XXXIV. PLANETARY NEBULÆ…

XXXIV. PLANETARY NEBULÆ

Planetary nebulæ seem to be intermediate between nebulous stars and annular nebulæ. Indefinite aureolas are replaced in them, as if through the spreading outward of nebulous matter towards a limiting spherical surface, by compact discs, and with the further advance of this process of exterior condensation, the discs become rings. A stellar nucleus persists throughout these phases. They may not be strictly phases of development. To establish an actual sequence of growth, facts of various orders should be considered. As a simple matter of fact, however, objects are found which combine so closely the visual features of planetary nebulæ and nebulous stars on the one side, and of planetary and annular nebulæ on the other, that their classification in the above order is prescribed inevitably, if only for mental convenience.

As a preliminary to the physical study of these objects, we must try to attain a clear conception of their real forms in solid space. This is not easy when their structural complexities are taken into account; but, setting these for the moment aside, we can gather some indications regarding the general plan of their fabrication. As a rule, planetary nebulæ are markedly elliptical. They may then be concluded to be spheroidal in shape; and even those sensibly circular are probably spheroids viewed along their shorter axes. That their compression is due to axial rotation is a fair inference, verifiable, possibly, by spectroscopic measurements. These were indeed ineffectually tried by Professor Keeler, 3rd April 1891, on a bright planetary in Hydra (N.G.C. 3242);[970] but he doubted whether the conditions of the experiment permitted the detection of a difference in the velocities of the advancing and retreating limbs of the nebula of less than seven or eight miles a second, and the movement of such bodies is likely to be excessively slow. Otherwise they could scarcely be supposed capable of holding together; for they are obviously of tenuous composition, and gravity at their equators can act very feebly in equilibrating centrifugal impulse.[971] And here an untrodden field opens to enterprising inquirers. A theoretical investigation might, to begin with, be attempted of the figures which should belong to rotating globes of the kind on certain probable assumptions as to their nature and modes of movement; and the results might be tested by the application to a number of promising objects of Keeler’s spectrographic method, in which the linear images of an equatorial slit serve, through the tilt imparted to them by the contrary motions of the opposite limbs, to measure, in miles per second, the speed of gyration. Small, lucid, strongly elliptical planetaries would be the most hopeful subjects for experiment; nor would a negative upshot be without value. The Hydra nebula, for instance, which is nearly as much flattened as the globe of Saturn, is equal in light to a seventh-magnitude star; it is about 20″ across, and somewhat diffuse at the edges.[972] With it may be compared a condensed planetary in Ophiuchus (N.G.C. 6572), sometimes called Struve No. 6 (Σ 6). Its light, which is comparatively intense, emanates from a small disc 8″ in diameter (exclusive of a hazy margin), with an ellipticity of about ¹⁄₂₀₀. Professor Keeler’s drawing of the object with the visible part of its spectrum is copied in Plate XXIII. Fig. 2. The nucleus is not stellar, although it gives strongly continuous light. The lines in its spectrum were found by Keeler to be displaced upward by an amount corresponding to a motion towards the earth of 6·3 miles a second;[973] but this motion should be considered to belong to the sun, not to the nebula. Judging by its appearance, axial acceleration may have advanced farther in it than in most members of the class, and might record itself in an equatorial spectrograph. The trial, at any rate, should be made. Anticipation of failure need not be allowed to paralyse effort. In science the rush of a forlorn hope often carries a fortress that has obstinately held out against a siege in regular form.

The uniformity of aspect at first supposed to characterise planetary nebulæ disappears before the searching scrutiny of the powerful telescopes now in use. Their surfaces prove to be full of suggestive detail. They are broken up by irregular condensations, or furrowed by the operation of antagonistic forces; they betray, here possibly the action of repulsive, there of attractive influences, and bear inscriptions of no less profound historical import than the contortions and faultings of terrestrial strata. They are quite commonly multiplex formations. One glimmering disc is superposed upon another, intimating the analogy of the successive filmy envelopes flung round the heads of active comets. With the twenty-seven-inch Vienna refractor Vogel succeeded, in 1883, in resolving the above-mentioned planetary in Ophiuchus (N.G.C. 6572) into three distinct layers,[974] doubtless representing concentric ellipsoids of unequal antiquity. A triple outpouring of matter at age-long intervals seems recorded. And the case is typical, although the nebulous shells are not often so clearly defined. This, however, is, on the one hand, a matter of telescopic seeing; on the other, perhaps of evolutionary progress. By way of illustration, let us take as the first term of a sequence an average nebulous star, such as that detected by Auwers in Auriga (N.G.C. 2175); next in order we might place an object near 16 Cygni (N.G.C. 6826), in which the “glow” has become compacted into a large, round, seemingly uniform disc.[975] Sir William Herschel described it as “a beautiful phenomenon, of a middle species between the planetary nebulæ and the nebulous stars.”[976] The following term of the series may be found in a planetary not far from γ Eridani (N.G.C. 1535), consisting of an eleventh-magnitude star, embossed upon two concentric nebulous shields; or rather, presumably, enclosed within a pair of nebulous globes. If we suppose these to have been produced by successive outflows, checked at a limiting surface, we could easily see that the occurrence of a third access of ejective energy would complete the model of the triple specimen in Ophiuchus. It may be added that the difficulty appears almost insuperable of explaining the growth of multifold planetaries on the hypothesis of simple contraction. A repulsive agency in a manner asserts its past activity.

Let us now imagine one of these spherical envelopes to be hollow. The effect to the eye would be that of a luminous ring. And, in fact, an appreciable proportion of planetary nebulæ show an interior circlet of dim radiance, which can hardly be otherwise interpreted than as the projection on a plane of a vast nebulous bubble. This mode of construction is fully carried out in “annular nebulæ”; it is partially realised in not a few specimens in which a ring within a disc is perceptible or conspicuous. All will be considered together in the next chapter; in this we are concerned only with examples devoid of—so to speak—symptoms of inflation.

The largest planetary in the heavens lies south-east of the second Pointer, β Ursæ Majoris. Discovered by Méchain in 1781, it was numbered 97 on Messier’s list (N.G.C. 3587), and has been extensively and carefully observed. Its dimensions were given by Lord Rosse in 1874 as 163″ by 147″;[977] but the major axis measures 203″ on a photograph taken by Dr. Roberts, with an exposure of four hours, 20th April 1895,[978] and reproduced, by his kind permission, in Plate XXIV. Fig. 1. The observational history of the object is extremely curious. Sir John Herschel saw “a large, uniform, nebulous disc, quite round, very bright, not sharply defined, but yet very suddenly fading away to darkness.”[979] On 2nd March 1848, however, Lord Rosse perceived a doubly perforated surface, with a star in each cavity,[980] and his observations were confirmed by Dr. Robinson. The drawing representative of them is a record of permanent interest. The resemblance to an owl’s face, given to it by the symmetrically placed oculi, is unmistakable; and the great planetary in Ursa Major became known, from the middle of the last century, as the “Owl Nebula.” The name has survived the similarity. On 9th March 1850 the two stars were noted at Parsonstown shining as usual in their respective excavations; five weeks later the fainter one had vanished;[981] nor could it ever again be found, though looked for about forty times during the ensuing quarter of a century. To Professor Keeler in 1891[982] the nebula wore indeed an entirely different aspect from that previously attributed to it. “There is but one nucleus,” he stated, “which is by estimation almost exactly central, at a place which in Lord Rosse’s drawing is occupied by a bridge of light between two dark openings. There is also but one central dark space.” Nevertheless, two appear in the Crowborough picture, like lagoons separated by an isthmus, and on the isthmus there is planted, as it were, a lighthouse, diffusing a brilliant illumination. Thus the interior vacancies remain in statu quo, while the stars that formerly occupied them have both faded out of sight, leaving the prodominance to a third, not identical with either. The change, in short—admitting that there has been change—relates, not to the structure of the nebula itself, but to the relative brightness of three connected stars. The question whether those at present extinct will ever become revivified, can be answered only by prolonged experience. Dr. Roberts thought that his photograph indicated for the nebula a combined ring and disc formation;[983] but this is not manifest. The picture affords no verification of the marginal inequalities recorded at Parsonstown; yet there is reason to believe that they were not illusory. The torn and jagged contour, which Professor Alexander expounded as the effect of “disruption and dispersion outward,”[984] may come into view in representations on a larger scale, taken with special precautions for the definition of minute details.

The suspected alteration of the Owl planetary accentuates the need for keeping watch over nuclear stars, more especially since they present unexplained peculiarities. Their light is of purely stellar quality, but of remarkable actinic power. They are very much brighter chemically than visually. Further, they often give nebulous images on the sensitive plate, while appearing sharp with the telescope. In some rare cases (as in N.G.C. 6781), they are eccentrically situated. Burnham is probably justified in regarding the possession of a central star as an essential feature of planetary nebulæ,[985] although it is occasionally undiscernible with the telescope. N.G.C. 6563 in Sagittarius, and N.G.C. 7354 in Cepheus, are examples of nebulous discs unrelieved to the eye by the sparkle of any stellar points; and a beautiful little planetary in Perseus, discovered by Barnard 11th December 1890,[986] is of similar aspect. The missing nuclei, however, of all such objects would, it is tolerably certain, become manifest in long-exposed photographs, which should accordingly be taken for the purpose of deciding a point of fundamental importance in the economy of planetary nebulæ.

Perhaps the most noted member of the class, from the numerous experiments of which it has been made the subject, is located in Draco (N.G.C. 6543), quite close to the pole of the ecliptic. To ordinary observation it presents a greenish-blue surface, 22″ by 18″, centred on a vividly white star of the tenth magnitude; but Professors Holden and Schaeberle recognised, with the Lick refractor, its “helical” conformation.[987] Two brighter intersecting hoops, perceived at a glance to diversify the disc, fell into position on closer scrutiny as the thread of a screw, uniting their curves into one continuous tri-dimensional spiral. The discernment of this novel form—believed to be typical—was a suggestive contribution towards what might be termed the solid geometry of nebulæ. Speculations as to its mode of origin would, however, be premature until, by the unerring testimony of the camera, it has been definitively proved to subsist.

The Draco planetary was observed on the meridian by Lalande 26th July 1790, and there are no good grounds for holding it to have since shifted appreciably from the place then assigned to it. Burnham, it is true, has found a progressive diminution of the distance between the central star of the nebula and one external to it, amounting to 0·033″ annually;[988] but the movement, if real, may confidently be ascribed to the disconnected star. The nebula is indeed very far from being stationary in space. Professor Keeler determined for it spectroscopically in 1891 a velocity of approach towards the solar system of forty miles a second;[989] and its apparent fixity on the sphere is doubtless only an effect of extreme remoteness. Some attempts to determine its annual parallax[990] have resulted only in showing it to be small “below compute.” From the prismatic examination of this object, 1st August 1864, Sir William Huggins learned the existence of gaseous nebulæ.

A nebula in Cygnus (N.G.C. 6826) was described by Mr. Burnham as “almost an exact duplicate” of the planetary in Draco. It is nearly circular, the longer diameter measuring nearly 27″, the shorter 24″. The nucleus is very bright (8·8 magnitude), and gives a strong continuous spectrum. The light from the disc, too, includes an unusually large white ingredient.[991] The radial motion of this nebula is very small; it approaches the sun by 3·3 miles a second. But the sun is travelling on its own account towards its place at a much higher rate of speed, so that the apparent sluggishness of the nebula indicates that it is really moving away from us, its measured pace representing only the velocity with which our system gains upon it.

The spectra of planetary nebulæ are fundamentally alike; they differ only in details. About forty bright lines have been determined in them, visually and photographically, and they are invariably fine and sharp, as if emitted by materials of great tenuity. They seem hazy only just where they cross certain diffuse nuclei; and this feature seems to imply a gradual condensation of the nebulous stuff towards the central mass, which, in such cases, cannot properly be called a star. The essential characteristic, however, of nebular light is the presence in it of the ray, or rays, of “nebulium.” This is the one sure criterion by which gaseous nebulæ can be distinguished from stars.[992] The chief nebulium line has a wave-length of λ 5007, and is of a clear green colour. With it is constantly associated a ray about one-third as bright at λ 4959, and the invariability of their relation lends strong probability to the opinion that both emanate from the same substance.[993] A strong ultra-violet line at λ 3727, photographed for the first time by Sir William and Lady Huggins in 1882, perhaps claims an identical origin. This, however, remains doubtful in the absence of decisive evidence that it is an unfailing constituent of the nebular spectrum. The green gas designated “nebulium” is unknown, so far, terrestrially; nor has it been observed to shine in any of the heavenly bodies except nebulæ. There is reason to believe it denser, or at any rate less diffusive than hydrogen. An object catalogued in the Southern Durchmusterung as a ninth-magnitude star under the title S.D.M. −12° 1172, was found by Mrs. Fleming in 1891 to give the spectrum of a planetary nebula. But hydrogen glows in it with unusual intensity. The relative brightness of the three green lines is estimated by Campbell[994] to be ordinarily 10 : 3 : 1, F (Ηβ) being the faintest and most refrangible. But the proportion in the planetary near Rigel is 10 : 3 : 7. In other words, hydrogen is of seven times its normal lustre comparatively to nebulium. Further, the three lines, when viewed through an open slit, form discs of severally 11″, 9″, and 14″ diameter. Here then, apparently, a hydrogen-envelope constitutes an outer shell to the nebulium-sphere; and a gas that rises higher than another is presumably specifically lighter, although, in view of the enormous altitudes attained by calcium vapour near the sun, the inference must be regarded as subject to qualification. Certainty on the point, and on many others connected with the physics and chemistry of nebulæ, may be said to be unattainable until nebulium is captured in the laboratory. And the prospect of this achievement, although not hopeless, is remote.

The following table gives the wave-lengths and origins, when they are known or can be conjectured, of forty bright lines in the spectra of planetary nebulæ. A few are common to all, notably the trio in the green, with about half a dozen of the blue and ultra-blue hydrogen lines, while others are more individual in their occurrence; but, on the whole, bodies of this class seem to be of remarkably uniform constitution.

LINES OBSERVED IN THE SPECTRA OF PLANETARY NEBULÆ.

Wave-Length. Origin. 6563 Hydrogen (Hα). 5876 Helium (D_{3}). 5751 Oxygen? 5680 Unknown. 5540 Unknown. 5412 Hydrogen; Pickering series? 5313 Unknown. 5183 Unknown. 5007 Nebulium. 4959 Nebulium? 4861 Hydrogen (Hβ). 4790 Unknown. 4743 Unknown. 4715 Unknown. 4688 Hydrogen; Rydberg series? 4662 Unknown. 4643 Nitrogen? 4610 Nitrogen? 4597 Nitrogen? 4574 Silicon? 4472 Helium. 4390 Helium. 4363[995] Unknown. 4341 Hydrogen (Hγ). 4265 Unknown. 4145 Unknown. 4122 Helium? 4102 Hydrogen (Hδ). 4067 Unknown. 4026 Helium. 3970 Hydrogen (Hε). 3968[995] Unknown. 3889 Hydrogen (Hζ). 3869[995] Unknown. 3836 Hydrogen (Hη). 3795 Hydrogen (Hθ). 3768 Hydrogen (Hι). 3727 Unknown. 3460 Unknown. 3390 Unknown.

Several of the Wolf-Rayet lines, it will be observed, are comprised in this list, notably those at λ 541, λ 469, and λ 464. And the absence of recognisable metallic rays strengthens the analogy with stars of that peculiar description. As a rule, the hydrogen spectrum in planetaries begins with the green line. The red line has been distinguished in only a few specimens, which appear more condensed than the rest. Yet it would be rash to assume that this is really the case.[996] The relative intensity of the hydrogen lines in stars and nebulæ is an intricate subject, the ramifications of which have yet to be tracked out. The coincident appearance of D_{3} with C is worth notice as a hint that the conditions favourable to the development of the slower light-vibrations are the same for helium as for hydrogen. They are markedly present in Struve’s planetary in Ophiuchus (N.G.C. 6572 = G.C. 4390), which gives a complex spectrum of at least thirty lines,[997] accompanied by faint continuous radiance. Three classes of fact regarding it are recorded in Professor Keeler’s sketch of the portion of it accessible to eye-observations (see Plate XXIII. Fig. 2): first, the positions of the component rays in the scale of wave-lengths; next, their relative lustre; thirdly, the extent of the nebula from which they are derived. And it is of interest to perceive that only those associated with nebulium, together with the green and blue lines of hydrogen (Hβ and Ηγ), seem to reach the limits of the disc, while the others are radiated only by its central parts. Still we have to remember that the length of the lines must depend to some extent upon their intensity; and that they may be short only because the sections of them given out by dim regions of the nebula are of evanescent faintness. Professor Keeler suspected the presence of dark bands interrupting its continuous light between D_{3} and λ 5007,[998] and they will, if verified, supply the only extant proof of absorption in gaseous nebulæ. The nucleus of Σ 6 apparently reinforces the emissions from the disc; but it is plainly not a genuine star. Further spectrographic investigation of the object is most desirable.

In the Draco planetary (N.G.C. 6543), on the contrary, the stellar and nebular elements of the spectrum are perfectly distinct. Some of the bright lines can indeed be seen only when the central star is outside the slit, and are therefore due, in Professor Campbell’s words, “to the nebula proper, as indeed are all the lines observed, and there is no evidence to show that they exist at all in the central star.”[999] A few of those registered in Struve’s planetary are missing here, particularly the hydrogen line (Pickering series) at λ 541; but C, D_{3}, and both the Wolf-Rayet blue radiations are perceptible, while the leading ultra-violet line at λ 3727 appeared conspicuously in Von Gothard’s photographs.[1000] The relative strength of the green lines in this nebula, as determined by Campbell, is 10 : 3 : 2. It is of a verd-antique hue, and, indeed, the nearly total suppression of red rays in their light gives to all planetaries a blue or greenish tinge.

One of the Durchmusterung stars in Cygnus (D.M. + 41° 4004) was noticed by the late Prebendary Webb, 14th November 1879, to have a hazy disc, some 10″ in diameter.[1001] Stephan, at Marseilles, independently detected its nebular character, and Winnecke compared it to a small comet with a tenth-magnitude nucleus at its preceding end. Moreover, the nucleus is double. Professor Keeler described the object (N.G.C. 7027) as follows:[1002]—“This is the brightest nebula that I have examined, and its spectrum is exceedingly interesting. The nebula is irregular in outline, and contains two central condensations, one of which has an oval and fairly well-defined outline. The other is much fainter and more diffuse.” His drawing of it, made at the great telescope, is copied in Plate XXIII. Fig. 3, together with a representation of its spectrum. The nuclei are obviously non-stellar. The continuous spectrum of even the more conspicuous member of the pair is not incomparably brighter than that derived from the disc, and it claims all the emission rays as properly, though not exclusively belonging to it. Professor Keeler accordingly regarded it as “in a much less condensed state than the nuclei of Σ 6 and many other nebulæ of its kind,” its exceptional brilliancy notwithstanding. The spectrum of Webb’s planetary is remarkable for the intensity of some usually quite subordinate lines, especially of the Rydberg hydrogen ray (λ 4688), and of the unknown lines at λ 4743 and λ 4363. Campbell could just identify the red glint of C,[1003] which by its faintness evaded Keeler’s survey; D_{3} was made out by both observers, and an unidentified line at λ 3869 was photographed by Von Gothard in 1892, although his plates were blank at the place where the significant λ 3727 was expected to appear.[1004]

A companion to Webb’s nebula, both in physical aspect and by vicinity in the sky (N.G.C. 7026), was discovered spectroscopically by Dr. Copeland in 1880.[1005] It had, however, been observed telescopically by Mr. Burnham seven years earlier.[1006] It is small, bright, duplex, a pair of diffuse nuclei 6″ apart sustaining a filmy structure which, viewed with the Lick refractor, suggested a comparison to two sheaves of corn laid side by side.[1007] Burnham refuses to admit the planetary nature of either of the objects in Cygnus,[1008] yet their spectra scarcely allow them to be relegated to a different class. The “Rydberg line” is equally prominent, relatively to their brightness, in both objects. From the cosmogonic point of view they are of high illustrative importance. We seem to have before our eyes double stars in slow course of formation, and preparing to break loose by the development of systemic revolutions from the trammels of a joint rotation.

Bi-nuclear planetaries are not uncommon. One such (N.G.C. 3195) was observed by Sir John Herschel in the south polar constellation of the Chameleon, and is depicted in his volume of Cape Results. The nuclei are fairly well matched in lustre, and will perhaps grow into a double star like γ Virginis. Their spectrum is still unrecorded; but it will be of considerable interest to determine whether it exhibits the peculiarities visible in those of the double planetaries in Cygnus. Two bright patches near opposite margins of the circumference give the nebula in Chameleon somewhat the air of a reduced copy of the celebrated “Dumb-Bell” in Vulpecula.

Of a planetary in the Poop of Argo, originally discovered by the elder Herschel, Lassell wrote at Malta about 1851:[1009]—“No description can do justice to this singular object,” which is “not beautiful, for it has no symmetry, but wonderful.” His drawing shows the disc, which had appeared to d’Arrest perfectly round, as pear-shaped, with multiple condensations. So that a quadruple or quintuple star in embryo may here be offered for our contemplation.

“Stellar” nebulæ have been mostly discovered by Pickering’s method of sweeping with a direct-vision spectroscope. There is probably no radical difference between them and planetaries, for their comparative minuteness may be a simple effect of distance. Or they may be constructed on a reduced scale. We should naturally expect to meet in nebulæ a variety of dimensions not inferior to that existing among stars. Moreover, they are all alike gaseous, and give—so far as is yet known—perfectly similar spectra. Nevertheless, Mr. Burnham writes of the stellar kind as “very small, bright, round nebulæ, which in a small instrument would resemble stars slightly out of focus, but do not appear to come within the planetary class.”[1010] An admirable specimen was detected spectroscopically by Pickering 16th July 1882. Previously registered as a 9·4 magnitude star, it took rank, on the strength of its bright lines, as a stellar nebula (N.G.C. 6790). The Lick thirty-six-inch showed it to be round and lucent, with a minute nuclear point.[1011] A miniature of Struve’s planetary in Ophiuchus seemed to float in the field of the telescope. Without a slit, the spectrum, examined by Keeler, resembled three tiny greenish stars, that formed on Hβ being much the faintest. This stellar nebula accordingly is analogous to Webb’s planetary in the feeble glow of its hydrogen constituent. Another pseudo-star in Aquila (N.G.C. 6891) disclosed itself prismatically to Dr. Copeland in 1884. It has a disc just 4″ across, and its spectrum, photographed by Von Gothard 27th October 1892,[1012] includes the usual range of nebular lines up to λ 3727, besides a fair admixture of continuous light. A nebula of the fourteenth magnitude, visually a finished star, was noticed for the quality of its light by Pickering, 25th November 1881, near the star b^2 Cygni.[1013] More conspicuous members of the class have been identified in considerable numbers on the Draper Memorial plates.

The crowd of small nebulæ photographed by Dr. Max Wolf in 1901 comprised a remarkable proportion of seeming planetaries. They were collected on his plates into pairs and groups in a manner recalling the distribution of Wolf-Rayet stars, but not previously observed to characterise that of planetary nebulæ. The question indeed arises whether they are really such? Or do they rather belong to “that much less interesting class of objects” designated by Mr. Burnham as “small circular patches of nebulosity”? The spectroscope alone can decide, and its verdict should be elicited without delay. Upon it will largely depend the conclusions to be drawn respecting the affinities of the planetary family, their mutual relations, and the mode of their scattering in space.

There is reason to believe them enormously remote. Four have been directly measured for parallax, namely, the helical nebula in Draco, the bi-nuclear planetary in Argo (N.G.C. 2440),[1014] Webb’s in Cygnus, and a structure with a ring and disc in Andromeda (N.G.C. 7663).[1015] None showed the least sign of perspective shifting when viewed from opposite sides of the earth’s orbit; and the demonstration thus afforded of their immense distance is confirmed by the insensibility of nebular proper motion. This, since it is continually progressive, must eventually prove determinable, and comparisons of its amount with the mean radial velocity of these bodies will supply a criterion for their absolute localisation. Professor Keeler derived from the line-displacements in the spectra of eleven nebulæ a value for this quantity of sixteen miles a second; and by sixteen miles a second they should, accordingly, on an average, progress along each of the other co-ordinates fixing their position in space. When corresponding angular advances have been established, their average distance can then at once be estimated.[1016] Let us assume, for instance, that the eleven nebulæ in question, taken one with the others, have a secular proper motion of three seconds of arc in declination, and as much in right ascension. This, although the outside of what is probable, implies that their mean distance corresponds to a light-journey of 580 years. A planetary not more than 10″ in diameter would, if thus remote, fill a globe about 600 times wider in girth than one circled by the orbit of Neptune, and 216 million times more capacious. The data supplied by Professor Keeler are indeed an obviously insufficient groundwork for extensive generalisations; but more of the same kind, and it may be hoped of not inferior quality, cannot fail to be forthcoming shortly; while the precise visual measurements executed by Burnham, Spitaler, Javelle, and others will surely serve, after some decades, for the detection of genuine nebular journeyings across the sky. A beginning of definite knowledge will then have been made regarding the true magnitudes and place in the sidereal scheme of these singular objects. It must not, however, be forgotten that many of the faint stars taken as fiducial points for their micrometrical determination may prove to be satellites of the neighbouring gaseous globes drifting and shifting in their company. This disquieting possibility, foreseen by Sir John Herschel and d’Arrest, would, of course, if realised, vitiate conclusions as to parallax or proper motion. Yet the work done for these purposes need not be looked upon as wasted. By its aid, should the slow revolutionary movements of stars round planetary nebulæ ultimately be brought to light, a new department of sidereal mechanics may be founded. Exact determinations in astronomy, made on a judicious plan, are rarely useless. If unprofitable for their designed aim, they are pretty sure to turn to account for some other, and perhaps a higher one.

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XXXV. ANNULAR NEBULÆ

The model annular nebula is the beautiful filmy ellipse situated between ß and γ Lyræ (N.G.C. 6720). Discovered by Darquier at Toulouse in 1779, it appeared to Sir William Herschel a simple hoop of light, quite dark within. The “gauze drawn over the hoop,” spoken of by Sir John Herschel, had been perceived by Schröter in 1797, and with long exposures the photographed ring fills up into a disc. Plate XXV. is taken from a singularly perfect representation obtained by Mr. W. E. Wilson in twenty minutes, while an hour’s exposure sufficed to blur beyond recognition the characteristic annular aspect of the structure. Yet the havoc thus wrought, in a pictorial sense, is compensated by the experimental significance of a result proving the interior of the ring to be far from vacuous of luminous or luminescent material.

The dimensions of the ring are about 87″ by 64″. It was shown in the Lick negatives as an exceedingly complex structure. “It seems,” Professor Keeler wrote,[1017] “to be made up of several narrower bright rings, interlacing somewhat irregularly, the spaces between them being filled with fainter nebulosity.” Many bright patches and condensations diversify the main annulus, which is, nevertheless, fundamentally continuous; it does not break up into detached knots of nebulosity. The light is strongest near the extremities of the transverse axis, and its failure at each vertex of the ellipse, conspicuously shown in our Plate, was, already in 1785, noticed by the elder Herschel.[1018] It is accompanied by symptoms of effusion, marking possibly equatorial outflows due to rotative acceleration. Lord Rosse in 1863, and Schultz in 1865, were alike struck with “nebulous radiations in the direction of the longer axis, which seemed momentarily almost to destroy the annular form.” They issued from the north-eastern side, while a similar appearance on the south-westerly side of the nebula was observed by Professor Holden at Washington in 1875.[1019] He perceived, too, with surprise that the south end of the minor axis, which Lord Rosse had represented as the best terminated, was unmistakably, after thirteen years, less clearly finished than the north end. Similarly, the Lick thirty-six-inch disclosed in 1888[1020] the whole southern margin as filamentous, by a kind of alternation with the corresponding state of the northern edge noticed at Parsonstown. This tufted or fringed effect was caught, for the first time photographically, by Professor Keeler. The oval imprinted on his plates was fringed on both sides, and his measurement of eleven nebulous projections from it will give the means of detecting any future variations in their luminosity or distribution.

The gauzy stuff in the interior is not a mere formless light-mist. A drawing published by Lord Rosse in 1844[1021] represented it as divided into longitudinal striæ, and their reality was confirmed by the Crossley photographs after fifty-five years. “I have tried,” Professor Keeler wrote, “to verify this band-structure by visual observation with the thirty-six-inch refractor, and have fancied at times that I could catch glimpses of it; but the observation is a most difficult one, the contrast of the bright and dark bands, exaggerated by the photograph, being almost too slight to affect the eye.” The surprising accuracy of the delineations made with the Rosse speculum, while its burnish continued unimpaired, was thus once more exemplified. It conveys a warning against lightly setting aside any of the earlier records concerning nebulæ obtained at Parsonstown, even when they imply seemingly improbable changes. It is certainly by no accident that the striæ within this nebula coincide in direction with its major axis; nor can the termination of the transverse axis by maxima, of the longitudinal axis by minima of brightness[1022] be regarded as casual features. All the details of the edifice, in fact, are arranged with obvious reference to its apparent shape, and this amounts to a demonstration that the elongation is real. The nebula, then, is not simply projected into an oval; it is not a circular formation viewed obliquely. There seems no escape from the conclusion that it is an ellipsoid of revolution—that the bands follow the line of the equator, and originate under conditions prescribed by the rotation of the body; while the partial interruption of luminosity at the ends of the oval mark outflows of matter where centrifugal velocity overrides the holding power of gravity. Everything, indeed, leads us to suppose that this nebula, like the rest of its kind, is actually a hollow spheroid of shining fluid, the marginal brightness resulting from the increased thickness of the luminous shell penetrated by the visual ray. The “hoop” and the “gauze” drawn round it are then two aspects of the same thing. Nebulous rings, as such, probably do not exist. They would be subject to perspective effects, no traces of which are to be found in the heavens. Annularity in nebulæ may accordingly be considered as a purely optical modification of a different structural plan.

A glittering point of light occupies the centre of the annulus in Lyra. Many anomalies are connected with its visibility. The first heard of it is in 1800, when Von Hahn of Remplin, in Mecklenburg, was surprised by its disappearance.[1023] He attributed the change, not to loss of light in the star, but to the nebulous clouding-over of the black background upon which he was accustomed to see it relieved. Unperceived by the Herschels, it was next seen by Lord Rosse in 1848,[1024] and attracted Father Secchi’s attention at Rome in 1855.[1025] A ten-inch Steinheil sufficed, in 1865 and 1867, even in unfavourable weather, to show Hahn’s star to Hermann Schultz;[1026] yet it unaccountably evaded the deliberate scrutiny, ten years later, of Professor Asaph Hall,[1027] armed though he was with the twenty-six-inch Washington equatorial. The same instrument, however, displayed its evasive sparkle to Mr. A. C. Ranyard, 23rd August 1878;[1028] while to Dr. Vogel, with the Newall telescope in 1875, and the Vienna twenty-seven-inch in 1883, it remained consistently imperceptible.[1029] Very remarkable, too, is its non-appearance to Dr. Spitaler at Vienna in 1885, when he carefully delineated the nebula, as well as in 1886, during frequently repeated verifying observations.[1030] The interior seemed then to contain only dimly luminous floccules; nevertheless on 25th July 1887 the star caught his eye at the first glance. It had, in the meantime, 1st September 1886, been photographed by Von Gothard, and has since abstained from capricious disappearances. It is indeed of such exceptional actinic power that the camera cannot easily lose sight of it. Fainter visually than the fifteenth magnitude, it needed only an exposure of one minute to come out distinctly on a Crossley negative, and it left a dim impression in half that time.[1031] With all exposures the image was clearly defined, although in photographs taken with other instruments it had usually presented a hazy disc. Its light appears to be of normal stellar quality. Its maximum of intensity, that is to say, falls in the yellow part of the spectrum. Both Keeler and Barnard agreed that, with the Lick and Yerkes refractors, the focus for the star was about one-fifth of an inch shorter than for the encircling nebula. And a similar disparity exists between the nuclei and discs of most planetaries. This, however, leaves their special chemical effectiveness unexplained; for Keeler’s suggestion of its being due to ultra-violet emissions lacks the support of known facts.

In the field with the Lyra nebula, Barnard perceived, 2nd October 1893, a second of about the fourteenth magnitude, 30″ in diameter, and somewhat irregular in shape.[1032] Professor Keeler’s longest-exposed negative showed the new object to be “a left-handed, two-branched spiral.”

In 1891 Mr. Burnham measured the nucleus of the ring in Lyra with reference to an external star of the twelfth magnitude which closely follows it. Eight years later, Professor Barnard employed the forty-inch Yerkes telescope to repeat his determinations,[1033] and was inclined to attribute a slight discrepancy to real motion in the nebula. But the Potsdam plates lent no confirmation[1034] to a suspicion which will probably remain long unverified. His experience was entirely negative as regards light-variability in any part of the object. Alleged changes in the stellar kernel he translated into genuine changes in visual facilities. “The fact,” he wrote, “that the nucleus is seen on a nebulous background makes steadiness of the atmosphere a most important factor in its distinctness—far more so than in the case of an ordinary star in the open sky. When the seeing is exceptionally good, the nucleus appears with a distinctness strikingly in contrast with its ordinary condition, so much so that one has to guard against deception in supposing a real change of light.”

Nine bright lines have been seen or photographed in the spectrum of the Lyra annulus, that of shortest wave-length at λ 3727 being, according to Von Gothard, the most intense.[1035] Hydrogen shines feebly; no C has been recorded, but the Rydberg ray at λ 4688 was detected by Campbell.[1036] Helium is unrepresented; since a violet line at λ 3869, which appears prominently and persistently in nebular spectra, cannot be ascribed to that substance. It chances, indeed, to coincide approximately with one belonging to a known series, but is itself of “rare and strange” origin.

Several analogues of the Lyra nebula have been discovered; none so large or so bright. The best imitation is situated in Cygnus (N.G.C. 6894). It measures 47″ by 41″, the inner vacuity 20″, and was marked “resolvable” at Parsonstown.[1037] Needless to say that the sparkling effect which conveyed the impression of a stellar constitution was altogether illusory. A fifteenth-magnitude star near the interior border of the ring to the north-west was measured by Burnham in 1891.[1038] The true nucleus is considerably fainter. First brought to view in a photograph taken by Dr. Roberts 31st August 1897,[1039] it duly reappeared in one of the Crossley pictures of 1899,[1040] and was visually discerned with the great Paris siderostat by M. Antoniadi, 17th July 1900.[1041] The elongation of the annulus from north-east to south-west was obvious to him no less than the faint haze with which it is filled. Nothing is known about the spectrum of the ring nebula in Cygnus. It doubtless resembles that of its prototype in Lyra.

Even more scanty is the information at hand regarding “a beautiful delicate ring” in Scorpio, about 40″ in diameter (N.G.C. 6337). It is in a field crowded with stars, two of which are projected upon, but may not belong to it. Neither occupies the proper position of a nucleus, and the one seen by Lassell at Malta had to him somewhat the aspect of a nebulous knot.[1042]

An annular nebula in Ophiuchus (N.G.C. 6369) was found by Mr. Burnham “very like the well-known example in Lyra, except in brightness.”[1043] A central star of 14·5 magnitude was probably then first noticed. The longest diameter of the ring measures 31″; the edges seemed to Sir John Herschel “a very little cottony,”[1044] and they are doubtless fringed with dim appendages, like those attached to its model in Lyra. This is again copied in an object discovered by Mr. Gale of New South Wales in 1897, or perhaps a little earlier. It is bright enough to make its late detection somewhat surprising.[1045]

Immersed in a fine cluster in Argo (Messier 46), a nebula of planetary aspect, about 60″ in diameter (N.G.C. 2438), drew the attention of Sir William Herschel. The Rosse reflector showed a central star dominating a vacuous interior, besides two stars sparkling on the condensed border.[1046] Lassell perceived in the object a resemblance to a large, dim, compound planetary in Eridanus (N.G.C. 1535), but with the qualifying circumstance that in N.G.C. 2438 only one “stratum of nebulosity” was discernible.[1047] This singleness of construction appears characteristic of perfected nebular rings, and such the inmate of the cluster in Argo has declared itself to be. In a photograph taken by Dr. Roberts, 24th February 1894,[1048] it is definitely and unmistakably annular. Three stars are projected upon, or contained within it, we cannot tell which; although one by its nuclear position gives some assurance of being there through organic relationship. Nor can we venture to assert that the nebula is really in the cluster. It may only be thrown accidentally into line with it. Still the fact that Sir John Herschel recorded in two cases similar collocations of planetary nebulæ with clustered stars[1049] inclines the balance of probability towards the side of genuine association. One of these groups (N.G.C. 5979) is situated in the constellation Circinus, the other (N.G.C. 2818) near the mast of Argo.

Certain complex formations, intermediate between planetary and annular nebulæ, have now to be considered. A striking specimen of the kind is met with in Andromeda (N.G.C. 7662). The disc, which includes perhaps more than one ring, measures 32″ by 28″. Alexander, about the middle of last century, and Lassell subsequently, thought the structure bi-annular. Vogel[1050] and Holden remarked its warped and twisted appearance, denoting possibly a multiple combination of rings thrown off in various planes as the outcome of long-past crises in a slow process of development. A central star surrounded by close spirals of nebulosity was seen at Parsonstown,[1051] but evaded the scrutiny of O. Struve in 1847, of Searle in 1866, and of Vogel in 1883. Lassell perceived it under the guise of a minute, bluish disc, Burnham as an ordinary fifteenth-magnitude star.[1052] Fig. 4 in Plate XXIII. reproduces Professor Keeler’s drawing of the Andromeda planetary with its visual spectrum.[1053] “This nebula,” he wrote, “is annular, with a bright inner ring and a very small nucleus. It is somewhat elongated north and south.” The fourth line in the spectrum is the “fundamental” of Rydberg’s hydrogen series; its unusual strength makes it the equal of the ordinary hydrogen lines on either side of it. The spectrum of the nucleus appears to be perfectly continuous, save for a possible bright knot about the place of D_{3}. Campbell, however, caught no glimpse of this radiation, although he determined, visually and photographically, eighteen bright lines in the spectrum of the nebula.[1054] We subjoin his list, with notes and comments between brackets.

BRIGHT LINES RECORDED IN N.G.C. 7662. ┌────────────┬────────────────────────────────────────────────────────┐ │Wave-length.│ Remarks. │ ├────────────┼────────────────────────────────────────────────────────┤ │ 540│Very faint, difficult (Wolf-Rayet line; Pickering │ │ │ series). │ │ 532│Very faint, difficult (possibly the chromospheric K │ │ │ 1474). │ │ 5007│First nebular line, very bright. │ │ 4959│Second nebular line, very bright. │ │ 4861│Hβ, very bright. │ │ 4744│Faint (origin unknown). │ │ 4715│Faint (unknown). │ │ 4688│Very bright (Rydberg hydrogen line). │ │ 4663│Very faint (unknown). │ │ 4643│Faint (nitrogen ?) │ │ 4472│Very faint (helium; prominent in Orion stars). │ │ 4364│Bright (unknown). │ │ 4341│Hγ, very bright. │ │ 4102│Ηδ, very bright. │ │ 4067│Very faint (unknown). │ │ 4026│Very faint (helium). │ │ 3969│Hε, very bright. │ │ 3869│Very bright (unknown). │ └────────────┴────────────────────────────────────────────────────────┘

The brilliancy of the last line, in view of the faintness and fewness of other helium emanations, confirms the inference that its association with that substance is inadmissible. Experiments in the laboratory would nevertheless be valuable on the behaviour, for instance, of the adjacent violet line of helium in a mixture of that gas with hydrogen.

Campbell perceived this nebula to consist “of two nearly concentric rings more or less broken up, with a fourteenth-magnitude stellar nucleus near its centre.” Ingall described it, 18th December 1885,[1055] as a “superb planetary,” of a bright greenish-blue tint, the centre not quite dark, and thus fitly to be called annular. On applying a power of 500, “an extraordinary structure appeared to unfold itself. The bright ring seemed very jagged and fringed at the edges, and the centre was mottled with unequal shades, often as if of two dark centres.” The note made on it at Harvard College was: “Somewhat annular; edges hazy.”[1056] The blue colour of the object faithfully corresponds to the actinic energy of its rays. In two seconds of exposure to them the Crossley reflector gave a weak image, including a barely visible central star,[1057] and Professor Keeler obtained finished pictures in 20, 30, and 60 seconds. At Potsdam, in 1892–93,[1058] thirty-three impressions were taken for the purpose of parallax-determinations, but they yielded no positive result. They afforded only the information that the nebula could not have an annual parallax so large as one-fifth of a second; and, indeed, the true value of the quantity, judging from other indications, may very well fall short of one-tenth the assigned maximum.

A “sky-blue likeness of Saturn” in Aquarius (N.G.C. 7009) is built very much on the lines of the Andromeda planetary, but with the addition of “ansæ.” These, in August 1888, were resolved by Holden and Schaeberle, with the aid of the Lick refractor, into a pair of attendant nebulosities, situated in line with the major axis of a strongly elliptical body, and subsensibly united to it by evasive gleams of illumination.[1059] Yet the likelihood is small of their being really satellite-globes revolving in the same track at an invariable interval of two right angles. The probabilities of the case oblige us to believe rather that the original interpretation of them as the extremities of an annular appendage came nearer the truth. An analogy indeed suggests itself between them and the nebulous effusions from the vertices of the ring formation in Lyra.[1060] And here again we are assured that they mark an equator—that the disc they seem attached to must be the projection of a rotating spheroid. Intricacies of interior arrangement are, however, visible, showing the progress of manifold activities. Two dark cavities, extended parallel to the major axis, and helical wisps of nebulosity, were observed by Vogel at Vienna in 1883.[1061] Keeler saw and photographed a somewhat distorted condensed ring measuring 26″ by 16″,[1062] and Scheiner’s plates recorded curious spoke-like projections from an intensely actinic central star.[1063] The spectrum differs from that of the Andromeda planetary only by the inclusion of the enigmatic “last line” at λ 3727. From the displacement of the green ray of nebulium Keeler determined for the Saturn nebula a movement of approach towards the sun at the rate of thirty-one miles a second, only a small proportion of which can be due to our own journey through space.

Sir William Herschel observed in the constellation Gemini in 1787 “a star of the ninth magnitude, with a pretty bright nebulosity equally dispersed all around” (N.G.C. 2392). Lord Rosse found in it a dark hole close to a slightly eccentric nucleus.[1064] D’Arrest thought the object might be called annular;[1065] Lassell perceived a ring surrounding a bluish disc;[1066] Secchi described it as a star with an annular aureola.[1067] H. C. Key, using an eighteen-inch silver-on-glass reflector, noticed about 1868[1068] a concatenation of bright and dark rings besides the patch of interior obscurity detected at Parsonstown. To Burnham the nebula in Gemini appeared “one of the most beautiful objects of the kind in the heavens.”[1069] He assigned to it a diameter of 45.″ Barnard, finally, was impressed by its “magnificent and beautiful” effect in the Yerkes telescope.[1070] It disclosed to him a ninth-magnitude star encircled not quite symmetrically by a brightish oval ring, partially incomplete towards the south. “This ring,” he continued, “which is well defined inside and out, is surrounded by a vacuity, and this in turn by an almost circular broad ring of light less intense than the inner ring, and with a distinct break in it north-preceding. It breaks up into a clouded or unequal surface, and is very irregular on its inner edge, but fairly uniformly circular on its outside edge. The inner ring is filled with a nebulous light which has a black spot in it, south-preceding the nucleus.”

That the perforated effect is no illusion may be taken as proved. Nor is it subject to change. It has obtruded itself now for half a century upon one observer after another, under divers conditions, both instrumental and climatic. This nebula then, and a few others like it, betray the action of some force strange to our experience, by which the matter contained in an extensive region is either expelled thence, or its light-giving faculty suppressed.

The annular nebula in Lyra is the only member of the class that has been satisfactorily investigated. Until spectrographic possibilities are further developed there seems little chance of dissipating the perplexities that still envelop its nature. That is, by direct means; for indirectly much may be done. Comparisons, for instance, of detailed results for sister-objects ought to prove highly instructive as to the laws governing the construction of all alike. Most of them have been singularly neglected, considering the interest attaching to their peculiarities. The hooped nebula in Cygnus, the “ghost” in Scorpio, Gale’s annulus, the ring in Ophiuchus, should be photographed with long exposures, on a scale sufficiently large to bring into view specialties of texture and build. Their agreement in certain fundamental relations would thus be tested, and its importance as a guide to theories of their mode of origin cannot be overrated. Their self-delineation would, however, doubtless accentuate besides that variety in similarity which, throughout the whole created world, illustrates the wealth of the resources disposed of by Nature, and the inexhaustible inventiveness of the Mind revealed in Nature.

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XXXVI. THE ORION NEBULA

The first place among irregular nebulæ is, by universal consent, accorded to the gleaming formation in the Sword handle of Orion. Although incidentally referred to by Cysatus of Lucerne in 1618, it received little attention until Huygens, in 1656, affixed his note of admiration, and executed a drawing of the bright central part, still known as the “Huygenian region.” Here is situated the trapezium, the hub, as it might be called, in which all the spokes of the great wheel are inserted. A photograph of the group is reproduced in Plate XXIV. Fig. 2. It was taken by Professor W. H. Pickering from Mount Wilson, California, 29th September 1889, with the thirteen-inch Boyden telescope. The exposure allowed was only ten minutes, and already the enveloping haze was beginning to cloud the images of the stellar sextett. The companions (which seem to vary in light[1071]) of the two brighter stars are distinct in the original negative, and can be made out in the figure, one as a tiny blotch, the other as a mere deformation of the lowest and largest disc. The physical association of these six stars may be assumed without much risk of error; but there is nothing to show that any real tie exists between them and four adjacent star-points, at the limit of vision with the Lick thirty-six-inch, detected by Barnard and Alvan Clark in 1889.

The nebula, as it developed on a plate exposed, under the same circumstances, during 2^h 36^m, is shown in Plate XXVI. The stars of the trapezium are here completely submerged; only their influence can be traced, or suspected, in the symmetrical arrangement of the expansive wings of light stretching away from their place. These are by no means vague or indefinite outflows. Some of the long streamers have sharp inner edges, peculiarly curved and notched. And the texture is generally filamentous, like that of solar prominences, the characteristic forms of which—as Mr. Ranyard effectively pointed out[1072] are faithfully imitated in some of the minor features of the nebula. An outlying mass to the north (N.G.C. 1977) not only claims affinity by some degree of structural resemblance, but is seen, on Professor Pickering’s plate, to be linked on to it by a faint intermediate extension. The gap between them is absolutely black to telescopic vision.

Improvements and modifications in photographic methods have led, by successive steps, to a remarkable increase in the compass assignable to this extraordinary formation. Portrait-lenses have played a leading part in the process. Professor Wadsworth’s reasonings[1073] have made it clear that small apertures, owing to the greater contrast afforded by them with the sub-sensible illumination of the sky, are positively advantageous for obtaining impressions of faint, extended objects. They have the further prerogative of a wide field of view, giving room for the grasp and delineation of large contours; so that, in certain branches of celestial portraiture, they render indispensable and invaluable services. By their aid the nebulous stars ι and c Orionis were proved to be dimly connected with the trapezium nebula; a sharply indented streamer became apparent, issuing towards it from the belt star, ζ Orionis, long recognised as a separate focus of nebulosity; while Max Wolf derived evidence of a luminous union between the nebulæ surrounding ζ and ε Orionis.[1074] Finally, the combined picture, shown in Plate XXVII., of all the nebulous patches in the constellation was obtained by W. H. Pickering, 14th January 1890.[1075] The instrument employed was a Voigtländer lens, 2·6 inches in aperture, and of 8·6 inches focal length; the time of exposure, 6^h 22^m. As the upshot, the fragmentary condensations previously known became united into a vast spiral formation, 15° across. Starting from near Bellatrix (which lies outside the field), it sweeps round by κ Orionis and Rigel to the south, then bends upward to η Orionis, and most probably effects a junction with the nuclear mass in the Sword handle, although the further course of the stream is rendered indistinct by the fogging of the plate. After an interval of nearly five years, Professor Barnard independently repeated and confirmed the Mount Wilson experiment.[1076] His small “lantern lens” showed, with exposures of 2^h and 1^h 15^m, “an enormous curved nebulosity encircling the belt and the great nebula, and covering a large portion of the body of the giant.” In comparison with it, he remarked, the old trapezium nebula “is but a pygmy.” The astonishing extension given to the luminous structure in Orion was, however, of less importance than the disclosure of its architectonic plan. Nor can it be supposed that we are, even now, fully acquainted with it. Further developments may be looked for. The nebulous connections of the various parts will doubtless be more clearly expounded in future photographs. Possibly an effective delineation might be obtained in sections with instruments, like the Crossley or the Meudon reflector, too restricted in angular grasp to embrace the whole at one view. The study of details would thus be made feasible, their subordination to the general design being at the same time kept well in sight. The relations of the members to the main body of this nebula offer a problem of extreme complexity. The stars of the trapezium, for instance, have an appreciable proper motion of about 5″ a century. The nebulous plumage attached to them must evidently partake in their displacement; but this cannot be unhesitatingly asserted of the appendages to the belt stars, still less of the ocean-river of nebulosity flowing outside them. Professor W. H. Pickering discussed the movements of nineteen stars involved in the spiral, though not of course necessarily connected with it.[1077] And so far as any common character could be ascribed to them, it was that of recession from the trapezium. It would be ridiculous to found any conclusion upon so flimsy a basis; yet the indication harmonises with hints, or nascent symptoms of a centrifugal tendency among clustered stars such as the Pleiades. The determination and comparison of their radial movements will be the best means of promoting knowledge on the subject; and since Pickering’s nineteen stars are all, except one, brighter than 6·2 magnitude, their spectroscopic measurement should present no difficulty.

Meanwhile the original “Fish-mouth” nebula raises, even when treated apart, questions of extreme intricacy. Seventy of the stars scattered through it were photographically determined by Scheiner, and compared in 1898[1078] with Gould’s similarly deduced places for them. The close agreement of the results showed them to have remained sensibly stationary during about a score of years, not sharing the drift of the central group. They must accordingly become, in course of time, detached from it, and from the encircling nebula. They constitute then no physical part of the structure, but are scattered casually over its surface. The trapezium-stars, on the contrary, are really embedded in the nebula. And the grounds for this assertion are twofold. Visual logic by itself certifies its truth. The wings of a bird do not start more definitely from the breastbone than the nebulous plumes from the stellar group at their base. The relation is emphasised by the circumstance that two of its members serve as the abutments of a luminous arch, photographed at Potsdam,[1079] which spans the interval between them. A similar nebulous bridge was remarked by Dr. Scheiner to connect two other stars in the outskirts of the formation. Then, as we have already seen, the spectrum of θ Orionis presents peculiarities indicative of a close affinity with the nebulous masses around, and indeed suggests, albeit doubtfully, an actual commingling of luminous substance. It follows that the nebula shares the proper motion of the stars, the small secular value of which (5″), at the probably vast distance of the involved objects, corresponds to a very considerable real velocity. Admitting for a moment the correctness of Professor Pickering’s estimate of one thousand years for the duration of the light-journey from the Orion nebula hither,[1080] we should have to ascribe to the trapezium and its belongings a lateral speed of forty-nine miles a second, the greater part of which would be inherent, since its direction precludes the supposition that it is a perspective effect of the solar journey. The receding radial motion of eleven miles per second, determined for the nebula by Keeler, seems, on the other hand, to belong almost entirely to the sun. Yet the line-displacements by which it is indicated vary, in some degree, from tract to tract of the formation. Recent spectrographic measures executed at Potsdam[1081] imply the progress of interstitial movements, the nature and laws of which it will be of the highest interest to ascertain.

Thirty bright lines, from λ 5007 up to the seventeenth member of the hydrogen series, have been photographed in the spectrum of the Orion nebula, besides the baffling groups faintly apparent on the Tulse Hill plate of 1888. The yellow helium ray (D_{3}), visually detected by Dr. Copeland in 1886, has at least three more refrangible associates; but several lines met with in planetary nebulæ are missing, notably those distinctive of the Wolf-Rayet class of stars. Thus the Pickering and the Rydberg series are alike unrepresented in this grand object. Nor does it glow with the light of any metallic vapour. Its hydrogen emissions are in some respects peculiarly conditioned. The red line is invisible; the green and blue lines are especially bright; beyond them there is a progressive decrease of intensity, though the complete series, up to Hρ, was photographed by Mr. S. A. Mitchell, with a concave grating of 11½ square inches, mounted on the twelve-inch refractor of the Yerkes Observatory.[1082] Thus the state of the nebula is such as to favour, apparently, the development of the quicker luminous vibrations. The inference has, indeed, been controverted. Dr. Scheiner maintained that the suppression of C in this spectrum is “due to purely physiological causes, and warrants no conclusions as to the physical conditions of luminosity,” in nebulæ.[1083] He regarded it, in fact, as an illustration of the “Purkinje effect,” by which the eye, owing to its differential colour-sensitiveness, loses sight of a red sooner than a green ray, when the light-source from which both proceed is gradually enfeebled. In the laboratory, accordingly, F survives alone in the spectrum of a hydrogen-tube at minimum illumination. The plausible deduction that there is no objective reality in the anomalous variations of comparative strength affecting the hydrogen lines derived from celestial objects is, nevertheless, untenable. It collapsed hopelessly, on being confronted with the simple fact, noted by Keeler,[1084] that the third hydrogen line (Hγ) always vanishes experimentally before the first, while in nebulæ it shines unfailingly, although F be imperceptible. The order of brightness, then, of the members of the series is not, in nebulæ, prescribed by physiological causes alone; the faintness of Hα is intrinsic. This is further proved by Professor Keeler’s observation of “Hβ, and several of the hydrogen series above it, glowing brilliantly in the spectrum of R Andromedæ, while not a trace of Hα could be found.” So that “in some of the variable stars we seem to have hydrogen in the same condition as in the nebulæ.”

Finally, Professor Campbell made the decisive experiment of directly comparing the nebulæ with an artificial spectrum.[1085] The two kinds of light being admitted through the upper and lower halves respectively of the same slit, their spectra were seen side by side in the eye-piece, and the corresponding lines in them could be at once equalised by merely altering the distance of the hydrogen-tube from the slit. Combined experiments with this apparatus by Keeler, Wright, and Campbell proved that (1) when the F lines from the nebula and tube were matched in brightness, the blue line above it (Hγ) was markedly stronger in the nebular than in the tube-spectrum; (2) the equalisation of the blue lines left the green line from the nebula conspicuously fainter than the same line from the tube. “The relative intensities,” it was concluded, “of the hydrogen lines from the nebula and from the tube are, therefore, not the same; the nebular lines are relatively the stronger toward the violet, the lines from the tube are relatively the stronger toward the red end of the spectrum.” Absolute measures showed the three principal lines in the spectrum of the Orion nebula to be of very low intensity. Their faintness completely neutralised, to Professor Campbell’s eye, their differences of tint. Hence the “Purkinje phenomenon,” which depends upon the perception of colour, cannot in any degree affect their comparative visibility. There is, however, another aspect to the question. The spectrum of the nebula varies, not alone from the standard of comparison supplied by the vacuum-tube, but also locally, within the formation itself. The relative strength of the constituent rays is different for its different sections. Dr. Runge of Hanover,[1086] who devoted special attention to the point during a visit to the Lick Observatory in September 1897, satisfied himself that the F line of hydrogen, which, near the trapezium, had only one-third (or possibly two-fifths) the intensity of the nebulium line, was ten times brighter in the faint outlying sections of the nebula. The relative gain was then twenty-five to thirtyfold; while the alleged physiological cause, admitting that it had full scope and play, could at the utmost have produced a gain of 1·8 times. The reality of the change was further certified by observing the second nebular line (λ 4959), which in the “Huygenian region” just equals F, to disappear as the slit was moved outward, while F continued to shine with a very sensible lustre. And since, in this case, the Purkinje effect was null (the lines being almost indistinguishable in colour), a demonstration was afforded of a genuine modification in the curve of emissive energy in passing from one part of the great nebula to another.

Professor Keeler devised a completely novel method of demonstrating its non-homogeneous character.[1087] Pictures of the nebula taken on orthochromatic plates protected by colour-screens from the blue radiations, were compared with impressions on ordinary unscreened plates, all being exposed with the Crossley reflector, though during very unequal times. The result was to show that, for equal intensity of the Huygenian region, that of the remote parts and outlying streamers fell greatly below its normal value in the screened photographs. “Conversely,” to quote the words of the ingenious operator,[1088] “where photographs made by the two methods, on the same night, show an equal extent of nebulosity, the Huygenian region is very much more intense on the orthochromatic plate. We infer, therefore, that in the remote parts of the nebula the two lowest nebular lines are weak, or the hydrogen lines strong, as compared with the Huygenian region. Thus the results of spectroscopic researches are confirmed, and are extended to parts of the nebula too faint for visual observation.” That is to say, the hydrogen image of the Orion nebula is larger than its image in nebulium. And this corresponds precisely with Campbell’s spectroscopic discovery about the planetary nebula S.D. −12° 1172,[1089] already adverted to. When nebulium comes to be examined in the laboratory—if that shadowy possibility be ever realised—the cause of the discrepancy may be laid bare. The most obvious is a difference of density between the two substances; yet we cannot unreservedly assume its validity, considering the noted effects of what we may call electrical preferences in modifying the spectra of attenuated gaseous structures.

Two facts, then, have been definitely ascertained regarding the hydrogen spectrum in the “Fish-mouth” nebula. The first is that its more refrangible constituents are preferentially developed as compared with the standard set by the vacuum-tube. The second is that of its persistence in regions of the nebula too dim to glow with any other species of light. Both peculiarities urgently demand explanations, which can probably be afforded only by arduous experimental work. Dr. Scheiner has not neglected this side of the inquiry; and although the outcome of his efforts is negative, it serves none the less to answer a fundamental question. He postulates in nebulæ extreme rarefaction and excessively low temperature; and to test the effect of the latter condition he plunged hydrogen-tubes into liquid air and examined the spectrum. He found it entirely unchanged by cooling to −200° centigrade.[1090] This, he pointed out, harmonises with the view that the luminosity of gases originates solely through internal movements of the individual molecules, and is hence independent of external temperature. Moreover, one of the few means available for terrestrially altering the relative strength of the red and green hydrogen lines is that of electrical differentiation. Professor J. J. Thomson’s observation[1091] of C bright, F invisible near the positive, F bright, and C invisible near the negative electrode, offers a clue which has not yet been followed up, for the threading of the labyrinth.

In another of its elements besides hydrogen, the spectrum of the Orion nebula is suspected to vary regionally. The ultra-violet ray at λ 3727 is second to none in importance when photographed with suitable apparatus. Nevertheless, it was missing from a plate exposed by Sir William and Lady Huggins 28th February 1889,[1092] although the impressions upon it of two much weaker lines near its place were clearly to be seen. The anomaly of its total absence has not recurred. Professor Keeler vainly went over the ground in 1892–93,[1093] groping with his slit for the blank district. Professor W. H. Pickering, it is true, derived indications of local diversities in the intensity of the line from a photograph taken without a slit, 10th July 1888; but they were rather suggestive than conclusive.

Suspected light-variations in the Orion nebula have not been confirmed by modern research. There is absolutely no photographic evidence of change, and visual discrepancies have not been attested with sufficient precision for the support of any positive inferences. Professor Holden was indeed disposed, after an exhaustive comparison of his own with numerous recorded observations, to believe in luminous instability of a partial kind;[1094] but the effects considered may have been only apparent. Their production is at once rendered intelligible by Professor Ormond Stone’s pertinent remarks:—“The general appearance of the Huygenian region,” he wrote in 1896,[1095] “is very much like that of a so-called ‘mackerel sky.’ Many of the condensations have pretty well-defined nuclei, whose light diffuses, blending with the surrounding nebulosity when the seeing grows poor. I have frequently been surprised to find how greatly the definition changes the relative brightness of the different condensations. This explains the many apparently contradictory estimates.” So far as it is possible to judge, then, the brightness of this nebula is not subject to change, either general or local. Moreover, its component parts seem absolutely fixed in outline and position. Internal movements, if in progress, will need the lapse of ages to become sensible to the eye. No relative shiftings of knots or nuclei have been detected; no alteration in shape of outgrowths and effusions. Yet they are of an eminently unstable aspect, and might be supposed no less essentially transitory than the appurtenances of comets. And so most probably they are, although the unit of time by which their duration is measured be long, and the scale of their construction unimaginably vast.

Some of the stars, however, scattered near the trapezium preserve anything but a constant lustre. One, catalogued as T Orionis, fluctuates irregularly from 9·7 to 13·0 magnitude; and others vary as unmistakably, though to a less extent. None are periodical, so that they belong to a different category from the flash-lights of globular clusters. An attentive study, photometric, photographic, and spectroscopic, of the Orion variables could not fail to be fruitful and instructive. Their instability is the more noteworthy from the whiteness of their light. T Orionis (also known as “Bond 822”) is perhaps a unique example of a star untinged with red losing and regaining nineteen-twentieths of its visible radiance. Among its obviously variable neighbours are the objects numbered by Bond in his survey of the nebula, 641, 647, 654, and 679. Scarcely any sustained attention has yet been paid to the group, notwithstanding the many questions of interest connected with it. Enough only is known to make it certain that its members exhibit no community of character in their vicissitudes save that of exemption from any traceable law of order. Thus Bond 654 was noticed by Holden, as it had been noticed by Otto Struve twenty years earlier, to rise occasionally, from habitual quasi-extinction, to brief maxima of about twelfth magnitude. But Ormond Stone recorded none of these sudden brightenings, although he observed the star, 30th September 1886, to rank higher than the twelfth magnitude. Possibly the manner of its variability is itself variable. The star Bond 647, on the other hand, has gained largely in average lustre since 1837, when Sir John Herschel’s measures, reduced to Struve’s scale, made it of 13·1 magnitude. Otto Struve chronicled its disappearance in 1863, after a prolonged maximum at 12·5 magnitude;[1096] Bond found it to be of 11·6 magnitude in 1867; Ormond Stone, of 10·9 magnitude, 1886 to 1894;[1097] and there is nothing to show that this steady rise has reached its culminating point. A compensatory decline may be anticipated, though not with entire confidence. The situation of these stars lends a special meaning to their variations. That they are physically connected with the nebula they are seen projected upon, cannot indeed be proved, but it may legitimately be assumed. Hence every advance in knowledge of their vicissitudes cannot but help to elucidate the still obscure relations of nebulous environment to stellar light-change.

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XXXVII. OTHER IRREGULAR NEBULÆ

Irregular nebulæ are objects of large size, indeterminate outlines, and capricious shapes. They become fully apparent, as a rule, only by photographic means, their exterior sections and subordinate parts shining too dimly for distinct visual perception. The chemical retina, however, sees them with comparative ease; for their light consists mainly of isolated short-period vibrations. They are hence known to be of gaseous composition, and all are situated in or near the Milky Way. Many observers, especially those armed with large reflectors, can pick them out at sight by their greenish tinge; but they never appear blue like planetaries. There seems, indeed, to be a definite difference of hue between the two classes, approximate spectral identity notwithstanding. This probably indicates superior strength of the nebulium lines in irregular nebulæ; or the maximum of intensity in the dim continuous spectrum may be situated lower in them than in the planetary kind. Discrimination is not easy; for impressions of colour are often too subtle to be analysed.

Irregular nebulæ are of surprising variety. Each specimen has individual peculiarities, for the most part inimitable by any other. No copy of the Orion nebula is to be found in the heavens, and the Argo nebula, which comes next to it in importance, is equally sui generis. This magnificent edifice has no corner-stone corresponding to the trapezium; instead, a black opening of a lemniscate form, and as sharp to the eye as if cut with a punching instrument, yawns in its brightest part. The operation by which it came to be produced was, moreover, repeated in a fainter nebulous tract farther south, and was hence evidently controlled by some definite and special combination of circumstances. With reason, then, in view of the unique character of this feature, the formation has been surnamed the “Key-hole Nebula.”

At the eastern edge of the northern key-hole lies the extraordinary variable, η Carinæ, the vicissitudes of which cannot but be related to the tumultuous processes of change doubtless going on in the seething chaos around. The general surface of the nebula is emblazoned besides with a multitude of ordinary small stars, historically and telescopically undistinguished. Their scattering, however, is not at random; it has marks of intention. Sir John Herschel pointed out their disposal along the margins of dark rifts in the nebula; and many such allineations were traced by Mr. Ranyard[1098] in Dr. Russell’s photographs, taken at Sydney in 1894. The object as a whole seemed to him “a very fine specimen of a nebulous cluster with a central condensation, associated with dark structures and radiating streams of stars.” These “are in most cases accompanied by narrow black channels in the general nebulosity, which run parallel to, and alongside of the star-streams.” One is reminded of the dark lanes bordered with stars in the Hercules cluster; and the analogy, if genuine, is of no slight significance. Setting aside for the moment its implications of affinity between stellar globes and nebulæ, it would afford a certainty that the stars distributed over the surface of the Argo formation are really in and of it; and since they are obviously galactic, this would amount to a demonstration that the nebula too is galactic—that it belongs to the Milky Way, not geometrically by projection, but physically by collocation.

The first photograph of this fine object was obtained by Dr. Russell with a six-inch portrait-lens in June 1891; but it had an experimental rather than a delineative value. This could not be said of one taken nine months later by Sir David Gill. An exposure of twelve hours, spread over four nights, with the thirteen-inch photo-refractor of the International Survey, yielded the remarkable picture which forms our frontispiece. The advantages of the autographic process could not be more forcibly exemplified than by comparing it with Sir John Herschel’s drawing of the same object.[1099] Months of labour at the telescope were of less avail than half a day’s “following” with the camera. The artist fully recognised the inability of even his skilful hand to delineate the endless gradations of light and shade which his eye perceived. The elaborate pains taken by him tended, indeed, as in most similar cases, to exaggerate contrasts, and so vitiate the general effect. In the photograph, the disclosed nebulous fields are not only wider, but they are more harmoniously related and more intelligibly arranged. Nor has the distinctive trait of the nebula evaded chemical portraiture. The “key-hole” is conspicuous on the plate, although deformed by luminous inflows; Herschel’s “kidney-bean” opening to the south (sixth square from the bottom, fourth from the left side of the Plate) is scarcely encroached upon by chemical diffusion; and a third vacuity of similar design, though less perfect execution, occurs to the north-west of the key-hole (fourth square from the top, seventh from the left). There is, nevertheless, one striking discrepancy between the drawing and the photograph—a discrepancy which, on the face of it, implies the occurrence of genuine and extensive change. An isolated, trident-shaped structure prominent in the former is imperceptible in the latter, or survives, at the most, fragmentarily. Its disappearance was due to no accidental defect in the Cape negative; the Arequipa plates, exposed with the Bruce twenty-four-inch lens, show a corresponding effacement. It had, in fact, taken place even to the eye already in 1871, when Dr. Russell failed to perceive the “swan” form (as Sir David Gill called it) with the Sydney eleven-inch refractor. The evidence of light-extinction is almost conclusive; yet it should not be admitted without further question. Visual study of the nebula would perhaps be the most promising road towards the end in view. Such objects are now rarely looked at; observers adapt their apparatus and devote their energies to the exposure of plates. Yet in some cases—and this is surely one of them—the direct and indirect methods should, for completeness, be employed concurrently. Photographic and photometric brightness are commonly disparate in stars; they differ in nebulæ still more widely; it remains to be proved whether their differences may not be irregularly distributed or even variable with time. The spectrum of the Argo nebula is of the usual gaseous type. Further particulars about it are wanting.

The Trifid nebula in Sagittarius (M 20 = N.G.C. 6514) affords another instance of ostensible change. Discovered by Messier in 1764, it appeared to Sir William Herschel in 1784 in the guise of “three nebulæ faintly joined into a triangle. In the middle,” he added, “is a double star.” And again, after two years, “About the double star is a black opening,” the combined effect of which recalled the Orion trapezium. He reiterated in 1811 that the position of the star was “in the middle” of the obscure space between the nebulæ.[1100] Sir John Herschel similarly assigned its place in 1827[1101] as “exactly in the central vacuity of a large irregular nebula, which appears to have been broken up into three portions by three rifts or cracks extending from its centre to its circumference, and whose directions meet at the double star.” Nothing could be more explicit; and his verbal description is authenticated by a rough sketch of high evidential value, though laying no claim to precision. Six years later, at Slough in 1833, he observed the double (really a sextuple) star to occupy “the centre of the trifid nebula.” Yet at the Cape in 1835, he drew it as adhering to the south-eastern lobe, and—stranger still—without comment on the alteration. And virtually under the same aspect the object was seen by the American observers, Mason and Smith, in 1839, as well as by Lassell at Malta in 1862. The complete immersion of the star-group in nebulosity, and its eccentric situation at the apex of a shining conical mass, are now patent to the merest tiro in telescopic scrutiny. Autographic impressions tell the same tale. One obtained by Dr. Roberts in ninety minutes, 13th July 1899, is shown in Plate XXVIII. Fig. 1. The open fan of nebulosity in the south-eastern quadrant has the multiple star at its apex, but indistinguishably, owing to the burnt-up condition of the plate in this bright region. The abruptness with which the luminous masses abut upon the dark rifts that divide them is most remarkable. On a Crossley plate of 6th July 1899,[1102] the small central block of nebulosity came out semi-detached, while in the Crowborough picture it appears as a simple prolongation of the great northern lobe. Here, too, by a further effect of light-concentration, the dependent nebula to the north is completely annexed by the adjacent triple structure. Its nuclear star would make an interesting subject for spectroscopic study.

Now Herschel’s Cape drawing of 1835 is in substantial agreement with the photographs of 1899. He saw about the same extent of nebulosity disclosed in them, distributed very much in the same way, and similarly related to the principal stars scattered through it. During sixty-four years, at any rate, fixity has prevailed. Mutual displacements are not sensibly in progress. The alleged variation must, we are driven to infer, have taken place suddenly between 1833 and 1835. This is certainly hard of credence; but it is still more difficult to admit that both Sir William and Sir John Herschel erred so egregiously as to locate the multiple star in the middle of a black space, if it really sparkled, as it does now, upon a background of lucent silver.

Professor Swift mentions having observed about 1888 “a luminous filament of the most delicate spider-like fineness stretched across the north-west cleft” of the Trifid nebula.[1103] It reminded him of a cable of the New York and Brooklyn suspension bridge, with the difference that it did not sag in the middle, but went straight from shore to shore. The installation of electric street lights at Rochester precluded him from keeping watch over this delicate and perhaps novel feature. It would be interesting to learn whether it continues visibly to span that strange abyss.

The spectrum of this nebula was observed by Professor Keeler in 1890 as “continuous but short, being apparently confined to the blue and green.”[1104] Only a “brightening near the middle” could be detected. Nevertheless, on 3rd August 1894, Professor Campbell perceived at a glance the three usual nebula lines, the third (F) being relatively strong.[1105] An auroral glow almost effaced whatever continuous light was present; and the same accident, singularly enough, recurred at the date of a second observation 24th May 1895. Much might be learned about the nature of the Trifid nebula by a searching spectrographic inquiry. The condition, for instance, of the unknown line at λ 3727 would be important to ascertain. Its variations from one object to another, or possibly from one region to the next of the same object, are doubtless of a significance only to be fathomed by the patient collection and comparison of facts. The spectrum of the multiple star, which seems to have a nuclear relation to the south-eastern division of the nebula, was stated by Keeler to be devoid of marked features. Yet such may present themselves when its thorough examination becomes practicable. The components of the object were successively discovered in the course of nearly a century. Sir William Herschel saw it double in 1784; Herschel and South about 1826 found it to be triple; and no more than three members of the group were distinguished by Lassell with his four-foot reflector. The fourth and fifth stars—neither much brighter than the thirteenth magnitude—were added by Professor Langley, using the fifteen-inch Harvard College refractor, in 1866; finally, the Washington twenty-six-inch disclosed the sixth to Professor Holden 5th August 1875.

A fantastic structure, known as the “Omega” or “Horse-shoe” nebula (M 17 = N.G.C. 6618), is situated on the border of Scutum Sobieski, and, like the Trifid, invites inspection from southern latitudes. Sir John Herschel perceived it at Slough in the figure of a Greek Omega (Ω), with the left-hand baseline turned upward.[1106] He was surprised to see at the Cape a second arch springing from the same level as the first,[1107] besides other suspected convolutions. The subordinate appendage was again noticed by Swift in 1883.[1108] In a photograph taken by Dr. Roberts, 5th August 1893, the “horse-shoe” resemblance is almost obliterated.[1109] Much greater prominence is given to the spindle-shaped axis originally noticed by Messier in 1764. On the plate it is found to be encompassed by a dim envelope, uniting the various patches of nebulosity into a large oval, 18′ by 12′, to the north-western end of which an abortive “horse-shoe” is appended like an excrescence. A picture obtained under more favourable circumstances might bring these somewhat incongruous parts into an intelligible mutual dependence. Professor Holden collected evidence suggestive of variation in the Omega nebula,[1110] but none that could be regarded as conclusive. “There has certainly not been any bodily shifting,” Dr. Dreyer pronounced in 1887.[1111] He was not, however, equally clear that partial fluctuations in brightness might not have taken place. The question remains an open one.

The spectrum of the Omega nebula was recorded as gaseous by Sir William Huggins in 1864. Nothing further is known about it.

Perhaps the most important of the nebulæ for purposes of comparative study is “30 Doradûs.” Situated in the Greater Magellanic Cloud, the “looped” nebula may exercise, in Professor Pickering’s opinion, a dominating influence over that extraordinary mixed assemblage. Yet it has the filmy and unsubstantial appearance of silver filagree torn in shreds and hung in the black sky. Its spectrum offers a remarkable combination of linear elements with strongly continuous radiance. Here, if anywhere, a frontier-instance between “white” and “green” nebulæ is to be found. Burton in 1874 affirmed the predominance in it of the fundamental nebular line; but the Harvard observers are less explicit. Professor Pickering briefly announced in 1892[1112] the spectrum of this object to be “unlike that of other gaseous nebulæ”; adding in 1897[1113] that its “constitution appears to be partly stellar and partly gaseous.” Further, a sixth-magnitude star in Libra (A.G.C. 20,937) is said to reproduce the peculiarities of its mixed light, a discovery ranking among the most profoundly instructive of those made by Mrs. Fleming. Its full import may, however, develop only through prolonged investigation.

Near the star ξ Persei on 3rd November 1885, Barnard discovered, at Nashville with a six-inch telescope, a “very faint, very large, diffused” nebula (N.G.C. 1499).[1114] Six years later it came prominently into notice through a photograph taken at Halensee, near Berlin, by Dr. Archenhold.[1115] The Willard lens was then repeatedly brought to bear upon it at Lick, and one of the resulting pictures, to which Barnard gave six hours’ exposure 21st September 1895, is reproduced in Plate XXIX. The nebulosity extends over at least two degrees, and includes many “angular condensations.” A round dark spot near the northern border strikes the eye at once. That it is “doubtless a hole in the nebula,” Professor Barnard avers. But in a gaseous mass a “hole” could neither be produced nor maintained. Light in the perforated region must be suppressed unless it be intercepted, and the latter alternative involves consequences that may fairly be called inadmissible.

A drawing of the ξ Persei nebula, published by Dr. Scheiner in 1893,[1116] embodies five photographic delineations obtained with a Voigtländer “euryscope” of four inches aperture, in times of exposure varying from one to six hours. He found it to be little inferior in size to the great Orion nebula, but totally different in plan of construction. It has strongly luminous borders, and these are connected by bright causeways crossing a comparatively obscure interior. There is no sign of a nucleus, nor any tendency towards the formation of one. Portrait-lenses, or some modification of them, seem to be the only kind of instrument with which impressions of this object can be secured; a plate exposed during six hours at the focus of the Potsdam thirteen-inch astrographic refractor showed no trace even of veiling from the prolonged impingement of its rays. They are equally ineffective, Dr. Archenhold states, upon orthochromatic plates—a fact reasonably held to imply that the nebula emits chiefly light-waves of short periods. In other words, it is a green nebula, and all but certainly gaseous.

A “vast and magnificent nebula” near Antares, seen imperfectly and fragmentarily, was disclosed in its entirety by Professor Barnard’s photographic researches in 1895.[1117] Its primary gathering-ground is about ρ Ophiuchi, a quadruple star of fourth magnitude giving a helium spectrum; but σ, γ^2, and 22 Scorpii, besides other smaller stars, form subordinate foci. Antares itself is involved in the trailing skirts of this cosmic cloud, but may in reality lie far away from them. Furrowed by an intricate system of rifts, and pierced by obscure cavities, the Antares nebula is evidently in an agitated and unstable condition; and its marked tendency to cling to individual stars suggests that its development will take the direction of accentuating such local condensations. The example of the Pleiades was recalled to Professor Barnard; and it may be that the formation in Scorpio presents us with an analogous aggregation in an earlier stage of growth.

One in many respects similar was photographed in Cepheus by Professor Barnard 13th October 1893. He traced in it “numerous irregular vacancies and zigzag lanes,” and noticed it to “mingle indefinitely with masses of small stars and become part of them.”[1118] This nebula is two degrees in diameter, and rudely circular in shape. Still more far-spreading and complex is a wonderful nebulous maze, vaguely centred at a point near ξ Cygni, but extending outward to a distance of at least eight degrees. Dr. Max Wolf, who virtually discovered this vast formation by his photographs of 1891, considers it to embrace all the stars, bright and faint, that come within its scope;[1119] and we cannot doubt that a heterogeneous system, partly stellar, partly nebular, is here presented to view. But the particularities of its composition evade for the present profitable inquiry.

Barnard’s circular nebula in Monoceros (N.G.C. 2237), which to the eye seems to draw a line of circumvallation round the cluster within, but loses in photographs all trace of annularity, may provisionally be classed as “irregular.” A fine picture taken by Dr. Roberts 5th March 1899[1120] shows the nebulosity to extend over a space about 77′ by 67′, in the form of a cloudy aggregation “broken up into wisps, streamers, and curdling masses, densely dotted with stars,” and including “many dark areas with and without either stars or nebulosity. Some remarkable black tortuous rifts meander through the nebulosity on the north-preceding half of the nebula; their margins are sharp and well defined in the midst of dense nebulosity. They are as clearly cut as we see the cañons of great rivers, but their width may in reality be millions of miles.”

Irregular, too, is a beautiful winged formation distantly resembling the Orion nebula, photographed by Schaeberle in the vicinity of Nova Aurigæ.[1121] The physical investigation of all these objects will prove an arduous but interesting task. The measurement of their radial movements, especially, should help to define ideas regarding their true status in the heavens.

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XXXVIII. NONDESCRIPT NEBULÆ

The “dumb-bell” nebula in Vulpecula (M 27 = N.G.C. 6853) might almost be called a hybrid between the planetary and irregular species. Its affinities are indeed so numerous as to embarrass its classification. Imperfectly seen, it appears double, and such it was considered to be by the elder Herschel. What survives of it when the fainter parts are obliterated by dim air or bad definition, suggests the bell-shaped receptacles detached from the neck of an hour-glass. And, in fact, the succession of instances is unbroken, from bi-nuclear planetaries through the “dumb-bell” stage to unmistakable pairs of nebulæ thinly connected by hazy strips. This point has, however, been already referred to. The dumb-bell nebula, moreover, possesses a nuclear star, situated at the narrowest point of the neck; and it is difficult to conceive of the neck as other than a temporary arrangement. Many indications favour the idea that it will, sooner or later, break up and disappear, leaving the star regent over the masses finally disjoined. And it is curious to remember that just such an interjacent star occurs in several compound nebular systems.

A tendency to form a marginal ring is, on the other hand, visible in this nebula. A photograph taken by Mr. W. E. Wilson of Daramona, Ireland, which we are permitted by him to reproduce in Plate XXX., shows an incomplete bright border to be a leading feature in its construction. Whether in the future it is destined to become predominant, we can scarcely venture to surmise. The dumb-bell nebula is about 8′ in diameter. The bright framework can be seen in the photograph to be filled out into an elliptical shape by faint supplementary luminosity, which, however, is much stronger in the original negative. Dim protrusions at the ends of the major axis, traceable in Dr. Roberts’s picture,[1122] intimate opposite outflows, similar in cause and character to those suspected to proceed from the ring nebula in Lyra. The texture of the “dumb-bell” is lumpy and irregular. Presumably it is of an ellipsoidal form, and rotates on its transverse axis.

The spectrum is purely gaseous, and is dominated visually, with more than the usual emphasis, by the green nebulium line, while in the photographic region the ray at λ 3727 was found by Von Gothard to be of quite peculiar strength.[1123] Symptoms of helium-emission should be looked for.

Several objects have been described as miniatures of the Vulpecula dumb-bell, but the likeness may prove, on a more searching examination, to be only skin-deep. Among its supposed analogues are: (1) N.G.C. 1365, situated in Fornax, and described in Sir John Herschel’s Cape Results.[1124] It belongs, he says, to the class of “annular nebulæ with centres,” but the ring breaks off, leaving the oval incomplete at its extremities, while the shorter axis terminates with two bright masses, between which lies a “resolvable” nucleus. (2) N.G.C. 5189. In Musca. A “general similitude” to the dumb-bell was noticed by J. Herschel.[1125] (3) N.G.C. 3195. In Chameleon. A drawing is given in the Cape Results. (4) N.G.C. 1978. One of the nondescript inmates of the Magellanic Cloud. (5) N.G.C. 6905. A gaseous nebula in Cygnus, with a star in the centre, and four attendant upon it, like satellites. (6) N.G.C. 3226, 3227. A double nebula, near γ Leonis, consisting of two hazy masses subsensibly united by a “neck,” and enclosed in a dim, elliptical sheath. All these various structures may not, indeed, be traversing the same line of development. In some, the annular tendency will perhaps eventually prevail, in some the disruptive. The balance between opposing forces is unlikely to incline everywhere the same way, and upon its poise depends by supreme appointment the special form to be assumed in the course of ages by each individual system.

The “Crab” nebula in Taurus, discovered by Dr. Bevis in 1731, was again noted by Messier, 12th September 1758,[1126] as a “whitish light, elongated like the flame of a taper.” He took it at first for the comet he was in search of; and, to avert future confusion of the kind, adopted the memorable expedient of constructing a catalogue of nebulæ.[1127] The specimen at present in question measures 5′ along its greatest diameter, and about 3′ across. Lord Rosse gave it its distinctive name from the protruding filaments resembling the claws of a crustacean, brought to view by his great reflector.[1128] These have not yet been photographed. On a plate exposed during three hours by Dr. Roberts, 2nd February 1892, there came out an ovate body composed of “dense masses of clouds, with fainter areas between them,” a deep bay on one side being counterbalanced by a projecting limb on the other.[1129] No tentacular appendages were disclosed.

The spectrum of the Crab nebula appears to be rather more strongly continuous than is usual in the gaseous class, to which it unquestionably belongs. For at Harvard College in 1868 the green nebulium ray was recorded, and was suspected to have less refrangible companions.[1130]

Another object of dubious relationships is Messier 77 (N.G.C. 1068), Lord Rosse’s “blue spiral” in Cetus.[1131] The description intimates an anomaly, since true spirals are “white,” and give a continuous spectrum. Now the colour of this object corresponds, as might have been expected, to a gaseous constitution, whether of the normal kind or in certain ways peculiar, remains to be proved. Its form, too, is ambiguous. A drawing published by Lord Rosse portrays a round, faint disc more than 2′ in diameter, upon which are relieved the brighter coils of a definitely separated central mass. In Dr. Roberts’s picture (see Plate XXVIII. Fig. 2), on the other hand, there is no trace of a convoluted structure. It shows “a stellar nucleus with projecting ansæ of dense nebulosity” surrounded by a faint zone, and that again “by a broad nebulous ring studded with strong condensations” like inchoate stars. A different aspect was, however, assumed by the object in a photograph taken by MM. Baillaud and Bourget in 1899 with the thirty-three-inch reflector of the Toulouse Observatory.[1132] The nucleus here again resolved itself into the winding folds seen at Parsonstown, and a paler spiral formation served for its background. Its character continues in many respects enigmatical.

M 77 is copied, in Professor Holden’s opinion, quite accurately by a nebula in the same constellation (N.G.C. 1084). Under the gaze of the camera it may resume the spiral shape obscurely visible to the Parsonstown observers, notwithstanding their final description of it as “a fine oval, with ragged edge and a mottled look,” indented by “a dark bay north of the nucleus.”[1133] Judging by analogy, it should yield a gaseous spectrum; though the circumstance, considering its remoteness from the Milky Way, would be exceptional.

Messier 78 (N.G.C. 2068), in Orion, is a “singular wispy nebula,” 6″ or 7″ across, enclosing a triple star, surmised to vary in light.[1134] An arrangement of the more lucent nebulous tufts along a spiral curve, affirmed visually,[1135] is denied photographically. Dr. Roberts’s plate exhibits a central cumulus, sharply terminated on one side, vaguely diffused on the other, the dense nuclear part being surrounded by dim floccules with wide dark spaces between.[1136] A smaller adjacent nebula (N.G.C. 2071) has a stellar focus, to which appurtenances like “mare’s tails” are attached. The distance from centre to centre of the two nebulæ is about 15′ of arc, and they can be made out to be in faint nebulous connection; but neither, Dr. Roberts expressly states, gives indications of possessing a spiral form. Their spectra, which are probably discontinuous, have scarcely been examined.

That of “a fine, pale-white object”[1137] in Canes Venatici is known to be continuous.[1138] Discovered by Méchain in 1781, M 94 (N.G.C. 4736) is large and bright;[1139] its nucleus, which is granular in texture, and evidently globular in shape, being surrounded by a zone of extremely dim, and that again by a zone of relatively intense luminosity. The outer annulus appears, in a photograph taken by Dr. Roberts,[1140] to be broken up into nearly a dozen irregular star-like condensations. Two abrupt opposite projections from it explain, possibly, the quasi-spiral aspect of the nebula in the Rosse reflector.

A circular object, one minute of arc in diameter, and of a “lucid pale-blue colour,” was met with by the elder Herschel in the constellation Hercules, and ranged in the planetary class[1141] (N.G.C. 6299). D’Arrest described it as a “nebulous disc,” invested with a “nebulous sheath,” so that its passing for a comet in 1819 is easily understood. Nevertheless, Sir William Huggins found it to shine with perfectly continuous light,[1142] and it was thought at Parsonstown, on 9th May 1872, to be resolved into a globular cluster.[1143] It should be added, however, that an undoubted planetary, N.G.C. 2022, was seen with the same instrument under a similar illusory aspect. There is indeed much difficulty in admitting the nebula in Hercules to be a genuine cluster. Its cometary envelope and the azure cast of its rays are almost contradictory of a stellar composition. But until a photograph of its spectrum has been obtained nothing can be definitely asserted on the subject. The quantity of light available for analysis is about equal to that given by a ninth-magnitude star.

A nebula in Virgo (N.G.C. 4900) looked, when ill-seen with the Rosse reflector, something like the Owl planetary in Ursa Major.[1144] A bright patch in the centre seemed to have dark spots on either side, the whole being surrounded by a lucid annulus or coil. Yet on a plate exposed at Crowborough, during three hours, no spiral structure emerged to view, although the nebulous condensations visible in the interior were judged to be of the kind usually found to accompany spirality.[1145] The formation, indeed, looks completely amorphous. It is probably of a non-gaseous nature.

The mutual relationships of many of the nebulæ just described are doubtless very close. Their elucidation offers a tempting and profitable field of research. Specimens in some respects anomalous are often the most instructive to study. Abortive features may be found in them, or half-developed characteristics, isolated from their accustomed surroundings, and thereby laid bare to scrutiny. Advantageous standpoints for comparison and correlation would thus be gained; and where these are effectively practicable, science cannot miss the path of progress.

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XXXIX. VARIABLE NEBULÆ

The occurrence of local changes of brightness is reasonably certain, as we have seen, in some of the great irregular nebulæ, and may be suspected in others. Variability affecting smaller objects in their entirety must then be admitted as possible. No doubt the phenomenon would introduce ideas difficult to adjust and unexpected; but the heavens are full of surprises. The immediate question to be put regarding it is, Does it really subsist? The answer must be given with extreme circumspection. The visibility of nebulæ depends upon contrast; the blackness of the sky has as much to do with it as the brightness of the filmy masses projected against it. They are besides apt to disappear with high magnification, and that for two reasons. First, because of the diffusion over a larger area of the same quantity of light; secondly, because of the restriction of the background in narrower fields of view. Hence there are drawbacks to the employment of large telescopes in nebular observation. The history of Tempel’s Merope nebula, marked by vicissitudes ascribed again and again to intrinsic causes, now fully recognised as non-existent, is a warning against hasty conclusions on so delicate a point. The lesson has indeed been so thoroughly learned that changes of the sort have of late been announced only with a certain timidity, and under reserve. Caution in the matter can, indeed, hardly be blamed for exaggeration, in view of Swift’s remark that, after the Krakatão eruption, many faint nebulæ absolutely disappeared.[1146] Nor need we go beyond Chacornac’s “temporary nebula” for an exemplification of optical caprices. On 19th October 1855 the French observer noticed a striated haze (N.G.C. 1988) attached to the star ζ Tauri, which seemed to have gained brightness in the ensuing January. No one else, however, saw it, and it had vanished by 20th November 1862. According to Tempel it never shone in the sky, but was a telescopic creation—a false image of an eleventh-magnitude star near ζ Tauri;[1147] and Burnham unhesitatingly adopts this opinion.[1148] Thirty years later its reality could have been tested by photographic means; but astronomers in those days had to rely upon the fallible human retina.

Chacornac’s phantom formation emerged near the site of two genuine nebular Novæ. On 11th October 1852, Hind detected, close to a star, then of the tenth magnitude, but since registered as an irregular variable under the designation T Tauri, a dim, round nebula (N.G.C. 1555) which brightened steadily until 1856, when it was obvious to general observation. A comparatively rapid decline ensued. Auwers[1149] could barely discern the object with the Königsberg heliometer in January 1858; to d’Arrest,[1150] using the eleven-inch Copenhagen refractor, it was wholly invisible 3rd October 1861; in 1862 it was vainly sought at Paris and at Malta with Foucault’s and Lassell’s great mirrors; Hind himself was unable to find it; Secchi, under the pure Roman sky, was equally unsuccessful; only at Pulkowa it continued to glimmer just perceptibly for a few months longer. From 1863 the sky in its place seemed a dead blank. At last, 15th October 1890, Mr. Burnham requested his colleague, Professor Barnard, to examine the region with the Lick thirty-six-inch, whereupon a nebulosity about 50″ in diameter, and so faint as to be at the limit of vision, was detected[1151] (see Fig. 49). Burnham too saw it, but believed that he could not have done so independently, his splendid powers of sight being better adapted to the discernment of concentrated than of diffused light-rays.

The nebula was again observed by Barnard—and with somewhat increased facility—in February 1895.[1152] Seven months later he was amazed to find it utterly gone! His search was repeated on three nights, under supremely good conditions, with the same negative result. And the object, so far from evading the grasp of large apertures, is peculiarly fitted for observation with them, owing to its small size and compact shape. Nevertheless, the forty-inch Yerkes refractor failed to show it at all in 1897, and barely enabled Barnard to catch a glimpse of it, 28th September 1898.[1153] Finally, Professor Keeler obtained faint images of it on two Crossley plates, exposed during four hours each, in December 1899.[1154] A copy of his drawing from them is given in Fig. 50. It exhibits the nebula as composed of three vaguely defined patches, united by a dim haze, the camera having, as usual, descried structural complexities inappreciable by the eye. On 20th January 1900, the great refractor just availed to bring it into view, and it has not since been heard of. To Professor Keeler it appeared inconceivable that in its present obscure state it could ever have been seen with small telescopes; and indeed the evidence of variability is conclusive. Will it ever recover any of its lost brightness? It may be doubted. The changes so far undergone by it have been, though comparatively slow, strictly analogous in character to those of “new” stars; a presumption hence arises that it will share their fate of permanent extinction. There is much reason to suppose that Hind noted in 1852 an early stage of its kindling; that its maximum in 1855–56 was solitary, its declension irretrievable.

Strange to say, the phenomenon was duplicated. While looking fruitlessly for Hind’s nebula, Otto Struve came upon another unfamiliar object (N.G.C. 1554) 4′ east of its predecessor’s empty place. This was early in 1868; and the Nova—for d’Arrest was sure of its previous non-existence[1155]—was kept in view until 1877, when absolute obliteration covered it. Even Barnard’s quest for it in 1890, 1895, and 1899 was ineffectual. Its former position is marked in Keeler’s drawing (Fig. 50) by the thirteenth-magnitude star b, but no nebulous impression was there made upon the plates. Dr. Roberts had indeed already, in 1890,[1156] vouched for its photographic disappearance. When last seen by Tempel,[1157] 8th November 1877, the nebula was 90″ across, and d’Arrest had expressly recorded the presence in it of an eccentrically situated, though definite nucleus.

Hind’s and Struve’s nebulæ were bright enough, as Professor Barnard recalled wonderingly,[1158] to be ranked, soon after the middle of the last century, in Herschel’s first and second classes respectively. Both were watched “and measured by the best observers then living,” and “were easily visible in ordinary telescopes.” Yet one survives only as a fitful shimmer; the other is utterly, and probably for ever, quenched. They are perhaps obscurely connected, and the system—if such it can be termed—may include the nebulous variable T Tauri; although nebulæ and star alike seem to fluctuate in complete independence one of the others.

These are the only authentic instances of temporary nebular developments; but allegations of nebular light-change are common. Some have proved groundless; not a few, however, rest on a solid substratum of fact. The following may serve as specimens.

On 17th October 1785 William Herschel discovered, not far from Algol, “a pretty bright star with two faint branches” (N.G.C. 1186). Sir John verified the observation, while estimating the star at only fourteenth magnitude (about twelfth on the modern scale). Yet neither it nor its appendages could be seen with the Parsonstown reflector, and d’Arrest, after diligent and repeated search, affirmed decisively, Nostra ætate in hac regione tale quid non exstat in cœlo.[1159] The lost object, nevertheless, came again into view in 1891. On 31st January and 26th February of that year, Bigourdan, using the thirteen-inch Paris equatorial, perceived it as a twelfth-magnitude star, with an unmistakable “fan” of nebulosity spreading from it over 1′ of arc. Only a fortnight later, Spitaler made a drawing of the object with the Vienna twenty-seven-inch, and described it as an elongated nebula, not regular enough to be called elliptical, 2′ in extent, and including a focal star of the eleventh magnitude.[1160] And Burnham, in the following August, “readily found in the proper place a tenth-magnitude star involved in a faint elongated nebula” measuring at least 2′ or 3′.[1161] During the course of 1891, accordingly, the star seems to have been progressively gaining light and the nebula compass. But if the star only were variable, the attached nebula would have appeared to shrink and become effaced as the bright point within it acquired intensity. Its simultaneous increase could not have been counterfeited. Proof was afforded by it that the growth, like the previous failures of luminosity, were due to influences diffused throughout every part of the formation.

The next variable nebula was a “find” of Barnard’s. It was conspicuous to him 30th November 1888. He judged it equal to a ninth or tenth-magnitude star, and remained convinced that its lucidity was of recent origin. Three years later it had parted with quite four-fifths of its lustre, and had faded down nearly to evanescence.[1162] It is situated in Cetus. No information is at hand as to whether its decline has continued since 1891. Unless arrested, it must, before the century closed, have carried it out of sight even of the chemical retina, and the object should then probably be relegated to the class of “temporaries.” The question is of great interest, and might be answered by taking one long-exposed photograph with a portrait-lens or a large reflector.

This is not the only case in which an accession of brightness has been thought to be demonstrated by the lateness of discovery. A nebula was encountered by Tuttle in Draco, 1st September 1859 (N.G.C. 6643), which, in d’Arrest’s opinion, should certainly have been caught in the meshes of the Herschelian nets unless in their time comparative obscurity had enveloped it. Similarly, a nebula in Camelopardalis detected by Barnard in 1889,[1163] and again independently by Denning in 1890,[1164] could not long, Barnard considered, have been thus readily apparent. He recommended its being kept under surveillance as a probable variable, but as yet it has shown no sign of being so;[1165] unless, indeed, Swift’s earlier observation of the same object, recorded without date by a simple entry on a star-map,[1166] indicated a previous maximum. Another instance of a possible rise in the scale of luminosity is afforded by a small, fairly bright nebula in the Camelopard, first observed by Denning 30th September 1891, and casually again four times in the ensuing month. Yet his many previous reviews of the sky-contents in that neighbourhood had failed to elicit any trace of its existence. Like Barnard’s and Tuttle’s “new” nebulæ, however, it has apparently come to stay; and since variability is in sidereal bodies usually an ineradicable property, the hypothesis of an ascent from invisibility cannot safely be accepted until a corresponding descent has been entered upon.

The irregular variability, on the other hand, of two nebulæ adverted to by Winnecke in 1877–78[1167] is almost incontestable. The first (N.G.C. 3666) is in Leo. Elliptic in shape, in size 90″ by 40″, it was marked “very bright” by the elder Herschel 15th March 1785, but “extremely faint,” 23rd March 1830, by the younger, who added the comment, “This nebula must have changed greatly if ever it belonged really to the first class.” But its waning was not definitive. Boguslawski inscribed it as a bright object in 1840 on the Berlin Academy star-map of that region; and Winnecke found it, 10th April 1878, of unquestionable primary rank. Yet meantime, in 1863, d’Arrest had described it as subobscura, and manifestly of third-class lustre; while again, on 24th May 1887, Dr. Dreyer perceived its diminished radiance only with the utmost difficulty. Its further history remains untold.

Winnecke’s second variable (N.G.C. 955) is an inmate of the crowded nebular district in Cetus. It consists, Burnham says,[1168] of long, narrow “nebulous wings on either side of a bright central condensation.” “On the whole,” he continues, “it is rather a curious object, and should be easily found and seen.” This was in 1891, and agrees quite well with Dreyer’s notice of the object in November 1887 as “fully of the second class.”[1169] The case for change rests upon its invisibility to Schönfeld in December 1861, and to Vogel in November 1865; although in 1856, 1863, and 1868 it had been seen at a glance by Schönfeld himself, no less than by d’Arrest and Winnecke.

Winnecke’s nebulæ were at first held by him to be periodical; but this they certainly are not. No fixed relation to time has so far been shown to govern nebular fluctuations. They either consist—according to the best evidence at command—of a solitary maximum, analogous to the outburst of a new star, or of irregular accessions and losses of light. No case of cyclical recurrence is on record. Photography is clearly destined to play an important part in the investigation of this difficult subject; its aid will be peculiarly welcome where visual faculties are often baffled, embarrassed, and deceived.

Nebular variability is indeed a phenomenon not only evasive to the senses, but startling to thought. It cannot be even remotely assimilated to the light-changes that progress in certain globular clusters; it is independent of geometrical conditions, of orbital movements, of planes and periods. Its cause defies conjecture; we can only be sure that it acts upon a prodigious scale. Thus Hind’s nebula in Taurus measured at least 2′ across. Its parallax was almost certainly less than one-tenth, and may not have exceeded one-hundredth of a second. The larger value would give, for the smallest admissible linear diameter of the object, 1200 astronomical units (radii of the earth’s orbit) or 111,000 million miles. Centred on the sun, it would extend on every side to twenty times the distance of Neptune, the equation of light within the vast formation being six and a half hours. Yet it kindled as a whole, through the pervading influence of some far-reaching event. Did another dark nebula sweep through it? We dare not pronounce. Its mysterious brightening, however, hints at the existence of an indefinite multitude of similar bodies lurking in the obscurity from which, by some rare chance, it emerged. It introduces us, in fact, to a realm of invisible nebulæ, impenetrable by observation, and hence pre-eminently adapted for the sports of scientific fancy.

Hind’s and Struve’s vanished nebulæ were presumably of gaseous composition, like the adjacent glow round T Tauri;[1170] Winnecke’s variable pair doubtless shine with the white light characteristic of the elliptic family to which they structurally belong. Their remoteness from the Milky Way points to the same conclusion. Luminous instability does not then appear to be associated in nebulæ with any special radiative quality. Those giving continuous, and those giving discontinuous spectra may equally be affected by it.

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XL. THE NATURE OF NEBULÆ

The relations of white to green nebulæ are obscure. Unitive links between the two classes have yet to be established. In most respects they stand at present widely apart. They present a superficial likeness, but their dissimilarities seem to be radical. They are unconnected by any marked spectral affinities; they differ organically in structure; their distribution on the sphere is regulated by opposite principles. Hence their genealogical precedence remains unsettled. It would be rash to say that either family developed from the other, or even that they are collateral offshoots from a common stock. That a line of continuity will, sooner or later, become traceable is more than likely, but we must wait for the guidance of facts with regard to it; premature divinatory efforts are usually good for less than nothing.

As to the constitution of white nebulæ, we seem on the verge of knowing something definite. Premonitions of their being a species of fine-grained star-cluster have become audible. The subject, however, is not ripe for discussion. A comparatively advanced stage has, on the other hand, been reached by the problems connected with gaseous nebulæ, since their spectra let us, to a certain extent, into the secret of their composition. This, on the whole, seems to be remarkably uniform. Individual differences, it is true, both physical and chemical, distinguish the various members of the class; but they are of a subordinate kind. We may then safely attempt to generalise as regards a few of their more obvious properties. Three of these can be at once enumerated:—

(1) Gaseous nebulæ are almost perfectly transparent.[1171]

(2) They shine with extreme feebleness.

(3) Their mass is vanishingly small in proportion to their bulk.

We will take each point separately. That nebulæ offer no appreciable obstacle to the transmission of light is attested by the unaltered radiance of stars shining through them. No absorption that can possibly be due to the cosmic fog in which they are plunged to depths of many millions of miles, is traceable in the spectra of such objects as Θ Orionis, of ρ Ophiuchi, of Maia or Merope in the Pleiades. Similarly, the central stars of planetary nebulæ shine through an interposed medium, the extent of which is measured by the radius of each gaseous globe; and this, by a rough minimum estimate, can rarely be less, and must often be a great deal more than 50,000 to 60,000 million miles. Yet from the heart of these extraordinary formations the light of their nuclei comes to us, so far as it is possible to judge, absolutely intact. The impotence of comets for light-stoppage is thus vastly enhanced in nebulæ.

The feebleness of their luminosity is a matter of direct observation. A shining superficies, unlike a shining point, loses none of its lustre with increased distance. Its area of course diminishes according to the law of inverse squares, but every minute element of that area continues to radiate with the same intensity as before. The sun, for instance, is no less bright as viewed from Neptune than when it crosses the meridian of Khartoum, but it is 900 times smaller. So with the nebulæ. They are really as faint as they appear. Using the best available data, Mr. Ranyard arrived at the conclusion that a planetary of the most vivid kind emits per square mile less than (1)/(22,000) millionth of the light sent abroad by the solar photosphere. This implies—adopting the result of Langley’s experiment at Pittsburg—that white-hot iron glows at least 4,000,000 times more powerfully than the bluish disc of the “Saturn” or the “Owl” nebula. Moreover, the differences in areal lustre between one nebula and another represent actual varieties of emissive strength. Remoteness has nothing to do with producing them. A debilissima—a “breath-stain” on the sky—may be as near to us as the great hiatus in the vault through which Huygens half-imagined the blazing of empyrean fires.

Finally, nebulæ being prodigiously voluminous and of apparently insignificant mass, must be of exceedingly low mean density. This fundamental fact was realised with uncompromising distinctness by Mr. Ranyard in 1892.[1172] Taking, for illustrative purposes, the Orion nebula to be a sphere 20′ in diameter, composed uniformly of materials 1,000,000 times rarer than atmospheric air at sea-level, he found that its mass would be such as to impart, to a body at a distance from its centre equal to that of α Centauri from ourselves, a circular velocity of 180, or a parabolic velocity of 255 miles per second. In the neighbourhood of the nebula, accordingly, there should be a marked prevalence of large proper motions. A star, travelling across the line of sight under the influence of its attraction at the rate of 100 miles a second, would, it was shown, have an annual displacement on the sphere of no less than 25·5″, and this independently of remoteness. For with the same angular dimensions, the solid contents of the nebula would increase as the cube of the distance assumed for it; while the seeming velocities of bodies in gravitational dependence upon it would undergo no change. This is rendered obvious by a moment’s consideration. For take any given star circulating round its centre of gravity at the rate, let us suppose, of 100 miles a second. And let us further suppose, to begin with, that the distance of the system is such that light would spend ten years on the journey thence to our eyes. Let us now double that distance and follow out the consequences. First, the nebula is eight times more massive than would have comported with the previous arrangement. Next, the revolving star is twice as far from it as before, since the apparent interval has not changed. Whence we easily gather, by the application of Kepler’s third law, that it now moves with double its previous speed. But its distance has, by hypothesis, also been doubled; consequently, its proper, or apparent motion remains just what it was. If, then, the stars about the Orion nebula were really in swift circulation, they should appear to be conspicuously progressive. This, however, is so far from being the case that the region is one of exceptional fixedness. The spectroscopic information at command is to a corresponding effect. The six brightest stars of the constellation, measured at Potsdam in 1892, proved to be all affected, in varying degrees, by the retreat of our system from that locality of the heavens, but gave no signs of travelling rapidly on their own account. The conclusion is inevitable that the Orion nebula—and it may be accepted as typical—contains inestimably less matter than should be comprised by it if its average density were that of a Crookes vacuum. Mr. Ranyard, indeed, assigned to it a consistence not exceeding (1)/(10,000) millionth that of air at standard pressure, which, he continued, “would about correspond to the mean density of the solar nebulous mass, supposing it to have been spherical when its radius was a little more than 107 astronomical units, or when the sun occupied a sphere with a radius of a little more than three and a half times the distance of Neptune.” The potential solar system in those days lay muffled in the haze of a small planetary nebula.

Yet it is impossible to conceive of nebulæ as formed simply of matter in an aerial condition. They are no mere vague effusions. They possess definite and characteristic structure. Lord Rosse,[1173] indeed, thought sharpness of contours distinctive of the gaseous kind. Mr. Maunder[1174] speaks of their “strange and complicate shapes, showing here and there strongly-marked outlines”; and he adverts to the difficulty of explaining this peculiarity in vast, uncontrolled extensions of rarefied gas. The abolition of this incongruity was one of the strong points of Sir Norman Lockyer’s “meteoritic hypothesis” of nebular constitution. The spectroscope, it is true, pronounced against it; nebular chemistry has very little in common with the chemistry of “uranoliths”; yet amid much that was precarious or unsound, the valuable idea was introduced that a proportion of solid matter must enter into the composition of nebulæ. Its condition and distribution, however, remain unknown. It would seem to be devoid of light, for the faint continuous spectrum accompanying the nebular bright lines would be displayed even by a homogeneous gaseous mass, unless its radiations were of purely superficial origin.[1175] But here we meet the unresolved enigma of nebular luminosity. How do they shine? Is it through the direct agency of heat? Experimental evidence does not countenance this view. In the laboratory, hydrogen and helium can be induced to give out their characteristic rays only under the stress of electrical excitement. The concomitant high temperature might—as Sir William Huggins pointed out in 1891[1176]—prevail only along the path of the discharge, while the surrounding gases remained cool, producing inequalities in heat-distribution similar to those believed to exist in vacuum tubes. If, on the other hand, nebular emissions were of simply thermal production, the whole radiating mass should be at nearly the same pitch of incandescence; for if the temperature were lower in some than in other of its parts, absorption lines or reversals would betray the fact, and the nebular spectrum bears no legible marks of selective light-stoppage. Yet it is eminently improbable that formations so circumstanced are, in their entirety, excessively hot. Their solid inclusions should, if they were, glow powerfully, and strong continuous radiance would replace the dim band, grey through faintness, actually seen. On the supposition, however, that electrical discharges cause the glow of nebulæ, their average temperature, judging by Dr. Scheiner’s experimental results, might approximate to absolute zero.

The word “temperature,” indeed, when applied to matter in the last stage of attenuation, has an extremely dubious meaning. Taking the kinetic theory of gases to represent the literal truth, we find the effects of heating upon them to be twofold, namely, increase (1) of translatory, (2) of internal energy, the latter being perhaps a consequence of the former. Since, then, their constituent particles travel faster when heat is applied, they come into mutual collision more frequently and more violently. That is to say, if their number per unit volume remains the same. Let, however, the density of the gas be reduced, other things being unchanged, what will ensue? There will be fewer encounters in equal times, but they will not be less violent, since the mean rate of the molecules has undergone no alteration. Hence, apart from one qualifying circumstance, the same species of vibration should be set going as when the gas was denser and equally hot. The qualifying circumstance is this. During the lengthened intervals between the collisions, the molecules are radiating—that is, imparting energy to the ether. If their free paths were sufficiently protracted, they would—if that be possible—lose all they possess, and need to be fully restocked at each encounter. The comparative isolation of the molecules would accordingly result in their being mostly dark, and what we must call cold; so that the extreme subtlety of matter seems incompatible with a high temperature in any intelligible sense of the term. But there is more. Reduction of pressure would have for its immediate result a general diminution of luminosity. In all probability, it would also produce a selective effect, for if, as seems likely, certain modes of molecular vibration are more persistent than others, they would, with the progress of cooling after each impact, tend to predominate, and the balance of intensity among the spectral lines of a glowing gas would thus be sensibly altered. Here, then, an explanation might be sought of the spectral anomalies of hydrogen in nebulæ. Nevertheless, it does not run quite smoothly, since up to the limit of possible rarefaction in vacuum tubes the crimson line of hydrogen gains consistently in strength.[1177] A corresponding difficulty presents itself in connection with the helium spectrum.[1178] “Green tubes” are at a higher degree of exhaustion than “yellow.” Yet in nebulæ the green ray is invisible; nebular helium radiates only D_{3} with a few members of the same “set.” By means of artificial exhaustion, accordingly, the nebular spectrum cannot apparently be imitated. The only experimental clue to its origin, in fact, is in Professor J. J. Thomson’s observation (already mentioned) that F and C are of interchangeable intensity at the negative and positive poles of a hydrogen tube.

To recapitulate. Two fundamental problems regarding gaseous nebulæ press for solution—one connected with their structural forms, the other with the nature of their light. Do they continuously fill the spaces they appear to occupy, or should we figure them to ourselves as collections of discrete bodies comparatively wide apart? In the former case the dark gaps and chasms which form one of their leading features should be ascribed, not to the absence of matter, but to defect of shining power, illuminative, not architectural contours being disclosed by them. In the latter, chiaroscuro effects would be reliable indices to the distribution of material.

Secondly, we are confronted with the mystery of their shining. Why and how are they lucent? Are they bright as a consequence of thermal or electrical stimulation, or is their radiance some undefined species of luminescence or phosphorescence? The answer must be given in view of the three following peculiarities of their spectra: (1) The leading hydrogen line in them is F, not C. (2) Helium shines in them as if in a “yellow” tube. (3) Reversals are absent; lines of absorption make no assured appearance, either as superimposed upon, or as subjacent to lines of emission. None of these points are easy to decide. Their consideration involves doubts and queries of an abstruse nature; but it will not therefore be neglected. Indeed, such a region of inquiry as is here presented, where we feel that at every step the Unknown may merge into the Unknowable, has a particular and an illimitable fascination.

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XLI. THE PHYSICS OF THE MILKY WAY

The Milky Way is an integral part of the great sidereal system. It marks the equatorial girdle of a sphere containing stars and nebulæ variously scattered and aggregated. The whole material creation is, to our apprehension, enclosed within this sphere. We know nothing of what may lie beyond. Thought may wander into the void, but observation cannot follow. And where its faithful escort halts, positive science comes to a standstill. Fully recognising the illimitable possibilities of Omnipotence, we have no choice but to confine our researches within the bounds of the visible world. That it has bounds is evident from the consideration that it possesses shape and parts. Indefinitely extended, star-filled space could have neither. Hence it offers to the human mind an intelligible problem—a problem perhaps too intricate for definitive solution, yet coming well within range of attack. The siege operations may be protracted through many a campaign; but in conducting them we shall climb from peak to peak of the Alpine chain of truth, and gain continually wider views of the majestic scene that encompasses and enchants us.

The structural relations of the cosmos may evidently be looked at from many sides; our immediate concern is with but one of its aspects. We have only to consider the nature of the materials used for the building of the edifice and the plan of their apportionment to its different sections. These materials consist of gaseous and white nebulæ in all their varieties; of star clusters, globular and irregular, and of the sundry species of stars; and even a cursory inspection shows that they are not piled together at random. Each class, on the contrary, obeys its own law of distribution; and the distribution of sidereal, as of animal species, is the outcome of their history, a test of their longevity, an index to their nature. They are where they are, because they are what they are.

There seemed, twenty years ago, very little reason to anticipate that the photographic method could ever be used to advantage for investigating the physics of the Milky Way; yet it has, especially in Professor Barnard’s hands, proved most effective in that difficult branch of inquiry. The requisites were peculiar. The galactic drifts are made up of very small stars—usually of fourteenth to sixteenth magnitude, or even fainter,—and the sensitive plate perceives them, not, as the human eye does, collectively, merged into a nebulous surface, but one by one as light-points. Hence their rays need to be powerfully concentrated in order to make any chemical effect with a moderately long exposure. Moreover, the field must be large enough to show the colossal forms into which these stellar units are grouped. For each picture a canvas of at least 100 square degrees must be available. Hence telescopes of the ordinary type, however powerful, are, for this purpose, entirely useless. An ordinary village photographer’s apparatus would be better adapted to it. Professor Barnard obtained his remarkable series of delineations with the “Willard lens,” a doublet of six inches aperture and thirty-one focal length, constructed at New York in 1859.[1179] They extend over a large part of the Milky Way, but leave lacunæ, the filling up of which should be the diploma-performance of the new Bruce lens. It might indeed be supposed that a few specimen sky-scapes in galactic regions would suffice to afford practical acquaintance with the entire round; but this is very far from being the case. No feature of the Milky Way is more surprising than its inexhaustible variety. No “law of condensation,” such as prevails in globular clusters, is there traceable. Each section follows its own method of aggregation. In one, cloud-forms are met with of the cirrus type; in another, they recall breaking waves or tossing spray;[1180] again, groups of irregular bright spots alternate with extensive ramifications and rifts; here the starry fabric is coarse-grained, there of microscopic fineness; while for heterogeneous scattering in many quarters, there is substituted in others an apparently designed arrangement of the stars into rings, chains, and ellipses.

Barnard’s photograph in Ophiuchus, reproduced in Plate XXXI., has characteristics demanding earnest attention. The bright star below the middle of the plate encircled by a halation ring is θ Ophiuchi, an object spectrally akin to Bellatrix, radiantly white and marked by helium absorption. A “long, dull vacancy,” visible to the naked eye, stretching east and west below (south of) Theta, comes out as “an irregular rift in the sheeting of stars,” of highly complex relations. It circuits the east as well as the north and south sides of the bright mass in which the star is placed, breaks up to the north into scattered dark openings, and straggles on to the western edge of the plate, whence—as other photographs of the series show—it communicates with some of “the great vacant lanes” in the “wonderful nebulous region about ρ Ophiuchi.”[1181] The most singular feature, however, of the galactic chasms visible in our figure is the presence in them of two gradations of obscurity. Darker details can be made out on the dark background, recalling the analogy (which did not escape Professor Barnard’s notice) of the “black holes” in the umbræ of active sun-spots. The conviction was thus, he added, almost enforced that the Milky Way rests here upon a far-reaching stratum of nebulous, or quasi-nebulous matter.

The reality, the frequency, and the determinateness of the black openings in the Milky Way, dimly seen with the naked eye, constitute one of the most important facts regarding the nature of that formation attested by the camera. It is true that their great exemplar in the southern heavens—the “Coalsack” in Crux—seemed, in a photograph taken by Dr. Russell of Sydney, 13th August 1890, to be embroidered with small stars over three-fourths of its area;[1182] but this is an unessential trait of the phenomenon. The fundamental circumstance connected with it is the extensive and complete perforation of the dense galactic stratum lying behind the starry network; and of this stratum there is no trace in the Sydney picture. Only the foreground of the scene is delineated in it; the backward stretching ranges remained inaccessible with the means employed. The true Coalsack, then, has not yet been photographed, and we are ignorant of the precise form which it will take upon the sensitive plate. The exposures made at Lick, however, effectively reached the piles of luminous dust which form the ultimate reality of the Milky Way, and brought into view with insistent clearness the pits and furrows of almost absolute darkness by which they are frequently interrupted. One is perpetually reminded, in looking at these autographic records, of Herschel’s exclamation of amazement when his telescope plunged suddenly into an unfathomable abyss in Scorpio: Da ist wahrhaftig ein Loch im Himmel!

Now “holes” of the kind have a very wide cosmical significance. The great rift, by which the entire galactic structure is divided throughout one-third of its circumference, is only a magnification of innumerable cracks and fissures yawning amid its component star-masses. And the dark lanes in globular clusters are plainly of kindred origin. Moreover, the riddled aspect of the Milky Way in certain of its sections is continually reproduced both in nebulæ and clusters—in nebulæ, whether gaseous or stellar, as well as in diverse species of clusters. The fact is a general one, that in all the forests of the universe there are glades and clearings. How they come to be thus diversified we cannot pretend to say; but we can see that the peculiarity is structural—that it is an outcome of the fundamental laws governing the distribution of cosmic matter. Hence the futility of trying to explain it as of incidental origin, as a consequence, for instance, of the stoppage of light by the interposition of obscure bodies, or aggregations of bodies, invisibly thronging space. That dark stars exist, singly and in systems, and dark nebulæ no less, we have been almost inevitably led to conclude; but the galactic clefts cannot be reckoned among their manifestations. They are, on the contrary, what they appear to be, intervals of starless space between neighbouring star-clouds, and suggest processes of disintegration[1183] advancing with inconceivable slowness towards unimagined issues. These wonderful collections are then in a state of flux; they are passing from one condition to another; Supreme Power is at work in dispersing or refashioning them, sending abroad their aggregated suns like flying sparks from the anvil.

But are those sparks indeed suns on the scale of our own? The answer to be given depends upon our estimate of their average distance. They are undoubtedly very remote. It may be taken as certain that they lie beyond the sphere occupied by the Durchmusterung stars—that is, by the ordinarily distributed stars of our system down to 9·5 magnitude. How much farther beyond, we are, however, unable to say. The thronging orbs of the Galaxy are by no means of uniform brightness. A recent inquirer[1184] concludes that they range from the sixth to the sixteenth magnitude; and the statement, which probably falls short of the truth, implies that stars enormously disparate in apparent lustre are at sensibly the same distance from ourselves; hence, that the crowd of small or medium-sized bodies collected in the Milky Way are dominated by veritable giants, more sparingly distributed. Professor Barnard endeavours to bring this state of things into conformity with received standards by levelling down the dimensions of galactic constituents.[1185] To the ruck of them he assigns scarcely more than planetary rank, so as to escape the necessity of admitting fabulous magnitudes for their leaders. But in truth we have no means of fixing a scale for such valuations. We know that there are suns hundreds, even thousands, of times larger than the sun. Why should there not be others larger still in a similar proportion? Non est naturæ mensura hominis electio, Kepler wrote to Herwart in 1599. Our estimates are, in many cases, minimum values; we can place no upper limit to the vastness of the orbs of space. It is only safe to affirm that the greatest and the least of them are associated and conjoined in the immense aggregations of “this gorgeous arch, with golden worlds inlaid, built with Divine ambition.”

The sidereal tribes are very differently affected towards the central plane of the system. Some, as it were, deliberately withdraw from it; others are exclusive in their preference for it; many press towards it, while maintaining a cosmopolitan status. That white stars largely preponderate in the Milky Way, is a fact made evident by Kapteyn’s discussion of the Cape Durchmusterung photographs.[1186] It might, nevertheless, as Mr. Monck of Dublin first noted, be due to their superior areal brilliancy, which would cause them, with increase of distance, to come into view preferentially, and at last exclusively. The percentage of white stars must, indeed, apart from very unlikely specialties of distribution, augment among the lower magnitudes, since a star emitting light of the Sirian quality would appear equally bright when fully twice as far off as one of the same size clothed with a solar atmosphere. Moreover, Kapteyn’s researches did not extend below the tenth magnitude, and the genuine galactic particles are of much lower grades of brightness. So that the nature of their spectra has not, up to the present, been disclosed to us. Yet a surmise regarding it may be hazarded. Mr. McClean perceived the bright helium stars included in his survey to be disproportionately numerous in the Milky Way zone. This is probably one among several symptoms of their great remoteness. They are sufficiently removed, perhaps, from the centre of the sidereal sphere to come within the sweep of the current of tendency setting in towards its equatorial regions. How it seems probable that fainter specimens of the type are similarly, but more strongly condensed; and a further step on the tempting road of conjecture leads us to the inference that the dim aggregations girdling the heavens are mainly composed of stars of the Orion family—of stars reversing the duplicate series of hydrogen lines, besides those of oxygen, helium, nitrogen, and silicon.

We have seen in an earlier chapter that the Milky Way is the chosen resort of Wolf-Rayet stars. They doubtless belong to it intimately and entirely; and the same may be said of Novæ. This involves the strange consequence that, amid the radiant galactic hordes, there must circulate a multitude of large obscure bodies, fitted on occasion to blaze into sudden conflagration, on a scale startling to intelligent beholders in every quarter of the universe. Must we then conclude that dark stars are relatively more plentiful in the Milky Way than elsewhere? Not necessarily, perhaps, for its scarcely infringed monopoly in the production of temporary stars might be explained equally well by the virtual limitation to it of the conditions needed for luminous explosions, as by the abundance in it of their appropriate fuel.

Very few gaseous nebulæ have any considerable galactic latitude; they are characteristically Milky Way objects. There are, indeed, exceptions. Some noted planetaries—those in Draco and Ursa Major for example—are situated far outside the zone of concentration, and they present the appearance of being nearer to the earth than most members of the class. The “stellar” variety, on the other hand, which are presumably small through remoteness, are, to the best of our knowledge, limited to the Milky Way; while irregular nebulæ occur either in the main stream or in some of its affluents. Among these the Magellanic Clouds are, in a sense, to be counted, though they should rather be described as pools, left behind as the waters contracted into their present bed. They are composed, like the Milky Way, of mixed ingredients, stellar and nebulous; they seem to reproduce its condition; they bear the same primitive stamp. Their globular shape, however, suggests an autonomous constitution, each being probably a self-regulated body; while the galactic aggregations may be supposed exempt from the efficient control of a central authority. Dr Russell’s photographs disclosed spiral tendencies in the Nubeculæ,[1187] destined perhaps to become more and more pronounced with time.

Through the agency of the portrait-lens and the sensitive plate, the nebulous affinities of the Milky Way have been more fully recognised than was possible by visual means alone. Several of Professor Barnard’s pictures exhibit an intermixture of vaguely diffused lucid matter with layers of minute stars, especially in parts of Cygnus, Cepheus, Perseus, Monoceros, and Scorpio. We see, then, that the great cosmic zone is not only frequented by nebulous objects, but is nebulous in itself. Yet it seems to repel from it the multitude of white nebulæ which tend to collect about its poles. This, at least, is the law of their visual distribution; but the camera threatens to abrogate it. Dr. Max Wolf holds that the results of his preliminary photographic surveys prove nebulæ to be in reality scattered pretty evenly over the heavens. Only their average brightness, he thinks, varies, the so-called “nebular regions” in Cetus and Virgo having acquired their reputation as such not because nebulæ are more numerous, but because they are there more conspicuous than in the intervening celestial tracts. The subject, in fact, of nebular distribution, which had been supposed practically disposed of, has been, by photographic explorations, reopened for fresh discussion.

The crowding of globular clusters upon the Milky Way is unmistakable; and the inwardness of their relation to its condensations is manifested by their avoidance of the vacuous rifts, and their adherence to the stream-lines of luminosity. Hence arises the logical necessity for their radical separation from white nebulæ, to which they seem, in most respects, so near akin that one might be led to believe mere difference of distance to occasion the distinction between resolvable and irresolvable objects. Only the opposite galactic proclivities of the two classes decisively place them apart.

Within sight of that ultimate problem, the structure of the sidereal universe, we pause. Our thoughts meet, but they cannot grapple with it; nor does it come within the scope of our present purpose to make the attempt. We must be content to register the marks of growth and change legible in the Milky Way; to note the evidence of its comparatively recent origin and inchoate state; to avow our impotence to comprehend the Supreme design which it is directed to realise; and to bend in awe and admiration before the unfathomable depths of difficulty and mystery towards which the study of sidereal development, in its larger bearings, inevitably leads. Die Schöpfung, as Kant discerned, ist niemals vollendet. Sie hat zwar einmal angefangen, aber sie wird niemals aufhören.

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