Rambles and reveries of a naturalist

Preface

little volume of Natural History sketches has very little to do with opinions or speculations. The aim of the writer has been to describe the facts and phenomena of Nature as they have appeared to present themselves to his own observation. They are life-pictures rather than compilations. So far as systematic arrangement is concerned, there has certainly been an effort to treat each subject taken up in such a way as to lay at least a stepping-stone on the path to a more serious contemplation of the sciences of the text-book and the laboratory, but the primary intention of the author has been rather to awaken or to stimulate a love for Nature in the minds of some who may not as yet have suspected what wondrous and ever-varying beauty lies everywhere about us in ditch and pond, in rock and stone, in river and sea, on earth and in the skies.

6 PREFACE.

The same objects have been described by other and abler pens, but facts and laws present themselves in different aspects to different minds, and the ceaseless and diversified operations of Nature cannot be too frequently portrayed. Even those who would not care to claim the character of students of Nature are not unwilling to be considered lovers of Nature, and for such the plain and non-technical descriptions of a few of her secrets, by those who tell the story of what they themselves have seen and felt in her presence, cannot fail to possess some degree of interest.

As to the value of scientific study in these days, it is almost superfluous to argue. As a means of mental discipline, the methods of inquiry and reasoning made use of in the pursuit of physical science are of great utility, while almost all matters affecting health and life are of such a sort as to be profitably dealt with only by a rigid adherence to those methods. What misery some bring upon themselves and others by dense ignorance of the most elementary laws of Nature ! And certainly the sum of happiness is much increased by some acquaintance, however superficial, with the results of the researches and doctrines of men of science which bear upon the vast problems concerned in the origin and history of life, the

PREFACE. 7

development and meaning of organic forms, and the principles upon which the government of the material universe is carried on. In regard to the spirit and aims of such studies, nothing can be more admirable than the following words of Charles Kingsley on this point, in which he sets forth the qualifications of the observer of Nature : " He should be brave and enterprising, and withal patient and undaunted ; not merely in travel, but in investigation ; knowing (as Lord Bacon might have put it) that the Kingdom of Nature, like the Kingdom of Heaven, must be taken by violence, and that only to those who knock long and earnestly does the great mother open the doors of her sanctuary. He must be of a reverent turn of mind also, not rashly discrediting any reports, however vague and fragmentary, giving man credit always for some germ of worth, and giving Nature credit for an inexhaustible fertility and variety, which will keep him his life long always reverent, yet never superstitious ; wondering at the commonest, but not surprised by the most strange ; free from the idols of size and sensuous loveliness ; able to see grandeur in the minutest objects, beauty in the most ungainly ; estimating each thing, not carnally, as the vulgar do, by its size, or its pleasantness to the senses, but

8 PREFACE.

spiritually, by the amount of Divine thought revealed to him therein ; holding every phenomenon worth the noting down ; believing that every pebble holds a treasure, every bud a revelation ; making it a point of conscience to pass over nothing through laziness or hastiness, lest the vision, once offered and despised, should be withdrawn ; and looking at every object as if he were never to behold it again."— Glaucw, p. 46.

This volume is mainly the record of pursuits which occupied holiday hours snatched from a life devoted to other and more serious tasks, but it has often been felt that these pursuits, while being a relief from the strain of other kinds of work, have yet been in a large measure in harmony with it, and helpful to its better performance. The preaching of the Gospel covers a wider ground than the mere interpretation of a written Word. Nature has revelations for man if he will try to read them intelligently. She may not be able to answer all the instincts of his complex being, but she has much to say that is vitally concerned in his moral, social, and spiritual welfare. It is the duty of the teacher of religion, we contend, to seek philosophical and scientific truth, as well as to expound and enforce theological doctrines. To leave out of our literature all that bears upon the exposition of Nature's

PREFACE. 9

processes and marvels is, we think, to betray our cause, and to show ourselves incapable of grasping its sacredness and its grandeur, and can only result in handing over our best and noblest sons, the flower and hope of the Christian Church, to influences that may be inimical to their faith and their devotion to God and humanity. It is considerations of this kind that have led to the inclusion of what we prefer to call Reveries amongst papers which are more properly described as accounts of Rambles of a Naturalist.

CONTENTS,

I. AMONGST THE SEAWEEDS

IL RAMBLES IN CORNWALL

III. ANCIENT SLATE-INSCRIPTIONS

IV. A VISIT TO THE CHANNEL TUNNEL . V. DEAD SEA-LILIES

VI. A BAY IN A QUARRY ....

VII. ST. HILDA'S SNAKE-STONES .

VIII. A VISIT TO GREENWICH OBSERVATORY

IX. MARVELS OF THE POND.

X. A RAMBLE THROUGH CATEKHAM VALLEY

XI. TIJIY ROCK-BUILDERS ....

XII. STAR-GAZING

XIII. AN EVENING AT THE MICROSCOPE

XIV. TBILOBITE HUNTING ....

XV. WHAT is CREATION? ....

XVI. THE MINISTRY OF NATURE .

PAGE

LIST OF ILLUSTBATIONS.

Frontispiece. Encrinite. Apiocrinus rotundus. 1. Seaweed. Fucus canaliculatus . . .

Fucus vesiculosus ...... 21

Fucus serratus ...... 22

Fucus nodosus ...... 24

Halidrys siliquosa ...... 25

Plocamium coccineum ..... 27

7. Diatom. Navicula didyma ...... 30

8. „ Pleurosigma formosum ..... 33

9. Graptolite. Didymograptus Murchisoni ... 52

10. Sea Fir. Sertularia fusca, with calycles ... 53

11. „ Sertularia abietina, calycles and capsule . 55

12. Magnified Calycles of Graptolite ..... 55

13. Double Graptolite. Diplograptus pristis ... 56

14. Silurian Encrinites. Marsupiocrinus ccelatus. Arms

and proboscis of ditto. Crotalocrinus rugosus.

Pelvis, stomach, and arms of ditto .... 75

15. Head of Encrinite. Poteriocrinus .... 76

16. Trilobite. Phacops caudatus ..... 94

I4 LIST OF ILLUSTRATIONS.

17. Ammonites communit ....... 103

18. Side view of ditto ........ 103

19. Pearly Nautilus ........ 104

20. Section of Shell of Ammonite ..... 105

21. Foliation of Ammonite Chambers ..... 106

22. Ammonites Jnfrons ........ 167

23. Side view of ditto ........ 107

24. Ammonites oUutus ....... 108

26. Side view of ditto ........ 108

26. Ammonites lawtus ........ 109

27. Ammonites varioosus and side view .... 9

30. Melicerta ringens ........ 127

31. Plumatetta repens ........ 129

32. Coral. Favorites GotMandica ..... 143

33. Tabulae of ditto ........ 143

34. Coral. LitKostrotion basaltiforme , . . .145

35. „ Halysites catenulatm ..... 147

36. „ Astrcea ananus ..... .14

37. „ Heliolites interstinctus ..... 149

38. CoraUites of ditto ........ 150

39. Coral. Section of Favosites cervicornis . . .151

40. „ Isastrtea insignis ...... 163

41. „ Thecosmilia annularis ..... 154

42. ,, Smilotrichus granulatus ..... 154

44. Desmid. Closterium striolatum ..... 181

45. Volvox ylobator ........ 185

46. Water-flea. Daphnia pulex. Male . . . .187

LIST OF ILLUSTRATIONS. 15

47. Water-flea. Daphnia pulex. Female . . . .187

48. Hydra viridis . * . 190

49. Stings of Hydra . 191

50. Trilobite. Trinucleu* fimbriatus 196

51. „ Paradoxides Tessini 196

52. „ Agnostus pisiformi». Upper side . . 197

54. King-Crab. Limulus. Under side . . . .201 56. Trilobite. Head-shield and Tail-shield of Phillipsw . 205

56. Eye of Trilobite. Asaphus caudatus .... 207

57. Fossil King-Crab. Belinurus trilolitioides . . .211

58. Trilobite. Prestwichia . . . . . .. .211

59. Larva of King-Crab 211

60. Larva of Trilobite 211

61. Shrimp Parasite. Bopyrus crangorum. Upper side . 213

62. Under side of ditto 213

63. Serolis Fabricii, with eye magnified . . . .214

I. Amongst The Sea Weeds

' Thou sea, who wast to me a prophet deep Through all thy restless waves and wasting shores, Of silent labour, and eternal change ; First teacher of the dense immensity Of ever-stirring life, in thy strange forms Of fish, and shell, and worm, and oozy weed : To me alike thy frenzy. and thy sleep Have been a deep and breathless joy."

KlNGSLEY.

jHERE are very few of my readers, probably, who have not found some degree of pleasure in collecting seaweeds during their holidays, even though there has been no sort of scientific interest in the occupation. Yet it is quite as entertaining, and assuredly more profitable, to turn these familiar things to account in the way of increasing our knowledge of Nature. To those for whom anything like a practical study of any branch of natural history is impossible save during the week or two which, once a year or so, is snatched from a busy life, a few notes on the subject of seaweeds, suggested by my own observations and wants, may be of some value. Although during one of my vacations I gave myself up almost exclusively to seaweed collecting, yet my sphere of operations was so limited, and the kind of shore which I worked was so uniform, that I must draw somewhat upon previous experiences as well as upon the results of others' work if I would be of any service.

FIG. 1. — Fucus canaliculatus, and spore containing sporules.

It may not be amiss, in the first place, to settle what we mean by a seaweed. The very common " grass-wrack," a sort of sea-grass, is not a true seaweed. This plant, which botanists call Zostera marina, is really a true flowering plant, whereas seaweeds proper are non-flowering, cellular plants, and constitute the lowest group of the great Linnean division of Cryptogamia, to which also belong lichens, fungi, mosses, and ferns. What we are concerned with, then, are marine algae, plants that have no vascular tissue, nor any root, stem, leaves, flowers, nor seeds. There are seaweeds which seem to possess roots, but these are mere sucker-like discs, and have no root-functions in regard to nutrition, while the so-called stems and leaves are, accurately speaking, fronds and their modifications. Those curious vesicles on so many marine algse, which look like pods or seed-vessels, and which the novice might suppose were concerned in reproduction, are in reality nothing but air-vessels, whose function is to keep the fronds floating. The higher algae are reproduced by spores, and others by zoospores. The former method is analogous to that which obtains amongst ferns, while zoospores are cells that separate from the parent plant. In general, when reproduction is by spores, there are organs which in some senses correspond to the anthers and pistils of flowers. In the spore cavity or conceptacle, are certain bodies called antheridia, which fertilize the spores proper. Many of the green seaweeds are reproduced by zoospores, which method has been thus described by Dr. Harvey, the well-known phycologist : " In an early stage the green matter, or endochrome, contained within the cells of these algse is of a nearly homogeneous consistence throughout, and nearly fluid ; but at an advanced period it becomes more and more granulated. The granules, when formed in the cells, at first adhere to the inner surface of the membranous wall, but soon detach themselves, and float freely in the cell. At first they are of irregular shapes, but they gradually become spheroidal. They then congregate into a dense mass in the centre of the cell, and a movement, aptly compared to that of the swarming of bees round their queen, begins to take place. One by one these active granules detach themselves from the swarm, and move about in the vacant space of the cell with great vivacity. Continually pushing against the sides of the cell-wall, they at length pierce it, and issue from their prison into the surrounding fluid, when their seemingly spontaneous movements are continued for some time. The vivacious granules, or zoospores, at length become fixed to some submerged object, where they soon begin to develop cells, and at length grow into algse similar to those from whose cells they issued."

Not much need be said as to the apparatus for collecting and mounting seaweeds. An old knife and a few bottles for the more delicate species, with some sort of vasculum or satchel for the larger kinds, are all that will be required. The collector must not be above going bare-footed into the water now and then, unless he is provided with a pair of strong knee-boots ; for beyond where the lowest tide leaves its line of sea-wrack may be found many rare and choice specimens. At the same time, the drift that lies over all those parts of the shore that have been covered by the tide will well repay scrutiny.

As soon as convenient the specimens should be well washed in fresh water ; floated on to paper with careful fingers, helped, in the case of delicate kinds, by a quill pen or small camel's hair brush. The plan I find to answer best is to lay a sheet of thickish paper at the bottom of a large meat dish, and, after filling the dish with water, to place the specimen over the paper, and tease it out into as graceful a form as time and patience will permit.

FIG. 2,—Fucus vesiculosus.

After this the mounts must be pressed^ and dried. When drained, spread over each a few strips of clean linen rags, which should be changed occasionally, care being taken to avoid disarranging. 'the plants, and keeping them for a day or two under pressure. Many seaweeds are gelatinous, and will adhere to the paper without any medium. The large, coarse, olive-coloured weeds are not easily mounted for the herbarium, but small portions of them may be fastened on paper like dried plants,

FIG. 3. — Fucus serratus, with conceptacle, spore andsporules.

while larger specimens merely require drying, and can be hung up for future reference, it being possible to restore to them much of their natural appearance by immersing them in warm water. For the identification of specimens a cheap and

AMONGST 7 HE SEAWEEDS. 23

portable book is the one by Shirley Hibberd, published by Groombridge, which, for all ordinary purposes, is more serviceable than many a far costlier work. Landsborough's book is also cheap and useful. For more advanced work it will be necessary to procure some such treatise as Greville's or Harvey's on British Algse.

The classification of seaweeds is a much simpler matter than is the case with more highly organized plants. It is curious that there is a direct relation between the colour and the mode of reproduction, so that, roughly speaking, the arrangement of seaweeds according to colour will correspond to botanical classification. The identification of genera and species, however, will require the microscope. There are three great primary divisions of marine

(1) Olive-coloured (Melanospermete) ;

(2) Red (Rkodospermece) ;

(3) Green (Chlorospermece}.

The olive-coloured kinds are mostly tough and strong ; many are furnished with air-vessels, and, in general, they grow near the shore. The commonest of these are the well-known Fuci (Greek phycos, a sea-weed), popularly called sea-wrack. The commonest of all is the Bladder-wrack, so called from the bladder-like air-vessels which cover it and by which it is kept floating in the water. Fucus nodosus (Fig. 4) is covered with large aircavities. F. serratus (Fig. 3) has saw-like edges to the fronds, and is destitute of air-vessels. Another

RAMBLES AND REVERIES.

familiar species is the channelled Fncus (Fig. 1). These with several others were once largely used in the manufacture ,of kelp on account of their alkaline constituents. F. vesiculosus and F. canaliculatus are equally common at most English sea-side places (Figs. 1,2). The celebrated Gulf-weed, Sargassum bacciferum, belongs to this order of Fucacece.

In the tide-pools near low-water mark occurs in many parts of our coasts the Desmarestia, whose fronds are flat and lance-shaped and pinnate, having spiny teeth on their edges.

This may be taken as typical of the Sporochnacece or second order of olive weeds.

The most attractive of all the seaweeds with airbladders is the Sea-oak or Halidrys (Fig. 5). It is found very generally on rocks and stones in the sea at nearly low water mark. Usually the fronds are about half a foot in length, though it is not uncommon to find them as long as three or four feet.

FIG. 4.— Fucv* nodosus.

It is very much branched, and the branches are ornamented with curiously shaped air-chambers, which look like the pods of vetches, or of the mustard plant. These pod-like structures are even marked with transverse depressions, just as if there

FIG. 5. — Halidrys siliquosa.

were actually seeds inside, and so the illusion is increased.

Several of these Fuci are very useful. The Scotch used to gather them for kelp-making, but that industry has been superseded. They are _still used as manure and as food for cattle, being mixed with meal. In the Hebrides the ashes of the

Bladder-wrack are used for drying cheeses, thus obviating the use of salt. These ashes contain about half their weight in alkaline salt. Its bladders also yield iodine, used largely for medicinal purposes. F. serratus is largely used for packing lobsters, and the channelled Fucus is eaten by cattle in the north of Britain.

The third order is called Laminar iace<z. The frond is flat and without midrib, and there is a kind of stalk. These grow below tide mark, but they are plentifully cast up, and very large specimens are dragged about by visitors to the seaside. Laminaria or Tangle is a good specimen for the study of the sucker-like "roots" which many algae possess.

L. fascia, or the Papery Tangle, has a very short thin stem, and widens out into a long broad frond ; one of the large tangles is called the Oar-weed. The Sweet Laminaria, or Sea-belt, makes a good hygrometer when hung up and will foretell rain.

The Dictyotacece include the Sea-endive, the pretty Padina pavonia, called so from its resemblance to a peacock's tail, the Girdle-weed or Zonaria, and Taonia or Peacock-weed, Dictyota or Netted sea-weed, Stilophora or Nettle-bearer, Punctaria and others.

The other orders of olive weeds are Chordariaceae and Ectocarpacea, which include several lovely feathery plants.

Rhodospermece or red seaweeds are the most beautiful of all. Red is the prevailing colour, but a few brown, purple, and orange-tinted species are included. The many-tubed Polysiphonia, whose fronds are made up of thread-like tubular cells, is well worth microscopical examination. There are a score of British species, and many more on other shores. Laurencia is abundant near low-water mark, and is of a deep red colour. The Coral weed, Corallina, has jointed pinnate fronds, and should be sought for in the deeper pools. Young and healthy specimens are dark purple, but as picked up they are usually white. This is a very curious plant, and scarcely looks like a plant, beingpointed, and clad with a coating of carbonate of lime. If a portion be placed in acid, the mineral will dissolve, and the real plant structure be plainly discerned. Nitophyllum^ or Shining Leaf, is a very

. ,, ,;. FiG.Q.—Plocamiumcoccineum.

fine weed, especially J\. punctatum, and may easily be found.

One of the commonest, and at the same time one of the prettiest, of the red seaweeds is the one called Plocamium (Fig. 6). Its popular name is the Braidedhair weed; indeed, plokamos means braided hair. Its colour is a delicate pinky red. On account of its elegant form and beautiful colour, it is one of the chief favourites of those who arrange seaweed ornaments. Although it lives beyond the shoreline, yet it is cast up in abundance everywhere along our coasts. The Griffithsias, called after Mrs. Griffiths, the celebrated algologist, are all of them exceedingly pretty.

One of the loveliest of the rhodosperms is Maugeria sanguined, more familiarly known by its older name of Delesseria sanguined. It is very common in the deep rocky pools. The Peacock's Tail, or Padina pavonia, is well known, and is largely used for ornamental patterns.

Ckondrus crispus, or Irish Moss, called, by the Irish, Carrageen, belongs to this second division of seaweeds. When seen growing in the quiet pools it appears very beautiful, and is of a deep brown or purple colour, but in shallower water is yellowish. The tasteless gelatine derived from it is largely used in cooking, and at one time it was credited with curative properties in cases of consumption. It is still used in making jellies, and even for blanc-mange, by confectioners. One of the species of chondrus constitutes the material from which Chinese birds make those edible nests which are so highly prized by Oriental epicures. Griffithsia, which has already been referred to, is a widely distributed and pretty genus. All the species are of delicate texture, and must not be kept too long in water, or they will lose their colour. The palmated Rhodymenia or Dulse is eaten in Ireland, and is also dried for use as tobacco.

The third division of marine algae consists mainly of green weeds.

The commonest of these, indeed one of the commonest of all seaweeds, is Codium tomentosum, the Woolly Skin-weed. It is of a fine dark green colour, and may be found in the deep rock-pools that are never left dry. Its texture is spongelike, and it is covered with delicate filaments. A prominent order of this group is that which comprises the Ulvacece. The best known of these are the black-green Porphyra, the Enteromorpha, nearly as common as Codium, called so because of its intestine-like tubular fronds, and the bright green, flat, and delicate Ulva, whose membranaceous frond, waved at the margin, grows sometimes to a length of two feet.

With one or two exceptions specimens of all the foregoing, besides many others not referred to, lie before me, the results of a few days' exploration of a mile or two of shore ; and as I glance over these plant-pictures shining out from their white background, they revive for a while the delight with which I sought and gathered them, and something of the awe which the great solemn ocean awakened in the mind while its lovely treasures were being culled comes upon me even now while I admire them. What produces devouter or more enduring impressions than to walk along the margin of the murmuring sea, stopping now and again to examine the wonders it casts up at our feet ; or to gaze over its expanse stretching out to where it seems to touch the sky, as though to bring into relief our littleness of knowledge, and at the same time to suggest the vastness of our intellectual capacities ?

3o RAMBLES AND REVERIES,

" Nature hath tones of magic deep, and colours iris bright, And murmurs full of earnest truth, and visions of delight ; 'Tis said, ' The heart that trusts in her was never yet

beguiled,'

But meek and lowly thou must be, and docile as a child. Then study her with reverence high, and she will give the

key, So ahalt thou learn to comprehend the secrets of the sea."

There are many marine algse besides these larger ones, which never fail to give delight to those who will take the trouble to look for them, minute and indeed invisible, and requiring the microscope for

FlG. l.—Navicvla didyma.

their detection. They are common enough, but they are passed over unconsciously by all but the thorough student of the wonders of the shore. These are the Diatoms, whose glassy coverings or valves are familiar to every one who possesses a microscope (Figs. 7, 8). It is scarcely possible to avoid collecting diatoms if anything at all wet is gathered. Weeds, muddy or sandy sediment from tidal pools, damp caves, stones, in fact everywhere, these interesting organisms are met with. If obtained in a living state, their curious locomotion, which led the older naturalists to class them amongst animalculsB, may be studied ; and if dead, their beautiful frustules provide valuable material for mounting for the microscope. Their movements seem to be wholly mechanical, and there is no trace of any self-directive power. They are of a jerky nature, and are quite aimless and uncertain.

So prolific are these tiny plants that their accumulated valves make large deposits in some places, as is the case at Dolgelly, Mull, and elsewhere in Britain. The entire city of Richmond, in Virginia, is built upon a bed of these diatoms eighteen feet in thickness. In Bohemia there is a deposit of them fourteen feet thick, from which is obtained that fine material used for delicate castings. I have a tin of diatomaceous earth from Hanover, known as Kieselguhr, or flint mud, which consists of very little else than the frustules of diatoms. This is the material which is used in the manufacture of dynamite, since it has the property of absorbing three times its weight of nitro-glycerine, thus presenting a remarkable illustration of combined feebleness and power. Tripoli powder and rotten stone, used for polishing, are similarly constituted. It is no exaggeration to say that, notwithstanding their extreme minuteness, these organisms have added more to the fossiliferous contents of the earth than all the great mammals put together.

The external covering or shell of diatoms is made up of two portions or valves, usually called frustules ; but there is no hinge connection, as in the case of molluscous bivalves. The two frustules are connected by a layer of silica, which runs round their edges. This flinty covering is perforated, in order to allow of communication with the outer world.

The minuteness of these plants is marvellous when we bear in mind how perfect they are in every detail of their form and markings. The giants amongst them are not more than about one-fiftieth of an inch in length, while the majority are less than one-thousandth of an inch. My friend, Mr. Isaac Robinson, of Hertford, who has made these organisms a special study, relates that one of his slides which contains the frustules from a single drop of water has upon it no less than 200,000 frustules, and another reaches the astonishing number of 430,000. And the water itself has to be taken into account in attempting to form an idea of the size of these infinitesimal inhabitants of that small world. If the bulk of the drop of water had consisted exclusively of frustules, there would have been, in the latter case, upwards of ten millions. How much truer it appears now than Shakespeare ever imagined when he made Hamlet say, " There are more things in heaven and earth, Horatio, than are dreamt of in our philosophy ! "

The utmost diversity of form is met with amongst these minute plants. They are boat-shaped, triangular, square, crescent-like, filamentous, and seaweed shaped ; in fact, there is scarcely any conceivable pattern which does not find its counterpart amongst these extraordinary objects.

The mounted frustules of Diatoms are extensively used as tests for the determination of the quality of microscopic lenses. An illustration of one of

AMONGST THE SEAWEEDS.

these is given. It is called Pleurosigma formosum (Fig. 8). For this object I must go to my cabinet and take out one of the slides there stored, and which has been properly prepared by a professional mounter.

The silicious valves of Diatoms are beautifully marked with ridges, called costse, connected with an inner membrane ; besides which there are numerous dots, more or less minute, along the outside membrane. Generally the dots are arranged in regular lines or series, radiating from the median line or costa. . With a low power these rows of dots look

FIG. 8. — Pleurosigma formosum.

like continuous lines or striae. Supposing that the diatom has been properly cleaned and well mounted, the striae can be resolved into dots by objectives of the requisite quality and magnifying power. As the fineness of the dots varies in different species (it is the character of these markings that determines the species), it must not, of course, be expected that a lens which will show the dots on one will necessarily do so on another. Certain well understood species, therefore, are usually adopted by the microscopist for testing lenses, and some of these are commonly referred to as to a sort of standard by which they describe the capabilities of an objective. The costa is always visible, then the lines or striae are brought into view ; and lastly, if the lens is equal to the proper test, the dots start ont into distinctness. It may be as well, however, to remark that the practised microscopist can often bring out dots by manipulating his light and condensers where the tyro would altogether fail. Some frustules do not exhibit the dot structure, but appear to be ornamented with ridges. But however highly magnified, the dots where they are shown are all geometrically perfect, and not one of the many thousands is ever found out of its proper place.

Pleurosigma formosum has been chosen as one of the most useful of these tests, and the appearances which it presents under the quarter inch, one-eighth inch, and one-twelfth inch objectives, may soon become familiar to the student. Another species, P. angulatum, is a good test for still higher powers ; while none but the very best lenses will resolve into dots the markings on Amphipleura pellucida, one of the fresh-water species. There are, of course, many other test objects besides diatoms, but these furnish a very easy and reliable means of examining lenses, and they have the great advantage of permanency in regard to shape and appearance, a vital feature of a reliable standard.