Digitized by the Internet Archive in 2015 https://archive.org/details/microscopeitsrev01carp FRONTISPIECE. April THE M ICROSCOPE AND ITS REVELATIONS WILLIAM B. CARPENTER, C.B. M.D. LLC. F.R.S. F.G.S. F.L.S. CORRESPONDING MEMBER OP THE INSTITUTE OF FRANCE, AND OF THE AMERICAN PHILOSOPHICAL SOCIETY, ETC., ETC. SIXTH EDITION ILLUSTRATED BY TWENTY-SIX PLATES AND FIVE HUNDRED WOOD ENGBAVINGS VOLUME I. NEW YOEK WILLIAM WOOD & COMPANY 56 & 58 Lafayette Place 1883 PEEFACE. The rapid increase which has recently taken place in the use of the Microscope, — both as an instrument of scientific research, and as a means of gratifying a laudable curiosity and of obtaining a healthful recrea- tion, — has naturally led to a demand for information, both as to the mode of employing the Instrument and its appurtenances, and as to the Objects for whose minute examination it is most appropriate. This information the Author has endeavored to supply in the following Treatise; in which he has aimed to combine, within a moderate compass, that information in regard to the use of his Instrument and its Appliances which is most essen- tial to the working Microscopist, with such an account of the Objects best fitted for his study as may qualify him to comprehend what he ob- serves, and thus prepare him to benefit Science whilst expanding and refreshing his own mind. The sale of five large Editions of this Manual, with the many spontaneous testimonies to its usefulness which the Author has received from persons previously unknown to him, justify the belief that it has not inadequately supplied an existing want; and in the prepa- ration of the new Edition now called-for, therefore, he has found no rea- son to deviate from his original plan, whilst he has endeavored to improve its execution as to every point which seemed capable of amended treat- ment. In his account of the various forms of Microscopes and Accessory Apparatus, the Author has not attempted to describe everything which is used in this country; still less, to go into minute details respecting the construction of foreign instruments. He is satisfied that in nearly all which relates both to the mechanical and the optical arrangements of their instruments, the chief English Microscope-makers are quite on a level with, if not in advance of, their Continental rivals; but, on the other hand, the latter have supplied instruments which are adequate to all the ordinary purposes of scientific research, at a lower price than such could until recently be obtained in this country. Several British makers, how- ever, are now devoting themselves to the production of Microscopes which shall be really good though cheap; and the Author cannot but view with great satisfaction the extension of the manufacture in this direction. In the selection of Instruments for description which it was necessary for him to make, he trusts that he will be found to have done adequate juS' iv PREFACE. tice to those ^rlio have most claim to honorable distinction. His princi- ple has been to make mention of such Makers as have distinguished themselves by the introduction of any oieiu pattern which he regards as deserving of special recommendation; those who have simply copied the patterns of others without essential modification, receiving no such recog- nition, — not because their instruments are inferior^ but because they are oiot original. In treating of the Applications of the Microscope, the Author has constantly endeavored to meet the wants of such as come to the study of the minute forms of Animal and Vegetable life with little or no previ- ous scientific prep^iration, but desire to gain something more than a mere sight of the objects to which their observation may be directed. Some of these may perhaps object to the general tone of his work as too highly-pitched, and may think that he might have rendered his descrip- tions simpler by employing fewer Scientific terms. But he would reply that he has had much opportunity of observing among the votaries of the Microscope a desire for such information as he has attempted to convey; and that the use of scientific terms cannot be easily dispensed with, since there are no others in which the facts can be readily expressed. As he has made a point of explaining these in the places where they are first introduced, he cannot think that any of his readers need find much diflB- €ulty in apprehending their meaning. The proportion of space allotted to the several departments has been determined not so much by their Scientific importance, as by their special interest to the amateur Microscopist; and the remembrance of this consider- ation will serve to account for much that might otherwise appear either defective or redundant. Thus, the Author has specially dwelt on those humble forms of Vegetable and Animal life, which the diligent collector is most likely to meet with, and which will fully reward his most atten- tive scrutiny. And he has endeavored, in his account of them, to inter- est his readers in the knowledge to be drawn from their study, as to those fundamental phenomena of living action which are now universally admit- ted to constitute the basis of Physiological science; thus giving to the portion of his Treatise which treats of Protophytic and Protozoic organ- isms, the character of a General Introduction to the study of Biology, which will, he hopes, prove specially useful to such as desire to follow this study into its higher walks. On the other hand, the Author has felt the necessity of limiting within a narrow compass his treatment of various important subjects which are fully discussed in Treatises expressly devoted to them (such, for example, as the structure of Insects, and Ver- tebrate Histology), in order that he might give more space to those on which no such sources of information are readily accessible. For the same reason, he has omitted all reference to the Embryonic Development of Vertebrated Animals, — a study that is second to none in scientific in- terest, but can only be advantageously taken up by the Microscopist who PREFACE. V lias been trained to the pursuit. And lie has found himself obliged to content himself with a mere indication of the new and important facts now being brought to our knowledge by Microscopic inquiry, in regard to the Deposits at present in progress on the bottom of the Deep Sea, the Mineral constitution of Sedimentary and Igneous Kocks, and other branches of Micro-Petrological inquiry, which are throwing a flood of new light on the past history of the Crust of the Earth. It has been the Author's object throughout, to guide the possessor of of a Microscope to the intelligent study of any department of Biology, which his individual tastes may lead him to follow-out, and his indi- vidual circumstances may give him facilities for pursuing. And he has particularly aimed to show, under each head, how small is the amount of trustworthy knowledge already acquired, compared with that which remains to be attained by the zealous and persevering student. Being satisfied that there is a large quantity of valuable Micro scope-power at present running to waste in this country, — applied in such desultory observations as are of no service whatever to Science, and of very little to the mind of the observer, — he will consider himself well rewarded for the pains he has bestowed on the production and revision of this Manual, if it should be tend to direct this power to more systematic labors, in those fertile fields which only await the diligent cultivator to bear abun- dant fruit. In all that concerns the worJcing of the Microscope and its appurten- ances, the Author has mainly drawn upon his own experience, which dates-back almost to the time when Achromatic Object-glasses were first constructed in this country. In his last Edition, he felt himself obliged by the demands which were made by Official duties upon his time and attention, to seek the aid of his friend Mr. H. J. Slack, in the prepara- tion of the portion of the work specially relating to the Microscope and its appliances. But having now, at last, the command of his own time, he has preferred that this, like the rest of the Treatise, should be the ex- pression of his own matured views; and has accordingly taken much trouble to acquaint himself thoroughly with such recent advances, alike in the theory and in the practice of Microscopy, as could be most fittingly introduced into it. Accordingly, he has introduced at pp. 156-161 a concise account of the ^ difl:raction-theory ^ of Prof . Abbe, which has now given the com- plete rationale of the relation between the ^angular aperture^ of Object- ives and their ^resolving power.' And he has followed this up by a dis- cussion of the question (pp. 161-173) whether the opening-out of the angular aperture to its extremest limits is the end to be specially aimed- at in the construction of Objectives for the highest kinds of Biological research; in other words, whether an Objective which resolves the most difficult Diatom tests, is on that account the one best suited for follow- ing the life-history of Monad, or for studying the development of a prob- vi PREFACE. lematical BaciUus-organism. Having the misfortune to differ in opinion on this point from certain American Microscopists, who are distinguished by their expertness in the resolution of lined tests by Objectives of the largest angular aperture, and who enthusiastically advocate the use of such Objectives as the only powers to be trusted for Biological research, he has requested his friend, Mr. Dallinger (than whom there can be no higher authority on such a question), to give him the benefit of his ex- j)erience thereon. And he is authorized by Mr. Dallinger to express his entire concurreiice in the opinion uniformly upheld by the Author, that great 'resolving power' is only exceptionally needed in the most difficult Biological investigations; what is especially required for the study of liv- ing and moving organisms being such crisp and clear definition, good working distance, and considerable focal depth, as high-jaower Objectives of the widest aperture cannot afford. These qualities are so admirably combined in the ' dry ' l-35th of ' moderate angle ' constructed to Mr. Dal- linger's order by Messrs. Powell and Lealand, that he has been able to do work (of the kind just specified) with this Objective, which it would have been simply impossible for him to do with the oil-immersion l-25thof the same makers, although this far surpasses their l-35tli in 'resolving' power. — When Prof. J. Edwards Smith, and those who side with him, shall have produced Biological work of anything like the same nature and quality as that of Mr. Dallinger, it will be interesting to know the results of their more extended experience. On another point of great practical importance, the Author has thought it worth while to avail himself of Mr. Dallinger's unrivalled experience, — the utility of ' deep eye-piecing.' For he has seen with astonishment that the enthusiastic American advocates of the widest angles for Objectives of moderate power, are claiming for such objectives the advantage that they may be worked-up to any amount of amplifica- tion by sufficiently 'deep eye-piecing;' solid eye-joieces of half or even a quarter of an inch being now spoken-of as in ordinary use. He does not for a moment doubt that difficult lined tests may be thus shown; but that it is far less trying to the vision, when exercised in continuous luork, to gain the needed amplification by a high Objective and shallow Eye- piece, than by a loiv Objective (however wide its angle) and deep Eye- piece, experience long ago satisfied him. Not having thus exercised his eyes, however, upon objects requiring the high amplifications used by Mr. Dallinger, he was fully prepared to submit his own judgment on this question to that of a gentleman who has so well earned his title to pro- nounce an authoritative verdict upon it; but, so far from having in the least to give way, the Author finds himself supported by Mr. D. in the most emphatic way. For he learns, not only that Mr. D. 's experience in the study of the most difficult Biological objects satisfies him of the immense | superiority of the highest Objective that admits of good working distance, combined with a low Eye-piece, over the 'strained amplification ' given PREFACE. vii by a 4-lOths, a l-4th, or even a l-8tli, with deep eye-pieces; but that Mr. D. is satisfied that if he had tried to do the work of the last ten years on the latter plan^ "ha would be now blind, instead of possessing as good and sensitive a sight as he had ten years ago/^ As it has been politely suggested by an American controversialist, that the Author's inability to appreciate the supreme value of wide aperture may be due to the senile deterioration of his vision, the Authof is happy to be able to state that, — thanks to his habit of using shallow Eye-pieces, and of never persisting in Microscope-work when he has felt visual fatigue, — his eyes are now as fit for Microscopy as they were when he began so to use them nearly half a century ago. He has only to add that he has endeavored, by a careful and thorough revision of the entire Treatise, to render it as serviceable as possible to those for whom it is specially intended. Besides introducing a large amount of new matter into the first four chapters, he has entirely re- written Chap, v., so as to embody in it an account of those methods of Hardening, Staining, Imbedding, and Section-cutting, which have com- pletely revolutionized many departments of Microscopic investigation, in the sections relating to the Protophytic forms of Vegetable life, much new matter has been introduced in regard so the ScMzomycetes or Bacte- rium group, the Myxomycetes, and others of those curious organisms which occupy the border-ground between Vegetable and Animal life. So, again, in the section on the Protozoic forms of Animal life, large ad- ditions have been made under the heads of Mojierozoa, Rhizopoda, Infu- soria (especially the flagellate smd suctor ial), and Radiolaria; and the sec- tion on Sponges has been entirely re-written. Some important additions have also been made (Chap. XXI.) in regard to the applications of the Microscope to Geological inquiry. — In many other instances, references have been made to the best sources of information upon recent discov- eries of interest, which a due regard to the necessary limits of his book made it requisite for the Author to dismiss with a mere mention. No fewer than fifty new Wood-engravings have been added (for the use of eleven of which the Author is indebted to the Council of the Lin- naean Society), besides the reproduction of Prof. Oohn's beautiful Plate of Volvox, which now forms the Frontispiece. To such as feel inclined to take up the use of the Microscope as a means of healthful and improving occupation for their unemployed hours, the Author would offer this word of encouragement, — that, notwithstand- ing the number of recruits continually being added to the vast army of Microscopists, and the rapid extension of its conquests, the inexhaustibility of Nature is constantly becoming more and more apparent; so that no apprehension need arise that the Microscopist's researches can ever be brought to a stand for vmnt of an object! London, May, 1881. TABLE OF OOIsTTEOTS. CHAPTER I. OPTICAL PRINCIPLES OP THE MICROSCOPE. PAGE Laws of Refraction: — Spherical and Chromatic Aberration, . . . . 1 Construction of Achromatic Objectives, 11 Immersion Systems, 16 Simple Microscope, 18 Compound Microscope 22 Prmciples of Binocular Vision 25 Stereoscopic Binocular Microscopies, 27 Nachet's, 28 Wenham's, . , , ^9 Stephenson's, 81 Tolles' Binocular Eyepiece, 33 Nachet's Stereo-pseudoscopic Binocular, 33 Special value of Stereoscopic Binoculars, 35 CHAPTER II. CONSTRUCTION OF THE MICROSCOPE. General Principles, * . .40 Simple Microscopes, . . .43 Ross's, 43 Quekett's Dissecting, . . 45 Siebert & Kraft's Dissecting, . 46 Laboratory Dissecting, . . 47 Beck's Dissecting and Nachet's Binocular, . . . .48 Field's Dissecting and Mount- ing, 49 Compound Microscopes, . . 51 Educational Microscopes, . . 53 Field's, 53 Crouch's, . ... . .53 Parkes's, 53 Students' Microscopes, . . .55 Baker's, 59 CoUins's, . , . . .59 Pilhscher's (International), . 59 Ross's (Zentmayer), . . 61 Wale's (New Working), . . 61 Nachet's, 63 Browning's (Rotating), . . 65 Crouch's (Binocular), . . 65 Baker's (Erecting ditto), . . 65 Second Class Microscopes, . . 67 Powell and Lealand's, . . 67 Beck's (Popular Binocular), . 68 Collins's (Harley Binocular), . 70 Swift's (Challenge), . . . 71 Browning's Smaller Stephenson Binocular, . . . .73 First Class Microscopes, . . .73 Ross's (Ross Model), . . . 73 Ross's (Jackson-Zentmayer), . 75 Powell and Lealand's, , . 77 Beck's, 77 Beck's (Improved), . . .80 Microscopes for Special Purposes . 80 Beale's Pocket and Demonstrat- ing, 81 Baker's Travelling, . . .81 Swift's Portable, . . .82 Nachet's Chemical, . . .82 Non-Stereoscopic Binoculars, . . 84 Powell and Lealand's, . . 85 Wenham's, . . • .85 X TABLE OF CONTENTS. CHAPTER III. ACCESSORY PAGE Amplifier, 86 Draw-tube, 87 Lister's Erector, , . • . 87 Micro-Megascope . , . ,88 Nachet's Erecting Prism, . . 88 Micro-Spectroscope, . . .89 Micrometric Apparatus, . . .92 Goniometer, 95 Diaphragm Eye-piece and Indica- tor, 95 Camera Lucida and other Drawing Apparatus, 96 Nose-piece, 99 Finders, 99 Diaphragms, 101 Achromatic Condensers, . . . 103 Webster Condenser, . . . 103 Obhque Illuminators, . . . 104 Amici's Prism, 106 Black-Ground Illuminators, • . 106 Table and Cabinet, . . . .130 Daylight and Lamps, . . .131 Position of Light, . , . .133 Care of the Eyes, . . . .134 Care of the Microscope, . , .134 General Arrangements, . . . 135 Focal Adjustment, . . . .137 Adjustment of Object-Glass, . .139 Materials, Instruments, and ApplU ances, 175 Glass Slides, . . . .175 Thin Glass, . . . .176 Varnishes and Cements, . .178 Cells for Mounting Objects, . 179 Wooden Slides for Opaque Ob- jects, 183 Turn-Table, . . . .184 Mounting Plate and Water Bath, 185 Slider-Forceps, Spring-Clip, and Spring-Press, . . . .185 Mounting Instrument, . .186 Dissecting Apparatus, . .186 Microtomes and Section-cut- ting, . . . . .188 APPARATUS. PAGE Wenham's Reflex Illuminator, . 109 Light Modifiers, . . . .110 Polarizing Apparatus, . . .111 Swift's Combination Sub-Stage, . 113 Side Illuminators for Opaque Ob- jects, 114 Parabolic Speculum, . . .116 Lieberkiihn, 117 Vertical Illuminators, . . .118 Stephenson's Safety Stage, . . 120 Stage-Forceps and Vice, . . . 120 Disk-holder and Object-holder, . 121 Glass Stage-Plate, . . . . 122 Growing Slides, .... 122 Aquatic Box and Cells, . . . 123 Zoophyte-Trough, . , , .125 Compressors, 126 Dipping Tubes, . . . .127 Glass Syringe, • . • • . 128 Forceps, 128 Arrangement for Transparent Ob- jects, 141 Arrangement for Opaque Objects, . 147 Errors of Interpretation, . . 150 Effects of Diffraction. . . .154 Relative Qualities of Objectives, . 161 Test-Objects, . . . . .165 Determination of Magnifying Power, 173 Preparation and Mounting of Ob- jects, 194 Imbedding Processes, . . 194 Grinding and Polishing Sec- tions of Hard Substances . 196 Decalcifying Process, . . 201 Hardening of Animal Substan- ces, ...... 202 Staining Processes, . . . 204 Chemical Testing, . . .208 Preservative Media, . . . 209 Mounting Thin Sections, . .212 Mounting in Canada Balsam, . 214 Mounting Objects in Cells, . 215 Labelling and Preserving of Ob- jects, 218 Collection of Objects, . . . 219 CHAPTER IV. MANAGEMENT OF THE MICROSCOPE. CHAPTER V. PREPARATION, MOUNTING, AND COLLECTING OF OBJECTS. TABLE OF CONTENTS. xi CHAPTER VI. MICROSCOPIC FORMS OF VEGETABLE LIFE '.—SIMPLER ALG^. Protoplasm— Vegetable and Ani- mal 222 Relation between Vegetable and Animal Kingdoms, . . . 223 Vegetable Cells in general, . . 224 Protophytic Algce, .... 229 Conjugateae, 236 Volvocineae, 237 Palmellaceae, 245 Ulvaceae, 246 Oscillatoriaceae, .... 247 Fungi differentiated from Algae, . 307 Schizomycetes, .... 307 Fermentative Action, . . .313 Alg^, 331 Hepaticae, 335 Mosses, 338 Sphagnaceae, • . . . 342 PAGE Nostochaceae, 249 Siphonaceae, 250 Confervaceae, 254 OEdogonieae, 257 Chaetophoraceae, .... 258 Batrachospermeae, .... 258 Characeae, ..... 259 Desmidiace^, 269 Pediastreae, .... 270 DlATOMACE.35, 273 Parasitic Fungi, .... 316 Myxomycetes, . . . .825 Lichens, 329 Ferns, 344 Equisetaceae, 349 Rhizocarpeae, 350 Lycopodiaceae, .... 350 CHAPTER VII. PROTOPHYTIC AND OTHER FUNGI.— LICHENS. CHAPTER VIII. MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. CHAPTER IX. MICROSCOPIC STUCTURE OF PHANEROGAMIC PLANTS. Distinctive Peculiarities of Phan- Structure of Epidermis and erogamia, 352 Leaves, , 577 Elementary Tissues, . . . 353 Structure of Flowers, . . . 382 Structure of Stem and Root, . . 367 Fertilization. — Seeds, . . . 385 THE MICROSCOPE. CHAPTER L OPTICAL PRINCIPLES OF THE MICROSCOPE. 1. Laius of Refraction: — Splierical and Chromatic Aberration. 1. All Microscopes in ordinary use, whether Simple or Compound, depend for their magnifying power on that influence exerted by Lenses, in altering the course of the rays of light passing through them, which is termed Refraction. This influence takes place in accordance with the two following laws, which are fully explained and illustrated in every elementary treatise on Optics: — I. A ray of light passing from a rarer into a denser medium, is refracted towards a line drawn perpendicularly to the plane which divides them; and vice versa. II. The sines of the angles of incidence and refraction (that is, of the angles which the ray makes with the perpendicular hefore and after its refraction) bear to one another a constant ratio for each substance, which is known as its index of refraction. Thus the ray E o (Fig. 1) passing from Air into Water, will not go on to E, but will be refracted towards the line c c' drawn perpendicularly to the surface A b of the water, so as to take the direction o w. If it pass into Glass, it will undergo a greater refraction, so as to take the direction 0 G. And if it pass into Diamond, the change in its course will be so much greater, that it will take the direction o D. The angle E o c is termed the ^ angle of incidence;' whilst the angles w o c', G o c', and r> o c' are the ^angles of refraction.' And whether the angle of incidence be large or small, its sine e e' bears a constant ratio in each case to the sine 10 or g or d d\ of the angle of refraction; and this ratio is what is termed the ^ index of refraction.' The ^ index of refraction ' is determined for different media by the amount of the refractive influence which they exert upon rays passing into them, not from air, but from a vacuum; and in expressing it, the sine of the angle of refraction is considered as the unit, to which that of the angle of incidence bears a fixed relation. Thus when we say that the ^index of refraction' of Water is 1.336, we mean that the sine e e' of the angle of incidence e o c of a ray passing into water from a vacuum, is to the sine w lo' of the angle of refraction w o c', as 1.336 to 1, or almost 1 2 THE MICROSCOPE AND ITS REVELATIONS. Pig.- 1. exactly as 1^ to 1, or as 4 to 3. So, again, the index of refraction for (flint) Glass, being about 1.6, we mean that the sine e e' of the angle of incidence of a ray E o c passing into Glass from a yacuum, is to the sine of g g' the angle of refraction G o c', as 1.6 to 1, or as 8 to 5. So in the case of Diamond, the sine e e' is to the sine d d' as 2.439 to 1, or almost ex- actly as 2 J to 1 , or as 5 to 2. Thus, the angle of incidence being given, the angle of refraction may be always found by divid- ing the sine of the former by the ^ index of refrac- tion,^ which will give the sine of the latter. In ac- cordance with these laws, a ray of light passing from one medium to another perpendicularly to the surface which divides them undergoes no re- fraction; and of several rays entering at different angles, those nearer the perpendicular are refracted less than those more inclined to the refracting surface. — When a pencil of rays, however, im- pinges on the surface of a denser medium (as when rays passing through Air fall upon Water or Glass), some of the incident rays are reflected from that surface, instead of entering it and undergoing refraction; and the pro- portion of these rays increases with the increase of their obliquity. Hence there is a loss of liglit in every case in which pencils of rays are made to pass through lenses or prisms: and this diminution in the brightness of the image formed by refraction will bear a proportion, on the one hand, to the number of surfaces through which the rays have had to pass; and, on the other, to the degree of obliquity of the incident rays, and to the difference of the refractive powers of the two media. Hence, in the passage of a pencil of rays out of Glass into Air, and then from Air into Glass again, the loss of light is much greater than it is when some medium of higher refractive power than air is interposed between the two glass surfaces; and advantage is taken of this principle in the construc- tion of Achromatic objectives for the Microscope, the component lenses of each pair or triplet (§ 14) being cemented together by Canada Balsam; as also in the interposition of Water or some other liquid between the covering-glass of the object and the front lens of the objective, in the * immersion lenses^ now coming into general use (§ 19). On the other hand, advantage is taken of the partial reflection of rays passing from air into glass at an oblique angle to the surface of the latter, in the construc- tion of the ingenious (non-stereoscopic) Binoculars of Messrs. Powell and Lealand and of Mr. Wenham (§ 81). 2. When, on the other hand, a ray, w o, emerges from a dense medium into a rare one, instead of following the straight course, it is bent from the perpendicular according to the same ratio; and to find the course of the emergent ray, the sine of the angle of incidence must be vmltiplied by the ^ index of refraction,^ which will give the sine of the OPTICAL PRINCIPLES OF THE MICROSCOPE. 3 angle of refraction. And thus, when an emergent ray falls very obliquely upon the surface of the denser medium^ the refraction which it would sustain in passing forth into the rarer medium, tending as it does to deflect it still farther from the perpendicular, becomes so great that the ray cannot pass out at all, and is reflected back from the plane which separates the two media, into the one from which it was emerging. This i7iternal reflection will take place whenever the product of the sine of the angle of incidence, multiplied by the index of refraction, exceeds the sine of 90°, which is the radius of the circle; and therefore the ^limiting angle,' beyond which an oblique ray suffers internal reflection, varies for differ- ent substances in proportion to their respective indices of refraction. Thus, the index of refraction of Water being 1.336, no ray can pass out of it into a vacuum,* if its angle of incidence exceed 48^ 28', since the sine li A' of that angle, H o c', multiplied by 1.336 equals the radius; and, in like manner, the ' limiting angle ' for Flint-glass, its index of refraction being 1.60, is 38° 41\ — This fact imposes certain limits upon the performance of microscopic Lenses, since of the rays which would otherwise pass out from glass into air all the more oblique are kept back; whilst, on the other hand, it enables the Optician to make most advan- tageous use of glass Prisms for the purpose of re-flection, the proportion of the light which they throw back being much larger than that returned from the best polished metallic surfaces, and the brilliancy of the reflected image being consequently greater. Such prisms are of great value to the Microscoj)ist for particular purposes, as will hereafter appear. (§§ 33- 38.) 3. The Lenses employed in the construction of Microscopes are chiefly convex ; those of the opposite kind, or concave, being only used to make certain modifications in the course of the rays passing through convex lenses, whereby their performance is rendered more exact (§§ 11, 13). — It is easily shown to be in accordance with the laws of refraction already cited, that when a bundle of parallel rays, passing through air, impinges upon a spherical surface of glass, these rays will be made to converge. For the perpendicular to every point of that surface is the radius drawn from the centre of the sphere to that point, and prolonged through it; so that, whilst any ray which coincides with the radial perpendicular will go on without change in its course towards the centre of the sphere; every ray which falls upon the spherical surface at an inclination to its pi'o- longed radius undergoes refraction in a degree proportionate (as already explained) to that inclination. And the effect upon the whole bundle will be such, that its rays will be caused to meet at a point, called the focns, some distance beyond the centre of curvature. — This effect will be somewhat modified by the passage of the rays into air again through a flane surface of glass, perpendicular to the axial ray (Fig. 2); and a lens of this description, called 2, plano-convex lens, will hereafter be shown to possess properties which render it very useful in the construction of Microscopes. — But if, instead of passing through a plane surface, the rays re-enter the air through a second convex surface, turned in the oppo- site direction, as in a double-convex lens, they will be made to converge ^ The reader may easily make evident to himself the internal reflection of Water, by nearly filling a wine-glass with water, and holding it at a higher level than his eye, so that he sees the surface of the fluid obliquely from beneath: — no object held above the water will then be visible through it, if the eye be placed beyond the limiting angle; whilst the surface itself will appear as if silvered, through its reflecting back to the eye the light which falls upon it from beneath. 4 THE MICROSCOPE AND ITS REVELATIONS. Fig, 2. Parallel rays, falling on a plano-convex lens of grlass, brought to a focus at the distance of the diameter of its sphere of curvature; and con- versely, rays diverging from that point, ren- dered parallel. Fig. 3. still more. This will be readily comprehended when it is borne in mind that the contrary direction of the second surface, and the contrary direc- tion of its refraction (this being fro7n the denser medium instead of i7ito it), antagonize each other; so that the second conyex surface exerts an influence on the course of the rays passing through it, which is almost exactly equiva- lent to that of the first. Hence the focus of 3j doicble-conyex lens will be at just half the distance, or (as commonly expressed) will be half the length of the focus of a ^:)Za^^o-conyex lens haviug the same curvature on one side (Fig. 3). 4. The distance of the Focus from the spherical surface will depend not merely upon its de- gree of curvature, out also upon the refracting power of the substance of which it may be formed; since the lower the index of refraction, the less will the oblique rays be de- flected towards the axial ray, and the more rernote will be their point of meeting; and conversely, the greater the refractive index, the more will the oblique rays be de- flected towards the axial ray, and the nearer will be their point of convergence. A lens made of any substance whose index of refrac- tion is 1.5, will bring parallel rays to a focus at the distance of its diameter of curvature, after they have passed through one convex surface (Fig. 2), and at the dis- tance of its radius of curvature, after they have passed through tiuo convex surfaces (Fig. 3); and as this ratio almost exactly expresses the refractive power of ordinary crown or plate Glass, we may for all practical purposes consider the ^ principal focus ' (as the focus for parallel rays is termed) of a douMe-couYex lens to be at the distance of its radius, that is, in the centre of curvature, and that of a plano-conYex lens to be at the distance of twice its radius, that is, at the other end of the diame- ter of its sphere of curvature. 5. It is evident from what has preceded, that as a Double-convex lens brings parallel rays to a focus in its centre of curvature, it will on the other hand cause those rays which are diverging from that centre before they impinge upon it, to assume a parallel direction (Fig. 3); so that, if a luminous body be placed in the principal focus of a double-convex lens, its divergent rays, falling on one surface of the lens, as a cone, will pass forth from its other side as a cylinder. If, however, the rays which fall upon a double-convex lens be diverging from the farther extremity of the diameter of its sphere of curvature, they will be brought to a focus at an Parallel rays, falling on a double-convex lens, brought to a focus in the centre of its sphere of curvature : conversely, rays diverging from that point rendered parallel. OPTICAL PKINCIPLES OF THE MICROSCOPE. 5 Rays diverging from the farther extremity of one diameter of curvature of a double-convex lens, brought to a focus at the same distance on the other side. equal distance on the other side of the lens (Fig. 4); but the more the point of divergence is approximated to the centre or principal focus, the farther removed from the other side will be the point of conver- Fig. 4, gence (Fig. 5), until, the point of divergence being at the cen- tre, there is no convergence at all, the rays being merely render- ed parallel (Fig. 3); whilst if the point of divergence be ie- yond the diameter of the sphere of curvature, the point of con- vergence will be within it (Fig. 5). The farther removed the point of divergence, the more nearly will the rays approach the parallel direction: until, at length, when the object is very distant, its rays in eflfect become parallel, and are brought together in the principal focus (Fig. 3). If, on the other hand, the point of divergence be tuitldn the princijDal focus, they will neither be brought to converge, nor be rendered parallel, but will diverge in a diminished de- gree (Fig. 6). And conversely, irio,^^ if rays already converging fall upon a double-convex lens, they will be brought together at a point nearer to it than its centre of curvature (Fig. 6). — The same principles apply equally to a plano-convex lens; allowance be- in^ made for the double distance of its principal focus. They also apply to a lens whose surfaces have different curvatures; the principal focus of such a lens be- ing found by multiplying the radius of one surface by the rad- ius of the other, and dividing this product by half the sum of the same radii. — The rules by which the foci of convex lenses may be found, for rays of different de- grees of convergence and diver- gence, will be found in works on Optics. 6. The refracting influence of concave lenses will evidently be precisely the opposite of that of convex. Eays which fall upon them in a parallel direction, will ^ , ^ . -u x,. . v i_-i.-^7. ././. Rays already converging, brought together by be made to diverge as ll from the a douUe-convex lens at a point nearer than its ■Drincinal foons which i<^ here principal focus; and rays diverging from a point n j-^V ^^^^^^^ within its principal focus, still diverging, through called the negative lOCUS. ihlS in a diminished degree. Rays diverging from points more distant than the principal focus of a double-convex lens on either side, brought to a focus beyond it ; the focus of convergence being within the diameter of cur- vature, if the focus of divergence be beyond it; and vice versa. 6 THE MICROSCOPE AND ITS REVELATIONS. will be for a plano-concave lens, at the distance of the diameter or the sphere of curvature; and for a double-concave, in the centre of that sphere. In the same manner, rays which are converging to such a degree, that, if uninterrupted, they would have met in the principal focus, will be rendered parallel; if converging more they will still meet, but at a greater distance; and if converging less, they will diverge as from a negative focus at a greater distance than that for parallel rays. If already diverging, they will diverge still more, as from a negative focus nearer than the principal focus; but this negative focus will ap- proach the principal focus, in proportion as the distance of the point of divergence is such that the direction of the rays approaches the parallel. 7. If a lens be convex on one side and concave on the other, forming what is called a me7iisciis, its effect will depend upon the proportion be- tween the two curvatures. If they are equal, as in a watch-glass, scarcely any perceptible effect will be produced; if the convex curvature be the greater, the effect will be that of a less powerful convex lens; and if the concave curvature be the more considerable, it will be that of a less powerful concave lens. The focus of convergence for parallel rays in the first case, and of divergence in the second, may be found by dividing the product of the two radii by half their difference. 8. Hitherto we have considered only the effects of lenses either on a ' bundle ' of parallel rays, or on a ^ pencil ^ of rays issuing from a single luminous point, and that point situated in the line of its axis. If the j)oint be situated above the line of its axis, the focus will be below it, and vice versa. The surface of every luminous body may be regarded as com- prehending an infinite number of such points, from every one of which a pencils of rays proceeds, to be refracted in its passage through a lens according to the laws already specified; so that a complete but inverted image or picture of the object is formed upon any surface placed in the focus and adapted to receive the rays. It will be evident from what has gone before, that if the object be placed at twice the distance of the princi- pal focus, the image, being formed at an equal distance on the other side of the lens (§ 5), will be of the same dimensions with the object : whilst, on the other hand, if the object (Fig. 7, a i) be nearer the lens, the object will be more brilliant in the same proportion. 9. A knowledge of these general facts will enable the learner to un- derstand the ordinary action of the Microscope; but the instrument is subject to certain optical imperfections, the mode of remedying which cannot be comprehended without an acquaintance with their nature. One of these imperfections results from the unequal refraction of the rays Formation of Images by Convex Lenses. Fig. 7. image A B will be farther from it, and of larger dimensions; but if the object a b be farther from the lens, the image a b will be nearer to it, and smaller than itself. Further, it is to be remarked that the larger the image in proportion to the object, the less bright will it be, because the same amount of light has to be spread over a greater surface; whilst an image that is smaller than the OPTICAL PRINCIPLES OF THE MICROSCOPE. 7 which pass through lenses whose curvatures are equal over their whole surfaces. If the course of the rays passing through an ordinary convex lens be carefully laid down (Pig. 8), it will be found that they do not all meet exactly in the foci already stated; but that the focus r of the rays ab, ab, which have passed through the marginal portion of the lens, is much closer to it than that of the rays al)j a by which are nearer the line of its axis. This may be shown experimentally, by ^ stopping out ^ either the cen- tral or the marginal portion of the lens; for it will then be diagram illustrating ^pZtencaZ ^6erra«on. found that the rays which are allowed to pass through the latter alone form a distinct image at f; whilst those which pass through the former alone form a distinct image at /. Hence, if the whole aperture be in use, and a screen be held in the focus f of the marginal portion of the lens, the rays which have passed through its central portion will be stopped by it before they have come to a focus; whilst, if the screen be carried back into the focus f of the latter, the rays which were most dis- tant from the axis will have previously met and crossed, so that they will come to it in a state of divergence, and will pass to c and d. In either case, therefore, the image will have a certain degree of indistinctness; and there is no one point to which all the rays can be brought by a single lens of spherical curvature. The distance f/, between the focal points of the central and of the peripheral rays of any lens, is termed its Spherical Aberration. — It is obvious that the desired effect could be produced by such an increase of the curvature round the centre of the lens, and such a diminution of the curvature towards its circumference, as would make the two foci coincident. And the requisite conditions may be theoreti- cally fulfilled by a single lens, one of whose surfaces, instead of being spherical, is a portion of an ellipsoid or hyperboloid of certain proportions. But the difficulties in the way of the mechanical execution of lenses of this description are such, that for practical purposes this plan of construc- tion is altogether unavailable; besides which, their performance would only be perfectly accurate for parallel rays. 10. Various means have been devised for reducing the aberration of lenses of spherical curvature. In the first place, it may be kept down by using ordinary lenses in the most advantageous manner. Thus the aber- ration of a Plano-convex lens whose convex side is turned towards paral- lel rays, is only lyy^-ths of its thickness; whilst, if its plane side be turned towards them, the aberration is 4^ times the thickness of the lens. Hence, when a plano-convex lens is used to form an image by bringing to a focus parallel or slightly-diverging rays from a distant object, its convex surface should be turned towards the object; but, when it is used to render parallel the rays which are diverging from a very near object, its plane surface should be turned towards the object. The single lens having the least spherical aberration, is a Double-convex whose radii are as one to six: when the flattest face of this is turned towards parallel rays, the aberration is nearly 3^ times its thickness; but when its most convex side receives or transmits them, the aberration is only Ijfo-ths of its thickness. — Spherical Aberration is further diminished by reducing 8 THE MICROSCOPE AND ITS REVELATIONS. the aperture or working-surface of the lens, so as to employ only the rays that pass through its central part, which, if sufficiently small in propor- tion to the whole sphere, will bring them all to nearly the same focus. Such a reduction is made in the Object-glasses of common (non-achro- matic) Microscopes; in which, Avhatever be the size of the lens itself, the greater portion of its surface is rendered inoperative by a stop, which is a plate with a circular aperture interposed between the lens and the rest of the instrument. If this aperture be gradually enlarged, it will be seen that, although the image becomes more and more illuminated, it is at the same time becoming more and more indistinct; and that, in order to gain defining power, the aperture must be reduced again. Now, this reduction is attended with two great inconveniences: in the first place, the loss of intensity of light, the degree of which will depend upon the quantity transmitted by the lens, and will vary therefore with its aperture; and, secondly, the diminution of the Angle of Aperture, that is, of the angle ab c (Fig. 10) made by the most diverging of the rays of the pencil issu- ing from any point of an object, that can enter the lens and take part in the formation of an image of it; on the extent of which angle (as will be shown hereafter) depend some of the most important qualities of a Micro- scope. 11. The Spherical Aberration may be approximately corrected, how- ever, by making use of combinations of lenses, so disposed that their op- posite aberrations shall correct each other, whilst magnifying power is still gained. For it is easily seen that, as the aberration of a concaye lens is just the opposite of that of a convex lens, the aberration of a con- vex lens placed in its most favorable position may be corrected by that of a concave lens of much less power in its most unfavorable position; so that, although the power of the convex lens is weakened, all the rays which pass through this combination will be brought to one focus. It is thus that the Optician aims to correct the Spherical Aberration, in the construction of those combinations of lenses which are now employed as Object-glasses in all Compound Microscopes that are of any real value as instruments of observation. But this correction is not always perfectly made: and the want of it becomes evident in the fog by which the dis- tinctness of the image, and especially the sharpness of its outlines, is im- paired; and in the eidola, or false images, on each side of the best focal point, which impair the perfection of the principal image, and can be themselves brought into view when proper means are used for their de- tection.^ The skill of the best constructors of Microscopic objectives has been of late years successfully exerted in the removal of the ' residual errors ' to which these eidola were due; so that objectives of the largest angular aperture are now made truly aplanatic, the corrections for Sphe- rical Aberration being applied with a perfection which was formerly sup- posed to be attainable only in the case of Objectives of small or moderate aperture. Still, the difficulty (and the consequent cost) of producing such objectives, constitutes one out of many reasons for the preference of objectives of moderate aperture, in which the correction for Spherical Aberration can be easily made complete, for all the ordinary purposes of scientific investigation (§ 17). 12. But spherical aberration is not the only difficulty with which the Optician has to contend in the construction of Microscopes; for one ^ See Dr. Royston Pigott's description of his Searcher for Aplanatic Images," and its uses, in the ''Philos. Transact." for 1870, p. 59. OPTICAL PRINCIPLES OF THE MICROSCOPE. 9 equally serious arises from the unequal refrangihiUty of the several Col- ored rays which together make \\^ White or colorless light, ^ so that they are not all brought to the same focus, even by a lens free from spherical aberration. It is this difference in their ref rangibility, which causes their complete separation or ' dispersion ' by the Prism into a sioedrum; and it manifests itself, though in a less degree, in the image formed by a convex lens. For if parallel rays of white light fall upon a convex surface, the most refrangible of its component rays, namely, the violet, will be brought to a focus at a point somewhat nearer to the lens than the princi23al focus, which is the meto of the whole; and the converse will be true of the i^ed rays, which are the least refrangible, and whose focus will therefore be more distant. Thus in Pig. 9, the rays of white light, A B, A." B^', Avhich fall on the peripheral j)ortion of the lens, are so far decomposed, that the violet rays are brought to a focus a c, and crossing there, diverge again and j)ass on towards p f, whilst the red rays are not brought to nG?.y. a focus until D, crossing the di- vergent violet rays at e e. The foci of the intermediate rays of the spectrum (indigo, blue, green, yellow, and orange) are interme- diate between these two extremes. The distance c D between the foci of the violet and of the red rays respectively, is termed Chromatic Aberration. If the image be re- Diagram illustrating Chromatic Aberration. ceivod upon a screen j)laccd at c — the focus of the violet rays — violet will predominate in its own color, and it will be surrounded by a prismatic fringe in which blue, green, yellow, orange, and red may be successively distinguished. If, on the other hand, the screen be placed at d — the focus of the red rays — the image will have a predominantly red tint, and will be surrounded by a series of colored fringes in inverted order, formed by the other rays of the spec- trum Avhich have met and crossed.^ The line e e, which joins the points of intersection between the red and the violet rays, marks the ^ mean focus,^ that is, the situation in which the colored fringes w^ll be narrow- est, the ^ dispersion ' of the colored rays being the least. As the axial ray a' b' undergoes no refraction, neither does it sustain any dispersion; and the nearer the rays are to the axial ray, the less dispersion do they suffer. Again, the more oblique the direction of the rays, whether they pass through the central or the peripheral portion of the lens, the greater will be the refraction they undergo, and the greater also will be their dispersion; and thus it happens that when, by using only the central part of a lens (§ 13), the chromatic aberration is reduced to its minimum, the central part of a picture may be tolerably free from false colors, whilst ^ It has been deemed better to adhere to the ordinary phraseology, when speaking of this fact, as more generally intelligible than the language in which it might be more scientifically described, and at the same time leading to no prac- tical error. 2 This experiment is best tried with a lens of long focus, of which the central part is covered with an opaque stop, so that the light passes only through a peri- pheral ring; since, if its whole aperture be in use, the regular formation of the fringes is interfered with by the spherical aberration, which gives a different focus to the rays passing through each annular zone. 10 THE MICROSCOPE AND ITS REVELATIONS. its marginal portion shall exhibit broad fringes, as is well seen in the pic- tures exhibited by non-achromatic Oxhydrogen-Microscopes. 13. Although the Chromatic aberration of a lens, like the Spherical, may be diminished by the contraction of its aperture, so that only its central portion is employed, the error cannot be got rid of entirely by any such reduction, which, for the reasons already mentioned, is in itself extremely undesirable. Hence it is of the first importance in the con- struction of a really efficient Microscope, that the chromatic aberration of its Object-glasses (in which the principal dispersion is liable to occur) should be entirely corrected, so that a large aperture may be given to these lenses without the production of any false colors. No such correc- tion can be accomplished, even theoretically, in a single lens; but it may be effected by the combination of two or more, advantage being taken of the different relations which the refractive and the dispersive powers bear to each other in different substances. For if we can unite with a convex lens, whose dispersive power is loio as compared to its refractive power, a concave of lower curvature, whose dispersive power is relatively high, it is obvious that the dispersion of the rays occasioned by the con- vex lens may be effectually neutralized by the opposite dispersion of the concave (§ 6); whilst the refracting power of the convex is only lowered by the opposite refraction of the concave, in virtue of the longer focus of the latter. — No difficulty stands in the way of carrying this theoretical correction into practice. For the ^ dispersive^ power of jiint'g\n>^^ bears so much larger a ratio to its refractive power than does that of crown- glass, that a convex lens of the former whose focal length is 7f inches, will produce the same degree of color as a convex lens of crown-glass whose focal length is 4J inches. Hence a concave lens of the former material and curvature will fully correct the dispersion of a convex lens of the latter; whilst it diminishes its refractive power to such an extent only as to make its focus 10 inches. — A perfect correction for Chromatic Aberration might thus be obtained, if it were not that although the extreme rays — violet and red — are thus brought to the same focus, the dispersion of the rest is not equally compensated; so that what is termed a secondary spectrum is produced; the images of objects, especially towards the margin of the field, being bordered on one side with a purple fringe, and on the other with a green. In the best constructed combinations, however, whether for the Telescope or the Microscope, the chromatic error is scarcely perceptible; the aberrations of the objective being so arranged as to be almost entirely compensated by the opposite aberrations of the eye-piece (§ ^7). 14. It was in the Telescope that the principle of correction for Chro- matic dispersion, which had been theoretically devised by Euler and other mathematicians, was first carried into practical application; an Achromatic object-glass having been constructed in 1733 by Hall, and a more perfect combination having been worked out in 1757 by DoUond, whose system, known as the ^telescopic triplet,^ remains in use to the present time. This triplet consists of a double-concave lens of flint-glass, interposed between two double-convex lenses of crown; such curves being given to their respective surfaces, as serve almost entirely to extinguish not only the Chromatic, but the Spherical aberration, in the case of rays preceding from distant objects, which fall on the surface of the object- glass in a direction that is virtually These rays form an image in the ' principal focus ' of the object-glass, the size of which varies with its distance from the lens; magnifying power being thus gained by OPTICAL PRINCIPLES OF THE MICROSCOPE. 11 lengthening the focus of the objective. — In the Microscope, on the other hand, the conditions are altogether different. For the object-glass receives rays which diverge very widely from a near object, and the size of the image formed by their convergence depends upon the propor- tionate distances of the object and the image from the lens (§ 8); mag- nifying power being thus gained by shortening the focus of the object- glass. And the chromatic and spherical aberrations resulting from the incidence of diverging rays can only be fairly corrected by a single- triplet combination, when its focus is long (giving a low magnifying power), and the divergence of those rays moderate, so that the angle of the aperture is small. 15. It has only been in comparatively recent times that the construc- tion of Achromatic object-glasses for Microscopes has been found prac- ticable; their extremely minute size having been thought to forbid the attainment of that accuracy which is necessary in the adjustment of the several curvatures, in order that the errors of each of the separate lenses which enters into the combination, may be effectually balanced by the opposite errors of the rest. The first successful attempt was made in a 6 c, its Angle of Aperture. this direction in the year 1823 by MM. Selligues and Chevalier, of Paris; the plan which they adopted being the combination of two or vaoY^jpairs of lenses, each pair consisting of a double-convex of crown-glass and a plano-concave of flint. — In the following year, Mr. Tulley, of London, without any knowledge of what had been accomplished in Paris, applied himself (at the suggestion of Dr. Goring) to the construction of Achro- matic object-glasses for the Microscope: and succeeded in producing a single combination of three lenses, on the telescopic plan, the corrections of which were extremely complete. This combination, however, was not of high power, nor of large angular aperture; and it was found that these advantages could not be gained without the addition of a second combination. — Prof. Amici at Modena, also, who had attempted the construction of microscopic object-glasses as early as 1813, but, despair- ing of success, had turned his attention to the application of the reflect- fi2 THE MICROSCOPE AND ITS REVELATIONS. PLATE I. 2 4 1 5 VARIOUS FORMS OF DIATOMACE^. Fig. 1. Actinocyclus Rolf sit. 2. Asterolampra concinna. 3. HeHopelta (as seen with black-ground illumination), 4. Asteromphalus Brookeii. 5. Aulacodiscus Or eg anus. OPTICAL PRINCIPLES OF THE MICROSCOPE. la Plate IT ECHINUS-SPINE (Original), and podura-scale (after R. Beck). Fig. 1. Transverse section of Spine of Echinometra heteropora. 2. Markings on Scale of Fodura, as seen by transmitted light under a well-corrected l-8th inch Objective. 3. Partial obliteration of the markings by the insinuation of moisture between the Scale and the Covering-glass. 4. Appearance of the markings, when the Scale is illuminated from above the oblique light falling at right angles to them. 5. The same, when the light falls on the Scale in the direction of the markings. 14 THE MICROSCOPE AND ITS REVELATIONS. ing principle to the Microscope, resumed his original labors on hearing of the success of MM. Selligues and Chevalier; and, by working on their plan, he produced, in 1827, an achromatic combination which surpassed anything of the same kind that had been previously executed. And these were soon rivalled by the objectives produced in London by Andrew Eoss and Powell. 16. It was in this country that the next important improvements originated; these being the result of the theoretical investigations of Mr. J, J. Lister,.^ which led him to the discovery of certain properties in Achromatic combinations that had not been previously detected. Under his guidance, Mr. James Smith, soon followed by other Opticians, succeeded in producing combinations far superior to any which had been previously executed, both in extent of aperture, flatness of field, and completeness of correction; and continued progress has been since made in the same direction by the like combination of theoretical acumen with manipulative skill. 17. The enlargement of the Angle of Aperture, and the greater com- pleteness of the corrections, first obtained by the adoption of Mr. Lister's princi23les, soon rendered sensible an imperfection in the performance of these lenses under certain circumstances, which had previously passed unnoticed; and the important discovery was made by Mr. A. Koss that a very obvious difference exists in the precision of the image, according as the object is viewed, with or without a covering of talc or thin glass; an Object-glass which is perfectly adapted to either of these conditions, being sensibly defective under the other. The mode in which this differ- ence arises is explained by Mr. Ross^ as follows: — Let o (Fig. 11) be any point of an object; o p the axial ray of the pencil that diverges from it: and o T, 0 t', two diverging rays, the one near to, the other remote from, the axial ray. Now if a G G G represent the section of a piece of thin glass intervening between the object and the object-glass, the rays o t and o t' will be refracted in their passage through it, in the directions t r, t' r'; and on emerging from it again, they will pass on towards E and e'. Now if the course of these emergent rays be traced backwards, as by the dotted lines, the ray e r will seem to have issued from x, and the ray e' r' from Y; and the difference x Y, which is called ' negative aberration,' is quite sufficient to disturb the previous balance of the aberrations of the composite lens of the object-glass. The requisite cor- rection may be effected, as Mr. Eoss pointed out, by giving to the front pair (Fig. 10, 1) of the three of which the Objective is composed, an excess of ^positive aberration' (^. e,, by under-correcting it, and by giving to the other two pairs (2, 3) an excess of ' negative aberration ' (i. e., by over-correcting them), and by making the distance between the former and the latter susceptible of alteration by means of a screw collar (§ 140). For when the front pair is approximated most nearly to the other two, and its distance from the object is increased, its positive aberration is more strongly exerted upon the other pairs than it is when the distance between the lenses is increased, and the distance between the front pair and the object is diminished. Consequently, if the lenses have been so adjusted that their correction is perfect for an uncovered object, the approximation of the front lens to the others will give to the whole combination an excess of positive aberration, which will neutralize ^See his Memoir in the Philosophical Transactions'* for 1829. Transactions of the Society of Arts," vol. li. OPTICAL PRINCIPLES OF THE MICROSCOPE. 15 the negative aberration occasioned by covering the object with a thin plate of glass. — This correction will obviously be more important to the perfect performance of the combination, the larger is its angle of aperture; since the wider the divergence of the oblique rays from the axial ray, the greater will be the refraction which they will sustain in passing through a plate of glass, and the greater therefore will be the negative aberration produced, which, if uncorrected, will seriously impair the distinctness of the image. It is consequently not required for loio powers whose angle of aperture is comparatively small, nor for medium powers, so long as their angle of aperture does not exceed 50°, and even objectives of \ of an inch focus, whose angle of aperture does not exceed 75", may be made to perform very well without adjustment, if their corrections be originally made perfect for the average thickness of glass used to cover objects of the finer kind. And objectives of much higher power and larger angle of aperture (especially suited for Students' Microscopes), are now constructed so as to work admirably without adjustment, being corrected for a standard thickness — such as 0.008 or 0. 006 inch — of the glass covers supplied by their makers. Such non-adjust- ing objectives, when less than \ inch focus, are best constructed on the immersion' system (§ 19). 18. For many years the best Microscopic objectives of moderate and high magnifying power were made by combining three superposed pairs of increasing focus and diameter (as in Fig. 10), each consisting of a double- convex lens of crown-glass partly achromatized by its own concave of flint; the two apposed surfaces of each pair being of the same curvature, and cemented together by Canada balsam. Various modifications of this arrangement, however, have been introduced at various times and by various constructors; some proceeding in the direction of simplification, whilst others have aimed at the greatest attainable perfection, irrespec- tive of complexity and constructive difficulty. It is obvious that there are great practical advantages on the side of any reduction in the number of component lenses, that is compatible with the good performance of the combination: liability to error, as well in the curved surfaces, as in the centering and setting of each, l3eing thereby diminished, while there is a like diminution in the loss of light which occurs whenever the rays pass out of one medium into another (§1). But, on the other hand, it seems certain that the highest theoretical perfection can be attained by an increase in the number of component lenses; so that, if the errors in work- manship are kept down to the lowest possible point, the performance of such complex combinations may be made superior to that of simpler ones. — The first important change in the direction of simplification con- sisted in the replacement of the fro7if combination by a single plano-con- vex of crown. This substitution, which seems to have been first devised by Amici, has been very generally adopted; a greater working distance from the object (which is very important in the case of the highest pow- ers) being attainable in this construction, than when the front is either a doublet or a triplet combination. But most makers who have used this method have added a lens to the back combination, making it a ^ tele- scopic trij)let,' still using a doublet in the middle; and admirable objec- tives on this construction (each consisting of two flint concave and four convex lenses of crown, with twelve surfaces in all) have been made by the best Opticians — English and American, French and German. — A further simplification has been recently carried into effect by Mr. Wenham; who 16 THE MICROSCOPE AND ITS REVELATIONS. has shown ^ that the whole color-correction may be effected in the mid- dle lens by a double-concave of dense flint between two convex lenses of crown, the back lens as well as the front being a single plano-convex of crown. Thus one double concave lens of flint is made to correct the chromatic aberrations of four convex surfaces of crown, the total number of surfaces being reduced to ten. There is a further advantage in this plan of construction, that no change of the front lens is needed to enable the combination to be used as an ' immersion ' objective (§ 19), the requi- site adjustment being effected by the screw-collar used for cover-correc- tion. — There can be no doubt that objectives of moderate angular aperture may be made on Mr. Wenham's system, so as to combine great excellence with comparative cheapness; but it does not seem equally suitable for first-class objectives, requiring for their greatest efficiency the widest attainable angular aperture. These have usually been made to consist of a front triplet, a middle doublet, and a back triplet, thus having eight lenses in all, with sixtee7i surfaces. But the first-class constructors in the United States (notably Messrs. Tolles, Spencer, and Wales) have added to these a single front plano-convex of crown, by means of which a longer working distance has been obtained; whilst the extraordinary excellence of their workmanship (only 'attain able, however, at a very high cost) has given to these very complex combinations a perfection of per- formance, which, to say the least, is unsurpassed by that of any objectives constructed for use in the ordinary manner, which is now distinguished as dry. 19. It was long since pointed out by Amici that the introduction of a drop of water between the front surface of the objective, and either the object itself or its covering-glass, would diminish the loss of light result- ing from the passage of the rays from the object or its covering-glass into air, and then from air into the object-glass. But it is obvious that when the rays enter the object-glass from water, instead of from air, both its refractive and its dispersive action will be greatly changed, so as to need an important constructive modification to suit the new condition. This modification seems never to have been successfully effected by Amici himself; and his idea remained unfruitful until it was taken up by Hart- nack and Nachet, who showed that the application of what is now known as the Immersion-system to objectives of high power and large angular aperture is attended with many advantages not otherwise attainable. For, as already pointed out (§ 1), the loss of light increases with the obliquity of the incident rays; so that when objectives of very wide angle of aperture are used ' dry,^ the advantages of its increase are in great degree nullified by the reflection of a large proportion of the rays falling very obliqely upon the peripheral portion of the front lens. When, on the other hand, rays of the same obliquity enter the peripheral portion of the lens from water, the loss by reflection is greatly reduced, and the ben- efit derivable from the large aperture is proportionally augmented. Again, the ^immersion system' allows of a greater working distance between the objective and the object, than is otherwise attainable with the same extent of angular aperture; and this is a great advantage, not merely in regard to convenience in manipulation, but also in giving a greater range of ^ penetration ' or ^ focal depth.' Further, the observer is rendered less dependent upon the exactness in the correction for the thickness of the covering-glass, which is needed where objectives of large ^ Proceedings of Eoyal Society," Vol. xxi., p. 111. OPTICAL PRINCIPLES OF THE MICROSCOPE, 17 angle are used ^ dry;' for as the amount of ^ negative aberration ' (§ 17) is far smaller when the rays which emerge from the covering-glass pass into water than when they pass into air, variations in its thickness pro- dace a much less disturbing effect. And thus it is found practically that Mmmersion ' objectives can be constructed with magnifying powers suffi- ciently high, and angular apertures sufficiently large, for all the ordinary purposes of scientific investigation, without any necessity for cover- adjustment; being originally adapted to give the best results with a covering-glass of suitable thinness, and small departures from this in either direction occasioning very little deterioration in their performance. For ^water-immersion^ objectives of the very largest aperture, however, to be used upon the most difficult objects, exact cover-correction is still necessary. — Whilst immersion '-objectives constructed on the original plan can only be employed ^ wet '(that is with the interposition of water), Messrs. Powell and Leland — followed by other makers — have so arranged their combinations, that by a change in the front lens they may be used ^dry,^ as m the ordinary manner. And in Mr. Wenham's sys- tem not even this change is required, the change from ^ web' to Mry,' ^r\di vice versa, being accomplished by an alteration in the distance of the front lens from the middle triplet, made by the screw-collar, as in ordi- nary cover-correction. 20. The immersion system 'has recently undergone a still further development, by the practical application of a method originally sug- gested by Mr. Wenham^ (but never carried by him into operation), and iudependently suggested by Mr. Stephenson^ to Prof. Abbe of Jena, under whose scientific direction it has been worked out by the very able German optician, Zeiss, with complete success. This method consists in the replacement of the water previously interposed between the covering- glass and the front surface of the objective, by a liquid having the same refractive and dispersive power as crown-glass; so that the rays issuing at any angle from the upper plane surface of the covering-glass, shall enter the plane front of the objective without any change either by refraction or dispersion, and without any sensible loss by reflection — even the most oblique rays proceeding in their undefiected course, until they meet the convex back surface of the front lens. It is obvious that all the advan- tages derivable from the system of water immersion are obtainable with still greater completeness by this system of Jiomogeneoiis immersion, pro- vided that a fluid can be found which meets its requirements. After a long course of experiments. Prof. Abbe found that oil of cedar- wood so nearly corresponds with glass, alike in refractive and in dispersive power, that it serves the purpose extremely well, except when it is desired to take special advantage of the most divergent or marginal rays, oil of fen- nel being then preferable. Objectives of yV^^^^ yV'^^^ mch focal length have been constructed on this plan by Zeiss; and it appears cer- tain that by its means a larger angle of aperture can be effectively ob- tained, than on any other construction. Whether any tests can be re- solved by its use, on which other objectives fail, is a point not yet satisfactorily determined. But there can be no doubt that the system of 'homogeneous immersion' will greatly facilitate the use of objectives pos- sessing the largest angular aperture, and capable of affording the highest magnifying power, for the ordinary purpose of scientific research, it is J Monthly Microscopical Journal," Vol. iii. (1870), p, 303. 2 Journ. of Royal Microsc. Society," Vol. i. (1878), p. 51. o 18 THE MICROSCOPE AND ITS REVELATIONS. precisely in the case of such objectives that the ' cover-correction ^ needs to be most exact. And although the practised microscopist lias no diffi- culty in making this, when the object at which he is looking (such as a Diatom, a Podura-scale, or a band of Nobert's ruled lines) is Icnown to him, yet the case is entirely different when the object is altogether unhnoion. For in examining such an object, he may be able only to sat- isfy himself after repeated trials, involving much expenditure of time and patience, as to the cover-correction which gives the truest represen- tation of the object; whilst, in using a Miomogeneous ' or ^oil-immer- sion ^ objective, he is able to feel an absolute certainty that, without any adjustment at all, the view which he gains of an unknown object is in every respect at least equal to that which he can obtain from the best ' dry ' or ' water-immersion ^ objective, most exactly adjusted- for thick- ness of cover. — This system has been taken up also by Messrs. Powell and Lealand, who liave constructed admirable ^oil-immersion' objectives ranging to l-25th inch focus, which, by a change of the front lens, may also be used ' dry.' 21. We are now prepared to enter upon the application of the Optical principles which have been explained and illustrated in the foregoing pages, to the construction of Microscopes. These are distinguished as Simple and Compound; each kind having its peculiar advantages to the Student of Nature. Their essential difference consists in this: — that in the former, the rays of light which enter the eye of the observer proceed directly from the object itself, after having been subjected only to a change in their course; whilst in the latter, an enlarged image of the object is formed by one lens, which image is magnified to the observer by another, as if lie were viewing the object itself. — The Simple Microscope may consist of a single lens; but (as will be presently shown) it may be formed of two, or even three: these, however, being so disposed as to pro duce an action upon the rays of light corresponding to that of a single lens. In the Compound Microscope, on the other hand, not less than two lenses must be employed: one, to form the enlarged image of the ob- ject, immediately over which it is placed, and hence called the object- glass; whilst the other again magnifies that image, and, being interposed between it and the eye of the observer, is called the eye-glass. A perfect Object-glass, as we have seen, must consist of a combination of lenses; and the eye-glass is best combined with another lens interposed between itself and the object-glass, the two together forming what is termed an eye-piece (§ 27). — These two kinds of instrument need to be separately considered in detail. 2. Simple Microscope. 22. In order to gain a clear notion of the mode in which a Single Lens serves to ^magnify' minute objects, it is necessary to revert to the phenomena of ordinary Vision. An Eye free from any defect has a con- siderable power of adjusting itself in such a manner as to gain a dis- tinct view of objects placed at extremely varying distances; but the image formed upon the retina will of course vary in size with the dis- tance of the object; and the amount of detail perceptible in it will follow the same proportion. To ordinary eyes, however, there is a limit within which no distinct image can be formed, on account of the too great divergence of the rays of the different pencils which then enter the eye; since the eye is usually adapted to receive, and to bring to a focus, rays which are parallel or but slightly divergent. This limit is vari- OPTICAL PRINCIPLES OF THE MICROSCOPE. 19 ous]y stated at from 5 to 10 inches; but though there are doubtless many persons whose vision is good at the shorter range, yet the longer is probably the real limit for persons of ordinary vision: who, though they may see an object much nearer the eye, discern little if any more of its details, what is gained in sizie being lost in distinctness. Now the utility of a convex lens interposed between a near object and the eye, consists in its reducing the divergence of the rays forming the several pencils which issue from it; so that they enter the eye in a state of moderate divergence, as if they had issued from an object beyond the nearest limit of distinct vision, a well-defined picture being thus formed upon the retina. Not only, however, is the course of the several rays in each pencil altered as regards the rest, but the course of the pencils themselves is changed, so that they enter the eye under an angle corre- sponding with that under which they would have arrived from a larger object situated at a greater distance; and thus the picture formed upon the retina by any object {a 5, Fig. 12), corresponds in all respects with one which would have been made by the same object increased in its di- mentions to A b, and viewed at the smallest ordinary distance of distinct vision. A ^short-sighted^ person, however, who can only see objects dis- tinctly at a distance of two or three inches, has the same power in his eye alone by reason of its great convexity, as that which the person of ordinary vision gains by the assistance of a convex lens which shall enable him to see at the same distance with equal distinctness. It is evident, therefore, that the magnifying power of a single lens, depending as it does upon the proportion between the distance at which it renders the object visible, and the nearest distance of unaided distinct vision, must be different to different eyes. It is usually estimated, however, by find- ing how many times the focal length of the lens is contained r^'-^^^-^^r in ten inches; since, in order to render the rays from the object j nearly parallel, it must be placed nearly in the focus of | the lens (Fig. 3); and the pic- ture is referred by the mind to j an object at the ordinary dis- tance. Thus, if the focal length of a lens be one inch, its magnifying power for each di- mension will be 10 times, and consequently 100 superficial; while if its focal distance be only one-tenth of an inch, its magnifying power will be 100 linear, or 10,000 superficial. 23. But the shorter the focus of the magnifying lens, the smaller must be the diameter of the sphere of which it forms part; and, unless its aperture be proportionateljr reduced, the distinctness of the image ; will be destroyed by the spherical and chromatic aberrations (§§ 9, 12) ] necessarily resulting from its high curvature. Yet notwithstanding the loss of light and other drawbacks attendant on the use of Single Lenses of high power, they proved of great value to the older Microscopists (among whom Leeuwenhoek should be specially named), on account of their freedom from the errors to which the Compound Microscope of the old construction was necessarily subject; and the amount of excellent Diagram illustrating the action of the Simple MicrO' scope ; ab object; a b its magnified image. / 20 THE MICROSCOPE AND ITS REVELATIONS. , work done by means of them surprises every one who studies the his- tory of Microscopic inquiry. — An important improvement on the single lens was introduced by Dr. Wollaston, who devised the doublet, still known by his name; which consists of two plano-convex lenses, whose focal lengths are in the proportion of one to three, or nearly so, having their convex sides directed towards the eye, and the lens of shortest focal length nearest the object. In Dr. W.'s original combination, no perfo- rated diaphragm (or ^stop') was interposed; and the distance between the lenses was left to be determined by experiment in each case. A great improvement was subsequently made, however, by the introduction of a ' stop ' between the lenses and by the division of the power of the smaller lens between two (especially when a very short focus is required), so as to form a U^iplet, as first suggested by Mr. Holland.^ When combina- tions of this kind are well constructed, both the spherical and the chro- matic aberrations are so much reduced, that the angle of aperature may be considerably enlarged without much sacrifice of distinctness; and hence for all save very low powers, such ^ doublets ' and ' triplets ' are far superior to single lenses. These combinations took the place of sin- gle lenses, among Microscopists (in this country at least) who were pro- secuting minute investigations in Anatomy and Physiology prior to the vast improvements effected in the Compound Microscope by the achro- matization of its object-glasses (§ 15); and, in particular, the admirable researches of Dr. Sharpey,^ on ciliary action in Animals (1830-35), and Mr. Henry Slack's beautiful dissections of the elementary tissues of Plants, and also his excellent observations on Vegetable cyclosis (1831),^ were made by their means. — The performance of even the best of these forms of Simple microscope, however, is so far inferior to that of a good Compound microscope, as now constructed, that no one who has the command of the latter form of instrument would ever use the higher powers of the former. And as it is for the prosecution of observations, and for the carrying on of dissections, which only require lotv powers, that the Simple microscope is chiefly needed, the WoUaston doublet has now almost gone out of use. 24. Another form of Simple magnifier, possessing certain advantages over the ordinary double-convex lens, is that commonly known by the name of the ' Coddington ' lens.^ The first idea of it was given by Dr. Wollaston, who proposed to apply two plano-convex or hemispherical lenses by their plane sides, with a ^stop' interposed, the central aperture of which should be equal to one-fifth of the focal length. The great ad- vantage of such a lens is, that the oblique pencils pass, like the central ones, at right angles to the surface, so that they are but little subject to aberration. The idea was further improved upon by Sir D. Brewster, who pointed out that the same end would be much better answered by taking a sphere of glass, and grinding a deep groove in its equatoriarl part, which should be then filled with opaque matter, so as to limit the central aperture. Such a lens gives a large field of view, admits a con- ^ Transactions of the Society of Arts," Vol. xlix. 2 See his article Cilia in the Cyclopaedia of Anatomy and Physiology," and the references under that head in the Index to the present work. 2 See his Memoir, with two beautiful Plates, in the ' ' Transactions of the Soci- ety of Arts," Vol. xhx., pp. 0, 7. ^ This name, however, is most inappropriate; since Mr. Coddington neither was, nor ever claimed to be, the inventor of the mode of construction by which this lens is distinguished. OPTICAL PRINCIPLES OF THE MICROSCOPE. 21 siderable amount of light, and is equally good in all directions; but its power of definition is by no means equal to that of an achromatic lens, or even of a doublet. This form is chiefly useful, therefore, as a Hand- magnifier, in which neither hign power nor perfect definition is required; its peculiar qualities rendering it superior to an ordinary lens, for the class of objects for which a hand-magnifier of medium power is required. Many of the magnifiers sold as ^ Coddington ' lenses, however, are not really portions of spheres, but are manufactured out of ordinary double- convex lenses, and are therefore destitute of the special advantages of the real ^ Coddington.' — The ^ Stanhope^ lens somewhat resembles the pre- ceding in appearance, but differs from it essentially in properties. It is nothing more than a double-convex lens, having two surfaces of unequal curvatures, separated from each other by a considerable thickness of glass; the distances of the two surfaces from each other being so adjusted, that when the most convex is turned towards the eye, minute objects placed on the other surface shall be in the focus of the lens. This is an easy mode of applying a rather high magnifying power to scales of but- terflies' wings, and the other similar flat and minute objects, which will readily adhere to the surface of glass; and it also serves to detect the presence of the larger animalcules or of crystals in minute drops of fluid, to exhibit the ^ eels' in paste or vinegar, etc., etc. — A modified form of the ^Stanhope' lens, in which the surface remote from the eye is plane instead of convex, has been brought out in France under the name of ' Stanhoscope,' and has been especially applied to the enlarge- ment of minute pictures photographed on its plane surface in the focus of its convex surface. A good ' Stanhoscope,' magnifying from 100 to 150 diameters, is a very convenient form of hand-magnifier for the recog- nition of Diatoms, Infusoria, etc. ; all that is required being to place a minute drop of the liquid to be examined on the plane surface of the lens, and then to hold it up to the light. ^ 25. For the ordinary purposes of Microscopic dissection, single lenses of from 3 to 1 inch focus answer very well. But when higher powers are required, and when the use of even the lower powers is continued for any length of time, great advantage is derived from the employment of Achromatic combinations now made expressly for this purpose by several Opticians. The writer has worked most satisfactorily for several years with the ^platyscopic lens,' magnifying about 15 diameters, made by Mr. Browning, who makes similar combinations of 20 and 30 diameters. And he can speak equally favorably of the ^ Steinheil doublets ' (con- structed by the eminent Munich optician of that name, and introduced into this country by Messrs. Murray and Heath), of which there are six, ranging from 2f inches to f inch focus. The Browning and the Stein- heil combinations give much more light than single lenses, with much better definition, a very flat field, longer working distance (which is very important in minute dissection), and, as a consequence, greater ' focal depth' or ^penetration' — i. e, a clearer view of those parts of the object which lie above or below the exact local plane. And only those who, like the writer, have carried on a piece of minute and difficult dissection through several consecutive hours, can appreciate the advantage in com- fort and in diminished fatigue of eye y which is gained by the substitution 1 See Quart. Journ. of Microsc. Science," Vol. vi., N.S. (1866), p. 263.—Of the Stanhoscopes sold by Toy-dealers at a very low price, only a part are really serviceable; care is requisite, therefore, in the selection. 22 THE MICROSCOPE AND ITS REVELATIONS. of one of these Achromatic combinations for a single lens of equivalent focus, even where the use of the former reveals no detail that is not dis- cernible by the latter. 3. Compound Microscope. 26. The Compound Microscope, in its most simple form, consists of only two lenses, the object-glass and the eye-glass. The former, c d (Fig. 13), receives the light-rays direct from the object, A b, brought into near proximity to it, and forms an enlarged but inverted and reversed' image, a'b', at a greater distance on the other side (§ 8); whilst the latter, L m, receives the rays which are diverging from this image, as if they pro- ceeded from an object actually occupying its position and enlarged to its dimensions, and brings these to the eye at E, so altering their course as to make that image appear far larger to the eye, precisely as in the case of the Simple microscope (§ 22). — It is obvious that, in the use of the very same lenses, a considerable variety of magnifying power may be obtained, by merely altering their position in regard to each other and to the ob- ject: for if the eye-glass be carried farther from the object-glass, whilst the object is approximated nearer to the latter, the image a' b' will be formed at a greater distance from it, and its dimensions will consequently be augmented; whilst, on the other hand, if the eye-glass be brought nearer to the object-glass, and the object removed farther from it, the distance of the image will be a much smaller multiple of the distance of the object, and its dimensions proportionately diminished. We shall hereafter see that this mode of varying the magnifying power of Com- pound Microscopes may be turned to good account in more than one mode (§§ 83, 84); but there are limits to the use which can be advantage- ously made of it. — The amplification may also be varied by altering the magnifying power of the Eye-glass; but here, too, there are limits to the increase; since defects of the object-glass which are not perceptible when its image is but moderately enlarged, are brought into injurious promi- nence when the imperfect image is amplified to a much greater extent. In practice, it is generally found much better to vary the power by em- ploying object-glasses of different foci: an object-glass of long focus form- ing an image which is not at many times the distance of the object from the other side of the lens, and which, therefore, is not of many times its dimension; whilst an object-glass of short focus requires that the object should be so nearly approximated to it, that the distance of the image is a much higher multiple of the object, and its dimensions are proportion- ably larger. — In whatever mode increased amplification may be obtained, two things must always result from the change: the proportion of the surface of the object of which an image can be formed must be dimin- ished; and the quantity of light spread over that image must be propor- tionably lessened. 27. In addition to the two lenses of which the Compound Microscope essentially consists, it is found advantageous to introduce another (r r. Fig. 14), between the object-glass and the image formed by it; the pur- pose of this lens being to change the course of the rays in such a manner, that the image may be formed of dimensions not too great for the whole of it to come within the range of the Eye-glass. As it thus allows more of the object to be seen at once, it has been called the field-glass; but it is now usually considered as belonging to the ocular end of the instru- ment — the eye-glass and the field-glass being termed the Eye-piece, Ya- OPTICAL PRINCIPLES OF THE MICROSCOPE. 23 rioi-is forms of this Eye- Krei 13. ^.14. piece have been proposed by different Opticians; and one or another will bo preferred, according to the purpose for which it may be required. That which it is most advan- tageous to employ with Achromatic object-glass- es, to the performance of which it is desired to give the greatest possible ef- fect, is termed the Hiiy- ghenian; having been em- ployed by Huyghens for his telescopes, although without the knowledge of all the advantages which its best construction ren- ders it capable of afford- ing. It consists of two plano-convex lenses (e e and F F, Fig. 14), with their plane sides towards the eye; these are placed at a distance equal to half the sum of their focal length; or, to speak with more precision, at half the sum of the focal length of the eye glass, and of the distance from the field-glass at which an image of the object- 2rlaSS would be formed ^ja^ram of simplest form ? . , A ^ J 9 T or Compound Microscope, by it. A ^stop' or dia- phragm, B B, must be placed between the two lenses, in the visual focus of the Eye-glass, which is, of course, the position wherein the image of the object will be formed by the rays brought into conver- gence by their passage through the field-glass. — Huyghens devised this arrangement merely to diminish the Spherical aberration; but it was subsequently shown by Boscovich that the Chromatic dispersion was also in great part corrected by it. Since the introduction of Achromatic object-glasses for Compound Microscopes, it has been further shown that nearly all error may be avoided by a slight over- correction of these; so that the blue and red rays may be caused to enter the eye in a parallel direction (though not actually co-incident), and thus to produce a colorless image. Thus let iS" M K (Fig. 15) represent the two extreme rays of three pencils, which, without the field-glass, would form a blue image convex to the eye-glass at b b, and a red one at R r; then, by the intervention of the field-glass, a blue image, concave to the eye-glass, is formed at b^ b', and a red one at r' r'. As the focus of the Eye-glass is shorter for blue rays than for red rays by just the diffcr- Diagram of complete Corn,' pound Microscope. 24 THE MICROSCOPE AND ITS REVELATIONS. ence in the place of these images, their rays, after refraction by it, enter the eye in a parallel direction, and produce a picture free from false color. If the object-glass had been rendered perfectly achromatic, the bhie rays, after passing through the field-glass, would have been brought to a focus at i, and the red at r; so that an error would be produced, which would have been increased instead of being corrected by the eye-glass. Another advantage of a well-constructed Huyglien- ^^^^^^^J^^^^^^^^ ian eye-piece is, that the image produced ^^^^^^HHH^^^^^B by the meeting of the rays after passing ^^^^HBHIHH^^^^H through the field-glass, is by it rendered ^^^^^^^^^^W^^^^H concave towards the eye-glass, instead of H|^H^^^^n^H|H convex, so that every part of it may be in ^^^^B^SS^H^^^^H focus at the same time, and the field of HH|^H|^H^H^HHH view thereby rendered fiat.^ — Two or ^^|^HH^H^H|^^H^B more Huyghenian Eye-pieces, of differ- ^^M^B^^mIH^H magnifying powers, known as A, ^^HHH^H|^H||B^H C, etc., are usually supplied with a Com- ^^^H^^HI^^HnH^H pound Microscope. The utility of the ^^HM^^H|^^HH|^^H higher powers will mainly depend upon ^^^^^^^^H^^^^^^H the excellence of the Objectives; for when ^HH^^^^H|^^^fl|^B an Achromatic combination of small ^^HH^^HI^HHlj^H aperture, which is sufficiently well cor- ^^HI^^BI^^HH^^H rected to perform very tolerably with a ^KBtHEUKKEtltl^ ' ^ ' shallow ^ eye-piece, is used with „ . „ _ , . „ . an eye-piece of hisfher maffnifyinsr power Section of Huygheman Eye-piece , a £ c ;\ y \ adanted to over-corrected Achroma- (commonly spokeu ot as a ^ deeper onc), tic Objectives. image may lose more in brightness and in definition than is gained by its amplification; whilst the image given by an Objective of large angular aperture and very perfect correction, shall sustain so little loss of light or of definition by ' deep eye-piecing,^ that the increase of magnifying power shall be almost clear gain. Hence the modes in which different Objectives of the same power, whose performance with shallow eye-pieces is nearly the same, are respectively affected by deep eye-pieces, afford a good test of their respective merits; since any defect in the corrections is sure to be brought out by the higher amplification of the image, whilst a deficiency of aperture is manifested by the want of light. — The working Microscopist will generally find the A eye-piece most suitable, B being occasionally employed Avhen a greater power is required to separate details, whilst 0 and other still deeper are useful for the purpose of testing the goodness of Objectives, or for special investi- gations requiring the highest amplification with Objectives of the finest quality. When great penetration or ' focal depth ^ is required, low Ob- jectives and deep Eye-pieces will often be found convenient. 28. For viewing large flat objects, such as transverse sections of Wood (Chap, ix.) or of Echinus-spines (Plate ii. Fig. 1), under low magnifying powers, the Eye-piece known as Kellner^s may be employed with advantage. In this construction, the field-glass, which is a double- convex lens, is placed in the focus of the eye-glass, without the interposi- tion of a diaphragm; and the eye-glass is an achromatic combination of a ^ Those who desire to gain more information upon this subject than they can from the above notice of ic, may be referred to Mr. Varley's investigation of the properties of the Huyghenian Eye-piece, in the 51st volume of the " Transactions of the Society of Arts;" and to tlie article Microscope," by Mr. Ross, in the Penny Cyclopaedia," reprinted, with additions, in the "English Cyclopaedia." OPTICAL PRINCIPLES OF THE MICROSCOPE. 25 plano-concave of flint with a double-conyex of crown, which is slightly under-corrected, so as to neutralize the over-correction given to the Ob- jectives for use with Huyghenian eye-pieces (§ 27). A flat well-illumi- nated field of as much as fourteen inches in diameter may thus be ob- tained with very little loss of light; but, on the other hand, there is a certain impairment of defining power, which renders the Kellner eye- piece unsuitable for objects presenting minute structural details; and it is an additional objection, that the smallest speck or smear upon the sur- face of the field glass is made so unpleasantly obvious, that the most care- ful cleansing of that surface is required every time that this Eye-piece is used. Hence it is better fitted for the occasional display of objects of the character already specified than for the ordinary wants of the working Microscopist. 29. A solid Eye-piece made on the principle of the ' Stanhope' lens (§ 24) is sometimes used in place of the ordinary Huyghenian, when high magnifying power is required for testing the performance of Objectives. The lower surface, which has the lesser convexity, serves as a Afield-glass;' whilst the image formed by this is magnified by the highly convex upper surface to which the eye is applied; the advantage supposed to be derived from this construction lying in the abolition of the plane surfaces of the two lenses of the ordinary eye-piece. A ' positive ' or Eamsden's Eye- piece — in which the field glass, whose convex side is turned upwards, is placed so much nearer the eye-glass that the image formed by the Objec- tive lies below instead of above it, — was formerly used for the purpose of Micrometry; a divided glass being fitted in the exact plane occupied by the image, so that its scale and that image are both magnified together by the lenses interposed between them and the eye. The same end, how- ever, may be so readily attained with the Huyghenian eye-piece (§ 91), that no essential advantage is gained by the use of that of Eamsden, the field of which is distinct only in its centre. 4. Stereoscopic Binocular Microscope. 30. The admirable invention of the Stereoscope by Professor Wheat- stone, has led to a general appreciation of the value of the conjoint use of both eyes in conveying to the mind a notion of the solid forms of objects, such as the use of either eye singly does not generate with the like certainty or effectiveness. And after several attempts, which were attended with various degrees of success, the principle of the Steroscope has now been applied to the Microscope, with an advantage which those only can truly estimate, who (like the Author) have been for some time accustomed to work with the Stereoscopic Binocular^ upon objects that are peculiarly adapted to its powers. As the result of this application cannot be rightly understood without some knowledge of one of the fundamental principles of Binocular vision, a brief account of this will be here introduced. — All vision depends in the first instance on the formation of a picture of the object upon the retina of the Eye, just as the Camera Obscura forms a picture upon the ground glass placed in the focus of its lens. But the two images that are formed by the two eyes respectively, of any solid object that is placed at no great distance in front of them, are far from ^ It has become necessary to distinguish the Binocular Microscope which gives true Stereoscopic effects by the combination of two dissimilar picture, from a Binpcular which simply enables us to look with both eyes at images which are essentially identical (§ 81). 26 THE MICROSCOPE AND ITS REVELATIONS. being identical; the perspective projection of the object varying with the point of view from which it is seen. Of this the reader may easily con- vince himself, by holding up a thin book in such a position that its back shall be at a moderate distance in front of the nose, and by looking at the book, first with one eye and then with the other; for he will find that the two views he thus obtains are essentially different, so that if he were to represent the book as he actually sees it with each eye, the two pictures would by no means correspond. Yet on looking at the object with the two eyes conjointly, there is no confusion between the images, nor does the mind dwell on either of them singly; but from the blending of the two a con- ception is gained of a solid projecting body, such as could only be other- wise acquired by the sense of Touch. Now if, instead of looking at the solid object itself, we look with the right and left eyes repectively at pic- tures of the object, corresponding to those which would be formed by it on the retinae of the two eyes if it were placed at a moderate distance in front of them, and these visual pictures are brought into concidence, the same conception of a solid projecting form is generated in the mind, as if the object itself were there. The Stereoscope — whether in the forms originally devised by Prof. Wheatstone, or in the popular modification long subsequently introduced by Sir D. Brewster — simply serves to bring to the two eyes, either by reflection from mirrors, or by refraction through prisms or lenses, the two dissimilar pictures which would accurately represent the solid object as seen by the two eyes respectively; these being thrown on the two retinae in the precise positions they would have occu- f ied if formed there direct from the solid Object, of which the mental mage (if the pictures have been correctly taken) is the precise counter- part.^ Thus in Fig. 16 the upper pair of pictures (a, b), when combined in the Stereoscope,^ suggest the idea of a projecting truncated Pyramid, ^ Although it is a comparatively easy matter to draw in outline two different perspective projections of a Geometrical Solid, such as those which are repre- sented in Fig. 16, it would have been quite impossible to delineate landscapes, buildings, figures, etc., with the same precision; and the Stereoscope would never have obtained the appreciation it now enjoys, but for the ready means supplied by Photography of obtaining simultaneous pictures, perfect in their perspective, and truthful in their lights and shades, from two different points of view so selected as to give an effective Stereoscopic combination. * This combination may be made without the Stereoscope, by looking at these OPTICAL PRINCIPLES OF THE MICROSCOPE. 27 with the small square in the centre, and the four sides sloping equally away from it; whilst the combination of the lower pair, c, D (which are identical with the upper, but are transferred to o})posite sides), no less vividly brings to the mind the visual conception of a receding Pyramid, still with the small square in the centre, but the four sides sloping equally towards it. 31. Thus we see that by simply crossing the picture in the Stereos- cope, so as to bring before each eye the picture taken for the other, a ^conversion of relief^ is produced in the resulting solid image; the pro- jecting parts being made to recede, and the receding parts brought into relief. In like manner, when several objects are combined in the same crossed pictures, their apparent relative distances are reversed; the remo- ter being brought nearer, and the nearer carried backwards; so that (for example) a Stereoscopic photograph representing a man standing in front of a mass of ice, shall, by the crossing of the picture, make the figure appear as if imbedded in the ice. A like conversion of relief may also be made m the case of actual solid objects by the use of the Pseudoscope; an instrument devised by Prof. Wheatstone, which has the effect of re- versing the perspective projections of objects seen through it by the two eyes respectively; so that the interior or a basin or jelly-mould is made to appear as a projecting solid, while the exterior is made to appear hollow. Hence it is now customary to speak of stereoscopic Vision as that m which the conception of the true natural relief of an object is called-up in the mind, by the normal combination of the two perspective projections formed of it by the right and left eyes respectively; whilst hj pseudosco- pic vision, we mean that • conversion of relief^ which is produced by the combination of two reversed perspective projections, whether these be obtained directly from the object (as by the Pseudoscope), or from ^crossed ^ pictures (as in the Stereoscope). It is by no means every solid object, however, or every pair of stereoscopic j)ictures, which can become the subject of this conversion. The degree of facility with which the ^ converted ' form can be apprehended by the Mind, appears to have great influence on the readiness with which the change is produced. And while there are some objects — the interior of a plaster mask of a face, for example — which can always be ^ converted^ (or turned inside-out) at once, there are others which resist such conversion with more or less of persistence.* 32. Now it is easily shown theoretically, that the picture of any pro- jecting object seen through the Microscope with only the right-hsmd half of an objective having an even moderate angle of aperture, must differ sensibly from the picture of the same object received through the fe/'^-hand of the same objective; and further, that the difference between such pictures must increase with the angular aperture of the objective. This difference may be practically made apparent by adapting a ^ stop ' to the objective, in such a manner as to cover either the right or the left half of its aperture; and by then carefully tracing the outline of the ob- ject as seen through each half. But it is more satisfactorily brought into view by taking two Photographic pictures of the object, one through each lateral half of the objective; for these pictures when properly paired in the Stereoscope, give a magnified image in relief y bringing out on a large figures with the axis of the eyes brought into convergence upon a somewhat nearer point, so that A is made to fall on B, and c on D. 1 For a fuller discussion of this subject, see the Author's Mental Physiology," §§ 168-170. 28 THE MICROSCOPE AND ITS REVELATIONS. scale the solid form of the object from which they were taken. What is needed, therefore, to give the true Stereoscopic power to the Microscope, is a means of so bisecting the cone of rays transmitted by the objective, that of its two lateral halves one shall be transmitted to the right and the other to the left eye. If, however, the image thus formed by the right half of the objective of a Compound Microscope were seen by the right eye, and that formed by the left half were seen by the left eye, the result- ant conception would be not stereoscopic hut 2)seudosco2nc; the projecting parts being made to appear receding, and vice versa. The reason of this is that as the Microscope itself reverses the picture (§ 26), the rays pro- ceeding through the right and the left hand halves of the objective must be made to cross to the left and the right eyes respectively, in order to correspond with the direct view of the object from the two sides; for if this second reversal does not take place, the effect of the first reversal of the images produced by the Microscope exactly corresponds with that produced by the ^crossing ' of the pictures in the Stereoscope, or by that reversal of the two perspective projections formed direct from the object, which is effected by the Pseudoscope (§ 31). It was from a want of due appreciation of this principle (the truth of which can now be practically demonstrated, § 38), that the earlier "attempts at producing a Stereosco- pic Binocular Microscope tended rather to produe a ^pseudoscopic con- version ' of the objects viewed by it, than to represent them in their true relief Arrangement of Prisms in Nachet's Stereoscopic Nachet's Stereoscopic Binocular. Binocular Microscope. 33. NacheVs Stereoscopic Binocular, — The first really satisfactory solution of the problem was that worked out by MM. Nachet; whose original Binocular was constructed on the method shown in Fig. 17. The cone of rays issuing from the back lens cf the objective masts the flat surface of a prism {p) placed above it, who section is an equilat- OPTICAL PRINCIPLES OF THE MICROSCOPE. 29 eral triangle; and is divided by reflection within this prism into two lat- eral halves, which cross each other in its interior. The rays a b that form the right half of the cone, impinging very obliquely on the internal face of the prism, suffer total reflection (§ 2), emerging through its left side perpendicularly to its surface, and therefore undergoing no refrac- tion; whilst the rays a' V forming the left half of the cone, are reflected in like manner towards the right. Each of these pencils is received by a lateral prism, which again changes its direction, so as to render it par- allel to its original course; and thus the two halves a b and a' V of the original pencil are completely separated from each other, the former being received into the left-hand body of the Microscope (Fig. 18), and the latter into its right-hand body. These two bodies are parallel; and, by means of an adjusting screw at their base, which alters the distance between the central and the lateral prisms, they can be separated from or approximated towards each other, so that the distance between their axes can be brought into exact coincidence with the distance between the axes of the eyes of the individual observer. — This instrument gives true Stereoscopic projection to the conjoint image formed by the mental fu- sion of the two distinct pictures; and with low powers of moderate angu- lar aperture its performance is highly satisfactory. There are, however, certain drawbacks to its general utility. First, every ray of each jDencil suffers tii^o reflections, and has to pass through /tv^/r surfaces; this neces- sarily involves a considerable loss of light, with a further liability to the impairment of the image by the smallest want of exactness in the form of either of the prisms. Second, the mechanical arrangements requisite for varying the distance of the bodies, involve an additional liability to derangement in the adjustment of the prisms. Third, the instrument can only be used for its own special purpose; so that the observer must also be provided with an ordinary single-bodied Microscope, for the ex- amination of objects unsuited to the powers of his Binocular. Fourth, the parallelism of the bodies involves parallelism of the axes of the ob- server's eyes, the maintenance of which for any length of time is fatiguing. 34. Wenhani^s Stereoscopic Binocular. — All these objections are over- come in the admirable arrangement devised by the ingenuity of Mr. Wenham; in whose Binocular the cone of rays proceeding upwards from the objective is divided by the interposition of a prism of the peculiar form shown in Fig. 19, so placed in the tube which carries the objec- tive (Figs. 20, 21 a) as only to interrupt one half, a c, of the cone, the other half, a b, going on continuously to the eye-piece of the prin- cipal or right-hand body R, in the axis of which the objective is placed. The interrupted half of the cone (Fig. 19, a), on its entrance into the prism, is scarcely subjected to any refraction, since its axial ray is per- pendicular to the surface it meets; but within the prism it is subjected to two reflections at b and c, which send it forth again obliquely in the line d towards the eye-piece of the secondary, or left hand body (Fig. 20, L); and since at its emergence its axial ray is again perpendicular to the surface of the glass, it suffers no more refraction on passing out of the prism than on entering it. By this arrangement, the image received by the right eye is formed by the rays which have passed through the left half of the objective, and have come on without any interruption whatever; whilst the image received by the left eye is formed by the rays which have passed through the right half of the objective, and have been subjected to two reflections within the prism, passing through only two surfaces of glass. The adjustment for the variation of distance 30 THE MICROSCOPE AND ITS REVELATIONS. between the axes of the eyes in different individuals, is made by draw ing-out or pushing-in the eye- pieces, which are moved consentaneously Fig. 20 Fig. Wenham's Prism. Wenham's Stereoscopic Binocular Microscope. by means of a milled-head, as shown in Fig. 21. — Jfow, although it may be objected to Mr. Wenham's method (1), that as the rays which pass through the prism and are obliquely reflected into the secondary body, traverse a longer distance than those which pass-on uninterrupedly into the principal body, the picture formed by them will be somewhat larger than that v/hich is formed by the other set; and (2), that the picture formed by the rays which have been subjected to the action of the prism must be inferior in distinctness to that formed by the uninterrupted half of the cone of rays, — these objections are found to have no practicable weight. For it is well known to those who have experimented upon the phenomena of Stereoscopic vision (1), that a slight difference in the size of the two pictures is no bar to their perfect combination; and (2), that if one of the pictures be good, the full effect of relief is given to the image, even though the other picture be faint and imperfect, provided that the outlines of latter are sufficiently distinct to represent its perspec- tive projection. Hence if, instead of the two equally half-good pictures which are obtainable by MM. Nachet's original construction, we had in Mr. Wenham's one good and one indifferent picture, the latter would be decidedly preferable. But, in point of fact, the deterioration of the sec- Olid picture in Mr. Wenham's arrangement is less considerable than that of hoth pictures in the original arrangement of MM. Nachet; so that the optical performance of the Wenhani Binocular is in every way superior. It has, in addition, these further advantages over the preceding, — First, the greater comfort in using it (especially for some length of time to- gether), which results from the convergence of the axes of the eyes at their usual angle for moderately-near objects; second, that this Binocu- lar arrangement does not necessitate a special instrument, but may be OPTICAL PRINCIPLIS OF THE MICROSCOPE. 31 applied to any Microscope which is capable of carrying the weight of the secondary body; the prism being so fixed in a movable frame that it may in a moment be taken out of tlie tube or replaced therein, so that when it has been removed, the principal body acts in every respect as an ordinary Microscope, the entire cone of rays ])assing uninterruptedly into it; and thirds that the simplicity of its construction renders its derange- ment almost impossible.^ 35. Stephenson'^ s Binocular. — A new form of Stereoscopic Binocular has been recently introduced by Mr. Stephenson,^ which has certain ad- vantages over both the preceding. — The cone of rays passing upwards from the object-glass, meets a pair of prisms (a a, Fig. 22) fixed in the tube of the microscope immediately above the posterior combination of the objective, so as to catch the light-rays on their emergence from it; these it divides into two halves, each of which is subjected to internal reflection from the inner side of the prism through which it passes; and the slight separa- tion of the two prisms at their upper end, gives to the two pencils B B, a divergence which car- ries them through two obliquely-placed bodies to their respective eye-pieces. By this internal reflection, a lateral reversal is produced, which antagonizes the lateral reversal of the Micro- scopic image; so that each eye receives the image formed by its own half of the objective, in the position required for the production of Stereo- scopic relief by the mental combination of the two. As the cone of rays is equally divided by the two prisms, and its two halves are similarly acted-on, the two picture are equally illuminat- ed, and of the same size; while the close ap- proximation of the prisms to the back lens of the objective enables even higher powers to be used Avith very little loss of light or of definition, provided that the angles and surfaces of the prisms Stephenson's Binocular Prisms. are worked with exactness. And as the two bodies can be made to con- verge at a smaller angle than in the Wenham arrangement, the observer looks through them with more comfort. But Mr. Stephenson's ingenious arrangement — which was first worked-out practically by the late Thomas Eoss, and has since been very successfully constructed by Brown- ing — is liable to the great drawback of not being con- vertible (like Mr. Wenham's) into an ordinary Mono- cular, by the withdrawal of a prism; so that the use of this form of it will be probably restricted to those who desire to work stereoscopically with high powers, — In order to avoid slight errors arising from the im- pinging of the central ray of the cone, at its emergence from the objective, against the double edge of the ^ The Author cannot allow this opportunity to pass without expressing his sense of the liberality with which Mr. Wenham freely presented to the Public this im- portant invention, by which there can be no doubt that he might have largely profited if he had chosen to retain the exclusive right to it. 2 "Monthly Microscopical Journal," Vol. iv. (1870), p. 61, and Vol. vii. (1872), p. 167. 32 THE MICROSCOPE AND ITS REVELATIONS. prism combination, Mr. Stephenson has devised a special form of sub- stage Condenser (also made by Mr. Browning), which causes the illumi- nating rays to issue from the object in two separate pencils, which will strike the surfaces of the two prisms. This consists of two deep cylin- drical lenses A and B, whose focal lengths are as 2.3 to 1, having their curved faced opposed to each other, as shown in section at c; the larger and less convex being placed with its plane side downwards, so as to re- ceive light from the mirror, or (which is preferable) direct from a lamp. Under this combination slides a movable stop, with two circular open- ings, as shown in Fig. 24. The lamp being placed in front of the instru- Double Stop for Stephenson Binocular. Stephenson's Erecting Prism. ment,'the two apertures admit similar pencils of light from it; so that each eye receives a completely equal illumination^ and no confusion can occur from the impinging of the rays on the lower edges of the prisms. With this arrangement the Podura-markings are shown as figured by the late Eichard Beck (Plate ii., fig. 2); while the curvatures of the scale come out with the distinctness peculiar to Binocular vision. 36. But one of the greatest advantages attendant on Mr. Stephenson's construction, is its capability of being combined with an erecting arrange- ment; which renders it applicable to purposes for which the Wenham Binocular cannot be conveniently used. By the interposition of a plane silvered mirror, or (still better) of a reflecting prism (Fig. 25), above the tube containing the binocular prisms, each half of the cone of rays is so deflected, that its image is reversed vertically ; the rays entering the prism through the surface c B, being reflected by the surface A B, so as to pass out again by the surface A c in the direction of the dotted lines. Thus the right and the left half cones are directed respectively into the right and the left bodies, which are inclined at a convenient angle, as shown in Fig. 26; so that — the stage being horizontal — the observer can look at his object at the inclination which he finds most comfortable. The angle to which the prism is worked can be varied to suit individual requirements; but if it should be desired to use the instrument with Polarized light, it will be found advantageous that the reflection from the surface A b should be at the polarizing angle of 56f since, by substi- tuting for the silvered mirror or prism a highly polished mirror of black glass, this will then act as an analyzer, with some decided advantages over the Nicol prism, except in being incapable of rotation. — The great value OPTICAL PRINCIPLES OF THE MICKOSCOPE. 33 of the Erecting Binocular consists in its Dia. 2tt applicability to the picking out of very minute objects, such as Diatoms^ Polycyst- ^ ina, or Foraminifera; and to the pro-* secution of minute dissections, especially when these have to be carried on in fluid. No one who has only thus worked monocii- larly, can appreciate the guidance derivable from iinocular vision, when once the habit of working with it has been formed. 37. Tolles^ Binocular Eyepiece, — An in- genious Eye-piece has been constructed by Mr. Tolles (Boston, U. S.), which, fitted into the body of a Monocular Microscope, converts it into an Erecting Stereoscopic Binocular. This conversion is effected by ^ , ^, ^. .,• « i p ' ^ Stephenson's Erecting Binocular, the interposition oi a system oi prisms similar to that originally devised by MM. Nachet (Eig. 17), but made on a larger scale, between an ^erector (resembling that used in the eye-piece of a day telescope) and a pair of ordinary Huyghenian eye- pieces; the central ov dividing prism being placed at or near the plane of the secondary image formed by the erector, w^hile the two eye-pieces are placed immediately above the two lateral prisms; and the combination thus making that division in the pencils forming the secondary image, which in the Nachet Binocular it makes in the pencils emerging from the objective. — As all the image-forming rays have to pass through the two surfaces of four lenses and two prisms, besides sustaining two internal reflections in the latter, it is surprising that Prof. H. L. Smith — while admitting a loss of light — should feel able to speak of the definition of this instrument as not inferior to that of either the Wenham or the Nachet Binocular. It is obviously a great advantage that this Eye-pieoe can be used with any microscope, and with Objectives of high power; but as its effectiveness must depend upon extraordinary accuracy of workmanship, its cost must necessarily be great. ^ 38. Nachefs Stereo-pseitdoscopic Binocular. — An ingenious modifi- cation of Mr. Wenham's arrangement has been introduced by MM. ^'achet; which has the attribute altogether peculiar to itself, of giving to the image either its true Stereoscopic projection, or a Pseudoscopic ^ con- version of relief,' at the will of the observer. This is accomplished by the use of two prisms, one of them (Eig. 27, a) placed over the cone of rays proceeding upwards from the objective, and the other (b) at the base of the secondary or additional body, which is here placed on the right (Eig. 28). The prism A has its upper and lower surfaces parallel; one of its lateral faces is inclined at an angle of 45°, whilst the other is vertical. When this is placed in the position 1, so that its inclined surface lies over the left half {I) of the cone of rays, these rays, entering the prism perpendicularly (or nearly so) to its inferior plane surface, under- go total reflection at its oblique face, and being thus turned into the horizontal direction, emerge through the vertical surface at right angles to it. They then enter the vertical face of the other prism b; and, after suffering reflection within it, are transmitted upwards into the right-hmd * See American Journal of Science." vol. xxxviii. (1864), p. Ill, and vol. xxxix. (1865), p. 212; and "Monthly Microsc. Journal," vol. vi. (1871), p. 45. 3 34: THE MICROSCOPE AND ITS REVCLATIONS. body r', passing out of the prism perpendicularly to the plane of emersion, which has such an inclination that the right-hand or secondary body (r, Fig. 28) may diverge from the left or principal body at a suitable angle. On the other hand, the right half (r) of the cone of rays passes upwards, without essential interruption, through the two parallel surfaces of the prism A, into the left-hand body {V), and is thus crossed by the other in the interior of the prism. But if the prism A be pushed over towards the right (by pressing the button a, Fig. 28), so as to leave the left half of the objective uncovered (as shown in Fig. 27, 2), that half \V) of the Arrangement of Prisms in Nachet's Stereo-pseudoscopic Binocular:—!, for Stereoscopic; 2, fop Pseudoscopic effect. cone of rays will go on without any interruption into the left'\\w.^ body (?'), whilst the right half (r r') will be reflected by the oblique face of the prism into the horizontal direction, will emerge at its vertical face, and, being received by the second prism B, will be directed by it into the right-h.^mdi body (r'). — Now, in the first position, the two Ixalves of the cone of rays being made to cross into the opposite bodies, true Stereo- scopic relief is given to the image formed by their recombination, just as in the arrangements previously described. But when, in the second position, each half of the cone passes into the body of its own side, so that the reveasal of the images produced by the Microscope itself (§ 26) is no longer corrected by the crossing of the two pencils separated by the prism A, a Pseudoscopic effect, or ^conversion of relief,^ is produced, the projections of the surface of the object being represented as hollows, and its concavities being turned into convexities. The suddenness with which this conversion is brought about, without any alteration in the position either of the object or of the observer, is a phenomenon which no intelligent person can witness without interest; whilst it has a very special value for those who study the Physiology and Psychology of Binocular vision.^ — As originally constructed, the adjustment for dis- ' The result of the numerous appHcations which the Author has made of this instrument to a great variety of Microscopic objects has led to a confirmation of the principle of Pseudoscopic vision, stated at the conclusion of § 31. — Where, as in the case of the saucer-like disks of the Arachnoidiscus (Plate xii.), the real and the converted forms are equally familiar, the * conversion ' either of the convex exterior or the concave interior is made both suddenly and completely. In more complex and less familiar forms, on the other hand, the conversion frequently requires time; being often partial in the first instance, and only gradually becom- ing complete. And there are some objects which resist conversion altogether, the only effect being a confusion of the two images. OPTICAL PRINCIPLES OF THE MICROSCOPE. 35 tance between the eyes was made by giving a horizontal traversing motion to the prism b and the secondary body placed above it, by means of a screw action. But this method was open to the two objections that the focal distance of the secondary body was thereby altered, and that the traversing fittings were liable to become loose by wear. To meet these, M. Nachet devised the construction represented in Fig. 28; in which the adjustment of the distance between the eye-pieces is effected by altering the angle of conver- gence between the bodies. This is done by turning the screw v, which is furnished with two threads of different speeds, whereby an inclination is given to the prism equal to half the angular displacement of the tube; an arrangement necessitated by the fact that the displacement of the rays reflected by a rotat- ing surface is double the angle described by that surface.^ — As an ordinary working instrument, how- ever, this improved Nachet Binocular can scarcely be equal to that of Wenham or Stephenson; whilst it must be regarded as inferior to the former in the following particulars : First, that as the uninterrupted half of the cone of rays (when the interposed prism is adjusted for Stereoscopic vision) has to pass through the hvo plane surfaces of the prism, a certain loss of light and deterioration of the picture are neces- sarily involved; whilst, as the interrupted half of the cone of rays has to pass through four surfaces, the picture formed by it is yet more unfavorably affected; second , that as power of motion must be given to loth prisms — to A, for the reversal of the images, and to b for the adjustment of the distance between the two bodies — there is a greater liability to derangement.^ It does not give the equal illumination of Mr. Stephenson's, is less free from optical error, and cannot, like his, be used with high powers. 39. The Stereoscopic Binocular is put to its most advantageouse use, when applied either to opaque objects of whose solid forms we are desirous of gaining an exact appreciation, or to transparent objects which have such a thickness as to make the accurate distinction between their nearer and their more remote planes a matter of importance. That its best and truest effects can only be obtained by objectives not exceeding 40^ of angular aperture, may be shown both theoretically and practically. Taking the average distance between the pupils of the two eyes as the base of a triangle, and any point of an object placed at the ordinary reading distance as its apex, the vertical angle inclosed between its two sides will be from 12° to 15°; which, in other words, is the angle of divergence between the rays preceding from any point of an object at Nachet's Stereo-pseudo- scopic Microscope. 1 Monthly Microscopical Journal," Vol. i. (1869), p. 31. 2 M. Nachet's arrangement, like Mr. Wenham's, can be adapted to any existing Microscope ; and it seems peculiarly suitable to those of French or German con- struction, in which the body is much shorter than in the ordinary English models. For in the application of the Wenham arrangement to a short Microscope, the requisite distance between the eye-glasses of its two bodies can only be obtained by making those bodies converge at an angle so wide as to produce great discom- fort in the use of the instrument, from the necessity of maintaining an unusual degree of convergence between the axes of the eyes. 36 THE MICROSCOPE AND ITS REVELATIONS. the ordinary reading distance to the two eyes respectively. This angle, therefore, represents that at which the two pictures of an object should be taken in the Photographic Camera, in order to produce the effect of ordinary binocular vision without exaggeration; and it is the one which is adopted by Portrait-photographers, who have found by experience that a smaller angle makes the image formed by the combinations of the pictures appear too flat ^ whilst a larger angle exaggerates its projection. Now, in applying this principle to the Microscope, we have to treat the two lateral halves (l, e, Fig. 29) of the objective as the two separate lenses of a double portrait-camera; and to consider at what angle each half should be entered by the rays passing through it to form its picture.' To any one acquainted with the principles of Optics, it must be obvious that the picture formed by each half of the objective must be (so to speak) an average or general resultant of the dissimilar pictures formed by its different parts. Thus, if we could divide the lateral halves or semi- lenses L, R, of the objective by vertical lines into the three bands ale and a' V and could stop off the two corresponding bands on either side, so as only to allow the light to pass through the remaining pair, we should find that the two j)ictures we should receive of the object would vary sensibly, according as they are formed by the bands a a', h J', or c c\ ^ The writer has been surprised to find that the advantages of the Stereoscopic Binocular have been treated by certain Microscopists of eminence as altogether chimerical; no real difference (they assert) being discernible between the right-hand and the left-hand pictures. — This assertion is obviously placed upon the limita- tion of the use of the instrument to thin transparent objects. It is where the surface is uneven (as is the case with most Opaque objects^ or where a Trans- parent object shows different structures in different planes of its thickness (as in injected preparations), that the special value of the Binocular shows itself. The dissimilarity of the two pictures of such objects received through the two halves of the objective, was long since demonstrated by Mr, Wenham, who, by covering with a diaphragm, first the right and then the left half of an objective of 2-3ds inch focus and 28° aperture, and carefully drawing the two images thus obtained, found them to be such as would combine stereoscopically, so as to bring out the object in reUef. See " Transact, of Microsc. Soc," N, S., Vol. ii. (1854), p. 1. OPTICAL PRINCIPLES OF THE MICROSCOPE. 37 For, supposing the pictures taken through the bands h V to be suffi- ciently dissimilar in their prospective projections, to give, Avhen com- bined in the Microscope, a sufficient but unexaggerated Stereoscopic relief, those taken through the bands a a' on either side of the centre would be no more dissimilar than two portraits taken at a very small angle between the cameras, and their combination would very inade- quately bring out the effect of relief; whilst, on the other hand, the two j)ictures taken through the extreme lateral bands c g% would differ as widely as portraits taken at too great an angle of divergence between the cameras, and their combination would exaggerate the actual relief of the object. Now, in each of the lateral halves, a spot v v' may be found by mathematical computation,* which may be designated the visual centre of ^ the whole Semi-lens; that is, the spot which, if all the rest of the semi- lens were stopped-off, would form a picture most nearly corresponding to that given iDy the whole of it. This having been determined, it is easy to ascertain what should be the angle of aperture (op Fig. 30) of the entire lens, in order that the angles v p v' between the ' visual centres' of its two halves should be 15^. The investigation of this question having been kindly undertaken for the author by his friend Dr. Hirst, the conclusion at which he arrived was that the angle of aperture of the entire lens should be about 36.6"^. This, which he gave as an approximate result only (the requisite data for a complete Mathematical solution of the question not having yet been obtained), harmonizes most remarkably with the results of experimental observations made upon opaque objects of knoiun shape, with Objectives of different angular apertures; so that the Stereoscoj)ic images produced by the several objectives may be compared, not only with each other, but with the actual forms which they ought to present. No better objects can be selected for this purpose than those which are perfectly spherical ; such as various globular forms of the Polycystina (Plate xix.), or the Pollen- grains of the Malvacece and many other Flowering-plants. When either of these is placed under a Stereoscopic Binocular, provided with an objective of half-inch or 4-lOths inch focus having an angular aperture of 80° or 90°, the effect of projection is so greatly exaggerated, that the side next the eye, instead of resembling a hemisphere, looks like the small end of an egg. If, then, the aperture of such an objective be reduced to 60° by a diaphragm placed behind its back lens, the exaggera- tion is diminished, though not removed; the hemispherical circle now looking like the large end 'of an egg. But if the aperture be further reduced to 40° by the same means, it is at once seen that the hemi- spheres turned towards the eye are truly represented; the effect of spherical projection being quite adequate, without being in the least exag- gerated. Hence it may be confidently affirmed — alike on theoretical and on practical grounds — that when an objective of wider angle than 40° is used with the Stereoscopic Binocular, the object viewed by it is repre- sented in exaggerated relief, so that its apparent form must be more or less distorted.^ — There are other substantial reasons, moreover, why ^ This position has been contested by observers who have used high powers binocularly with transparent objects, and who, in their zeal for large angles of aperture, affirm that no exaggeration of Stereoscopic effect is produced by the combination of the two pictures thus obtained. But it seems to be forgotten that such objects cannot afford the actual measure of Stereoscopic effect, which is given by opaque objects of known form — as above described. And, so far as tihe Author's experience extends, every competent observer who makes use of a good 38 THE MICROSCOPE AND ITS REVELA.TIONS. Objectives of limited angle of aperture should be preferred (save in particular cases) for use with the Stereoscopic Binocular. ^ As the special value of this instrument is to convey to the mind a notion of the solid forms of objects, and of the relations of their parts to each other, not merely on the same, but on different planes, it is obvious that those Objectives are most suitable to produce this effect, which possess the greatest amount of penetration or focal depth ; that is, which most dis- tinctly show, not merely what is precisely in the focal plane, but what lies nearer to or more remote from the objective. Now, as will be explained hereafter (§ 158, ii.), increase of the angle of apei'ture is necessarily attended with diminution of ^penetrating' power; »o that an objective , of 60° or 80° of aperture, though exhibiting minute surface-details which an objective of 40° cannot show, is much inferior to it in suitability to convey a true conception of the general form of any object, the parts of which project considerably above the focal plane or recede below it. 40. In concluding these general observations upon the use of the Ste- reoscopic Binocular, the Author would draw attention to two important advantages he has found it to possess; his own experience on these points being fully confirmed by that of others. — In the^r^^ place, the penetrat- ing poioer or focal depth of the Binocular is greatly superior to that of the Monocular microscope; so that an object whose surface presents consider- able inequalities is very much more distinctly seen with the former than with the latter. The difference may in part be attributed to the practical reduction in the angle of aperture of the Objective, which is produced by the division of the cone of rays transmitted through it into two halves; so that the picture received through each half of an Objective of 60^ is formed by rays diverging at an angle of only 30°. But that this optical explanation does not go far to account for the fact, is easily proved by the simple experiment of looking at the object in the first instance through each eye separately (the prism being in place), and then with both eyes together; the distinctness of the parts which lie above and beneath the focal plane being found to be much greater when the two pictures are combined, than it is in either of them separately. In the absence of any adequate optical explanation of the greater range of focal depth thus shown to be possessed, by the Stereoscopic Binocular, the Author is inclined to attribute it to an allowance for the relative distances of the parts, which seems to be unconsciously made by the mi7id of the ob- server, when the solid image is shaped out in it by the combination of the two pictures. — This seems the more likely from the second fact to be half-inch Objective of 40° aperture — resembling the one first constructed to his order by Messrs. Powell and Lealand, and now procurable from several excellent rnakers— in the study of Folycystina, the smaller Foraminifera, or the larger discoidal Diatoms, viewed as opaque objects, soon becomes sensible of its advan- tage over Objectives of the same power but of larger angular aperture, in giving (1) unexaggerated relief, (2) much greater focal depth, and (3) such a working distance as enables side-illumination to be conveniently used. Having lately had occasion to give much attention to the structure and development of Isthmia (Chap. VII.), the writer has found great advantage from the use of a l-4th objective, constructed by Zeiss, of what will be considered by many the absurdly low angle of 40°; the truth of the conception it gives of the solid forms of the frustules (when viewed as opaque objects), which is capable of easy verification, being in striking contrast with the violent exaggeration of relief which is pro- duced when the same objects are similarly viewed through a l-4th inch of 90° or 120° aperture. Doubtless the elementary structure of the frustule can only be properly studied by an Objective of large angle; but this is an altogether different inquiry. OPTICAL PRINCIPLES OF THE MICROSCOPE. 39 now me'iitioned: namely, that when the Binocular is employed upon objects suited to its powers, the prolonged use of it is attended with very much less fatigue than is that of the Monocular Microscope. This, again, may be in some degree attributed to the division of the work between the two eyes; but the Author is satisfied that, unless there is a feeling of dis- comfort m the eye itself, the sense of fatigue is rather mental than visual, and that it proceeds from the constructive effort which the observer has to make, who aims at realizing the solid form of the object he is examin- ing, by an interpretation based on the flat picture of it presented by his vision, aided only by the use of the focal adjustment, which enables him to determine what are its near and what its remote parts, and to form an estimate of their difference of distance. Now, a great part of this con- structive effort is saved by the use of the Binocular; which at once brings before the Mind's eye the solid image of the object, and thus gives to the observer a conception of its form usually more complete and accurate than he could derive from any amount of study of a Monocular picture.* ' It has happened to the Author to be frequently called on to explain the ad- vantages of the Binocular to Continental (especially German) savans, who had not been previously acquainted with the instrument. And he has been struck with finding that when he exhibited to them objects with which they had already become familiar by careful study, and of whose solid forms they had attained an accurate conception, they perceived no advantage in the Stereoscopic combina- tion, seeiiig such objects with it (visually) just as they had been previously accus- tomed to see them (mentally) without it. But wlien he has exhibited to them suitable objects with which they had not been previously familiarized, and has caused them to look at these in the first instance monocularly, and then stereo- scopically, he has never failed to satisfy them of the value of the latter method, except when some visual imperfection has prevented them from properly appre- ciating it. He may mention that he has found the wing ©f the little Moth known as Zenzera CEsculi, which has an undulating surface, whereon the scales are set at various angles, instead of having the usual imbricated arrangement, a pecu- liarly appropriate object for this demonstration. The general inequality of its surface, and the individual obliquities of its scales, being at once shown by the Binocular, with a force and completeness which could not be attained by the most prolonged and careful Monocular study. THE MICKOSCOFE AND ITS REVELATIONS. OHAPTEE II. CONSTRUCTION OF THE MICROSCOPE. 41. The ojjtical jDrinciples whereon the operation of the Microscope depends having now been explained, we have next to consider the mechani- cal provisions whereby they are brought to bear upon the different purposes which the instrument is destined to serve. And first, it will be desirable to state those general principles which have now received the sanction of universal experience, in regard to the best arrangement of its constituent parts. — Every cornplete Microscope, whether Simple or Com- pound, must possess, in addition to the lens or combination of lenses which affords its magnifying power, a stage whereon the Object may securely rest, a concave mirror for the illumination of transparent objects from beneath, and a condensing -lens for the illumination of opaque objects from above. I. Now, in whatever mode these may be connected with each other, it is essential that the Optical part and the Stage should he so disposed, as either to he altogether free from tendency to vibratio7i, or to vibrate together; since it is obvious that any movement of one, in which the other does not partake, will be augmented to the eye of the observer in proportion to the magnifying power employed. In a badly-constructed instrument, even though placed upon a steady table resting upon the firm floor of a well- built house, when high powers are used, the object is seen to oscillate so rapidly at the slightest tremor — such as that caused by a person walking across the room, or by a carriage rolling-by in the street — as to be fre- quently almost indistinguishable: whereas in a well-constructed instru- ment, scarcely any perceptible effect will be produced by even greater dis- turbances. Hence, in the choice of a Microscope, it should always be subjected to this test, and should be unhesitatingly rejected if the result be unfavorable. If the instrument should be found free from fault when thus tested with high powers, its steadiness with low powers may be assumed; but, on the other hand, though a Microscope may give an image free from perceptible tremor when the lower powers only are employed, it may be quite unfit for use with the higher. — The Author has found no test for steadiness so crucial as the vibration of a paddle-steamer going at full speed against a head-sea; and the result of his comparison between the two principal ^models' generally used in this country will be stated hereafter (§ 49). II. The next requisite is a capahility of accurate adjust^nent to every variety of focal distance, without movement of the object. It is a principle univorsally recognized in the construction of good Microscopes, that the stafj'3 whereon the object is placed should be a fixture; the movement by which the focus is to be adj 3ted being given to the optical portion. This movement should be such as to allow free range from a minute fraction CONSTRUCTION OF THE MICK06COPE. 41 of an inch to three or four inches, with equal j)ower of obtaining a delicate adjustment at any part. It should also be so accurate, that the optic axis of the instrument should not be in the least altered by any move- ment in a yertical direction; so that if an object be brought into the centre of the field with a low power, and a high power be then substi- tuted, the object should be found in the centre of its field, notwithstand- ing the great alteration in the focus. In this way much time may often be saved by employing a low power as a finder for an object to be examined by a higher one; and when an object is being viewed by a succession of powers, little or no readjustment of its place on the stage should be required. For the Simple Microscope, in which it is seldom advantageous to use lenses of shorter focus than l-4th inch (save where ^ doublets^ are employed, § 23), a rack-and-pinion adjustment, if it be made to work both tightly and smoothly, answers sufficiently well; and this is quite adequate also for the focal adjustment of the Compound body, when objectives of low power only are employed. But for any lenses whose focus is less than half-an-inch, a ' fine adjustment,' or ' slow motion,' by means of a screw-movement operating either on the object-glass alone or on the entire body, is of great value; and for the highest powers it is quite indispensable. In some Microscopes, indeed, which are provided with a ^fine adjustment,' the rack-and-pinion movement is dispensed with, the ' coarse adjustment ' being given by merely sliding the body up and down in the socket which grasps it; but this plan is only admissible where, for the sake of extreme cheapness or portability, the instrument has to be reduced to the form of utmost simplicity. III. Scarcely less important than the preceding requisite, in the case of the Compound Microscope, especially with the long body of the ordinary English model, is the capability of being placed in either a vertical or a horizontal position^ or at any angle with the horizon, without deranging the adjustment of its parts to each other, and without placing the eye-piece in such a position as to be inconvenient to the observer. It is certainly a matter of surprise, that some Microscopists, especially on the Continent, should still forego the advantages of the inclined position, these being attainable by a very small addition to the cost of the instru- ment; but the inconvenience of the vertical arrangement is much less when the body of the microscope is short, as in the ordinary Continental model; and there are many cases in which it is absolutely necessary that the stage should be horizontal. This position, however, can at any time be given to the stage of the inclining Microscope, by bringing the optic axis of the instrument into the vertical direction. And even with the stage horizontal, a convenient inclination may be given to the visual axis, not merely by such modifications in general construction as constitute the special features of the erecting Binocular of Mr. Stephenson (§ 36) or the Inverted Microscope of Dr. Lawrence Smith (§ 80), but by the application to the ordinary vertical body of the erecting eye-piece of M. Natchet (§ 86). — In ordinary cases an inclination of the body at the angle of about 55° to the horizon will usually be found most convenient for unconstrained observation; and the instrument should be so constructed, as, when thus inclined, to give to the stage such an elevation above the table, that, when the hands are employed at it, the arms may rest con- veniently upon the table. In this manner a degree of support is attained, which gives such free play to the muscles of the hands, that movements of the greatest nicety may be executed by them; and the fatigue of long- continued observation is greatly diminished. Such minutiae may appear 42 THE MICROSCOPE AKD ITS EEYELATIONS. too trivial to deserve mention; but no practised Microscopist will be slow to acknowledge tlieir value. — For otlier purposes, again, ifc is requisite that the Microscope should be placed horizontally, as when the Camera Lucida is used for drawing or measuring. It ought, therefore, to be made capable of every such variety of position; and the Stage must of course be provided with some means of holding the object, when it is itself placed in a position so inclined that the object would slip down unless sustained. lY. The last principle on which we shall here dwell, as essential to the value of a Microscope designed for ordinary work, is Simplicity in the construction and adjustment of every part. Many ingenious mechanical devices have been invented and executed, for the purpose of overcoming difficulties which are in themselves really trivial. A moderate amount of dexterity in the use of the hands is sufficient to render most of these superfluous; and without such dexterity, no one even with the most com- plete mechanical facilities, will ever become a good Microscopist. Among the conveniences of simplicity, the practised Microscopist will not fail to recognize the saving of time effected by being able quickly to set up and put away his instrument. Where a number of parts are to be screwed together before it can be brought into use, interesting objects (as well as time) are not unfrequently lost; and the same cause will often occasion the instrument to be left exposed to the air and dust, to its great detri- ment, because time is required to put it away; so that a slight advantage on the side of simplicity of arrangement often causes an inferior instru- ment to be preferred by the working Microscopist to a superior one. Yet there is, of course, a limit to this simplification; and no arrangement can be objected-to on this score, which gives advantages in the examination of difficult objects, or in the determination of doubtful questions, such as no simpler means can afford. — The meaning of this distinction will be- come apparent, if it be applied to the cases of the Mechanical Stage and the Achromatic Condenser, For although the Mechanical Stage many be considered a valuable aid in observation, as facilitating the finding of a minute object, or the examimation of the entire surface of a large one, yet it adds nothing to the clearness of our view of either; and its place may in great degree be supplied by the fingers of a good manipulator. On the other hand, the use of the Achromatic Condenser not only con- tributes very materially, but is absolutely indispensable, to the formation of a perfect image, in the case of many objects of a difficult class: the want of it cannot be compensated by the most dexterous use of the ordinary appliances; and consequently, although it may fairly be considered super- fluous as regards a large proportion of the purposes to which the Microscope is directed, whether for investigation or for display, yet as regards the particular objects just alluded to, it must be considered as no less neces- sary a part of the instrument than the Achromatic Objective itself. Where expense is not an object, the Microscope should doubtless be fitted with both these valuable accessories; where, on the other hand, the cost is so limited that only 07^6 can be afforded, that one should be selected which will make the instrument most useful for the purposes to which it is likely to be applied. In the account now to be given of the principal forms of Microscope readily procurable in this country, it will be the Author's object, not so much to enumerate and describe the various patterns which the several CONSTRUCTION OF THE MICROSCOPE. 4:3 Makers of the instrument have produced; as, by selecting from among them those examples which it seems to him most desirable to make known, and by specifying the peculiar advantages which each of these presents, to guide his readers in the choice of the kind of microscope best suited on the one hand, to the class of investigations they may be desirous of following out, and, on the other, to their pecuniary ability. He is anxious, however, that he should not be supposed to mark any preference for the particular instruments he has selected, over those constructed upon the same general plan by other Makers. To have enumerated them all, would obviously be quite incompatible with the plan of his Treatise; but he has considered it fair (save in one or two special cases) to give the preference to those Makers who have worked out their own plans of con- struction, and have thus furnished (to say the least) the general designs which have been adopted with more or less of modification by others. SIMPLE MICROSCOPES. 42. Under this head, the common Hand-Magnifier or pocket lens first claims our attention; being in reality a Simple Microscope, although not commonly accounted as such. Although this little instrument is in every one's hands, and is indispensable to the Naturalist — furnishing him with the means of at once making such preliminary examinations as often afford him most important guidance — yet there are comparatively few who know how to handle it to the best advantage. The chief difficulty lies in the steady fixation of it at the requisite distance from the object; especially when the lens employed is of such short focus, that the slight- est want of exactness in this adjustment produces evident indistinctness of the image. By carefully resting the hand which carries the glass, however, against that which carries the object, so that both, whenever they move, shall move together, the observer, after a little practice, will be able to employ even high powers with comparative facility. The lenses most generally serviceable for Hand-magnifiers range in focal length from two inches to half an inch; and a combination of two or three such in the same handle, with an intervening perforated plate of tortoiseshell (which serves as a diaphragm when they are used together), will be found very useful. When such a magnifying power is desired, as would require a lens of a quarter of an inch focus, it is best obtained by the substitution of a ' Coddington' (§ 24), or, still better, of the Browning or the Stein- heil Doublet (§ 25), for the ordinary double-convex lens. The handle of the magnifier may be pierced with a hole at the end, most distant from the joint by which the lenses are attached to it; and through this may be passed a wire, which, being fitted vertically into a stand or foot, serves for the support of the magnifying lenses in a horizontal posi- tion, at any height at which it may be covenient to fix them. Such a little apparatus is a rudimentary form (so to speak) of what is commonly understood as a Simple Microscope; the term being usually applied to those instruments in which the magnifying powers are supported other- wise than in the hand, or in which, if the whole apparatus be supported by the hand, the lenses have a fixed bearing upon the object. 43. Rosses Simple Microscope, — This instrument holds an intermedi- ate place between the Hand -magnifier and the complete microscope ; be- ing, in fact, nothing more than a lens supported in such a manner as to be capable of being readily fixed in a variety of positions suitable for dissecting and for other manipulations. It consists of a circular brass 44 THE MICROSCOPE AND ITS REVELATIONS. foot, •wherein is screwed a short tubular pillar (Fig. 31), which is ^ sprung ' at its upper end, so as to grasp a second tube, also ^ sprung,^ by the draw- ing-out of which the pillar may be elongated to about 3 inches. This carries at its upper end a jointed socket, through which a square bar about 3^ inches long slides rather sfciffly; and one end of this bar carries another joint, to which is attached a ring for holding the lenses. By lengthening or shortening the pillar, by varying the angle which the square bar makes with its summit, and by sliding that bar through the socket, almost any position and elevation may be given to the lens, that can be required for the purposes to which it may be most usefully ap- plied ; care being taken in all instances, that the ring which carries the lens should (by means of its joint) be placed horizontally. At A is seen the position which adapts it best for picking out minute shells, or for other similar manipulations ; the sand or dredgings to be examined being spread upon a piece of black paper, and raised upon a book, a box, or some other support, to such a height that when the lens is adjusted Fig. 31. A S Ross's Simple Microscope. thereto, tne eye may De applied to it continuously without unnecessary fatigue. It will be found advantageous that the foot of the microscope should not stand upon the paper over which the objects are spread, as it is desirable to shake this from time to time in order to bring a fresh por- tion of the matters to be examined into view ; and generally speaking, it will be found convenient to place it on the opposite side of the object, rather than on the same side with the observer. At B is shown the po- sition in which it may be most conveniently set for the dissection of ob- jects contained in a plate or trough, the sides of which, being higher than the lens, would prevent the use of any magnifier mounted on a Horizontal arm. — The powers usually supplied with this instrument are one of an inch focus, and a second of either a half or a quarter of an inch. By unscrewing the pillar, the whole is made to pack into a small flat case, the extreme portability of which is a great recommendation. Although the uses of this little instrument are greatly limited by its want of stage, CONSTRUCTION OF THE MICROSCOPE, 45 mirror, etc., yet, for the class of purposes to wliicli it is suited, it has advantages over perhaps every other form that has been devised. 44. QueJcetfs Dissecting Micro- scope, — By the Scientific investiga- tor who desi;.^es a large flat stage, combined with portability, the ar- rangement devised by Mr. John Quekett (Fig. 32) will^be found ex- tremely convenient. The Stage, whicli constitutes the principal part of the apparatus, is a plate of brass (bronzed') nearly six inches square, screwed to a piece of mahogany of the same size, and about 5-8ths of an inch thick; underneath this is a fold- ing flap four inches broad, attached on each side by hinges; and the two flaps are so shaped, that, when folded together, one lies closely upon the other, as shown at b, Fig. 32, whilst, when opened, as shown at A, they give a firm support to the stage at a convenient height.^ At the back of the stage-plate is a round hole, through which a tubular stem works vertically with a rack-and-pinion movement, carrying at its summit the horizontal Arm for the magnify- ing powers ; and into the underside '^''''''''l^l^Zl^i.f^'Z^l^r^tl. of the stage-plate there screws a stem which carries the mirror-frame. From this frame the mirror may be re- moved, and its place supplied by a convex lens, which serves as a condenser for opaque objects, its stem being then fitted into a hole in the stage, at one side or in front of its central perforation. The instrument is usually furnished with three magnifiers — namely, an inch and a half -inch ordinary lenses, and a quarter-inch Coddington; and these (or the combinations of equivalent foci already mentioned, § 25), will be found to be the powers most useful for the purposes to which it is specially adapted. As a black back-ground is often required in dissecting objects which are not transpa- rent, this may be most readily provided by attaching a disk of dead-hl^ck paper to the back of the mirror. The lenses, mirror, condenser, vertical stem, and milled-head, all fit into a drawer which shuts into the under- side of the stage ; so that, when packed together, and the flaps kept down by an elastic band, as shown at b, Fig. 32, the instrument is extremely portable, furnishing (so to speak) a case for itself. It may be easily made with an addittional arm carrying a light Compound body. ^ The Stage-plate is sometimes made of plate-glass or ebonite ; and this is decidedly advantageous where Sea- water or Acids are used. ^ The Stage is now more generally supported, either (as in Mr. Ladd's model) on four legs of strong brass wire, which screw into its underside, and are packed in its drawer when dismounted ; or (as made by Mr. Swift and Messrs. Parkes of Birmingham) on four brass legs which fold beneath it ; — either of these construc- tions remedying the chief disadvantage of the original model, which consists in the exclusion of side light from the mirror. 46 THE MICBOSCOPB AND ITS KEVELATIONS. furnished with objectives suitable for the examination of dissections or other preparations made upon the stage, without disturbing them by moval to another instrument. 45. Sieiert and Kraft's Dissecting Microscope. — In making minute dissections, however, the hands are most advantageously rested, not on the stage itself, but on supports at a level intermediate between that of the Siebert and Kraft*s Dissecting Microscope, as folded in case. stage and that of the table. Such a support, in some Continental Dissect- ing Microscopes — as those of Nachet and Zeiss — is attached to each side of the stage of an ordinary Simple Microscope ; but this arrangement is subject to the disadvantage of causing the whole weight of the hands to bear on the stage, so as, by depressing it, to throw the object out of focus, unless the stage be made of extraordinary solidity, or be supported in front as CONSTRUCTION OF THE MICROSCOPE. 47 well as behind. Hence the Author regards the arrangement adapted by Messrs. Siebert and Kraft (Fig. 33) as preferable ; in which the supports for the hands are oblique wooden blocks, altogether disconnected from the stage. These, being hinged to the wooden base of tlie pillar, can be made to turn up for portability (as shown in Fig. 34), so that the instru- ment packs into a very small compass. 46. Laboratory Dissecting Microscope, — Where, on the other hand, portability may bo altogether sacrificed, and the instrument is to be adapted to the making of large dissections under a low magnifying power, some such form as is represented in Fig. 35 — constructed by Messrs. Baker on the basis of that devised by Prof. Huxley for the use of his Practical Class at South Kensington — will be found decidedly preferable. The framework of the instrument is solidly constructed in mahogany, all its surfaces being blackened; and is so arranged as to give two uprights for the support of the stage, and two oblique rests for the hands. Close to the summit of each of these uprights is a groove into which the stage-plate slides; and this may be either a square of moderately thick glass, or a plate of ebonite having a central perforation into which a disk of the same material maybe fitted so as to lie flush with its surface; one of those being readily substituted for the other, as may best suit the use to be made of it. The magnifier is carried on an arm working on a racked stem, which is raised or lowered by a milled-head pinion attached to a pillar at the fur- ther right-hand corner of the stage. The length of the rack is sufficient to allow the arm to be adjusted to any focal distance between 2 inches and l-4th of an inch. But as the height of the pillar is not sufficient to allow the use of a lens of 3 inches focus (which is very useful for large dissections) the arm carrying the lenses is made with a double bend, which, when its position is reversed (as is readily done by unscrewing the milied-head that attaches it to the top of the racked stem), gives the additional inch required. As in the Quekett Microscope, a Compound body may be easily fitted, if desired, to a separate arm capable of being pivoted on the same stem. The mirror frame is fixed to the wooden basis of the instrument; and places for the magnifiers are made in grooves beneath the hand- Laboratory Dissecting Microscope. 48 THE MICROSCOPE AND ITS REVELATIONS. supports. — The advantages of this general design have now been satis- factorily demonstrated by the large use that has been made of it ; but the details of its construction (such as the height and slope to be given to the hand-rests) may be easily adapted to individual requirements. 47. BecFs Dissecting Microscope, iviih NacUefs Binocular, — A sub- stantial and elaborate form of Dissecting Microscope, devised by the late Mr. R. Beck, is represented in Fig. 36. From the angles of a square ma- hogany base, there rise four strong brass pillars, which support, at a height of 4 inches^ a brass plate 6^ inches square, having a central aper- Xto^ sa ture of 1 inch across ; upon this rests a circular brass plate, of which the diameter is equal to the side of the preced- ing, and which is at- tached to it by a revolv- ing fitting that sur- rounds the central aper- ture, and can be tight- ened by a large milled- head beneath ; whilst above this is a third plate, which slides easi- ly over the second, be- ing held down upon it by springs which allow a movement of 1^ inch in any direction. The top- plate has an aperture of Beck's Dissecting Microscope, with Nachet's Binocular 1^ iucll for the reception Microscope. of yarious glasscs and troughs suitable for containing objects for dissection; and into it can also be fitted a spring-holder, suitable to receive and secure a glass slide of the ordinary size. By turning the large circular plate, the object under observation may be easily made to rotate, without disturbing its relation to the optical portions of the instrument; whilst a traversing movement may be given to it in any direction, by acting upon the smaller plate. The left-hand back pillar contains a triangular bar with rack- and-pinion movement for focal adjustment, which carries the horizontal aim for the support of the magnifiers; this arm can be turned away towards the left side, but it is provided with a stop which checks it in the opposite direction, when the magnifier is exactly over the centre of the stage-aperture. Beneath this aperture is a concave mirror, which when not in use, lies in a recess in the mahogany base, so as to leave the space beneath the stage entirely free to receive a box containing apparatus; whilst from the right-hand back corner there can be raised a stem car- rying a side condensing-lens, with a ball-and-socket movement. In addi- tion to the Single lenses and Coddington ordinarily used for the purposes of dissection, a Binocular arrangement was devised by Mr. K. Beck,^ on the principle applied by MM. Nachet, about the same date, in their Stereo-pseudoscopic Microscope (§ 38). Adopting Mr. Wenham's method of allowing half the cone of rays to proceed to one eye without interrup- ^ Transactions of the Microscopical Society," N. S., Vol. xii. (1864), p. 3. CONSTRUCTION OF THE MICROSCOPE. 4:9 tion, he caused the other half to be intercepted by a pair of prisms dis- posed as in Fig. 27, 2, and to be by them transmitted to the other eye. It will be readily understood that this arrangement, though pseudosco- pic for the Compound Microscope, is stereoscopic for the Simple Micro- scope, in which there is no reversal of the pictures; and the Author can testify to the fidelity of the eflfect of relief obtainable by Mr. E. Eeck's apparatus, which, being carried on an arm superposed upon that which bears the magnifier, can be turned aside at pleasure. But he has found its utility to be practically limited by the narrowness of its field of view, by its deficiency of light and of magnifying power, and by the inconve- nience of the manner in which the eyes have to be applied to it. — An ar- rangement greatly superior in air these particulars having been since worked out by MM. Nachet, the Autlior has combined this with Mr. E. Beck's Stand and finds every reason to be satisfied with the result; the solidity of the stand giving great firmness, whilst the size of the stage- plate affords ample room for the hands to rest upon it. The Objective in Nachet's arrangement is an achromatic combination of three pairs, having a clear aperture of nearly 3-4ths of an inch, and a power about equal to that of a single lens of one inch focus; and immediately over this is a pair of prisms, each resembling a. Fig 27, having their inclined surfaces opposed to each other, so as to divide the pencil of rays passing upwards from the objective into two halves. These are reflected horizontally, the one to the right and the other to the left; each to be received by a lateral prism corresponding to B, and to be reflected upwards to its own eye, at such a slight divergence from the perpendicular as to give a natural con- vergence to the axes when the eyes are applied to the eye-tubes super- posed on the lateral prisms — the distance between these and the central prisms being made capable of variation, as in the Compound Binocular of the same makers (§ 38). The magnifying power of this instrument may be augmented to 35 or 40 diameters, by inserting a concave lens in each eye-piece, which converts the combination into the likeness of a Galilean telescope (or opera-glass); and this arrangement (originally sug- gested by Prof. Briicke of Vienna) has the additional advantage of increasing the distance between the object and the object-glass, so as to give more room for the use of dissecting instruments. — To all who are engaged in investigations requiring very minute and delicate dissection, the Author can most strongly recommend MM. Nachet's instrument. No one who has not had experience of it, can estimate the immense advantage given by the Stereoscopic view, not merely in appreciating the solid form of the object under dissection, but also in precisely estimating the relation of the dissecting instrument to it in the vertical direction. This is especially important when fine scissors are being used horizon- tally; since the course of the section can thus be so regulated as to pass through the plane desired, with an exactness totably unattainable by the use of any monocular magnifier. 48. Field'' s Dissecting and Mounting Microscope, — This instrument, constructed on the plan of Mr. W. P. Marshall, is a combination of a Dis- secting Microscope with a set of apparatus and materials for the prepara- tion and mounting of microscopic objects; the whole being packed in a small cubical case about seven inches each way, convenient both for general use, but more particularly as a travelling case for carrying the several requisites for the examination and mounting of objects into the country, or to the seaside. — The Microscope can be used either Simple or Compound, as shown in Fig. 37; and is fitted with a mirror, side-condenser, and stage- 4 50 THE MICROSCOPE AND ITS REVELATIONS. forceps, and witli metal and glass stage-plates; a dissecting-trougli, lined with cork, also fits into the opening of the stage. The Simple Micro- scope, as used for dissecting and mounting, is shown in the lower figure; it has two powers used singly or in combination, which are carried by the smaller arm of the stand. The Compound body, as shown in the upper figure, screws into the larger arm of the stand, and has a divided objective, giving a range of three powers; the nose is made with the standard screw, so as to fit any first-class objectives. A telescope sliding arm, fitting into a socket on either side of the stage, can also be used to carry the simple- microscope powers, as well as a larger low-power lens, that serves also as a Field's Dissecting and Mounting Microscope. hand-magnifier; and the arm can be readily fixed in any desired position for examining objects away from the instrument. A watch-glass holder used upon the glass stage-plate, gives the means of sliding steadily upon the stage in any direction objects that are under examination in a watch- glass. A turn-table for mounting purposes is carried upon a long spindle that works through the corner of the stage (as shown in the lower figure), the arm of the stand serving as a support for the hand whilst using the turn-table; the top is made of the size of an ordinary glass slide, and the slide is held upon it by an india-rubber band. A hot plate fits into the opening of the stage, and is heated by a spirit-lamp placed in the posi- tion of the mirror, which is then turned to one side; and the larger arm CONSTRUCTION OF THE MICROSCOPE. 51 serves also as a watch-glass holder for preparing crystals hj evaporation over the spirit-lamp. A selection of materials required in preparing and mounting objects is supplied in a rack of bottles sliding in the case; and a set of instruments — dissecting-needles, knife, forceps, dipping tubes, brushes, etc. — with a supply of cover-glasses, cells, etc., are carried in the three drawers; all the different contents of the case being readily accessible when it is set open, as shown in the lower part of the figure. ^ COMPOUND MICROSCOPES. 49. Of the various forms of Compound Microscope, the greater num- ber may be grouped with tolerable definiteness into three principal Classes : the First consisting of those high-class instruments in which the greatest possible perfection and completeness are aimed at, without regard to cost ; the Second including those which are adapted to all the ordinary requirements of the observer, and which can be fitted with the most important Accessories ;^ whilst to the Third belong the Students' and Educational Microscopes, in which simplicity and cheapness are made the primary considerations. Besides these, there is a class of Micro- scopes devised for special purposes, but not suited for ordinary use. — In all, save the last, the same basis of support is adopted ; namely, a tripod ^foot,' carrying a pair of uprights, between which the Microscope itself is swung in such a manner, that the weight of its different parts may be as nearly as possible balanced above and below the centres of suspension in all the ordinary positions of the instrument. This double support was first introduced by Mr. George Jackson, who substituted two pillars (a form which Messrs. Beck retain in their Large Microscope, Plate vii., and is now adopted by Messrs. Eoss, Plate v.) for the single pillar, con- nected with the Microscope itself by a ^ cradle joint,^ which was previously in use, and which is still employed in many Continental models (Fig. 45). But in place of pillars screwed into the tripod base, the uprights are now usually cast in one piece with the base, both for greater solidity and for facility of construction (Pig. 39); while in most of the more recent models an open framework is adopted (more or less resembling that first devised by Mr. Swift, Pig. 50), which combines great steadiness with lightness. Messrs. Powell and Lealand, it will be observed, adopt a tri- pod support of a different kind (Pig. 48 and Plate vi.); still, however, carrying out the same fundamental principle of swinging the Microscope itself between two centres. An entirely new and very effective mode of swinging the body has lately been introduced by Mr. George Wale of New York (Fig. 44). — Two different modes of giving support and motion to the ' body ^ will be found to prevail. In the first, which may be called the Ross model (as having been originally adopted by Mr. Andrew Boss), the ^ body ' is attached at its base only to a transverse ^ arm,^ which, being pivoted to the top of the ' stem,^ is raised or lowered with it by the rack- and-pinion action that works in the pillar to which the stage is fixed (Fig. 52). The fundamental objection to this method is, that unless the 1 The whole of this apparatus is supplied complete at the moderate cost of £4. or, without the Compound body and inclined movement of the stand, at £3 10s. 2 It is true that the most important of these accessories may be applied to some of the smaller and lighter kind of Microscopes ; but when it is desired to render the instrument complete by the addition of them, it is far preferable to adopt one of those larger and more substantial models, which have been devised with express reference to their most advantageous and most convenient employment. 52 THE MICROSCOPE AND ITS REVELATIONS. transverse arm and the body are constructed with great solidity, the ab- sence of support along the length of the latter leaves its ocular end sub- ject to vibration, which becomes unpleasantly apjiarent when highpoweri35 are used, giving a dancing motion to the objects. With the view of preventing this vibration, the top of the ^ body ^ is sometimes connected with the back of the transverse arm by a pair of oblique ^ stays ^ (Fig- 48); but the usual plan is to obtain the requisite firmness by the thickness and weight of the several parts. In the other, which may be termed the Jackson model, and which was first adopted by Mr. James Smith (the predecessor of Messrs. Beck), the body is supported along a great part of its length on a solid ^ limb ' whereby its ^ vibration ^ is reduced to a minimum ; and the rack, which is acted on by a pinion working in that limb, is attached to the body itself ; a construction that gives a great smoothness and easiness of working (Plate Yii.). — Having made use of instruments constructed by the best makers on both models, the Author has no hesitatation in expressing his preference for the second, which is now employed by most English makers (having been adopted by Messrs. Eoss themselves in their more recent instruments), and by nearly all American. He regards it as certain that greater freedom from vibration can be obtained in lightly-framed Microscopes constructed on the Jack- son model, than in any but the most solid and cumbrous of the old Ross pattern ; and feels assured that the principle of supporting the ' body^ along a great part of its length (which may be applied in a variety of modes) will in time supersede that of fixing it by its base alone, which is oviously the mode least adapted to prevent vibration at its ocular end. In describing the Instruments which he has selected as typical of the several groups above enumerated, the Author wishes not to be under- stood as giving any special preference to these, above what may be the equally good instruments of other Makers. The number of those who now construct really excellent Microscopes has of late years increased greatly ; but their models are for the most part copied more or less closely from those previously adopted for their high-class Microscopes by the three principal Firms which long had exclusive possession of the field. Where any individual Maker has introduced a real novelty, either in plan of construction, or in simplification leading to reduction of price, the Author has thought this worthy of special notice ; whilst the limits within which he is restricted oblige him to content himself with a bare mention of other Makers whose productions are favorably known to him. It will be found most advantageous to commence with the Educational and Students' Microscopes, as the most simple in construction ; and to pro- ceed from these through the Second to the First- Class Microscopes, reserving to the last the group of instruments adapted for Special pur- poses. THIRD-CLASS MICROSCOPES. 50. Very important contributions to our knowledge of Nature have unquestionably been made by the assistance of instruments not surpassing the least jierfect of those now to be described. And there is this advan- tage in commencing Microscope-work with a simple and low-j^riced instrument — that the risk of injury to a more costly Microscope, which necessarily arises from want of experience in its use, is avoided; whilst the inferior instrument will still be found serviceable for many purposes. CONSTRUCTION OF THE MICROSCOPE. • 53 after a better one has been acquired. Microscopes, of whatever Class, should be provided with the ' Society's screw ^ now used not only by British and American, but also by several Continental Makers; so that any of their Objectives may be fitted to them. (See Note, p. 58.) Educational Microscopes. 51. FieWs Educational Microscope, — This instrument is known as the ' Society of Arts Microscope,^ in consequence of its having gained the medal awarded by that Society, in 1855 (at the suggestion of the Author) for the best three-guinea Compound Microscope that was then produced. It has two Eye-pieces, and two achromatic Objectives, Condenser, Live- box, etc., and retains its place amongst useful instruments of low price. It is within the knowledge of the Author, that the production of this instrument has greatly promoted the spread of Microscopy among many to whom the pursuit has proved most valuable as a refreshing and elevat- ing occupation for hours that might have been otherwise either spent in idleness or turned to much worse account. 52. Crouches Educational Microscope, — This is a very simple and at the same time serviceable, instrument (Fig. 38); well suited for the dis- play of Botanical objects, small Insects or parts of larger ones, Zoophytes and Polyzoa that may be picked up on almost any sea-shore, or the Cir- culation in a Frog's foot. In order to minimize its cost, the ordinary modes of focal adjustment are dispensed with; the ^ coarse ' adjustment being made by sliding the body through the tube which grasps 'Em^S^ it, and which is lined with velvet to secure a smooth and equable ' slip;' and the ' fine ' by slight- ly drawing-out the Eye-pieces. This method answers very well for the low powers for which this insbrument is intended; and it has the advantage of not allowing the adjustment which a Teacher has made, to be readily disturbed by the PujDils to whom an object is being exhibited. It is provided with a side-condenser for illumi- nating opaque objects; and with a diaphragm-plate fitted into a tube which is screwed into the aperture of the stage, and which is adapted also to receive a po- larizing prism and spot-lens.* 53. Parkes^s Educational Microscope, — Such as desire a large and more substantial in- strument, which may be advan- tageously used for higher pow- ers, and made to serve a greater variety of purposes, will find the crouch's Kducational Microscope. Microscope represented in Fig. ^ The cost of this instrument, with a dividing object-glass of \ inch and 1 inch focus, in mahogany case, is only £2 10s. 54 • THE MICROSCOPE AND ITS REVELATIONS. 39 very suitable to such requirements. It is solidly built, without being unduly weighty, carries a body of full diameter (which can be lengthen- ed by a draw-tube to ten inches), and stands well upon its^ base. The ' coarse ^ adjustment is made (as in the preceding case) by sliding the body within the tube that grasps it, the lining of which with cloth makes it work very easily (Fig. 39); but a rack and pinion movement may be added at a Parkes's Educational Microscope. small additional cost. The ^ fine ' adjustment is made by a screw (turned by the milled-head at the top of the vertical pillar), which acts on the car- nage of the body; and as this carriage slides between dove-tailed grooves, the adjustment is made with entire freedom from ' twist. ^ The Microscope is furnished with two eye-pieces, of which the lower is preferable for objects requiring good definition; whilst the higher gives a flat field of eight inches CONSTRUCTION OF THE MICROSCOPE. 55 diameter, suitable for Sections of Wood and other like objects viewed with the low-power objective. The powers usually supplied are a separating com- bination of 2 inch and 1 inch, which, by the use of the two eye-pieces and the di*aw-tube, gives a range of magnifying power from 15 to 110 diame- ters; and a l-4tli inch of 70° aperture, from which, by the same means, a range of magnifying power can be obtained from 140 to 450 diameters. The aperture of the stage is furnished with a cylindrical fitting, which carries two diaphragms (one with a small aperture, the other with a larger) for regulating the quantity of light reflected from the mirror to the object, a ground-glass for the equable diffusion of the light over a large field, and a ' spot-lens ^ for black-ground illumination. The mir- ror is plane on one side, and concave on the other; and a condenser for opaque objects is attached by a jointed arm, giving universal motion, to the tube that carries the body. The Objectives of this Microscope, as of most of those constructed by the same Makers, are made to fit into the nozzle of the body by their ' patent sliding adapter,^ which enables one power to be exchanged for another without any screwing or unscrewing. But their Microscopes can be used with any objective carrying the ^ Society's screw, ^ by simply unscrewing the special nozzle from the end of the body. And by sliding the special nozzle upon either of its own objectives, this may be used with any other instrument furnished with that screw. ^ Students^ Microscopes. 54. The principle is now universally recognized, that the form of Microscope best adapted to the wants of the Medical or Biological Stu- dent, is one in which simplicity and compactness of general construction are combined with excellence in optical performance. The demand for instruments of this kind was first met by Continental Opticians; and at the time when Messrs. Ross, Powell and Lealand, and Smith and Beck — then almost the only constructors of Microscopes in this country — sold no Objectives but such as would stand the highest tests and were costly in proportion, recourse was necessarily had, by such as desired simpler and cheaper instruments, to the Opticians of France and Germany; among whom MM. JSTachet, Oberhauser (succeeded by Hartnack), and Kellner (succeeded by Gundlach), long shared the chief English demand. A large number of new Makers, however — many of them trained in one or other of the three principal establishments just named — have now entered the field; and have put themselves in fair competition with Continental Opticians, and with each other, alike in the excellence of their work (both mechanical and optical), and in moderation of price. A distinct class of ^ Students' Microscopes^ of English construction, more or less framed upon Continental models, has thus come into general use; afford- ing ample choice, in the varieties of their pattern, to such as may have a preference for one or other of them as most suitable to the work on which they may be engaged. With few exceptions, the Microscopes properly belonging to this class have the small short ' body ' (capable, however, of being lengthened by a ' draw-tube ') of the Continental instruments; and this is grasped by a tube attached to the ' limb,^ in such a manner as ^ The price of this Microscope with the above-named Accessories, in a vsrell- made mahogany Case, is £6 10s. An Objective of l-6th inch focus, giving a max- imum power of 560 degrees, or one of l-7th inch giving a maximum power of 700 diameters, may be substituted for the l-4th inch at a very small advance of cost. A Polariscope and Achromatic Condenser can be easily added. 66 THE MICROSCOPE AND ITS REVELATIONS. to give a support that is free from vibration even when high powers are in use. In the simplest models, such as that of Messrs. Baker (Fig. 40), there is no rack-and-pinion movement for the ' coarse ' adjustment, which can be very easilj^ made by sliding the body through the tube which holds it, provided that this be lined with cloth or velvet; but the rack move- ment can generally be added at a small cost. A^fine^ adjustment for exact focussing, by means of a micrometer-screw worked by a milled-head, is always provided; and this movement may be given in different Avays. In the Continental models, the screw is usually contained within the pil- lar that supports the arm or limb to which the carriage of the body is attached, the milled-head being at its summit (Figs. 40, 45); this answers well if due provision be made to prevent ' twist ^ of the movable portion (causing lateral displacement of the image), without interfering with its freedom of vertical motion. By many British and American makers, the fine adjustment is made to act on a tube within the ' nose^ of the body, into which the objective is screwed; this being raised or lowered, either by a lever contained within the arm, which is acted-on by the milled-head carried by it (as in the original Ross model, Fig. 52), or by a shorter lever at the lower end of the body, to which the milled-headed screw is attached (as in Messrs. Beck's Large Microscope, Plate yii.). This method is subject to two disadvantages: (1) that the focussing tube which carries the objective can scarcely be made to work with the requisite facility, without a liability to ' twist,' which becomes very perceptible after much wear, in the displacement of the image when a high magnifying power is in use; and (2) that by the vertical movement thus given to the focussing tube, the working length of the body, and consequently the magnifying power undergoes change in every adjustment for focus. The plan of fine adjustment which has been adopted from an American model by Messrs. Ross (Plate v.) and is employed in Wale's New Working Microscope (Fig. 44), seems to the Author in everyway preferable. Here a lever contained within the limb, and acted-on by a micrometer-screw at ifs back, gives motion to a long slide, working in dove-tailed grooves, behind the racked slide which carries the body; and this can be made to work very easily, without either ^ twist' or 'lost time.' — The Stage of Students' Microscoj)es is often a simple plate of brass, with a couple of springs for holding down the object-slide; but in some models (Figs. 40, 45) there is an ' upper stage-plate ' of glass, rotating in the optic axis of the body. Into the aperture of the stage a cylindrical fitting is usually screwed, for the purpose of receiving the Accessories required for giving varied illumination; the most indispensable of these being Diaphragms of different apertures. These should be so fitted that they can be brought up flush with the level of the stage; the limitation of the illuminating pencil for the purpose of obtaining the best definition being much more effectively made by a very small aperture (not exceeding a large pin-hole) close to the under side of the object-slide, than by a wider aperture at some distance beneath it. For the same reason, if a rotating ' dia- phragm-plate ' (§ 98) be employed, containing a graduated series of aper- tures, it should be attached to the under side of the Stage itself, and not to the bottom of the cylindrical fitting beneath it. For perfect regula- tion of the light, nothing is so effective as the ^Iris-diaphragm (§ 98); and this, as Mr. Wale has shown (§ 60), may be constructed so cheaply, that its general adoption seems very desirable. — The mirror should be double, one of its surfaces plane and the other concave; and it should be so attached (I) that its distance from the stage may be varied sufficiently, CONSTRUCTION OF THE MICROSCOPE. 57 to allow the rays reflected from the concave side to be either brought to a focus on the object, or to give a uniform illumination over a larger field, and this alike with the parallel rays of daylight, and the diverging rays of a lamp; and (2) that it may be thrown so far out of the optic axis, as to reflect rays of considerable obliquity. The first of these objects is answered by making the mirror-frame slide upon a stem fixed into the bottom of the pillar (Fig. 41); but this does not give sufficient obliquity. The second is readily provided-for by attaching the mirror-frame to a swinging-bar, pivoted to the under side of the stage (Fig. 42); this gives any amount of obliquity, but does not enable the distance of the mirror from the stage to be varied. If the mirror-frame be made to slide on a stem, it should be mounted on a jointed arm, so as to be made capable of reflecting very oblique light; or, if attached to a swinging bar, this bar should be made capable of elongation by a sliding piece working in a dove-tail groove (as in Wale's Microscope, Fig. 44), so as to allow its dis- tance from the stage to be varied. — A very ingenious arrangement of the rotating ^ upper stage' has been devised by Mr. John Phin (of New York). It is so fitted with a short tube, that it may be slid into the cylindrical fitting, not only from above, but also from 'below; and as the object-slide rests upon the springs which press it upwards against the stage-plate, not only may light of any degree of obliquity be throvv^n upon it, but the advantage of a ^safety-stage' (§ 117) is obtained, since the springs that support the slide readily yield to any pressure exerted on it by the objective. A Student's Microscope fitted with this form of rotat- ing stage, and with either Wenham's ^ disk illuminator,' or ' Woodward's prism' (§ 101), and having the mirror hung in the manner just recom- mended, will be found capable — if furnished with good Objectives — of resolving all but the most difficult Diatom-tests. 55. In regard to the qualities of the Objectives desirable for a Stu- dent's Microscope, the Author feels assured that he expresses the convic- tion of the most experienced workers in various departments of Biological inquiry, when he re-affirms the doctrine of which nearly half a century's varied experience has satisfied him, but which has been of late vehe- mently contested (not always very cautiously) by Microscopists whose range of study has been less extended — that good definition, with mode- rate angle of aperture, is the essential requisite; Objectives of this class being not only much more easy to use by the inexperienced, but fre- quently also giving much more information even to the experienced (in virtue of their greater ^penetration' or ^ focal depth'), than can be ob- tained from Objectives of the very wide angles required for the resolution of difficult diatom-tests (see § 161). Every one who is at all conversant with the recent history of Micro-Zoology, Micro-Botany, Micro-Geology, or Animal or Vegetable Histology, must know that at least ninety-nine hundredths of the enormous additions made to each of these departments of inquiry during the last quarter of a century, have been Avorked-out by Objectives of the kind here recommended; and those who affirm that all this work is so imperfect that it will have to be done over again with Ob- jectives of excessively wide aperture, have to prove the fact. Doubtless neio methods of preparation are constantly revealing novelties in whole classes of objects which (it was supposed) had been already studied exhaustively; and no one can affirm that he has made out everything, in any object, which it is capable of being thus made to show. But the Author feels confident that no such extension of our knowledge is likely to take place in this direction, as will require the habitual use of the very 58 THE MICROSCOPE AND ITS REVELATIONS. costly wide-angled Objectives, which certain Microscopists, especially in the United States, are now extolling as alone trustworthy/ In confirma- tion of the foregoing remarks, the following additional authorities may be cited: — Dr. Beale, whose Histological experience no one can call in question, says (^^How to Work with the Microscope,'^ 5th ed., p. 10): — '^For ordinary work it will be found inconvenient if the object-glass, ^' when in focus, comes too close to the object. This is a defect in glasses having a high angle of aperture. Such glasses admit much light, and define many structures of an exceedingly delicate nature which look confused when examined with ordinary powers. For gen- eral microscoinc work, however, glasses of medium angular aperture ''are to be recommended. Glasses having an angle of 150° and upwards ^^are valuable for investigations upon many very delicate and thin struc- ^^tures, such as the Diatomacece; hut such powers are not well adapted ''for ordinary work,^^ So Dr. Heneage Gibbes, who has been trained under Dr. Klein, one of the most distinguished Histologists of the pres- ent day, recommends the Student Practical Histology and Pathology," p. 6) to get some good Microscopist to test the object-glasses he thinks of purchasing; " and he should see that they are tested on some Histological object, and not on Diatoms, as the wide angles necessary for resolving "test DiatomacecB are the reverse of useful to the you7ig histologistJ^ And Dr. Leidy, of Philadelphia, everywhere well known as a most able Biological worker of large and varied experience, who has lately produced an admirable Monograph (illustrated by 48 beautiful quarto-plates) on the " Fresh-water Ehizopods of North America," makes a point, in his Introduction (p. 3), of informing Students that Microscopic observa- " tions, such as those which form the basis of the present work, do not require elaborate and high-priced instruments;" the Student's Micro- scopes of Zentmayer, Beck, or Hartnack, with a power of l-4th or l-5th inch, and the occasional use of a l-8th or 1-lOth inch, furnishing all that is needed. ^^I give the above statement," he adds, ^^not with any dis- ^^positionto detract from the value of the various magnificent in stru- ^^ments so much in vogue, but with the object of dispelling a common impression widely prevalent, at least among those with whom I habitu- ^^ally come into contact, that the kind of work such as I now put forth ^^can be done only with the help of elaborate and expensive instru- ^^ments."2 ^ The cost of the Objective of l-4th inch focus and 170° aperture, made by Mr. Tolles, of Boston, is 70 dollars (about £14); which would purchase a very good English Student's Microscope, with a series of excellent Objectives up to 1-lOth * immersion. 2 Now that the requirements of a Student's Microscope are so definitely under- stood, the Author would suggest whether it would not be better that a new stand- ard screw of much smaller size than the * Society's' should be adopted for it, so as to enable Students' * Objectives ' to be set in the small light ' mounts ' used on the Continent, instead of in the massive mounts which the Socciety's screw neces- sitates; especially as, on the construction already recommended (§ 17) no adjust- ment for thickness of covering-glass is required, even for high powers. A small light ' nosepiece,' for change of Objectives, could then be added at a low cost, — to the great convenience of the worker. Such Microscopists as, commencing with * Students' Microscopes,' afterwards provide themselves with more complete instruments, would readily employ their Students' objectives with the latter by means of an * adapter.' But the Author's experience would lead him to recom- mend any one engaged in research to keep his Student's Microscope, with its own series of objectives, constantly on his table; and to have recourse to his larger instrument, with its first-class Objectives and varied methods of Illumination, only for the more complete scrutiny of the preparations h^ has made with his simpler model. CONSTRUCTION OF THE MICROSCOPE. 59 56. Baher^s Students Microscope. — Most of the conditions above specified as desirable, are well fulfilled in the instrument represented in Fig. 40; which might easily be brought into entire conformity with them. It is extremely light and handy; and is so well hung as to be very steady in all positions. It is provided with a rotating glass stage; and this car- ries a cj^lindrical fitting (not represented in the figure) for the usual Accessories, ^ Baker's Student's Microscope. CoUins's Student's Microscope. 57. Collinses Student^ s Microscope. — This instrument (Fig. 41) is con- structed on a plan altogether different; the body having the diameter of that of the larger Microscopes by the same maker (Fig. 49), so as to receive their eye- pieces, and being capable of elongation by a draw-tube to the full ordinary length. It is provided with a rack-movement acting on a carriage attached along the length of the body (as in the Jackson model); and the top of this carries the milled-head for the fine adjustment, which acts upon a lever near the bottom of the carriage, so as to raise or lower a focussing tube within the nozzle of the body. 58. Pillischer^s International Microscope, — The Student who may be willing to incur a slight additional expense, for the sake of obtaining a substantial and well-constructed instrument, cannot do better (in the Author's judgment) than possess himself of the International Micro- scope of Mr. Pillischer (Fig. 42), in which the advantages of British and Continental methods are ingeniously combined. The pillar, carrying a ^ The price of this instrument, with one Eye-piece and two Objectives (1 inch and l-4th inch), in Case, is 5 guineas; or, with rack movement for coarse adjust- ment, 6 guineas 60 THE MICROSCOPE AND ITS REVELATIONS. rack-movement with double milled-head, is swung on two uprights set on a solid foot, in such a manner as to be well balanced; and at the top of the racked stem is the milled-head that works the screw for fine adjustment, raising or lowering the horizontal arm which carries the body, without twist or loss of time. This arm carries a tube firmly screwed into it, through which the body slides; and while this arange- ment, by giving additional support to the lower part of the body, effect- ually antagonizes vibratiou, it allows the body to be raised to a height that permits the use of objectives of 3 or 4 inches' focus, for which the rack-movement is not long enough to provide. On the outside of this tube is a clip having attached to it a jointed arm that carries a condens- Pillischer's International Microscope. Ross's (Zentmayer) Student's Microscope. ing lens for opaque objects; which, by raising or lowering the clip, or turning it round the tube, can be brought into any required position. The stage is simple, and carries a rotating diaphragm-plate on its under side. The mirror is attached to a swinging bar, which might easily be made to elongate like that of the Wale Microscope (§ 60). — The special merit of this model (of which the Author can speak from considerable experience of its use), lies in the facility with which both the coarse and the fine movements may be worked with either of the hands, while rest- ing on the table in the position most convenient for manipulating the CONSTRUCTION OF THE MICROSCOPE. 61 object on the stage, an advantage which every real tuorher with a simple instrument of this class will appreciate.* 59. Rosses {Zentmayer) Shtdenfs Microscope. — Another instrument of superior make (Fig. 43), has lately been introduced by Messrs. Eoss, with the view of affording to the Student the advantage of the ' swinging tail-piece for oblique ilUimination/ devised by Mr. Zentmayer; of which a fuller description will be given in its application to their First-class Microscope (§ 72). This tail-piece swings round a pivot which serves for the attachment of the stage to the limb; and at the back of the limb is a milled-head working on the projecting end of this pivot, by tightening which the stage may be firmly fixed in its ordinary horizontal position, whilst by loosening it the stage may be made to incline to one side or the other. The ' tail-piece ' carries, between the mirror and the stage, a ' sub- stage,' fitting into which may be screwed an ordinary 1 inch, 1\ inch, or 2 inch Objective, which answers the purpose of an Achromatic condenser; and when a pencil of light reflected from the mirror has been made by it to focus in the object, the swinging of the ^tail-piece' to one side or the other will give any degree of obliquity to the illuminating pencil that may be desired, without throwing its focus off the object, as this lies in the plane of the centre round which it turns. The ' tail-piece' may even be carried round above the stage, so that light of various degrees of obliquity may be concentrated upon opaque objects. The object-plat- form of the stage is of glass, and rotates round the optic axis of the microscope; so that the object may be illuminated by oblique rays from any azimuth. A mechanical stage may be added, if desired. — The work- manship of this simple model is of the highest class; and there is little real tvorh, of which, in the hands of an observer who knows how to turn the instrument to the best account, it may not be made capable, by the addition of a Polariscope, Paraboloid, and other accessories, which its Sub-stage adapts it to receive.^ 60. Wale's Neio Working Microscope, — A Student's Microscope lately brought out by Mr. George Wale (IJ. S.), deserves special notice, on account of several ingenious improvements which he has introduced into its construction. — In the first place, the Himb ' which carries the body and the stage, instead of being swung by pivots — as ordinarily — on the two lateral supports (so that the balance of the Microscope is greatly altered when it is much inclined), has a circular groove cut on either side, into which fits a circular ridge cast on the inner side of each sup- port. The two supports, each having its own fore-foot, are cast separately (in iron), so as to meet to form the hinder foot, where they are held together by a strong pin; while by turning the milled-head on the right support, the two are drawn together by a screw, which thus regulates the pressure made by the two ridges that work into the two grooves on the limb. When this pressure is moderate, nothing can be more satis- factory than either the smoothness of the inclining movement, or the balancing of the instrument in all positions; while, by a slight tighten- 1 The cost of the above Microscope, with two Eye-pieces (B and C), and two Objectives (5-8ths and l-7th inch) giving — vrith the Draw-tube — a range of powers from 50 to 423 diameters, packed in a very compact Case, is only £7 10s. Od., or, with the addition of an A Eye-piece, a 1^ or 2-inch Objective, Polarizing Apparatus, and Beale's Drawing Camera, 10 guineas. 2 The price of the Microscope, as above figured, in Case, is 10 guineas. None but first-class Objectives are supplied by Messrs. Ross; but the Student wlio finds these too costly may obtain elsewhere such as suit his requirements. 62 THE MICROSCOPE AND ITS REVELATIONS. ing of the screw, it can be firmly fixed either horizontally, verticallyj, or at any inclination. The ' coarse ' adjustment is made by a smooth- working rack; whilst the ^fine/ made by a milled-head at the back of the ^limb/ raises or lowers the body by acting on the slide that carries the rack-and-pinion movement. The body is furnished with a long draw-tube, which carries a screw at its lower end for the reception of objectives of foci too long to be worked from the nose of the outside body. The stage, though thin enough to admit very oblique light, is very firm; Wale's New Working Microscope. it is circular, and has an all-round groove near its edge, alike on its upper and its under side. Into this groove there fits a spring-clip for holding down the slide upon the stage; and this may not only be turned round into any position above the stage, but may be reversed so as to hold the slide against its imder side, thus enabling light of any degree of obliquity to be thrown on the object. A removable * Iris-diaphragm ' (§ 98), which is made to open or close by a screw-action, is fitted into the stage in such a manner that its aperture is very close to the under side of the CONSTRUCTION OF THE MICROSCOPE. 63 object-slide — an arrangement than which, in the Anther's opinion, nothing can be better. This may be replaced by a cylindrical fitting for the reception of a Polariscope, Paraboloid, etc. The donble mirror is carried npon an arm which swings on a pivot from the front of the limb beneath the stage, and is capable of extension by a dovetail sliding bar. — Altogether, this instrument (so far as its mechanical arrangements are concerned), comes nearer than any others that the Author has seen, to his idea of a model Student's Microscope.* 61. NaclieVs Studenfs Microscope. — This instrument deserves special mention for certain peculiarities of construction which distinguish it from the ordinary Continental model of Microscopes of this class. While most of these can be used only in the vertical position, the Micro- scope of MM. Nachet is attached to the supporting pillar by a cradle- joint, which allows it to be inclined at any angle. The body is furnished with a draw-tube, by which it is shortened for packing; and. is embraced by a tube which carries the rack, so that it is well supported, and maybe readily drawn out and replaced by the Binocular already described. (§ 38, Fig. 28). The ^slow motion^ is given by a milled-head placed at the top of the sliding-stem, so as to be near that which gives the rack- and-pinion adjustment. The chief peculiarity of this instrument, how- ever, lies in its Stage, which the Author has no hesitation in pronouncing to be the most perfect of its Tcind that has been yet devised.'^ Its base is formed of a thick plate, 3J inches square, having a large circular aperture; and on this is superposed a circular plate of 3 inches in diameter, to which a rotary movement, concentric with the optic axis of the Microscope, can be given with great facility. In this circular plate a disk of thin plate-glass is cemented with black cement, the united thickness of the two around the central aperture being not more than l-8th of an inch, so that light of the greatest obliquity can be trans- mitted to the object from beneath. The rotating plate is furnished with a projection at the back, to which is attached a strong V-shaped pair of springs, having their extremities armed beneath with small ivory knobs, which press down on the Object-carrier. This last consists of a brass frame furnished with tongues and springs projecting forward for the reception of the slide, and also wi^h a pair of knobs, to v^liich the fingers may be applied in giving motion to it; whilst the frame incloses a piece of plate-glass a little thicker than itself. Thus the under surface of the glass-plate of the Object-carrier slides over the upper surface of the circular glass stage-plate; being held down upon it and retained in any position by the pressure of the ivory knobs. The advantages of this arrangement lie (1) in the perfect facility with which the Object-carrier may be moved, and the steadiness with which it keeps its place when not unduly weighted; (2) in the facility with which it can be readjusted, in case the movement should become too easy, by bending down the V springs; and (3) by the^absence of liability to derangement by rust — a point of great importance when work is being done with sea-water or chemicals. The front portion of the rotating plate bears a small pro- ^ This Microscope, with two Eye-pieces, and with fairly good Objectives of 2-3ds and l-5th inch, is sold in New York for 35 dollars, or little more than £7. It could probably be made in this country (if there were a considerable demand for it) for 5 guineas. 2 This Stage, which, on the Author's recommendation, has been copied, first by Mr. Crouch, and now by other English opticians, seems to have been originally invented by Mr. Zentmayer of Philadelphia. 64 THE MICROSCOPE AND ITS REVELATIONS. jecting piece on either side, into which may be screwed a pin that carries a sliding-sprin^; this arrangement is suited for securing a Zoophyte-trough or other piece of apparatus not suitable to being received by the object carrier, which can be easily slipped away from beneath the ivory knobs, thus leaving the stage free. To the under side of the stage is firmly pivoted a broad bar, into which is screwed a short sprung tube, that becomes exactly concentric with the optic axis of the instrument, when the bar (which is shown turned away in the figure) is pushed beneath the stage until checked by a firm stop; and as this bar is composed of two pieces, held together by a pair of screws working through slots, the centering of the tube may be precisely readjusted if it should at any time become faulty. Into this tube may be inserted another that carries either (1) a Diaphragm, sliding with caps of differ- Nachet's Student's Microscope. Browning's Rotating Microscope. ent apertures; (2) a Polarizing prism; (3) a Ground-glass for diffusing the light, which may be either plane, or a plano-convex lens ground on its flat side Avhich is directed upwards; (4) au Achromatic Condenser; and (5) a Glass Cone, having its apex pointing downwards, and a large black spot in the centre of the convex base directed towards the object, which gives au excellent ' black-ground ^ illumination. Lastly, the Mirror is attached to a stem which is so jointed as to enable it to reflect rays of very great obliquity. — To those who wish a compact instrument of great completeness and capability, which may be worked advantage- ously even with high powers, the Author can strongly recommend this C?OJ^STRUCTION OF THE MICROSCOPE. 65 Microscope. The Objectives supplied with it are of great excellence and very moderate cost, and are quite adequate for all the ordinary purposes of scientific investigation. 62. Broionincf s Rotating Microscope. — The peculiarity of this instru- ment is that, as in many of the Continental models, the object-platform (b), with the limb carrying the body above it, revolves together; whilst the lower ])Lite of the stage (o), with any apparatus fitted into it, as like- wise the mirror, remains fixed. Thus the object is enabled to receive illumination in every azimuth without any derangement either in its cen- tering, or in its focal adjustment. Tho body is supported, as in the Jack- son model, upon a limb. A, which is firmly fixed to the rotating plate B of the stage. In the simplest form of the instrument, shown in the figure, the rotation is effected by pressing a finger on the projecting pins attached to b; but if required, B can be made to move by a pinion and toothed wheel, with graduated scale attached; and a sub-stage for carrying illumi- nating apparatus can be fixed to an arm below c. This Microscope is fur- ther characterized by the solidity of its several parts, and the care" taken in its construction to secure it against derangement from an accidental strain. It is particularly adapted to the use of those who work with high powers upon objects requiring the varied illumination for which this rotating arrangement gives special facilities. 63. Croucli's Student's Binocular. — This instrument (Plate in.) was devised at a time when the construction of the Binocular was still almost exclusively confined to the makers of First-class instruments; and it had the great merit of bringing within reach of the Student a convenient and well-constructed Binocular, at a moderate cost. With the improvements it has since received, it still remains one of the best instruments of its class; and the Author, after considerable use of it, can strongly recom- mend it to such as desire to possess a Binocular at once cheap, good, and portable. Its general arrangement is shown in Plate in., but a mechani- cal stage can be substituted, if desired. The rotating stage and object- holder resemble those of MM. Nachet's Microscope (Fig. 45). — An Achro- matic Condenser, Paraboloid, Polarizing apparatus, etc., can be added to this instrument; or it may be fitted with Mr. Crouch's ' Universal Sub- stage Illuminator,' which, like that of Mr. Swift (Fig. 85), combines the different Accessories ordinarily required for the examination of trans- parent objects.' 64. Baker's Student's Erecting Binocular. — With a special view to the wants of Students in various departments of Biology, Messrs. Baker have adapted a Stephenson Binocular (§ 35) to the stand of their Student's Microscope, as shown in Fig. 47; with which the stand of their Laboratory Dissecting Microscope (Fig. 35) may be so combined as to afford the requisite support to the hands, when they are engaged in dissecting (or otherwise manipulating) objects on the stage of the Binocu- lar. An ordinary Monocular body may be readily substituted for the Binocular; and the same Eye-pieces and Objectives serve for both. The low cost at which this instrument is made, will doubtless cause many to possess themselves of it, whose pursuits will be specially facilitated by its use.^ ^ The price of this instrument, with one pair of Eye-pieces, two Objectives (a best 1-inch and a l-4th of 110°), and a Condenser for opaque objects, in case, is £12 15s. Od. 2 The price of this Binocular, with one pair of Eye-pieces, a dividing Objective of 1 inch and 2 inches, and a l-4th inch of 70% in Case, is 10 guineas. 5 crouch's students' binocular. CONSTRUCTION OF THE MICROSCOPE. 67 Excellent Students' Microscopes are now produced by many other Makers; among whom Messrs. Beck should be particularly mentioned, as having led the way in supplying low-priced but really serviceable instruments, such as could at that time only be obtained on the Continent. Their * Economic ' Microscope framed on the Continental model, and furnished with good Objectives of 1 inch and l-4th inch, is sold for 5 Guineas; and other Objectives specially constructed, for it, ranging to the l-16th inch, with a complete set of Accessories, are supplied' at a very moderate cost. The same Makers supply an * Economic' Wenhamj Binocular, having two pairs of Eye-pieces, three Objectives, a glass rotating' Stage, and a jointed lengthening arm to the Mirror (which allows it to be used above the Stage for the illumination of opaque objects) for 10 Guineas. — Mr* Collins also supplies a 10 guinea Wenham Binocular, with Objectives of 1 inch and l-4th inch (80''), the latter being specially adapted for use with the Binocular, by a short mount which brings it close to the Wenham prism. — Mr. Swift makes Baker's Student's Erecting Binocular. a * College ' Microscope, in which the Stage is fitted with a revolving diaphragm- plate of ingenious construction, that brings its apertures up to the level of the object-slide. Of Mr. Crouch's and Messrs. Parkes's Students' Microscopes also, the Author can speak with approval, as regards both the mechanical and the optical part of their work. 65. Second-Class Microscopes. — ^Under this head may be ranked those instruments which combine first-rate workmanship with simplicity in the, plan of construction; and which may be consequently designated as ^ Supe- rior Students' Microscopes.' Among these the first place should be given to Messrs. Powell and Lealand's Smaller Microscope (Fig. 48), which was long the favorite instrument of British Histologists, and which, though not adapted for objects requiring very oblique light, is still in demand 68 THE MICROSCOPE AND ITS REVELATIONS. among those who value first-rate workmanship, with all convenient appli- ances for ordinary Biological research. A Sub-stage (not shown in the figure) carrying every kind of illuminating apparatus, can be attached beneath the stage; and the large angular aperture now given by Messrs. Powell and Lealand to their Immersion Achromatic Condenser, enables , this instrument to resolve the most difficult test-objects. The stand is \ well suited to carry a Bino- cular body; which may be fitted not *^only with the or- dinary stereoscopic ^ Wen- ham^ prism, but also with the non-stereoscopic arrange- ment of these Makers (§ 81), which enables even the highest powers to be used binocularly, though not ste- reoscopicaily. 66. The value of Stereo- scopic Binocular vision in Scientific investigation be- ing now admitted by all who have really worked with it tipon siiitable objects, the Author would earnestly re- commend every one about to provide himself with even a Second-class Microscope, to incur the small expense of the Binocular addition. This addition, however, will lose an important element of its value, if the Stage of the instrument be not adapted to rotate in the oi3tic axis of the Body; so that objects which are being viewed by incident light may be pre- sented to the illuminating rays in every direction. Among the first to recog- nize this principle, and to apply it in practice, were Messrs. Beck; whose Popular Microscope (Plate IV.), devised by the late Mr. E. Beck, will be found very suitable to the wants of such as work with low and moderate powers upon objects for the study of which Binocular vision is peculiarly advantageous; and especially serviceable to Travellers, as the ingenious way in which it is framed and supported enables it to bear a good deal of rough usage without injury. The original Eoss model here adopted in the support and movement of the body, is sufficiently steady when only moderate powers are employed; and the stem that forms the centre of the whole, is swung immediately behind the stage on a broad stay G, which, again, is attached by a pair of centres at its lower angles to the triangular base r. The lower end h of the stem carries a stout projecting pin, which fits into various holes along the median line of the base; whereby the instru- Powell and Lealand's Smaller Microscope. CONSTRUCTION OF THE MICROSCOPE. 69 EDATE ly. beck's popular microscope. 70 THE MICROSCOPE AND ITS REVELATIONS. ment may be firmly steadied in positions more or less inclined, or may be fixed upright. It may be also fixed in the horizontal position required for drawing with the Camera Lucida; for the pin at the bottom of the stem then enters the hole at the top of the stud k, and the stay g falls flat down, resting on the top of the stout pin L. The advantages of this construction are that it is strong, firm, and yet light; that the instru- ment rests securely at the particular inclination desired, which is often not the case on the ordinary construction when the joint has worked loose; and that in every position there is the needful preponderance of balance. The Stage D is circular, and upon it fits a circular plate T, which rotates in the optic axis of the Microscope. On the plate T there slides the Object-holder u, which is so attached to it by a wire spring that bears against its under surface, as to be easily moved by either or both hands; and as access can be readily gained to this spring by detach- ing the plate T from the stage, it may either be removed altogether so as to leave the stage free, or may be adjusted to any degree of stiffness desired by the observer. The object-holder has a ledge V for the support of the slide; and it is also provided with a small spring w, attached to it by a milled-head, by turning which the spring may be brought to bear with any re- quired pressure against the edge of the slide laid upon the object-holder, so as to prevent it from shifting its place when rotation is given to the stage, or when, the instrument being placed in the horizontal posi- tion, the stage becomes verti- cal. The central tube of the Stage is furnished with a rota- ting Diaphragm-plate, and is adapted to receive various ^ther fittings; and a Side-Con- denser on a separate stand is also supplied. * 67. Collinses Harley Bin- ocular. — This instrument, as represented in Eig. 49, is sub- stantially framed and well hung on the Eoss model; but is now made also on the Jack- son model at the same price. The caps of the Eye-i)ieces are provided with shades, which cut off the outside lights from each eye; these can be adapted to any instrument, and the Author can speak strongly of coiiins's Harley Binocular. their value from his own ex- i ; ^ ^ The price of this instrument, with two pairs of Eye-pieces, three Objectives (a 2-inch of 10% a 1-inch of 22% and a l-4th of 75°), and Side-Condenser on stand, in Case, is £16 10s. CONSTRUCTION OF THE MICROSCOPE. 71 perience. The Wenham prism at the common base of the bodies is fitted into an oblong box, which slides through the arm that carries them; this contains, in addition, a Nicol analyzing prism, and is also pierced with a , vacant aperture; so that, by merely sliding this box transversely until its aperture comes into the axis, the instrument may be used as an ordinary Monocular; or, if the analyzing prism be made to take the place of the Wenham, whilst the polarizing prism beneath the stage is brought into position by rotating the Diaphragm-plate in which it is fixed, it is at once converted into a Polarizing Microscope — with the disadvantage, however, of not being then Binocular. It has also a ^nose-piece' carrying two Objectives, by a sliding movement of which one power may be substi- tuted for the other/ 68. Swiff s Challenge Micro- scope, — The instrument con- structed under this designation by Messrs. Swift, is one of which it may be fairly said that it is surpassed by no other of its price in the excellence of its work- manship, and its suitability to the general wants of the Micro- scopist. The support on which it is hung is extremely firm and substantial without being heavy; and when the limb is brought to the horizontal position, resting on the cross plate between the two uprights, the instument is still well balanced. The rack and pinion movement is made with oblique teeth; a construc- tion which favors smoothness and sensitiveness in the adjust- ment, so that a l-4th inch objec- tive may be focussed by it alone. The fine adjustment is made by the milled-head at the lower end of the body. — It is a peculiarity in this instrument, which espe- cially fits it for those who work much with Polarized light, that Swift's challenge Microscope. the analyzing prism is fitted into the body above the Wenham prism, in such a manner that, when its fitting is drawn out (without being removed), it is completely out of the way of the light-rays; whilst, when the use of the Polariscope is required, the prism can be at once pushed into the body, working in conjunction with the Wenham prism. This mode of mounting the analyzer is found to interfere much less with the definition of the objec- tive, than the insertion of it between the objective and the Wenham ^ The price of this instrument, with Mechanical rotatinj^ Stage, two pairs of Eye-pieces, two Objectives (either a 2-inch of 12°, or a 1-inch of 18°, with a l-4th of 95°), Side-Condenser on Stand, and Polarizing apparatus in Cabinet, is £19. Accessories of various kinds can be readily fitted to it. — A * first-class ' Binocular is also constructed by the same Maker on the Jackson model. 72 THE MICROSCOPE AND ITS KEVELATIONS. prism. The stage rotates in the optic axis; and may either bear (as in the figure) a sliding object-carrier, or may be furnished with mechanical actions* The mirror is attached to the stem by a crank-arm, allowing it to be so placed as to reflect light of considerable obliquity. Beneath the Stage is a broad horizontal dovetail groove, into which is very exactly fitted a firm (sprung) slide that cames a Sub-stage for illuminating appa- ratus, fitted with a vertical rack movement, and with horizontal center- ing screws; this arrangement (devised by Mr, Swift) enables the sub- Browning's Smaller Stephenson Binocular. stage to be placed in position or removed, without disturbing either the stage or the mirror. The extremely ingenious Universal Sub-stage — combining Achromatic Condenser, Black-ground Illuminator, and Pola- rizer with varied adaptations — devised by Mr. Swift for this Microscope, but capable of being applied to any other, will be described hereafter (§ 112). The Author, having had his instrument (thus fitted) in constant CONSTRUCTION OF THE MICROSCOPE. 73 use for several years past, feels justified in unreservedly expressing his high appreciation of it/ 69. Browning^s Smaller 8teplienso7i Binocular. — This instrument, represented in Fig. 61, is of more substantial build than the Students' Binocular of Messrs. Baker (§ 64); and is further distinguished by.its special adaptation for use with Polarized light. In place of the reflecting prism at the junction of the inclined bodies, a plane piece of dark glass, silvered on one face, is hung on a horizontal axis at the polarizing angle; its silvered face being turned in front v^hen it is used for ordinary pur- poses, so as to reflect into the two inclined bodies, the light-rays which proceed to it from the pair of dividing prisms; whilst, when it is to act as an analyzer, it is turned on its axis by means of a milled-head so as to bring the dark-glass surface to the front. Further, by fixing into the arm the tube which carries the objective, with its fine adjustment, and by making that which contains tlie dividing prisms and mirror, and which also carries the double body, slide over it, the latter can either be turned half round, so as to point the eye-pieces in the reverse direction (for the exhibition of the object to an observer sitting at the opposite side of a small table) without any disturbance of the adjustments; or it can be lifted off altogether, and replaced by an ordinary Monocular body,^ FIEST-CLASS MICROSCOPES. 70. "We now pass to an entirely different class of Instruments— those of which the aim is, not simplicity, but perfection; not the production of the best effect compatible with limited means, but the attainment of everything that the Microscope can accomplish, without regard to cost or complexity. To such, of course, the Stereoscopic Binocular is an in- dispensable addition; and it is not less essential that the Stage should have a rotatory movement in the Optic axis of tlie instriment; — not only for the due examination of opaque objects, as already mentioned (§ 66), but also because this movement is requisite for the effective examination of very delicate transparent objects by Oblique light, allowing the effect of light and shadow to be seen in every direction; and, in addition, be- cause in the examination of objects under Polarized light, a class of ap- pearances is produced by the rotation of the object between the prisms, which is not developed by the rotation of either of the prisms themselves. 71. Boss's First-class Microscope,— As what is known as the Boss model is still made, being preferred by some purchasers, we shall com- mence with a notice of the original form of the instrument which has gained so high a celebrity. — The general plan of this Microscope, as shown in Fig. 52, is carried out with the greatest attention to solidity of construction, in those parts especially which are most liable to tremor, ^ The price of this instrument in the simple form here figured, with one pair of Eye-pieces and best 1-inch and l-4th inch (80°) Objectives, and Condensing lens on separate stand, in Case, is £14. A mechanical stage costs £2 10s, additional, and the sub-stage (without fittings) £2 2s. — A very ingenious * swinging sub- stage ' has been lately devised by Mr. Swift ('' Journ. of Koy. Microsc. 8oc.," vol. iii., 1880, p. 867) for obtaining illumination of any degree of obliquity, even by two pencils at once. The Condenser is made to slide on an arc-piece (as in Mr. Grubb's arrangement, § 72), which is prolonged above the Stage for opaque illumination; and with this may be combined a second arc-piece at right angles to the first, carrying a second Condenser, which is found serviceable in the reso- lution of difficult Diatom-tests. ^ The price of this instrument, with one pair of Eye-pieces and Objectives of 1 inch (16°) and l-4th inch (75°), is £20. Any Accessories can readily be added to it. 74 THE MICROSCOPE AND ITS REVELATIONS. as also to the due balancing of the weight of its different parts upon the horizontal axis. Any inclination may be given to it; and it may be fixed in any position by a clamping screw, turned by a short lever on the right- hand upright. The ^fine^ adjustment is effected by the milled-head on the transverse arm just behind the base of the ' body;' this acts upon the ' nose ' or tube projecting below the arm, wherein the objectives are screwed. The other milled-head, seen at the summit of the stem, serves the circular rack, moreover, enables it to be used as a Goniometer (§ 92). Below the stage, and in front of the stem that carries the mirror, is a dovetail sliding-bar, which is moved uj) and down by the milled-head shown at its side; this sliding-bar carries what is termed by Mr. Boss the ^Secondary stage ' (shown separately at b), which consists of a tube for the reception of the Achromatic Condenser, Polarizing prisms, and other fittings. To this secondary stage a traversing movement of limited extent is given by means of two screws, one on the front and the other on the left-hand side of the frame which carries it, in order that its axis may be brought into perfect coincidence with the axis of the body; and a rotatory movement also is given to it by the turning of a milled-head, which is occasionally useful, and the exact amount of which is measured by a graduated circle. — The special advantages of this instrument consist in Ross's First-class Microscope. Tig. 52. to secure the transverse arm to this, and may be tightened or slackened at pleasure, so as to regulate the traversing movement of the arm; this movement is only allowed to take place in one direction, namely, towards the right ride, being checked in the opposite by a ' stop,' which secures the coincidence of the axis of the principal ^body' with the centre of the stage, and with the axis of the illuminating appa- ratus beneath it. The ob- ject-platform, to which rect- angular traversing motions are given by the two milled- heads at the right of the stage, is also made to rotate in the optic axis by a milled- head placed underneath the stage on the left-hand side; this turns a pinion which works against a circular rack, whereby the whole apparatus above is carried round about two-thirds of a revolution, without in the least disturbing the place of the object, or removing it from the field of the Micro- scope. The graduation of CONSTRUCTION OF THE MICROSCOPE. 75 its general steadiness, in the admirable finish of its workmanship., and in the variety of movements which may be given both to the object and to the fittings of the secondary or sub-stage. Its disadvantages consist in the want of portability that necessarily arises from the substantial mode of its construction; and in the liability to tremor in the image, when the liighest powers are used, through the wanfe of support to the body along its length (§ 49). — This last consideration has induced Messrs. Boss to adopt the * Jackson-model^ in their more recent Microscopes; the newest and most complete form of which will be next described. 72. Boss's Improved Jackson- Zeiitmuyer Microscope. — In this admir- able instrument (Plate v.) the Jackson-model is followed as to general construction, whilst it is improved-on in various important particulars. The ^limb^ that supports the principal body with the usual rack-and- pinion slide for coarse adjustment, carries also a second (or focussing) slide at the back of the first, to which a slow up and-down movement is given by a lever passing through a channel in the limb, which is acted- on by a micronometer screw with a large milled-head placed in a very acces- sible position. This arrangement renders the fine adjustment quite free from either ^ twist' or ^ loss of time,' whilst permitting it to work with sufficient freedom; and has the advantage of not affecting the mag- nifying power by altering the length of the body. Further, if a divided scale (with a vernier) be engraved on the edge of the limb, the thickness of any uncovered object lying on the stage can be measured with great exactness. The rotating stage-plate (graduated at its edge to serve as a Goniometer), is supported upon a firm ring composed of metal of pecu- liar inflexibility; and to this it can be secured in any azimuth by a clamping-screw beneath. Its single traversing platform is moved in rectangular directions by two milled-heads placed on the same axis, that work a combination of screw and pinion (devised by the ingenuity of Mr. Wenham), which is placed above instead of beneath it; and in this device more oblique light (it is affirmed) can be brought to bear upon the lower surface of the object, than in any other mechanical stage yet con- structed. The stage-ring is not immovably fixed to the limb, but is at- tached to a conical stem, which passes through the tubular pivot of the swinging ' tail-piece ' to be presently described, and is clamped at the back of the instrument by a strong screw and nut. Thus the stage may be made to incline toward either side at any angle, so that a view may be gained of the sides and edges of a solid object, as well as of its front; or it may be removed altogether, and replaced by any other form of ob- ject-support more suitable to the special requirements of the individual Microscopist. — The most important novelty, however, consists in the adoption of the (patented) Zentmayer method of giving to the entire illuminating apparatus any desired degree of obliquity. The Mdea' is by no means new; and it was carried- out many years ago by the late Mr. Grubb of Dublin, who fixed beneath the stage a sector or arc-piece of nearly a semi-circle having its centre in the object, upon which the at- tachments of the mirror and condenser were made to slide. But tlie arrangement devised by Mr. Zentmayer is not only far simpler, but also more effective. It consists in swinging the ' tail-piece' which carries the mirrow and the secondary or sub-stage, upon a pivot placed at the back of the stage, the horizontal axis of which is in a line with the point of intersection of the optic axis of the body with the plane of the object on the stage; so that the axis of the condenser shall always pass through that point, whatever may be its inclination to the perpendicular. By 76 THE MIOKOSCOPE AND ITS REVELAl'IOilS. OOfTPTRUCTION OF THE MICROSCOPE. 7T means of this arrangement, every kind of illuminating apparatus adapted to the sub-stage can be made to act at any obliquity whatever; and as the tail-piece may be swung round on the side opposite to that of the milled-heads of the traversing stage, until it is brought considerably above the stage, oblique illumination may be thrown by the condenser, not only on the under but also on the upper surface of any object. It is one great advantage of this method, that condensers of large angle of aperture are not required for the purpose of oblique illumination; the con- verging pencils given by ordinary Objectives of 1 inch or 1 J inch focus, used as condensers, being fully adequate. Further, the swinging tail-piece may be used to measure the angular aperture of Objectives in the manner to be hereafter described, its inclination to the optic axis being marked by a divided arc on its upper segment, which also enables the illuminating angle at which any particular object is best seen to be observed and re- corded. — Altogether, it may be unhesitatingly affirmed, that the Zent- mayer system enables the best results of oblique illumination to be ob- tained with greater facility than any other of equal effectiveness; while the simplicity of the construction of the whole instrument enables Messrs. Eoss to reduce its cost considerably below that of the old Eoss or Eoss- Jackson models. 73. Poioell and Leland^s Large Microscope, — These eminent Makers have not made any essential modification in the construction of their large Microscope, represented in Plate yii. ; preferring to furnish the very oblique illumination now in general demand by enlarging the angu- lar aperture of their Achromatic Condenser (§ 99). The chief peculiar- ity of their model consists in the attachment both of the Stage and Sub- stage to a large solid brass ring, which is firmly secured to the stem of the instrument. The upper side of this ring bears a sort of carriage that supports the stage; and to this carriage a rotatory movement around the optic axis of the principal body is given by a milled-liead, the amount of this movement (which may be carried through an entire revolution) being exactly measured by a graduated circle. The stage, which is furnished with the usual traversing movements, worked by two milled-heads on the same axis, is made thin enough to admit of the mirror being so placed, by means of its extending arm, as to reflect light on the object from out- side the large brass ring that supports the stage and sub-stage. Light of the greatest obliquity, however, may be more conveniently obtained by an Amici's prism (§ 102) placed above the supporting ring. The sub-stage is furnished with rotatory and rectangular, as well as with ver- tical movements. The instrument is so well balanced on its horizontal axis, that it remains perfectly steady without clamping, in whatever po- sition it may be placed. 74. Beclc's First-class Microscope, — It was by this Firm that the Jackson model was first adopted, for which the Author has already ex- pressed his preference (§ 49). Besides the steadiness imparted to the double body by the support given to it by the limb along the greater part part of its length, it is an additional advantage of this construction, that by continuing the limb beneath the stage, the secondary body or Sub- stage (which carries the illuminating apparatus) is made to work in a dovetailed groove that is ploughed-out in continuity with that in which the rack of the principal body slides, an arrangement obviously favorable to exactness of centering. The Stage has a nearly complete rotation in the optic axis of the instrument, motion being given to it by a milled- head beneath the stage, the pinion attached to which can be readily 78 THE MICKOSCOPE AND ITS KEVELAVIONS. CONSTRUCTION OF THE MICROSCOPE. ELATE YIL POWELL AND LEALAND'S LARGE MICROSCOPB. 80 THE MICROSCOPE AND ITS REVELATIONS. thrown out of gear when a more rapid rotation of the stage by hand is desired; and it bears a graduated circle at its margin for the measure- ment of angles. It is fitted immediately beneath the object-platform with an iris-diaphragm^ worked by a lever action. 75. Bgc¥s Improved First- class Microscope, — In order to meet the demand for very oblique illumination, and to supply this in a mode yet more perfect than the Zentmayer system, Messrs. Beck have adapted to the preceding instrument a swinging sub-stage, carried by an arm that works radially upon a large vertical disk attached to the limb, on the plan originally suggested by Mr. Grubb; his semi-circle being extended, how- ever, into a nearly complete circle, so as to allow the arm carrying the sub-stage and mirror to be brought round to the upper side of the stage, for the illumination of opaque objects. The essential feature of their construction, however, which differentiates it from every other yet devised, consists in a provision for adjusting the illuminating apparatus to the thickness of the glass slide on which the object is mounted. This is effected by making the disk with its radial arm, slide vertically in a dove- tail fitting; the illuminating apparatus attached to it, at whatever degree of obliquity it may be placed, being raised or lowered (by a lever-handle) in the optical axis of the instrument, so as to enable the illuminating cone to be exactly focussed in the object itself — which on the Zentmayer model, can only be done with precision when the upper surface of the slide is exactly in the plane of the horizontal axis of the swinging ^ tail- piece.' — The Stage also, in this elaborate instrument, is so attached to the limb by a firm pivot, as to be capable not only of being inclined to- ward either side at any angle, but also of being turned completely over, so as to allow the object to be viewed from its under side — a provision to which the Author's experience makes him attach a special value. First-class Binocular Microscope-Stands, copied (more or less closely) from either the Ross or the Jackson models, are also made by Messrs. Baker, Collins, Crouch, Pillischer, and Swift, as well as by other makers of whose work the Author has no personal knowledge. — That of Mr. Crouch is distinguished by a provision for meeting the difficulty which is continually experienced, of keeping the image in place during the rotation of the stage, especially with high powers; the adjustment which suits one Objective, not being good for another somewhat differently centered. This defect presents itself still more frequently when a * nose-piece ' is in use; its centering being rarely so exact as to be free from an error that makes itself very perceptible when a high power is exchanged for a low one. By means of two diagonal screws beneath the stage, worked by two milled-heads at its hinder margin, Mr. Crouch affords a ready means by which the observer can adapt the centering of his stage to any objective he may have in use. — Mr. Browning also constructs a First-class Stand for his Stephenson Binocular. MICROSCOPES POR SPECIAL PURPOSES, Of the large number of instruments which have been ingeniously de- vised, each for some particular use, it would be quite foreign to the pur- pose of this Treatise to attempt to give an account. A few forms, how- ever, may be noticed, as distinguished either by their special adaptiveness to very common wants, or by the ingenious manner in which the require- ments of particular classes of investigators have been met. 76. Dr, Beale's Pocket Microscope. — This instrument consists of an ordinary Microscope-body, the Eye-piece of which is fitted with a draw- tube that slides smoothly and easily; whilst its lower end is fitted into an outer tube, of which the end projects beyond the objective. Against this projecting end the object-slide is held by a spring, as shown in Fig. 53, CONSTBCCTION OF THE MICKOSCOPE. 81 being fixed (if necessary) by a screw-clip. The coarse adjustment is made by sliding the body through the outer tube which carries the ob- ject; and the fine adjustment by sliding the eye-tube in or out. The object, if transparent, is illuminated either by holding up the Microscope to a window or lamp, from which the rays may pass directly through it, or by directing it towards a mirror laid on the table at such an angle as to reflect light from either of these sources: if opaque, it is allowed to receive direct light through an aperture in the outer tube. The extreme simplicity and portability of this instrument (which when closed is only six inclics long) constitutes its special recommendation. "With due care even high powers may be use, the eye-piece adjustment giving the power of very exact focussing. Hence this Pocket Microscope may be conve- niently applied to the ]nirposes of Clinical observation (the examination of Urinary Deposits, Blood, Sputa, etc.), either in hospital or in private practice; whilst it may also be advantageously used by the Field Natural- ist in examining specimens of Water for Animalcules, Protophytes, etc. Dr. Beale's Demonstrating Microscope. 77. Dr. Beale^s Demonstrating Microscope. — The same instrument may be used for the purposes of Class-demonstration, by attaching its outer tube on a wooden support to a horizontal board, which also carries a small lamp attached to it in the required position (Fig. 53). The object having been fixed in its place, and the coarse adjustment made by sliding the body in the outer tube, these parts may then be im- movably secured, nothing being left movable except the eye-tube, by sliding which m or out the fine adjustment may be effected. Thus the whole apparatus may be passed from hand to hand with the greatest facil- ity, and without any probability of disarrangement; and every observer may readily ^ focus ' for himself, without any risk of injuring the object.^ 78. Bahefs Travelling Microscope, — An instrument has been devised by Mr. Moginie, which is but little inferior in portability to the Pocket Microscope of Prof. Beale, and has some advantages over it. The body (Fig. 54) slides in a tube which is attached to a stem that carries at its end a small Stage and Mirror. The stem itself contains a fine adjustment that is worked by a milled-head at its summit; and near to this is attached by pivot-joint a pair of legs, which, when opened-out, form with the stem a firm tripod support. The coarse adjustment having been made by sliding the body through the tube which grasps it, the fine adjustment is ^ The price of Dr. Beale's Clinical Microscope, as made by Mr. Collins, without Objectives, is £1 lis. 6d. That of the same instrument fitted up as a Demonstrat- ing Microscope, is £3 3s. — Mr. Collins also makes another Class and Demonstra- tion Microscope, or a pattern of Dr. Lawson's for £3 10s., without Objectives. 6 82 THE MICROSCOPE AND ITS REVELATIONS. made by the milled-head; and thus even high powers may be very con- viently worked. The legs being tubular, one of them is made to hold glass dipping-tubes, whilst the other contains needles set in handles, with three short legs of steel wire, by screwing which into the stem and stage, the Microscope may be used (though not without risk of overturn) in the marvel of ingenuity; while its workmanship is so excellent that its joints do not easily become loosened by wear, and can all be readily tightened when required. It is so steady as to bear being worked (as a Monocular) with even high powers; but its great advantage consists in its suitability to the Traveller, who either wishes (as often happens to the Author) to display to scientific friends in other countries a set of objects that can be most advantageously seen by the Binocular under low powers, or to avail himself of opportunities of examining on the spot any interesting speci- mens he may meet with. The instrument also carries Mr. Swift's Combination Sub-stage (Fig. 85), which can be packed, together with three Objectives, Side-Condenser, and several other Accessories, into a Case only 11 inches long, G|- inches wide, and 3|- inches deep, the whole weighing only 7^ lbs. 80. NaclieVs Chemical Microscope, — The inverted Microscope origi- nally constructed by MM. Nachet on the plan devised by Dr. J. Lawrence Smith, of Louisiana, U.S., for the purpose of viewing objects from their under side when heat or re-agents are being applied to them,^ has lately been improved by its constructor with a special view to meeting the re- quirements of observers engaged in the ' cultivation ' of the minute organ- ^ Instruments nearly resembling the above are made by Messrs. Murray and Heath, Mr. Browning, and Mr. Swift. ^ This idea was suggested at nearly the same time by Dr. Leeson; and was carried out in an instrument constructed for him by Messrs. Smith and Beck, vertical position. This in- strument may be specially recommended to those who, already possessing a superior Microscope, desire neither to encumber themselves with it whilst travelling, nor to ex- pose it to risk of injury, but wish to utilize its Objectives by means of a simple and portable arrangement.^ Baker's Travelling Microscope. 79, Simft's Portable Bin- ocular. — Carrying still further an idea originally worked-out by Messrs. Powell and Lea- land, Mr. Swift has devised a very complete Portable Bin- ocular, which can be folded into a very small compass, without any screwing or un- screwing, and can be thus set up, as in Pig. 55 A, or packed away, as at Fig. 55 b, with great facility, when once the manner of doing so has been learned. Its construction is a CONSTRUCTIOK OF THE MICROSCOPE. isms which act as fer- ments. The general ar- rangement of this instru- ment is shown in Fig. 56. On the table which forms its base, there rests a box containing a glass mirror silvered on its upper sur- face, which is phiced at such an angle as to reflect the light-rays received through the inverted Ob- jective mounted on the top of the box, into the body fixed into its oblique face. Over the objective is placed tlie Stage, above which again is the Mirror for reflecting light down- wards through the object placed upon it. The focal adjustment is made in the first place by means of a sliding tube which carries the objec- tive, and then by the micromet er-screw v, Avhich raises or lowers the stage. The platform on which the optical appar- atus rests, can be moved in rectangular directions by the two milled-heads, 0, t; and is furnished with two graduated scales, by means of which it may be brought with exact- ness into any position pre- viously recorded, so that any point of the object may be immediately re- found — an arrangement of special value in cultiva- t i o n - experiments. On the stage is a circular glass cell, G, for holding the fluid to be examined; in the bottom of this in a n aperture, which i s closed by a piece of thin cover-glass well cemented round its edges, thus al- lowing the use of high magnifying powers hav- Swift's Portable Binocular, as set up for use. Swift's Portable Binocular, as packed in case. 84 THE MICROSCOPE AND ITS REVELATIONS. ing a short yery focus; while its top is ground flat, so that a cover of thin plate-glass may be closely fitted to it by the intervention of a little grease or glycerine; the whole being secured in its place by three small uprights. The cell is furnished also with two small glass taps, r, R, with which india-rubber tubes are connected. By this cell, which may be made to serve as a moist, a warm, and a gas-chamber, experi- ments on the rarefaction and compression of air, and on the absorption of gases, can be made with great facility. For ' cul- tivation ' experiments, smaller cells are provid- ed, which are attached to brass-plates so arranged as to have always a fixed position on the stage. ^ 81. Non- Stereoscopic Binoculars, — The great comfort which is expe- rienced by the Microsco- pist from the conjoint use of both eyes, has led to the invention of more than one arrangement by which this comfort can be secured, when those high powers are required which cannot be employ- ed with the ordinary Stereoscopic Binocular. This is accomplished by Messrs. Powell and Lea- land by taking advantage of the fact already ad- verted to (§ 1), thatwhen Nachet's Chemical Microscope. a pencil of Pays falls obliquely upon the sur- face of a refracting medium, a part of it is reflected without entering that medium at all. In the place usually occupied by the Wenham prism, they interposed an inclined plate of glass with parallel sides, through which one portion of the rays proceeding upwards from the whole aper- ture of the Objective passes into principal body with very little change in its course, whilst another portion is reflected from its surface into a rectangular prism so placed as to direct it obliquely upwards into the secondary body (Fig. 57). Although there is a decided difference in brightness between the two images, that formed by the reflected rays being the fainter, yet there is marvellously little loss of definition in either, even when the 25th-inch objective is used. The disc and prism are fixed in a short tube, which can be readily substituted in any ordinary Binocu- lar Microscope for the one containing the Wenham prism. — Other arrange- ' A Mineralogical Microseope specially contrived by M. Nachet for minute Petrological researches, will be described at the end of Chap. xxi. CONSTRUCTION OF THE MICROSCOPE. 85 ments were long since devised by Mr. Wenham,^ with a yiew to obtain a greater equality in the amount of light-rays forming the two pictures; and he has lately carried one of these into practical effect^ with the advan- tage that the compound prism of which it consists, has so nearly the same shape and size as his ordinary stereoscopic prism, as to be capable of being mounted in precisely the same manner, so that the one may be readily ex- changed for the other. The axial ray preceding upwards from the objective, enters the prism a b d e f (Fig. 58) at right angles to its lower face, and passes on to where it meets the inclined face A b, at which this prism is nearly in contact with the oblique face of the right-angled prism ABC. By internal reflection from the former, and external reflec- tion from the latter, about half the beam h is ^j^^ ^ reflected within the first prism in the direction c h, while the other half pro- ceeds straight onwards through the second prism, in the direction c a\ so as to pass into the "principal body. The reflected half, meeting at d the oblique (silvered) surface d e, of the first prism, is again reflected in the direction d V ; and passing out of that prism perpendicular- ly to its surface a F, prO-PowellandLea- ceeds towards the J^^^scopic b^^^ g being 23erpendicular to each other. The rays emerging from the field-glass enter this prism by Its lower surface, and are reflected at i, upon the face a b h g e, from which they are again ^ Journal of Quekett Microscopical Club," July, 1879. ^ The Author has thus exhibited to his friends a Microscopic view of the Moon. ACCESSORY APPARATUS. 89 reflected upon the lower surface at the point k, and thence to the point L upon the vertical face c d G H, and lastly at the point m, upon the other yertical face D e f g; from which the image normally and com- pletely erected, i^ again sent back, to issue by the superior surface upon which the eye-glass is placed. All the reflections are total except tlie first at i; and the loss of light is far less than would be anticipated. — The ob- liquity which this Prism gives to the visual rays, when the Microscope is placed yertically for dissecting or for the examination of objects in fluid, is such as to bring them to the eye at an angle very nearly corresponding with that at which the Microscope may be most conveniently used in the inclined position (§ 41, iii.); so that, instead of being an objection, it is a real advantage. 87. Sorby-Broiuning Ificro-jSpectroscope,'— When the Solar ray is decomposed into a colored spectrum by a prism of sufficient dispersive power to which the light is admitted by a narrow slit, a multitude of dark lines make their appearance. The existence of these was originally noticed by Wollaston; but as Fraunhofcr first subjected them to a thor- ough investigation, and mapped tlicm out, they are known as Fraicn- Nachet's Erecting-Prism. hofer lines. The greater the dispersion given by the multiplication of prisms in the Spectroscope, the more of these lines are seen; and they bear considerable magnification. They result from the interruption or absorption of certain rays in the Solar atmosphere, according to the law, first stated by Angstrom, that ''rays which a substance absorbs are pre- cisely those which it emits when made self-luminous/' KirchhofE showed that while the incandescent vapors of Sodium, Potassium, Lithium, etc., give a spectrum Avith characteristic Iriglit lines, the same vapors intercept portions of white light, so as to give darh lines in place of the bright ones, absorbing their own special color, but allowing rays of other colors to pass through. — Again, when ordinary light is made to pass through colored bodies (solid, liquid, or gaseous), or is reflected from their sur- faces, so as to affect the eye with the sensation of color, its spectrum is commonly found to exhibit absorption bands, which differ from the Fraun- liofer lines, not only in their greater breadth, but in being more or less nebulous or cloudy, so that they cannot be resolved into distinct lines by magnification, while too much dispersion thins them out to indistinct- * For general information on the Spectroscope and its uses, the student is re- ferred to Professor Roscoe's " Lectures on Spectrum Analysis," or the translation of Dr. Schellen's ''Spectrum Analysis." 90 THE MICROSCOPE AND ITS REVELATIONS. ness. Now, it is by the character of these bands, and by their position in the spectrum, that the colors of different substances can be most ac- curately and scientifically compared; many colors whose impressions on the eye are so similar that they cannot be distinguished, being readily discriminated by their spectra. The purpose of the Micro-Spectroscope is to apply the spectroscopic test to very minute quantities of colored sub- stances; and it fundamentally con- sists of an ordinary Eye-piece (which can be fitted into any Microscope) with certain special modifications. As originally devised by Mr. Sorby, and worked-out by Mr. Browning, the Micro-Spectroscope is constructed as follows (Fig. 61):— AboYC its Eye- glass, which is achromatic, and made Micro-specti^scope. capable of focal adjustment by the milled-head b lor rays oi dmerent re- frangibilities, there is placed a tube A, containing a series of five prisms, two of flint-glass (Fig. 62, F f) interposed between three of crown (c c c), in such a manner that the emergent rays r r, which have been separated by the dispersive action of the flint-glass ^-^^^ prisms, are parallel to the rays which enter the combination. Below the eye-glass, in the place of the ordinary stop, is a dia- phragm with a narrow slit, which limits the Arrangement ofprismsin Spectroscope admission of light; this cau be adjusted in Eye-piece. Vertical positiou by the milled-head h, whilst the breadth of the slit is regulated by c. The foregoing, with an Objective of suitable power, would be all that is needed for the examination of the spectra of objects placed on the stage of the Microscope, whether opaque or transparent, solid or liquid, provided that they transmit a sufficient amount of light. But as it is of great importance to make exact compa- risons of such artificial spectra, alike with the ordinary or natural Spec- trum, and with each other, provision is made for the formation of a second spectrum, by the insertion of a right-angled prism that covers one-half of this slit, and reflects upwards the light transmitted through n aperture seen on the right side of the eye-piece. For the production vyf the ordinary spectrum, it is only requisite to reflect light into this aperture from the small mirror i, carried at the side; whilst for the pro- duction of the spectrum of any substance through which the light re- flected from this mirror can be transmitted, it is only necessary to place the slide carrying the section or crystalline film, or the tube containing the solution, in the frame D d adapted to receive it. In either case, this second spectrum is seen by the eye of the observer alongside of that pro- duced by the object viewed through the body of the Microscope, so that the two can be exactly compared. 88. The exact position of the Absorption-bands is as important as that of the Fraunhofer-lines; and some of the most conspicuous of the latter afford fixed points of reference, provided the same Spectroscope be employed. The amount of dispersion determines whether the Fraunhofer- lines and Absorption-bands are seen nearer or farther apart; their actual positions in the field of view varying according to the dispersion, while their relative positions are in constant proportion. — The best contrivance ACCESSORY APPARATUS. 91 for measuring the spectra of absorption bands is Browning's Bright-line Micrometer shown m Fig. 63. At R is a small mirror by which light from the lamp employed can be reflected through E D to the lens c, which, by means of a perforated stop, forms a bright pointed imago on the surface of the upper prism, whence it is reflected to the eye of the observer. The rotation of a wheel worked by the milled-head M carries this bright point over the spec- trum, and the exact amount of motion may be read off to l-10,000th inch on the graduated circle of the wheel. To use this apparatus, the Fraunhof er lines must be viewed by sending bright day- light through the spectroscope, and the positions of the principal lines care- fully measured, the reading on the micrometer-wheel being noted down. A Spectrum-map may then be drawn on cardboard, on a scale of equal parts; and the lines marked on it, as shown in the upper half of Fig. 64. The lower half of the same figure shows an Ab- sorption-spectrum, with its bands at certain distances from the Fraunhofer lines. The cardboard Spectrum-map, when once drawn, should be kept for reference.' 89. A beginner with the Micro- Spectroscope should first hold it up to the sky on a clear day, without the intervention of the microscope, and note the effects of opening and closing the slit by rotating the screw c (Fig. 61); the lines can only be well seen when the slit is reduced to a narrow open- ing. The screw H diminishes the length of the slit, and causes the spectrum to be seen as abroad or a narrow ribbon. The screw e (or in some patterns two small sliding-knobs) regulates the quantity of light admitted through the square aperture seen between the points of the springs D D. — Water tinged with port wine, madder, and blood, are good fluids with which to commence this study of absorption-bands. They may be placed in small test tubes, in flat glass cells, or in wedge-shaped cells.^ As each color varies in refrangibility, the focus must be adjusted by the screw b. Fig. 61, according to the part of the spectrum that is examined. — When it is desired to see the spectrum of an exceedingly Bright-line Spectro-Micrometer. ^ Mr. Swift has devised an improved Micro-Spectroscope, in which the Micro- metric apparatus is combined with the ordinary Spectroscopic Eye-piece, and two spectra can be brought into the field at once. — Otiier improvements devised by Mr. Sorby, and a new form devised by Mr. F. H. Ward, have been carried into execution , by Mr. Hilger. (See Journ. of Roy. Microsc. Soc," Vol. i., 1878, p. 326, and Vol. ii., 1879, p. 81.) Another construction possessing some advantages over the origi- nal form, has been devised by Zeiss of Jena (See ''Journ. of Roy. Microsc. Soc," Vol. iii., 1880, p. 703). -A series of specimens, in small tubes, for the study of Asorption- spectra, is kept on sale by Mr. Browning ; and the directions given in his *' How to Work with the Micro-Spectroscope " should be carefully attended to. 92 THE MICROSCOPE AND ITS EEVELATIONS. minute object, or of a small portion only of a larger one, tlie prisms are to be removed by withdrawing the tube containing them ; the slides should then be opened wide, and the object, or part of it, brought into the centre of the field ; the vertical and horizontal slits can then be partly shut, so as to inclose it, and if the prisms are then replaced, and a suitable objective employed, the required spectrum will be seen unaffected by adjacent objects. For ordinary observations, Objectives of from 2 inches to 2- ^ 3ds inch focus will be found most suitable ; but for very minute quan- tities of material a higher power must be employed. Even a single Eed Blood-corpuscle may be made to show the characteristic Absorption -bands represented (after Prof. Stokes) in Fig. 65/ 90. MicrometriG Apparatus, — Although some have applied their micrometric api)aratus to the Stage of the Microscope, yet it is to the Eye -piece that it may be most advantageously adapted.^ The Cohweb Micro?netery invented by Eamsden for Telescopes, is probably, when well constructed, the most perfect instrument that the Microscopist can em- upper half, Map of Solar Spectrum, showing Fraunhofer lines. Lower half, Absorption Spefe trum, showing position of Bands in relation to lines. ploy. lb is made by stretching across the field of an Eye-piece two verj delicate parallel wires or spider's threads, one of which can be separated from the other by the action of a micrometer screw, the head of which is divided at its edge into a convenient number of parts, which success- ively pass-by an index, as the milled-head is turned. A portion of the field of view on one side is cut off at right angles to the filaments, by i\ scale formed of a thin plate of brass having notches at its edge, whoso distance corresponds to that of the threads of the screw, every fifth notch being made deeper than the rest for the sake of ready enumeration. The object being brought into such a jiosition that one of its edges seems ^ For further information on The Spectrum Method of Detecting Blood," see an important paper by Mr. Sorby, in Monthly Micros. Journal," Vol. vi. (1871), p. 9. 2 The Stage-Micrometer constructed by Fraunhofer is employed by many Continental Microscopists ; but it is subject to this disadvantage— that any error in its performance is augmented by the whole magnifying power employed ; whilst a like error in the Eye-piece Micrometer is increased by the magnifying power of the eye-piece alone. — Dr. Royston-Pigott has pointed out (" Monthly Micros. Journ.," Vol. ix., 1873, p. 2.), that by placing the Cobweb Micrometer at some distance beneath the stage, and by forming an aerial image of it (by an interposed lens) in the plane of the object ; the delicacy and accuracy of its measurements may be greatly increased ; the numerical value of each division being reduced, in proportion to the reduction in the size of the aerial image, which will of course be determined by the focal length of the lens that forms it, and by the distance of the Micrometer beneath it. ACCESSORY APPARATUS. 93 to touch the stationary filament, the other thread is moved by the micro- meter-screw until it appears to lie in contact with the other edge of the object ; the number of the entire divisions on the scale shows how many complete turns of the screw must have been made in thus separating the filaments, while the number to which the index points on the milled- head shows what fraction of a turn may have been made in addition. It is usual, by employing a screw of 100 threads to the inch, to give to each division of the scale the value of 1-lOOth of an inch, and to divide the milled -head into 100 parts ; but the absolute value of the divisions is of little consequence, since their micrometric value depends upon the Objective with which the instrument may be employed. This must be determined by means of a ruled slip of glass laid upon the stage ; and as the distance of the divisions even in the best-ruled slips is by no means uniform, it is advisable to take an avarage of several measurements, both upon different slips, and upon different parts of the same slip. Here the Drawtube Avill be of es- sential use, in enabling the Micro- scopist to bring the value of the divisions of his Micrometer to even numbers, — The Microscopist who applies himself to researches requiring micrometric measure- ment, should determine the value of his Micrometer with each of the Objectives he is likely to use for the purpose ; and should keep a table of these determinations, recording in each case the extent to which the tube has been drawn out, as marked by the graduated scale of inches which it should possess. And he should also make an accurate estimate of the thick- ness of the Cobweb-threads them- selves : since, if this be not prop- ^ ^ ^ ^ , ^ in T P • -L J 1 Spectroscopic appearance or fresn bcarlet erly allowed for, a serious error bw- 2 of Deoxydized Biood (cmorine) ; 3, of will be introduced into the mea- H^Bmatm; oj^tained by acting on cruorine with an . T , , 1 • • , acid : 4, of Hsematm reoxydized. surements made by this instru- ment, especially when the spaces measured are extremely minute. (See Michell, in Transact. Micros. Soc/' N. S., Vol. xiv., p. 71.) 91. The costliness of the Cobweb Micrometer being an important obstacle to its general use, a simpler method (devised by Mr. G. Jackson) is more commonly adopted; which consists in the insertion of a transpar- ent scale into an ordinary Huyghenian Eye-yiece in the focus of the eye- glass, so that the image of the object is seen to be projected upon it. This scale is ruled like that of an ordinary measure {i.e., Avith every tenth line lo7ig, and every fifth line half its length) on a slip of glass, which is so fitted into a brass frame (Pig. 66, b), as to have a slight motion towards either end; one of its extremities is pressed-upon by a fine milled-head screw which works through the frame, and the other by a spring (con- cealed in the figure) with antagonizes the screw. The scale thus mounted is introduced through a pair of slits in the Eye-piece tube, immediately above the diaphragm (Eig. 66, a), so as to occupy the centre of the field; THE MICROSCOPE AND ITS REVELATIONS. and it is brought accurately into focus by unscrewing the eye-glass until the lines of the scale are clearly seen. The value of the divisions of this scale must be determined by means of a ruled Stage-micrometer, as in the former instance, for each Objective employed in micrometry, the use of the Draw-tube enabling the proportions to be adjusted to even and con- venient numbers); and this having been accomplished, the scale is brought to bear upon the object to be measured, by moving the latter as nearly as possible into the centre of the field, and then rotating the Eye-piece in such a manner that the scale may lie across that diameter which it is de- sired to measure. The pushing screw at the extremity of the scale being then turned until one edge of the object appears to be in exact contact with one of the long lines, the number of divisions which its diameter occupies is at once read-off by directing the attention to the other edge — the operation be- ing nothing more than laying a rule across the body to be measured.^ This method of measurement may be made quite exact enougli for all ordi- nary purposes, provided, in the first place, that the Eye-piece scale be divided with a fair de- gree of accuracy; and secondly, that the value of its divisions be ascertained (as in the case of the Cobweb-Micrometer) by sev- eral comparisons with a ruled scale laid upon the Stage. Thus if, by a mean of numer- Jackson's Eye-piece Micrometer. ^^^g observations, WC establish the value of each division of the eye-piece scale to be 1-12, 500th of an inch, then, if the image of an object be found to measure tS^ of those divisions, its real diameter will be 3^+ ysItto Wtt ii^ch.^ With an Objective of l-12th-inch focus, the value of the divisions of the Eye-j)iece scale may be reduced to 1-25, 000th of an inch; and as the eye can estimate a fourth part of one of the divisions with tolerable accuracy, it follow that a mag- nitude of as little as 1-100, 000th of an inch can be measured with a near approach to exactness. — Even this exactness may be increased by the Application of the diagonal scale (Fig. 67) devised by M. Hartnack. The ^ Dr. Royston-Pigott (Zoc. cit.) prefers to introduce into the aperture of the diaphragm a plano-convex lens of very long focus, with the lines engraved upon its flat surface. The advantage of the screw-movement is sacrificed, but a greater distinctness of the lines is obtained. ® The calculation of the dimensions is much simplified by the adoption of a Decimal scale; the value of each divison being made, by the use of the Draw-tube adjustment, to correspond to some aliquot part of a ten-thousandth or a hundred- thousandth of an inch, and the dimensions of the object being then found byl simple multiplication: — Thus (to take the above example) the value of each divi- sion in the decimal scale is .00008, and the diameter of the object is .00028. The Metric system being now universally employed on the Continent, many British and American Microscopists prefer to record their observations in parts of a Milli- metre; and with a view to their convenience Messrs. Beck supply Stage-Microme- ters ruled on one side of a median line to lOOths and lOOOths of an Inch, and on the other side to lOOths of a Millimetre. ACCESSORY APPARATUS. 95 J / •"•1 , ■ I ■ 6 ill ili::-ltilt£t liilLuliAMini : ; vertical lines are crossed by two parallel lines, at a distance from each other of five divisions of the vertical scale; and the parallelogram thus formed is crossed by a diagonal. It is obvious from this construction, that the lengths of the lower segments of the 50 vertical lines, cut off by the diagonal, will progressively increase from .1 to 5.0; so that when it is desired to obtain an exact measurement of an object between these limits it is only requisite to find the segment Avhose length precisely coincides with the diameter to be taken, which it Avill then give in tenths of the value of the vertical divisions, whatever these may be. Thus, at a, the length of the segment will be 1.8; at i it will be 3.4. — Whatever method be adopted, if the measure- ment be made in the Eye- Pi<3.,67. piece and not on the stage, it will be necessary to make allowance for the adjust- ment of the Object-glass to the thickness of the glass that covers the object, since its magnifying power is considerably affected by Hartnack's Eye-piece Micrometer. the separation of the front pair of lenses from those behind it (§ 17). It Avill be found convenient to compensate for this alteration by altering the Draw-tube in such a manner as to neutralize the effect produced by the adjustment of the Objective; thus giving one uniform value to the divisions of the Eye- piece scale, whatever may be the thickness of the covering-glass; the amount of the alteration required for each degree must of course be deter- mined by a series of measurements with the Stage-micrometer. — Micro- metric measurements may also be made with the Camera Lucida, in the manner to be presently described, or with Dr. Beale's neutral tint reflector (§ 94). 92. Goniometer, — When the Microscope is employed in researches on minute Crystals, their angles may be measured by adapting a Goniometer to the Eye-piece; but as all First-Class Microscopes are now provided with rotating Stages graduated at their edges, with tlie addition of a Vernier-scale if desired, the measurement may be more conveniently made by giving rotation to the object. An Eye-piece is required whose field is traversed diametrically by fixed line (either a filament stretched across it, or a line ruled on glass), and is turned so as to bring this line into coincidence with one of the lines forming the angle to be measured, when the Stage is at zero; the stage is then rotated until the fixed line coincides with the other line of the angle, and the amount of movement is read off on the scale. — If a higher degree of precision be required than either of these methods is fitted to afford, the Douile Refracting Gonio- meter, invented by Dr. Leeson, may be substituted.^ 93, Diapliracpn Eye-piece, — It is often useful to cut off the light sur- rounding the object or part of the object to be examined; for the sake alike of avoiding glare that is injurious to the eye, and of rendering the features of the object more distinct. This may be accomplished on the plan of Mr. Slack, by the introduction, just above the ordinary ' stop,' of ^ For a description of this instrument, see Dr. Leeson 's description of it in Part xxxiii. of the "Proceedings of the Chemical Society," and Mr. Eichard Beck's *• Treatise on the Microscope," p. 65. 96 THE MICROSCOPE AND ITS REVELATIONS. four small shutters, worked by as many milled-lieads projecting slightly beyond the flange of the eye-piece. By combining the movements of these shutters in various ways, it is easy to form a series of symmetrical aper- tures, bounded by straight lines, and of any dimensions required. As remarked by its inventor, this Diaphragm Eye-piece may also be used to isolate one out of many objects that may be on the same slide, and thus to show that object alone to persons who might not otherwise distinguish it. — For this last purpose the Indicator of Mr. Quekett may also be used; which is a small steel hand placed just over the diaphragm, so as to point to nearly the centre of the field, whilst it may be turned back when not required, leaving the field of view quite free. The particular object or portion of the object to which it is desired to direct attention, being brought to the extremity of the hand, is thus at once ^ indicated ^ to any other observer. Microscope arranged with Camera Lucida, for Drawing or Micrometry, 94. Camera Lucida and otlier Drcming Apparatus. — Various contri- vances may be adapted to the Eye-j)iece, in order to enable the observer to see the image jorojected upon a surface whereon he may trace its out- lines. The one most generally employed is the Camera Lucida prism contrived by Dr. Wollaston for the general purposes of delineation; this being fitted on the front of the eye-piece, in place of the ' cap ^ by which it is usually surmounted. The Microscope being placed in a horizontal position, as shown in Fig. 68, the rays which pass through the eye-piece into the prism sustain such a total reflection from its oblique surface, that they come to its upper horizontal surface at right angles to their previous direction; and the eye being so placed over the edge of this surface as to receive these rays from the prism through part of the pupil, whilst it looks with the other half beyond the prism down to a white paper surface on the table, it sees the image so strongly and clearly projected upon that surface, that the only difficulty in tracing it arises from a certain incapa- city which seems to exist in some individuals for seeing the image and the tracing-point at the same time. This difficulty (which is common to all instruments devised for this purpose) is lessened by the interposition of a slightly convex lens in the position shown in the figure, between the ACCESSOKY APPARATUS. 97 eye and the paper, in order that the rays from the paper and tracing- point may diverge at the same angle as those which are received from the prism; and it may be generally got over altogether, by experimentally modifying the relative degrees of light received from the object and from the paper. If the image bo too bright, the paper, the trp-cing- point, and the outline it has made, are scarcely seen; and either less light may be allowed to come from the object, or more light (as by a taper held near, may be thrown on the paper and tracing-point. Sometimes, on the other hand, measures of the contrary kind must be taken. — Another in- strument for the same jourpose, invented by the celebrated anatomist Soemmering, and ])ref erred by some Microscopists, is a flat speciilujn of polished steel or speculum-metal, of smaller diameter than tli^ ordinary j)upil of the eye, fixed at an angle of 45° in front of the eye-piece. The rays from the eye-piece are reflected vertically upwards to the central part of the pupil placed above the mirror, whilst, as the eye also receives rays from the paper and tracer in the same direction, through the peripheral Eis. Pig.' 70^ Chevalier's Camera Lucida. Nachet's Camera Lucida, portion of the pupil, the image formed by the Microscope is visually pro- jected downwards. — In another form of Camera Lucida, devised by Amici, and adapted to the horizontal microscope by Chevalier, the eye looks through the Microscoj)e at the object (as in the ordinary view of it), instead of looking at its projection upon the paper, the image of the tracing-point being projected upon the field — an arrangement which isiu many respects more advantageous. This is effected by combining a per- forated steel mirror with a reflecting prism; and its action will be under- stood by the accompyaning diagram (Pig. 69). The ray a b proceeding from the object, after emerging from the eye-piece of the Microscope, passes through the central perforation in the oblique mirror m, which is placed in front of it, and so directly onwards to the eye. On the other hand, the ray a' proceeding upwards from the tracing-point, enters the prism P, is reflected from its inclined surface to the inclined surface of the mirror M, and is by it reflected to the eye at i% in such parallelism to the rap b proceeding from the object, that the two blend into one image. — The 98 THE MICROSCOPE AND ITS REVELATIONS. same effect is produced by a contrivance which has been devised by MM. Nachet for use with vertical Microscopes, and is much employed on the Continent. It consists of a prism of a nearly rhomboidal form (Fig. 70), which is placed with one of its inclined sides A c, over the eye-piece of the Microscope; to this side- is cemented an oblique segment e, of a small glass cylinder, which presents to the ray a i, proceeding directly upwards from the object, a surface at right angles to it; so that this ray passes into the small cylinder E, and out from the side A B, of the larger prism, with- out sustaining any refraction, and with very little loss by reflection from the inclined surfaces at which they join. But the ray a' V , which comes from the tracing point on a paper at the end of the base of the Micro- scope, entering the rhomboidal prism, is reflected from its inclined side B D, to its inclined side A c, and thence it is again reflected to h, in coin- cidence with the ray which has directly proceeded from the object. As the ray a' V is necessarily oblique, the picture visually projected on the paper will be distorted, unless the right side of the drawing-board bo raised, so that its plane shall be at right angles to a* V, — Of the nume- rous contrivances for drawing from the Microscope, the simplest and by no means the least effective, is the Neutral Tint Reflector^ recommended by Dr. Beale, which consists of a piece of neutral-tint glass, set in a cap fitted on the Eye-piece, with which it makes an angle of 45^^. The Microscope being arranged as in Fig. 68, the eye, looking downwards, receives at the same time the image-forming rays from the eye-piece, wdiich come to it by reflection from tlie surface of the glass, and those from the paper, tracing-point, or rule, which pass to it through the glass. A simple and inexpensive substitute for this, which its inventor (Mr. T. B. Jennings, U.S.) has found very efficient, maybe made by taking a flat cork about 1^ inch in diameter, cutting a hole in it sufficiently large to enable it to fit tightly on the Eye-piece (without its cap), and then making a transverse slit beneath the hole, into which is to be inserted a thin-glass cover at an angle of 45°. 95. With one or other of the foregoing contrivances, every one may learn to draw an outline of the Microscopic image; and it is extremely desirable for the sake of accuracy, that every representation of an object should be based on such a delineation. Some persons will use one instrument most readily, some another; the fact being that there is a sort of a knack'' in the use of each, which is commonly acquired by practice alone, so that a person accustomed to the use of any one of them does not at first work well with another. Although some persons at once acquire the power of seeing the image and the tracing-point with equal distinctness, the case is more frequently otherwise; and hence no once should allow himself to be baffled by the failure of his first attempt. It will sometimes happen, especially when the AVollaston prism is employed, that the want of power to see the pencil is due to the faulty position of the eye, too large a part of it being over the prism itself. When once a good position has been obtained, the eye should be held there as steadily as possible, until the tracing shall have been completed. It is essential to keep in view that the proportion between the size of the tracing and that of the object is affected by the distance of the eye from the paper; and hence that if the Microscope be placed upon a support of different height, or the Eye-piece be elevated or depressed by a slight inclination given to the body, the scale will be altered. — This it is, of course, peculiarly important to bear in mind, when a series of tracings is being made of any set of objects which it is intended to delineate on a uniform ACCESSORY APPARATUS. 99 scale; or wlien the Camera Lucida (or any similar arrangement) is em- ployed for the purpose of Micrometry. All that is requisite to turn it to this account, is an accurately divided Stage-micrometer, which, being ])laced in the position of the object, enables the observer to see its lines projected upon the surface upon which he has drawn his outline; for if the divisions be marked upon the paper, the average of several taken, and the paper then divided by parallel lines at the distance thus ascer- tained (the spaces being subdivided by intermediate lines, if desirable), a very accurate scale is furnished, by which the dimensions of any object drawn in outline under the same power may be minutely determined. Thus, if the divisions of a Stage-micrometer, the real value of each of Avhich is a 100th of an inch, should be projected on the paper with such a magnifying power as to be at the distance of an inch from one another, it is obvious that an ordinary inch-scale applied to the measurement of an outline would give its dimensions in lOOths of an inch, whilst each tenth of that scale would be the equivalent of a 1,000th of an inch. When a sufficient magnifying power is used, and the dimensions of the image are measured by the ^ diagonal^ scale (which subdivides the inch into 1,000 parts), great accuracy may be obtained. It was by the use of this method, that Mr. Gulliver made his admirable series of measure- ments of the diameters of the Blood -corpuscles of different animals. — In using Nachet's vertical Camera for Micrometry, care must be taken so to adjust the slope of the drawing-board, that the Micrometer scale shall be projected on the paper without distortion. 96. Nose-piece. — It is continually desirable to be able to substitute one objective for another with as little ex- penditure of time and trouble as possible; so as to be able to examine under a higher magnifying power the details of an ob- ject of which a general view has been obtained by means of a lower; or to use the lower for the purpose of finding a minute object (such as a particular Dia- tom in the midst of a slide-full) which we wish to submit to high amplification. This is effected by the Nose-piece of Mr. C. Brooke, which, being screwed into the object-end of the body of the Micro- scope, carries two objectives, either of which may be brought into position by turning the arm on a pivot. In its origi- ^^^^^'^ improved Nose-piece, nal form, the arm was straight; so that the Objective not in use was often brought own upon the Stage, unless the relative lengths of the two objec- tives were specially adjusted. This inconvenience is avoided, however, in the construction adopted by Messrs. Powell and Lealand, and further sim- plified by Mr. Swift (Fig. 71); the bend given to the arm having the effect of keeping the Objective not in use completely off the stage. ^ The work- ing Microscopist will scarcely find any Accessory more practically useful to him than this simple piece of apparatus. 97. Finders. — All Microscopists occasionally, and some continually, feel the need of a ready means of -finding, upon a glass slide, the particu- lar object, or portion of an object, Avhich they desire to bring into view; and various contrivances have been suggested for the purpose. Where different magnifying powers can be readily substituted one for another^ 100 THE MICROSCOPE AND ITS REVELATIONS. as by the use of the Erector (§ 84) or of the Nose-piece, no special means are required; since, when the object has been found by a low power, and brought into the centre of the field, it is rightly placed for examination by any other Objective. Even this slight trouble, however, may be saved by the adoption of more special methods; among the simplest of which is marhing the position of the object on the surface of the thin glass which covers it. The readiest mode of doing this, when the object is large enough to be distinguished by the naked eye or under the Simple Microscope, is to make a small ring round it with a fine cameFs- hair pencil dipped in Asphalte, or Brunswick black (Indian ink being objectionable, as liable to be washed off when water-immersion Objectives are in use); but when the object is not thus visible, the slide must be laid in position on the stage, the object ^ found ^ in the Microscope, the Condenser adjusted to give a bright and defined circle of light, and then, the Microscope-body being withdrawn, the black ring is to be marked around the illuminated spot. Tliis method, however, has the disadvan- tage of concealing any other objects that may lie in dose proximity to the one around which the circle is drav/n; and recourse must be had in such cases to some other plan. Tlie Mechanical Stage may be easily turned to account as n> finder, by engraving upon it two scales, horizontal and vertical, by which the object-platform may be exactly set to any desired position; this platform being itself provided with a removable ^ stop, ^ against which the glass slide (resting on its lower edge) may so abut, as always to occupy the same place on the platform. Now sup- posing an observer to be examining a newly-mounted slide, containing any object which he is likely to wish to find on some future occasion, he first lays the slide on the object-platform, with its lower edge resting on the ledge,, and its end abutting against the lateral stop, and brings the object=platform itself to the zero of the scales; then, whenever, on moving the slide by the traversing action, he meets with any particular form worthy of note, he reads off its position upon the two scales, and records it in any convenient mode. The scale may be divided to 50ths of an inch, and each of these spaces may be again halved by the eye; and 26 the record may perhaps be best made thus, — Triceratium favus ^-^^ the upper number marking the ^ latitude^ of the object on the vertical scale, and the lower its longitude ^ on the horizontal. Whenever the Micro- scopist may wish again to bring this object under examination, he has merely to lay the slide in the same ])ositon on the platform, and to adjust the platform by its scales according to the recorded numbers.^ — The ^ finder^ most commonly used is that invented by Mr. Maltwood,^ which consists of a glass slide 3 inches by 1^ inch, on which is photographed a scale that occupies a square inch and is divided by horizontal and vertical lines at l-50th of an inch apart into 2,500 squares, each of which contains two numbers, one marking its ^ latitude^ or place in the vertical series, and the other its ' longitude ^ or place in the horizontal series. ^ This plan, first suggested by Mr. Okeden, might be adopted with so little trouble or expense in every Microscope possessed of a Mechanical stage, that it would be very desirable for every such Microscope to be furnished with these graduated scales. If the different Makers would agree to use the l-50th inch scale, Observers at a distance from one another, who might wish to examine each other's objects, would have no difficulty in finding them by the record of their positions accompanying each slide. 2 'Transactions of the Microscopical Society," N. S., Vol. vi. (1858), p. 59. ACCESSORY APPARATUS. 101 The slide, when in use, should rest upon the ledge of the stage of the Microscope, and be made to abut against a stop about 1^ inch from the centre of the stage. — In order to use this ^finder/ the Object-slide must be laid upon the Stage in such a manner as to rest upon its ledge and to abut against the stop; and when some particular object, whose place it is desired to record, has been brought into the field of view, the object- slide being removed and the ^finder' laid down in its place, the numbers of the square then in the field are to be read off and recorded. To find that object again at any time, the finder^ is to be laid in its place on the stage, and the stage moved so as to bring the recorded number into view; and the object-slide being then substituted for the finder, the desired object will present itself in the field. As care is taken in the production of each MMaltwood,' that the scale shall be at an exact dis- tance from the bottom and left-hand end of the glass slide, the Micro- scopist may thus enable any other observer provided with a similar ^finder' to bring into view any desired object, by informing him of the numbers that mark its latitude and longitude. These numbers may either be marked upon the object-slide itself, or recorded in a separate list.^ 98. Diajjhragms. — Every Microscope should be provided with some means of regulating the amount of light sent upwards from the Mirror through transparent objects under examination. This is usually accom- plished by means of a Diaphragm-plate, perforated by apertures of dif- ferent sizes (the smallest of which should be no larger than a pin-hole), and pivoted to a removable fitting attached to the under side of the Stage, in such a manner that by rotating the plate, either of the aper- tures can be brought into the optic axis of the instrument. The larg- est of its apertures should be made to carry a ground-glass (so fitted as to be removable at pleasure), the use of which is to diffuse a soft and equable light over the field when large transparent objects are under examination with, a low power; Avhile between the smallest and the largest aperture there should be an unperforated space, to serve as a dark background for Opaque objects. The edge of the Diaphragm-plate should be notched at certain intervals, and a spring-catch fitted so as to drop into the notches, in order that each aperture may be brought into its proper cen- tral position. When the Diaphragm-plate is used to imjorovc the defini- tion of high powers, it loses much of its value if its aperture be not very close to the under side of the object-slide; and any arrangement which sets it at some distance beneath the stage is consequently objectionable. Its best position is in tlie thichyiess of the stage, which, for receiving it, is ^ The only drawback to the utility of the Maltwood finder lies in the fact that a single square more than covers the field taken in by l-4th Objective with the A eye-piece; so that with powers many times as great, the proportion of the square viewed at once is so small, as to make it impossible to fix the place of the object with any precision. To obviate this difficulty, Mr. W. Webb* proposes a fixidev ruled with lines only 1 -200th of an inch apart, so as to divide a square of only 3-4th of an inch into 22,500 squares. As it would be impossible to mark dis- tinguishing numerals within squares of such minuteness, he rules stronger lines at intervals, so as to divide the whole area into ' blocks' of 100 squares in each; and any individual square can be easily described (1.) by the block in which it lies, and (2) by its position in that block. ("Journ. of Roy. Microsc. Soc," Vol. iii., 1880, p. 750). — To those who prefer the simplicity given by the numbering of each square in the Maltwood finder, the Author would suggest that Cxie object may be always * found' by it with the l-4th Objective; and that, if thus brought into the centre of its field, the object will lie within the field of any Objective of higher power, provided the centering of the two be conformable. 102 THE MICROSCOPE AND ITS REVELATIOiNS. made of two plates screwed together.— A 'different arrangement may be adopted with advantage, when the Stage is provided with a cyhndrical fitting for the reception of Illuminating and Polarizing apparatus. A short tube sliding into this may carry a shoulder at its upper end, upon which may be fitted two or more caps with apertures of different sizes, so that these perforated caps may be either pushed up flush with the sur- face of the stage, or may be lowered to any distance beneath it, according as the best effect is produced. A ground-glass for diffusing light may also be adapted to lie on the shoulder in the place of the perforated caps; and there should also be an mipevforsited cap to serve as a back- ground to opaque objects. — Such great advantage is often derivable from a gradational modification of the light, that the Microscopist who de- sires to avail himself of this will do well to provide himself with one of the forms of graduating diaphragm which have been recently^ntroduced. That long ago invented by DoUond for Telescopic purposes is equally applicable to the Microscope; the circumstance that its aperture is square instead of round, not constituting any practical objection to its use. In another form, introduced by Mr. Collins (Fig. 72), four shutters are made to move inwards simultaneously, by acting on a lever-handle, so as to narrow the aperture, the shape of which always remains more nearly cir- cular than square. And in the ' Iris Diaphragm ^ devised by Mr. J. II. Brown, ^ the multiplication of the number of shutters makes the aperture practically circular. The new construction of this, devised by Mr. G-eo. Wale, U. S., is so simple, inexpensive, and effectual, that its general adop- tion in place of the Diaphragm-plate may be anticipated. 99. Achromatic Condensers, — In almost every case in which an Objec- tive of l-4th inch or any shorter focus is employed, its performance is greatly improved by the interposition of an Achromatic combination between the mirror and the object, in such a manner that the rays re- flected from the former shall be brought to a focus in the spot to which the object is directed. A distinct picture of the source of light is thus thrown on the object, from which the rays emanate again as if it were self-luminous. The Achromatic combination, which (at least in all First-class Microscopes) is one specially adapted to the purpose, is fur- nished with a Diaphragm-plate immediately beneath its lowest lens (Fig. 73); and this is pierced Avith holes of such forms and sizes as to cut off in various degrees, no merely the peripheral but also the central part of Collins's Graduating Diaphragm. Beck's Achromatic Condenser. ^ ''Transactions of the Microscopical Society," Vol. xv , p. 74. ACCESSORY APPARATUS. 103 the illuminating pencil, or to allow oblique light to pass only in some one azimuth, or in two azimuths at right angles to each other. The Achromatic Condenser of Messrs. Beck is a combination of three pairs, of which the first and second are removable, so that the back pair may be used alone for the illumination of objects viewed with low or medium powers. — The Achromatic Condenser of Messrs. Powell and Lealand has an angular aperture of 170°, and thus transmits rays of extreme obliquity through objects mounted on thin ghiss; all other rays being excluded (if desired) by a special arrangement of stops. The Diaphragm-plate being perforated by apertiires of different sizes, the largest of these (which transmits the entire pencil) can be partially closed by centric or eccentric stops attached to a separate arm, any one of which can be brought into the optic axis; and thus, whilst the graduated apertures of the dia- phragm-plate limit periplieral portion of the pencil, the stops cut off its central, allowing the transmission either of its entire peripheral por- tion, or of the rays proceeding only from some special part or parts of it. The same eminent makers have lately introduced a JSTon-achromatic Oil- immersion Condenser; which, at a much lower cost, serves for the reso- lution of the most difficult tests, their illumination by colored rays not being found practically objectionable. — In the Achromatic Condenser now made by Messrs. Ross, extreme obliquity of the illuminating rays is not provided for, this being obtained by means of their swinging ' tail- piece' (§ 72). Its combination has a focus of about 4-lOths inch; and beneath its back-lens, which has an aperture of half an inch, is an Iris- diaphragm for reducing it in any desired degree, with a rotating dii> phragm-plato having a set of stops adapted to limit the aperture and to give a ' black-ground ' illumination under objectives of different angular apertures. — Messrs. Beck have recently introduced a new Achromatic Condenser with a front revolving eccentrically (Fig. 72), by which means its focus may be varied, and a ' black-ground*' illumination may be obtained suitable for objectives having angles as high as 120°. 100 . Weister Condenser, — Though the original idea of the arrangement which has come into general use under this designation, and which is at the same time comparatively inexpensive and applicable to a great variety of purposes, was given by Mr. J. Webster (''Science Gossip," April 1st, 1865), it has received important modifications at the hands of the Opticians by whom the instrument is manufactured; and has, per- haps, not even yet undergone its full development. In its present form 104 THE MICROSCOPE AND ITS REVELATIONS. the arrangement of the lenses strongly resembles that used in the Kell- ner eye-piece (§ 28); the field-.s^lass of the latter serving as a condenser to receive the cone of rays reflected upwards from the mirror, and to make it converge upon a small Achromatic combination, whicli consists of a double-convex lens of crown, with a plano-convex lens of flint, the plane side of the latter being next the object. These lenses are of large size and deop curvature; so that when their central part is stopped-out, the rays transmitted from their peripheral portion meet at a wide angle of convergence, and have the elfect of those transmitted through the peri- pheral portion of the ordinary Achromatic Condenser. When, on the other hand, this combination is used with a diaphragm that allows only the central rays to pass, these rays meet at a small angle; and the illu- mination thus*^ given is very suitable for objects viewed with low powers. Again, by stopping-out the central portion of the combination; and re- moving the Condenser to a short distance beneath the object, the effect of a ^ blaok-ground ' illumination (§104) can be very satisfactorily ob- tained witli Objectives of low or moderate angular aperture. Further, by stopping-out not only the central but also a great part of the periphe- ral rays, so as only to allow the light to enter from a small portion or portions of the margin, illumination of considerable obliquity can be ob- tained. All this can be provided for by a Diaphragm-plate made to rotate at as short a distance as possible beneath the condensing-lens; but as the number of apertures in this plate is necessarily limited, a greater variety is obtained by the use of a Graduating Diaphragm (§ 98) for the regulation of the centric aperture, and by making the apertures in the rotating plate subservient to the other purposes already named, as is done in the arrangement of Mr. Collins (Fig. 75). — Still greater variety can be obtained by substituting for the Diaphragm-plate a short tube sliding within the one that carries the lenses; its summit being furnished with a socket into which may be inserted a diaphragm of blackened card or of thin metal, with an aperture or apertures of any shape or size that may be desired. In this manner the diaphragm may be carried up quite close to the condensing lens, which is a great advantage; and when oblique illumination is desired, the light may be transmitted from any azimuth, by giving rotation to the tube carrying a diaphragm with a marginal aperture. — The Webster Condenser thus improved (which may also be used in combination with the Polariscope) will be found one of the most universally-useful accessories with which a Student's Microscope can be provided.^ 101. OUiqiie Illuminators, — The extremely oblique illumination re- quired for the resolution of the more difficult lined ^ tests,' may be pro- vided, as has been shown, either by the employment of a Condenser of very wide angular aperature (§ 99); or by giving to the whole Illuminat- ing apparatus (as originally suggested by Mr. Grulfb, of Dublin) a posi- ^ A form of condenser specially adapted for very oblique and also for ' black- ground ' illumination was devised a few years ago by Prof. Abbe of Jena ("Monthly Microsc. Journ.," Vol. xiii., 1875, p 77), and has since been specially adapted by him for use with 'homogeneous immersion ' objectives, being fitted to the microscope-stands constructed by Zeiss; but not being found easily appli- cable to Microscopes of the ordinary English models, it has not been taken up by Makers of this country. It seems to the Author, however, that the sZzdmg-plate, l>y which any degree of eccentricity can be given to the apertures that the opti- cal combination admits of, might, in combination with the Iris-dia^jhragm for limiting the angle of the pencil, be advantageously substituted for the rotating diaphragm-plate. ACCESSORY APPARATUS. 105 tion of sucli obliquity to the optic axis of the Microscope, that even its axial ray shall fall upon the object-slide at a very low inclination — as in the Ross-Zentmayer Microscopes (§§ 59, 72), and in the arrangements of Messrs. Beck (§ 75) and Mr. Svvift (§ 68). It is considered by Mr. Wen- ham that there is no better method of utilizing this arrangrement, than by making the Sub-stage carry an ordinary Objective of about 1-inch focus, and throwing its pencil upon a hemispherical lens of half an inch diameter, the plane side of which has a film of glycerine interposed between itself and the object-slide. The lens may either be held in this position by its own adhesion, or it may be so fitted into a thin stage, that its plain surface shall lie flush with the surface of the object- plat- form. This (as also the Disk-Illuminator to be next described) may be made to work well with any form of Students' Microscope, which, like Wale's (§ 60), has a thin stage and a mirror so swung as to be capable of reflecting rays of great obliquity. — For the illumination of objects by a line of light thrown upon them very obliquely, Mr. Wenham has devised the simple Illuminator shown in Fig. 76. This consists of a semi-circu- lar disk of glass (somewhat resembling the half of a button) of half an inch in diameter, the sides of which are flattened, while the circular edge is rounded and well polished to a transverse radius of 1-lOth of an inch. This concentrates the light thrown upon any part of its circumference, upon an object mounted on a slide of the usual thick- ness, with whose under side it is brought into Wenham's Disk-niuminator. immersion-contact by the intervention of either water, glycerine, or a more refractive oil. As it should be so fitted to the Microscope as to illuminate the objects from any azimuth, it should have its flat sides grasped in a clip, which may either be mounted on the Sub-stage, or attached to under side of the Stage — in either case having its diametric section brought up to the under surface of the object-slide. By giving rotation to the object, the illuminator remaining fixed, the illuminating beam may be made to cross the former in any direction that is fitted to bring out its markings. With this simple Illuminator, even Amphipleura pellitcida may be resolved without the aid of a Condenser, the mirror alone sufficing. ^ — Another simple and effective appliance for the same purpose, is the Woodward Prism: a small obtuse-angled triangle of glass, whose long face must be brought into immersion-contact with the object-slide by a film of interposed glycerine. Originally devised as a right-angled prism, it was suited only for the illumination of objects seen under immersion Objectives of widest angular aperture; but by reducing its oblique angles to less than 45°, so as to open-out the two equal sides, it may be adapted to Objectives of much smaller aperture. In using it, the light is made to enter one of the oblique facets perpendicularly to its surface; and by looking in the like direction through the other side of the prism, the observer can see when the face of the object is best illumi- nated, by the rays reflected on it from the inner surface of that facet. — This prism can be made to hang to the under surface of the object-slide by the film of interposed glycerine; but as it is very apt to slip when the microscope is inclined, and as its full advantage can only be obtained when the object is made to rotate so as to meet the illuminating beam in ^ For the mode of constructing this Illuminator, see Journ. of Roy. Microsc. See," Vol. iii. (1880), p. 246. 106 THE MICROSCOPE AND ITS REVELATIONS. every azimuth, it should be mounted, like the Disk-illuminator just de- scribed, in an independent fitting.' 102. The A mici PrUm, which causes the rays to be at once reflected by a plane surface and concentrated by lenticular surfaces, so as to answer the purpose of Mirror and Condenser at the same time, is much approved by many who have used it. Such a Prism may be either mounted on a separate base, or attached to some part of the Microscope-stand. The mounting shown in Fig. 77, is a very simple and convenient one; this consists in attaching the frame of the prism •EnL.v:7 to a sliding bar, which works in dovetail grooves on the top of a cap that may be set on the ^secondary body' beneath the stage; the slide serves to regulate the distance of the prism from the axis of the microscope, and consequently the obliquity of the illum- ination; whilst its distance beneath the stage is adjusted by the rack-movement of cylindri.- cal fitting. In this manner, an illuminating pencil of almost any degree of obliquity that Amici's Prism. is permitted by the construction of the Stage may be readily obtained ; but there is no pro- vision for the correction of its aberations. In order to use this oblique illumination to the greatest advantage, either the prism or the object should be made to rotate, thus causing the oblique rays to fall upon the latter from every azimuth in succession, so as to bring out all its markings (§ 145). 103. Black- Ground Illuminators. — When the rays are directed with such obliquity as not to be received into the Object-glass at all, but are suflBciently retained by the Object to render it (so to speak) self-luminous, we have what is known as the Black-ground illuminatio7i. For low powers whose angular aperture is small, and for such objects as do not require any more special provision, a sufficiently good ' black-ground illumination may be obtained by turning the concave Mirror as far as possible out of the axis of the microscope, especially if it be so mounted as to be capable of a more than ordinary degree of obliquity. In this manner it is often possible, not merely to bring into view features of structure that might not otherwise be distinguishable, but to see bodies of extreme transparence (such, for instance, as very minute Animalcules) that are not visible when the field is flooded (so to speak) by direct light; these presenting the beautiful spectacle of phosphorescent points rapidly sailing through a dark ocean. It is one of the great advantages of this kind of illumination, that, as the light radiates from each part of the object as its proper source, instead of m^vQly passing through it from a more remote source, its different parts are seen much more in their normal relations to one another, and it acquires far more of the aspect of solidity. The rationale of this is easily made apparent, by holding up a glass vessel with a figured surface in front of a lamp or a window, at some dis- tance from the eye, so that it is seen by transmitted light alone: for the figures of its two surfaces are then so blended together, that unless their form and distribution be previously known, it can scarcely be said with certainty which markings belong to either. If, on the other hand, an opaque body be so placed behind the vessel that no rays are transmitted abid.. Vol. i. (1878). p. 246. ACCESSORY APPARATUS. 107 directly through it, whilst it receives adequate illumination from the light around, its form is clearly discerned, and the two surfaces are dis- tinguished without the least difficulty. 104. A simple method of obtaining ' black-ground ' illumination, which works well with objectives of low power and small angular aper- ture, consists in fixing into the top of a short tube that slides into the ' cylindrical fitting' usually carried beneath the stage in Educational and Students' Microscopes, a small ' bull's eye ' lens, the plane surface of which (placed uppermost) has its central portion covered by a black spot. When light reflected by the mirror falls on the lower surface of this Spot- Lens, only the rays that fall on its marginal ring are allowed to pass; and these, owing to its high curvature, are so strongly refracted inwards, as to cross eacii other in the object (when the lens is focussed for it), and then diverge again at an angle sufficiently wide to pass beyond the mar- gin of the objective, like those transmitted by the Paraboloid to be pres- ently described (Fig. 79, F G, f h). Thus the field is left dark; whilst the light stopped by the object gives it a luminosity of its own.— The same effect is gained by the use of the Webster Condenser (§ 100) with a central stop placed immediately behind the lower lens or upon the flat surface of the upper.— Neither of the foregoing plans, however, will answer well for objectives of high power, having such large angles of aperture that the light must fall very obliquely to pass beyond them alto- gether. Thus if the pencil formed by the 'spot-lens' have an angle of 50°, its rays will enter a 4-lOths objective of 60°, and the field will not be darkened. 105. A greater degree of obliquity, suited to afford ' black-ground ' il- lumination with Objectives of larger angular aperture, may be obtained by the use ot the Parabolic lUummator' (Fig. 78); which consists of a Parabolic Illuminator was first devised by Mr. Wenham, who, however, employed a Silver speculum for the purpose. About the same time, Mr. Shad- bolt devised an Annular Condenser of Glass for the same purpose (see Transact, of Microsc. Soc," Ser. I , Vol. iii., 1852, pp. 85, 132). The two principles are com- bined in the Glass Paraboloid. 108 THE MICROSCOPE AND ITS REVELATIONS. Paraboloid of glass that reflects to its focus the rays which fall upon its internal surface* A diagrammatic section of this instrument, showing the course of tlie rays through it, is given in Fig. ?9, the shaded portion representing the Paraboloid. The parallel rays r r' r'\ entering its lower surface perpendicularly, pass on until they meet its parabolic sur- face, on wliich they fall at such an angle so as to be totally reflected by it(§ 2) and are all directed towards its focus, F. The top of the parabo- loid being ground out into a spherical curve of which r is the centre, the rays in emerging from it undergo no refraction, since each falls perpen- dicularly upon the part of the surface through which it passes. A stop placed at s prevents any of the rays reflected upwards by the mirror from ]).issing to the object, which, being placed at F, is illuminated by the rays reflected into it from all sides of the Paraboloid. Those rays which pass through it diverge again at various angles; and if tlie least of these, G F H, be greater than the angle of aperture of the Object-glass, none of them can enter it. The stop s, is attached to a stem of wire, which passes vertically through the Paraboloid and terminates in a knob beneath, as shown in Fig. 78; and by means of this it may be pushed upwards so as to cut off the less divergent rays in their passage towards the object, thus giving a black-ground illumination with Objectives of an angle of aperture nmch wider than G f h. — In using the Paraboloid for delicate objects, the rays which are made to enter it should be parallel, con- sequently the 'plane Mirror should always be employed; and when, instead of the parallel rays of daylight, we are obliged to use the diverg- ing rays of a lamp, these should be rendered as parallel as possible, pre- yiously to their reflection from the mirror, by the interposition of the M)uirs eye' Condenser (Fig. 87) so adjusted as to produce this effect. There are many cases, however, in which the stronger light of the concave Mirror is preferable. — When it is desired that the light should fall on the object from one side only, the circular opening at the bottom of the wide tube (Fig. 78) that carries the Paraboloid, may be fitted with a diaphragm adapted to cover all but a certain portion of it; and by giving rotation to this diaphragm, rays of great obliquity may be made to fall upon the object from every azimuth in succession.^ — A small glass cone, with the apex downwards, and the base somewhat convex, with a stop in the cen- tre, is fitted by MM. Nachet to their Microscopes for the same purpose; and performs very effectively. lU6. In order to adapt the Paraboloid for black-ground illumination under Objectives of wide angle of aperture, Mr. Wenham^ long since constructed ^ flat-topped paraboloid, fitted to rellect only rays of such extreme obliquity, that they would not pass out of the flat surface of the l)araboloid into the under surface of the slide, unless a film of either water or of some liquid of higher refractive index (such as turpentine, or oil of cloves) was interposed between them. When thus enabled to enter the slide, these rays pass on until they meet the cover, from which (in the case of dry-front objectives) they are reflected downwards upon the surface of the object, giving it a bright illumination on a perfectly dark field. The special value of this instrument, however, not being then understood, it was not constructed for sale. — The same prin- ^ By the use of such a diaphragm, or of a large stop with an eccentric per- foration, Mr. G. Williams has succeeded in resolving the transverse striae of Amphipleura pellucida with water-immersion Objectives. See Journ. of Boy. Microsc. Soc," Vol. lii. (1880), p. 524. 2 Transact, of Microsc. Soc," N. S., Vol. iv. (1856), p. 59. ACCESSORY APPARATUS. 109 ciple, however, haying been more recently taken up by Dr. Edmunds, an Immersion Paraboloid specially devised by him for use with immersion Objectives of large aperture, has been constructed by Messrs. Powell & Lealand, with results so satisfactory, that it now ranks among the Accessories most valued by such as habitually work with Objectives of that highest class. ^ 107. Wenliam's Reflex Illuminator, — Another very ingenious and valuable illuminator for high powers has been devised by Mr. Wenham,^ and constructed by Messrs. Eoss. It is composed of a glass cylinder (Fig. 80, a) half-an-inch long, and four-tenths of an inch in diameter; one side of Avhich, starting from the bottom edge, is worked to a polished face at an angle of 64° with the base. The top of the cylinder is polish- ed flat, whilst its lower surface is convex, being polished to a radius of 4-lOths of an inch; close beneath this last is set a plano-convex lens of 1^ inch focus; and the combination is set eccentrically in a fitting, i ?*, adapted to be received into the Sub-stage. The parallel rays, / / /, reflected up into it from the mirror, are made to con- verge, by the convex surfaces at the base of the cylinder, at such an angle, that if their course were continued through glass they would meet at the point A, above the glass slide c ; but by impinging on the inclined polished surface, they are reflect- ed to the flat segmental top, from which again they would be reflected obliquely downwards Wenham's Reflex illuminator. so as to meet m the ponit iy but for its being brought into ^ immersion-contact ' with the under side of the slide. Passing upwards through the slide, they meet in a point, g, a little above its upper surface, in the optic axis of the Micro- scope, to which point the object must be brought; and by giving rota- tion either to the object or to the illuminator, it may be illuminexl from every azimuth. For convenience of centering, a black half-cylinder e, is so fixed by the side of the cylinder, that if a dot upon its upper surface be brought into the centre of the field of view of a low-power objective, its focus g, will lie in the optic axis. — Some skill and practice are required to use this apparatus to advantage, but it will amply repay the trouble of mastering its difficulties. It is best suited to thin flat objects; with those that are thick and irregular, distortion is unavoidable. ^ Monthly Journ. of Microsc. Sci.," Vol. xviii., p. 78. UUd,, Vol. vii., p. 239. 110 THE MICROSCOPE AND ITS REVELATIONS. Although specially designed as a ' black-ground ' illuminator, it may also be made useful in the resolution of difficult Test-objects by transmitted light,' the illuminator being lowered until a colored spectrum appears in the field, the rays of which bring out their markings with remarkable distinctness. — For use with either of these arratigements for ^black- ground ' illumination, it is better that the objects should be mounted * dry,' especially when they are to be viewed under ' immersion ' objec-^ tives; balsam-mounted objects being thus seen better with dry-front objectives. 108. The following directions are given by Mr. Schulze (^^ English Mechanic," 1877, No. 661) for the use of two illuminators last described: — First, rack up the Sub-stage, until the plane top of the illaminator is level with the stage; centre carefully; put a drop or two of glycerine on the under side of the slide, taking care that no air-bells are formed; and place the slide on the stage. If, now, rays parallel to the optic axis are thrown up by the plane mirror or rectangular prism, a luminous spot w^ill appear on the slide if an object lies in the optic axis. Next focus; and by adjusting the mirror or rectangular prism more carefully, the object will be brilliantly illuminated by very oblique rays on a black ground. ... I generally use one of How's common Microscope lamps filled with good paraffin oil, and having a wick half an inch broad; bat for the highest powers I have recourse to the Dallinger lamp (§ 131). After I have obtained the best results, I interpolate a bulTs-eye Con- denser to increase the light, focussing carefully a miniature image of the flame on the slide. I invariably use the narrow side of the flame turned towards the mirror or prism, when resolving lined tests. It is, however, by sunlight that the performances of the Immersion Paraboloid and Eeflex Illuminator seem to eclipse any resolution that can be obtained by transmitted light." [This was written before Mr. Schulze had found out the mode of working these instruments already noticed.] In regard to the relative values of the two illuminators, Mr. Schulze states as the result of careful comparative trials of them: — " The Paraboloid is a trifle easier managed, gives a little more light by lamplight, and is somewhat cheaper than the Reflex Illuminator. Both perform equally well on dark ground by sunlight; but the Reflex Illuminator can also be used on balsamed slides and with immersion lenses for the examination of objects by transmitted very oblique white light." 109. Liglit-Modifiers. — For (1) reducing the intensity either of Solar- light or Lamp-light, (2) for correcting the yellowness of the latter, and (3) for the equable diffusion of either light over a large field, it is often convenient to employ interposed media, the nature of which must be varied according to the particular purpose to be attained. — The direct rays of the Sun are very little employed by Microscopists, except for Photography or some other special purpose. But when recourse is had to them in ordinary Microscopy, it is well to take advantage of ' Rainey's Light-modifier,' which is a combination of one thickness of dark-blue glass free from any tint of red, another of very pale blue w^itli a slight shade of green, and two of thick white plate-glass, all cemented together by Canada balsam. This is mounted by Messrs. Powell and Lealand on a separate stand; and may be used with Lamp-light as with sunlight. — Some observers use Lamp-chimneys of either neutral-tint or bluish glass ^ See Schulze in *'Journ. Roy. Microsc. Soc," Vol. i. (1878), p. 45; and Col. Dr. Woodward in same Vol., p. 248. ACCESSORY APPARATUS. Ill for the purpose of moderating the glare of the flame or of correcting its yellowness; but as the chimney cannot be conveniently changed when- ever the full light is required, the Author much prefers making such night-modifiers^ a part of the Illuminating apparatus attached to the Microscope itself: and this may be done in different modes, according to the construction ol the instrument. Thus, when the Webster Condenser (§ 100) is in use, it may be furnished with three caps made to slide upon its upper portion; one of them fitted with a disk of blue-glass, second with one of neutral-tint glass, and the third with a finely-ground glass. And in Swift's Combination Sub-stago (§ 112) similar disks maybe made to drop into the openings of the rotating plate; so that one may readily be changed for another, or, if all three be placed in the plate at once, an object may be examined under any one of them by merely rotating the plate. Every ordinary Diaphragm-plate (§ 98) ought to have its largest aperture fitted, by means of a projecting shoulder, to carry such a set of disks. — The three arms on which the rotating Selenites are attached to the Sub-stage of Messrs. Beck's First-class Microscope (Fig. 82), may be fitted with similar disks, each of which may then be used either sepa- rately or in combination with one or both of the others. — Every ' Light- modifier' should be so constructed and worked, that the light should be made as nearly as possible to resemble that of a bright white cloud. For this purpose a Avhite-cloud Eeflector may be easily made — either flat, by casting a Plaster of Paris disk upon the plane surface of the mirror — or concave, by casting it on the surface of a glass globe; the light reflected from the surface of the plaster requiring to be condensed for the illum- ination of small objects. — Very pleasant white-cloud effects may be obtained by methods adopted by Mr. Slack. For large objects, viewed with powers of 1^ to 4 inches, he places under the stage a tube holding a large disk (1|- inch diameter) of ground glass, the ground surface being protected by a plain glass cover over it. By this means the peculiar tint of the freshly ground surface is permanently retained. For 2-3ds and half-inch powers he employs a glass slide carrying a disk or square of thin paper, saturated with spermaceti, and protected from dirt by a thin glass cover that adheres to it. This slide, disk downwards, is placed under the object. Under still higher powers, some objects may be very conveniently illuminated by a small bull's-eye finely ground on its flat surface, and fixed with its convex face downwards in a tube that slides into the Sub-stage fitting. 110. Polarizing Apparatus, — In order to examine transparent objects by Polarized Light, it is necessary to employ some means of polarizing the rays before they pass through the object, and to apply to them, in some part of their course between the object and the eye, an analyzing medium. These two requirements may be provided for in different modes. polarizer m^^^ either a bundle of plates of thin glass, used in place of the mirror, and polarizing the rays by reflection; or it may be a ^single image ^ or ^ NicoP prism of Iceland Spar, which is so constructed as to transmit only one of the two rays into which a beam of ordinary light is made to divaricate by passing through this substance. Of these two methods, the 'Nicer prism is the one generally preferred, the objection to the reflecting polarizer being that it cannot be made to rotate. This polarizing prism is usually fixed in a tube (Fig. 81, a, a), furnished with a large milled-head, c, at the bottom, by which it is made to rotate in a collar, 5, that screws into the Sub-stage fitting. For the analyzer a second 'Nicor prism is usually employed; and this, fixed in a 112 THE MICROSCOPE AND ITS REVELATIONS. short tube, may be fitted either into a collar interposed between the lower end of the body and the Objective, or into a cap placed over the Eye-piece (Fig. 81, b), in the stead of the ordinary eye-piece cap. ^ The former arrangement, which is specially adapted for use with the Binocu- lar Microscope, has the advantage of not limiting the field, but it stops a good deal of light; while * in the latter, the image A ^ is brighter, but a good deal of the margin of the field is cut off. In the Harley Binocular (§ 68) the analyzing prism is fit- ted into a slide below the Wenham prism, which is> drawn out when thepolari- scope is not in use; while in Swift's Challenge Bi- A, Fitting of Polarizing B, Fitting of Analyzing n hp n 1 ^i r n <5i m i 1 ji r 1 1 rl p i «5 Prism in Sub-stage. Prism above Eye-piece. ^^^^^^^^^ ^ SimUdl Sliue IS fitted into the body above the Wenham prism. In these arrangements, such advantage as is obtainable by the rotation of the analyzing prism is of course foregone; and the same sacrifice is made, when, in the Stephenson Binocular (§ 36), the Iceland spar analyzer is replaced by a refiector. — The Polarizing apparatus may be worked in combination either with the Achromatic Condenser (by which means it may be used with high power Objectives), or with either of the ^black-ground' Illuminators (§§ 104, 105), which show many objects — such as the horny polyparies of Zoophytes — gorgeously projected in colors upon a dark field. 111. For bringing out certain effects of Color by the use of Polarized Light (Chap, xxii.), it is desirable to interpose a plate of Selenite be- tween the polarizer and the object; and it is advantageous that this should be made to revolve." A very convenient mode of effecting this, is to mount the Selenite plate in a revolving collar, which fits into the upper end of the tube that receives the Polarizing prism. In order to obtain the greatest variety of coloration with different objects, films of Selenite of different thickness should be employed; and this may be ac- complished by substituting one for another in the revolving collar. A still greater variety may be obtained by mounting three films, which separately give three different colors, in collars revolving in a frame re- sembling that in which hand-magnifiers are usually mounted; this frame being fitted into the Sub-stage in such a manner, that either a single Selenite, or any combination of two Selenites, or ail three together, may be brought into the optic axis above the polarizing prism (Fig. 82). As many as thirteen different tints may thus be obtained. — When the con- struction of the Microscope does not readily admit of the connection of the Selenite plate with the Polarizing prism, it is convenient to make use of a plate of brass (Fig. 83) somewhat larger than the glass slides in which objects are ordinarily mounted, with a ledge near one edge for the slide to rest against, and a large circular aperture into which a glass is fitted, having a film of Selenite cemented to it; this ' Selenite stage' or object- carrier being laid upon the Stage of the Microscope, the slide containing the object is placed upon it; and, by an ingenious modification contrived by Dr. Leeson, the ring into which the Selenite plate is fitted being made movable, one plate may be substituted for another, whilst rotation may ACCESSORY APPARATUS. 113 be given to the ring by means of a tangent-screw fitted into the brass- plate. — The variety of tints given by a Selenite-film nnder Polarized light, is so greatly increased by the interposition of a rotating film of Mica, that two Selenites — red and Uue — with a Mica-film, are found to give the entire series of colors obtainable from any number of Selenite- films, either separately or in combination with each other. The Revolv- Darker's Selenites, as fitted by Messrs. Beck. ing Mica-Selenite Stage (Fig. 84) devised by Mr. Blankly, and made by Mr. Swift, furnishes a very simple and effective means of obtaining these beautiful effects; the Mica film being set in a diaphragm which can be made to rotate by applying the finger at the front edge of the stage; whilst the two Selenites are so placed in a slide, that either of them can be brought under the aperture as desired. Fig. 84. Blankley's Revolving Mica-Selenite Stage. 112. Swiff s Combination Sui-stage. — In this ingenious piece of ap- paratus (Fig. 85) are combined the advantages of (1) an Achromatic Condenser, a, centred by two milled-headed screws, c c, and having an angle of 140", which fits it for use with Objectives of very wide angular aperture, whilst, by removing the upper combination, it is made to suit lower powers; (2) a contracting Diaphragm worked by the lever b; (3) a revolving Diaphragm, E, with four apertures, into which can be fitted ; either {a) a series of three central stops, giving a Black-ground illumina-' tion scarcely inferior to that of the paraboloid, and capable of being used with the small angled l-5th, {h) tinted or ground-glass Moderators, or {c) two Selenite-films for the Polarizing apparatus; (4) a Polarizing prism, F, mounted on an eccentric arm, so as to be brought under the axis of the condenser when not in use, and thrown out when not wanted; and 8 lU THE MICROSCOPE AND ITS REVELATIONS. (5) an upper arm carrying two revolving cells geared together by fine teeth (one of them shown at d, while the other is under the condenser), SO that a revolving motion may be given to either by acting on the other; one of these cells carries a plate of Mica, the revolution of which over the selenite-films gives a great variety of color- tints with Polarized light; while the other serves to receive oblique-light disks, to which rota- tion can be given by the same means. — The special advantage of this Condenser Hps in its having the polarizing prism, the selenite- and mica-films, the black-ground and oblique-light stops, and the moderator, all brought close under the back lens of the Achromatic; whilst it combines in itself all the most important applianctes which the Sub-stage of Secondary body of First-class Microscopes is able to afford. It may be specially recom- mended to such as make much use of Polarized light. 113. Illuminators for Opaque Objects, — All objects through which sufficient light cannot be transmitted to enable them to be viewed in the modes already de- scribed, require to be illuminated by rays, which, being thrown upon the surface under examination, shall be reflected from it into the Microscope; and this mode of view- ing them may often be advantage- ously adopted in regard to semi- transparent or even transparent objects, for the sake of the diverse aspects it affords. Among the va- rious methods devised for this purpose, the one most generally adopted con- sists in the use of a Condensing Lens (Fig. 86), either attached to the Microscope, or mounted upon a separate stand, by which the rays pro- ceeding from a lamp or from a bright sky are made to converge upon the object. — For the efficient illumination of large opaque objects, however, it is desirable to employ a BulVs eye Condenser (which is a plano-convex lens of short focus, two or three inches in diameter), mounted upon a separate stand, in such a manner as to allow of being placed in a great variety of positions. The mounting shown in Fig. 87, is one of the best that can be adopted: the frame which carries the lens is borne at the bot- tom upon a swivel joint, which allows it to be turned in any azimuth; whilst it may be inclined at any angle to the horizon, by the revolution of the horizontal tube to which it is attached, around the other horizgntal tube which projects from the stem; by the sliding of one of these tubes within the other, again, the horizontal arm may be lengthened or short- ened; the lens may be secured in any position (as its weight is apt to drag Swift's Combination Sub-stage. ACCESSORY APPARATUS. 115 it down when it is inclined, unless the tubes may bo made to work, the one into the other, more stiffly than is convenient) by means of a tio-ht- ening collar milled at its edges; and finally the horizontal arm is attached to a sprung socket, which slides up and down upon a vertical stem. The optical effect of such a * bull's-eye' differs according to the side of it turned towards the light, and the condition of the rays which fall upon it. The position of least spherical aberration is when its convex side is turned towards parallel or towards the least diverging rays: consequently, when used by Daylight, its plane surface should be turned towards the ohject; and the same position should be given to it when it is used for procuring converging rays from a lamp, this being placed four or five times farther IFia. 87. Condensing Lens. Bull's-eye Condenser. off on one side than the object is on the other. But it may also be em- ployed for the purpose of reducing the diverging rays of the Lamp to par- allelism, for use either with the Paraboloid (§ 105) or with the Parabolic speculum to be presently described; and the plane side is then to be turned towards the lamp, which must be placed at such a distance from the ' bull's-eye,' that the rays which have passed through the latter shall form a luminous circle equal to it in size, at whatever distance from the lens the screen may be held. For viewing minute objects, under high powers, the smaller Condensing lens may be used to obtain a further con- centration of the rays already brought into convergence by the ^bull's- eye.' — An ingenious and effective mode of using the ^bull's-eye' con- denser, for the illumination of very minute objects under high-power 116 THE MICROSCOPE AND 1T8 KEVELATIONS. Objectives, has been devised by Mr. James Smith. The Microscope being in position for observation, the lamp should be placed either in the front or at the side (as most convenient), so that its flame, turned edge- ways to the stao^e, should be at a somewhat lower level, and at a distance of about three inches. The bull's-eye should be placed between the stage and the lamp, with its plane surface uppermost, and with its con- vex surface a little above the stage. The light entering its convex sur- face near the margin turned towards the lamp, falls on its plane surface at an angle so oblique as to be almost totally reflected towards the oppo- site margin of the convex surface, through which it passes to the object, a little above the plane of the stage, on which it should cast a sharp and brilliant wedge of light. The adjustment is best made by first placing a slip of white card on the stage, and when this is well illuminated, substi- tuting the object-slide for it; making the final adjustment while the object is being viewed under the Microscope. No difficulty is experienced in getting good results with powers of from 200 to 400 diameters; but high powers require careful manipulation. Mr. Smith states, that he has succeeded in illuminating by this simple method, minute objects (such as Beck's Parabolic Speculum. Crouch's Adapter for Parobolic Speculum. Diatoms and scales of Lejnaopiera), very brilliantly and clearly, upon a dark field, under an immersion l-16th inch Objective. But he considers that it answers better for objectives of moderate than of very wide angu- lar aperture. ^ 114. The Illumination of Opaque objects may be effected by reflection as well as by refraction; and the most convenient as well as most efficient instrument yet devised for this purpose is the Parabolic Speculum of Mr. E. Beck (Fig. 88), which is attached to a spring-clip that fits upon the Objectives {% inch, \\ inch, 1 inch, 2-3ds inch) to which it is espe- cially suited, and is slid up or down, or turned round its axis, when the object has been brought into focus, until the most suitable illumination has been obtained. The ordinary rays of diffused Daylight, which may be considered as falling in a parallel direction on the Speculum turned towards the window to receive them, are reflected upon a small object in its focus, so as to illuminate it sufficiently brightly for most purposes; ^ See Journ. Roy. Micro.c. Soc," Vol. iii. (1880), p. 398. ACCESSORY APPARATUS. 117 but a much stronger light may be concentrated on it, when the Speculum receives its rays from a lamp placed near the opposite side of the stage, a ^bulFs-eye^ being interposed to give parallelism to the rays. For the sake of Microscopists who may desire to use this admirable instrument with Objectives to which it has not been specially fitted, an adapter is made by Mr. Crouch, consisting of a collar (Fig. 89, a) interposed between the lower end of the body of the Microscope and the objective; on this is fitted the ringB, which turns easily round it, and carries the horizontal arm c c, jointed at each end; whilst the stem D, which can be lengthened or shortened at pleasure, hanging from this, carries at its lower end the Speculum f attached to it by the ball-and-socket joint e. By this arrange- ment the Parabolic Speculum may be used not only with the objectives already named, but also with those of one-half or 4-lOths inch focus, if these do not approach the object so nearly as to interfere with the reflec- tion of the illuminating rays from the Speculum. 115. LieberMilm. — A mode of illuminating opaque objects by a small concave Speculum reflecting directly down upon them the light reflected Diagram of Lieberkuhn up to it from the Mirror, was formerly much in use, but is now compara- tively seldom employed. This concave Speculum, termed a ' Lieber- kuhn' from the celebrated Microscopist who invented it, is made to fit upon the end of the Objective, having a perforation in its centre for the ])assage of the rays from the object to the lens; and in order that it may receive its light from a mirror beneath (Fig. 90, a), the object must be so mounted as only to stop-out the central portion of the rays that are reflected upwards. The curvature of the Speculum is so adapted to the focus of the Objective, that, when the latter is duly adjusted, the rsys reflected up to it from the mirror shall be made to converge strongly upon the part of the object that is in focus: a separate speculum is conse- quently required for every objective. The disadvantages of this mode of illumination are chiefly these: — first, that by sending the light down upon the object almost perpendicularly, there is scarcely any shadow, so that the inequalities of its surface and any minute markings which it might present, are but faintly or not at all seen; second, that the size of the ob- 118 THE MICROSCOPE AND ITS REVELATIONS. ject must be limited by that of the speculum, so as to allow the rays to pass to its marginal portion; and third, that a special mode of mounting is required, to allow the light to be reflected from the mirror around the margin of the object. The first objection may be in some degree removed by turning the mirror considerably out of the axis, so as to reflect its light obliquely upon the Lieberkiihn, which will then send it down obliquely upon the object (Fig. 90, b); or by covering one side of the Lieberkiihn by a diaphragm, which should be made capable of rotation, so that light may be reflected from the uncovered portion in every azi- muth: the illumination, however, will in neither case be so good as that which is afforded with powers up to 2-3ds inch, by the Parabolic Spec- ulum just described. The mounting of Opaque objects in wooden slides (Fig. 124), which affords in many cases the most convenient means of preserving them, completely prevents the employment of the Lieberkiihn in the examination of them; and they must be set for this purpose either upon disks which afford them no protection, or in cells (§ 169) with a blackened background. The cases wherein the Lieberkiihn is most use- ful, are those in which it is desired to examine small opaque objects, such as can be held in the Stage-Forceps (§ 118) or mounted on small disks (§ 119), or laid upon a slip of glass, with objectives of half-inch focus or less; since a stronger light can be thus concentrated upon them, than can be easily obtained by side-illumination. In every such case, a bhick background must be provided, of such a size as to fill the field, so that no light shall come to the eye direct from the mirror, and yet not large enough to create any unnecessary obstruction to the passage of the rays from the mirror to the speculum. With each Lieberkiihn is com- monly j)rovided a blackened stop of appropriate size, having a well-like cavity, and mounted upon a pin which fits into a support connected with the under side of the stage; but though this ^dark well^ serves to throw out a few objects with peculiar force, yet, for all ordinary pur- poses, a spot of black paper or black varnish will answer the required purpose very effectually, this spot being either made on the underside of the cell which contains the object, or upon a separate slip of glass laid upon the stage beneath this. 116. Vertical Illuniination for High Powers, — ^Various attempts have been made by Mr. Wenham and others to view opaque objects under powers too high for the advantageous use of the Lieberkiihn, by employ- ing the Objective itself as the illuminator, light being transmitted into it downwards from above. By Prof. H. L. Smith, of Geneva College, U. S., a pencil of light admitted from a lateral aperture above the objective, was reflected downwards upon the object through its lenses, by* means of a small silver speculum placed on one side of its axis and cutting off a portion of its aperture. By Messrs, Powell and Lealand, a piece of plane glass was placed at an angle of 45° across a tube placed like an adapter between the Objective and the body of the Microscope; and whilst a pencil of light, entering at the side aperture and striking against this inclined surface, is reflected by it downwards through the objective on to the object, the rays proceeding upwards from the object pass upwards (with some loss by reflection) through the plane glass into the body of the Microscope. For this fixed plate of glass, Mr. E. Beck substituted a disk of thin glass attached to a milled-head (Fig. 91, b), by the rotation of which its angle may be exactly adjusted; and this is introduced by a slot (shown at Fig. 91, a) into the interior of an adapter that is inter- posed between the objective (c, d) and the nose {c) of the Microscope. 4 ACCESSORY APPARATUS. 119 The light which enters at the lateral aperture (a, a) falling upon the oblique surface of the disk (c, I), is reflected downwards, and is concen- trated by the lenses of the Objective upon the object beneath. The lateral aperture may be provided with a diaphragm, having a series of apertures, for diminishing the false light to which this method is liable; or a screen with a small aperture may be placed at any distance between the lamp and the Illuminator, that is found to produce the best effects. In using this illuminator, the lamp should be placed at a distance of about 8 inches from the aperture; and when the proper adjustments have been made, the image of the flame should be seen upon the object. The illumination of the entire field, or the direction of the light more or less to either side of it, can easily be managed by the interposition of a small Condensing lens placed at about the distance of its own focus from the lamp. The Objects viewed by this mode of illumination with dry- front objectives, are best uncovered; since, if they are covered with thin glass, so large a proportion of the light sent down upon them is reflected from the cover (especially when Objectives of large angle of aperture A c Beck's Vertical Illuminator. are employed) that very little is seen of the objects beneath, unless their reflective power is very high. With immersion objectives, however, covered objects may be used; and the author has seen a more perfect resolution of difficult tests by this mode of viewing them (first suggested by Mr. Morehouse, of Wayland, New York) than by any other.' — Another method of Vertical Illumination long since devised by Mr. Tolles has recently been brought into notice by Prof. W. A. Eogers, of Boston, U. S. It consists in the introduction of a small rectangular prism, resembling that of Nachet's Binocular (a. Fig. 27), at a short dis- tance behind the front combination of the Objective; so that parallel rays entering its vertical end-surface, pass on between the parallel horizontal surfaces, until they meet the inclined surface by which they are reflected downwards. In passing through the front combination of the objective. Journ. of Roy. Microsc. Soc," Vol. ii. (1879), pp. 194, 266. 120 THE MICROSCOPE AND ITS REVELATIONS. they are deflected towards its axis; but as their angle of convergence is less than the angle of divergence of the rays proceeding from the object, the reflected rays will not meet in the focal point of the lens, but will be so distributed as to illuminate a sufficient area. By altering the extent to which the prism is pushed in, or by lifting or depressing its outer end by means of a milled-head screw, the field of illumina- j^ c)2^ tion can be regulated. The working of this prism with immersion objectives is stated by Mr. ToUes to be pecu- liarly satisfactory.^ 117. Stephenson's Safety Stage. — In examining ob- jects with those higher powers which focus extremely close to the covering glass, the slightest inadvertence is likely to lead to a fracture of the glass, and perhaps to the destruction of a valuable slide. This is a serious matter with Moller's Diatom Type Slide, or Nobert's Test Lines, or with many others that are expensive or perhaps impossible to replace. To remove this source of danger, Mr. Stephenson contrived the safety stage," shown in Fig. 92. The frame on which the slide car- Safety-stage. I'yii^g the object rcsts, is hinged at its upper part, and kept in its true position by slight springs, which give way directly the slide is pressed by the objective. It is found that springs firm enough to insure the steadiness required for high powers, may yet be sufficiently flexible to give way before very thin glass is endangered, and a glance at the stage shows if it is made to deviate from the nor- mal position in which its upper and lower edges are parallel,— (See also § 54.) Section 2. Apparatus for the Presentation of Objects, 118. Stage- Forceps and Vice, — For bringing under the Object-glass in different positions such small opaque objects as can be conveniently held in a pair of forceps, the Stage- Forceps (Fig. 93) supplied with most ^^cj^^ Microscopes afford a ready means. These are mounted by means of a joint upon a pin, which fits into a hole either in the corner of the Stage itself or in the object-platform; the ob- stage-Forceps. j^^^ inserted by pressing the pin that projects from one of the blades, whereby it is separated from the other; and the blades close again by their own elasticity, so as to retain the object when the pressure is withdrawn. By sliding the wire stem which bears the Forceps through its socket, and by moving that socket vertically upon its joint, and the joint horizontally upon the pin, the object may be brought into the field precisely in the position required; and it may be turned round and round, so that all sides of it may be examined, by simply giving a twisting movement to the wire stem. The other extremity of the stem often bears a small brass box filled with cork, and perforated with holes in its side; this affords a secure hold to commmon pins, to the heads of which small objects can be attached by gum, or to which disks of card, etc., may be attached, whereon objects are mounted for being viewed with the Lieberkiihn (§ 115). This ' •* Journ. of Roy. Microsc. Soc," Vol. iii., pp. 526, 754. ACCESSORY APPARATUS. 121 method of mounting was formerly much in vogue, but has been less employed of late, since the Lieberkiihn has fallen into comparative dis- use. — The Stage Vice, as made by Mr. Ross for Mr. Slack, was contrived for the purpose of holding small hard bodies, such as Minerals, apt to be jerked out by the angular motion of the blades of the forceps, or very delicate substances that will not bear rough compression. In this appa- ratus the blades meet horizontally, and their movements can be regulated to a nicety with a fine screw. The Stage Vice fits into a plate, as is the case with Beck's disk-holder. Fig. 94. 119. For the examination of objects which cannot be conveniently held in the stage-forceps, but which can be temporarily or permanently attached to disks, no means is comparable to the Disk-holder of Mr. E. Beck (Fig. 94) in regard to the facility it affords for presenting them in every variety of position. The object being attached by gum (having a small quantity of glycerine mixed with it) or by gold-size, to the surface of a small blackened metallic Disk, this is fitted by a short stem project- ing from its under surface into a cylindrical holder; and the holder carry- ing the disk can be made to rotate around a vertical axis by turning the milled-head on the right, which acts on it by means of a small chain that works through the horizontal tubular stem; whilst it can be made to in- cline to one side or to the other, until its plane becomes vertical, by turn- ing the whole movement on the horizontal axis of its cylindrical Tlik^i» socket.^ The supporting plate be- ing perforated by a large aperture, the object may be illuminated by the Lieberkiihn if desired. The disks are inserted into the holder, or are removed from it, by a pair of Forceps constructed for the purpose; and they may be safely Beck's Disk-holder, put away, by inserting their stems ^i, • 4. into a plate perforated with holes. Several such plates, with inter- vening guards to prevent them from coming into too close apposition, may be packed into a small box. To the value of this little piece of apparatus the Author can bear the strongest testimony from his own experience, having found his study of the Foraminifera greatly tacili- tated by it.— A less costly substitute, however, which answers suthciontly well for general purposes, is found m the Object-holder of Mr. Morris (Fig. 95), which consists of a supporting plate that carries a ball-and- socket joint in its centre, into the ball of which can be fitted by a taper- ing stem either a holder for small cardboard disks, or a larger holder suitable for carrying an ordinary slide. By the free play ot the ball- and-socket joint in different directions, the object may either be made to rotate, or may be so tilted as to be viewed obliquely or almost laterally. This instrument can, of course, be used only by side illumination; and in order to turn it to the best account, the objects to be viewed by it must be mounted on special disks; but it has an advantage over the preceding, in being applicable also to objects mounted m ordinary slides.— 1 he same purpose is answered, in the Ross Zentmayer Microscopes (^^ &y, 72), and in the Improved Beck Microscope (§ 65), by turning the stage round its horizontal axis, so that an object mounted on a slide may be 1 A small pair of Forceps adapted to take up minute objects may be fitted into the cylindrical holder, in place of a disk. 122 THE MICROSCOPE AND ITS REVELATIONS. viewed at any desired angle or inclination, when it has been brought into the most suitable azimuth by the rotating of the stage round its vertical axis. 120. Glass Stage-plate, — Every microscope should be furnished with a piece of Plate-glass, about 4 in. by 1-^ in., to one margin of which a nar- row strip of glass is cemented, so as to form a ledge. This is extremely useful, both for laying objects upon (the ledge preventing them — to- gether with their covers, if used — from sliding down when the Miscro- scope is inclined), and for preserving the stage from injury by the spill- Fig: 95. Morris's Object-holder. ing of sea-water or otner saline or corrosive liquids, when such are in use. Such a plate not only serves for the examination of transparent, but also of opaque objects; for if the Condensing-lens be so adjusted as to throw a side-light upon an object laid upon it, either the Diaphragm- plate or a slip of black-paper will afford a dark back-ground; whilst ob- jects mounted on the small black disks suitable to the Lieberkuhn may conveniently rest on it, instead of being held in the Stage-forceps. 121. Growing Slide, — A number of contrivances have been devised of late years, for the purpose of watching the life-histories of minute aquatic organisms, and of ' cultivating ' such as develop and multiply themselves in particular fluids. One of the simplest and most effective, that of Mr. Botterill, represented in Fig. 96, — consists of a slip of ebonite, three inches by one, with a central aperture of 3-4ths of an inch at its under side; this aperture is reduced by a projecting shoulder, whereon is ce- Botterill's Growing-Slide. mented a disk of thin glass, which thus forms the bottom of a cell hol- lowed in the thickness of the ebonite slide. On each side of this central cell, a small lateral cell communicating with it and about l-4th inch in diameter, is drilled-out to the same depth; this serves for the reception of a supply of water or other fluid, which is imparted, as required, to the ACCESSORY APPARATUS. 123 IFiG central ^ growing^ cell, which is completed by placing a thin-glass cover over the objects introduced into it, with the interposition of a ring of thin paper, or (if a greater thickness be required) of a ring of cardboard or vulcanite. If the fluid be introduced into one of the lateral cells, and be drawn- off from the others — either by the use, from time to time, of the small glass syringe to be hereafter described (§ 127), or by threads so arranged as to produce a continuous drip into one and fro7n the other — a constantly renewed supply is furnished to the central cell, which it enters on one side, and leaves on the other, by capillary attraction. ^ — Dr. Mad- Maddox's Growing-siide. dox^s Groioing- Slide will be understood from the annexed sketch. The shaded parts are pieces of tinfoil fastened with shellac glue to a glass slide. The minute fungi or spores to be grown are placed on a glass cover large enough to cover the tinfoil, with a droplet of the fluid re- quired. This, after examination to see that no extraneous matter is introduced, is placed over the tinfoil, and the edges fastened with wax softened with oil, leaving free the spaces x x for entrance of air. Grow- ing-slides of this description could be made cheaply with thin glass instead of tinfoil.^ — For an account of a more elaborate apparatus devised by Messrs. Dallinger and Drysdale for the prosecution of their admirable researches hereafter to be noticed (Chap, xi.), the reader is referred to ^he description and figures given by them in the Monthly Microscopi- cal Journal," Vol. xi., 1874, p. 97. 122. Aquatic Box, — The Live-Box or Animalcule-cage (Fig. 98, a) consists of a short piece of wide brass tube, fixed perpendicular into an aperture of its own diameter in a flat-plate of brass, and closed-in at its top by the object- tablet, a disk of glass with bevelled edges (b); ovet this box there slides a cover, consisting of another piece of brass tube hav- ing a disk of thin glass fixed into its top. The cover being taken of, a drop of the liquid to be examined, or any thin object which can be most advantageously looked-at in fluid, is placed upon the lower plate; the cover is then slipped over it, and is pressed down until the drop of liquid be spread out, or the object be flattened, to the degree most convenient for observation. If the glass disk which forms the lid be ce- mented or burnished into the brass ring which carries it, a small hole should be left for the escape of air or superfluous fluid; and this may be ^ For descriptions of other forms of Growing-Slide, see Transact, of Microsc. Society/' Vol. xiv., N.S., p. 34, and *' Quart. Journal of Microsc. Science," N.S., Vol, vii , p. 11. ^ See his paper on Cultivation of Microscopic Fungi, in Monthly Microscopi- cal Journ.," Vol. iii. (1870), p. 14. — Dr. Maddox recommends the following fluid as sufficiently hygrometric to keep the spores moist, and as adapted to Fungoid growths: — Dextrine 2 grains. Phosphate of Soda and Ammonia 2 Saturated Solution of Acetate Potash 12 drops. Grape Sugar 16 grains. Freshly distilled water . 1 oz. The water is to be boiled in a large test-tube or beaker for 15 minutes, and covered whilst boiling and cooling; when settled, it should be poured into per- fectly clean 2-drachm stoppered bottles, and kept for use. 124 THE MICROSCOPE AND ITS REVELATIONS. closed np with a morsel of wax, if it be desired to prevent the included fliiid from evaporating. But as it is desirable that the cover-glass should ] 0 thin enough to allow a l-4th or a l-6th inch Objective to be em- [)loyed, and as such thin glass is extremely apt to be broken, it is a much better plan to furnish the brass cover with a screw-cap, which holds the glass disk with sufficient firmness, but permits it to be readily replaced. It is always desirable, if possible, to pre- vent the liquid from spreading to the edge of the disk, since any objects it may contain are very apt in such a case to be lost under the opaque ring of the cover: this is to be avoided by limit- ing the quantity of liquid introduced, by laying it upon the centre of the lower plate, and by pressing down the cover with great caution, so as to flatten the drop equally on all side , stopping short when it is spreading too close to the margin. If the Live- box be well constructed, and the glass disks be quite flat, they will come in- to such close contact, that objects of extreme thinness may be compressed between them; and it may thus be Aquatic Box or Animalcule-Cage, as seen in made, with a little practice, to SCrVC perspective at A, and in secDion at B and c. the purposc of a Compressor (§ 125). In its ordinary form, however, the elevation of the object-tablet above the stage prevents the Live-box from being used with the Achromatic Condenser or Paraboloid: but another form is made by Mr. Swift, in which the object-tablet is fixed at the bottom of the tube, flush with the surface of the plate (as shown at c); and as the covering disk is fixed to the 'bottom of the cover-tube, and thus slides inside the box-tube, the ob- ject can be illuminated by any of the means applicable to objects con- tained in ordinary flat cells (§ 123). The only disadvantage of this con- struction is that the cover-disk must fixed in the tube which carries it. 123. Infusoria, minute Algse, etc., however, can be well seen by plac- ing a drop of the water containing them, on an ordinary slide, and laying a thin piece of covering-glass on the top. And objects of somewhat greater thickness can be examined by placing a loop or ring of fine cotton-thread upon an ordinary slide, to keep the covering-glass a small distance from it; and the object to be examined being placed on the slide with a drop of water, the covering-glass is gently pressed down till it touches the ring. Still thicker objects may be viewed in the various forms of ' cells ' hereafter to be described (§§ 171-3); and as, when the cells are filled with fluid, their glass covers will adhere by capillary attraction, provided the superfluous moisture that surrounds their edges be removed by blot- ting-paper, they will remain in place when the Microscope is inclined. — An Annular Cell, that may be used either as a ^live-box' or as a ^grow- ing-slide,' has lately been devised by Mr. Weber (U. S.). It is a slip of plate-glass of the usual size and ordinary thickness, out of which a cir- cular ^cell' of 3-4ths inch diameter is ground, in such a manner that its bottom is convex instead of concave, its shallowest part being in the centre, and the deepest round the margin. A small drop of the fluid to be examined being placed upon the central convexity (the highest ACCESSORY APPARATUS. 125 part of which should be almost flush with the general surface of the plate), and the thin glass-cover being placed upon is, the drop spreads itself out in a thin film, without finding its way into the deep furrow around it; and thus itholds-on the covering-glass by capillary attraction, while the furrow serves as an air chamber. If the cover be cemented, down by a ring of gold-size or dammar, so that the evaporation of the fluid IS prevented, either Animal or Vegetable life may thus be main- tained for some days, or, if the two should be balance d (as in an Aqua- rium), for some weeks. ^ — An improvement has been devised by Dr. Ed- monds in the form of this Annular Cell; which he also makes to serve as a ^gas-chamber' for the introduction of gases or vapors into the Annu- lar space. The central prominence is shaped as a truncated paraboloid; and while, by focussing in the object a 2-inch objective used as a conden- ser, a bright field is obtained, this may be exchanged for a dark field by ])utting the condenser out of focus (so that its light is thrown on the sides of the paraboloid), and by gumming a black disk on the centre of its under surface. A straight groove being cut in the slide, parallel to its long side, and tangentially to the annular groove which it should equal in depth, two fine glass tubes are cemented in it; one of them, which is left projecting beyond the end of the slide, being connected with a slender elastic tube through which gases or vapors may be pro- jected into the annular space, while the other serves to convey them away.^ 124. Zoopliyte Trougli. — For the examination of larger aquatic Ani- mals or Plants under low or moderate powers, recourse may be advanta- geously had either to the original Zoophyte-trough of Mr. Lister (which is still kept on sale by most Makers), or to a form lately devised by Mr. Botterill, which has several advantages over the older one. Fig. 99.. This consists of two plates of vulcanite, a back and a front, shaped as in Fig. 99, connected together by three brass screws; these, being fixed in the back plates, pass through the front, where their projecting ends are furnished with small milled- heads. Between these plates Botteriirs Zoophyte Trough. are two rectangular plates of glass, cut to such a length as to lie between the two side-screws of the vulcanite plates, and having such a breadth that while their lower edges rest on the bottom-screw, their upper are flush with the top of the vulcanite disks. The glass plates are kept apart by a half-ring of vul- canized india-rubber, of such a diameter as to lie just outside of the semi- circular margin of the vulcanite plates; and they thus form the sides and bottom of a trough, which is made water-tight by a moderate pressure exerted by turning the milled-heads. The space between the two glass plates may be varied by using half -rings of different thicknesses; whilst, if it be desired to use a higher power than will work through ordinary glass, a front plate of tliin glass may be substituted.— One great advan- tage of this arrangement is the facility with which the pieces composing ^ Journ. Roy. Microsc. Society," Vol. ii. (1879), p. 55. 2 Ihid,, Vol. iii. (1880), p. 585.— This Parabolized Oas-Slide is made by Messrs, Beck. 126 THE MICROSCOPE AND ITS REVELATIONS. it may be taken apart, either for cleaning or for the repair of a fracture — an accident to which the use of thin glass of course renders it specially liable. 125. Compressor. — The purpose of this "nstrument is to apply a gradual pressure to objects whose structure ..an only be made out when they are thinned by extension, while their organization is so delicate as to be confused or altogeiner destroyed by the slightest excess of pressure. For the examination of such, an instrument in which the degree of compression can be regulated with precision is almost indispensable. The Compressorium represented in i'ig. 100 was originally devised by Schick of Berlin; whilst its details were modified by M. de Quatrefages, who constantly employed it in his elaborate and most suc- cessful researches on the or- ganization of the Marine Worms. Being, however, deficient in any provisions for securing the parallelism of the approximated sur- faces, it has been superseded that view. — In Bosses Improved upper plate d is attached to Schick's Compressor. ,oy other forms devised expressly with Compressor^ shown in Fig. 101, the p slide that works between grooves in the vertical piece c, so that, when raised or lowered by the milled-head, it always maintains its parallelism to the lower plate A. Pig. 101. The thin glass carried by the upper plate D (which can be turned aside on a swivel joint, as shown in the lower figure) is a square that slides into grooves on its under side, so as to be easily replaced if broken. The glass to which it is opposed is a circular disk lodged in a shallow socket in plate B, which is received into a part of the lower plate a that is sunk below the rest. The plate b carrying the lower glass can be drawn out (as shown in the lower figure) and laid upon the Dissecting Microscope, to be replaced in the Compressorium after the object has been prepared for compression. The only drawback to the use of this instrument lies in the inconvenience of using it in the reversed position so as to look at the object from its under side. — This reversion is provided for in the two forms of the instrument made by Messrs. Beck, which are shown in Figs. 1C2, 104. In both, the upper and the lower glasses are fixed, upon a plan devised by Mr. Slack, by means of flat-headed screws, two to each glass (Fig. 103, a), the heads fitting into holes of the opposite frame, so as to permit the close approxi- mation of the two glass surfaces. In their Parallel Plate Compressor (Fig. 102) the constant parallelism of the two plates is secured by the two parallel bars, a, a; while the degree of their approximation and pressure Ross's Improved Compressor, ACCESSORY APPARATUS. 127 is regulated by the screw h, which works out of centre in a conical hole of the lower frame, so that, the further it is introduced, the more closely the two frames, with their glasses, are approximated. This pattern works equally well whichever side is uppermost. In the Reversible Cell Coin- pressor of the same makers (Figs. 103 B, J 04) the upper glass is held down by a ring a, which screws-on to that which bears the lower one, giving any degree of pressure that may be required. When screwed together, they form a cell that fits into the plate Z>, and is attached to it by the milled-head c; by unscrewing which the cell can be instantly detached and replaced in a reverse position. — In all these Compressors, it is easy to vary the thickness of the glass within convenient limits; and the ob- FiG. 105. Fig. 102. ADC Dipping Tubes. Glass Syringe. server should be always provided with a stock of glass slips and disks of the requisite sizes and of different thicknesses, suitable to the kind of investigation he may be prosecuting. As thin glasses, when used for compression, are very liable to fracture, the power of immediately re- placing them without the employment of cement (as in Mr. Slack's con- struction) is a great convenience. 126. Dipping Tubes. — In every operation in which small quantities of liquid, or small objects contained in liquid, have to be dealt with by the Microscopist, he will find it a very great convenience to be provided with a set of Tubes of the forms represented in Fig. 105, but of some- 128 THE MICROSCOPE AND ITS REVELATIONS. what larger dimensions. These were formerly designated as ^fishing tubes;' the purpose for which they were originally devised having been the fishing-out of Water-fleas, aquatic Insect-larvae, the larger Animal- cules, or other living objects distinguishable either by the unaided eye or by the assistance of a magnifying-glass, from the vessels that may contain them. But they are equally applicable, of course, to the selection of minute Plants; and they may be turned to many other no less useful pur-v poses, some of which will be specified hereafter. — When it is desired to secure an object which can be seen either with the eye alone or with a magnifying-glass, one of these tubes is passed down into the liquid, its upper orifice having been previously closed by the forefinger, until its lower orifice is immediately above the object; the finger being then re- moved, the liquid suddenly rises into the tube, probably carrying the object up with it; and if this is seen to be the case, by putting the finger again on the top of the tube, its contents remain in it when the tube is lifted out, and may be deposited on a slip of glass, or on the lower disk of the Aquatic-box, or, if too copious for either receptacle, may be dis- charged into a large glass cell (Fig. 120). In thus fishing in jars for any but minute objects, it will be generally found convenient to employ the open-mouthed tube c; those with smaller orifices, B, c, being employed for ^fishing' for Animalcules, etc., in small bottles or tubes, or for selecting minute objects from the cell into which the water taken up by the tube a has been discharged. It will be found very convenient to have the tops of these last blown into small funnels, which shall be cov- ered with thin sheet India-rubber; for their action (like that of the stop- per of the Dropping-bottle, Fig. 138) can then be regulated with the greatest nicety by the pressure of the finger. 127. Glass Sifringe. — In dealing with minute Aquatic objects, and in a great variety of other manipulations, a small Glass Syringe of the pat- tern represented in Fig. 106, and of about double the dimensions will be found extremely convenient. When this is firmly held between the fore and middle fingers, and the thumb is inserted into the ring at the summit of the piston-rod, such complete command is gained over the piston, that its motion may be regulated with the greatest nicety: and thus minute quantities of fluid may be removed or added, in the various operations which have to be performed in the preparation and mounting of Objects (Chap, v.); or any minute object may be selected (by the aid of the simple Microscope, if necessary) from amongst a number in the same drop, and transferred to a separate slip. A set of such Syringes, with points drawn to different degrees of fineness, and bent to different curvatures, will be found to be among the most useful ' tools ' that the working Microscopist can have at his command. riG;.lQ7. Forceps. 128. Forceps,~kxioi\\QT instrument so indispensable to the Micro- scopist as to be commonly considered an appendage to the Microscope, is the Forceps for taking up minute objects; many forms of this have been devised, of which one of the most convenient is represented in Fig. 107, ACCESSORY APPARATUS. 129 of something less than the actual size. As the forceps, in Marine re- searches, have continually to be plunged into sea-water, it is better that they should be made of brass or of German silver than of steel, since the latter rusts far more readily; and as they are not intended (like Dissecting- f creeps) to take a firm grasp of the object, but merely to hold it, they may be made very light, and their spring-portion slender. As it is es- sential, however, to their utility, that their points should meet accurately, it is well that one of the blades should be furnished with a guide-pin passing through a hole in the other. The foregoing constitute, it is believed, all the most important pieces of Apparatus which can be considered in the light of Accessories to the Microscope. Those which have been contrived to afford facilities for the preparation and mounting of Objects, will be described in a future chap- ter (Chap. v.). And the simple and efficient substitute which the Author has been accustomed to use for the Frog-Plate thought essential by many Microscopists, will be described in Chap. xx. under the head of Circulation of the Blood. 9 130 THE MICKOSCOPE AND ITS KEVELATIONS. CHAPTEE IV. MANAGEMENT OF THE MICROSCOPE. 129. Tahle. — The Table on which the Microscope is placed when in use, should be one whose size enables it also to receive the various appur- tenances which the observer finds it convenient to have within his reach, and whose steadiness is such as to allow of his arms being rested upon it without any yielding ; it should, moreover, be so framed, as to be as free as possible from any tendency to transmit the vibrations of the building or floor whereon it stands. The working Microscopist will find it a matter of great convenience to have a Table specially set apart for his use, fur- nished with drawers, in whicli are contained the various Accessories he may require for the preparation and mounting of objects. If he should desire to carry about with him all the apparatus he may need for the prosecution of his investigations in different localities, and for the mounting of his preparations on the spot, he will find it very convenient to provide himself with a small Cabinet, fitted with drawers in which every requisite can be securely packed, and of such a height, that, when laid upon an ordinary table, it may bring up the Quekett or other Dis- secting Microscope placed upon it to the position most convenient for use.* — If the Microscope be one which is not very readily taken out from and put back into its case, it is very convenient to cover it with a large bell- glass ; which may be so suspended from the ceiling, by a cord carrying a counterpoise at its other end, as to be raised or lowered with the least possible trouble, and to be entirely out of the way when the Microscope is in use. Similar but smaller bell-glasses (wine-glasses whose stems have been broken answer very well) are also useful for the protection of objects which are in course of being examined or prepared, and which it is desir- able to seclude from dust. — For the purpose of Demonstration in the Lecture-room, a small traversing platform may be constructed to run easily upon rollers, and to carry the Microscope and Lamp securely clamped down upon it, so as to be passed from one observer to another. For Demonstration to a small party sitting round a circular table, it is convenient to employ a -shaped platform, the vertical angle of which is pivoted to a weight placed in the centre of the table, whilst the angles * The dimensions of the Cabinet which the Author has had constructed for himself (its size being so adapted to that of the box of his Crouch's Binocular that the two are received into the same travelling-case) are 14 inches long, 7 inches broad, and 4^ inches high. In the middle there are five shallow drawers, 5 inches broad, containing dissecting apparatus, large flat cells, glass-covers, syringes, etc. ; on one side are two drawers, each 3^ inches broad, the upper one, containing slides, cells, etc., rather more than one inch deep inside, the lower, for larger pieces of apparatus, 2 inches deep ; on the other side is a single drawer of the same breadth and 3:^ inches deep, for bottles containing solutions, cements, etc. MANAGEMENT OF THE MICROSCOPE. 131 at the base are supported upon castors, so that the platform may run round to each observer in succession. Or the table itself, if not too large, may rotate (like a dumb-waiter) upon its central pillar, as made by Messrs. Beck. 130. Light, — Whatever maybe the purposes to which the Microscope is applied, it is a matter of the first importance to secure a pure and ade- quate Illumination. For the examination of the greater proportion of objects, good daylight is to be preferred to any other kind of light ; but good lamplight is preferable to bad daylight, especially for the illumina- tion of opaque objects. When daylight is employed, the Microscope should be placed near a window, whose aspect should be (as nearly as may be convenient) opposite to the side on which the sun is shining; for the light of the sun reflected from a bright cloud is that which the experienced Microscopist will almost always prefer, the rays proceeding from a cloud- less blue sky being by no means so well-fitted for his purpose, and the dull lurid reflection of a dark cloud being the worst of all. The direct light of the sun is far too powerful to be ordinarily used with advantage, unless its intensity be moderated, either by reflection from a plaster of Paris mirror, or by jDassage through some ^Modifier* (§109) ; it is, how- ever, occasionally used by some observers to work out intricate markings or fine color, and may sometimes be of advantage for these purposes, but without great care would be a fertile source of error. — The young Micro- scopist is earnestly recommended to make as much use of daylight as possible ; not only because, in a large number of cases, the view^of the object which it affords is more satisfactory than that which can be ootained by any kind of lamplight, but also because it is much less trying to the eyes. So great, indeed, is the difference between the two in this respect, that there are many who find themselves unable to carry on their obser- vations for any length of time by lamplight, al- though they experience neither fatigue nor strain from many hours' continuous work by daylight. Even ordinary daylight may be considerably im- proved by the interposition of a glass globe of about six inches in diameter, filled with water; and this may also bo advantageously used for the illu- mination of transparent objects by lamp-light, if the water be very slightly tinged with ammonio- sulphate of copper, which takes off the yellow glare. 131. Lamps, — When recourse is had to Artifi- cial light, it is essential, not only that it should be of good quality, but that the arrangement for fur- nishing it should be suitable to the special wants of the Microscopist. The most useful light for ordinary use is that furnished by the steady and constant flame of a flat- wicked Lamp, fed with one of the best varieties of Paraffin oil. This (with its chimney-shade) should be so mounted on a stem rising from a secure base, as to be capable of ad- justment to any height above the table; and on the same stem should also slide a telescope-arm having a bulFs eye condenser attached to it by a ball-and-socket joint, in such a manner as to be adjustable in any posi- tion in regard to the flame, and at the same time to be carried upwards or downwards with the^lamp — an arrangement originally devised by Mr. 132 THE MICROSCOPE AND ITS REVELATIONS. Beckett (Fig. 108). It is preferable, however, to surround the glass chimney by a cylinder of porcelain, having a large aperture on one side for the passage of the light; and this may be advantageously blackened on the outside, contracted above into a cone, and furnished with a shade over the aperture (as in Mr. Swift's construction, Fig. 109), so that as little light as possible may enter the eye of the observer, except that which proceeds from the object. The lamp should be so hung as to be capable of being rotated on its own vertical axis; so that either the whole breadth of the flame, or its edge only, may be turned towards the mir- ror or condenser, according as diffused or concentrated light is required. In Mr. Swift's Lamp (Fig. 110), the Bull's-eye is mounted on a separate Tia. m Tig, lio. Chimney and Shade of Swift's Swift's Microscope -Lamp. Microscope-Lamp. stem, capable both of vertical elevation and of horizontal adjustment, which rises from one end of an arm that is pivoted beneath the base of the brass cylinder that carries the lamp; and from the other end of this arm there rises a second stem, carrying a speculum, from which addi- tional light may be reflected when desired. By rotating this arm on its pivot, the speculum and condenser are shifted together, so as to direct the full power of the flame wherever it may be required; an arrangement especially convenient for the illumination of opaque objects. — As it ]6 often found extremely difficult to obtain an exact centering of the illumi- MANAGEMENT OF THE MICROSCOPE. 133 nating beam, when yery high powers are employed, by mere hand-shif t- ings of the lamp and its condenser, Messrs. Dallinger and Drysdale, in the admirable investigations of whose results a summary will be given hereafter (Chap, xi.), have found great advantage from the use of a Lamp mounted on a base to which a traversing horizontal movement can be given in any direction by rectangular screws, and furnished with an upright standard carrying two racks, on which the lamp itself and the bull's-eye condenser can be separately raised or lowered by milled-head pinions. By this more exact method of adjustment, the observer is able, after a little experience in its use, to secure that most perfect position of the flame and condenser, which ordinary hand-adjustment might not suc- ceed in attaining until after a great expenditure of time and patience.^ 132. Position of the Light, — When the Microscope is used by day- light, it will usually be found most convenient to place it in such a man- ner that the light shall be at the left hand of the observer. It is most important that no light should enter his eye, save that which comes to it through the Microscope; and the access of direct light can scarcely be avoided, when he sits with his face to the light. Of the two sides, it is more convenient to have the light on the left; first, because it is not in- terfered with by the right hand, when this is employed in giving the requisite direction to the mirrr, or in adjusting the illuminating appara- tus; and, secondly, because, as most persons in using a Monocular Micro- scope employ the right eye rather than the left, the projection of the nose serves to cut off those lateral rays, which, when the light comes from the right side, glance between the eye and the eye-piece. The side-shades fitted by Mr. Collins to the eye-pieces of his Harley Binocular (Fig. 49) may be advantageously employed with every instrument of that class. — When Artificial light is employed, the same general precautions should be taken. The Lamp should always be placed on the left side, unless some special reason exist for placing it otherwise; and if the Object under ex- amination be transparent, the lamp should be placed at a distance from the eye about midway between that of the stage and that of the mirror. In the examination of objects of the greatest delicacy and difficulty, however, in which it is important to get rid of the reflection from the front surface of the Mirror, a rectangular Prism should be substituted for it, when the conditions of the observation necessitate the use of the Microscope in the vertical position; but when the instrument can be inclined, the Lamp may be most advantageously placed in the axis of the Achromatic Condenser or other Illuminator, so that its light may be transmitted to the object without intermediate reflection. If, on the other hand, the Object be opaque, the Lamp should be at a distance about midway behind the eye and the stage; so that its light may fall on the object at an angle of about 45° with the axis of the Microscope. — The passage of direct rays from the flame to the eye should be guarded against by the interposition of the lamp-shade; and no more light should be diffused through the apartment, than is absolutely necessary for other purposes. If observations of a very delicate nature are being made, it is desirable, alike by daylight and by lamplight, to exclude all lateral rays from the eye as completely as possible; and this may be readily accom- * See Monthly Microsc. Jour.," Vol. xv. (1876), p. 165.— As the directions given by these excellent observers for centering the illuminating beam are too long for citation, such as desire to profit by their experience must learn its results from their own account of them. 134 THE MICROSCOPE AND ITS REVELATIONS. plished by means of a shade made like the upper part of a Mask, and lined witii black cloth or velvet, which should be fixe(i on the ocular end of the Microscope. 133. Care of the Eyes. — Although most Microscopists who habitually work with the Monocular microscope acquire a habit of employing only one eye (generally the right), yet it will be decidedly advantageous to the beginner that he should learn to use either eye indifferently; since by employing and resting each alternately, he may work much longer without incurring unpleasant or injurious fatigue, than when he always employs the same. — Whether or not he do this, he will find it of great importance to acquire the habit of keeping open the unemployed eye. This, to such as are unaccustomed to it, seems at first very embarrassing, on account of the interference with the microscopic image, which is occasioned by the picture of surrounding objects formed upon the retina of the second eye; but the habit of restricting the attention to that impression only which is received through the microscopic eye, may generally be soon acquired; and when it has once been formed, all diffi- culty ceases. Those who find it unusually difficult to acquire this habit, may do well to learn it in the first instance with the assistance of the shade just described; the employment of which will permit the second eye to be kept open without any confusion. — So much advantage, however, is derived from the use of the Binocular arrangement, either stereoscopic or non-stereoscopic, that the Author would strongly recommend its use to every observer, save in cases of exceptional difficulty. There can be no doubt that the habitual use of the Microscope, for many hours together, especially by lamp-light, and with high magnifying powers, has a great tendency to injure the sight. Every Microscopist who thus occupies himself, therefore, will do well, as he values his eyes, not merely to adopt the various precautionary measures already specified, but rigor- ously to keep to the simple rule of not continuinn to observe any longer than he can do so without fatigue.^ 134. Care of the Microscope, — Before the Microscope is brought into use, the cleanliness and dryness of its glasses ought to be ascertained. If dust or moisture should have settled on the Mirror, this can be readily wiped off. If any spots should show themselves on the field of view, when it is illuminated by the mirror, these are probably due to parti- cles adherent to one of the lenses of the Eye-piece: and this maybe determined by turning the eye-piece round, which will cause the spots also to rotate, if their source lies in it. It may very probably be sufficient to wipe the upper surface of the eye-glass (by removing its cap), and the lower surface of the field-glass; but if, after this has been done, the spots should still present themselves, it will be necessary to unscrew the lenses from their sockets, and to wipe their inner surfaces; taking care to screw them firmly into their places again, and not to confuse the lenses of different eye-pieces. Sometimes the eye-glass is obscured by dust of ^ The Author attributes to his rigorous observance of the above rule his entire freedom from any injurious affection of his visual organs, notwithstanding that, of the wh le amount of Microscopic study which he has prosecuted for forty-five years past, a large proportion has been necessarily carried on by Artificial light, most of his daylight hours having been occupied in other ways. He has found the length of time during which he can ' microscopize ' without the sense of fatigue, to vary greatly at different periods, half-an-hour's work being sometimes sufficient to induce discomfort, whilst on other occasions none has been left by three or four hours' almost continuous use of the instrument — his power of visual endurance being usually in relation to the vigor of his general system. MANAGEMENT OF THE MICROSCOPE. 135 ex^iT.me fineness, wliieli may be carried off by a smart puff of breath; the vapor which then remains upon the surface being readily dissipated by rapidly moving the glass backwards and forwards a few times through the air. And it is always desirable to try this plan in the first instance; since, however soft the substance with which the glasses are wiped, their polish is impaired in the end by the too frequent repetition of the pro- cess. The best material for wiping glass is a piece of soft wash-leather, from which the dust it generally contains has been well beaten out. — If the Object-glasses be carefully handled, and kept in their boxes when not in use, they will not be likely to require cleansing. One of the chief dangers, however, to which they are liable in the hands of an inexperi- enced Microscopist, arises from the neglect of precaution in using them with fiuids; which, when allowed to come in contact with the surface of the outer glass, should be wiped off as soon as loossible. In screwing and unscrewing them, great care should be taken to keep the glasses at a distance from the surface of the hands; since they are liable not only to be soiled by actual contact, but to be dimmed by the vaporous exhalation from skin which they do not touch. This dimness will be best dissipated by moving the glass quickly through the air. It will sometimes be found, on holding an Object-glass to the light, that particles either of ordinary dust, or more often of the black coating of the interior of the Microscope, have settled upon the surface of its back-lens; these are best removed by a clean and dry camel's-hair pencil. If any cloudiness or dust should still present itself in an object-glass, after its front and back surfaces have been carefully cleansed, it should be sent to the maker (if it be of English manufacture) to be taken to pieces, as the amateur will seldom succeed in doing this without injury to the work; the foreign combinations, however, being usually put together in a simpler manner, may be readily unscrewed, cleansed, and screwed together again. Not unfrequently an objective is rendered dim by the cracking of the cement by which the lenses are united, or by the insinuation of moisture between them; this last defect occasionally arises from a fault in the quality of the glass, which is technically said to * sweat.' In neither of these cases has the Microscopist any resource, save in an Optician experienced in this kind of work; since his own attempts to remedy the defect are pretty sure to be attended with more injury than benefit. 135. General Arrangement of the Microscope for Use, — The inclined position of the instrument, already so frequently referred to, is that in Avhich observation by it may be so much more advantageously carried-on than in any other, that recourse should always be had to it, unless par- ticular circumstances render it unsuitable. The precise inclination that may prove to be most convenient w^ill depend upon the ^ build ' of the Microscrope, upon the height of the observer's seat as compared with that of the table on which the instrument rests, and lastly, upon the sitting height of the individual; and it must be determined in each case by his own experience of what suits him best — that which he finds most com- fortaUe being that in which he will be able not only to work the longest, but to see most distinctly. — The selection of the Objectives and Eye- pieces to be employed must be entirely determined by the character of the object. Large objects presenting no minute structural features should always be examined in the first instance by iYiQ lowest powers, whereby a general view of their nature is obtained; and since, with lenses of com- paratively long focus and small angle of aperture, the precision of the focal adjustment is not of so much consequence as it is with the higher 136 THE MICROSCOPE AND ITS REVELATIONS. powers, not only those parts can be seen which are exactly in focus, but those also can be tolerably well distinguished which are not precisely in that plane, but are a little nearer or more remote. Wlien the general aspect of an object has been sufficiently examined through low powers, its details may be scrutinized under a higher amplification; and this will be required in the first instance, if the object be so minute that little or nothing can be made out respecting it save when a very enlarged image is formed. The power needed in each particular case can only be learned by experience; that which is most suitable for the several classes of ob- jects hereafter to be described, will be specified under each head. In the general examination of the larger class of objects, the range of power that is afforded by Zeiss's Adjustable Low-power Objective (§ 159, I.) will often be found useful; whilst for the ready exchange of a low power for a higher one, great convenience is afforded by the Nose-piece (§ 96). 136. When the Microscopist wishes to augment his magnifying power, he has a choice between the employment of an Objective of shorter focus and the use of a deeper Eye-piece. If he possess a complete series of Objectives, he will frequently find it best to substitute one of these for another without changing the Eye-piece for a deeper one; but if his ^powers' be separated by wide intervals, he will be able to break the abruptness of the increase in amplification which they produce, by using each Objective first with the shallower and then with the deeper Eye- piece. Thus, if a Microscope be provided only with fwo Objectives of 1-inch and l-4th inch focus respectively, and with two Eye-pieces, one nearly double the power of the other, such a range as the following may be obtained — 60, 90, 240, 360 diameters; or, with two Objectives of somewhat shorter focus, and with deeper Eye-pieces (as in some French and German instruments) — 88, 176, 350, 700 diameters. In the exami- nation of large Opaque objects having uneven surfaces, it is generally preferable to increase the power by the Eye-piece rather than by the Objective; thus a more satisfactory view of such objects may usually be obtained with a 3-inch or 2-inch Objective and the B Eye-piece, than with a l|-inch or 1-inch Objective and the A Eye-piece. The reason of this is, that in virtue of their smaller Angle of Aperture, the Objectives first named have a much greater amount of ^penetrating power' or ^ focal depth' than the latter (§ 158, i.); and in the case just specified this quality is of the first importance. The use of the Draw-tube (§ 83) enables the Microscopist still further to vary the magnifying power of his instrument, and thus to obtain almost any exact number of diameters he may desire, within the limits to which he is restricted by the focal length of his 01)jectives. The advantage to be derived, however, either from ^deep Eye-piecing' or from the use of the Draw-tube, will mainly depend upon the quality of the Object-glass. For, if it be imperfectly corrected, its errors are so much exaggerated, that more is lost in definition than is gained in amplification; whilst, if its apertures be small, the loss of light is an equally serious drawback. On the other hand, an Objective of perfect correction and adequate angle of aperture will sustain this treat- ment with so little impairment in the perfection of its image, that » magnifying power may be obtained by its use, such as, with an inferior instrument, can only be derived from an Objective of much shorter focus combined with a shallow Eye-piece. — The author thinks it a great mistake, however, to attempt to make an Objective of medium power ordinarily do the work on which an Objective of high power should properly be employed. For not only can it not be brought up to this MANAGEMENT OF THE MICROSCOPE. 137 without such an increase of its angle of aperture as unfits it for its own proper work, but the "deep eye-piecing' required cannot be had recourse to habitually without exposing the eyes to severe overstrain. The advan- tage of loio Eye-pieces and deej:) Objectives, as compared with deep Eye- pieces and loiv Objectives, has been very well ])ut by likening it to the comfort of reading large print without spectacles, or with spectacles suited to the sight, and reading small print with a magnify ing-glass. 137. In making the Focal Adjustment, when low powers are used, it will scarcely be necessary to employ any but the coarse adjustment, or ^ quick motion;' provided that the rack be well cut, the pinion work in it smoothly and easily, without either ^spring,' ^loss of time,' or Uwist,' and the milled-head be large enough to give the requisite leverage. All these are requisites which should be found in every well-constructed instrument; and its possession of them should be tested, like its freedom from vibration, by the use of high powers, since a really good coarse adjustment should enable the observer to ^ focus' an Objective of l-8th inch with precision. — What is meant by ^spring' is the alteration which may often be observed to take place on the withdrawal of the hand; the object which has been brought precisely into focus, and which so remains as long as the milled-head is between the fingers, becoming indistinct when the milled-head is let go. The source of this fault may lie either in the rack-movement itself, or in the general framing of the instrument, which is so weak as to allow of displacement by the mere weight or pres- sure of the hand: should the latter be the case, the ^spring' may be in a great degree prevented by carefully abstaining from hearing on the milled-head, which should be simply rotcHed between the fingers. — By Hoss of time ' is meant the want of sufficient readiness in the action of the pinion upon the rack, so that the milled-head may be moved slightly in either direction without affecting the body; thus occasioning a great diminution in the sensitiveness of the adjustment. This fault may sometimes be detected in Microscopes of the best original construction, which have gradually worked loose owing to the constancy with which they have been in employment; and it may often be corrected by tight- ening the screws that bring the pinion to bear against the rack. — And by Hwist' it is intended to express that apparent movement of the object across the field, which results from a real displacement of the axis of the body to one side or the other, owing to a want of correct fitting in the working parts. ^ As this last fault depends entirely on bad original workmanship, there is no remedy for it; but it is one which most seriously interferes with the convenient use of the instrument, however excellent may be -its optical performance. — In the use of the coarse adjustment with an Objective of short focus, extreme care is necessary to avoid bringing it down upon the object, to the injury of one or both; for although the spring with which the tube for the reception of the object- glass is furnished, whenever the ^fine adjustment' is immediately applied to this, takes off the violence of the crushing action, yet such an action, even when thus moderated, can scarcely fail to damage or disturb the object, and may do great mischief to the lenses. Where the fine adjust- ment is otherwise provided for, still greater care is of course required, ^ In testing either the ' coarse' or the * fine ' adjustment for ' twist/ care should be taken that the light reflected from the mirror is axial not oblique ; since, if the illuminating rays are inclined to the optic axis, the object, when thrown out of focus, will appear to vanish laterally, which it does not do (provided the adjustments work well) when illuminated axially. 138 THE MICROSCOPE AND ITS REVELATIONS. unless a spring ^safety-tube' be provided, into which the Objectives are screwed. — It is here, perhaps, well to notice, for the guidance of the young Microscopist, that the actual distance between the Objective and the object, when a distinct image is formed, is always considerably less than the nominal focal length of the objective. — One more precaution it may be well to specify; namely, that either in changing one object for another, or in substituting one Objective for another, save when powers of such focal length are employed as to remove all likelihood of injury, the Body should have its distance from the Stage increased by the ^coarse adjustment.' This precaution is absolutely necessary when Objectives of short focus are in use, to avoid injury either to the lenses or to the object; and when it is habitually practised with regard to these, it becomes so much like an ^acquired instinct,' as to be almost invariably practised in other cases. 138. In obtaining an exact Focal Adjustment with Objectives of less than half-an-inch focus, it will be generally found convenient to employ t\iQ fine adjustjnent or ^slow motion;' and as recourse will frequently be had to its assistance for other purposes also, it is very important that it should be well constructed and in good working order. The points to be l^articularly looked to in testing it, are for the most part the same with those already noticed in relation to the coarse movement. It should Avork smoothly and equably, ]oroducing that graduated alteration of the distance of the Objective from the object which it is its special duty to effect, without any jerking or irregularity. It should be so sensitive, that any movement of the milled-head should at once make its action apparent by an alteration in the distinctness of the image, when high powers are employed, without any ^oss of time.' ^ And its action should not give rise to any twisting or displacing movement of the image, which ought not to be in the least degree disturbed by any number of rotations of the milled-head, still less by a rotation through only a few degrees. — One great use of this adjustment consists in bringing into view different strata of the object, and this in such a gradual manner that their con- nection with one another shall be made apparent. AVhether an Opaque or a Transparent object be under examination, only that part which is exactly in focus can be perfectly discerned under any power; and when high powers of large angular aperture are employed, this is the only part that can be seen at all. A minute alteration of the focus often causes so different a set of appearances to be presented, that, if this alteration be made abruptly, the relation of each to its predecessors can scarcely be even guessed at; and the gradual transition from the one to the other, which the ^slow motion' alone affords, is therefore necessary to the correct interpretation of either. To take a very simple case: — The transparent body of a certain animal being traversed by vessels lying in different planes, one set of these vessels is brought into view by one adjustment, another set by ^focussing' to a different plane; and the connection of the two sets of vessels, which may be the point of most importance in the whole anatomy of the animal, may be entirely over- looked for want of a ^fine adjustment,' whose graduated action shall enable one to be traced continuously into the other. What is true even 1 It will sometimes happen that the * slow motion ' will seem not to act, merely because it has been so habitually worked in one direction rather than the other, that its screw has been turned too far. In that case, nothing more is required for its restoration to good working order, than turning the screw in the other direction, until it shall have reached about the middle of its range of action. MANAGEMENT OF THE MICROSCOPE. 139 of low and medium powers, is of course true to a still greater degree as to high powers; for although the ^ quick motion^ may enable the observer to bring any stratum of the object into accurate focus, it is impossible for him by its means to secure that transitional ^focussing' which is often much more instructive than an exact adjustment at any one point. A clearer idea of the nature of a doubtful structure is, in fact, often derived from what is caught sight of in the act of changing the focus, than by the most attentive study and comparison of the differ- ent vrews obtained by any number of separate ^focussings.' The experi-' enced Microscopist, therefore, whilst examining an object of almost any description, constantly keeps his finger on the milled-head of the ' slow motion,^ and watches the effect produced by its revolution upon every feature which he distinguishes; never leaving off until he be satisfied that he has scrutinized not only the entire surface, but the entire thick- ness of the object. It will often happen that, where different structural features present themselves on different planes, it will be difficult or even impossible to determine with the Monocular microscope which of them is the nearer and which the more remote, unless it be ascertained by the use of the ^slow motion,* when they are successively brought into focus, whether the Objective has been moved towards or away from the object.^ Even this, however, will not always succeed in certain of the most diffi- cult cases, in which the difference of level is so slight as to be almost inappreciable; as, for instance, in the case of the markings on the silice- ous valves of the Diatoms (Fig. 166). 139. When Objectives of short focus and of wide angular aperture are in use, something more is necessary (save in the case of ' homogeneous- immersion* lenses, § 20), than exact focal adjustment; this being the adjustment of the Objective itself, which is required to neutralize the disturbing effect of the glass cover upon the course of the rays pro- ceeding from the object (§ 17), — unless (as in the Objectives now commonly made for Students' Microscopes) they are construct- ed for working only with cover- glasses of a certain standard thick- cncQvcrctlf ness. For such adjustment, it 'toemu will be recollected, a power of al- tering the distance between the front pair and the remainder of the combination is required; and this power is obtained in the fol- lowing manner: — The front pair of lenses is fixed into a tube (Fig. Ill, a), which slides over an interior tube (b) by which the other two pairs are held; and it is drawn up or down by means of a col- lar (c), which works in a furrow cut in the inner tube, and upon a screw- thread cut in the outer, so that its revolution in the plane to which it is fixed by the one tube gives a vertical movement to the other. In one Section of Adjusting Object-Glass. ^ It is in objects of this kind that the great advantage of the Stereoscopic Binocular arrangement makes itself most felt (§§ 30-40). 14:0 THE MICROSCOPE AND ITS REVELATIONS. part of the outer tube an oblong slit is made, as seen at into which projects a small tongue screwed on the inner tube; at the side of the former two horizontal lines are engraved, one pointing to the word ^ un- covered/ the other to the word ^covered;' whilst the latter is crossed by a horizontal mark, which is brought to coincide with either of the two lines by the rotation of the screw-collar, whereby the outer tube is moved up or down. When the mark has been made to point to the line ^uncov- ered/ it indicates that the distance of the lenses of the object-glass is such as to make it suitable for viewing an object without any interference from thin glass; when, on the other hand, the mark has been brought by the revolution of the screw-collar into coincidence with the line ^cov- ered,' it indicates that the front lens has been brought into sach proxi- mity with the other two, as to produce a ^positive aberration^ in the Objective, fitted to neutralize the ' negative aberration ^ produced by the interposition of a glass cover of extremest thickness. But unless this cor- rection be made, with the greatest precision, to the thickness of the par- ticular cover in use, the enlargement of the Angle of Aperture, to which Opticians have of late applied themselves with such remarkabe success, becomes worse than useless; being a source of diminished instead of in- creased distinctness in the details of the object, which are far better seen with an Objective of greatly inferior aperture, possessing no special ad- justment for the thickness of the glass. The following general rule is given by Mr. Wenham for securing the most efficient performance of an Object-glass with any ordinary object: — " Select any dark speck or oj)aque portion of the object, and bring the outline into perfect focus; then lay the finger on the milled-head of the fine motion, and move it briskly backwards and forwards in both directions from the first position. Ob- serve the expansion of the dark outline of the object, both when within and when without the focus. If the greater expansion, or coma, is when the object is without the focus, or farthest from the Objective, the lenses must be placed farther asunder, or towards the mark ^ uncovered.^ If the greater coma is when the object is tuithin the focus, or nearest to the Objective, the lenses must be brought closer together, or towards the mark ^ covered.^ When the object-glass is in proper adjustment, the ex- pansion of the outline is exactly the same both within and without the focus.'' A different indication, however, is afforded by such ' test-ob- jects ' as present (like the Podura-scale and the Diatomaceae) a set of dis- tinct dots or other markings. For '^if the dots have a tendency to run into lines when the object is placed witliout the focus, the glasses must be brought closer together; on the contrary, if the lines appear when the object is within the focal point, the lenses must be farther separated."^ AVhen the Angle of Aperture is very wide, the difference in the aspect of any severe test under different adjustments becomes at once evident; markings which are very distinct when the correction has been exactly made, disappearing almost instantaneously when the screw-collar is turned a little way round. 2 1 See Quart. Journ. of Microsc. Science," Vol. ii. (1854), p. 138. 2 Mr. "Wenham remarks (loo. cit.), not without justice, upon the difficulty of making this adjustment even in the objectives of our best Opticians; and he states that he has himself succeeded much better by making the outer tube the fixture, and by making the tube that carries the other pairs slide within this; the motion being given by the action of an incHned slit in the revolving collar upon a pin that passes through a longitudinal slit in the outer tube, to be attached to the inner. — The admirable Objectives in the first-class American Opticians, are (the Author believes) always constructed so that the adjustment is effected by the movement of the hack combinations, as long since recommended by Mr. Wenham. MANAGEMENT OF THE MICROSCOPE. 141 140. Although the most perfect correction required for each particu- lar object (which depends not merely upon the thickness of its glass cover, but upon that of the fluid or balsam in Avhich it may be mounted) can only be found by experimental trial, yet for all ordinary purposes, the following simple method, first devided by Mr. Powell, will suffice. The object-glass, adjusted to ^uncoyered,' is to be ^focussed' to the ob- ject; the screw-collar is next to be turned until the surface of the glass cover comes into focus, as may be perceived by the spots or striae by which it may be marked; the object is then to be again brought into fo- cus by the ' slow motion.^ The edge of the screw-collar being graduated, the particular adjustment which any object may have been found to re- quire, and of which a record has been kept, may be made again without any difficulty. — By Messrs. Smith and Beck, however, who first intro- duced this graduation, a further use is made of it. By experiments such as those described in the last paragraph, the correct adjustment is first found for any particular object, and the number of divisions observed through which the screw-collar must be moved in order to bring it back to 0^, the position suitable for an uncovered object. The thickness of the glass cover must then be measured by means of the ^ slow motion '; this is done by bringing into exact focus, first the object itself, and then the surface of the glass cover, and by observing the number of divisions through which the milled-head (which is itself graduated) has passed in making this change. A definite ratio between that thickness of glass, and the correction required in that particular Objective, is thus established; and this serves as the guide to the requisite correction for any other thickness, which has been determined in like manner by the ^slow mo- tion.^ Thus, supposing a particular thickness of glass to be measured by 12 divisions of the milled-head of the ^slow motion,^ and the most perfect performance of the Objective to be obtained by moving the screw- collar through 8 divisions, then a thickness of glass measured by 9 divi- sions of the milled-head would require the screw-collar to be adjusted to 6 divisions in order to obtain the best effect. The ratio between the two sets of divisions is by no means the same for different combinations; and it ought to be determined for each Objective by its maker, who will generally be the fittest judge of the best ^ points^ of his lenses; but when this ratio has been once ascertained, the adjustment for any thickness of glass with which the object may happen to be covered, is readily made by the Micro- scopist himself. Although this method appears somewhat more complex than that of Mr. Powell, yet it is more perfect; and when the ratio be- tween the two sets of divisions has been once determined, the adjustment does not really involve more trouble. — Another use is made of this ad- justment by Messrs. Smith and Beck; namely, to correct the disturbance in the performance of Objectives, which is made by the increase of dis- tance between the Objective and the Eye-piece, occasioned by the use of the Draw-tube (§ 83). Accordingly they mark a scale of inches on the Draw-tube (which is useful for many other purposes), and direct that for every inch the body is lengthened, the screw-collar of the Objective shall be moved through "^a certain number of divisions. 141. Arrangement for Transparent Objects. — If the Object be already ' mounted ^ in a slide, nothing more is necessary, in order to bring it into the right position for viewing it, than to lay the slide upon the Object- platform of the Stage, and so to support it by means of the spring-clips, sliding-ledge, or other contrivance, that the part to be viewed is, as nearly as can be guessed, in the centre of the aperture of the stage, and 142 THE MICROSCOPE AND ITS REVELATIONS. therefore in a line with the axis of the body. If the object be not ^mounted/ and be of such a kind that it is best seen dry, it may be sim- ply laid upon the glass Stage-plate (§ 120), the ledge of which will pre- vent it from slipping off when the Microscope is inclined; and a plate of thin glass may be laid over it for its protection, if its delicacy should seem to render this desirable. If, again, it be disposed to curl up, so that a slight pressure is needed to flatten or extend it, recourse maybe had to the use of the Aquatic Box (§ 122 ) or the Compressor (§ 125), without the introduction of any liquid between the surfaces of glass. In a very large proportion of cases, however, either the objects to be examined are al- ready floating in fluid or it is preferable to examine them in fluid, on account of the greater distinctness with which they may be seen. If such objects be minute, and the quantity of liquid be small, the drop is sim- ply to be laid on a slip of glass, and covered with a plate of thin glass; if the object or the quantity of liquid be larger, it will be better to place it in a concave slide or cell; whilst, if the object have dimensions which render even this inconvenient, the Zoophyte Trough (§ 124) will afford the best means for its examination. — In the case of minute living ani- mals, whose movements it is desired limit (so as to keep them within the field of view) without restraining them by compression, the Author has found the following plan extremely convenient. The drop of water taken up with the animal by the Dipping- tube being allowed to fall into a concave slide (Fig. 122), the whole of the superfluous water may be re- moved by the Syringe (§ 127), only just as much being left as will keep the animal alive. If the animal be very minute, it is convenient to effect this withdrawal by placing the slide on the stage of the Dissecting Mi- croscope (§ 44), and working the Syringe under the magnifier; and it will be found after a little practice, that the complete command which the operator has over the movements of the piston, as well as over the place of the point of the syringe, enables him to remove every drop of super* fluous water without drawing the animal into the syringe. When, on the other hand, it is desired to isolate a particular animal from a number of others, the syringe may be conveniently used, after the same fashion, to draw it up and transfer it to another slide; care being, of course, taken that the syringe so employed has a sufficient aperture to receive it freely. — If it be wished to have recourse to compression, for the expansion or flattening of the object, this may be made upon the ordinary slide, by pressing down the thin-glass cover with a pointed stick; and this method which allow the pressure to be applied at the spot where it is most re- quired, will generally be found preferable for delicate portions of tis- sue which are easily spread out, and which, in fact, require little other compression than is afforded by the weight of the glass cover, and by the capillary attraction which draws it into proximity with the slide beneath. A firmer and more enduring pressure may be exerted by the dexterous management of a well-constructed Aquatic Box; and this method is pecu- liarly valuable for confining the movements of minute animals, so as to keep them at rest under the field of the microscope, without killing them. It is where a firm but graduated pressure is required, for the flattening-out of the bodies of thin semi-transparent animals, without the necessity of removing them from the field of the Microscope, that the Compressor is most useful. 142. In whatever way the Object is submitted to examination, it must be first brought approximately into position, and supported there, just as if it were in a mounted Slide. The precise mode of effecting this will MANAGEMENT OF THE MICROSCOPE. 143 differ, according to the particular plan of the instrument employed- thus, in some it is only the ledge itself that slides along the stage* in others it is a carriage of some kind, whereon the object-slide rests; in others, again, it is the entire platform itself that moves upon a fixed plane beneath. Having guided his object, as nearly as he can do by the unassisted eye, into its proper place, the Microscopist then brings his light (whether natural or artificial) to bear upon it, by turning the Mirror in such a direction as to reflect upon its under surface the rays which are received by itself from the sky or the lamp. The concave mirror is that which should always be first employed, the plane being reserved for special purposes; and it should bring the rays to convergence in or near the plane in which the object lies (Fig. 112). The distance at which it should be ordinarily set beneath the stage, is that at which it brings parallel rays to a focus; but this distance should be capable of elongation, by the lengthening of the stem to which the mirror is attached, since the rays diverging from a lamp at a short distance are not so soon brought to a focus. The correct focal adjustment of the Mirror may be judged by its formation of images of window-bars, chim- neys, etc., upon any semi-transparent medium placed in the plane of the object. It is only, however, when small objects are being viewed under high magnifying powers, that such a concentration of the light reflected by the Mirror is either necessary or desirable; for, with large objects seen under low powers, the field would not in this mode be equally illuminated. The diffusion of the light over a larger area may be secured, either by shifting the Mirror so much above or so much below its previous position, that the pencil will fall upon the object whilst still converging, or after it has met and diverged; or, on the other hand, by the interposition of a disk of Ground-glass in the course of the converging pencil, — this method which is peculiarly appropriate to lamp-light, be- ing very easily had recourse to, if the diaphragm-plate have had its larger aperture fitted to receive such a disk Arrangement of Microscope for Transparent Objects. (§ 98). The eye being now applied to the Eye-piece, and the body being ^focussed,^ the object is to be brought into the exact position required by the use of the transversing movement, if the stage be provided with it; if not, by the use of the two hands, one moving the object-slide from side to side, the other pushing the ledge, fork, or holder that carries it, either forwards or backwards as may be required. — It is always to be remembered, in making such adjustments by the direct use of the hands, that, owing to the inverting action of the Microscope, the motion to be given to the object, whether lateral or vertical, must be precisely opposed to that which its image seems to require, save when Erectors (§§ 84, 86) are employed. When the object has been thus brought fully into view, the 144: THE MICROSCOPE AND ITS REVELATIONS. Mirror may require a more accurate adjustment. What should be aimed-at is the diffusion of a clear and equable light over the entire field; and the observer should not be satisfied until he has attained this end. If the field should be darker on one side than on the other, the Mirror should be slightly turned in such a direction as to throw more light upon that side; perhaps in so doing, the light may be withdrawn from some part previously illuminated; and it may thus be found that the pencil is not large enough to light up the entire field. This may be owing to one of three causes: either the cone of rays may be received by the object too near to its focal apex, the remedy for which lies in an alteration in the dis- tance of the Mirror from the stage; or, from the very oblique position of the mirror, the cone is too much narrowed across one of its diameters, and the remedy must be sought in a change in the position either of the Microscope or of the Lamp, so that the face of the Mirror may not be turned so much away from the axis of vision; or, again, from the centre of the Mirror being out of the optic axis of the instrument, so that the illuminating cone is projected obliquely, — an error which can be rectified without the least difficulty. If the cone of rays should come to a focus in the object, the field is not unlikely to be crossed (in the day-time) by window-bars or chimneys, or (at night) the form of the lamp-flame may be distinguished upon it; the former must be got rid of by a slight change in the inclination of the Mirror; and if the latter cannot be dis- sipated in the same way, the lamp should be brought a little nearer. 143. The equable illumination of the entire field having been thus obtained, the quantity of light to be admitted should be regulated by the Diaphragm-plate (§ 98). This must depend very much upon the nature of the object, and upon the intensity of the light. Generally speaking, the more transparent the object, the less light does it need for its most perfect disjolay; and its most delicate markings are frequently only made visible, when the major part of the cone of rays has been cut off. Thus the movement of the cilia — those minute vibratile filaments with which almost every Animal is provided in some part of its organism, and which many of the humbler Plants also possess in the early stages of their exist- ence — can only be discerned in many instances when the light is admitted through the smallest aperture. On the other hand, the less transparent objects usually require the stronger illumination which is afforded by a wider cone of rays; and there are some (such as semi-transparent sections of Fossil Teeth) which, even when viewed with low powers, are better seen with the intenser light afforded by the Achromatic Condenser. — In every case in which the object presents any considerable obstruction to the passage of the rays through it, great care should be taken to pro- tect it entirely from incident light; since this extremely weakens the effect of that which is received into the Microscope by transmission. It is by daylight that this interference is most likely to occur; since, if the precautions already given (§ 132) respecting the use of lamp-light be observed, no great amount of light can fall upon the upper surface of the object. The observer will be warned that such an effect is being pro- duced, by perceiving that there is a want not only of brightness but of clearness in the image, the field being veiled, as it were, by a kind of thm vapor; and he may at once satisfy himself of the cause, by inter- posing his hand between the stage and the source of light, when the immediate increase of brilliancy and distinctness will reveal to him the source of the previous deficiency in both. Nothing more is necessary for its permanent avoidance, than the interposition of an opaque screen MANAGEMENT OF THE MICROSCOPE. 146 (blackened on the side towards the stage) between the window and the object; care being of course taken that the screen does not interfere with the passage of light to the mirror. Such a screen may be easily shaped and adapted either to be carried by the stage itself, or by the stand for the condenser; but it is seldom employed by Microscopists, as it inter- feres with access to the left side of the stage; and the interposition of the hand, so often as it may be needed, is more frequently had recourse to in preference, as the more convenient expedient. The young Micro- scopist who may be examining transparent objects by daylight, is recom- mended never to omit ascertaining whether the view which he may obtain of them is in any degree thus marred by incident light. 144. Although the illumination afforded by the Mirror alone is quite adequate for a very large proportion of the purposes for which the Micro- scope may be profitably employed (nothing else having been used by many of those who have made most valuable contributions to Science by means of this instrument), yet, when high magnifying powers are employed, and sometimes even when but a very moderate amplification is needed, great advantage is gained from the use of a Condenser. The form which has been described under the name of the Webster Condenser (§ 100) answers so well for most purposes, and may in addition be so easily converted into a ^ black ground^ Illuminator, that the working Microscopist will find it convenient to keep it always in place; substitut- ing an Achroinatic Condenser of greater power (§ 99) only when specially needed. Special care is needed in the use of this last, both as to the coincidence of its optic axis with that of the Microscope itself, and as to its focal distance from the object. The centering may be most readily accomplished by so adjusting the distance of the Condenser from the Stage (by the rack-and-pinion action or the sliding movement with which it is always provided), that a sharp circle of light shall be thrown on any semi-transparent medium laid upon it; then, on this being viewed through the Microsope with an Objective of sufficiently low power to take in the whole of it, if this circle be not found concentric with the field of view, the axis of the Condenser must be altered by means of the milled- head tangent-screws with which it is provided. Or a cap with a minute central aperture may be fitted on the top of the Condenser, and this aperture centered in the field of an objective of medium power. Or, again, a diaphragm with a very minute central perforation may be placed at a little distance beneath the Achromatic Condenser, and the image of this may be brought into the centre of the field of a l-4th objective, which is the best arrangement when it is to be used with very high powers. The focal adjustment of the Condenser, on the other hand, must be made under the Objective which is to be employed in the examination of the object, by turning the Mirror in such a manner as to throw upon the visual image of the object (previously brought into the focus of the Micro- scope) an image of a chimney or a window-bar, if daylight be employed, or of the top, bottom, or edge of the lamp-flame, if lamp-light be in use; the focus of the condenser should then be so adjusted as to render the view of this as distinct as possible; and the direction of the Mirror should then be sufficiently changed to displace the image, and to substitute for it the clearest light that can be obtained. It will generally be found, how- ever, that although such an exact focussing gives the most perfect results by Daylight, yet that by Lamp-light the best illumination is obtained when the Condenser is removed to a somewhat greater distance from the object, than that at which it gives a distinct image of the lamp. In every 10 146 THE MICROSCOPE AND ITS REVELATIONS. case, indeed, in which it is desired to ascertain the effect of variety in the method of illumination, the effects of alterations in the distance of the condenser from the object should be tried; as it will often happen that delicate markings become visible when the condenser is a little out of focus, which cannot be distinguished when it is precisely in, focus. The regulation of the amount of light transmitted through the object is often of the very first importance; and no means of accomplishing this is so convenient as a Graduating or Iris Diaphragm (§ 98). For some objects of great transparence, the White-Cloud illumination (§ 109) may be had recourse to with advantage. 145. There are many Transparent Objects, however, whose pecu- liar features can only be distinctly made out, when they are viewed by light transmitted through them obliquely instead of axially; and this is especially the case with such as have their surfaces marked by very deli- cate and closely-approximated furrows, the direction of the oblique rays being then a matter of primary importance. Thus, suppose that an object be marked by longitudinal striae too delicate to be seen by ordinary direct light; the oblique light most fitted to bring them into view will be that ^ proceeding in either of the directions c or D; that which falls upon it in the directions A and B tending to obscure the striae rather than to disclose them. But if the striae ^ ^ should be due to furrows or prominences which have one side inclined and the other side abrupt, they will not be B brought into view indifferently by light from C or from D, but will be shown best by that which makes the strongest shadow; hence, if there be a projecting ridge, with an abrupt side looking towards c, it will be best seen by light from d; whilst if there be a furrow with a steep bank on the side of c, it will be by light from that side that it will be best displayed. But it is not at all unfrequent for the longitudinal striae to be crossed by others; and these transverse striae will usually be best seen by the light that is least favorable for the longitudinal; so that, in order to bring them into distinct view, either the illuminating pencil or the object must be moved a quarter round. The simplest mode of obtaining this end, is to make the Mirror capable of being turned into such a position as to reflect light into the object from one side and at a very oblique angle (which is best done by the Zentmayer arrangement); and to give the Stage a rotatory movement, so that the object may be presented to that light under every azimuth. 146. For objects of greater difficulty, however, it is better to have recourse to the Accessories already described (§§ 101-108), which are specially provided to furnish oblique illumination in the most effectual manner. A good example of the variety of appearances which the same object may exhibit, when illuminated from different azimuths, and with slight changes of focussing, is shown in Fig. 113, which represents por- tions of a valve of Pleurosigma formosum as seen under a power of 1300 diameters; the markings shown at A, B, and c, being brought-out by ollique light in different directions, which, however, when carefully used, does not produce these erroneous aspects; whilst at D, is shown the effect of axial illumination with the Achromatic Condenser. — It cannot be too strongly impressed on the young Microscopist, however, that the special value of very oblique illumination is limited to the resolution of ' test- objects;' and that for the ordinary purposes of scientific study and research, axial illumination is generally preferable. As in regard to the qualities of Objectives (§ 55), so in respect to Illumination, may MANAGEMENT OF THE MICROSCOPE. 147 Fig, Biological ifc be confidently asserted that tlie solution of the most difficult Biological problems to which the Microscope has been yet applied, lias been attained by arrangements by no means the most favorable to the discernment of the markings on Diatom-valyes or the lines on Robert's test-plate; and that, converseTy, the arrangements specially effective for the ^resolutions of the most difficult U7iecl ^ tests ^ have not, as yet, been shown to have much value in " " " investigation (§ 158). 147. There are many kinds of Transpa- rent objects — especially such as either con- sist of thin plates, disks, or spicules of Siliceous or Calcareous matter, or contain such bodies — which are peculiarly well seen under the Black-ground illumination (§§ 104, 105); for not only does the brilliant luminosity which they then present, in contrast with the dark ground behind them, show their forms 'to extraordinary advan- tage; but this mode of illumination imparts to them an appearance of solidity which they do not exhibit by ordinary transmitted light (§ 103); and it also frequently brings out surface-markings which are not other- wise distinguishable. Hence when any ob- ject is under examination that can be sup- posed to be a good subject for this method, the trial of it should never be omitted. For low powers, the use of the Spot-lens or the Webster Condenser with the central stop will be found suflSciently satisfactory; for the higher, the Paraboloid or the Eeflex Il- luminator should be employed. — Similar general remarks may be made respecting the examination of objects by Polarized y^^^^ Pieurosigma formosum, light (S 110). Some OI the most strikms^ with portions a, b, c, d, showing diverse effects of this kind of illumination are "^'"'^ produced upon bodies whose particles have a crystalline aggregation; and hence it may often be employed with great advantage to bring such bodies into view, when they would not otherwise be distinguished: thus, for example, the r aphides of Plants are much more clearly made out by its means, in the midst of the tissues, than they can be by any other. But the peculiar effects of Polarized light are also exerted upon a great number of other Organized substances, both animal and vegetable; and it often reveals differences in the arrangement or in the relative den- sity of their component particles, the existence of which would not other- wise have been suspected; hence the Microscopist will do well to have recourse to it, whenever he may have the least suspicion that its use can give him an additional power of discrimination. 148. Arrangement for Opaque Objects, — There are many objects of the most interesting character, the opacity of which entirely forbids the transmission of light through them, and of which, therefore, the surfaces only can be viewed by means of the incident rays which they reflect. These are, for the most part, objects of comparatively large dimensions. 148 THE MICROSCOPE AND ITS REVELATIONS. for which a low magnifying power suffices; and it is specially important, in the examination of such objects, not to use a lens of shorter focus than is absolutely necessary for discerning the details of the structure; since, the longer the focus of the Objective employed, the less is the indistinctness produced by inequalities of the surface, and the larger, too, may be its aperture, so as to admit a greater quantity of light, to the great improve- ment of the brightness of the image. Objectives of long focus are espe- cially required in Microscopes that are to be used for Educational purposes;^ and an endless variety of ^ common objects ' suitable to these may be found by such as will take the trouble to search for them. — The mode of bringing Opaque objects under view will differ according to their ^mounting,' and to the manner in which it is desired to illuminate them. If the object be mounted in a ^ slide ' of glass or wood, upon a large Opaque surface, the slide must be laid on the stage in the usual manner, and the object brought as nearly as possible into position by the eye alone (§ 141). If it be not so mounted it may be simply laid upon the glass Stage-plate, resting against its ledge; and the Diaphragm-plate must then be so turned as to afford it a black background, light being thrown upon it by a Condensing Lens or Bull's-eye placed as in Fig. 114, or (still better) by Beck's Para- bolic Speculum, which gives a far better illumination by diffused daylight ^j^j^j^ ^yji^ determined by Arrangement of Microscope for Opaque Objects. the size of the SUrface to be il- luminated and by the kind of light required. If the magnifying power employed be high, and the field of view be consequently limited, it will be desirable so to adjust the lens as to bring the cone of rays to a point upon the part of the object under examination; and this adjustment can only be rightly made whilst the object is kept in view under the Microscope, the condenser being moved in various modes until that position has been found for it in which it gives the best light. If, on the other hand, the power be low, and it be desired to spread the light equably over a large field, the Con- denser should be placed either within or beyond its focal distance; and ^ The makers of Educational Microscopes supply at a small cost, single (triplet) combinations of 3 inches, 2 inches, or 1^ inch focus, or dividing combinations of half inch and 1 inch, 1 inch and 2 inchs, or inch and 3 inches, which are quite adequate for ordinary requirements. distance from the object than can be obtained by any other means yet devised, and which is equally well adapted to lamp-light, when used in combination with the Bull's- eye (§ 114). Direct sunlight cannot be employed without the production of an injuri- ous glare, and the risk of burning the object; but the sunlight reflected from a bright cloud is the best light possible. When a Condens- ing Lens is used, it should always be placed at right angles to the direction of the illuminating rays, and at a MANAGEMENT OF THE MICROSCOPE. 149 here, too, the best position will be ascertained by trial. It will often be desirable also to vary both the obliquity of the light and the direction in which it falls upon the object; the aspect of which is greatly affected by the manner in which the shadows are projected upon its surface, and in which the lights are reflected from the various points of it. Many objects, indeed, which are distinguished by their striking appearance when the light falls upon them on one side, are entirely destitute both of brilliancy of color and of sharpness of outline when illuminated from the opposite side. Hence it is always desirable to try the effect of chang- ing the position of the object; which, if it be ^mounted,' may be first shifted by merely reversing the place of the two ends of the slide, and then, if this be not satisfactory, may be more completely as well as more gradu- ally altered by making the object-platform itself to rotate. With regard to the obliquity of the illuminating rays, it is well to remark, that if the object be ^ mounted ' under a glass cover, and the incident rays fall at too great an angle with the perpendicular, a large proportion of them will be reflected, and the brilliancy of the object will be greatly impaired; and hence when Opaque objects are being examined under high powers with a very oblique illuminating pencil, they should always be uncovered. 149. The same general arrangement must be made when Artificial light is used for the illumination of Opaque objects; the Lamp being placed in such a position in regard to the Stage that its rays may fall in the direction indicated in Fig. 114, and these rays being collected and concentrated by the Condenser, as already directed. Since the rays pro- ceeding from a lamp within a short distance are already diverging, they will not be brought by the Condenser to such speedy convergence as are the parallel rays of daylight; and it must, therefore, be farther removed from the object to produce the same effect. By modifying the distance of the Condenser from the lamp and from the object respectively, the cone of rays may be brought nearly to a focus, or it may be spread almost equably over a large surface, as may be desired. And the same effect may be produced by shifting the position of the Condenser, when the Parabolic Speculum is employed in combination with it. No more effective illumination can be desired for objects viewed under the low powers to which the Parabolic Speculum is adapted, than that which is afforded by this combination; the Beckett Lamp (Fig. 108) supplying a most convenient means of using it, as the Author can testify from a very large experience. In the illumination of Opaque objects. Artificial light has the advantage over ordinary daylight, of being more easily concen- trated to the precise degree, and of being more readily made to fall in the precise direction, that may be found most advantageous. Moreover, the contrast of light and shadow will^be more strongly marked when no light falls upon the object except that proceeding from the Lamp used for its illumination, than it can be when the shadows are partially light- ened by the rays which fall upon the object from every quarter, as must be the case if it be viewed by daylight. — If a more concentrated light be required, the flame of the lamp may be turned edgewise to the object, and the small Condensing-lens may be used in combination with the Bull's eye ; being so placed as to receive the cone projected by it, and to' bring its rays to a more exact convergence. It was in this way that Mr.l Beck obtained the views of the Fodura'scsle given in plate II., Figs. 4, 5. In this manner very minute bodies may be viewed as Opaque objects under high magnifying powers, provided that the brasswork of the ex- tremities of the Objectives be so bevelled-off as to allow the illuminating 150 THE MICROSCOPE AND ITS REVELATIONS. cone to have access to the object. As none but a yery oblique illumina- tion, however, can be thus obtained, the view of the object will be by no means complete, unless it be supplemented by that which maybe obtained by means of the Vertical Illummator (§ 116), which supplies for high powers the kind of illumination that is given by the Lieberkuhn for the lower. 150. There are many Opaque objects, such as Foraminifera, wh h it is desirable to view from all sides, in order that their features may be completely made out. This may be readily done with objects mounted in slides, when the Microscope is provided with the Zentmayer stage, by inclining the stage to one side or the other (first taking care that the ob- ject is well secured upon it), and then giving rotation to the object-plat- form. For such objects as can be conveniently attached to small disks. Beck's Disk-holder (Fig. 94), affords by far the most convenient and effect- ive mode of presenting them in every variety of aspect; but the disks may also be held by attached pins, either in the Stage-forceps, or by the inser- tion of the pins into the cork-box at its other end (§ 118), a variety of movements being given in either case by turning the forceps in its tube. So, again, many small objects, such as parts of Insects, may be grasped in the Stage-forceps itself, and by a little care in manipulation, each aspect may be brought into view successively. In either of these cases, the Lieberkuhn may be employed for their illumination; and light of con- siderable obliquity may be obtained by its means, either by turning the Mirror out of the axis, or by covering part of the reflecting surface of the Lieberkiihn by a cap, or by a combination of both methods. Whenever the Lieberkuhn is employed, care must be taken that the direct light from the Mirror is entirely stopped-out by the interposition of a ^ dark well ^ or of a black disk, of such a size as to fill the field given by the partic- ular Objective employed, but not to pass much beyond it. Opaque objects that are permanently mounted either upon cardboard disks, or in the slides specially provided for them, may be presented to the Microscope in a considerable variety of directions by means of Morris's Object-holder (Fig. 95); which, however, can only be employed with side-illumination. If it be desired to make the most advantageous use of this appliance, ob- jects mounted in slides should be so placed that the parts to be brought into view by its tilting movement may look towards the long edges of the slide ; since it is obvious that a much greater inclination may be given to it in either of these directions, than in the direction of either of its ex- tremities. 151. Errors of Interpretation. — The correctness of the conclusions which the Microscopist will draw regarding the nature of any object, from the visual appearances which it presents to him when examined in the various modes now specified, will necessarily depend in a great degree upon his previous experience in Microscopic observation, and upon his knowledge of the class of bodies to which the particular specimen may belong. Not only are observations of any kind liable to certain fallacies, arising out of the previous notions which the observer may entertain in regard to the constitution of the objects or the nature of the actions to which his attention is directed, but even the most practised observer is apt to take no note of such phenomena as his mind is not prepared to appreciate. Errors and imperfections of this kind can only be corrected, it is obvious, by general advance in scientific knowledge; but the history of them affords a useful warning against hasty conclusions drawn from a too cursory examination. If the history of almost any scientific investi' MANAGEMENT OF THE MICROSCOPE. 151 gation were fully made known, it would generally appear that the sta- bility and completeness of the conclusions finally arrived-at had only been attained after many modifications, or even entire alterations, of doctrine. And it is, therefore, of such great importance as to be almost essential to the correctness of our conclusions, that they should not be finally formed and announced until they have been tested in every conceivable mode. I It is due to Science that it should be burdened with as few false facts and false doctrines as possible. It is due to other truth-seekers that they should not be misled, to the great waste of their time and pains, by our errors. And it is due to ourselves that we should not commit our repu- tation to the chance of impairment, by the premature formation and publication of conclusions, which may be at once reversed by other ob- servers better informed than ourselves, or may be proved to be fallacious at some future time, perhaps even by our own more extended and care- ful researches. The suspension of the judgment, whenever there seems room for doubt y is a lesson inculcated by all those Philosophers who have gained the highest repute for practical wisdom; and it is one which the Microscopist cannot too soon learn, or too constantly practise. — Besides these general warnings, however, certain special cautions should be given to the young Microscopist, with regard to errors into which he is liable to be led, even when the very best instruments are employed. 152. Errors of interpretation arising from the imperfection of the focal adjustment are not at all uncommon amongst young Microscopists. With lenses of high power, and especially with those of large angular aperture, it very seldom happens that all the parts of an object, however minute and flat it may be, can be in focus together; and hence, when the focal adjustment is exactly made for one part, everything that is not in exact focus is not only more or less indistinct, but is often wrongly repre- sented. The indistinctness of outline will sometimes present the appear- ance of a pellucid border, which, like the diffraction-band, may be mis- taken for actual substance. But the most common error is that which is produced by the reversal of the lights and shadows resulting from the refractive powers of the object itself; thus, the bi-concavity of the blood- disks of Human (and other Mammalian) Blood occasions their centres to appear dark when in the focus of the Microscope, through the divergence of the rays which it occasions; but when they are brought a little within the focus by a slight approximation of the object-glass, the centres appear brighter than the peripheral parts of the disks. An opposite reversal presents itself in the markings of certain Diatoms^ which, like Fleuro- sigma angulatum, present, when ex- actly focussed, the aspect of rows of hemispherical beads (Fig. 166, a). When the surface is viewed a little inside the focus, its aspect is that shown at A, Fig. 115; whilst, when the surface is slightly beyond the focus (b), the hexagonal areolae are dark, and the intervening partitions light. — The experienced Microscopist False hexagonal areolation of Pleuro- will find in the optical eilects produced Z^mllo itoooX^^eterf"'"- by variations of focal adjustment the most certain indications in regard to the nature of such inequalities of surface as are too minute to be made apparent by the use of the Stereo- scopic Binocular. For superficial elevations must necessarily appear 152 THE MICROSCOPE AND ITS REVELATIONS. brightest when the distance between the Objective and the object is in- creased, whilst depressions must apper.r brightest when that distance is diminished. — The Student should be warned against supposing that in all cases, the most positive and strihing appearance is the truest; for this is often not the case. Mr. Slack's optical illusion, or silica-crack slide,^ illustrates an error of this description. A drop of water holding colloid silica in solution is allowed to evaporate on a glass slide, and, when quite dry, is covered with thin glass to keep it clean. The silica deposited in this way is curiously cracked; and the finest of these cracks can be made to present a very positive and de/eptive appearance of being raised bodies like glass threads. It is also easy to obtain diflraction-lines at their edges, giving an appearance of duplicity to that which is really single. 153. A very important and very frequent source of error, which some- times operates even on experienced Microscopists, lies in the refractive influence exerted by certain peculiarities in the internal structure of ob- jects upon the rays of light transmitted through them; this influence being of a nature to give rise to appearances in the image, which suggest to the observer an idea of their cause that may be altogether different from the reality. Of this fallacy we have a ^ pregnant instance ^ in the misinterpretation of the nature of the lacicnce and canaliculi of Bone, which were long supposed to be solid corpuscles with radiating filaments of peculiar opacity, instead of being, as is now universally admitted, minute chambers with diverging passages excavated in the solid osseous substance. For, just as the convexity of its surface will cause a transpa- rent cylinder to show a bright axial band,^ so will the concavity of the internal surfaces of the cavities or tubes hollowed-out in the midst of highly-refracting substances, occasion a divergence of the rays passing through them, and consequently render them so dark that they are easily mistaken for opaque solids. That such is the case with the so-called ^ bone corpuscles,' is shown by the effect of the infiltration of Canada balsam through the osseous substance; for when this fills np the excava- tions, being nearly of the same refractive power with the bone itself, it obliterates them altogether. — So, again, if a person who is unaccustomed to the use of the Microscope should have his attention directed to a preparation mounted in liquid or in balsam that might chance to contain air-buiUes, he will be almost certain to be so much more strongly im- pressed by the appearances of these than by that of the object, that his first remark will be upon the number of strange-looking black rings which he sees, and his first inquiry will be in regard to their meaning. 154. Although no experienced Microscopist could now be led astray by such obvious fallacies as those alluded to, it is necessary to notice them as warnings to those who have still to go through the same education. The best method of learning to appreciate the class of appearances in question, is the comparison of the aspect of globules of Oil in water, with that of globules of Water in oil, or of bubbles of Air in water or Canada balsam. This comparison may be very readily made by shaking up some oil with water to which a little gum has been added, so as to form an emulsion; or by simply placing a drop of oil of turpentine (colored by magenta or carmine) and a drop of water together on a slip of glass, lay- * Monthly Microscopical Journal," Vol. v. (1872), p. 14. ^ This was the appearance which gave rise to the erroneous notion that long prevailed amongst Microscopic observers, and still lingers in the Public mind, of the tubular structure of the Human Hair, MANAGEMENT OF THE MICROSCOPE. 158 ing a thin-glass cover upon them, and then moving the cover several times backwards and forwards upon the slide. Now when such a mix- ture is examined with a sufficiently high magnifying power, all the glo- bules present nearly the same appearance, namely, dark margins with bright centres; but when the test of alteration of the focus is applied to them, the difference is at once revealed; for whilst tlie globules of Oil surrounded by water become darher as the object-glass is depressed, and lighter as it is raised, those of Water surrounded by oil become more luminous fis the object-glass is depressed, and darker as it is raised. The reason of this lies in the fact that the high refracting power of the Oil causes each of its globules to act like a ^ow^X^-convex lens of very short focus; and as this will bring the rays which pass through it into conver- gence above the globule (i. e., between the globule and the Objective), its brightest image is given when the object-glass is removed somewhat farther from it than the exact focal distance of the object. On the other hand, the globule of Water in oil, or the minute bubble of air in water or balsam, acts, in virtue of its inferior refractive power, like a double- concave lens; and as the rays of this diverge from a virtual focus heloio the globule {i, e., between the globule and the mirror), the spot of great- est luminosity will be found by causing the object-glass to approach tvitliin the proper focus. A thorough mastery of these appearances is very im- portant in the study of the ^protoplasm' of Plants — the ^sarcode' of Animals, — which includes oil-particles, together with spaces occupied by a watery fluid, which (having been at one time supposed to be void) are known as Vacuoles.' 155. Among the sources of fallacy by which the young Microscopist is liable to be misled, one of the most curious is the movement exhibited by very minute particles of nearly all bodies that are sufficiently finely di- vided, when suspended in water or other fluids. This movement was first observed in the fine granular particles which exist in great abundance in the contents of the Pollengrains of plants (sometimes termed the fovilla), and which are set free by crushing them; and it was imagined that they indicated the possession of some special vital endowment by these particles, analogous to that of the Spermatozoa of animals. In the year 1827, however, it was announced by Dr. Eobert Brown that numer- ous other substances. Organic and Inorganic, when reduced to a state of equally minute division, exhibit a like movement, so that it cannot be regarded as indicative of any endowment peculiar to the fovilla granules; and subsequent researches have shown that there is no known exception to the rule that such motion takes place in the particles of all substances, though some require to be more finely divided than others before they will exhibit it. The closer the conformity between the specific gravity of the solid particles and that of the liquid, the less minute need be that reduction in their size which is a necessary condition of their movement: and thus Carmine, Indigo, or Gamboge rubbed up with water, show it extremely well; whilst the particles of Metals, which are from seven to twenty times as heavy as water, require to be reduced to a minuteness many times greater, before they will exhibit it. The movement is chiefly of an oscillatory kind ; but the particles also rotate backwards and for- wards upon their axes, and gradually change their places in the field of view. The movement of the smallest particles is the most energetic, and the largest (exceeding l-5000th of an inch) are quite motionless, whilst those of intermediate size move with comparative inertness. A drop of common ink which has been exposed to the air for some weeks, or a drop 154 THE MICROSCOPE AND ITS REVELATIONS. of fine clay (such as the prepared haolin used by Photographers) shaken- up with water, is recommended by Prof. Jevons/ who has recently studied this subject, as showing the movement (which he designates pedesis) extremely well. But none of the particles he has examined are so active as those of pumice-stone that has been ground-up in an agate mortar; for these are seen under the microscope to leap and swarm with an incessant quivering movement, so rapid that it is impossible to follow the course of a particle which probably changes its direction of motion 15 or 20 times in a second. The distance through which a particle moves at any one bound is usually less than l-5000th of an inch. This ' Brownian movement ' (as it is commonly termed) is not due to evapora- tion of the liquid : for it continues, without the least abatement of energy, in a drop of aqueous fluid that is completely surrounded by oil, and is therefore cut off from all possibility of evaporation; and it has been known to continue for many years in a small quantity of fluid inclosed between two glasses in an air-tight case. And, for the same reason, it can scarcely be connected with chemical change. But the observations of Prof. Jevons (loc. cit.) show that it is greatly affected by the admixture of various substances with water; being, for example, increased by a small admixture of gum, while it is checked by an extremely minute admixture of sulphuric acid or of various saline compounds, these (as Prof. J. points out) being all such as increase the conducting power of water for Elec- tricity. The rate of subsidence of finely-divided clays or other particles suspended in water, thus greatly depends upon the activity of their ^Brownian movement;' for, when this is brought to a stand, the particles aggregate and sink, so that the liquid clears itself. — In any case in which the motions of very minute particles, of whatever kind, are in question, it is necessary to make allowance for this ^ molecular^ movement; and the young Microscopist will therefore do well to familiarize himself with its ordinary characters, by the careful observation of it in such cases as those just named, and in any others in which he may meet with it.^ 156. Diffraction. — The course of Light-rays is altered not only by refraction when they pass from one transparent medium into another, and by reflection when they fall on polished surfaces which they do not enter, but also by inflection at the edges of objects by which they pass; and as the differently colored rays which altogether make up white light are affected by such inflection in different degrees, they are separated by it (as by refractive ^dispersion ') into colored bands; the phenomenon be- ing altogether known as diffraction. This may be made evident by caus- ing abeam of sunlight to enter a darkened room through a very narrow slit, and to fall on a white screen; for the narrow line of white light will show a border of colored fringes, which become wider as the slit is nar- rowed; and if these fringes be viewed through a piece of colored glass, Avhich allows only rays of its own color to pass, they will appear as a suc- cession of bands alternately bright and dark. This alternation is pro- duced by the interference of the Light- waves; just as the alternations of sound and comparative silence termed ^ beats,' which are heard when two slightly different tones are being sounded together, are due to the inter- 1 Quarterly Journal of Science," N. S., Vol. viii. (1878), p. 172. '-^ See also the Rev. J. Delsaulx **On the Thermo-Dynamic Origin of the Brownian Motions" in Monthly Journ. of Microsc. Sci ," Vol. xviii. (1877), p. 1; and Dr. W. M. Ord '*On some Causes of Brownian Movements" in Journ. of Roy. Microsc. Soc," Vol. ii. (1879), p. 656. MANAGEMENT OF THE MICROSCOPE. 155 ference of the Sound-waves.^ The colored fringes are produced by the superposition of all these bands. — When, again, a small opaque plate of any substance is interposed in the course of the pencil of solar light ad- mitted into a darkened room through a very small hole in a card, or di- verging from the point at which it has been collected by a convex lens of short focus, the shadow thrown by it on the screen will be surrounded by a series of colored fringes, and the shadow itself will be larger than the geometrical shadow.— But, further, if a piece of glass be ruled by a dia- mond with parallel lines, some hundreds or thousands to an inch, so as to form what is called a ^grating,' and the narrow beam proceedmg from the slit be looked-at through this grating (so held that the direction of its lines is parallel to that of the slit), a number of spectra come into view, ranged at nearly equal distances on both sides of the slit.^ Now, it is manifest that when a beam of light is made to pass through an object that is being examined Microscopically, the light and shade in the picture seen by the eye must be occasioned by the greater or less transparence of the different parts of that object; and that, wherever there are definite lines or margins sufficiently opaque to throw a definite shadow, such Fia. 116. Scale of Gnat showing beaded markings; photographed by Dr. Woodward. shadow must be bordered more or less obviously by ^interference ^ or ' diffraction ' spectra, especially in the case of objects having strongly- marked lines with very transparent intermediate spaces. There are many objects of great delicacy, in which ^diffraction-bands^ are liable to be mistaken for indications of actual substance; whilst, on the other hand, the presence of an actual substance of extreme transparence may some- times be doubted or denied, through its image being attributed to diffrac- tion. No rules can be given for the avoidance of such errors; since they can only be escaped by the discriminating power which education and habit confer on the experienced Microscopist.— A good example of this kind is afforded by the minute beading presented in the scales of the Gnat and Mosquito (Fig. 116). These scales are composed of a very delicate double membrane, strengthened by longitudinal ribs on either side, those ^ The colors of thin plates, — as seen wlien the sun shines on a soap-bubble or on a film of oil spread out over a surface of water — or when we look at a window through two glasses separated by an attenuated film of air, — are familiar exam- ples of * interference-fringes,' which, when displayed annularly, are known as ' Newton's rings.' Such ' gratings' are now much used in Spectroscopic observation; and afford the best means of determining the wave-lengths of the rays of the several parts of the spectrum. 156 THE MICROSCOPE AND ITS KEVELATION8. of the opposite sides uniting at the broad end of the scale, where they generally project as bristle-like appendages beyond the intermediate mem- brane; and they are crossed transversely by fine markings, which are probably ridge-like corrugations of their membrane, these also existing on both surfaces of the scale. The attention of Dr. Woodward having been drawn by Dr. Anthony to the presence in these scales of three ujii- form parallel rows of beads in every interspace between two adjoinijig ribs, he was at first inclined to believe that the markings are real, represent- ing an actual structure in the scale; but having obtained an excellent Photograph of it by monochromatic sun-light, under a power of 1,350 diameters, he was led to alter his opinion, and to regard them as pro- duced by the crossing of the transverse markings by longitudmal diffrac^ tioii'lines, conditioned by the longitudinal 7'ibs and parallel to them.^ His chief reasons for so regarding them were (1), that ^^the longitudinal diffraction-lines are clearly seen alike in the Microscopic image and in the Photographs, to extend into empty space beyond the contour of the scales, almost as far as the ends of the bristles in which the parallel ribs terminate;'' and (2), ^^that they vary in number with varying ob^ liquity of illumination, so that in the same scale two, three, four, and five rows of beads can be seen, and photographed at pleasure, in every intercostal space.'' The true appearance. Dr. Woodward considers, is given when the Achromatic Condenser is so adjusted that its light is either central or slightly oblique in the longitudinal direction of the scale. 157. The recent researches of Prof. Abbe of Jena appear to have con- clusively proved that Diffraction has a most important share, previously altogether unsuspected, in the formation of the Microscopic images of very closely approximated lines or other markings, in objects viewed under high magnifying powers of large Angular aperture. — All that has been hitherto said of the formation of Microscopic images, relates to such as are produced, in accordance with the laws of refraction, by the alteration in their course which the Light-rays undergo in their passage through the lenses interposed between the object and the eye. These dioptric images, when formed by lenses free from Spherical and Chro- matic aberration, are geometrically correct pictures, truly representing the appearances which the objects themselves would present, were they en- larged to the same scale, and viewed under similar illumination. And we are fully justified, therefore, in drawing from such Microscopic images (provided that they are free from diffraction-spectra) the same conclu- sions in regard to the structure of the objects they picture, as we should draw from the direct vision of actual objects having the same dimensions. There is, however, an optical limit as to the completeness of such images in regard to minute detail; as it appears from the theoretical researches of Prof rs. Helmholtz and Abbe, that no amount of magnifying power can separate dioptrically two lines, apertures, or markings of any kind, not more than l-2500th of an inch apart. The visual separation or ^ reso- lution ' of more closely approximated lines or other markings is entirely the result of diffraction; the Objective receiving and transmitting, not only the ordinary dioptric rays, but the ' inflected ' rays whose course has been altered in their course through the object by some peculiarity in the disposition of its particles. These rays, when acted-on by the Objective, produce ' diffraction-spectra;' the number and relative position of which * Monthly Microsc. Joum.," Vol. xv. (1876), p. 253. MAl^AGEMENT OF THE MICROSCOPE. 157 bear a relation to the structural arrangement on which their production depends.^ If the Objective be perfectly corrected, and all the diffraction- spectra lie within its field, they will be re-united by the Eye-piece to form u secondary or ' dilfraction ' image, lying in the same focal plane with the dioptric image, and coinciding with it, while filling up its outlines by supplying intermediate details. But where the markings (of whatever nature) are so closely approximated as to produce a wide dispersion of the interference-spectra, only a part of them may fall within the range of the Objective; and the re-combination of these may produce a diffraction- image differing more or less completely (perhaps even totally) from the real structure; whilst, if they should lie entirely outside the field of the Objective, no secondary or diffraction image will be produced. Thus, whilst the dioptric image represents the actual object, a diffraction- image formed by the reunion of some of the interference-spectra is only an optical expression of the result of their partial re-combination, which may represent something entirely different from the real structure; — the sa)ne arrangement of lines (for example) being presented to the eye by dif- /i Aperture" (§ 10), by the allowance made for the modification in the course of MANAGEMENT OF THE MlCROSCori: and (2) that, as there is a like increase in the number of separate diffraction spectra which can be combined with the dioptric image, the representations of minute structure given by Objectives of widest Angular aperture are more trustworthy than those given by those of narrower. 158. Relative Qualities of Objectives. — In estimating the comparative values of different Objectives, regard must always be had to the purpose for which each is designed; since it is impossible to construct a combina- tion which shall be equally serviceable for every requirement. It is commonly assumed than an Objective which will show certain Test- objects, musb be very superior for everything else to a glass which will not resolve these; but this is known to every practised Microscopist to be a complete mistake, the qualities which enable it to resolve some of the more difficult ^ tests,^ not being by any means identical with those which make it most useful in all the ordinary purposes of Scientific investiga- tion. Five distinct attributes have to be specially considered in judging of the character of an Objecfc-glass, viz. — (1) its working-distance, or actual interval between its front lens and the object on which it is focussed; (2) its defining power, or power of giving a clear and distinct image of all well-marked features of an object, especially of its bounda- ries; (3) it penetrating power, or focal depth, by which the observer is enabled to looh into the structure of objects; (4) its resolving poimr, by which it enables closely-approximated markings to be distinguished; and (5) the flatness of the field which it gives. I. The ^ Working distance ^ of an Objective has no fixed relation to its ^ focal length;' the latter being estimated by its equality in magnify- ing power with a single lens of given curvature;* while the former varies with the mode in which the combination is constructed, and with the angular aperture given to it. Of two Objectives of 1-inch focus and the same angle of aperture (say 25^), one may have, in virtue of its construc- tion, a much longer ^ working distance ' than the other; and this is not only an advantage in facilitating the side-illumination of opaque objects, but also in admitting (as will presently appear) of greater ' focal depth ' or ^penetration.' But it is especially in the case of high powers that ^ working distance ' comes to be of essential importance. The widening of angular aperture which is required to give them their highest degree of ^resolving' power (iv.) necessitates a very close approximation of the front lens to the. object; and whilst it is an absolute necessity that the interval should be sufficient for the interposition of a cover of the thin- nest glass, or (if this be inadmissible) of a film of mica, every addition to this interval is a clear gain, not only in convenience of working, but also in regard to the 'penetrating' power (iii.) of the Objective. — The increase of ^ working distance ' obtainable by the use of the Immersion system is by no means the least of its advantages. II. The ' Defining power ' of an Objective depends upon the com- pleteuess of its correctio?is for Spherical and Chromatic aberration (§§ 9- the rays, by the medium— whether Air, Water, Glycerine, Balsam, or Oil- through which they pass in their course from the object into the Objective. (See Appendix.) ^ Owing to the want of some common standard, Objectives constructed by different Makers of the same nominal focal length, often differ considerably from each other in magnifying power; and the proportional amplification given by the different Objectives of any one Maker's series is often very different from that indicated by their nomenclature. It is therefore greatly to be wished that some uniform standard could be agreed on; such as that of Magnifying power under an Eye-piece of definite focal length, at a fixed distance from the Objective. 11 162 THE MICROSCOPE AND ITS KEVELATIONS. 15), especially the former; and it is an attribute essential to the satisfac- tory performance of any Objective, whatever be its other qualities. Good definition may be more easily obtained with lenses of small or moderate, than with lenses of large angular aperture; and as it is impos- sible to construct ^dry^ Objectives of very wide angle, without some sacrifice of perfect correction (Abbe), there is a limit which, where ^definitions is of primary importance, cannot be advantageously passed. On the immersion system, however, and especially on the ' homogeneous immersion' system (§§ 19, 20), Objectives can be constructed of very much wider angle, without any injurious sacrifice of definition arising from inadequate correction. But here there comes in another source of impairment — the difference in the perspective views of every object not a 7nere mathematical point or line, which are received through the different parts of the area of the Objective. The picture given by the entire area is — so to speak — the ^general resultant' of the dissimilar pictures recived through these several parts;^ and as this dissimilarity obviously increases with the angle of operture of the Objective, its defining power mnst be proportionately impaired. This theoretical conclusion has been experi- mentally verified by Dr. Koyston Pigott; who has found that by com- paring Objectives of large with those of moderate apertures, on such objects as the cracks in Mr. Slack's silica-films (§ 152), or the aerial image formed by the Achromatic Condenser of a hair stretched before the light at some distance, the advantage was decidedly on the side of the latter. He has shown^ ^^that the black margins or black marginal annuli of refracting spherules constantly displayed by small aperture Objectives, are attenuated gradually to invisibility as the apertures are widened to the utmost; that the black margins of cylinders, tubules, or semi-tubules, also suffer similar obliterations; and that, in consequence, minute detcdls are concealed or destroyed till the aperture is sufficiently reduced.^^ — It is also the experience of Messrs. Dallinger and Drysdale, that for the definition of the immeasurably-minute reproductive granules of the Monadine forms whose life-history they have studied (§ 418), or of the flagella of Bacterium termo (§ 305), which may be characterized as the highest feats of Biological Microscopy yet performed, moderate angles of aperture are unquestionably uo be preferred (vi.) — An experienced Micro- scopist will judge of the defining power of an Objective by the quality of the image it gives of any fitting object with which he is familiar; no test being, in the Author's judgment, more suitable than the Po^Z^^ra-scale (§ 162). Any imperfection in Defining power is exaggerated, as already pointed out (§§ 26, 136), by Meep Eye-piecing;' so that, in determin- ing the value of an Objective, it is by no means sufficient to estimate its performance under a low Eye-piece — an image which appears tolerably ^ This point has been long kept before the mind of the Author, by his studies in Stereoscopic Microscopy ; the condition of the effect of relief in the Binocular image being the dissimilarity of the pictures of any object not absolutely flat, that are formed by the right and the left halves of the Objective respectively (§ 39). And he is glad to tind his view of its importance confirmed by so able a practical Optician as Mr. Zentmayer ; vrho. in a Lecture on the Elementary Prop- erties of Lenses, published in the Journal of the Franklin Institute " for May and June, 1876, and cited in the Monthly Microscop. Journ.," Vol. xvi. (1876), p. 317, called attention prominently to the confusion of images necessarily attendant upon large apertures, except when viewing absolutely flat objects, from the fact that the image formed by pencils transmitted by one side of the lens are unavoid- ably different from corresponding images formed by the opposite side of the lens. "2 Proceeding of Royal Society," June 19th, 1879, MANAGEMENT OF THE MICROSCOPE. 163 dear when moderately magnified, being often found exceedingly deficient in sharpness when more highly amplified. The use of the Draw-tube (§83) affords an additional means of testing the Defining power; but recourse cannot be fairly had to this, unless an alteration be made in the adjustment for the thickness of the glass that covers the object (§ 139), in proportion to the nearer approximation of the object to the Objective whicli the lengthening of the body involves. III. The Penetrating power or ' focal depth ' of an Object-glass may be defined as consisting in the vertical range through whicli the parts of an object not precisely in the focal plane may be seen with sufiicient distinctness to enable their relations with what does lie precisely in that plane to be clearly traced out; just as we could do by ordinary vision, if the object were itself enlarged to the size of its Microscopic image. — Now this is a quality which is very differently valued by different observers, according to the nature of the work on which they may be severally engaged. The Histologist who is scrutinizing the elementary compo- nents of a tissue that is spread out in the thinnest possible film between two plane surfaces of glass, considers ^penetration' rather an evidence of imperfection in his Objective, which (he affirms) cannot show him anything save what is exactly in the focal plane, without a sacrifice of its highest attainable capacity for doing the latter. On the other hand, the Anatomist who is studying the general organization of some minute Plant or Animal, or the structure of individual organs in a larger one, finds a certain amount of ' penetration ' essential to his recognition of the relations between the several parts of the object whicli are suc- cessively brought into distinct views by alterations of the focal adjust- ment. And the Physiologist who is watching the actions that are going on in a living Organism or in some component part of it (as, for exam- ple, the internal movements of an Amoeba, or the cyclosis in a leaf-cell of Vallisneria) could form no satisfactory conception of such phenomena, if, instead of passing gradationally (as an Objective of good ' penetration ' allows him to do) from one focal plane to another, he can only get a series of ' dissolving views ' with an interval of ' chaos ' between each, as he does when working with an Objective whose ' penetration' has been sacri- ficed to Angular aperture. — For the study of opaque objects which present such inequalities of surface as to render it impossible to appre- hend their true forms unless much more can be seen than is precisely in focus at once, good ' penetrating' power is obviously essential; and this is indispensable to the advantageous use of the Stereoscopic Binocular, which grossly exaggerates the effect of projection, when objects are viewed under Objectives of too wide an angle (§ 39). — No definite rule can be laid down as to the relation which the ' focal depth ' of an Objective bears to its Svorking distance' and its ^angular aperture;' because much depends upon the mode of their construction. But it may be stated generally that Objectives of longest working distance have the greatest ^penetration;' whilst the widening of the Angular aperture diminishes penetration at a rapidly increasing rate. * ^ The Author is informed by Prof. Abbe, that, theoretically— the plan of con- struction remaining the same — the ' penetration ' of an Objective decreases, as the square of the Angular aperture increases.— It is perfectly well-known to Photo- graphers, that a good picture of the interior of a long Sculpture-gallery, showing both the near and the distant parts with tolerable distinctness, can only be obtained by a lens of very narrow angle. — The singular assertion lately made by Dr. Blackham On Angular Aperture of Objectives," New York, 1880), that 164 THE MICROSCOPE AND ITS REVELATIONS. IV. The ^Resolving power ' by which very minute and closely approx- imated markings — whether lines, striae, dots, or apertures — are separ- ately discerned, has now been clearly shown to depend upon Angular aperture (§ 157); and this, not so much — as formerly supposed — on account of the greater obliquity of the rays Avhich large-angled Objectives will admit, as because of their capacity to receive and recombine the ' dif- fraction-spectra Hhat lie Avithout the range of Objectives of more lim- ited angle. In comparing the ^ resolving^ powers of different Objec- tives, it must be borne in mind that the advantages of wide aperture will be lost, if the obliquity of the illumination does not correspond with that of the most divergent rays which enter the Objective to take part in the formation of the image. But when the question is not of the reso- lution of surface-markings (such as those of Diatom-valves), but of the determination of internal structure (as, for example, in the study of the process of division in cell-nuclei), axial illumination is decidedly to be preferred, as being attended with less liability than oblique to produce deceptive api3earances. — It appears from the theoretical researches of Prof. Abbe, that the maximum attainable resolving power with an An- gular aperture of 180° should separate 118,000 lines to the inch; and this agrees well with what has been actually accomplished (§ 160). But the loss of ^ resolving^ power consequent upon the contraction of the aperture from 180° to 128^° is only 10 per cent.; while a further reduc- tion to 106 J° only lowers the number of separable lines to 94,400 per inch. V. The ^ Flatness of the field ^ afforded by the Object-glass is a con- dition of great importance to the advantageous use of the Microscope, since the real extent of the field of view practically depends upon it. Many Objectives are so constructed, that, even with a perfect flat ob- ject, the foci of the central and of the peripheral parts of the field are so different, that when the adjustment is made for one, the other is ex- tremely indistinct. Hence, when the central portion is being looked at, no more information is gained respecting the peripheral, than if it had been altogether stopped out. With a really good Object-glass, not only should the image be distinct even to the margin of the field, but the marginal portion should be as free from color as the central. In many microscopes of inferior construction, the imperfection of the Objectives in this respect is masked by the contraction of the aperture of the dia- phragm in the Eye-piece (§ 27), which limits the dimensions of the field; and the performance of one Objective within this limit may scarcely be distinguishable from that of another, although, if the two were com- pared under an Eye-piece of larger aperture, their difference of excellence would be at once made apparent by the perfect correctness of one to the margin of the field, and by the entire failure of the other in every part save its centre. In estimating the relative merits of two lenses, there- fore, as regards this condition, the comparison should be made under an Eye-piece giving a large field. VI. The most perfect objective for general purposes, is obviously that which combines all the preceding attributes in the degree in which they are mutually compatible. But it seems to be now clear that the highest perfection of the two primary qualities, ^defining' power and ^resolving * depth of focus ' has no relation to Aperture, but depends on residual " {i. 6., uncorrected) Spherical Aberration, and that " the less the lens has of it, the bet- ter the lens," does not require serious refutation. MANAGEMENT OF THE MICROSCOPE. 165 power/ cannot be obtained in the same combination; so that the choice between two Objectives, one distingnished by the former of these attri- butes, and the other by the latter, will depend upon the kind of work on which it is to be employed. If the resolutions of the markings on Dia- tom-valves is the Microscopist's special pursuit/ he will rightly prefer an Objective of the largest attainable angle, with the best definition that is compatible with it. But if he be engaged upon difficult Biological in- vestigations, he will do well to make perfect 'definition^ his stJie qua 11071, and to be content with the largest angle that can be obtained with- out a sacrifice of this. It is, as already stated, in admitting of perfect correction for Spherical Aberration, even to an aperture of 180°, that the great superiority of the ^immersion system ' consists; but the great- est perfection in the construction of even an immersion Objective, cannot (in the nature of things) prevent that impairment of defini- tion, which has been experimentally as well as theoretically shown by Dr. Eoyston Pigott to be consequent upon excessive widening of the angle of aperture. The most serviceable Objectives for the most dif- ficult Biological investigations, therefore, will (in the Author's judg- ment) be such as possess the combination of qualities attributed by Mr. Dallinger to the l-35th inch constructed specially for his work by Messrs. Powell and Lealand; ''the angle is moderate; its definition very crisp and clear; and its penetration, considering its magnifying power, very considerable.^^ 159. Test'Ohjects, — It is usual to judge of the optical perfection of a Microscope by its capacity for exhibiting certain objects, which are re- garded as Tests of the merits of its Object-glasses; these tests being of various degrees of difficulty, and that being accounted the best instru- ment which shows the most ' difficult' of such tests. Now it must be borne in mind that of the qualities which have been just enumerated, the ' tests ^ usually relied-on have reference almost exclusively to two — viz., definition and resolving potoer; and that the greater number of them, being objects whose surface is marked by lines, striae, or dots, are tests of resolving power, and thus of Angular aperture only. Hence, as already shown, an Objective may resolve some very difficult test-objects, and yet may be very unfit for ordinary use. Moreover, these 'difficult' tests are only suitable to Object-glasses of very short focus and high mag- nifying power; whereas the greater part of the real tvorh of the Micro- scope is done with Objectives of low and medium power; and the enlargement of the Angular aperture, which enables one of these to re- ^ It is assuredly neither the only nor yet the chief vs^ork of the Microscope (as some appear to suppose) to resolve the markings on the siliceous valves of Dia- toms ; in fact, the interest which attaches to observations of this class is entirely confined to the value of these objects as * tests ' of the performance of Objectives (§ 159). If one-tenth of the attention which has been devoted to the scrutiny of these objects with instruments of the highest class, had been given to the study of the Life-history of the minute Plants which furnish them, with such a Stu- dent's microscope as thirty years ago enabled Mr. Thwaites to discover their * conjugation,' it cannot be doubted that vast benefit would have accrued to Bio- logical Science. — It has been urged that the acquirement of the power of dis- playing ' difficult ' Diatom-tests, is a valuable ' gymnastic ' for the training of Mi- croscopists; but the experience of the Author, and of every Biological teacher he knows, is that a much better training for the Student is to begin with the study of such easy objects — e, g., the Yeast-Plant, and Colorless Blood-Corpuscles, — as afford him the experience which it is absolutely essential that he should acquire in the first instance, and to proceed gradually from these to the more difficulty gaining new knowledge at every stage. 166 THE MICROSCOPE AND ITS REVELATIONS. solve (under deep Eye-pieces) many objects which were formerly consid- ered adequate tests for higher powers, is for ordinary purposes rather injurious than beneficial, detracting from the value of the Objective for the work to which it is specially adapted. For Microscopists of large .Biological experience know perfectly well that every ^ power ^ has its own proper range and capacity; and that they work most satisfactorily with the ^powcr ^ most suitable to the investigation on which they may be en- gaged. In estimating the vahie of an Object-glass^ it should always be considered for tohat purpose it is intended; and its merits should be judged-of according to the degree in which it fulfils that purpose. We shall therefore consider what are the objects proper to the several ^ powers ' of Object-glasses — loio, medium, and high; and what are the objects by its mode of exhibiting which, each may be fairly judged. I. By Objectives of loiv power we may understand any whose focal length is greater than Half -an inch. The ^ powers^ usually made in this country are known as 4 inch, 3 inch, 2 inch, \\ inch, 1 inch, and 2 3ds inch focus; and they give a range of amplification of from 10 to 70 dia- meters with the A eye-piece, and of from 16 to 120 diameters with the B eye-piece. An ^ adjustable^ low power is made by Zeiss of Jena (ob- tainable from Messrs. Baker), in which, by varying the position of the front-lens by means of a screw-collar, a range of power is obtainable from about 8 to 16 diameters with the A eye-piece, and from 12 to 24 with the B eye-piece. This has been found by the Author extremely convenient for the display of large opaque objects, of which it is desired to show the whole under as high an amplification as will make their images fill the field. Objectives of Ioid power are most used in the examination of opaque objects, and of Transparent objects of large size and of compara- tively coarse texture; and the qualities most desirable in them are a sufii- ciently large aperture to give a bright image, combined with such accu- rate definition as to give a clear image, with ^ focal depth ^ suflicient to prevent any moderate inequalities of surface from seriously interfering with the distinctness of the entire picture, and with perfect ^ flatness ' of the image when the object itself is flat. For the 3 inch, 2 inch, or 1^ inch Objectives,^ no ground of judgment is better than the manner in which it shows such an injected preparation as the interior of a Frog's Lung (Fig. 485) or a portion of the villous coat of the Monkey's Intes- tine (Fig. 479); for the aperture ought to be sufficient to give a bright image of such objects by ordinary daylight, without the use of any illu- minator; the border of every vessel should be clearly defined, without any thickness or blackness of edge; every part of such an object that comes within the field should be capable of being made-out when the focal ad- justment is adapted for any other part; whilst, by making that adjust- ment a medium one, the whole should be seen without any marked in- distinctness. If the Aperture be too small, the image will be dark: but if it be too large, details are brought into view (such as the separateness of the particles of the vermilion injection) which it is of no advantage to see; whilst, through the sacrifice of penetration, those parts of the object which are brought exactly into focus being seen with over-minuteness, the remainder are envelo])ed in a thick fog through which even their gen- eral contour can scarcely be seen to loom. If the corrections be imper- ^ These are ordinarily composed of two pairs of lenses only, as the corrections can be ad«lq[uately made by this combination for an Angular aperture of 33°, which is the largest that is found practically useful for the 1^ inch. MANAGEMENT OF THE MICROSCOPE. 167 fectly made, no line or edge will be seen with perfect sharpness. For Defining power, the Author has found the Pollen-grains of the Holly- hock or any other flower of the Malloiu kind (Fig. 277, a) viewed as an opaque object, a very good test; the minute spines with which they arc beset being but dimly seen with any save a good Object-glass of these long foci, and being really-well exhibited only by adding such power to the Eye-piece, as will exaggerate any want of definition on the part of an inferior lens. For Flatness of field no test is better than a section of Wood (Fig. 253) or a large Echinus spine (Fig. 369), under an Eye-piece that will give a field of the diameter of from 9 to 12 inches. The general performance of Object-glasses of 1-inch and 2-3ds inch focus, may be partly judged-of by the manner in which they show such injections as those of the Gill of the Eel (Fig. 484), or of the Bird's Lung (Fig. 486), which require a higher magnifying power for their resolution than those previously named; still better, perhaps, by the mode in which they ex- hibit a portion of the wing of some Lepidopterous Insect having well marked scales. The same qualities should here be looked-for, as in the case of the lowest powers; and a want of either of them is to be distin- guished in a similar manner. The increaso of Angular aperture which these Objectives may advantageously receive up to 30% should render them capable of resolving all the easier ^test' scales of Lepidoptera, such as those of the Morplio menelaits (Fig. 414), in which, with the B eye- piece, they should show the transverse as well as the longitudinal mark- ings. The Proboscis of the Blow-fly (Fig. 428)' is one of the best trans- parent objects for enabling a practised eye to estimate the general performance of Object-glasses of these powers; since it is only under a really good lens, that all the details of its structure can be well shown. In particular, all the outlines and edges should be seen clearly and sharply, without any haze or fringe; the tracheal spires and rings should be well-defined, without any color between them; and there should be no indication of general mist. An Objective which shows this well, may be trusted for any other object of its kind. For Flatness of field, sections of small Echinus-spines (Plate IT., fig. 1) are very good tests. The exact- ness of the corrections in lenses of these foci may be judged-of by the ex- amination of objects which are almost sure to exhibit Color if the correc- tion be otherwise than perfect. This is the case, for example, with the so-called glandulce of Coniferous wood (Fig. 248), the centres of which ought to be clearly defined under such objectives, and to be quite free from color; and also with the tracliece of Insects (Fig. 432), the spires of which ought to be distinctly separated from each other, without any ap- pearance of intervening chromatic fringes. II. We may consider as Objectives of medium power the Half-inch, 4-lOths inch, l-4th inch, and l-5th inch; the magnifying power of which ranges from about 90 to 250 diameters under the A eye-piece, and from about 150 to 400 diameters with the B eye-piece. The first three, when used by reflected light, can be advantageously employed in the examina- tion of such small opaque objects as Diatoms, Polycystina, portions of small Feathers, capsules of the lesser Mosses, Hairs, etc. ; they should be so mounted on cones as to allow of side illumination; and the l-4th should have suflBcient working distance to permit its easy use for these ^ This object should be mounted in Glycerine- jelly; for when mounted in Bal- sam, the parts are usually flattened-out and squeezed together, so that their real forms and relative positions cannot be seen. 168 THE MICROSCOPE AND ITS REVELATIONS. purposes, with an aperture not exceeding 80°. Larger-angled l-4ths and l-5ths cannot be conveniently used for opaque objects, unless these are shown by Prof. Smith's or some analogous illumination (§ 116). — The great value of these powers lies in the information they enable us to obtain regarding the details of organized structures and of living actions, by the examination of properly-prepared transparent objects by trans- mitted light; and it is to them that the remarks already made respecting Angular aperture (§ 158, ii.) especially apply; since it is here that the greatest difference exists between the ordinary requirements of the Scientific investigator, and the special needs of those who devote them- selves to the particular classes of objects for which the greatest ^resolv- ing ' power is required. A moderate amount of such power is essential to the value of every Objective within the above-named range of foci: thus, even a good half-inch should enable the markings of the larger scales of the Polyommatus argiis (^azure-blue Butterfly') to be well dis- tinguished — these being of the same kind with those of the Menelaus, but more delicate — and should clearly separate the dots of the small or ^battledoor' scales (Fig. 41G) of the same Insect, which, if unresolved, are seen as coarse longitudinal lines; a good 4:-10ths inch should resolve the larger scales of the Podura (Plate II., fig. 2) without difficulty; and a good l-4th or l-5th-inch should bring out the markings on the smaller scales of the Podura, and should resolve the markings on the Pleuro- sifjma angulatum into lozenge-lines, the B and 0 eye-pieces being used when the scales are very small and their markings delicate. Even the half-inch or the 4-lOths inch may be made with angles of aperture suffi- ciently wide to resolve the objects named as difficult tests for the powers above them ;^ but for the reasons already stated, the Author thinks it most undesirable that they should be thus forced up to the work altogether unsuited to their powers, by a sacrifice of those very qualities which constitute their special value in the study of the objects whereon they can be most appropriately and effectively employed. And he is decidedly of opinion that an angular aperture of 50° is as great as should be given to a IIalf-?nch, 60° to a 4:-10ths inch, and 90° to a l-4th inch, that are destined for the ordinary purposes of Scientific investigation: whilst his own experience would lead him to prefer an angle of 40° for the Half-inch (§ 39), and of 80° for the l-4th inch, provided the correc- tions are perfect. Objectives of these apertures should show the easier tests first enumerated, with perfect Definition, a fair amount of Pene- trating power, and complete Flatness of field. No single object is so useful as the Podura-scale for the purpose of testing these qualities in a l-4th inch or l-5th inch Objective; and it may be safely said that a lens ^ By Mr. Tolles (Boston, N.E.) the Angle of the half-inch is carried to 80''; and that of the 4-lOths to 145°. And it has lately been seriously maintained that an Objective of the latter focus supplies almost every need of the Biologist, since, as even diflScult Diatom-tests can be shown by it, it can be worked up by deep Eye- piecing to the highest power that he requires, except for special investigation f. But the resolution of a Diatom is one thing, while the prosecution of investiga- tion continued through several hours at a time is quite another; and the Author, regarding the advice of this writer as most dangerous to the eyes of those who may follow it, deems it his duty to enter his protest against it. — Many excellent makers now make first-class Objectives of narrow as well as wide angles; thus, Messrs. Powell and Lealand, followed by several others, make the half-inch of 40° (first constructed for the Author, to be used with the Stereoscopic Binocular), as well as a half -inch of 70°; Messrs. Beck make a 4-lOths of 55°, as well as one of 90° ; and Mr. Crouch a l-4th of 60°, another of 105^, and another of 140°. I MANAGEMENT OF THE MICROSCOPE. 169 which brings out its markings satisfactorily will suit the requirements of the ordinary working Microscopist, although it may not resolve difficult Diatoms. In every case, the Objective should be tried with the B and C as well as with the A eye-piece; and the effect of this substitution will be a fair test of its merits. Where markings are undistinguishable under a certain Objective, merely because of their minuteness or their too close approximation, they m.ay be enlarged or separated by a deeper Eye-piece, provided that the Objective be well corrected. But if, in such a case, the image be darkened or blurred, so as to be rather deteriorated than improved, it may be concluded that the Objective is of inferior quality, having either an insufficient Angular aperture, or being imperfectly corrected, or both. HI. All Object-glasses of less than l-5th inch focus may be classed as liigh powers; the focal lengths to which they are ordinarily constructed being l-6th, l-8tli, 1-lOth, l-12th, l-16th, l-20th, l-25th, l-40th, and l-50th of an inch respectively; the l-12th, l-16th, ]-25th, and l-50th being made by Messrs. Powell and Lealand, and the 1-lOth, l-20th, and l-40th by Messrs. Beck. The magnifying powers which Objectives from l-6th to l-25th inch focus are fitted to afford, range from about 320 to 1250 diameters with the shallower Eye-piece, and from 480 to 1850 dia- meters with the deeper: but by the use of still deeper Eye-pieces, or by the Objective of l-50th inch, or the l-80th recently constructed by Messrs. Powell and Lealand, a power of 4000 or more may be obtained. It is seldom, howeyer, that anything is really gained thereby. — The in- troduction of ifmnersion-lenscs (§ 19) has considerably increased the utility of what may be called moderately high powers, such as l-8th, 1-lOth, and l-12th. These, if really good, can be used when necessary with deep Eye-pieces; and yery little of scientific importance that is be- yond their reach has yet been seen by higher Objectives, though the lat- ter have, no doubt, special value in certain circumstances when skilfully employed. With these and higher powers not intended for exclusive use upon ^yexatious' Diatoms, the Angle of aperture should be so pro- portioned to focal length, as not to sacrifice the ' definition ' and ' penetra- tion ^ required to show the internal organs of small Eotifers, large Infu- soria, minute Worms, etc. An Objective that will show surfaces only, may be broadly stated to be of little use for Biological investigation. Dry-front l-8ths or l-12ths with an aperture closely approaching 170°, are of very limited utility, from want of penetration, and from focussing extremely close to their objects; while with 30° to 40^ less aperture and good corrections, they are much more serviceable, losing yery little (as already shown, § 158, iv.) in ^ resolving^ power, and gaining much in working distance and penetration. 160. For Eesolving power, the best tests are afforded by the lines ar- tificially ruled by M. Nobert, and by the more ^ difficult^ Diatoms. — What is known as Noierfs Test is a plate of glass, on a small space of which, not exceeding one-fiftieth of an inch in breadth, are ruled from ten to nineteen series of lines, forming as many separate bands of equal breadth. In each of these bands, the lines are ruled at a certain known distance; and the distances are so adjusted in the successive bands, as to form a regular diminishing series, and thus to present a succession of tests of progressively increasing difficulty. The distances of the lines differ on different plates; all the bands in some series being resolvable under a good Objective of l-4th inch focus, whilst the closest bands in others long defied the resolving power of l-12th inch Objectives of large 170 THE MICROSCOPE AND ITS REVELATIONS. Aperture. On the ninefceen-band Test-plate the lines are ruled at the following distances, expressed in parts of a Paris line, which, to an Eng- lish inch, is usually reckoned as .088 to 1.000, or as 11 to 125: — Band 1. 1-lOOOth. Band 8. l-4500th. Band 14. l-7500th. 2. l-1500th. " 9. l-5000th. " 15. l-SOOOth. 3. l-2000th. 10. l-5500th. 16. l-8500th. " 4. l-2500th. 11. l-6000th. " 17. l-OOOOth. > " 5. l-3000th. 12. l-6500th. 18. l-9500th. 6. l-3500th. " 13. l-7000th. 19. MOOOOth. 7. l-4000th. The following exact estimates of the numbers of the lines to the Eng- lish inch, in some of the Bands, are given by Dr. Koyston Pigott:^ — Band. Band, of spaces ^^^^ No, of spaces per inch. per inch. per inch, I. 11,259.51358. IX. 56,297.56790. XV. 90,076.10864. III. 22,519.02716. XL 67,557.08148. XVII. 101,335.62222. IV. 33,778.54074. XIII. 78,816.59506. XIX. 112,595.13580. VII. 45,038.05432. In objects like Nobert's Test-plate, spurious diffraction lines are easily mistaken for genuine resolution; and the difficulty of resolving the higher bands of his series was formerly supposed to be an optical impossibility. The more recent investigations of Helmholtz and Abbe, however, have disposed of this theoretical objection; and the ^resolution ^ of Robert's 19th band, which was long supposed to be a sort of crux of Microscopy, is now easily demonstrable. 161. It cannot be questioned that the recognition of the value of the markings on the siliceous valves of the Diatoms as Test-objects (first made by Messrs. Harrison and SoUitt, of Hull, in 1841) has largely con- tributed to the success of the endeavors which have since been so effectu- ally made, to perfect high-power Objectives, and to devise new methods of using them to the best advantage. But it has now been demonstrated, both theoretically and practically, that the power of ^resolving' these markings essentially depends on the Angular aperture of the Objective: so that, as a lens which possesses it in a high degree may be very defi- cient in ^definition,' and will probably have an inconveniently short ^ working distance' with very little ^penetration' — qualities essential to an Objective to be employed in Biological investigation, — the resolution of difficult Diatom-tests by no means proves the fitness of an Objective for the ordinary work of the Microscopist. — Still, these tests are of great value for the purpose to which they are really adapted; and it Avill there- fore be desirable here to specify their relative degrees of ^difficulty,' which is indicated by the closeness of their lineation, leaving for future discussion (§ 277) the nature of the structure to which that lineation is due. The greater part of the Piatoms now in use for this purpose, are comprehended in the genus Pleurosigma of Prof. W. Smith; which includes those Naviculm whose ^frustules' are distinguished by their sigmoid (S-like) curvature (Pig. 165). ^ '* Monthly Microscopical Journal," Vol. ix. (1878), p. (53.— A much larger number of lines to the inch has been assigned to Nobert's Test-plate by Mr. J. Allan Broun ("Proceedings of Royal Society," Vol. xxiii., 1875, p. 531), on the basis of his measurement of Photographs taken by Dr. E. Carter (Surgeon U. S. Army); but there seems strong ground to believe that either from diffraction, or from some mistake in the magnifying power employed, Mr. Broun's estimate must be greatly in excess of the reality. MANAGEMENT OF THE MICEOSCOPE. 171 Direction Strice in 1-I00f7i of an inch. OiUl 1 JEl« SOLLITT. 1. Pleurosigma formosum ... uidguiiai • . — /iy) o /Q, strigile • • • tx dllO V Cl otJ . . 36 OA Q Balticum . . . transverse . . ACi OA A 4, attenua.tum 40 A(\ OK OO Ft o. hippocampus . • . . tldllO VtJJ OtJ . , 40 A w . . . 40 A A — 40 6. strigosum ... diagonal . . AA. QCi A A — 40 7. quadratum ... LUdgUIldi . . — OO 8. elongatum H 1 Q rr/^n q 1 . . 1 Llict^vJlicli . . AH 9. lacustre . . . transverse 48 10. angulatum . . . diagonal ... 52 ...51 — 46 11. aestuarii . . . diagonal ... 54 12. fasciola ... 64 ... , . 90 — 50 13. Navicula rhomboides ... 85 . . , , . Ill — 60 14. Nitzschia sigmoidea . . . transverse . . .,. 85 15. Amphipleura pellucida. ...130 —120 (Navicula acus). Good specimens of the first ten of the foregoing list may be resolved, with Judicious management, by good small-angled l-4th or l-5th inch Objectives, and even, with very oblique illumination, by Objectives of one-half and 4-10 ths inch, having an angular aperture of 90°; the re- mainder require the larger aperture proper to the l-8th inch or higher power, for the satisfactory exhibition of their markings. The first column of measurements in the above table gives the numbers stated by Prof. W. Smith as averages; the second column gives the numbers subsequently assigned as the extreines by Mr. Sollitt,^ who pointed out that great dif- ferences exist in the fineness of the markings of specimens of the same species obtained from different localities — a statement now so abundantly confirmed, as to be entitled to rank as an established fact. It is in regard to Amphipleura pellucida^ however, that the greatest diversity of opinion has existed; and the conclusion which the Author had expressed in the earlier editions of this Manual, that Mr. Sollitt's estimate was much too high (having been based on ^spurious' lineation), has been fully con- firmed by Col. Dr. Woodward; who, having succeeded in obtaining very perfect Photographs of this Diatom, under powers of 1500 and 16*50 diam- eters, has found that the stria3 on the largest valves w^ere never more than 91 in 1-lOOOth of an inch, while those on the smallest never exceeded 100 in the lOOOth inch.^ The ^resolution' of the lines on this test may be made without much difficulty by ^immersion' Objectives of l-8th inch without any excessive Aperture; but the resolution of the lines into dis- tinct dots is a severe test for Objectives of largest Aperture. — Several very difficult tests of this description have been furnished by the late Prof. Bailey^ of West Point (U.S.); among them the very beautiful Gram- matophora subtilissimu and the Hyalodiscus suMilis, the latter being of discoid form, and having markings which radiate in all directions, very much like those of an engine-turned watch. — To these may be added the Surirella gemr)ia, which presents appearances of a very deceptive character. These appearances, as represented by M. Hartnack, are shown in Fig. 118, A, b; the upper part of the valve A being illuminated by oblique light in the direction of its axis, and the lower part by oblique light in a * * On the Measurement of the Striae of Diatoms,' in Quart. Journ. of Microsc. Science," Vol. viii. (1860), p. 48. 2 Monthly Microsc. Journ.," Vol. v. (1871), p. 163. 2 See his interesting Memoirs in Vols. ii. and vii. of the Smithsonian Contri- butions to Knowledge." On Hyalodiscus subtilis, see Hendry, in Quart. Journ. of Microsc. Science," Vol. i., N.S. (1861), p. 179. 172 THE MICROSCOPE AKD ITS REVELATIONS. direction transverse to its axis; while b shows a portion more highly magnified under the last illumination. This Diatom, however, has been successfully photographed by Dr. Woodward (Fig. 118, c), who says of it: A careful examination of specimens mounted dry, has satisfied me that Hartnack's interpretation is erroneous. The fine striae are, I think, rows of minute hemispherical beads; the appearance of hexagons is tho optical result of imperfect definition or of unsuitable illumination. For photographing this object, I have selected a frustule of somewhat less than the medium size. It measures 1 290th of an inch in length. Longitudinally the fine striae count at the rate of 72,000 to the inch. These striae are resolved into beaded appearances, which count laterally 84,000 to the inch."' 162. As a test for those qualities of Objectives which best fit them for the general purposes of Biological investigation, the Author remains of the opinion (which he finds to be shared by many able and experienced Valve of Surirella gemma, with portion (b) more highly magnified, showing: two systc-ms ot markings a and 6, as represented by Hartnack; while c is copied from a photograph taken oy Dr. Woodward. Microscopists, and by Makers specially familiar with their requirements) that nothing is better than the scale of the Lepidocyrtus cervicolUs, commonly known as the Podura (Fig. 419). It is a fact perfectly familiar to such Makers, that an Objective may serve, in virtue of its wide Angular aperture, to resolve Diatom- tests of considerable difficulty, and may yet fail utterly on the Podura-scale, in consequence of its inferior defining power; and such an Objective can be of very little ser- vice to the Biological investigator. On the other hand, although the exact structure of the Podura-scale is still (like that of the Diatom-valve) a matter of discussion, yet all are agreed as to the appearances it presents under Objectives that combine in the fullest degree the attributes already specified as best qualifying them for Scientific work; so that any glass which shows these appearances satisfactorily, may be safely accounted suitable for that purpose. The surface of this scale, when viewed under 1 Monthly Microsc. Journ.," Vol. vi. (Ib71), p. 100. - MANAGEMENT OF THE MICROSCOPE. 173 a sufficiently high amplification, is seen to be covered with the peculiar markings shown in Plate ii., Figs. 2, 3, which are sometimes designated ^ spines,"^ but are more commonly known as ^ notes of admiration ' or 'ex- clamation-markings.' These should be clearly separated from each other, and their margins well defined. An Objective of small angle (such as a l-4th inch of 60°) will show the ^spines' dark throughout; a l-4th inch of 100 will show a light streak extending from the large end, down the centre of each marking; and a further enlargement of the aperture will show an extension of this streak through the entire length of each ' spine.' The degree in which these markings retain their brightness and distinctness under deep Eye-piecing, may be considered a most valuable test of the excellence of the defining power of the Objective. As it is impossible that large-angled Objectives used *dry,' should be perfectly corrected for spherical aberration (so as to possess the greatest possible defiyiing power) without some residuum of chroinatic aberration, all the best defining glasses will show the thick part of the spines tinged with either blue or red. Perfect Achromatism, on the other hand, is only attainable with 'dry' lenses at some sacrifice of resolving and defining power; and many Microscopists prefer to keep the latter to their highest point, even at the expense of complete color-correction. Most Physiolo- gists, hower, will prefer the highest attainable achromatism, at some sac- rifice of aperture. But it ^is one of the advantages of the * immersion- system,' that the residual aberrations of even large-angled Objectives can be much more perfectly compensated than they can be in 'dry' Objec- tives; so that on this as on several other accounts, their use is to be re- commended whenever permitted by the nature of the research. 163. Determination of Magnifyijig Power. — The last subject to be here adverted to is the mode of estimating the magnifying power of Microscopes, or, in other words, the number of times that any object is magnified. This will of course depend upon a comparison of the real size of the Object with the apparent size of the Image; but our estimate of the latter will depend upon the distance at which we assume it to be seen; since, if it be projected at different distances from the Eye, it will present very different dimensions. Opticians generally, however, have agreed to consider ten inches as the standard of comparison; and when, therefore, an object is said to be magnified 100 diameters, it is meant that its visual image projected at ten inches from the Eye (as when thrown down by the Camera Lucida, § 94, upon a surface at that distance beneath), has 100 times the actual dimensions of the object. The mea- surement of the magnifying power of Simple or Compound Microscopes by this standard is attended with no difficulty. All that is required is a Stage-Micrometer accurately divided to a small fraction of an inch (the 1-lOOth will answer very well for low powers, the 1-lOOOth for high), and a common foot-rule divided to tenths of an inch. The Micrometer being adjusted to the focus of the Objective, the rule is held parallel with it at the distance of ten inches from the eye. If the second eye be then opened whilst the other is looking through the Microscope, the circle of light included within the field of view crossed by the lines of the Micro- meter will be seen faintly projected upon the rule; and it will be very Basy to mark upon the latter the apparent distances of the divisions on the Micrometer, and thence to ascertain the magnifying power. Thus, supposing each of the divisions of 1-lOOth of an inch to correspond with 1| inch upon the rule, the linear magnifying power is 150 dianieters: if it corresponcj with half an inch, the magnifying power is 50 diameters. 174: THE MICROSCOPE AND ITS REVELATIONS. If, again, each of the divisions of the 1-lOOOth inch Micrometer corre- sponds to 0.6 of an inch upon the rule, the magnifying power is 600 dia- meters; and if it corresponds to 1.2 inches, the magnifying power is 1200 diameters. In this mode of measurement, the estimate ot parts of tenths on the rule can only be made by guess; but greater accuracy may be obtained by the use of the Diagonal scale (Fig. 67), or still better, by projecting the Micrometer-scale with the Camera Lucida at the distance of ten inches from the eye, marking the intervals on paper, taking an average of these, and repeating this with the compasses ten times along the inch-scale. Thus, if the space given by one of the divisions of the 1-lOOOth-inch Micrometer, repeated ten times along the rule, amounts to 6 inches and 2^ tenths, the value of each division will be .625 of an inch, and the magnifying power 625. — It is very important, whenever a high degree of accuracy is aimed at in Micrometry, to bear in mind the caution already given (§ 91) in regard to the difference in magnifying power produced in the adjustment of the Objective to the thickness of the glass that covers the object. — The superficial Magnifying power is of course estimated by squaring the linear; but this is a mode of statement never adopted by Scientifio observers. • PREPAEiiTIOJSr, MOUJSTING, AND COLLECTION OP OBJECTS. 175 CHAPTER V. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 164. Under this head it is intended to give an account of those Materials, Instrnments, and Appliances of various kinds, which have been found most serviceable to Microscopists engaged in general Biological re- search, and to describe the most approved methods of employing them in the preparation and mounting of Objects, for the display of the minute structures thus brought to our knowledge. Not only is it of the greatest advantage that the discoveries made by Microscopic research should — as far as possible — be embodied (so to speak) in 'preparations,' which shall enable them to be studied by every one who may desire to do so; but it is now universally admitted that such ^preparations' often show so much more than can be seen in the fresh organism, that no ex- amination of it can be considered as complete, in which the methods most suitable to each particular case have not been put in practice. — It must be obvious that in a comprehensive Treatise like the present, such di, gen- eral treatment of this subject is all that can be attempted, excepting in a few instances of peculiar interest. And as the Histological student can find all the guidance he needs in the numerous Manuals now prepared for his instruction, the Author will not feel it requisite to furnish him with the special directions that are readily accessible to him elsewhere. Section 1. — Materials, Instruments^ and Appliances. 165. Glass Slides. — The kind of Glass best suited for mounting ob- jects, is that which is known as ^patent plate;' and it is now almost invariably cut, by the common consent of Microscopists in this country, into slips measuring 3 in. by 1 inch. For objects too large to be mounted on these, the size of 3 in. by 1| in. may be adopted. Such slips may be purchased, accurately cut to size, and ground at the edges, for so little more than the cost of the glass, that few persons to whom time is an ob- ject, would trouble themselves to prepare them; it being only when glass slides of some unusual dimensions are required, or when it is desired to construct 'built-up cells' (§ 174), that a facility in cutting glass with a glazier's diamond becomes useful. The glass slides prepared for use should be free from veins, air-bubbles, or other flaws, at least in the cen- tral part on which the object is placed; and any whose defects render them unsuitable for ordinary purposes, should be selected and laid aside for uses to which the working Microscopist will find no difficulty in putting them. As the slips vary considerably in thickness, it will be advantage- ous to separate the tliin and the thick from those of medium substance. The first may be employed for mounting delicate objects to be viewed 176 THE MICROSCOPE AND ITS REVELATIONS. by the high powers with which the Achromatic Condenser is to be used, so as to avoid any unnecessary deflection of the illuminating pencil by the thickness of the plate which it has to traverse beneath the object; the second should be set aside for the attachment of objects which are to be ground-down, and for which, therefore, a stronger mounting than usual is desirable; and the third are to be used for mounting ordinary objects. Great care should be taken in washing the slides, and in removing from them every trace of greasiness by the use of a little soda or potass solu- tion. If this should not suffice, they may be immersed in the solution recommended by Dr. Seller, composed of 2 oz. of Bichromate of Potass, 3 fl. oz. of Sulphuric Acid, and 25 oz. of Water, and afterwards thor- oughly rinsed. (The same solution may be advantageously used for cleansing Cover-glasses, § 132.) Before they are put away, the slides should be wiped perfectly dry, first with an ordinary ' glass-cloth,^ and afterwards with an old cambric handkerchief. And before being used, each slide should be again carefully wiped, so as to remove all adherent dust. Where slides that have been already employed for mounting preparations are again brought into use, great care should be taken in completely removing all trace of adherent varnish or cement; first by scraping (care being taken not to scratch the glass), then by using an appropriate solvent, and then by rubbing the slide with a mixture of equal parts of alcohol, benzole, and liquor sodae, finishing with clean water. 166. Thin Glass, — The older Microscopists were obliged to employ thin laminae of talc for covering objects to be viewed with lenses of short focus: but this material, which was in many respects objectionable, is now only employed for Objectives of exceptionally short focus (such as l-50th or 1-75 th inch), being entirely superseded for other purposes by the thin glass manufactured by Messrs. Chance of Birmingham, which may be obtained of various degrees of thickness, down to l-500th of an inch. This glass, being un-annealed, is very hard and brittle; and much care and some dexterity are required in cutting it. This should be done with the tvriting diamond; and it is advantageous to lay the thin glass upon a piece of wetted plate-glass, as its tendency to crack and ' star ^ is thereby di- minished. For cutting square or other rectangular covers, nothing but a flat rule is required. The cutting of rounds by unaccustomed hands is usually attended with so much breakage, that it is really a saving of money as well as of time to purchase them from the dealers; who usually keep them in several sizes, and supply any others to order. The differ- ent thicknesses are usually ranked as 1, 2, and 3; the first being used for covering objects to be viewed with low powers, the second for objects to be viewed with medium powers; and the third for objects requiring high powers. The thinnest glass is of course most difficult to handle safely, and is most liable to fracture from accidents of various kinds; and hence it should only be employed for the purpose for which it is absolutely needed. The thickest pieces, again, may be most advantageously em- ployed as covers for large Cells, in which objects are mounted in fluid (§§ 171-174) to be viewed by the low powers whose performance is not sensibly affected by the aberration thus produced. The working Micro- scopist will find it desirable to provide himself with some means of mea- suring the thickness of his cover-glass; and this is especially needed if he is in the habit of employing Objectives without adjustment, which are corrected to a particular standard (§ 17). A small screw-gauge of steel, made for measuring the thickness of rolled plates of brass, and sold at PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 177 the Tool-shops, answers this purpose yery well; but Boss's Lever of Con- tact (Fig. 119), devised for this express purpose, is in many respects pre- ferable. This consists of a small horizontal table of brass, mounted upon a stand, and having at one end an arc graduated into 20 divisions, each of which represents Ross's Lever of Contact. 1-lOOOth of an inch, ' ^^ic;.li9. so that the entire arc measures l-50th of an inch; at the other end is a pivot on which moves a long and deli- cate lever of steel, whose extremity points to the graduated arc, whilst it has very near its pivot a sort of projecting tooth, which bears at* against a ver- tical plate of steel that is screwed to the horizontal table. The piece of thin-glass to be measured being inserted between the vertical plate and the projecting tooth of the lever, its thickness in thousandths of an inch is given by the number on the graduated arc to which the ex- tremity of the lever points. Thus, if the number be 8, the thickness of the glass is .008 or l-125tli of an inch.^ — It will be found convenient to sort the covers according to their thicknesses, and to keep the sortings apart, so that each may be used for the powers to which it is the most suitable. For Objectives whose angle of aperture is between 40° and 75°, glass of .008 is not too thick; for Objectives of between 75° and 120° of aperture, the thickness may range from .006 to .004; but for Objectives whose angle of aperture exceeds 120% and whose focus is less than 1-lOth of an inch, only covers of from .004 to .002 should be used. 167. On account of the extreme brittleness of the Thin-glass, it is desirable to keep the covers, when cut and sorted, in some fine and soft powder, such as Starch. Before using a cover, however, the Microscopist should be careful to clean it thoroughly; not merely for the sake of removing foulness which would interfere with the view of the object, but also for the sake of getting rid of adherent starch -grains, the presence of which might lead to wrong conclusions; and also to free the surface from that slight greasiness, which, by preventing it from being readily wetted by water, frequently occasions great inconvenience in the mounting of objects in fluid. The thicker pieces may be washed and wiped without much danger of fracture, if due care be employed; but the thinner require much precaution; and in cleansing these, a simple instrument devised by Mr. W. W. Jones will be found very useful. This consists of a small tube of brass about an inch in diameter and the same in height (a stout pill-box makes a good substitute), into which fits loosely a weighted-plug, to the flat bottom of which is cemented a piece of chamois leather. Another piece of soft leather is stretched upon a flat tablet of wood or plate-glass; and by placing the cover-glass (damped by the breath) under the plug; within the end of the tube, and keeping the tube well-down on the tablet, the glass can be rubbed between the two leather surfaces with perfect security, the weight of the plug affording sufficient pressure.' 1 Another form of gauge, in which the measurement is obtained with great precision and facility by the sliding of a wedge, is described in the Journ. of the Roy. Microsc. Soc," Vol. ii. (1879), p. 65. 1 In the improved form of this little instrument made by Messrs. Hunter & 12 178 THE MICROSCOPE AND ITS REVELATIONS. 168. Varnishes and Cements. — There are three very distinct purposes for which Cements that possess the power of holding firmly to Glass, and of resisting not merely water but other preservative liquids, are required by the Microscopist; these being (1) the attachment of the glass covers to the slides or cells containing the object, (2) the formation of thin ^ cells,' of cement only, and (3) the attachment of the ' glass-plate ' or ^ tube-cells ' to the slides. The two former of these purposes are answered by liquid cements or varnishes, which may be applied without heat; the last re- quires a solid cement of greater tenacity, which can only be used in the melted state. — Among the many such Cements that have been recom- mended by different workers, the following may be specially named as having stood the test of a large experience, both as to general utility and permanent value: — a. Japanners* Gold size — This, which may be obtained at every Color-shop, is (according to the Author's experience) the most trustworthy of aU cements for closing-in mounted objects of almost any description. It takes a peculiarly firm hold of glass; and when dry it becomes extremely tough, without brittleness. When new, it is very liquid and * runs' rather too freely; so that it is often advan- tageous to leave open for a time the bottle containing it, until the varnish is somewhat thickened. By keeping it still longer with occasional exposure to air, it is rendered much more viscid: and though such * old ' Gold-size is not fit for ordinary use, yet one or two coats of it may be advantageously laid over tlie films of newer varnish, for securing the thicker covers of large cells (§§ 171 — 4). When- ever any other varnish or cement is used, either in making a cell or in closing it in, the rings of these should be covered with one or two layers of Gold-size ex- tending beyond it on either side, so as to form a continuous film extendmg from the marginal ring of the cover to the adjacent portion of the glass slide.' 5. Asphalts Varnish.— This is a black varnish made by dissolving half a drachm Caoutchouc in mineral naphtha, and then adding 4 oz. of Asphaltum, using heat if necessary for its solution. It is very important that the Asphaltum should be genuine, and the other materials of the best quality. Some use Asphalte as a substitute for gold-size; but the Author's experience leads him to recommend that it should only be employed either for making shallow * cement-cells ' 170), CT for finishing-off preparations already secured with gold-size. For the former purpose it may advantageously be slightly thickened by evaporation. c. Black Japan. — The varnish sold at the Color-shops under this name, may be used for the same purposes as the preceding. When it is used for making * cement-cells,' the slides to which it has been applied should be exposed for a time to the heat of an oven, not raised so high as to cause it to blister; this wiil increase its adhesion to the glass slide, and will flatten the surface of the rings. d. Dammar Cement , which is made by dissolving gum dammar in benzole, and adding about one-third of gold-size, has the advantage of drying very quickly; and may be preferably used for a first coat when glycerine is used as the material for mounting. e. BelVs Cement may be recommended on the same grounds; but it ' runs ' so freely, that for ordinary purposes the Author much prefers gold-size or dam- mar. /. Canada Balsam is so brittle when hardened by time, that it cannot be safe- ly used as a cement, except for the special purpose of attaching hard specimens to glass, in order that they may be reduced by grinding, etc. Although fresh soft balsam may be hardened by heating it on the slide to which the object is to be attached, yet it may be preferably hardened en masse by exposing it in a shal- low vessel to the prolonged but moderate heat of an oven, until so much of its volatile oil has been driven off that it becomes almost (but not quite) resinous on cooling.^ If, when a drop is spread out on a glass and allowed to become quite cold, it is found to be so hard as not to be readily idented by the thumb-nail, and Sands, the leather is not cemented to the bottom of the plug, but merely strained over it, so as to be easily renewable. ^ The Author has fluid preparations mounted with Gold-size nearly forty years ago, which have remained perfectly free from leakage; the precaution having been taken to lay on a fresh coat every two or three years. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 179 yet not so hard as to * chip,' it is in the best condition to be used for cementing. If too soft, it will require a little more hardening on the slide, to which it should be transferred in tlie liquid state, being brought to it by the heat of a water-bath; if too hard, it may be dissolved in chloroform or benzole, for use as a mounting * medium ' (§ 205). g. Shell-lac Cement is made by keeping small pieces of picked Shell-lac in a bottle of rectified spirit, and shaking it from time to time. It cannot be recom- mended as a substitute for any of the preceding; as, when dry and hard, it has little hold on glass. But it answers very well for making cells for dry- mounting (§ 167). — What is known as Liquid-glue is an inferior kind of the same cement, made by dissolving inferior shell-lac or some commoner resin, in naphtha. It cannot be trusted for a permanent hold; and those who employ it are likely to find themselves disappomted in regard to the durability of their preparations.^ h. Marine Glue, which is composed of Shell-lac, caoutchouc, and naphtha, is distinguished by its extraordinary tenacity, and by its power of resisting solvents of almost every kind. Different qualities of this substance are made for the several purposes to which it is applied; and the one most suitable to the wants of the Microscopist is known in commerce as G K 4. The special value of this cement, which can only be applied hot, is in attaching to glass slides the glass or metal rings which thus form * cells ' for the reception of objects to be mounted in fluid; no other cement being comparable to it either for tenacity or for durability. The manner of so using it will be presently described (g 171)c i. Various colored Varnishes are used to give a finish to mounted preparations, or to mark on the covering-glasses of large preparations the parts containing special kinds of noteworthy structure. A very good black varnish of this kind is made by working up very finely powdered lamp-black with gold-size. For red, sealing-wax varnish made by dissolving red sealing-w ax (the best is alone worth using) in rectified spirit, is commonly used; but it is very liable to chip and leave the glass, when hardened by time. The red varnish specially prepared for Micro- scopic purposes by Messrs. Thompson & Capper (of Liverj)ool) seems likely to stand better, but the Author's experience of it has been short. For white, ' zinc cement ' answers well: which may be made by dissolving 1 oz. of gum dammar in 1 oz. of oil of turpentine by the aid of heat; rubbing up 1 drachm of oxide of zinc with an equal quantity of oil of turpentine (adding the latter by drop) into a creamy mixture perfectly free from lumps or grii^ and then mixing the two fluids, which must be well stirred together, and strained through a piece of fine muslin pre- viously wetted with turpentine. Blue or green pigments may be worked-up with this, if cements of those colors be desired. k. For attaching labels and covering papers to slides either of glass or wood, and for fixing-down small-objects to be mounted ' dry ' (such az Foraminifera, parts oi Insects, etc.), the Author has found nothing; preferable to ci rather thick mucilage of Gum Arabic, to which enough Glycerine has been added to prevent it from drying hard, with a few drops of some Essential oil to prevent the develop- ment of mouldc The following formula has also bf-a recommended:— Dissolve 2 oz. of Gum Arabic in 2 oz. of water, and then add l-4th oz. of soaked gelatine (for the solution of which the action of heat will be required), 30 drops of gly- cerine, and a lump of camphor.— The further advantage is gained by the addition of a slightly increased proportion of Glycerine to either of the foregoing, that the gum can be very readily softened by water; so that covers may be easily removed (to be cleansed if necessary) and the arrangement of objects (where many are mounted together, § 175) altered. 169. Cells for Dry-mounting. — ^Wliere the object to be mounted ' dry' {ix. not immersed either in fluid or in any 'medium') is so thin as to require that the cover should be but little raised above the slide a ' cement ceir (§ 170) ansv^ers this purpose very well; and if the application of a gentle warmth be not injurious, the pressing down of the cover on the softened cement will help both to fix it, and to prevent the varnish applied round its border from running in. Where a somewhat deeper cell is ' From the appearance and smell of the Hollis's Glue recommended by Dr. Heneage Gibbs, the Author cannot but believe that its nature is essentially the same as that of ordinary ' liquid glue,' and that it is therefore liable to the same objection. 180 THE MICROSCOPE AND ITS EEYELATIONS. required, it can be made in the manner suggested by Prof. H. L. Smitli (U.S.) specially for the mounting of Diatoms. A sheet of thin writing- l)aper dipj^ed into thick shell-lac varnish is hung up to dry; and rings are then cut out from it by punches of two different sizes. One of these rings being laid on a glass slide, and the cover, with the object dried upon it, laid on the ring, it is to be held in its place by the forceps or spring-clip, and the slide gently warmed so as cause a slight adhesion of the cover to the ring, and of the ring to the slide; and this adhesion may then be rendered complete, by laying another glass slide on the cover, and pressing the two slides together, with the aid of a continued gentle heat. — Still deeper cells may be made with rings punched out of tin-foil of various thicknesses; and cemented with shell-lac varnish on either side. And if yet deeper cells are needed, they may be made of turned rings of vulcanite or ebonite, cemented in the same manner. — It is always safer to protect such dry mounts by attaching paper covers to the slides; as the tendency of the rings to start at any ^jar,^when the shell lac lias re- acquired its resinous hardness, is thereby greatly diminished. — Small objects, such as Diatoms and Polycysiina, which are to be viewed by Lieberktihn illumination (§ 115), should be mounted on disks punched out of thin black card-board, whose diameter scarcely exceeds the field of the Objective under which they are to be shown; and the protecting cell should be large enough to allow an ample opening for the light-rays to pass up from the mirror to the speculum, between the inner edge of its ring and the outer margin of the disk. 170. Cement-Cells, — Cells for mounting tliin objects in any watery medium, may be readily made with Asphalte or Black Japan varnish, by the use of Mr. Shadbolt's ' Turn-table' (§ 176) or one of its modifications. The glass slide being placed under 'Bm.WDs, its springs, in such a manner that its two edges shall be equidistant from ^ the centre (a guide to which posi- tion is afforded by the circles traced on the brass), and its four corners equally projecting beyond tho cir- cular margin of the plate, a camel s hair pencil dipped in the varnish "B is held in the right hand, so thvX its point comes into contact with the glass over whichever of the circles may be selected as the guide to the size of the ring. The turn- table being made to rotate by the ^ application of the left fore-finger to the milled-head beneath, a ring of varnish of a suitable breadth is made upon the glass; and if this be set aside in a horizontal position, it will be found, when hard, to ^ present a very level surface. If a greater thickness be desired than , a sino^le application will conveni- Tube-Cells, Round and Quadrang-ular. l^ ^ ^ ^ ^ i i i, V au^uiai. ently make, a second layer may be afterwards laid on. It will be found convenient to make a considerable number of such cells at once, and to keep a stock of them ready prepared PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 181 for use. If the surface of any ring should not be sufficiently level for a covering-glass to lie flat upon it, a slight rubbing upon a piece of fine emery-paper laid upon a flat table (the ring being held downwards) will make it so. 171. Ring-cells. — For mounting objects of greater thickness, it is de- sirable to use cells made by cenenting rings, either of glass or metal, to the glass slides, with marine glue. Glass-rings of any size, diameter, thickness, and breadth are made by cutting transverse sections of thick walled tubes; the surfaces of these sections being ground flat and parallel. Not only may round cells (Fig. 120 A, b) of various sizes be made by this simple method, but, by flattening the tube (when hot) from which they are cut, the sections may be made quadrangular or square, or oblong (c, d). For intermediate thicknesses between cement-cells and glass ring- cells, the Author has found no kind so convenient as the rings (sold by Mr. Collins) stamped out of tin, of various thicknesses. These, after being cemented to the slides, should have their surfaces made perfectly flat by rubbing on a piece of fine grit or a corundum-file, and then smoothed on a "Water of Ayr stone; to such surfaces the glass covers will be found to adhere with great tenacity. * The Glass Slides and Cells v^hich aio to be attached to each other, must first be heated on the Mounting piate; and some small cuttings of Marine glue are then to be placed either upon that surface of the cell which is to be attached, or upon that portion of the slide on vs^hich it is to lie, the former being perhaps prefer- able. When they begin to melt, they may be worked over the surface of attach- ment by means of a needle point; and m this manner the melted glue may be uniformly spread, care being taken to pick out any of the small gritty particles which this cement sometimes contains. When the surface of attachment is thus completely covered with liquefied glue, the cell is to be taken up with a pair of forceps, turned over, and deposited in its proper place on the slide; and it is then to be firmly pressed down with a stick (such as the handle of the needle), or with a piece of flat wood, so as to squeeze out any superfluous glue from beneath. If any air-bubbles should be seen between the cell and the slide, these should if pos- sible be got rid of by pressure, or by slightly moving the cell from side to side; but if their presence results, as is sometimes the case, from deficiency of cement at that point, the cell must be lifted off .again, and more glue applied at the required spot. Sometimes, in spite of care, the glue becomes hardened and black- ened by overheating; and as it will not then stick well to the glass, it is pref era- able not to attempt to proceed, but to lift off the cell from the slide, to let it cool, scrape off the overheated glue, and then repeat the process. When the cement- ing has been satisfactorily accomplished, the slides should be allowed to cool gradually in order to secure the firm adhesion of the glue; and this is readily ac- complished, in the first instance, by pushing each, as it is finished, towards one of the extremities of the plate. If two plates are in use, the heated plate may then be readily moved away upon the ring which supports it, the other being brought down in its place, and as the heated plate will be some little time in cooling, the firm attachment of the cells will be secured. If, on the other hand, there be only a single plate, and the operator desire to proceed at once in mount- ing more cells, the slides already completed should be carefully removed from it, and laid upon a wooden surface, the slow conduction of which will prevent them from cooling too fast. Before they are quite cold, the superfluous glue should be scraped from the glass with a small chisel or awl; and the surface should then be carefully cleansed with a solution of potash, which may be rubbed upon it with a piece of rag covering a stick shaped like a chisel. The cells should next be washed with a hard brush and soap and water, and may be finally cleansed by rubbing with a little weak spirit and a soft cloth. In cases in which apxjearance is not of much consequence, and especially in those in which the cell is to be used for mounting large opaque objects, it is decidedly preferable not to scrape off the glue too closely round the edges of attachment; as the *hold' is much firmer, and the probability of the penetration of air or fiuid much less, if the im- mediate margin of glue be left both outside and inside the cell. — To those to whom time is of value, it is recommended that all cells which require Marine- glu^ cementing be purchased from the dealers in Microscopic apparatus. 182 THE MICROSCOPE AND ITS REVELATIONS. 172. Plate-Glass Cells. — Where large sliallow cells vnth flat bottoms are required (as for mounting Zoophytes, small Medusce, etc.), they may be made by drilling holes in pieces of plate glass of various sizes, shapes, and thicknesses (Fig. 121, a), which are then cemented to glass slides with marine glue. By drilling two holes at a suitable distance and cutting out the piece between them, any required elongation of the cavity may be obtained (b, c, d). 173. Sunk Cells.— "Eict- 121. name is given to round or oval hollows excavated by grinding ^ in the substance of glass slides, which, for this purpose, should be thicker than ordinary. Such cells have the advantage not only of comparative cheapness, ^ but also of durability, as they are not liable to injury by a sud- den jar, such as sometimes causes the detachment of a cemented D plate or ring. For objects whose shape adapts them to the form and depth of the cavity, such cells will be found very conve- nient; thus the Author has a p series of young Coinatulce (Fig 378) thus mounted, which are Plate-Glass Cells. extremely well displayed, alike on their upper and on their Ekx . under surfaces. It naturally suggests itself as an objection to the use of such cells, that the concavity of their bottom must ^ so deflect the light rays, as to distort or obscure the image; but as the cavity is filled either with water or some other liquid of higher refractive power; the deflection is so slight as to be I practically inoperative. Before mounting objects in such cells, the Microscopist should see that their concave surfaces are free from scratches or roughness. 174. Built-up Cfe«^\— When cells are required of forms or dimensions not otherwise pro- curable, they may be huilt up of separate pieces of glass cemented together. Large shallow Cells, suitable for mounting Zoophytes Sunk Cells. similar flat objects, may be easily constructed after the following method: — A piece of plate-glass, of a thickness that shall give the desired depth to the cell, is to be cut to the dimensions of its outside PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 183 wall; and a strip is then to be cut off with the diamond from each of its edges, of such breadth as shall leave the interior piece equal its dimen- sions to the cavity of the cell that is desired. This piece being rejected, the four strips are then to be cemented upon the glass slide in their original position, so that the diamond-cuts shall fit together with the most exact precision; and the upper surface is then to be ground flat with emery upon a pewter plate, and left rough. — The perfect construction of largo deep cells of this kind (Fig. 123, A, b), however, requires a nicety of workmansliip which few amateurs possess, and the expenditure of more time than Microscopists generally have to spare; and as it is con- • rrc. i2.:>\ sequently preferable to obtain them ready-made, directions for making them need not be here given. 175. Wooden Slides for Opaque Objects. — Such ^dry^ objects as Foraminifera, the capsules of Mosses y parts of Insects, and the like, may be conveniently mounted in a very simple form of wooden ^ slide (first devised by the Author and now come into general use), which also serves as a jirotective 'cell.' Let a number of slips Built-up ceiis. of mahogany or cedar be pro- vided, each of the 3-inch by 1-inch size, and of any thickness that may be found convenient, with a corresponding number of slips of card of the same dimensions, and of pieces of deadAA^ick paper rather larger than the aperture of the slide. A piece of this paper being gummed to the middle of the card, and some stiff gum having been previously spread over one side of the wooden slide (care being taken that there is no superfluity of it immediately around the aperture), this is to be laid down upon the card, and subjected to pressure. ^ An extremely neat ' cell ' will thus be formed for the reception of the object (Fig. 124), the depth of which will bo determined by the thickness of the slide, and the diameter by the size of the perforation; and it will be found convenient to provide slides of various thicknesses, with apertures of different sizes. The cell should always be deep enough for its wall to rise above the object; but, on the other hand, it should not be too deep for its walls to interfere with the oblique incidence of the light upon any object that may be near its periphery. The object, if flat or small, may be attached by Gum-muci- lage (§ 168 ^); if, however, it be large, and the part of it to be attached have an irregular surface, it is desirable to form a ^bed' to this by gum thickened with starch. If, on the other hand, it should be desired to mount the object edgeways (as when the mouth of a Foraminifer is to be brought into view), the side of the object may be attached with a little gum to the loall of the cell. — The complete protection thus given to the Object is the great recommendation of this method. But this is by no ^ It will be found a very convenient plan to prepare a large number of such Slides at once: and this may be done in a marvellously short time, if the slips of card have been previously cut to the exact size in a bookbinder's press. The slides, v^^hen put together, should be placed in pairs, back to back; and every pair should have each of its ends embraced by a Spring-press ;Fig. 129) until dry. 184 THE MICROSCOPE AND ITS REVELATIONS. means its only convenience. It allows the slides not only to range in the ordinary Cabinets, but also to be laid one against or over another, and to be packed closely in cases, or secured by elastic bands; which plan is extremely convenient not merely for the saving of space, but also for preserving the objects from dust. Should any more special protection be required, a thin glass cover may be laid over the top of the cell, and secured there either by a rim of gum or by a perforated paper cover attached to the slide; and if it should be desired to pack these covered slides together, it is only necessary to interpose guards of card somewhat thicker than the glass covers. 176. Turn-table. — This simple instrument (Fig. 125), devised by Mr. Wooden Slide for Opaque Objects. Shadbolt's Turn-table for making Cement-Cells. Shadbolt, is almost indispensable to the Microscopist who desires to pre- serve preparations that are mounted in any ^ medium ^ beneath circular covers; since it not only serves for the making of those ^Cement-cells' (§ 170) in which thin transparent objects can be best mounted in any kind of ^ medium' but also enables him to apply his varnish for the secur- ing of circular cover-glasses not only with greater neatness and quickness, but also with greater certainty than he can by the hand alone. As the method of using it for the latter purpose is essentially the same as that already described under the former head, it need not be here repeated; the only special precaution to be ob- served, being that the cover-glass, not the slide, should be ^centered;' which can be readily done, if several concentric circles have been turned on the rotating-table, by making the cover-glass correspond with the one having its own diameter. — A number of ingenious modifications have been devised in this simple instrument, with the view of securing exact cen- tering; the simplest of them (which has the advantage of being applic- able at a trifling expense to any ex- isting turn-table) being that of Mr. 0. S. Rolfe.^ But as it is often re- quisite to use this instrument with slides not accurately cut to size and shape, or of greater breadth than the ^regulation ' 1-inch, the Author is dis- posed to prefer the form devised by Mr. Dunning ^ (Fig. 126). The circular Dunning's turn-table. ^ Journal of the Quekett Microscopical Club," Vol. v., p. 249. 2 Op. cit., vol. vi., p. 81. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 185 table, made rather thicker than usual, has a dovetail groove ploughed out across its diameter, in which work two sliding guides A, a, the ends of which are cut and ' sprung/ so as to have a sufficiently firm hold. These guides carry the two clips b, b; one of which is fixed at right angles to its guide, whilst the other is pivoted, in order that it may adjust itself to any irregularity in the form of the slide. — When Cement-cells arc being made either with this or the ordinary Turn-table, it is convenient to mark the centre of each slide with a dot of ink on its under surface; this may be easily applied in its right place by laying on it a slip of card cut to the regulation size, with a small central perforation; and by so laying down the slide that the dot lies on the centre of the rotating plate, much trouble may afterwards be saved. 177. Moimting Plate and Water-hath, — Whenever heat has to be applied either in the cementing of Cells or in the mounting of Objects, it is desirable that the slide should not be exposed direct to the ilame, but that it should be laid upon a surface of regulated temperature. As cementing with Marine Glue or hardened Canada Balsam requires a heat above that of boiling water, it must be supplied by a plate of metal; and the Author's experience leads him to recommend that this should be a piece of iron not less than six inches square and half an inch thick; and that it should be supported, not on legs of its own, but on the ring of a Eetort-stand, so that by raising or lowering the ring, any desired amount of heat may be imparted to it by the lamp or gas-flame beneath. The advantage of a plate of this size and thickness consists in the gradational temperature which its different parts afford, and in the slowness of its cooling when removed from the lamp. When many cells are being cemented at once, it is convenient to have two such plates, that one may be cooling while the other is being heated. — The Eetort-stand also serves for the support of the Water-bath, which affords the heat required for liquefying and mixing the fats employed in the imbedding process (§ 189), for melting the glycerine jelly or other media used in mounting, and for a variety of other purposes. A circular-bottomed flat tin vessel, 6 inches in diameter and 2^ inches deep, with a handle like that of a saucepan, and two covers, — one a flat plate of 8 inches square (its edges guarded by being turned over wire) for slides to lie upon, having a hole large enough to admit a small bottle of cement or medium, — the other fitting the vessel, but with an opening large enough for a porcelain basin, — will answer every purpose. 178. Slider- Forceps, Spring Clip, and Spring-Press. — For holding slides to which heat is being applied, especially while cementing objects to be ground-down into thin sections, the wooden Slider-Forceps (Fig. 127) will be found extremely convenient. This, by its elasticity, affords a secure grasp to a slide of any ordinary thickness, the wooden blades being separated by pressure upon the brass studs; while the lower stud, with the bent piece of brass at the junction of the blades, affords a level support to the forceps, which thus, while resting upon the table, keeps Slider-Forceps. 186 THE MICROSCOPE AND ITS REVELATIONS. the heated glass from contact with its surface. For holding-down cover- glasses whilst the balsam or other medium is cooling, if the elasticity or the object should tend to make them spring-up, the wire Spring-Clip (Fig. 128), sold at a cheap rate by dealers in Microscopic apparatus, will be found extremely convenient. Or, if a stronger pressure be required, recourse may be had to a simple Spring- Press made by a light alteration of the ' American clothes peg ' which is now in general use in this country for a variety of purposes; all that is necessary being to rub down the opposed surfaces of the ^clip^ with a flat file, so that they shall be parallel to each other when an ordinary slide with its cover is interposed between them (Fig. 129). One of these convenient little implements may also be Spring-Clip. Spring-Press. easily made to serve the purpose of a Slider-forceps, by cutting back the upper edge of the clip, and filing the lower to such a plane that when it rests on its fiat side, it shall hold the slide parallel to the surface of the table, as in Fig. 127. 179. Ifounting Instrument, — A simple mode of applying graduated pressure concurrently with the heat of a lamp, which will be found very convenient in the mounting of certain classes of objects, is afforded by the Mounting instrument devised by Mr. James Smith. This consists of a plate of brass turned up at its edges, of the proper size to allow the ordinary glass slide to lie loosely in the bed thus formed, this plate has a large perforation in its centre, in order to allow heat to be directly applied to the slide from beneath, and it is attached by a stout wire to a handle (Fig. 130). Close to this handle there is attached by a joint an upper Smith Mounting Instrument. wire, which lies nearly parallel to the first, but makes a downward turn just above the centre of the slide-plate, and is terminated by an ivory knob; this wire is pressed upwards by a spring beneath it, whilst, on the other hand, it is made to approximate the lower by a milled-head turning on a screw, so as to bring its ivory knob to bear with greater or less force on the covering glass. The special use of this arrangement will be ex- plained hereafter (§ 210). ^ 180. Dissecting Apparatus, — The mode of making a dissection for Microscopic purposes must be determined by the size and character of the object. Generally speaking, it will be found advantageous to carry on PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 187 the dissection under Water, with which Alcohol should be mingled where the substance has been long immersed in spirit. The size and depth of the vessel should be proportioned to the dimensions of the object to be dissected; since, for the ready access of the hands and dissecting-instru- ments, it is convenient that the object should neither be far from its walls, nor lie under any great depth of water. Where there is no occa- sion that the bottom of the vessel should be transparent, no kind of Dis- secting trough is more convenient than that which every one may readily make for himself, of any dimension he may desire, by taking a piece of sheet Gutta-percha of adequate size and stoutness, warming it sufficiently to render it flexible, and then turning-up its four sides, drawing out one corner into a sort of spout, which serves to pour away its contents when it needs emptying. The dark color of this substance enables it to fur- nish a back-ground, which assists the observer in distinguishing delicate membranes, fibres, etc., especially when magnifying lenses are employed; and it is hard enough (without being too hard) to allow of pins being fixed into it, both for securing the object, and for keeping apart such portions as it is useful to put on the stretch. When glass or earthenware troughs are employed, a piece of sheet-cork loaded with lead must be pro- vided, to answer the same purposes. In carrying-on dissections in such a trough, it is frequently desirable to concentrate additional light upon the part which is being operated-on, by means of the smaller Condensing- lens (Fig. 86); and when a low magnifying power is wanted, it may be supplied either by a single lens mounted after the manner of Eoss's Simple Microscope (Fig. 31, b), or by a pair of Spectacles mounted with the ^semi-lenses,^ ordinarily used for Stereoscopes.* Portions of the body under dissection being floated-ofl when detached, may be conveniently taken up from the trough by placing a slip of glass beneath them (which is often the only mode in which delicate membranes can be satisfactorily spread out); and may be then placed under the Microscope for minute examination, being first covered with thin glass, beneath the edges of which is to be introduced a little of the liquid wherein the dissection is being carried-on. Where the body under dissection is so transparent, that more advantage is gained by transmitting light through it than by looking at it as an opaque object, the trough should have a glass bot- tom, and for this purpose, unless the body be of unusual size, some of the Glass Cells already described (Figs. 121-123) will usually answer very well. The finest dissections may often be best made upon ordinary slips of glass; care being taken to keep the object sufficiently surrounded by fluid. For work of this kind no simple instrument is more generally serviceable than the Laboratory Dissecting Microscope (Fig. 35), which will carry any power from 3-inch to a l-4th inch; whilst the Stephenson Erecting Binocular (Fig. 47) may be used with the like supports for the hands, when a higher power is preferred. 181. The Listruments used in Microscopic dissection are for the most part of the same kind as those which are needed in ordinary minute Au- atomical research, such as scalpels, scissors, forceps, etc.; the fine instru- ^ The author can strongly recommend these Spectacles, as useful in a great variety of manipulations which are best performed under a low magnifying power, with the conjoint use of both eyes. — Where a higher power is needed, re- course may be advantageously had to Messrs. Beck's 3-inch Achromatic Binocu- lar Magnifier, which is constructed on the same principle, allowing the object to be brought very near the eyes, without requiring any uncomfortable convergence of their axes. 188 THE MICROSCOPE AND 1TB REVELATIONS. ments used in operations upon the eye, however, will commonly be found most suitable. A pair of delicate scissors, curved to one side, is extremely convenient for cutting open tubular parts; these should have their points blunted; but other scissors should have fine points. A pair of very fine* pointed Scissors (Fig. 131), one leg of which is fixed in a light handle, and the other kept apart from it by a spring, so as to close by the pressure of the finger and to open of itself, will be found (if the blades be well sharpened) much superior to any kind of knives, for cutting through delicate tissues with as little disturbance of them as possible. — A pair of small straight Forceps with fine points, and another pair of curved forceps will be found useful in addition to the ordinary dissecting forceps. 182. Of all the instruments contrived for delicate dissections, however, none are more serviceable than those which the Microscopist may make for himself out of ordinary needles. These should be fixed in light rjG. T31. Fig. 132, Spring-Scissors. Curved Scissors for Cutting thin Sections. wooden handles' (the cedar sticks used for camel-hair pencils, or the handles of steel-j)enholders, or small Porcupine-quills, will answer ex- tremely well), in such a manner that that their points should not project far,^ since they will otherwise have too much ^spring;' much may be done by their mere tearing action; but if it be desired to use them as cutting instruments, all that is necessary is to harden and temper them, and then give them an edge upon a hone. It will sometimes be desirable to give a finer j^oint to such needles than they originally possess; this also may be done upon a hone. A needle with its point bent to a right angle, or nearly so, is often useful; and this may be shaped by simply heating the point in a lamp or candle, giving to it the required turn with a pair of pliers, and then hardening the point again by reheating it and plunging it into cold water or tallow. 183. Section-cutting. — The young Microscopist will do well to practise the cutting of thin Sections of soft Vegetable and animal substances with a sharp razor: considerable practice is needed, however, to make effect- ual use of it; and some individuals acquire a degree of dexterity which ^ The handles of ladies Crochet-needles have been recommended for this pur- pose; and although they afford the facility of lengthening of shortening the act- ing point of the needle at will, and also of carrying a reserve store of needles at the other end, yet the Author would decidedly recommend the use of the wooden handles, of which it will be found convenient always to have several at hand, mounted with needles of different sizes. 2 The following is the mode in which the Author has found it convenient to mount his needles for this and other purposes : — The needle being held firmly in a pair of pliers grasped by the right hand, its point may be forced into the end of a cedar or other stick held in the left, until it has entered to the depth of half an inch or more; the needle is then cut off to the desired length (the eye-end being thus got rid of); and being then drawn out of the stick, the truncated end is forced into the hole previously made by the point, until it cannot be made to penetrate farther, when it will be found to be very securely fixed. The end of the handle which embraces it may then be bevelled-away round its point of insertion. PREPARATION J MOdNTING, AND COLLECTION OF OBJECTS. 189 Tig. others never succeed in attaining. The making of hand-sections will be greatly facilitated by the previous use of the hardening and imbeddincr processes robe hereafter described (§§ 189, 199); but the best of them rarely equal good sections cut by a Slicrotome. — For the preliminary examination of any soft structure, such a pair of Scissors as is represented in Fig 132 will often be found very useful; since, owmg to the cur- vature of the blades, the two extremities of a section taken from a flat surface will generally be found to thin away, although the middle of it may be too tliick to exhibit any structure. The two-bladed Knife contrived by Prof. Valentin was formerly much used for cutting microscopic sec- tions of soft tissues: but as such sections can be cut far more ett'ectively by the methods to be presently described, a mere mention of this instru- ment will here suffice. 184. Microtome. —There is a large class of substances, of moderate hard- ness, both Animal and Vegetable, of which extremely thin and uniform slices can be made by a sharp-cutting instrument, if they be properly held and supported, and the thickness of the section be regulated by a mechan- ical contrivance; such are, in particular, the Stems and Eoots of Plants, and the Horns, Hoofs, Cartilages, and similarly firm structures of Animals. Various costly machines have been devised for this purpose, some of them characterized by great ingenuity of contrivance and beauty of work- manship, but most of the purposes to which these are adapted will be found to be answered by a very simple and inexpensive little instrument, I which may either be held in the hand, or (as is preferable) may be firmly attached by means of a T-shaped piece of wood (Fig. 133), to the end of a table or work-bench. This instrument essentially consists of an upright hol- low cylinder of brass, with a kind of piston which is pushed from below upwards by a fine-threaded or ' micro- meter ' screw turned by a large milled- head, at the upper end the cylinder terminates in a brass table, which is T)laned to a flat surface, or (which is preferable) has a piece of plate-glass Simple Microtome, cemented to it, to form its cutting bed. At one side is seen a small milled- head, which acts upon a ^binding screw,' whose extremity projects into the cavity of the cylinder, and serves to compress and steady anything that it holds. For this is now generally substituted a pair of screws, work- ing through the side of the cylinder, as in Fig. 120. A cylindrical stem of wood, a piece of horn, whalebone, cartilage, etc., is to be fitted to the interior of the cylinder so as to project a little above its top, and is to be steadied by the ^binding screw; ^ it is then to be cut to a level by means of a sharp knife or razor laid flat upon the table. The large milled-head is next to be moved through such a portion of a turn as may very slightly ^ It is difficult to convey by a drawing the idea of the real curvature of this instrument, the blades of which, when it is held tti front view, curve — not to either side— but towards the observer; these scissors being, as the French instru- ment-makers say, courb4s sur leplat. 190 THE MICROSCOPE AND ITS REVELATIONS. elevate the substance to be cut, so as to make it project in an almost insen- sible degree above the table, and this projecting part is to be sliced off with a knife previously dipped in water. For many purposes an ordinary razor will answer sufficiently well; but thinner and more uniform sections can be cut by a special knife having its edge parallel to its back, its sides slightly concave, and its back with a uniform thickness of rather less than l-4th inch. Such a knife should be 4 or 5 inches long, and 7-8ths inch broad; and should be set in a box-wood handle about 4 inches long (Dr. S. Marsh). The motion given to its edge should be a combination of drawing 'd>ndi pressing, (It will be generally found that better sections are made by working the knife /rom the operator, than toiuards him). When one slice has been thus taken off, it should be removed from the blade by dipping it into water, or by the use of a camel-hair brush; the milled-head should be again advanced, and another section taken: and so on. Different substances will be found both to lear and to require different degrees of thickness; and the amount that suits each can only be found by trial. It is advantageous to have the large milled-head gradu- ated, and furnished with a fixed index; so that this amount having been once determined, the screw shall be so turned as to always produce the exact elevation required. — Where the substance of which it is desired to obtain sections by this instrument is of too small a size or of too soft a texture to be held firmly in the manner just described, it may be placed between the two vertical halves of a cork of suitable size to be pressed into the cylinder; and the cork, with the object it grasps, is then to be sliced in the manner already described, the small section of the latter being care- fully taken-off the knife, or floated-away from it, on each occasion, to prevent it from being lost among the lamellae of cork which are removed at the same time. Vertical sections of many Leaves may be successfully made in this way; and if their texture be so soft as to be injured by the pressure of a cork, they may be placed between two half-cylinders of car- rot or elder-pith. 185. Hailes^s Microtome, — The foregoing simple form of Microtome has received, at various hands, numerous modifications of detail, without any essential change in its plan of construction. Its chief defect is, that as the body to be cut is directly acted-on by the screw at the bottom of the cylinder, its motion (if it be tightly held by the binding screws) is apt to be jerky and irregular. To remedy this defect, Mr. II. P. Hailes has devised an improved model, the essential feature of which is that the body to be cut is secured within an inner tube, which, sliding freely within the outer cylinder, is raised smoothly and equally by the micrometer screw attached to the base of the latter, as shown in Fig. 134 (1, 2). The cut- ting-bed formed by the flange B, is provided with two slips h of hardened steel, on which, in ordinary section-cutting, the knife or razor slides horizontally, as in the ordinary Microtome. But by the addition shown in 3, 4, this instrument can also be effectively adapted for cutting thin sections of substances hard enough to require the use of the saw. At the back of the cutting-bed, there can be secured (by means of the screw and and steadying-pins) a metal block, h\ which carries two guides of hard steel; and these, when thus attached, lie over the two similar strips fixed on the cutting-bed. By passing the blade of a fine saw between the movable guides and the fixed strips, and screwing down the former (which are raised by a spring) as far as will confine the saw without im- peding its working, sections of Bone, Teeth, etc., may be cut as thin as the nature of the substance will allow, and with a uniformity that with- PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 191 out such guidance cannot be attained. — When the Microtome is employed for this last purpose, the saw may be most conveniently worked vertically; and this is readily done by detaching the instrument from the table, and holding it down upon its clamp-side, which is so shaped as to afford a level support. 186. In what is known as the Strashicrg Microtome^ invented by Prof. Schieflerdecker, the substance to be cut is fixed in the cylinder by binding-screws, while the circular cutting-bed, instead of being fixed on the upper end of the cylinder, is made to screw upon it, so as to be raised or lowered by turning it round. Thus, after a section has been taken, a slight lowering of the cutting-bed, measured by the graduation of its margin, prepares it for the cutting of the next.^ — The simplicity of this 5 fni J ^ 1 Ilailes'G Microtome. The two upper figures show the instrument (1) as seen from the side, (2) as seen in section a, outer cyUnder, carryin,- u;.; :r flan?;-e c, on whose surface he two strips of hard steel, 6, h; this flange fixed t^* the bar c, v/hich carries a clar_ip and screw for attaching the Microtome to a table: in the sec'onalfi^n (2) is seen the inne.- tubo c, within which the substance to be cutis fixed by tht twc binding screws, r, c, which work through a slot in the outer cylinder; to the bottom of the inner tube is fixed a block d. thror.(;:h which works the micrometer-screw e, turned by the milled-hdad o in the bracket f attached to the bottom of the outer cylinder, and having a graduated collar/. The two lower figures show the additional Saw-guide, seen from the side at 3, and from above at 4 :— &i, metal block with a screw to secure it on cutting-bed; h\ b^, steel guides. instrument, which is made to be held in one hand whilst the section is cut with the other, is its great recommendation. 187. Imledding and Freezing Microtomes, — Y or making thin sections of soft tissues, however, preference is now generally given to Microtomes in which the substance to be cut is so imbedded in some material that fills the cylinder, that it does not need to be fixed by binding-screws, being pushed upwards by the action of the micrometer-screw beneath ^ Quart. Journ. of Microsc. Science," Vol. xvii. (1877), p. 35.— Another Micro- tome, suggested by the preceding, is described by Mr. W. Teesdale in "Journ. of Roy. Microsc. Soc," Vol. iii. (1880), p. 1035. 192 THE MICROSCOPE AND ITS REVELATIONS. upon the imbedding plug. This plug may be either a cylinder of carrot, turnip, potatoe, or elder-pith, cut to fit the well of the Microtome, and excavated to receive the substance to be cut; or it may be a cast of the interior, made either by pouring into it paraffine or some similar sub- stance liquefied by heat (§ 189), or by filling it with thick gum-mucilage which is then rendered dense by cold (§ 191). The latter plan was first devised by Prof. Kutherford, whose Freezing Microtome, in which the upper part of the cylinder is surrounded by a well filled with a freezing- mixture, has now come into general use. — The substitution of ether-spray for ice-congelation was suggested by Mr. Bevan Lewis; and an improved model, which can be used either as a Freezing or as an Imbedding Microtome, has been devised by Messrs. Beck. An ingenious method of so attaching the cutting-blade by a ^ parallel motion,' as to make its edge at the same time move tangentially and transversely to the plane of section, has been devised by Prof. Seiler, of Philadelphia, and has found much approval, as well in this country as in the United States.^ Rivet-Leiser Microtome;— a, as seen from the front; b, as seen from behind. 188. Rivet-Leiser Microtome, — For the cutting of very thin sections of soft Animal or Vegetable substances which may be advantageously imbedded in paraffine or some other hard fat (§ 189), no instrument is more eifective than that represented in Fig. 135, which is known as the ' Leipzig ^ or ' Kivet-Leiser ' Microtome. This has for its base an oblong solid metal plate, from which rises a vertical plate, of which the upper edge is inclined at a gentle angle. From either side of this vertical plate, there projects a smoothly-planed plate, like a shelf sloping inwards; but while the edge of one of these shelves is parallel to the base, that of the other is parallel to the inclined margin of tho vertical plate. On the former slides a carrier bearing a Knife, the position of which can be adjusted and fixed by means of a binding-screw that works through a slot 1 " Journ. of Roy. Microsc. Soc," Vol. ii. (1879), p. 329. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 193 in its handle; whilst on the latter there slides an Object-carrier, consist- ing of a clamp, whose opening is controlled by a binding-screw, for hold- ing the block of paraffine in which the substance to be cut is imbedded. From this description, it will be obvious that when the carrier that bears the knife (as seen at a) is slid from one end of its shelf to the other, the knife always remains on the same level; but that when the Object-carrier is similarly slid (from right to left in Fig. b), it gradually rises, always keeping at the same height in relation to the inclined edge of the vertical plate. This edge being graduated, and a ^ vernier^ being engraved on the carriage, the progressive elevation of the surface from which the section is to be taken can be measured with the most minute exactness; as the substitution of the inclined plane for the screw altogether does away with the ' lost time ' from which the action of the latter is seldom entirely free. The manner in which the knife is attached to its carriage, enables it to be so fixed as to give any proportion that may be desired between the sliding and the pressing cut. — The simple model here described is extensively used on the Continent; and the Author can indorse its reputation from large personal experience. Certain modifica- tions have been recently made in it, however which must not be passed without notice. One of these re- lates to the mode in which the Eto^ISs;?, block of paraffine is held in its car- rier, so that the position of the body imbedded in it may be varied, without taking the block out of the clamp. The screw a (Fig. 136), working through the fixed piece h, brings the movable piece c (which is guided by two pins that work through h) against the fixed piece d, and thus secures the body to be cut. The clamp is connected by means of the bent arm e with the block /, the upper surface of which is rounded ; and on this it can improved object Carrier for the Rivet-LeiseP , T . ' n 1 J i 1 Microtome. be moved m a plane parallel to the middle plate of the instrument, so as to take a position more or less oblique in which it may be fixed by the binding-screw g. The block / again is connected with the fixed block li, by a pivot passing through the latter; and on this it may be rotated in a plane at right angles to the middle plate, being fixed in any position by the binding-screw i. By the combination of these two movements, the object can be placed (and then fixed) in such a position that the sectional plane shall traverse it in any desired direction. — The knife-carrier is also furnished with screws that etiable the inclination of the blade to be regulated with great pre- cision. And, if desired, the object-carrier may be advanced up its incline by a screw traversing the entire length of the instrument, instead of by hand; an addition, however, which seems to the Author quite unnecessary, and certainly not worth its cost. ^— This Microtome can bo made in hard wood at a lower cost than in metal, and with very little sacrifice (if any) of efficiency; and it has lately been recommended that the body of the instrument should be divided longitudinally, and its two ''♦Joum. of Roy. Microsc. See," Vol. ii, (1880), p. 334. 13 194 THE MICROSCOPE AND ITS REVELATIONS. halves attached at one end, but made to diverge at the other at any angle, being there fixed by a clamping screw. ^ Section" 2. — Preparation and Mounting of Objects. 189. Imiedding Processes. — The preparation of soft Organic sub- stances for Section-cutting by ^imbedding/ may be made in two modes> the choice between which will depend upon the consistence of the sub- stance. If (1) it be compact, like a piece of liver or kidney, it only needs to be surrounded by the imbedding mass, which will afford it as a iuliole the requisite support. But if (2) it be partly occupied, like a piece of lung, by interstitial cavities, it must be penetrated by the imbedding substance, so that every part may be duly supported. — For simple im- bedding, nothing is so suitable as the firmer fats; which must not, how- ever, be so hard as to be brittle. Thus, if white Wax be used, it should be melted with an equal weight of olive oil; if Paraffine or Spermaceti, it should be melted with about one-fifth of its weight of lard or soft tallow. The latter is generally to be preferred, as shrinking less in cooling; the cylinder formed by the hardened wax being liable to become loose in the well of the Microtone. Either mixture being kept in stock, carefully secluded from dust, a small quantity of it should be melted for use in a porcelain basin floated in a water-bath. To avoid injury to the tissue, its temperature should not be raised more than is requisite for its thor- ough liquefaction. The substance to be cut, having been previously hardened (§ 199), should be taken out of the spirit in which it is pre- served; and a piece of suitable size having been cut off, this should bj placed on blotting-paper, so that the spirit may drain away, and its sur- face may become dry. It is then to be dipped (as recommended by Dr, Sylvester Marsh), in a very weak solution — 20 grains to the ounce — of Gum Arabic, care being taken in doing so not to squeeze out the spirit so as to remoisten the surface; and the superfluous liquid being then again removed by blotting-paper, the surface will in a few minutes become dry and glazed with a thin film of gum, the use of which is to keep the imbedding substance from adhering to it. The plug of the Microtome (which may advantageously have a large-headed screw inserted into its upper side, to furnish a ' hold ^ for the imbedding substance) being set at the depth of about an inch beneath the cutting-bed, melted wax or paraffine is to be poured into it to about half this depth; and the sub- stance to be cut being then held in the tube in the best position (which is not its centre, but nearer the side next the operator), the imbedding material is to be slowly poured in, until the imbedded substance is entirely covered, and the cavity completely filled. When the imbedding material has become quite solidified by cooling, the cutting of sections may be proceeded with. 190. When, however, it is necessary that the substance to be cut should be entirely penetrated by the imbedding material, a much longei preparatory process is necessary. In many cases in which the sections are required to display rather the goieral than the minute structure, satisfactory results may be obtained by keeping the substance (previously steeped in pure water) immersed for a lengthened period at a gentle warmth, either in a strong mucilage of Gum Arabic, or in a solution of Gelatine that will ^set' on cooling, its cavities having been laid open 1 Brandt in Zeitschrift fiir Mikrosk.," Bd. ii. (1880), p. 172. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 195 sufficiently for the gradual penetration of the liquid to their interior. The entire mass being then exposed to the air, the slow evaporation of its water will at last reduce it to a consistence sufficiently firm to enable sections of it to be taken; or the water may be drawn out by steepino- in Alcohol. This plan has been found to answer for the entire bodied of Insects, Stems of herbaceous Plants, and the like. — But when the sec- tions are to be cut of the extreme thinness required for showing minute* histological detail, it is much better to use either Paraffine slightly soft-" ened with lard, or Cacao-butter, which last has been much recommended for the imbedding of structures of extreme delicacy. The material to be cut must be first dehydratedy or deprived of its Water; which is done by letting it lie for a time in ordinary Spirit, then transferring it to Recti- fied spirit, and at last treating it with absolute Alcohol. From this it is to be transferred to some volatile oil; oil of bergamot being used for delicate objects; oil of turpentine answering sufficiently well for larger bodies. When this has completely replaced the spirit, the body is to be immersed for some little time in a hot saturated solution of paraffine in oil of turpentine. When it has lain sufficiently long in this to be thor- oughly penetrated, it is to be immersed in the melted paraffine, which should not be more heated than is necessary to keep it quite liquid; and it should be moved about in this for some little time, an occasional gentle squeeze being given to it with the forceps, so that the solution may be replaced as completely as possible by the liquefied paraffine. When hardened by cooling, the substance thus prepared may be ' imbedded ' in any ordinary cylinder Microtome, in the manner already described; the coating with gum being of course omitted. But if the sections are to be made either with the Rivet-Leiser Microtome, or by hand, it is necessary to provide a mould into which the imbedding; material can be poured. This may be made of cylindrical form, by twisting a strip of paper round the end of a small ruler; or a brick-shaped block may be cast in a mould made by turning up the edges of a suitably-sized piece of paper, and pinning together the cross-folds at the two ends. But it is generally more convenient to use for this purpose small boxes of tin 2 inches long, and 3-4ths of an inch in breadth and depth, with removable bottoms. A small piece of filtering paper being placed between the bottom and the sides of the box, and the substance to be imbedded being held in it in the most suitable position, the paraffine is poured in until the box is completely filled, and this is set aside to cool. When the paraffine has perfectly solidified, the box it to be lifted off its bottom; and the block, being pushed out of it, is thm ready for cutting.— In using the section- knife, care should be taken to keep it constantly wetted with methylated spirit; and it is desirable that each section should be removed from it before another is taken. When, for the study of the anatomy of an animal, sections are being taken m series, and it is important that their order should be preserved, a set of watch-glasses should be previously provided, each about half filled with spirit, and the sections successively taken should be dropped singly into them; care being taken in the arrangement of the glasses to maintain the relative position of the sections. In order to dissolve out the imbedding material, the sections should be soaked in oil of turpentine with about one-fourth part of creasote; and if its structure is suitable for examination with high powers, it may be cleared by a short immersion in oil of cloves. They are then to be mounted either in Canada balsam solution (§ 209) or in Dammar cement. 196 THE MICROSCOPE AND ITS REVELATIONS. 191. When the freezing process is employed, the substance to be cut (which may either be fresh, or have been hardened by some of the processes to be hereafter described, § 199) must be thoroughly penetrated by a thick solution of gum; for this, when frozen, does not become crystalline, and may be cut like cheese. If the substance to be cut has ^ been immersed in p^lcohol, this must be completely removed in the first 'instance by immersion in water for from six to twenty-four hours, according to the size of the mass; for the gum will not penetrate any part which is still alcoholized. And the substance should be then immersed in the gum-solution for from twelve to twenty-four hours before it is frozen; in order that every part may be permeated by the gum, and no water be left to form crystals of ice. If the freezing Microtome of Prof. Rutherford * be employed, the freezing-box should be filled with alternate spoonsful of salt and either snow or finely powdered ice, which are to be stirred round the well previously filled with the gum solution. With the Ether-spray Microtome, the freezing is produced by the rapid evaporation of the liquid injected into the freezing chamber. In either case, the substance to be cut is to be introduced into the well, as soon as the gum begins to harden at its periphery; and should be held in place until fixed by the advancing congelation. In cutting the sections, no wetting of the knife is necessary; as it is kept sufficiently wetted by the thawing gum. The sections should be placed in methylated spirit diluted with twice its volume of water; and this soon not only dissolves out the gum, but removes any air-bubbles the section may contain. If the section is to be at once mounted (which should always be done if it is very delicate and liable to be spoiled by manipulation) it should be placed on a slide before it has thawed, and washed by forming around it a little pool of dilute spirit, which may be readily changed two or three times by the glass syringe (§ 127). Sections cut by the freezing process may for the most part be mounted in glycerine jelly, for which no other preparation will be needed than the use (if desired) of the Staining process hereafter to be described (§ 202). But if, for the sake of rendering the sections more transparent, mounting them in Canada balsam or Dammar is preferred, they must be treated first with strong spirit, then with absolute alcohol, and then with either oil of cloves or oil of turpentine. — It is claimed by Dr. Rutherford as the special advan- tage of the freezing process, that " delicate organs, such as the retina, the embryo, villi of the intestines, lung, trachea with its ciliated epithe- lium, may all be readily cut without fear of their being destroyed by the imbeddinfic agent. When imbedded in paraffine, very delicate structures are more liable to damage; the villi of the intestine, for instance, being often denuded of their epithelium, and sometimes themselves torn. 192. Grinding and Polishing Sections of Hard Substances, — Sub- stances which are too hard to be sliced in a Microtome — such as Bones, Teeth, Shells, Corals, Fossils of all kinds, and even some dense Vegetable Tissues — can only be reduced to the requisite thinness for Microscopical examination, by grinding-down thick sections until they become so thin as to be transparent. General directions for making such preparations ^ This instrument has received various improvements since it was first devised, and should be obtained from Mr. Gardner, South Bridge, Edinburgh, — the maker recommended by its inventor. It may be employed also as an ordinary * imbed- ding' Microtome, when the 'imbedding' is thought preferable to the freezing process. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 197 will be here given;^ but those special details of management which par- ticular substances may require, will be given when these are respectively described. — The first thing to be done will usually be to procure a section of the substance, as thin as it can be safely cut. Most substances not sili- ceous may be divided by the fine Saws used by artisans for cutting brass; and these may be best worked either by a mechanical arrangement such as that devised by Dr. Matthews/ or, if by hand, between ' guides,' such as are attached for this purpose to Hailes's and some other Microtomes. But there are some bodies (such as the Enamel of Teeth, and Porcellane- ous Shells), which, though merely calcareous, are so hard as to make it very difiicult and tedious to divide them in this mode; and it is much the quicker operation to slit them with a disc of soft iron (resembling that used by the Lapidary) charged at its edge with diamond-dust, which disc may be driven in an ordinary lathe. Where waste of material is of no ac- count, a very expeditious method of obtaining pieces fit to grind down, is to detach them from the mass with a strong pair of ^ cutting pincers, or, if they be of small dimensions, with ^ cutting pliers;' and a flat sur- face must then be given to it, either by holding them to the side of an ordinary grindstone, or by rubbing on a plate of lead (cast or planed to a perfect level) charged with emery, or by a strong-toothed file; the former being the most suitable for the hardest substances, the latter for the toughest. There are certain substances, especially Calcareous Fossils of Wood, Bone, and Teeth, in which the greatest care is required in the performance of these preliminary operations, on account of their extreme friability; the vibration produced by the working of the saw or the file, or by grinding on a rough surface, being sufiicient to disintegrate even a thick mass, so that it falls to pieces under the hand; such specimens, therefore, it is requisite to treat with great caution, dividing them by the smooth action of the wheel, and then rubbing them down upon nothing rougher than a very fine ^grit,' or on the ^corundum-files ' now sold in the tool-shops, which are made by imbedding corundum of various degrees of fineness in a hard resinous substance. Where (as often hap- pens) such specimens are suflBciently porous to admit of the penetration of Canada Balsam, it will be desirable, after soaking them in turpentine for a while, to lay some liquid balsam upon the parts through which the section is to pass, and then to place the specimen before a fire or in an oven for some little time, so as first to cause the balsam to run-in, and then to harden it; by this means the specimen will be rendered much more fit for the processes it has afterwards to undergo. — It not unfre- quently happens that the small size, awkard shape, or extreme hardness of the body, occasions a difficulty in holding it either for cutting or grind- ing; in such a case, it is much better to attach it to the glass in the first instance by any si'de that happens to be flattest, and then to rub it down by means of the 'hold' of the glass upon it, until the projecting portion has been brought to a plane, and has been prepared for permanent attach- ment to the glass. This is the method which it is generally most conve- nient to pursue with regard to small bodies; and there are many which can scarcely be treated in any other way than by attaching a number of ^ The following directions do not apply to Siliceous substances; as sections of these can only be prepared by those who possess a regular Lapidary's apparatus, and have been specially instructed in the use of it. 2 Journ. Quekett Microsc. Club," Vol. vi. (1880), p. 83. 198 THE MICROSCOPE AND ITS REVELATIONS. them to the glass at once, in such a manner as to make them mutually support one another/ 193. The mode in which the operation is then to be proceeded with, depends upon whether the section is to be ultimately set up in Canada balsam (§ 210), or is to be mounted ' dry' (§ 169), or in fluid (§ 211). In the former case, the following is the plan to be pursued: — The flat- tened surface is to be polished by rubbing it with water on a ^ Water-of- Ayr ' stone, or on a hone or ' Turkey '-stone, or on an ' Arkansas '-stone; the first of the three is the best for all ordinary purposes, but the two lat- ter, being much harder, may be employed for substances which resist it.^ When this has been sufficiently accomplished, the section is to be attached with hard Canada balsam to a slip of thick well-annealed glass; and as the success of the final result will often depend upon the completeness of its adhesion to this, the means of most effectually securing that adhesion will now be described in detail. The slide having been placed on the cover of the Water-bath, and the previously-hardened balsam having been softened by the immersion of the jar containing it in the bath itself, a sufficient quantity of this should be laid on the slide to form, when spread out by liquefaction, a thick drop somewhat larger than the surface of the object to be attached. The slide should then be allowed to cool, in order that the hardness of the balsam should be tested. If too soft, as indicated by its ready yielding to the thumb-nail, it should be heated a little more, care being taken not to make it boil so as to form bubbles; if too hard, which will be shown by its chipping, it should be re-melted and diluted with more fluid balsam, and then set aside to cool as before. When it is found to be of the right consistence, the section should be laid upon its surface with the polished side downwards; the slip of glass is next to be gradually warmed until the balsam is softened, special care being taken to avoid the formation of bubbles; and the sec- tion is then to be gently pressed down upon the liquefied balsam, the pressure being at first applied rather on one side than over its whole area, so as to drive the superfluous balsam in a sort of wave towards the other side, and an equable pressure being finally made over the whole. If this be carefully done, even a very large section may be attached to glass with- 1 Thus, in making horizontal and vertical sections of Foraminifera^ as it would be impossible to slice them through, they must be laid close together in a bed of hardened Canada Balsam on a slip of glass, in such positions, that when rubbed down, the plane of section shall traverse them in the desired directions; and one flat surface having been thus obtained for each, this must be turned downwards, and the other side ground away. The following ingenious plan was suggested by Dr. Wallich (**Ann. of Nat. Hist., July, 1861, p. 58), for turning a number of minute objects together, and thus avoiding the tediousness and difficulty of turn- ing each one separately; — The specimens are cemented with Canada Balsam, in the first instance, to a thin film of mica, which is then attached to a glass slide by the same means; when they have been ground-down as far as may be desired, the slide is gradually heated just sufficiently to allow of the detachment of the mica- film and the specimens it carries; and a clean slide with a thin layer of hardened balsam having been prepared, the mica-film is transferred to it with the ground surface downwards. When its adhesion is complete, the grinding may be pro- ceeded with; and as the mica-film will yield to the stone without the least diffi- culty, the specimens, now reversed in position, may be reduced to requisite thinness. 2 As the flatness of the polished surface is a matter of the first importance, that of the Stones themselves should be tested from time to time; and whenever they are found to have been rubbed down on any part more than on another, they should be flattened on a paving-stone with fine sand, or on the lead-plate with emery. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 199 out the intervention of any air-bubbles; if, however, they should present themselves, and they cannot be expelled by increasing the pressure over the part beneath which they are, or by slightly shifting the section from side to side, it is better to take the section entirely olf, to melt a little fresh balsam upon the glass, and then to lay the section upon it as before. 194. When the section has been thus secured to the glass, and the attached part thoroughly saturated (if it be porous) with hard Canada balsam, it may be readily reduced in thickness, either by grinding or filing, as before, or, if the thickness be excessive, by taking oif the chief part of it at once by the slitting wheel. So soon, however, as it approaches the thinness of a piece of ordinary card, it should be rubbed down with water on one of the smooth stones previously named, the glass slip being held beneath the fingers with its face downwards, and the pressure being applied with such equality that the thickness of the sec- tion shall be (as nearly as can be discerned) equal over its entire surface. As soon as it begins to be translucent, it should be placed under the Microscope (particular regard being liad to the precaution specified in § 143), and note taken of any inequality; and then, when it is again laid upon the stone, such inequality may be brought down by making special pressure with the forefinger upon the part of the slide above it. When the thinness of the section is such as to cause the water to spread around it between the glass and the stone, an excess of thickness on either side may often be detected by noticing the smaller distance to which the liquid extends. In proportion as the substance attached to the glass is ground away, the superfluous balsam which may have exuded around it will be brought into contact with the stone; and this should be removed with a knife, care being taken, however, that a margin be still left round the edge of the section. As the section approaches the degree of thin- ness which is most suitable for the display of its organization, great care must be taken that the grinding process be not carried too far; and fre- quent recourse should be had to the Microscope, which it is convenient to have always at hand when work of this kind is being carried on. There are many substances whose intimate structure can only be displayed in its highest perfection, when a very little more reduction would destroy the section altogether; and every Microscopist who has occupied himself in making such preparations, can tell of the number which he has sacrificed in order to attain this perfection. Hence, if the amount of material be limited, it is advisable to stop short as soon as a good section has been made, and to lay it aside — letting well alone' — whilst the attempt is being made to procure a better one; if this should fail, another attempt may be made, and so on, until either success has been attained, or the whole of the material has been consumed — the first section, however, still remaining: whereas, if the first, like every subsequent section, be sacri- ficed in the attempt to obtain perfection, no trace will be left ^^to show what once has been." In judging of the appearance of a section in this stage under the Microscope, it is to be remembered that its transparence will subsequently be considerably increased by mounting in Canada balsam: this is particularly the case with Fossils to which a deep hue has been given by the infiltration of some coloring matter, and with any sub- stances whose particles have a molecular aggregation that is rather amorphous than crystalline. When a sufficient thinness has been at- tained, the section may generally be mounted in Canada balsam; and the mode in which this must be managed will be detailed hereafter (§ 210). 200 THE J^lICROSCOl-E ANV ITS REVELATIONS. 195. By a slight variation in the foregoing process, sections may be made of structures, in which (as in Corals) hard and soft parts are com- bined, so as to show both to advantage. Small pieces of the substance are first to be stained thoroughly (§ 202), and are then to be ' dehydrated ' by alcohol (§ 190). A thin solution of copal in chloroform is to be pre- l^ared, in which the pieces are to be immersed; and this solution is to be concentrated by slow evaporation, until it can be drawn out in threads which become brittle on cooling. The pieces are then to be taken out, and laid aside to harden; and when the copal has become so firm that the edge of the finger-nail makes no impression, they are to be cut into slices, and ground down attached to glass, in the manner already described, the sections being finally mounted in Canada balsam. — The sections (attached to glass) may be partially or completely decalcified, the soft parts remaining in situ, by first dissolving out the copal with chloro- form; when, after being well washed in water, they should be again stained, and mounted either in weak spirit, or (after having been dehy- drated) in Canada balsam.* 196. A different mode of procedure, however, must be adopted when it is desired to obtain sections of Bone, Tooth or other finely tubular structures, i^/^penetrated by Canada balsam. If tolerably thin sections of them can be cut in the first instance, or if they are of a size and shape to be held in the hand whilst they are being roughly ground down, there will be no occasion to attach them to glass at all: it is frequently conveni- ent to do this at first, however, for the purpose of obtaining a ' hold ^ upon the specimen; but the surface which has been thus attached must after- wards be completely rubbed away, in order to bring into view a stratum which the Canada balsam shall not have penetrated. As none but sub- stances possessing considerable toughness, such as Bones and Teeth, can be treated in this manner, and as these are the substances which are most quickly reduced by a coarse file, and are least liable to be injured by its action, it will be generally found possible to reduce the sections nearly to the required thinness, by laying them upon a piece of soft cork or wood held in a vice, and operating upon them first with a coarser and then with a finer file. When this cannot safely be carried farther, the section must be rubbed down upon that one of the fine stones already mentioned (§ 193) which is found best to suit it: as long as the section is tolerably thick, the finger may be used to press and move it; but as soon as the finger itself begins to come into contact with the stone, it must be guarded by a flat slice of cork, or by a piece of gutta-percha, a little larger than the object. Under either of these, the section may be rubbed down to the desired thinness; but even the most careful working on the finest- grained stone will leave its surface covered with scratches, which not only detract from its appearance, but prevent the details of its internal structure from being as readily made out as they can be in a polished section. This polish may be imparted by rubbing the section with putty-powder (peroxide of tin) and water upon a leather strap, made by covering the surface of a board with buff-leather, having three or four thicknesses of cloth, flannel, or soft leather beneath it: this operation must be performed on both sidefs of the section, until all the marks of the scratches left by the stone shall have been rubbed out; when the ^ See Koch in Zoologischer Anzeig.," Bd. i., p. 36.— The Author, having seen (by the kindness of Mr. H. N. Mosely) some sections of Corals prepared by this process, can testify to its complete success. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 201 specimen will be fit for mounting ^dry' after having been carefully cleansed from any adhering particles of putty-powder. 197. Decalcification. — When it is desired to examine the structure of the Organic matrix, in which the Calcareous salts are deposited that give hardness to many Animal and to a few Vegetable structures (such as the true Corallines), these salts must be dissolved away by the action of some Mineral Acid, which may be either Nitric or Hydrochloric. This should be employed in a very dilute state, in order that it may make as little change as possible in the soft tissue it leaves behind. When the Lime is in the state of Carbonate (as, for example, in the skeletons of Echino- ^?er??ZcS Chap, xiy.), the body to be decalcified should be placed in a glass jar or wide-mouthed bottle holding from 4 to 6 oz. of water, and the acid should be added drop by drop, until the disengagement of air- bubbles shows that it is taking effect; and the solvent process should be allowed to take place very gradually, more acid being added as required. When, on the other hand, much of the lime is in the state of Phosphate, as in Bones and Teeth, the strength of the acid solvent must be increased; and for the hardening of the softer parts of the organic matrix, it is desirable that Chromic acid should be used. In the case of small bones, or delicate portions of large (such as the cochlea of the ear), a half per cent solution of chromic acid will itself serve as the solvent; but larger masses require either Nitric or Hydrochloric acid in addition, to the extent of 2 per cent of the former or 5 per cent of the latter. By some the chromic and the nitric or muriatic acid are mixed in the first instance; while by others it is recommended that the bone should lie first in the chromic acid solution for a week or ten days, and that the second acid should be then added. If the softening is not completed in a month, more acid must be added. When thoroughly decalcified, the bone should be transferred to rectified spirit; and it may then be either sliced in the Microtome, or torn into shreds for the demonstration of its lamellae. — Acid solvents may also be employed in removing the outer parts of Calcareous skeletons, for the display of their internal cavities (a plan which the Author has often found very useful in the study of Fora- minifera); or for getting rid of them entirely, so as to bring into com- plete view any ^internal cast^ which may have been formed by the silicification of its originally soft contents (Figs. 332, 337).^ It has been in this mode, even more than by the cutting of thin sections, that the structure of Eozo'on Canadense (Plate xvii. ) has been elucidated by Pro- fessor Dawson and the Author. For the first of these purposes, strong acid should be applied (under the Dissecting Microscope) with a fine camel's hair pencil; and another such pencil charged with water should be at hand, to enable the observer to stop the solvent action wheneve;* he thinks it has been carried far enough. For the second, it is better that the acid should only be strong enough for the slow solution of the shelly substance; as the too rapid disengagement of bubbles often produces displacement of delicate parts of the substituted mineral, whilst, if the acid be too strong, the ' internal cast ' may be altogether dissolved away. 198. Preparation of Vegetable Substances.— Little preparation is required, beyond steeping for a short time in distilled water to get rid of saline or other impurities, for mounting in preservative media specimens of the minuter forms of Vegetable life, or portions of the larger kinds of AlgcB, Fungi, or other succulent Cryptogams. But the Woody struc- tures of Phanerogams are often so consolidated by gummy, resinous, cr 202 THE MICROSCOPE AND ITS REVELATIONS. other deposits, that sections of them should not be cut until they have been softened by being partially or wholly freed from these. Accordingly, pieces of stems or roots should be soaked for some days in water, with the aid of a gentle heat if they are very dense, and should then be steeped for some days in methylated spirit, after which they should again be transferred to water. The same treatment may be applied to hard-coated seeds, the ^stones' of fruit, ^vegetable ivory,' and other like substances. — Some Vegetable substances, on the other hand, are too soft to be cut sufficiently thin without previous hardeniiig, either by allowing them to lose some of their moisture by evaporation, or by drawing it out by steep- ing them in spirit. Either treatment answers very well with such sub- stances as that which forms the tuber of the Potato; sections of which display the starch-grains in situ. Where, on the other hand, it is desired to preserve color, spirit must not be used; and recourse may be- had to Gum-imbedding (§ 191), which is particularly serviceable where the substance is penetrated by air-cavities, as is the case with the Stem of the Rush, the thick leaves of the Water-lily , etc. But where the stain- ing process is to be employed (§ 200), the substance should be previously bleached by the action of chlorine (preferably by Labarraque's chlorinated soda), and then treated with Alcohol for a few hours. 199. Hardening of Animal Substances. — Save in the case already treated of (§ 192), in which the tissues are consolidated by Calcareous deposit, the preparatory treatment of Animal substances consists in hardening them. The very soft tissues of which most of the lower Animals are composed, contain so large a proportion of Water, that the withdrawal of this by immersion in strong spirit causes them to shrink so much as completely to obscure their structure. Nothing has yet been found so serviceable in preserving them as Osmic acid ; the poisonous action of which at once kills living Infusoria, etc., Echinoderm or Annelid larvae, and the like; and hardens their delicate organisms, so as to allow them to be afterwards stained and preserved with very little change; and thus many points of their structure can be better made out in their ^mounted' than in their living state. The special procedures which have been successfully worked out by M. Oertes for Infusoria, and by Mr. Percy Sladen for Echinoderm larvce, will be described under those heads. — The hardening of the general body-substance of the larger Invertehrata is for the most part sufficiently effected by the action of the Alcoholic spirit in which they are usually preserved; and this may be carried farther, if required, by steeping them for a time in absolute Alcohol. For hardening particular tissues, however, such as Nerves, recourse must be had to some of those hardening agents, used in the preparation of the Tissues of the higher Animals, which will be now specified: — a. Alcohol. — For hardening purposes, Rectified spirit should be used in pref- erence to methylated; and its action is (as a rule) most beneficial after some of the other hardening agents have been employed. The substance to be hardened should be first placed for a day or two in a mixture of equal parts of rectified spirit and water, then transferred for about 48 hours to rectified spirit, and from this to absolute alcohol. — One injurious effect of this treatment is, that by the coagulation of their albuminous components many textures are rendered opaque: but, as Dr. Beale pointed out, this may be corrected by the addition of a little caustic Soda, which must be made, however, with great caution. — When the Alcoholic treatment is used merely for so dehydrating sections pre- viously immersed in watery solutions, that they may be mounted in Canada balsam or Dammar, they may be transferred at once from rectified spirit to oil of turpentine, without treating them with absolute alcohol. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 203 &. Chromic Acid, which is one of the most generally useful of hardening agents, is most conveniently kept in a 1 per cent solution, which may be diluted with several times its volume of water, with or without the addition of spirit. Although its hardening action may be effected by a strong solution in two or three days, it is far better to prolong the process by using the menstruum weak, especially when the substance is in mass; since, if its exterior be so hardened as to prevent the penetration of the fluid, its interior will soften and decay. The following is the mode of procedure most generally approved: — The menstruum having been prepared by mixing two parts of a l-6th per cent solution of chromic acid and one part of methylated spirit, the material must be cut into small pieces about half an inch square, and put into a wide-mouthed stoppered bottle holding from 6 to 10 ozs. of the fluid; this fluid should be changed at the end of 24 hours, and then every third day; and the material will be probably found sufficiently hardened (which must be ascertained by trying whether a tol- erably thin hand-section can be made with a razor) in the course of from 8 to 13 days. If not, the process must be continued, care being taken that it be not so prolonged as to render the substance brittle. The hardening may afterwards be completed by transferring the substance first into dilute and then into stronger spirit; and this will get rid of the color given by the chromic acid, as well as of other flocculent matter. The spirit must be changed as often as it becomes foul and discolored; and when it remains bright and clear, the specimens will be ready for cutting. c. Bichromate of Potass, in a 3 per cent watery solution, may be used where very slow and prolonged hardening is required. With the addition of 1 per cent of sulphate of soda, it constitutes Muller's Fluid, which may be conveniently used to harden large pieces that may be left in it for several weeks; no change of the fluid being necessary after the first week. — The hardened substance, after being well washed, is to be treated with spirit, as in the preceding case. d. Picric or Carbazotic Acid is used for the same purposes as Chromic acid; its hardening power is not so great, but it does not shrivel the tissues as much, its action is more rapid, and it may be advantageously used where * decalcifica- tion ' is necessary (§ 197). As it is but slightly soluble in water, a cold-water so- lution must be saturated; and the quantity of liquid should be large in propor- tion to that of the substance to be acted-on. — Picric acid is used, in combination with Carmine or Aniline-blue, as a staining material (§ 203, b). e. Kleinenberg's Fluid. — The following method of preparing delicate and per- ishable tissues is strongly recommended by Kleinenberg, who has had much experience of it in his investigations on the anatomy of the lower Invertebrata: — To a saturated solution of picric acid in distilled water, add 2 per cent of con- centrated sulphuric acid; all the picric acid which is precipitated must be re- moved by filtration. One part of the filtrate is to be diluted with 3 parts of water; and, finally, as much pure kreosote must be added as will mix. The object to be preserved must remain in this liquid for 3, 4, or more hours; and is then to be transferred for 5 or 6 hours into 70 per cent alcohol, and thence removed into 90 per cent alcohol, which should be changed until it ceases to acquire a yellow tint. /. Osmic Acid — This agent is one of peculiar value to the Microscopist whose studies lie among the lower forms of Animal and Vegetable life; as its applica- tion immediately kills them, without producing any retraction or shrinking of their parts, and only not preserves their tissues, but brings out differences in those which might otherwise escape observation. It is sold in the solid state in sealed* tubes; and is most conveniently kept as a 1 per cent solution in distilled water. The solution should be preserved in well-stoppered bottles secluded from the light; and should be used with great caution, as it gives forth a pungent vapor which is very irritating to the eyes and nostrils. It is recommended by Dr. Pelletan,! M. Certes,^ and M, Raphael Blanchard,^ for fixing and preserving Animalcules {Infusoria and Rotifera), Desmidiece, Diatomacece, Bacteria, and Vi- briones, etc.; by Dr. Vignal^ for Noctiluca ; by Mr. T. Jeffrey Parker^ for Ento- mostraca and other small Crustacea ; and it has been successfully used also in the preparation of Insect structures. To the Histologist its special value lies m 1 Journ. of Rov. Microsc. Soc," Vol. i. (1878), p. 189. 2 Ibid., Vol. ii. (1879), p. 331, and *Comptes Rendus,' 1879, p. 433. 3 Ibid , Vol. ii. (1879), p. 463. 4 Robin's Archives de Physiologic," Tom. xiv. (1878), p. 586. 6 Journ. of Roy. Microsc. Soc," Vol. ii. (1879), p. 381. 204: THE MICROSCOPE AND ITS REVELATIONS. its blackening of fatty matters and the medullary substance of nerve-fibres. And the Embryologist finds it of peculiar value in giving firmness and distinct- ness to the delicate textures with which he has to deal. Various degrees of dilu- tion of the 1 per cent solution will be needed for these different purposes. Mr. Parker further states (Zoc. cit.) that he has found this agent very service- able in the preparation of delicate Vegetable structures. The acid seems to be taken up by each granule of the protoplasm, and these to be decomposed, giv- ing to the granule the characteristic gray color, thus at the same time both hardening and staining." — A mixture of 9 parts of a l-4th per cent, solution of Chromic acid, with one part of a 1 per cent solution of Osmic acid, answers for many purposes better than osmic acid alone, the brittleness produced by its use being completely avoided. — After being subjected to this agent, the specimens should be treated with 30 per cent alcohol, gradually increased in strength to absolute. 200. Staining Processes, — Much attention has been given of late years to the use of agents, which, either by simply dyeing, or by chemi- cally acting on Organic substances, in different modes and degrees, serve to differentiate the different parts of organs or tissues of complex struc- ture, and to render more distinct such delicate features in preparations mounted in transparent media, as might otherwise escaps notice. The agents which merely dye the tissues are for the most part Coloring mat- ters of Vegetable or Animal origin; those which act upon them chemically are Mineral substances. The dyes need generally to be ^ fixed ^ by some ^mordant;' but the effects of chemical agents are usually permanent. The staining-processes may be used either before or after section -cutting, according to circumstances. Where the substance is in mass, and is not readily penetrable by the staining fluid (which is especially liable to be the case where it has been hardened in chromic acid), it is generally bet- ter to stain the sections after cutting, if they hold sufficiently well together to bear being transferred from one fluid to another. And if the substance is to be imbedded in gum, and cut with the freezing Micro- tome, it is generally preferable to stain the sections after they have been cut; as the processes necessary for the removal of the gum would be likely also to remove the dye. But where the substance to be cut has to be penetrated by wax or paraffine, it is better that the staining should be effected in the first instance. As a general rule, it is better that where the substance is to be stained en masse, the staining fluid should be weak and its action slow; because in that mode the stain is more equably dif- fused. When, on the other hand, the process is made use of with thin sections, it is convenient that the action should be more rapid, and the staining fluid may therefore be stronger; but unless its operation be care- fully watched so as to be stopped at the right stage, the whole tissue may .be deeply dyed, and the value of the selective staining altogether lost. 201. It will generally be found convenient to carry-on the staining of thin sections either in watch-glasses, or in small cups of white porcelain; but care must be taken not to place many sections together so as to lie one upon the other, as this will prevent the staining from being uniform. Small delicate sections may often be advantageously stained upon the glass slides upon which they are to be mounted; a pool of the staining fluid being made upon the slide, to be removed, when the staining has proceeded far enough, by the small glass Syringe (§ 127). It is even possible to stain a section after it has been covered with thin glass, by depositing the fluid in contact with one edge of the glass cover, and drawing it through by applying a bit of blotting-paper to the opposite margin; and the process may thus be performed while the section is actu- ally under observation on the stage of the Microscope, the staining liquid PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 205 being withdrawn in the same manner when the desired effect has been produced, and being replaced by the preservative medium. — For taking- up sections without injury to them, and transferring them from one ves- sel to another, recourse may be advantageously had to the ^lifter' of Dr. Sylvester Marsh ' (Fig. 13?); which is a strip of German silver or copper of the thickness of stout cardboard, 7 inches long and 5-8ths inches broad, each end of which, carefully smoothed and rounded, is to be turned at the distance of 5-8ths inch to an angle of about 35 °. One end is to be left plain, for lifting the section with some of its fluid, when it is to be deposited on a slide; while -the other is perforated for letting the fluid escape, when the section is to be floated-off into a vessel filled with some different fluid. 202. The relative value of different Stain- ing Agents, the best modes of applying them, and the benefits derivable from their use in the study of the minute structure of Man and the higher Animals/ have now been pretty fully determined by Histologists; and consid- Marsh's Section-Lifter. erable progress has also been made in the ap- plication of the differential straining process to the various parts of the higher Vegetable fabrics.' But there is still a wide field which has been as yet but little cultivated, in the application of the staining process to the study of the lower Organisms of both Kingdoms; and every one who is engaged in the minute investigations of any particular group, must work out for himself the modifications which the ordinary methods may require. All that can be here attempted is to give such directions as to the agents to be employed, and the best modes of using them, as are likely to be most generally useful. a. Carmine, — This was one of the first Dyes employed for staining purposes ; >,nd its value was speciahy insisted on by Dr. Beale, as enabling living Pro- toplasm (by him designated * germinal matter,' or * bioplasm ') to be distinguished ;rom any kind of ' formed material.' It has a special affinity for cell-nuclei (pro- toplasts) and the axial cylinders of white nerve-fibres; and thus, if the substance lo be stained be only left in the carmine fluid long enough for it to dye these «jubstances, they are strikingly differentiated from all others. It is essential that ihe fluid should have a sHght alkaline reaction, especially where the substance has been hardened with chromic acid. The presence of too much alkali is inju- rious ; the want of it, on the other hand, causes the dye to act on the tissues generally, and thus negatives its differentiating effect. Dr. Beale directs it to be prepared as follows:— Ten grains of Carmine in small fragments are to be placed in a test-tube, and half a drachm of strong Liquor Ammonias added ; by agitation and the heat of a spirit-lamp the carmine is soon dissolved, and the liquid, after boiling for a few seconds, is to be ahowed to cool. After the lapse of an hour, much of the excess of ammonia wiU have escaped; and the solution is then to be mixed with 3 oz. of DistUled Water, 2 oz. of pure Glycerine, and \ oz. of Alcohol. The whole may be passed through a filter, or, after being allowed to stand for 1 See his useful little Treatise on Section-Cutting." ^ , ^ «See the Treatises on Practical Histology" by Prof. Putherford, Prof. Schafer, Dr. Heneage Gibbes, Prof. Ranvier, Prof. Frey, and others; How to Work with the Microscope " by Dr. Beale; and Davies's Preparation and Mount- ing of Microscopic Objects " (2d Edition, edited by Dr. Matthews). 3 This has been chiefly carried out in the United States by Dr. Beatty, Mr. Walmsley, and Mr. Merriman, whose processes are described in the successive volumes of the American Journal of Microscopy." 206 THE MICROSCOPE AND ITS REVELATIONS. some time, the perfectly clear supernatant fluid may be poured off and kept for use. If, after long keeping, a little of the carmine should be deposited through the escape of the Ammonia, the addition of a drop or two of Liquor Ammonias will redissolve it. Prof. Rutherford recommends that, for slow but more certain staining, the liquid should at once be^put into a stoppered bottle, so as not to allow the ammonia to evaporate, and should be diluted by the addition of from two to seven volumes of water. Carmine is used as a general stain in * double staining ' (§ 203); and a suitable fluid for this purpose is made by mixing 30 grains of carmine with 2 drachms of borax, and 4 fl. oz. of water, and pouring off the clear supernatant fluid. To fix the stain of carmine, the section should be immersed for a few minutes in a mixture of five drops of glacial Acetic acid and 1 oz. of water. b. Picro-Carminate of Ammonia, known as Picro-Carmine, is a very excellent staining material, which is applicable to a great variety of purposes. Being somewhat difficult to prepare, it is best purchased ready for use (from Martin- dale, New Cavendish Street). About ten drops should be filtered into a watch- glass, and diluted with distilled water; the sections should remain in the solution for from 20 to 30 minutes; and if at the end of that time they should not be suf- ficiently stained, a little more picro-carmine should be added. This dye, used alone, produces a double staining; nuclei fixing upon the carmine, while other tissues are colored yellow by the picric acid. If the sections be placed in methy- lated spirit, they may be kept without loss of color, and may be afterwards subjected to other processes. If placed in water, the picric acid stain is removed, while the carmine is left. c. Hcematoxylin, or Extract of Logwood, is now employed more generally than carmine (which it much resembles in action), its violet color being more pleasant to the eye. The following is given by Kleinenberg as the best mode of preparing it: — Make a saturated solution of crystallized chloride of calcium in 70 per cent alcohol; mix one volume of this solution with from 6 to 8 volumes of a saturated solution of alum in 70 per cent alcohol; and having half-filled a watch- glass with this mixture, pour into it as many drops of a concentrated solution of Haematoxylin in absolute alcohol as will serve to give the required intensity of color. The object must remain in the dye for a period varying from a few minutes to six hours, according to its size and the nature of the tissues composing it, and is then to be washed in water. If it should be stained throughout, and it be desired that only tissues to be specially distinguished should retain their color, the diffused stain may be removed by immersion in rectified or methylated spirit, or in a 1-half per cent solution of alum. — The following is another formula given by Dr. Gibbes: — Mix 6 grammes of Extract of Logwood (as obtainable from Martin- dale, New Cavendish Street) with 18 grammes of alum, and add 28 cub. centim. of distilled water. Filter, and add to the filtrate 1 drachm of spirit. Keep in a stoppered bottle a week before using. If what remains on the filter be mixed with 14 cub. centim. of distilled water, and, after soaking for an hour or two, be filtered, and ^ drachm of spirit be then added, a second solution will be made aa strong as the first. From 7 to 10 drops of this solution are to be diluted with a watch-glassful of distilled water; the best degree of dilution being only to be found by trial. All staining fluids of this kind are liable to change by keeping; a portion of the coloring matter passing out of solution, and being deposited on the sides and bottom of the vessel containing it. A deposit of the same kind is liable to occur on the specimen during the staining, especially if the process be prolonged; and it is better in such cases at once to transfer the specimen to a fresh solution. When sufficiently stained, the specimens may be treated with methylated spirit, which will fix the color; whilst, if the staining has been carried too far, the excess of color may be removed by the Acetic acid mixture which is used to flx carmine. — If the substance to be stained with Logwood should have been previously hardened with chromic acid, it should be previously steeped in a weak solution of bicarbonate of soda. d. Magenta has nearly the same selective staining property as carmine; and is useful in the examination of specimens for which rapid action and sharp definition are required. But, like other Aniline dyes, it is liable to fade; and should, there- fore, not be employed for permanent preparations. Ordinary magenta fluid may be prepared by dissolving 1 J grains of magenta crystals in 7 fl. oz. of distilled water, and adding ^ fl. oz. of rectified spirit. The color of a section stained with this may be preserved for some time, by immersing it in a l-3d per cent watery solution of corrosive sublimate. e. Eosin, which dyes the tissues generally of a beautiful garnet-red color, PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 207 should be used in a strong watery solution; and the sections must be well washed in water after staining. Its chief use is in ' double staining' (§ 203). /. For blue and green staining, the various Aniline dyes are principally used. They are, for the most part, however, rather fugitive in tlieir effects; not forming durable combinations with the tissues they stain. Some of them are soluble in water, others only in spirit; and the selection between the dyes of these two classes will have to be guided by the mode in which the preparations are treated. These dyes are for the most part best fixed by benzole; and as the sections treated with this fluid may be at once mounted in Canada balsam, there is greater probability of their colors being preserved. Besides blue and green, the Aniline series fur- nishes a deep rich brown, known as Bismarck's Brown; and a blue-blacky which has been recommended for staining nerve-cells. g. A good blue stain (tending to purple) is also given by the substance termed Indigo-Carmine; which is particularly recommended for sections of the brain and spinal cord that have been hardened in chromic acid. A saturated solution of the powder in distilled water having been prepared, this may either be used with the addition of about 4 per cent of oxalic acid ; or, if an alcoholic fluid be pre- ferred, methylated spirit may be added to the aqueous solution, the mixture being filtered to remove any coloring matter that may have been precipitated. If sec- tions thus stained have an excess of color, this may be removed by the action of a saturated solution of oxalic acid in alcohol. h. A beautiful green hue is given by treating with a saturated solution of Picric acid in water, sections previously stained with Aniline blue; or the two agents may be used together, 4 or 5 parts of a saturated solution of the latter be- ing added to a saturated aqueous solution of the former. This picro-aniline, it is believed, may be relied on for permanence; and it acts well in double staining with picro-carmine. i. Two chemical agents, Nitrate of Silver and Chloride of Gold, are much used by Histologists for bringing-out particular tissues; the former being especially valuable for the staining of Epithelium-cells; the latter for staining Nerve-cells, Connective-tissue corpuscles, Tendon-cells, and Cartilage-cells. The most advan- tageous use of these can only be made by the careful observance of the directions which will be found in treatises on Practical Histology. k, Molyhdate of Ammonia is recommended as affording a cool blue-gray or neutral-tint general stain, which affords a pleasant ' ground ' to pares strongly colored by bright selective stains. 203. Double and Triple Staining, — Very instructive as well as beauti- ful effects are produced by the simultaneous or successiye action of two or three staining fluids; which will respectively pick out (so to speak) the parts of a section for which they have special affinities. Thus, if a section through the base of the tongue of a cat or dog, be stained with picro- carmine, rosein, and iodine-green, the muscles-fibres will take the first, the connective tissue and protoplasm of cells will be colored by the second, while the third will lay hold of the nuclei in the superficial epithelium, serous glands, and non-striated muscle in the vessels; and, further the mucous glands will show a purple formed by the combined action of the red and green (Gibbes).' A very striking contrast of the like kind is shown in the double staining of the frond of a Pern with log- wood and aniline blue; the 80ri taking the latter, and standing out brilliantly on the general surface tinged by the former.— The effects produced by using one stain after the other, are generally much better than those obtained by simultaneous staining. The selective action of a second stain is not prevented by a previous general staining; for the dye which gives the lat- ter seems to be more weakly held by the parts which take the former, so as to be (as it were) displaced by it. Thus, if a section of a Stem be stained throughout by a solution of Eosin (2 grains to 1 oz.), and be then placed, after washing in strong alcohol, in a half -grain solution of Nichol- 1 See his Practical Histology," Chap, v., and his Paper in Journ. of Hoy. Microsc. Soc," Yol. iii. (1880), p. 390. 208 THE MICROSCOPE AND ITS REVELATIONS. son's blue made neutral, the blue will in no long time entirely drive out the red; but by carefully watching the process, it will be seen that the different tissues will change color in different times, the softer cells giving up their red and taking-in the blue more quickly than the harder; so that by stopping the process at the right point (which must be determined by taking-out a section, dipping it in alcohol, and examining it under the microscope), the two kinds of cells are beautifully differentiated by their coloring/ The best effects are usually produced by Carmine and Indigo- carmine, Logwood and Picro-carmine, Carmine or Logwood and Aniline- blue or Aniline-green. But very much has yet to be learned on this sub- ject; and the further investigation of it will be likely to produce results that will amply repay the time and labor bestowed. 204. Chemical Testing, — It is often requisite, alike in Biological and in Mineralogical investigations, to apply Chemical Tests in minute quan- tity to objects under Microscopic examination. Various contrivances have been devised for this purpose; but the Author would recommend, from his own experience, the small glass Syringe already described (Fig. 106), with a fine-pointed nozzle, as the most convenient instrument. One of its advantages is the very precise regulation of the quantity of the test to be deposited, which can be obtained by the dextrous use of it; whilst another consists in the power of withdrawing any excess. Care must be taken in using it, to avoid the contact of the test-liquid with the packing of the piston. — Whatever method is employed, great care should be taken to avoid carrying away from the slide to which the test-liquid is applied, any loose particles which may lie upon it, and which may be thus trans- ferred to some other object, to the great perplexity of the Microscopist. For testing Inorganic substances, the ordinary Chemical Eeagents are of course to be employed; but certain special Tests are required in Biologi- cal investigation, the following being those most frequently required : a. Solution of Iodine in water (1 gr. of iodine, 3 grs. of iodide of potassium, 1 oz. of distilled water) turns Starch blue and Cellulose brown; it also gives an in- tense brown to Albuminous substances. h. Dilute Sulphuric Acid (one of acid to two or three parts of water), gives to Cellulose thsit has been previously dyed with iodine a blue or purple hue; also, when mixed with a solution of sugar, it gives a rose-red hue, more or less deep, with Nitrogenous substances and with bile (Pettenkofer's test). c. What is known as Schulze's Test is a solution of Chloride of Zinc, Iodine, and Iodide of Potassium, made in the following way : — Zinc is dissolved in Hydrochloric acid, and the solution is permitted to evaporate in contact with metallic zinc, until it attains the thickness of a syrup; this syrup is then saturated with iodide of potassium, and iodine is last added. This solution serves . like the preceding, to detect the presence of Cellulose ; and has the advantage over sul- phuric acid of being less destructive to the tissues. Each will sometimes succeed where the other fails; consequently, in doubtful cases, both should be employed, d. Concentrated Nitric Acid gives to Albuminous substances an intense yel- low. e. Acid Nitrate of Mercury (Millon's Test) colors Albuminous substances red. /. Acetic Acid^ which should be kept both concentrated and diluted with from 3 to 5 parts of water, is very useful to the Animal Histologist from its power of dissolving, or at least of reducing to such a state of transparence that they can no longer be distinguished, certain kinds of membranous and fibrous tissues, so that other parts (especially nuclei) are brought more strongly into view. gr. Solution of Caustic Potass or Soda (the latter being generally preferable) has a remarkable solvent effect upon many Organic substances, both Animal and Vegetable; and is extremely useful in rendering some structures transparent, » See Journ. of Roy. Microsc. Soc," Vol. iii. (1880), p. 694. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 209 while others are brought into view,— its special action being upon Tiorn?/ textures, whose component cells are thus rendered more clearly distinguishable. h Ether dissolves Resins, Fats, and Oils; but it will not act on these through membranes penetrated with watery fluid. i. Alcohol dissolves Resins and some Volatile Oils; but it does not act on ordi- nary Oils and Fats. It coagulates Albuminous matters, and consequently renders more opaque such. textures as contain them. The opacity, however, may be re- moved by the addition of a small quantity of Soda. 205. Preservative Media, — We have now to consider the various modes of preserving the preparations that have been made by the several methods now indicated; and shall first treat of such as are applicable to those minute Animal and Vegetable organisms, and to tliose Sections or Dis- sections of large structures, which are suitable for being mounted as transparent objects. A broad distinction may be in the first place laid down between resinous and aqueous preservative media; to the former belong only Canada Balsam and Dammar; whilst tlie latter include all the mixtures of Avhich Water is component. — The choice between the two kinds of media will partly depend upon the nature of the processes to which the object may have been previously subjected, and partly upon the degree of transparence which may be advantageously imparted to it. Sections of substances which have been not only imbedded in, but pene- trated by paraffine, wax, or cacao-butter, and have been stained (if desired) previously to cutting, are, as a rule, most conveniently mounted in Canada balsam or Dammar; since they can bo at once transferred to either of these from the menstruum by which the imbedding material has been dissolved-out. The durability of this method of mounting makes it preferable in all cases to which it is suitable; the exception being where it renders a very thin section too transparent, which is specially liable to happen with Dammar. — When it is desired to mount in either of these media Sections of structures that have been imbedded in gum or gelatine, these substances must first be completely dissolved-out by steeping in water; the sections must then be dehydrated ^ by subjecting them to mixtures of spirit and water progressively increased in strength to absolute alcohol; and after this has been effected, they are to be transferred to tur- pentine, and thence to benzole. In this process much of the staining is apt to be lost; so that stained sections are often more advantageously mounted in some of those aqueous preparations of Glycerine, which approach the resinous media in transparance and permanence. — When Canada balsam was first employed for mounting preparations, it was em- ployed ia its natural semifluid state, in which it consists of a solution of resin in volatile oil of turpentine; and unless a large proportion of the latter constituent was driven off by heat in the process of mounting (bubbles being thus formed of which it was often difficult to get rid), or the mounted slide was afterwards subjected to a more moderate heat of long continuance, the balsam would remain soft, and the cover liable to displacement. This is avoided by the method now generally adopted, of previously getting rid of the turpentine by protracted exposure of the balsam to a heat not sufficient to boil it, and dissolving the resin tlius obtained either in benzole or chloroform, the solution being made (with the aid of gentle heat) of such viscidity as will allow it to 'run 'freely when slightly warmed. Either of these sol vents evaporates so much more quickly than turpentine, that the balsam left behind hardens in a com- paratively short time. — The natural Balsam, however, may be preferably used (with care to avoid the liberation of bubbles by overheating) in mounting sections already cemented to the slides by hardened balsam 14 210 THE MICROSCOPE AND ITS REVELATIONS. (§ 193); and also for mounting the chitinous textures of Insects, which it has a peculiar power of rendering transparent, and which seem to be penetrated by it more thoroughly than they are by the artificially-prepared solution (§ 210). — The solution of Dammar in benzole is very convenient to work with, and hardens quickly. 206, The following are the principal aqueous media whose value has been best tested by general and protracted experience: — a. Fresh specimens of minute Protophytes can often be very well preserved in in Distilled Water saturated with Camphor; the complete exclusion of air serving botli to check their living actions and to prevent decomposing changes. When the preservation of color is not a special object, about a tenth part of Alcohol may be added, and this will be found a suitable medium for the preservation of many delicate Animal textures. b. Aqueous Solution of Carbolio Acid. — Even the very small quantity of this agent which cold water will take up, has a powerful preservative effect; and the solution may be advantageously employed for mounting preparations of many delicate structures, both Animal and Vegetable. c. The same may be said of Salicylic Acid, which has been very successfully employed for delicate preparations in th3 small proportion that will dissolve in cold water. For coarser structures a stronger solution is preferable; and this may be made by combining with the acid a small quantity either of borax dissolved in glycerine or of acetate of potass. d. Where the preservation of minute histological detail is not so much desired, as the exhibition of larger structural features of objects to be viewed by reflected light, nothing is better than Dilute Spirit; the proportion most generally service- able being 1 of Alcohol to 4 or 5 of water; and an even weaker mixture serving to prevent further change in tissues already hardened by strong Alcohol. The Author has a series of the beautiful Pentacrinoid larvae of Comatula (Plate xxi.) thus preserved in cells twenty years ago; which are as perfect as when f ist mounted. These weaker mixtures have no action on Gold-Size. Of late years, Glycerine has been largely used as a preservative; either alone, according to the method of Dr. Beale (§ 208), or diluted with water, or mixed with gelatinous substances. — It is much more favorable to the preservation of color than most other media; and is therefore spe- cially useful as a constituent of fluids used for mounting Vegetable objects in their natural aspects. It has also the property of increasing the trans- parence of Animal structures, though in a less degree than resinous sub- stances; and may thus be advantageously employed as a component of media for mounting objects that are rendered too transparent by Balsam or Dammar. — Two cautions should be given in regard to the employment of Glycerine; -first, that, as it has a solvent power for Carbonate of Lime, it should not be used for mounting any object having a calcareous skeleton; and second, that in proportion as it increases the transparence of organic substances, it diminishes the reflecting power of their surfaces, and should never be employed, therefore, in the mounting of objects to be viewed by reflected light, although many objects mounted in the media to be pres- ently specified are beautifully shown by ^black-ground' illumination. e. A mixture of one part of Glycerine and two parts of Camphor- water may be used for the preservation of many Vegetable structures. /. For preserving soft and delicate Marine Animals which are shrivelled-up, so to speak, by stronger agents, the Author has found a mixture of 1 part of Gly- cerine and 1 of Spirit with 8 or 10 parts of Sea Water, the most suitable preser- vative. g. For preserving minute Vegetable preparations, the following method, devised by Hantzsch, is said to be peculiarly efficient: — A mixture is made of 3 parts of pure Alcohol, 2 parts of Distilled Water, and 1 part of Glycerine; and the object, laid in a cement-cell, is to be covered with a drop of this liquid, and then put aside under a bell-glass. The Alcohol and Water soon evaporate, so that the Glycerine alone is left; and another drop of the liquid is then be added, and a PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 211 second evaporation permitted; the process being repeated, if necessary, until enough Glycerine is left to fill the cell, which is then to be covered and closed in the usual mode. ^ h. The Glycerine Jelly prepared after the manner of Mr. Lawrence may be strongly recomaiended as suitable for a great variety of objects. Animal as well as Vegetable, subject to the cautions already given: — Take any quantity of Nel- son's Gelatine, and let it soak for two or three hours in cold water, pour off the superfluous water, and heat the soaked gelatine until melted. To each fluid ounce of the Gelatine add one drachm of Alcohol, and mix well; then add a fluid drachm of the white of an egg. Mix well while the Gelatine is fluid, but cool. Now boil until the albumen coagulates, and the Gelatine is quite clear. Filter through fine flannel, and to each fluid ounce of the clarified Gelatine add six fluid drachms of Price's pure Glycerine, and mix well. For the six fiuid drachms of Glycerine, a mixture of two parts of Glycerine to four of Camphor-water may be substituted. The objects intended to be mounted in this medium are best prepared by being immersed for some time in a mixture of one part of Glycerine with one part of diluted Alcohol (1 of alcohol to 6 of water)." ^ A small quan- tity of Carbolic acid may be added to it with advantage. When used, the jelly must be liquefied by gentle warmth, and it is useful to warm both the slide and the cover-glass previously to mounting — This takes the place of what was for- merly known as Deane's Medium, in which honey was used to prevent the har- dening of the gelatine. i. For objects which would be injured by the small amount of heat required to liquefy the last-mentioned medium, the Glycerine and Gum Medium of Mr. Farrants will be found very useful. This is made by dissolving 4 parts (by weight) of picked Gum Arabic in 4 parts of cold Distilled Water, and then add- ing 2 parts of Glycerine. The solution must be made without the aid of heat, the mixture being occasionally stirred, but not shaken, whilst it is proceeding: after it has been completed, the liquid should be strained (if not perfectly free from impurity) through fine cambric previously well washed out by a current of clean cold water; and it should be kept in a bottle closed with a glass stopper or cap (not with cork), containing a small piece of Camphor. — The great advantage of this Medium is that it can be used cold, and yet soon viscifies without crack- ing; it is well suited to preserve delicate Animals as well as Vegetable tissues, and in most cases increases their transparence. It often is quite impossible to predicate beforehand what preservative medium will answer best for a particular kind of preparation; and it is consequently desirable, where there is no lack of material, to mount simi- lar objects in two or three different ways, marking on each slide the method employed, and comparing the specimens from time to time, so as to judge the condition of each. 207. In dealing with the small quantities of fluid media required in mounting Microscopic objects, it is essential for the operator to be pro- vided with the means of transferring very small quantities from the ves- sels containing them to the slide, as well as of taking up from the slide what may be lying superfluous upon it. Where some one fluid, such as Diluted Alcohol or the Carbolic acid solution, is in continual use, it will be found very convenient to keep it in the small Dropping-bottle repre- sented in Fig. 138. The stopper is perforated, and is elongated below into a fine tube, whilst it expands above into a bulbous funnel, the mouth of which is covered with a piece of thin Vulcanized India-rubber tied firmly round its lip. If pressure be made on this cover with the point of the finger, and the end of the tube be immersed in the liquid, in the bottle, ' See the Rev. W. W. Spicer's Handy-book to the Collection and Preparation of Freshwater and Marine Algae, etc.," pp. 57-59. ''Nothing," says Mr. Spicer, "can exceed the beauty of the preparations of Desmidiacece prepared after Herr Hantzsch's method; the form of the plant and the coloring of the endochrome having undergone no change whatever." 2 A very pure Glycerine jelly, of which the Author has made considerable use, is prepared by Mr. Rimmington, chemist, Bradford, Yorkshire. 212 THE MICROSCOPE AND ITS KEVELATIONS. this will rise into it on the removal of the finger; if, then, the funnel be inverted, and the pressure be re-applied, some of the residual air will be forced out, so that by again immersing the end of the tube, and remov- ing the pressure, more fluid will enter. This operation may be repeat- ed as often as may be necessary, until the bulb is entirely filled; and when it is thus charged with fluid, as much or as little as may be needed is then readily expelled from it by the pressure of the finger on the cover, the bulb being always refilled if care be taken to immerse the lower end of the tube before the pressure is withdrawn. The Author can speak from large experience of the value of this little implement; as he can also of the utility of the small Glass Syringe (§ 127) for the same pur- pose, and this not only for fine Aqueous liquids, but also for Glycerine jelly, and Canada balsam. For these media having been poured, when liquefied by warmth, each into its own syringe (its piston having been previously drawn out), can be forced out as oc- casion requires, by pressure on the replaced piston, which may be graduated with great nicety, when the syringe has been gently warmed by lying for a short time on the AVater-bath cover (§ 177). Farrants's medium may be conveniently used in the same manner. But the solutions of Canada Balsam and Gum Dammar in volatile fluids will not be sufficiently secure from change by evaporation through the point of the syringe; and are better kept in wide-mouthed capped jsiYS, the liquid being taken-out on a pointed glass rod, or ^ stirrer^ cut to such a length as will enable it to stand in the jar when its cap is in place. — Great care should be taken to keep the inside of the cap and the part of the neck of the jar on which it fits, quite dean^ so as to prevent the fix- ation of the neck by the adhesion between these two surfaces. Should such adhesion take place, the cautious application of the heat of a spirit- lamp will usually make the cap removable. In taking out the liquid, care should be taken not to drop it prematurely from the rod, — a mis- chance which may be avoided by not taking up more than it will properly carry, and by holding it in a horizontal position, after drawing it out of of the bottle, until its point is just over the slip or cover on which the liquid is to be deposited. 208. Mountijig Thin Sectio?is. — The thin sections cut by the Micro- tome, or membranes obtained by Dissection, do not require to be placed in cells when mounted in any viscid medium; since its tenacity will serve to keep off injurious pressure by the cover-glass. When the preparation has been previously immersed in Aqueous liquids, and is to be mounted in glycerine, glycerine jelly, or Farrants's medium, the best mode of placing it on the slide is to float it in a saucer or shallow capsule of water, to place the slide beneath it, and, when the object lies in a suitable position above it, to raise the slide cautiously, holding the object in place by a needle, until it is entirely out of water. The slide is then to be wiped by an absorbent cloth, taking care not to touch the object with it; and the small quantity of liquid still surrounding the object is to be carefully drawn off by a bit of blotting-paper, care being taken not to touch the object with it (as its fibres are apt to adhere), or to leave any loose fibres on the side. Before the object is covered, it should be looked at under a Dissecting or Mounting Microscope, for the purpose of improving (if desirable) its disposition on the slide, and of removing any foreign particles PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 213 that may be accidentally present. A drop of the medium (liquefied, if necessary by a gentle warmth) is then to be placed upon it, and another drop placed on the cover and allowed to spread out. The cover being then taken up with a pair of forceps, must be inverted over the object, and brought to touch the slide at one part of its margin; the slide being itself inclined in the direction of the place of contact, so that the medium accumulates there in a little pool. By gently letting down the cover, a little wave of the medium is pressed before it; and, if enough of the medium has been deposited, the whole space beneath the cover will be filled, and the object completely saturated. If air-bubbles should unfor- tunately show themselves, the cover must be raised at one margin, and a further quantity of the medium deposited. If, again, there are no air- bubbles, but the medium does not extend itself to the edge of the cover, the cover need not be raised, but a little may be deposited at its edge, whence it will soon be drawn in by capillary attraction, especially if a gentle warmth be applied to the slide. It will then be advantageous again to examine the preparation under the Dissecting Microscope, for it will often happen that an opportunity may thus be found of spreading it better, by the application of gentle pressure to one part or another of the covering-glass, which may be done without injurious effect either with a stiff needle or by a pointed stick, a method whose peculiar value, when viscid media are employed, was first pointed out by Dr. Beale. — The slide should then be set aside for a few days, after which its mounting may be completed. Any excess of the medium must first be removed. If Gly- cerine has been employed, much of it may be drawn off by blotting-paper (taking care not to touch the edge of the cover, as it will be very easily displaced); and the remainder may be washed away with a camel-hair brush dipped in water, which may be thus carried to the edge of the cover. The water having been drawn off, a narrow ring of liquefied glycerine- jelly may be made around—not on — the margin of the cover (according to the suggestion of Dr. S. Marsh) for the purpose of fixing it before the cement is applied; and when this has set, the slide may be placed on the Turn-table (§ 176), and the preparation ' sealed ^ by a ring either of Dam- mar or of Bell's cement, which should be carried a little over the edge of the cover, and outside the margin of the ring of glycerine- jelly. This 'ringing' should be repeated two or three times; and if the preparation is to be viewed with ^oil-immerson^ lenses, it should be finished off with a coat of Hollis's glue, which is not attacked by cedar-oil. Until the cover has been perfectly secured, a slide carrying a glycerine preparation should never be placed in an inclined position, as its cover will be almost sure to slide by its own weight. — If Glycerine-jelly or Farrants's medium have been employed, less caution need be used, as the cover-glass, after a few days' setting, will adhere with sufficient firmness to resist displace- ment. The superfluous medium having been removed by the cautious use of a knife, the slide and the margin of the cover may be completely cleansed by a camel-hair brush dipped in warm water; and when quite dried, the slide, placed on the Turn-table, may be sealed with Gold-size, — any other Cement being afterwards added either for additional security or for ^appearance.' 209. When, on the other hand, the Section or other preparation is to be mounted in a Kesinous medium, it must have been previously prepared for this in the modes already described (§§ 190, 191), which will present it to the mounter either in Turpentine or some other essential oil, or in Alcohol. From either of these it may be transferred to the slide by the 214: THE MICROSCOPE AND ITS REVELATIONS. ^ lifter^ (§ 201); its unpevforeited end being employed, so as to carry with the object a small pool of the fluid from which it has been taken. — This will greatly facilitate the transfer of the object from the lifter to the slide; as it may be readily floated off with the aid of a slight touch of a needle. The fluid thus deposited with it having been drained away by blotting- paper, the object may be treated (if desirable for thoroughly clearing it) with a drop of Clove-oil, which should be deposited, not on the object, but near it, and made to run to it by inclining the slide, so as, by running lender it, to rise through it and saturate it thoroughly. After about two minutes, the clove-oil is to be drained away, and the Balsam or Dammar solution applied by the glass rod; one drop being placed on the object, and another on the cover, which is then to be turned and lowered-down on the object in the manner already described. The presence of a few air-bubbles may be here disregarded, as they will ultimately disappear; but care must be taken that the resinous solution not only fills the space between the cover and the slide, but extends beyond its entire margin, as much shrinkage will be produced by the evaporation of the solvent. If this precaution be attended-to, and ' appearance ^ is not a serious consid- eration, nothing more is requisite for the protection of the preparation; since the margin of resin left by the evaporation of its solvent forms an adequate cement, especially if the cover be secured by gummed-paper from being loosened by a ^ jar.' But if it be desired to replace this by a black or colored cement,^ the resin must first be scraped away with the edge of an awP carried alo7ig (not towards) the margin of the cover; and the slide, being then cleaned with benzole, and finally wiped with methy- lated spirit, may finally be ^ ringed ' on the Turn-table. 210. Mounting Objects in Canada Balsam, — Although it is prefer- able for Histological purposes to employ a solution of hardened Balsam, yet as there are many objects for mounting for which the use of the ' natural ' Balsam is preferable, it will be well to give some directions for its use. — When Sections of hard substances have been ground down on the slides to which they have been cemented (§ 194), it is much better that they should be mounted without being detached, unless they have become clogged with the abraded particles, and require to be cleansed out — as is sometimes the case with sections of the shells, spines, etc., of Echinoderms, when the balsam by which they have been cemented is too soft. If the detachment of a specimen be desirable, it may be loosened by heat, and lifted off with a camel-hair brush dipped in Oil of Turpen- tine. But, where time is not an object, it is far better to place the slide to steep in Ether or Chloroform in a capped jar, until the object then falls off of itself the solution of its cement. It may be thoroughly cleansed by boiling it in methylated spirit, and afterwards laid upon a piece of blotting-paper to dry; after which it may be mounted in fresh balsam on a slide, just as if it had remained attached. The slide having been warmed on the water-bath lid, a sufficient quantity of balsam should be pressed out from the syringe on the object; and care should be taken that this, if previously loosened, should be thoroughly penetrated by it. If any air-bubbles arise, they should be broken with the needle-point. ^ The great Scientific investigators of Germany, who cut an entire Worm into thin transverse sections, carefully mounted in their order, would scorn to spend time in such a mere * finish,' which they would consider only worthy of Amateurs. 2 The Author has found this implement, mounted in a small handle, far less liable to disturb the cover, than the ' old penknife,' the slipping of whose point in chip ping-away hard resin has oftened occasioned him much mischief. PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 215 The cover having been similarly warmed, a drop of balsam should be placed on it, and made to spread over its surface; and the cover should then be turned over and let down on the object in the manner already described. If this operation be performed over the water-bath, instead of over the spirit-lamp, there will be little risk of the formation of air- bubbles. However large the section may be, care should be taken that the Balsam is well-spread both over its surface and that of its cover; and by attending to the precaution of making it accumulate on one side by sloping the slide, and letting down the cover so as to drive a wave before it to the opposite side, very large sections may thus be mounted without a single air-bubble. (The author has thus mounted sections of Eozoon three inches square.) — In mounting minute Balsam-objects, such as Diatoms, Polycystina, Sponge-spicules, and the beautiful minute spines of Opliiurida, great advantage will be obtained from following the plan suggested by Mr. James Smith, for which his Mounting Instrument (Fig. 130) is specially adapted. The slide being j^laced upon its slide- plate, and the object having been laid upon the glass in the desired jDOsition, the covering glass is laid upon this, and the ivory knob is to be screwed down, so as, by a very slight pressure on the cover, to keep in its place. The slide is then to be very gently warmed, and the Balsam to be applied at the edge of the cover, from which it will be drawn in by capillary attraction, penetrating the objects, and leaving no bubbles if too much heat be not applied. In this manner the objects are kept exactly in the places in which they were at first laid; and scarcely a particle of superfluous balsam, if due care has been employed, remains on the slide. — When the chitinous textures of Insects are to be thus mounted, they must be first softened by steeping in Oil of Turpentine; and a large drop of Balsam being placed on a warmed slide, the object, taken up in the forceps, is to be plunged in it, and the cover (balsamed as before) let down upon it. It is with objects of this class, that the Spring- Clip (Fig. 128) and the Spring-Press (Fig. 129) prove most useful in holding down the cover until the balsam has hardened suffi- ciently to prevent its being lifted by the elasticity of the object. — Various objects (such as the palates of Gasteropods), which have been prepared by dissection in water or weak spirit, may be advantageously mounted in Balsam; for which purpose they must be first dehydrated, and then transferred from rectified Spirit into Turpentine. Carholio Acid lique- fied by heat has been lately recommended by Dr. Ealph^ as most efficient in drawing out water from specimens to be mounted in Balsam or Dammar, which afterwards readily take its place. — Sections of Horns, Hoofs, etc., which afford most beautiful objects for the Polariscope, are best mounted in natural Balsam, which has a remarkable power of increasing their transparence. — It is better to set aside in a warm place the slides which have been thus mounted, before attempting to clean off the superfluous Balsam; in order that the covers may be fixed by the grad- ual hardening of what lies beneath them. 211. Mou7iting Objects in Aqueous Liquids. — By far the greater number of preparations which are to be preserved in liquid, however, should be mounted in a Cell of some kind, which forms a of suitable depth, wherein the preservative liquid may be retained. This is absolutely necessary in the case of all objects whose thickness is such as to prevent 1 See the accout of Dr. Ralph's method in Journ. of Roy. Microsc. Soc," VoL iii. (1880), p. 858. 2J6 THE MICROSCOPE AND ITS REVELATIONS. the glass-cover from coming into close approximation with the slide; and it is desirable whenever that approximation is not such as to cause the cover to be drawn to the glass-slide by capillary attraction, or whenever the cover is sensibly kept apart from the slide by the thickness of any portion of the object. Hence it is only in the case of objects of the most extreme tenuity, that the Cell can be advantageously dispensed with; the danger of not employing it, in many cases in which there is no difficulty in mounting the object without it, being that after a time the cement is apt to run-in beneath the cover, which process is pretty sure to continue when it may have once commenced. When Cement-cells (§ 170) are em- ployed for this purpose, care must be taken that the surface of the ring is perfectly flat, so that when the cover- glass is laid-on, no tilting is pro- duced by pressure on any part of its margin. As a general rule it is desirable that the object to be mounted should be steeped for a little time previously in the preservative fluid employed. — A sufficient quantity of this fluid being deposited from the Syringe or Dropping-bottle to over-fill the cell, the object is to be introduced into it either with the Forceps or the Dipping-tube (§ 126); and the slide should then be examined on the Dissecting Microscope, that its entire freedom from foreign particles and from air bubbles may be assured, and that its disposition may be corrected if necessary. The cover should then be laid on very cautiously, so as not to displace the object; which in this case is best done by keeping the drop highest in the centre, and keeping the cover parallel to the slide whilst it is being lowered, so as to expel the superfluous fluid all around. This being taken up by the syringe, the cement ring and the margin of the cover are to be dried with blotting-paper, especial care being taken to avoid drawing-ofl too much liquid, which will cause the gold-size to run- in. It is generally best to apply the first coat of Gold-size tlmi, with a very small and flexible brush worked with the hand; this will dry suffi- ciently in an hour or two, to hold the cover whilst being ' ringed ^ on the Turn-table. And it is safer to apply a third coat a day or two afterwards: old Gold-size, which lies thickly, being then applied so as to raise the the ring to the level of the surface of the cover. As experience shows that preparations thus mounted, which have remained in perfectly good order for many years, may be afterwards spoiled by leakage, the Author strongly recommends that to prevent the loss of valuable specimens, an additional coating of gold-size be laid-on from time to time. 212. Mou7iting of Objects in Deep Cells.— objects which require deep cells are, as a rule, such are as to be viewed by reflected light; and are usually of sufficient size and substance to allow of air being entangled in their tissues. This is especially liable to occur where they have under- gone the process of decalcification (§ 198); which will very probably leave behind it bubbles of Carbonic acid. For the extraction of such bubbles, the use of an Air-pump is commonly recommended; but the Author has seldom found this answer the purpose satisfactorily, and is much disposed to place confidence in a method lately recommended— steeping the speci- men in a stoppered jar filled with freshly boiled water, which has great power of drawing into itself either Air or Carbonic acid. Where the structure is one which is not injured by Alcohol, prolonged steeping in this will often have the same effect. — The next point of importance is to select a cover of a size exactly suitable to that of the ring, of whose breadth it should cover about two-thirds, leaving an adequate margin uncovered for the attachment of the cement. And the perfect flatness