their geometrical aspect, derived results strongly in favour of the Wilsonian hypothesis, computing for twenty-three especially symmetrical formations an average umbral depth of rather more than a thousand kilometers. Yet the discussion of the long series of Stonyhurst drawings led Father Sidgreaves[139] to ascribe to most spots a convex rather than a concave shape. Professor Hale[140] allowed small weight to testimony so contradictory as that regarding the apparent width of the penumbra at various distances from the sun’s limb, but was inclined to consider the advocates of the Wilsonian doctrine as having rather the better of the argument. “In any case,” he added, “they will hardly be ready to admit that the umbra is at a higher level than the penumbra, for it cannot be doubted that the penumbral filaments overlie the umbra, and frequently unite to form bridges extending completely across it.”

One of the most singular details of spot-phenomena is the occasional appearance of a large umbra as a notch on the limb. This implies its projection in a dark mass against the sky, the encroachment upon the bright disc being perhaps only an effect of irradiation. Its inconsistency with a depressed form was pointed out both by Mr. Howlett and by Father Sidgreaves.

Where, then, is truth to be found in this remarkable controversy? How can the jostling facts be reconciled? Compromises have been resorted to. Spots, it is averred, are cloud-like at certain stages of their growth, crateriform at others. Or individual spots belong to one or the other type, according to the circumstances of their origin. But these are subterfuges; let us take a broader view. The concavity, at any rate, of bridged spots is indisputable. The attribution to them of a “mountainous” character would throw the arrangement of their parts into utter confusion. Moreover, De la Rue obtained in 1861, by stereoscopic means, ocular proof of depression in one such object. The experiment might easily and usefully be renewed. A pair of photographs, taken at an interval of twenty-six minutes, gives, through the sun’s rotation, just the right amount of difference in aspect for combination into one picture in relief. The moot question, “concave or convex,” might thus receive a direct answer. Then if, in a long succession of instances, the answer preserved a uniform tenour, it might safely be concluded that anomalous appearances of lifted umbræ in spots seen obliquely are illusory and of purely optical production.

But we cannot even so escape from the entanglements of the subject. It has different bearings, which have all to be taken into account. Spots are very hot relatively to their light, and their thermal radiations are peculiarly conditioned. Professor Frost’s determinations,[141] carried out at Potsdam in 1892, showed that absorption does not take increasing effect upon them with approach to the limb to anything like the extent that it does upon the corresponding radiations from the photosphere. “The reasonable inference from this,” he wrote, “is that the spots are at a higher level than the photosphere, and hence less subject to the absorption of the sun’s atmosphere.” Mr. W. E. Wilson’s[142] more delicate series of observations in 1893–4 showed likewise that “the radiation from the umbræ of spots does not suffer absorption when near the limb in the same manner as a point on the photosphere.” Nevertheless Professor Langley[143] had found in 1874–5 “the decrement of heat in approaching the limb” to be very nearly in the same ratio for photosphere and spots. This flagrant contradiction between results equally authoritative may not be without meaning, since they were obtained at nearly opposite phases of solar activity: Langley’s, three years after a maximum, when it was verging towards stagnation; Frost’s and Wilson’s, during a period of culminating disturbance. The two latter recommended systematic observations throughout an eleven-year cycle for the purpose of investigating the nature of the relationship, and they respectively threw out the alternative suggestions that, during its course, the thermal condition of spots, or the level at which they are formed, may undergo progressive changes.

A more promising explanation was offered by Egon von Oppolzer.[144] Spot-umbræ, he reminded perplexed solar physicists, are surmounted by abnormally hot chromospheric layers, certainly exempt from absorption. Hence the indiscriminate sum of their radiations and those of the underlying spots gains by comparison with those from the simple photosphere, at and near the marginal parts of the disc. Now the flame-stratum develops chiefly above spots of an active type; and spots of an active type predominate at epochs of maximum. This consideration at once removes the discrepancy between Langley’s results on one side, and Frost’s and Wilson’s on the other. In the first series, absorption produced its full and due effects, because quiet spots being presumably in question, no appreciable overlying source of heat was present. In the second and third, the overlying source was so strong as in great measure to efface the gradations of heat-stoppage suffered by the object beneath. The disproportionate thermal power of spots may be similarly accounted for. Our instruments measure, not only their direct radiations, but also those sent out by ignited materials, to some extent enveloping them.

The solar rotation is a subject much too important to be disposed of in a paragraph; it need here only be said that a by-product of its detailed study has been to throw further doubt upon orthodox opinions as to the location of spots. The rates of axial movement deduced by Stratonoff and Wolfer from their progression round the sun appear, at least primâ facie, to compel the inference that they are veritably situated at a level higher than that of the photosphere.[145] Yet here again some fallacy is likely to be involved. In view of all these complications it is scarcely to be wondered at that the Where? has almost superseded the What? in recent discussions about solar maculæ. The upshot, so far, seems to be that they are essentially depressions, although depressions very shallow relatively to their superficial extent. Their abnormal geometrical behaviour is due, in part, as Mr. Maunder has suggested, to the cavities being over-filled, and the umbræ consequently dome-shaped;[146] in part to the optical elevation into view of bottoms which should, but for refractive action, be concealed by shelving sides. Their radiative irregularities, again, are explicable by the influence of their coronas of hot flames. As to their rotational anomalies, they must stand over for future consideration, with the remark that, to deduce the position of spots from the degree of their conformity to a supposed law of solar rotation, is to attempt the solution of one enigma by proposing another still more arduous.

The movements of sun-spots are of three kinds. There are first those that belong to them collectively, as objects attached to a rotating globe. With these we are not at present concerned. Next, they have individual “proper motions” of transport over that globe. Finally, they show internal movements variously connected with the processes of their development and decay. The last are mostly spiral or circular, and they evidently ensue upon inrushes of photospheric matter. They are sometimes performed round “black nuclei” as centres; and black nuclei are probably, as we have seen, interspaces between obscurely luminous umbral effusions. But the whirling of spots is not systematic or innate; it does not characterise them essentially; it occurs incidentally, and as a result of disturbance. No fixed rules prescribe its mode or direction. Opposite gyrations have been simultaneously observed in different members of the same group of umbræ, and even successively in a single spot. They are executed in other cases intermittently by fits and starts. A revolution is not often completed; the description of large angles is exceptional. Spots cannot then be described, in any true sense, as “solar cyclones”; the vorticose motions occasionally exhibited by them spring from temporary impulses, and cease when the force of these is exhausted.

The proper motions of spots are indicative of much more than has yet been learned from them. Three kinds of influence seem to be effective in producing them; namely, mutual action, action from without, and action from within. After segmentation, in the first place, umbræ repulse one another; they separate with great velocities. They behave like similarly electrified masses, but whether they really are such or not is an open question. In the second place, growing spots in general move rapidly forward. They share the common drift, but with an acceleration often amounting to three or four hundred miles an hour. It seems as if cooled materials, pouring down upon them from above, drove them forward with the added speed due to a wider circle of rotation. If this were actually the fact, however, macular increase and macular advance should always go together; and they are not uncommonly disunited. Processes of extension in spots may even be accompanied by retrogression over the sun’s surface.[147] Indeed, the conduct of these strange objects is governed by no invariable rules. Strong tendencies visibly influence it; yet none that are irresistible. They can be annulled or reversed by countervailing circumstances. Hence the special need for guarded inferences in treating of this subject.

The movements of sun-spots in latitude are not visibly related to their drift in longitude. They are highly irregular, and not often conspicuous. Carrington, however, perceived in them a kind of inchoate method. Spots, according to his generalisation, situated within the solar tropics (so to speak) tend to approach the equator; spots outside the north and south limit of twenty degrees, to depart farther from it. But the exceptions observed are so numerous as sometimes to go near disproving the rule. We have said that spot-movements in longitude bear some marks of being communicated by exterior agencies. Those in latitude, on the contrary, suggest interior action. They are connected, most likely, with the hidden system of circulation prevailing in the body of the sun, and reflect its local perturbations.

A remarkable feature of photospheric commotions was referred to by Father Cortie at the meeting of the Royal Astronomical Society, 11th May 1900.[148] He termed it “alternation” in disturbance. A group of spots generally includes two chief members, posted respectively in the van and rear of the array. These take it in turns to develop. We are reminded of the reciprocal flickerings of the fragments of Biela’s comet. Analogous pulsations, but on a larger scale, manifest themselves in responsive disturbances north and south of the solar equator. Mr. Maunder had already pointed out in 1894 that an “active train” of spots is often “accompanied by a feebler copy of itself a few degrees north or south. An outbreak of the first magnitude,” he continued, “will indeed reproduce itself in several directions.”[149] Somewhat similar correspondences are noticeable between volcanic foci on the earth; yet the inferences they suggest might prove misleading.

The occultation of a spot 107,500 miles across was observed during the solar eclipse of 15th March 1858. Its vast dimensions were, however, exceeded in the same year by those of an object with the “record” diameter of 143,500 miles. An enormous double spot, which appeared in June 1883, covered an area of 2500 million square miles; and the great spot of February 1892, with its dependants and outliers, spread still further afield. But such gigantic formations are rarely stable. Their history is one of tumults and vicissitudes. Comparatively small circular spots possess individually a much more lasting character, although great outbreaks are the longest lived in their successive modifications. The maximum duration so far registered was for a “composite disturbance,” consisting of four very large spot-groups, and thirteen others of smaller dimensions, which appeared seriatim, and in obvious association, on a restricted region of the solar surface.[150] The manifestation continued for 527 days, from 25th September 1891 to 5th March 1893, while the sun completed nearly twenty-one rotations. About two rotation-periods represent, according to Father Cortie, the average life of a spot.

Most of what we know about sun-spots has been learned by a statistical method of inquiry. Nor can such methods be dispensed with in the future. But they do not alone suffice. They must be supplemented and reinforced by individualisation. Each notable spot should be studied in itself and in all its relations, singly, specifically, and generically. Efforts should be made to determine its nature, as though it were a solitary specimen. Can it, without doing violence to plain facts, be regarded as an excavation in the photosphere? Or, if apparent inconsistencies with this view be present, are they such as might be due to refraction? The hypothesis can only be tested by confronting it with particular cases, and trying definitely how far it avails to meet their exigencies. Refractive possibilities in the sun have been, until lately, almost ignored; they are now in some quarters vastly exaggerated. Still allowance has to be made for their realisation, in ways perhaps corroborative, rather than subversive of received theories.

There are many other doubts to be set at rest besides those regarding the interpretation of perspective effects. The record, indeed, of no significant structural detail should be omitted; and what detail of these enigmatical objects can be called insignificant? Above all, variations in their parts and features, whether simultaneous, successive, or alternative, claim the closest attention; since the establishment of a course of correlated changes comes very near to the detection of the underlying causal nexus.

The comparison of one spot with another is a natural sequel to the investigation of each spot in itself. Do their peculiarities, it may be asked, depend in any way upon heliographical position? Do they vary periodically? Can certain traits in sun-spots be classed together as inevitably associated, certain others as mutually exclusive? Wider questions, too, suggest themselves as to the place of spots in the general solar economy, and as to the nature of their connection with faculæ, prominences, coronal streamers, and the totality of solar phenomena. Attempts have been made, both by speculative and practical means, to throw light on these obscure topics, but with results not as yet wholly satisfactory. Meantime, additional facts are needed—facts systematically collected, methodically sifted and compared. Isolated observations are rarely of any considerable value in such complex matters. Meaning accrues to them just in proportion as they can be allied to others made in correspondence with them, but under modified conditions. “Correlate and compare” should be the watchword of astrophysicists.

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VIII. THE SPECTRUM OF SUN-SPOTS

Sun-spots give a remarkably compounded spectrum. It appears to sum up five different sets of effects. That is to say, the obscure longitudinal stripe corresponding to the umbra owns a quintuple origin. Each of its elements might be made the subject of a somewhat prolonged discussion. For the sake of clearness we will briefly enumerate them. They consist of: (1) A bright background of ordinary photospheric light; (2) a nearly continuous band of dense absorption, extending from the infra-red to the ultra-violet; (3) a select array of widened Fraunhofer lines; (4) nearly all the Fraunhofer-lines under a normal aspect; (5) a restricted number of bright lines.

Dunér of Upsala ascertained in 1891[151] that the fundamental radiance of spots is indistinguishable from that of the general surface of the sun. Their darkness is then due to increased absorption, not to diminished radiation. This fact decisively negatives some current theories, and thus limits the field of speculation as to the nature of spots. They are shown by it unmistakably to be regions where cooled materials of some kind accumulate. Of what kind those materials are, we can learn something—although not by any means all that could be wished—from their peculiar modes of arresting light.

A section of a spot-spectrum in the yellow-green is portrayed in Fig. 10 from a photograph by Professor Young. The belt of strong absorption which is its leading feature seems, but is not really, continuous. Young himself discovered in 1883 that it is made up of innumerable fine dark lines, set very closely together, or even actually overlapping. Each individual in the multitude is, he tells us, “spindle-shaped—i.e. thicker in the middle where the spectrum is darkest—and tapers to a fine hair-like mark at each end; most of them can be traced across the penumbra-spectrum, and even out upon the general surface of the sun.”[152] These observations, which require a high resolving power in the apparatus employed, were amply confirmed by Dunér. He perceived further that the lines are collected into groups, leaving chinks of undimmed photospheric brilliancy between.[153] Within the limits of the “b-group” alone, no less than 300 of these dusky fibres were counted; above F, however, they become merged together by crowding, and below E by diffusion. Their separation and arrangement are most evident in quiescent round spots with intensely black nuclei—in spots, that is to say, commonly regarded as of the minimum type. Now such a spectrum as they constitute cannot be produced by liquid or solid matter, however minutely subdivided; it decisively claims a gaseous origin. Hence the darkening in spots is not merely an intensification of the “smoky” absorption veiling the entire disc; it is special and peculiar. So much can be safely asserted.

Perhaps the most distinctive part of the spot-spectrum is the collection, included in it, of accentuated Fraunhofer lines.[154] They are picked out to be widened and darkened on some recondite principle of selection, which varies from spot to spot, and from epoch to epoch. This was early noticed by Sir Norman Lockyer, and he pursued the inquiry with striking results. The discussion in 1886 of observations upon the spectra of seven hundred sun-spots, made at South Kensington on a fixed plan during six years, led him to the following conclusions:—[155]

(1) “The most widened lines in sun-spots change with the sun-spot period.”

(2) “At, and slightly after the minimum, the lines are chiefly known lines of the various metals.”

(3) “At, and slightly after the maximum, the lines are chiefly of unknown origin.”

In other words,[156] “As we pass from minimum to maximum, the lines of the chemical elements gradually disappear from among those widened, their places being taken by lines of which we have at present no terrestrial representatives.” “Dissociation,” in short, was the mot de l’énigme. As the sun’s temperature increased with the growth of disturbance, substances in a terrestrial sense “elementary” were supposed to split up into exotic constituents, giving spectral lines strange to laboratory experience.

The evidence for the progressive change thus interpreted was indeed slight, except as regarded iron; and iron alone was taken account of in the confirmatory Stonyhurst observations. So far as they went, however, they were decisive, and all the more so that they covered a different spectral range (B to D) from that (D to F) examined at South Kensington. They showed demonstratively that, throughout the disturbed interval between January 1884 and October 1886, iron lines were all but completely replaced by “unknown lines” in the list of those affected in spots, while they duly reappeared upon the restoration of photospheric tranquillity. In connection with their behaviour, nevertheless, Father Cortie established an important distinction. Their presence or absence he found to be determined, not by the general flow of solar commotion, but by the nature of individual spots. In those of rent and ragged aspect and tumultuous proclivities, iron lines are ousted by unidentified faint rays; but in tranquil spots the iron spectrum is at all times prominent. And since the former sort prevail at maximum, the latter at minimum, the statistical outcome is that the spectral variations appear to depend simply upon the great cyclical pulsation of the solar globe. Only on special examination they prove to be determined more locally and particularly than this would imply. In some unquiet spots, for instance, which developed near the minimum of 1889, the effacement of iron lines was as complete as if the epoch had been one of maximum. In fact (as the Stonyhurst astronomer remarked),[157] the widening of unknown lines is common to all stages of solar activity, provided spots of an appropriate character be at hand. This is not a distinction without a difference. It cuts the ground from under the assumption of periodical vicissitudes in the general chemistry of the sun. Iron is not everywhere, and inevitably reduced there to its elements as temperature and disturbance culminate together, but—if at all—only as a special effect in the hottest spot-craters. And this again brings up difficulties connected with relative temperature—difficulties which, in one form or another, perpetually recur in the study of astrophysics.

But there is more to be said. Further inquiries have materially altered the aspect of the case, for they have led to the transference from the “unknown” to the “known” class of so many spot-lines that the completion of the process may be confidently anticipated. Rowland’s photographic comparisons have contributed most effectually to its advance. Young and Cortie have traced a crowd of sun-spot rays to vanadium; titanium claims as many, or more; and others perhaps originate from allied “rare” metals. This singular line of identification is very strongly traced. Thus all the vanadium lines, twenty-eight in number, between C and D are by turns broadened in spot spectra, although of evanescent faintness in the photosphere; nor does the conjecture seem unwarranted that the high temperature compounds with nitrogen and oxygen, both of this metal and titanium, may yet be recognised in umbral chemistry. The distension in a spot of two vanadium lines, at λ 5728 and λ 5731 respectively, is well shown in Fig. 10. They are of quite minor importance in the Fraunhofer spectrum.

Father Cortie surmises that the vapours absorbing in spots may be associated by their approximate conformity to a certain standard of density. “The level of sun-spots,” he suggests, “is possibly the level of the faint lines of such metals as have an atomic weight about 50.”[158] Iron, nickel, titanium, and vanadium, all assiduous frequenters of umbral cavities, belong to this category. But the rule is compromised by exceptions and incongruities.

The actual state of the case is this. There is no evidence of elemental dissociation in sun-spots, but spectral diversities are obvious and persistent. They indicate the disappearance of iron from tumultuous formations, and the emergence in them of titanium and vanadium. There are doubtless concomitant changes, but they await ascertainment and particularisation. A caveat, however, has to be entered. The principle upon which these inquiries have been conducted is imperfectly assured. It is commonly taken for granted that the widened lines constitute the spot-spectra; that they, and they alone, represent the emanations of the constipated vapours blotching the lustrous disc. But this is a somewhat arbitrary assumption. The theory of line-expansion by pressure is very imperfectly understood. The phenomenon does not occur uniformly and invariably. Lines of different substances are differently affected by it; lines even of the same substance are unlike in their susceptibility to its influence. The inferential building up then, of spot-spectra out of widened lines is subject to many qualifications. These do not lessen the importance of the observed relation, but they importantly modify it.

The iron lines intensified in spots, presumably by the specific action of their nuclear vapours, are often unsymmetrically broadened. They are usually diffuse towards the violet side, sharp to the red. This may point to the presence of chemical compounds;[159] since the flame-spectra of metals and of their oxides seem to be differentiated just by the development, in the latter, of these peculiar shadings. The possibility must accordingly be admitted that iron-oxides exist in the sun. Yet the implied temperature is improbably low, since they can be broken up here on the earth by the simplest metallurgical processes. But they might perhaps form transiently (so to speak) in spots, as a result of the local chilling of swiftly circulating material. It may be added that the iron lines distinctive of spot-spectra are so-called “low-temperature lines.” They are brilliant in the electric arc, but tend to be outshone by others in the higher excitement of the spark.

Further complexity was imparted to spot-absorption by the appearance of certain dusky bands, of which nine, situated below D, were observed early in 1885 at Stonyhurst,[160] and no fewer than seventeen more refrangible in 1880–3 at Greenwich. One proved identical with a fluting drawn by Young in 1872,[161] and all were resolvable into densely packed lines. Nothing is known, or can even be conjectured, as to their origin; but they are clearly symptoms of disturbance, since, with the sudden advent of a solar calm in October 1886, they at once utterly vanished.

Mr. Evershed considers the majority of the unaffected Fraunhofer lines in spot-spectra to be possibly spurious;[162] they may, he thinks, be inherent, not in umbral light, but in the photospheric glare diffused equally over spots and the surrounding sky. Some of the ordinary dark solar lines, however, thin out in crossing umbræ, and a few show traces of partial brightening. Moreover, the radiations from spots cannot escape transmission through the reversing layer, and are hence subject to precisely the same absorption exerted upon sunlight in general, so that the Fraunhofer spectrum in its integrity truly belongs to spots, notwithstanding the reinforcement of some of its components and the enfeeblement of others through influences special to them.

One of the most remarkable features of spot-absorption has still to be noticed. Helium, as we have seen, makes no show in the Fraunhofer spectrum. Yet a helium-envelope surrounds the sun to a depth of five thousand miles. Every ray of sunshine sent abroad into space has been sifted through this huge volume of gas, which, by its anomalous inertness, bids defiance to “Kirchhoff’s law.” Emitting complex ranges of vibrations, it nevertheless exacts no corresponding toll of absorption. Its transparency seems absolute. Either it is so hot that it replaces the light arrested, or its arresting power is nullified by rarefaction. The former alternative is excluded by the consideration that an excess of temperature should be notified by the presence of bright helium lines in the general spectrum of the sun, and they are no more visible in it than dark ones. Hence the absorptive incapacity of chromospheric helium may provisionally be attributed to extreme attenuation.[163] This view has gained plausibility through the discovery that helium in or near spots acts at times normally upon light, for the vapours and gases producing umbral obscurity are assuredly, on any theory of spot-formation, denser within than outside the apparent cavity. The shading then at D_{3}, like the fusiform shape of the sodium pair below it (see Fig. 11), results from increased pressure. It is usually significant of vehement disturbance. In twenty or thirty spots with flaming appendages—and mostly in their penumbral regions—Professor Young has seen the yellow helium ray reversed;[164] it was similarly visible to Professor Naegamvala in the huge vortex of February 1892,[165] and to Mr. A. A. Buss of Manchester, on 17th March 1899,[166] in a spot the incessant activity of which was the more remarkable on account of its occurrence near an imminent minimum. Absorption by the deep red helium ray at λ 6678 was three times observed in spot-spectra by Father Perry during 1883,[167] but its chemical meaning was then unsuspected. Now that the helium-spectrum has been unravelled, further particulars might easily be learned as to the associates of D_{3} in spots. Its isolated occurrence is improbable.

Spot-spectra are crowned and completed by the frequent superposition upon them of vivid rays. These originate from the gaseous effusions often accompanying the formation and transformation of spots. They are readily identifiable. Hydrogen lines and the H and K of calcium are the most frequently brightened; D_{3} is sometimes bright over the umbra, dark in the penumbra, of the same spot; and “double reversals” of the sodium “D pair” are quite commonly observed. The phenomenon is illustrated in Fig. 11. The brilliant ray shining at the core of the fuzzy spindle, into which each of the coupled lines is broadened, evidently proceeds from an overlying hotter and rarer stratum of sodium-vapour. The magnesium group “b” is occasionally affected in the same way. The “rosy veils” in spot umbræ give out, as might be expected, hydrogen rays, and “bridges” are also loci of emission. The ultra violet members of the hydrogen series are never present, bright or dark, in spots; and the fifth line (Hε), which falls just within the region of visibility, has often been looked for in vain. In some of Professor Hale’s spectrographs, however, of the giant spot of February 1892 it showed faintly bright beside the more conspicuous H of calcium.[168] So far the record stands alone. It has a particular interest from the ambiguous position occupied by this ray in the solar spectrum.

The agitation prevailing in spots is often betrayed by line-distortions, telling of the swift recession or approach of vapours congregated in them. The condition of the C-line, as sketched by Professor Hale, 13th February 1892, in the same spot, is shown in Plate VI., Fig. 2. The brilliant patch over the umbra is of normal wave-length; it was derived from a flame radially immobile; but the hook-like appurtenance of the dark line testifies to an extraordinary outrush of cooler gas from the lower part of the formation. We can, to a certain extent, trace its course. It started outward with a uniform velocity away from the earth of about 120 miles a second. This slackened unequally, as can be seen by the breadth of the “hook” at its junction with the line; and the whole mass of hydrogen came to rest at a distance of thirty to forty thousand miles from the point of issue, which (it is worth noting) was at the very middle of the nucleus. The nature of the force raising this brief but tremendous storm cannot readily be imagined. Its abrupt development marked an acute crisis of disturbance, to which the earth responded with magnetic twitches and auroral illuminations.

Motions in spots seem to be limited and local. They can be inferred to prevail with great violence at certain levels, while complete tranquillity reigns at others. This, at least, is the only explanation of the chemical peculiarities of solar hurricanes. Nothing, for instance, is commoner than the raging of hydrogen-storms amid profound calcium-calms. Nay, lines belonging to the same substance may indicate for it simultaneously rest and motion. Thus a few iron lines are at times observed to be displaced or twisted through the effects of rapid approach or recession, while the remainder maintain their usual positions and aspect. The anomaly is most striking, and challenges persistent attention. Sir Norman Lockyer meets it with the dissociation-hypothesis; but this raises more difficulties than it removes.

Enough has been said to show that numerous and most curious problems await solution by students of sun-spot spectra. The subject is wide enough to occupy a band of specialists, and its remoter implications can still be only surmised. Nevertheless, definite conclusions are not wholly out of reach. First, as to the cause of nuclear darkness. It is certainly to be found in augmented, and (so to speak) reiterated absorption. Spots are not simply rents in a shining veil, exposing an obscure substratum. They are not super-heated regions, where processes of condensation are suspended. The photosphere is screened, not perforated, by them. Moreover, the screening is by interposed vapours. Umbral absorption is mainly, if not altogether, of the gaseous kind. It is essentially linear and banded. No part of it can be safely attributed to the action of a foggy precipitate such as modifies elsewhere the “surpassing glory” of the disc. They probably differ in this respect from “pores” and “veiled spots,” but specific inquiries on the point have yet to be made.

There are strong indications that spot-spectra originate under conditions of increased pressure and diminished temperature. Still the coolest umbræ must be hotter than the reversing layer, for otherwise the Fraunhofer lines would show bright against them, and, as we know, they cross them in dusky array. This circumstance is fundamental in solar thermal relations, yet has been generally overlooked. The ordering aright of such relations is a prime desideratum in solar physics, and should serve as an indispensable guide to the interpretation of spectral diversities.

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IX. FACULÆ AND PROMINENCES

Faculæ are outgrowths from the photosphere. This is known by direct observation. They have been seen and photographed jutting from the limb as rotation brought them into, or carried them out of view. It is also inferred from their superior brilliancy, which might serve to measure the opacity of the veil spread over the surface they surmount. On the other hand, they escape none of the Fraunhofer absorption; the whole range of solar dark lines is invariably present in their analysed light. Hence their position can be defined as intermediate between the “smoke veil” and the reversing layer. This consideration affords a safe holding-ground for reasonings about the status of these remarkable objects. They testify to internal commotions of the same nature as those giving rise to spots, but exempt from their heliographic limitations. Faculæ are not confined to the spot-zones; they develop all over the solar globe. Since, however, spots invariably claim their attendance, they are most numerous in the latitudes frequented by such disturbances, while showing independent maxima much nearer to the poles.[169] But their imperfect visibility upon the disc greatly restricted their observation, until Professor Hale and M. Deslandres almost simultaneously invented a method for spectrographically recording them.[170]

It depends essentially upon the use of a double slit—Janssen’s valuable invention for isolating spectral rays. Celestial objects can in this way be photographed in monochromatic light—that is to say, their forms in each separate chemical element can be distinctively recorded. The importance of the fresh start thus made is difficult to exaggerate. With a double slit and a sensitive plate, the comparative distribution of glowing vapours in the sun can be satisfactorily investigated, and anomalies connected with their distribution, if not removed, at least fully recognised and defined. All spectral rays, nevertheless, are not equally available for all purposes. In chromospheric photography, for instance, the calcium H and K offer immense advantages, not only because of their actinic efficiency, but still more on account of the broad bands of shadow rendering them conspicuous as Fraunhofer lines. These serve to protect against atmospheric glare the bright lines superposed upon them at the edge of the sun; and glare is the worst foe of daylight photography. In systematic work, moreover, K is, for more than one reason, always preferred to H. And so came to be established, in two continents at once, a new branch of astronomical art—the art of picturing the sun and its surroundings in calcium-light of a single quality.

The first and immediate object in view was the day-by-day photography of prominences; but it was very soon found practicable to extend the work from the limb to the disc. One slit was caused to travel across the sun’s image, which had a diameter of two inches in the Chicago twelve-inch refractor, while the motion of the second was adjusted so that it exactly kept pace with the K-line, admitting it alone, through a chink just two thousandths of an inch in width, to impress the sensitive plate. At Chicago, the first experiment of the kind was made 28th December 1891, and a similar mode of procedure was described by M. Deslandres before the Paris Academy of Sciences, 8th February 1892.[171] The upshot in each case was the discovery that K is doubly reversed over extensive tracts of the photosphere. Plate IV., Fig. 1, shows the sun self-portrayed in calcium light, 11th April 1894. The lines crossing the photograph are accidental imperfections, one set originating from dust-particles in the jaws of the slit, the other from irregularities in the movement of the siderostat. The regions of reversal are not confined to the neighbourhood of spots, but spread irregularly over the sun’s surface. The individual bright forms they include are often bent into spirals or doubly curved. They correspond very closely, both in aspect and position, with faculæ directly seen, and were at once, by Professor Hale, identified with them. M. Deslandres, however, classing them as a species of hybrid between faculæ and prominences, bestowed upon them the compound name, expressive of this mixed quality, of “facular flames.”[172] Hale’s view, in other words, was that the novelty disclosed by his “spectroheliograph” consisted in the emission by faculæ of bright “H and K” by way of supplement to their regular photospheric spectrum, while Deslandres considered that the new investigation applied to a distinct kind of objects, neither prominences nor faculæ, although partaking of the nature of both. The question raised is difficult; let us briefly examine its bearings.

The calcium-flames photographed at Paris and Chicago are certainly not prominences. Their positions do not coincide with those of the chromospheric offshoots. They appear where prominences are not, and vice versâ. Nor are prominences bright enough—unless by a rare exception—to show in relief against the sun; they are strictly objects for marginal spectroscopic study. Finally, motion-displacements are conspicuous in them, and are absent from the sinuous shapes on the disc. The objections to identifying these with faculæ are less obvious, yet seem equally insuperable.

Faculæ belong no less unmistakably to the photosphere than the Himalayas do to the earth’s crust. They are flung upward from it; they subside back into it. Their light is of the same brilliantly continuous quality; it is marked by the same kind and amount of linear absorption. It has then been sifted through all the vaporous strata amassed above the surface of the sun. This gives the key to the position; for it compels the inference that while faculæ are situated beneath the reversing layer, the allied gaseous forms must be located above it. The argument for their comparative elevation is cogent. The reversing layer, as we know, is rich in calcium vapour cooler than the photosphere, and, à fortiori, cooler than the flames detached by their superior lustre from the photosphere. Their calcium-rays should, accordingly, be effectually stopped by the reversing layer; they could not by any possibility be transmitted through it. Hence the bright “K” in which the solar disc can be depicted must originate above the region where Fraunhofer absorption takes place. The wreathing forms emitting it belong to a different locality from that of true faculæ. They are chromospheric, not photospheric, developments. Yet, as just stated, they cannot be assimilated to prominences. They probably lie at the base of the chromosphere, and they follow the distribution of faculæ with close, though not rigid exactitude. Hydrogen is at times represented in their spectra by a crimson glimmer of C, so they are not composed of calcium solely; but their real nature and place in the sun’s economy remain admittedly enigmatical. We are only sure that they spring up in close relation with faculæ as symptoms of the same disturbances. That their relation to them is one of actual identity seemed indeed at first sight scarcely open to doubt; yet for the reasons just assigned they must be regarded as separate phenomena.

The connection of faculæ with spots is not, as we have seen, of the “mutual” kind. Their reciprocity—as a typical Irishman might say—is one-sided. Outside the spot-zones, where solar commotions have a less vehement character than nearer to the equator, faculæ lie “extended many a rood” in sluggish inactivity during intervals long enough for the working out of manifold changes in the brilliant circumvallations of spots. These are very often crowned with prominences, which, however, develop somewhat tardily. Nascent umbræ are rarely accompanied by them, while moribund umbræ offer a favoured site for their growth. Thus a remarkably brilliant prominence, agitated by violent motion, towered above the eastern limb as it was passed, on 3rd March 1892, by the shrunken remnant of the vast spot-group which had slipped out of sight round the western limb a fortnight previously.[173]

Prominences are related to the chromosphere in very much the same way that faculæ are related to the photosphere. They arise from it by effluence or eruption. Now the chromosphere itself has a markedly eruptive aspect. It presents no billowy ocean surface, but resembles rather a Tartarean meadow planted with stalks and grass blades of fire, waving under some unimaginable furnace-blast. With clear definition, a filiform texture is everywhere apparent. The “straw-thatch” effect of penumbræ denotes a similar peculiarity in photospheric materials, and that it is shared by prominences can be inferred from their frequent construction, as if out of the untwisted strands of a rope.[174] The red rim of the eclipsed sun is conspicuously jagged, and its saw-like outline suggested the name Sierra, originally bestowed upon it by Airy in 1851. It does not, then, represent a fluid envelope in a state of equilibrium. There is another reason why this is impossible. The chromosphere does not preserve a uniform depth. Averaging about 5000 miles, it is subject to irregular and temporary variations, as yet imperfectly observed and entirely unexplained. General subsidences are even affirmed to take place. One such was noted with surprise by Trouvelot, June to August 1875.[175] He had frequently observed extensive local depressions, but never before a universal shoaling. He attributed to the sharper views of the photosphere afforded by this removal of part of the interposed medium, his discovery of “veiled spots,” imperfect umbral formations, owing their abortive character, perhaps, to their unfavourable situation near the poles. In 1887, the height of the chromosphere, measured thirty-two times by Dr. Fényi, S.J.,[176] was found to diminish above the spot-zones, and sensibly to increase outside their limits. It would be of interest to determine whether this condition prevails commonly, or whether it is restricted to epochs of minimum.

Prominences are of two varieties—eruptive and quiescent. These differ chemically, visually, and heliographically. Eruptive prominences are of a more mixed constitution than the quiescent sort; they include many more ingredients; they give an intense carmine light, have jet-like or upspringing shapes, and are mostly confined to the spot-zones. They are genuine fire-fountains, while quiescent prominences frequently resemble cloud-banks. The former are individual outbreaks; the latter unite into communities, counterfeiting banyan groves, ranges of jungle, fields of cirrus. Usually connected with the chromosphere by stems or trunks, they in some cases, not only float free in isolated masses, but, still more remarkably, are generated at great elevations above it, as if by the spontaneous illumination of pre-existing material. The analogy with terrestrial condensations in an azure sky is obvious, but may be quite misleading.

Quiescent prominences develop on an enormous scale, nor always tranquilly. Formations, at least, which are in some respects cognate with them, become, on occasions, the scenes of striking explosive accidents. So that their generic title must be understood in a restricted sense. Thus the cloud-like character of a huge object photographed by Deslandres,[177] 31st May 1894, seemed vouched for by its vicinity to the south pole, and by the large extent of the solar limb garnished by it. At 2 P.M., when it was first pictured—as usual, in calcium light—its height, apart from foreshortening, was 2′ 20″, or 63,000 English miles. At 4^h 27^m it had sprung up to 135,000, and 73^m later still to 270,000 miles (see Plate IV., Fig. 2), an elevation far beyond any recorded for objects of the kind situated more than sixty-five degrees from the solar equator, while this wonderful structure spread from the seventieth to the eighty-first southern parallel. Perceptibly filamentous, it seemed to grow by the elongation of its component threads or ribands, and included within its vast bulk probably the minimum conceivable quantity of matter. On the same plates, a group of smaller but intensely active prominences registered themselves at the solar antipodes.

An example of a transient apparition, difficult to classify, is given in Fig. 12. It simulates a portentous conflagration. From bottom to summit the red pillar of hydrogen measured 166,000 miles, according to Dr. Fényi’s observation at 2^h 20^m, 19th September 1893. But it wholly lacked any interior principle of stability. Opposite, and excessively swift movements in line of sight of the base and shaft betrayed the progress of destructive change. The catastrophe was not long delayed. Within less than half an hour all was over. A rush upward set in at the rate of 132 miles a second, carrying the frail edifice to a height of 224,000 miles. A minute and a half later, it had faded and dissolved, “like the baseless fabric of a vision,” leaving just “a rack behind” in the shape of an insignificant protuberance hedged in with faculæ. “Throughout the course of its appearance,” Dr. Fényi wrote,[178] “the entire object consisted simply of very bright luminous bands or strips scattered one after another in ragged forms, and apparently lying nearly at right angles to the limb of the sun. They were strikingly bright even in the highest parts of the prominence. The form as a whole was also like a band or stripe, which had no pronounced inclination, but stood erect nearly in the direction of the sun’s radius.”

This amazing outburst was repeated with enhancements, nineteen hours later, at a point on the sun’s limb almost diametrically opposite to its predecessor. The height in this case attained was 300,000 miles, the mean velocity of ascent being 214 miles a second, besides which, retreat from the earth was indicated for the entire mass at the rate of close upon 160 miles a second. A convulsion at least equally violent was witnessed by the same observer on 24th December 1894,[179] when a brilliant and wide-spreading, yet tolerably tranquil prominence, 56,000 miles high, suddenly began to mount, and attained in thirty-five minutes the towering stature of 300,000 miles. Speedy disorganisation ensued. Less than two hours after the explosion its scene was vacant. The body affected by it had been shattered out of existence by its destructive violence.

The Kalocsa observations were made with a visual spectroscope. Simultaneous photographs would have been of especial value for comparison with them, but leisure was not afforded for combining methods. The three objects they referred to were palpably identical in character. All showed precisely the same kind of structure. They were made up of glowing vertical bands, collected into loose sheaves, or scattered in outlying detachments. Each enormous aggregation, too, stood erect during the tumultuous processes of expansion and collapse. They were related to spot-groups, if at all, only by diametrical opposition. The singular counterbalancing tendency of solar disturbances is frequently conspicuous, and perhaps rarely or never absent. Two gigantic, although ephemeral apparitions, thus coupled, were, for instance, observed by Trouvelot, 26th June 1885;[180] and on 16th August of the same year he noted the apparent connection of a prodigious chromospheric outburst with a spotted condition of the limb 180° distant. A relief of pressure may be concerned in these phenomena, as in volcanic eruptions; if so, the lift, or diminution of gravity, must act right across the solar globe, quickening convection-currents and facilitating the antipodal delivery of extra supplies of heat. In point of fact, the usual premonitory symptom of the explosive development of prominences is a disengagement of light.[181] Exceptional brilliancy is the forerunner of abnormal activity. No satisfactory cause, however, can be assigned for the imagined relief of pressure, since tidal influences are fairly out of the question. Only one undeniable inference can be derived from the contrary symmetry of solar commotions. It is that they are extremely deep-seated. They have their roots in the hidden profundities of the great globe they agitate exteriorly. They are not then mere local accidents; they make an intimate part of the solar economy. The recognition of this characteristic is interesting and important.

The tallest prominences are ordinarily the least coherent in structure. An example is shown in Fig. 13, from a drawing made by M. Fényi, 3rd October 1892. The object it portrays was almost unique in its fantastic and colossal form. Overarching thirty degrees of the limb, it rose above it to a height of nearly a quarter of a million of miles. Its fragmentary composition was patent.[182] Under the eyes of the delineating artist it was “in the act of being blown to shreds.” Photographs happily secured at Chicago seven hours later, exhibited it as then fallen to less than one-quarter of its high estate, while of augmented lateral spread. These represented, of course, its calcium aspect, the Kalocsa drawing its shape in red hydrogen; but there was no evidence of the two vapours being differently distributed throughout this bubble edifice. Facular patches marked, on the sensitive plates, the points both of rise and re-descent of the materials constituting it. Next morning only some insignificant wreckage strewed its place; virtual annihilation had overtaken it.

The tremendous velocities observed in prominences constitute a formidable problem in solar physics. They not infrequently exceed the critical rate of 383 miles per second; that is to say, they transcend the limit of the sun’s gravitational controlling power, and, apart from possible resistance by a medium, should carry the substances animated by them finally away into space. The shapes and motions of prominences, however, clearly indicate some retardative action,[183] although its excessive feebleness, at least in coronal regions, can be inferred from the unimpeded circulation of comets passing within a hundred thousand miles of the sun’s surface. Hence it is only a surmise, not a certainty, that chromospheric outbursts are attended by irrevocable loss of matter. The swiftest so far recorded took place, 17th June 1891, in connection with a spot just disappearing through rotation. A concentration of vivid luminosity gave the signal that something unusual was impending, “and after six P.M., Kalocsa mean time,” M. Fényi wrote,[184] “the point at 281° shone with so great a brilliancy that its reddish light seemed to become white; an enormous displacement of the spectrum towards the blue, at a medium height above the sun’s limb, indicated at the same time an approach of the hydrogen in our direction with a prodigious velocity.” The entire object, which was composed of glowing filaments, lay, in fact, on the more refrangible side of the C-line, as the result of approaching speed up to 550 miles a second. A vertical ascent at the rate of 300 miles being meanwhile directly visible, a total velocity (neglecting an uncertain third component) of about 680 miles a second must have been attained in this amazing explosion. Trouvelot’s observation a few hours earlier of a brilliant and peculiar facular blaze at the place where it occurred[185] was significant of the intense energy pent up in the spot-group, waiting release by the figurative trigger-touch.

This prominence easily wins the prize for rapidity of movement; yet the competition has been keen. Velocities of the same order, arising under similar circumstances, have been frequently determined. They appear, indeed, improbable, and have been stigmatised as “fabulous.” No actual transport of matter, it is alleged, can be in question, but merely a swift transference, through gases previously obscure, of a luminous condition.[186] But this does not account for the conspicuous shiftings of spectral lines, which could not ensue from the progress of incandescence through stationary matter. They demand the strict application of Doppler’s principle; real velocities must correspond to them. Nor, even if there could be deception about the movements of prominences in the line of sight, is illusion possible about those across it. And both kinds are of the same order of speed; they are complementary; they represent different aspects of identical disturbances. This is not all. Prominences are often visibly twisted; they are composed of spirally mounting flames; while to this helical conformation correspond gyratory movements, at times disclosed by the spectroscope. Here, at any rate, we have to do with bodily transportations of matter; “luminescence” cannot be propagated vortically. Besides, the measured speeds are as difficult to explain on one hypothesis as on the other. Chemical action does not spread instantaneously. Through the tenuous gases of the chromosphere, a maximum rate of one mile a second might be assigned to its progress—a rate, that is to say, some hundreds of times slower than the velocities to be explained. That they are somehow of electrical production is a safe assertion, likely to be true, if not in an immediate, then in a remote sense. Yet we are little the wiser for the admission. The “floating of an idea” in the mind does not constitute knowledge; and a speculation is only valuable when it offers a definite starting-point for practical research.

Total eclipses have ceased to be indispensable for the prosecution of chromospheric studies. Day by day the red rim of the sun, with the strange forms protruding from it, can be viewed spectroscopically; and day by day the same objects vestured in violet can be photographed under the broad shelter of the Fraunhofer K-line. Nevertheless, noontide darkness, when it comes, brings very appreciable help. Differences are noticeable between what can be seen in and out of eclipse. According to the late Professor Tacchini,[187] the chromosphere always appears deeper under cover of the interposing moon, because it is surmounted by a pink-white margin, giving continuous light, and therefore spectroscopically invisible in daylight. Some prominences are probably of analogous composition. Only their skeleton-forms come out in the crimson radiance of hydrogen; they are compacted and clothed with white materials, the shining of which is effaced by the glare of common day. A spectroscopic survey of the chromosphere and its appendages should hence always be made immediately before and after every eclipse, for comparison with the direct photographic records obtained during the corresponding totalities. For the present the information acquired by daylight work at the edge of the sun remains under a partial slur of incompleteness.

The objects called “white prominences” belong indeed wholly to the pageantry of eclipses. First noticed by Tacchini at Caroline Island, 6th May 1883, they showed as lucid jets about a hundred thousand miles high, with a surface like granulated silver. Attempts made, after the return of daylight, to view them prismatically proved fruitless; they gave forth no hydrogen or helium rays. Again at Grenada, 29th August 1886, a gigantic helical structure, described by Mr. Maunder as “of the intensest silver whiteness,”[188] towered three hundred thousand miles above the limb of the moon. Its spectrum, photographed by W. H. Pickering, included bright H and K, and Professor Hale accordingly entertained the hope that such objects might come within the range of his spectroheliograph; but so far no trace of them has been caught outside total eclipses.

Dark chromospheric forms are not unknown. A “black protuberance,” observed by Trouvelot at Meudon, 7th October 1892,[189] might have been only a negative impression, like “black” flashes of lightning; yet it in a measure falls into line with eclipse-records certainly not due to illusion. In the great “anvil protuberance” disclosed 7th August 1869, Dr. Lewis Swift saw “many black lines crossing in different directions, and inasmuch,” he added, “as they must have been, at least, fifty thousand miles long and a thousand miles broad, it would appear to be important to understand the cause of this phenomenon, and (to ascertain) if these markings are always present.”[190] Corroborative observations were made by Alvan G. Clark and Professor George Davidson. With them may be usefully compared M. Liais’s description of a black-edged but colourless prominence watched at Paranagua, in Brazil, during the eclipse of 7th September 1858.[191] The composite effect was not due to contrast, since the obscure summit stood out alone when the bright lower portion had disappeared behind the advancing moon. These singular phenomena excited little comment, and lapsed into oblivion, so that dusky ramifications, connecting and defining prominences in the eclipse-photographs of 9th August 1896,[192] seemed entirely novel features. They were explained by M. Hansky, a member of the Russian party on the Amur, as outflows of hydrogen cooled by expansion;[193] but inadequately. Hydrogen below the temperature of luminosity is transparent, and the photographed veinings were densely opaque. They afford a hint, that will certainly not be disregarded, of the workings of unknown activities in connection with prominence-development.

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