THE MICROSCOPE. Polariscope < Objects. L58 I nihil Plate VIM. THE MICROSCOPE: ITS HISTORY, CONSTBUCTION, AND APPLICATION: BEING A FAMILIAR INTRODUCTION TO THE USE OP THE INSTRUMENT, AND THE STUDY OF MICROSCOPICAL SCIENCE. BY JABEZ HOGG, F.L.S., F.RM.S. SECRETARY, ROYAL MICROSCOPICAL SOCIETY ; MEMBER OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND; AUTHOR OF "ELEMENTS OF NATURAL PHILOSOPHY,' " » v>\nu OF OPHTHALMOSCOPIC SURGERY " " WITH UPWARDS OF FIVE HUNDRED ENGRA TINGS, AND COLOURED ILLUSTRATIONS BY TUFFEN WEST. $10^ d&Jbiiwn. LONDON : GEORGE ROUTLEDGE AND SONS, NEW YORK : 416, BROOME STREET. . . "■ 1871. jg6J 4 7. • ■ PREFACE TO THE SIXTH EDITION". N" issuing the Sixth Edition of this Work on the Microscope, we may state that it has been thoroughly revised and for the most part re-written. Eight carefully and beautifully executed Plates are added, which were drawn by Tuffen West from natural objects, engraved and printed by Edmund Evans in the first style of colour- printing. The Author cannot but express his grateful surprise at the extraordinarily popular reception which his 1x)ok has met with : a sale of fifty thousand is an unprecedented event for a work of the kind. This circumstance is extremely gratifying to him, because it affords reasonable grounds for believing that his work has been useful, and encourages renewed effort to make the volume still more acceptable. It has been his endeavour to bring the information contained in its pages up to the most recent discoveries ; although, in a daily progressing field of science, it is almost impossible to keep pace with the advance of knowledge in all its ramifications. The passing remarks the Author has seen occasion to make upon the various objects that have fallen under his notice are intended to serve, as would a finger-post or VI PREFACE. guide in a country abounding in treasures. It was impossible to have attempted a more detailed descrip- tion than has been given in dealing with the vast expanse of natural objects presented to his contemplation. If, however, the notices have been sufficiently ample and precise to assist the study of the reader, the Author will have accomplished the most cherished object he had in view in presenting the work to the public. I, Bedford Square, October, 1867. PREFACE TO THE FIRST EDITION. HE Author of this Publication entered upon his task with some hesitation and diffidence ; but the reasons which influenced him to undertake it may be briefly told, and they at once explain his motives, and plead his justification, for the work which he now ventures to submit to the indulgent con- sideration of his readers. j£3[ It had been to him for some time a sub- ject of regret, that one of the most useful and fascinating studies — that which belongs to the do- main of microscopic observation — should be, if not wholly neglected, at best but coldly and indifferently appreciated by the great mass of the general public ; and he formed a strong opinion, that this apathy and inattention were mainly attributable to the want of some concise, yet suffi- ciently comprehensive, popular account of the Microscope, both as regards the management and manipulation of the instrument, and the varied wonders and hidden realms of beauty that are disclosed and developed by its aid. He saw around him valuable, erudite, and splendid volumes , which, however, being chiefly designed for circulation amongst a special class of readers, were necessarily pub- Vlll PREFACE. lished at a price that renders them practically unattainable by the great bulk of the public. They are careful and beautiful contributions to the objects of science, but they cannot adequately bring the value and charm of micro- scopic studies home, so to speak, to the firesides of the people. Day after day, new and interesting discoveries, and amplifications of truth already discerned, have been made, but they have been either sacrificed in serials, or, more usually, devoted to the pages of class publications ; and thus this most important and attractive study has teen, in a great measure, the province of the few only, who have derived from it a rich store of enlightenment and gratification : the many not having, however, parti- cipated, to any great extent, in the instruction and enter- tainment which always follow in the train of microscopical studies.1 The manifold uses and advantages of the Microscope crowd upon us in such profusion, that we can only attempt to enumerate them in the briefest and most rapid manner in. these prefatory pages. It is not many years since this invaluable instrument was regarded in the light of a costly toy ; it is now the inseparable companion of the man of science. In the medical world, its utility and necessity are fully appre- ciated, even by those who formerly were slow to perceive its benefits ; now, knowledge which could not be obtained even by the minutest dissection is acquired readily by its assistance, which has become as essential to the anatomist and pathologist as are the scalpel and bedside observation. The smallest portion of a diseased structure, placed under a Microscope, will tell more in one minute to the ex- perienced eye, than could be ascertained by long examina- (1) At the time this work was written, scarcely a book of the kind had been published at a price within the reach of the working classes. PREFACE. IX tion of the mass of disease in the ordinary method. Microscopic agency, in thus assisting the medical man, contributes much to the alleviation of those multiplied " ills which flesh is heir to." So fully impressed were the Council of the Eoyal College of Surgeons with the import- ance of the facts brought to light in a short space of time, that, in 1841, they determined to establish a Professorship of Histology, and to form a collection of preparations of the elementary tissues of both animals and vegetables, healthy and morbid, which should illustrate the value of microscopical investigations in physiology and medical science. From that time, histological anatomy deservedly became an important branch of the education of the medical student. In the study of Vegetable Physiology, the Microscope is an indispensable instrument ; it enables the student to trace the earliest forms of vegetable life, and the functions of the different tissues and vessels in plants. Valuable assistance is derived from its agency in the detection of adulterations. In the examination of flour, an article of so much importance to all, the Microscope enables us to judge of the size and shape of the starch-grains, their markings, their isolation and agglomeration, and thus to distinguish the starch-grains of one meal from those of another. It detects these and other ingredients, invisible to the naked eye, whether precipitated in atoms or aggre- gated in crystals, which adulterate our food, our drink, and our medicines. It discloses the lurking poison in the minute crystallisations which its solutions precipitate. " It tells the murderer that the blood which stains him is that of his brother, and not of the other life which he pretends to have taken ; and as a witness against the criminal, it on one occasion appealed to the very sand on which he trod at midnight." I PREFACE. The zoologist finds in the Microscope a necessary co- operator. To the geologist it reveals, among a multiplicity of other facts, " that onr large coal-beds are the ruins of a gigantic vegetation ; and the vast limestone rocks, which are so abundant on the earth's surface, are the catacombs of myriads of animal tribes, too minute to be perceived by the unaided vision." By " conducting the eye to the confines of the visible form," the Microscope proves an effective auxiliary in defining the geometric properties of bodies. Its influence as an instrument of research upon the structure of bodies has been compared to that of the galvanic battery, in the hands of Davy, upon Chemistry. It detects the smallest structural difference, heretofore inappreciable, and, as an ally of Chemistry, enables us to discover the very small changes of form and colour effected by test- fluids upon solids ; and dissects for us, so to speak, the most multiplex compounds. It opens out to the mind an extended and vast tract, opulent in wonders, rich in beauties, and bound- less in extent. The Microscope not only assists studies, and develops objects of profound interest, but also opens up innumer- able sources of entertainment and amusement, in the ordinary conventional acceptation of these terms ; — dis- erosrng to us peculiarities and attractions in abundance ; — impressing us with the wonderful and beautifully-skilful adaptation of all parts of creation, and filling our minds with additional reverence and admiration for the beneficent and Almighty Creator. The Author will conclude these prefatory observations with a few words in explanation of his arrangements, and by way of acknowledgment to those to whom he is indebted. He has sought, in the volume that he now lays before the public, to point out and elucidate at once in a PREFACE. XI practical manner and in a popular style, the vast fund of utility and amusement which the Microscope affords, and has endeavoured to touch upon most of the interesting subjects for microscopic observation as fully as the restric- tions of a limited space, and the nature of the succinct summary, would permit. To have dwelt upon each in complete detail would have necessitated the issue of many expensive volumes — and this would have entirely frustrated the aim which the writer had in view ; he has, therefore, con- tented himself with the humble, but, he trusts, not useless, task of setting up a finger-post, so to say, to direct the inquirer into the wider road. In the section of the work devoted to the minuter portion of creation, he has ventured to dwell somewhat longer, in the belief that that depart- ment ;6 more especially the province of the microscopist. He has arranged his topics under special headings, and in separate chapters, for the sake of perspicuity and precision ; and has brought the ever-welcome aid of illustrations to convey his explanatory remarks more vividly to the minds of his readers. He is peculiarly indebted to Professor Quekett, whose valuable lectures, delivered annually in the Eoyal College of Surgeons, and whose multifarious and successful researches, have pre-eminently distinguished him as the microscopist of the day. Prom notes made during the lectures spoken of, and from the many admirable papers which this gentleman has published, much sound information has been gleaned ; and the Author has to thank him, in the most sincere and cordial manner, for placing at his disposal the mass of contributions with which he has enriched microscopical science. A free use has been made of the researches of scientific investigators generally — Leeuwenhoek, Ehrenberg, Carpenter, Johnston, Ralfs, Busk, Gosse, Huxley, Hassall, Lobb, Davies, and other members of the Microscopical Society of London. His Xll PREFACE. acknowledgments are likewise due to Mr. George Pearson, for the great care he has bestowed upon the engravings which illustrate these pages. Finally, it is the Author's hope that, by the instru- mentality of this volume, he may possibly assist in bring- ing the Microscope, and its most valuable and delightful studies, before the general public in a more familiar, com- pendious, and economical form than has hitherto been attempted ; and that he may thus, in these days of a diffused taste for reading and the spread of cheap pub- lications, supply further exercise for the intellectual faculties, — contribute to the additional amusement and instruction of the family circle, — and aid the student of nature in investigating the wonderful and exquisite works of the Almighty. If it shall be the good fortune of this work, which is now confided with great diffidence to the consideration of the public, to succeed in however slight a degree, in furthering this design, the Author will feel sincerely happy, and will be fully repaid for the attention, time, and labour that he has expended. London, May, 1854. PAET I. HISTORY OF THE INVENTION AND IMPROVEMENTS OF THE MICROSCOPE. CHAPTER I. PACK HISTORY OP THE INVENTION AND IMPROVEMENTS OF THE MICROSCOPE 1 CHAPTER II. mechanical and optical principles involved in the construction of the microscope — lenses— mode of estimating their power — micrometers — polarised light — camera ltjcida — binocular instrument — photographic-drawing — m1crospectroscopy — achro- matic lenses — magnifying power — wollaston's doublet — coddington's lens — simple and compound microscopes — quekett's, ross's, powell and lea- land's, baker's, pillischer's, ladds', Murray's, high- ley's, COLLINS' S, AND OTHER MICROSCOPES 15 '33 and r r, also reverses the curvature of the images as there shown, and gives them the form best adapted for distinct vision by the eye-glass e e. The field-glass has at the same time brought the blue and red images closer together, so that they are adapted to CONSTRUCTION OF THE MICROSCOPE. 49 pass uncoloured through the eye-glass. To render this important point more intelligible, let it be supposed that the object-glass had not been over-corrected, that it had been perfectly achromatic ; the rays would then have become coloured as soon as they had passed the field-glass ; the blue rays, to take the central pencil, for example, would converge at b", and the red rays at r", which is just the reverse of what the eye-lens requires ; for as its blue focus is also shorter than its red, it would demand rather that the blue image should be at r", and the red at 6". This effect we have shown to be produced by the over- correction of the object-glass, which protrudes the blue foci b b as much beyond the red foci r r as the sum of the distances between the red and the blue foci of the tield-lens and eye-lens ; so that the separation b r is exactly taken up in passing through those two lenses, and the whole of the colours coincide as to focal distance as soon as the rays have passed the eye-lens. But while they coincide as to distance, they differ in another respect, — the blue images are rendered smaller than the red by the superior refractive power of the field-glass upon the blue rays. In tracing the pencil I, for instance, it will be noticed that, after passing the field-glass, two sets of lines are drawn, one whole and one dotted, the former repre- senting the red, and the latter the blue rays. This is the accidental effect in the Huyghenian eye-piece pointed out by Boscovich. The separation into colours of the field- glass is like the over-correction of the object-glass, — it leads to a subsequent complete correction. For if the differently coloured rays were kept together till they reached the eye-glass, they would then become coloured, and present coloured images to the eye ; but fortunately, and most beautifully, the separation effected by the field- glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to the spherical figure, the refractive power is less than at the margin, that that spherical error of the eye-lens constitutes a nearly perfect balance to the chromatic dispersion of the field-lens, and the blue and red rays I' and l" emerge sensibly parallel, presenting, in consequence, the perfect defini- tion of a single point to the eye. The same reasoning E 60 CONSTRUCTION OF THE MICROSCOPE. Fig. 33. is true of the intermediate colours and of the other pencils. The eye-glass e e not only brings together the images b' V, r r, but it likewise has the most important effect of rendering them flat, thus at once correcting both the chromatic and spherical aberration. . The Huyghenian eye-piece, which we have described, is the best for merely optical purposes ; but when it is required to measure the magnified image, ST Tfta we use the eye-piece invented by Mr. Ramsden, and called by him the micrometer eye-piece. The arrangement may be readily understood upon reference to fig. 33. The field-glass has now its plane face turned towards the object ; the rays from the ob- ject are made to converge immediately in front of the field-glass ; and here is placed a plane-glass, on which are engraved divisions of l-100th of an inch or less. The markings of these divisions come into focus, therefore, at the same time as the image of the object, and both are distinctly seen together. The glass with its divisions is shown in fig. 34, on which, at a, are seen some magnified grains of starch. Thus the measure of the magnified image is given by mere inspection ; and the value of such measurements, in reference to the real object, when once obtained, is con- stant for the same object-glass. It is affirmed by Mr. Ross, that if the achromatic prin- ciple were applied to the construction of eye-pieces, the latter is the form with which the greatest perfection would be obtained. That such an adaptation might be produc- tive of valuable results, appears from Mr. Brooke's state- ment, that he has employed as an eye-piece, a triplet objective of one inch focus, the definition obtained by it being superior to that afforded by the ordinary Huyghe- nian eye-piece. Some of the lowest French achromatic i =9 A — — ' • ' . Fig. 34. CONSTRUCTION OF THE MICROSCOPE. 51 object-glasses answer extremely well for this purpose ; and as the sets are usually made removable, the front pair can be readily separated for the experiment. Mr. Lister places on the stage of his microscope a divided scale, the value of which is known ; and viewing the scale as the microscopic object, observes how many of the divisions on the scale attached to the eye-piece corre-' spond with One of those in the magnified image. If, for instance, ten of those in the eye-piece correspond with one of those in the image, and if the divisions are known to be equal, then the image is ten times larger than the object, and the dimensions of the object are ten times less than indicated by the micrometer. If the divisions on the micrometer and on the magnified scale are not equal, it becomes a mere rule-of-three sum ; but in general this trouble is taken by the maker of the instrument, who furnishes a table showing the value of each division of the micrometer for every object-glass with which it may be used. Mr. Jackson invented the simple and cheap form of micrometer, represented in fig. 35, which he described in the Microscopical Society's Transactions, 1840. It consists of a slip of glass placed in the focus of the eye- glass, with the divisions sufficiently fine to have the value of the ten-thousandth of an inch with the quarter-inch object-glass, and the twenty-thousandth with the eighth ; at the same time the half, or even the quarter of a division may be estimated, thus affording the means of attaining all the accuracy that is really available. It may therefore entirely supersede the more complicated and expensive screw-micrometer, being much handier to use, and not liable to derangement in inexperienced hands. The positive eye-piece gives the best view of the micro- meter, the negative of the object. The former is quite free from distortion, even to the edges of the field ; but the object is slightly coloured. The latter is free from colour, but is slightly distorted at the edges. In the centre of the field, however, to the extent of half its diameter, there is no perceptible distortion ; and the clearness of the definition gives a precision to the measure- e 2 52 CONSTRUCTION OP THE MICROSCOPE. ment which is very satisfactory. For this reason Mr. Jackson gives it the preference. Fig. 35. — Mr. Jackson's Micrometer eye-piece. Short bold lines are ruled on a piece of glass, a, fig. 35 ; and to facilitate counting, the fifth is drawn longer, and the tenth still longer, as in the common rule. Very finely levigated plumbago is rubbed into the lines, to render them visible ; and they are covered with a piece of thin glass, cemented by Canada balsam, to secure the plumbago from being wiped out. The slip of glass thus prepared is placed in a thin brass frame, so that it may slide freely ; and is acted on at one end by a pushing-screw, and at the other by a slight spring. Slips are cut in the negative eye-piece on each side, b, so that the brass frame may be pressed across the field in the focus of the eye-glass, as at m ; the cell of which should have a longer screw than usual, to admit of adjust- CONSTRUCTION OF THE MICROSCOPE. 53 ment for different eyes. The brass frame is retained in its place by a spring within the tube of the eye-piece ; and in using it the object is brought to the centre of the field by the stage movements ; and the coincidence between one side of it and one of the long lines is made with great accuracy by means of the small pushing-screw that moves the slip of glass. The divisions are then read off as easily as the inches and tenths on a common rule. The operation, indeed, is nothing more than the laying a rule across the body to be measured; and it matters not whether the object be transparent or opaque, mounted or not mounted, if its edges can be distinctly seen, its diameter can be taken. Previously, however, to using the micrometer, the value of the divisions should be ascertained with each object- glass ; the mode of doing which is best performed as follows : — " Lay a slip of ruled glass on the stage ; and having turned the eye-piece so that the lines on the two glasses are parallel, read off the number of divisions in the eye- piece which cover one on the stage. Repeat this process with different portions of the stage-micrometer, and if there be any difference, take the mean. Suppose the hundredth of an inch on the stage requires eighteen divi- sions in the eye-piece to cover it ; it is quite plain that an inch would require eighteen hundred, and an object which occupied nine of these divisions would measure the two- hundredth of an inch. This is the common mode of expressing microscopical measurements ; but I am of opinion that a decimal notation would be preferable, if universally adopted. " Take the instance supposed, and let the microscope be furnished with a draw-tube, marked on the side with inches and tenths. By drawing this out a short distance, the image of the stage micrometer may be expanded until one division is covered by twenty in the eye-piece. These will then have the value of two-thousandths of an inch, and the object which before measured nine will then mea- sure ten ; which, divided by 2,000, gives the decimal fraction -005. " Enter in a table the length to which the tube is drawn 54 CONSTRUCTION OF THE MICROSCOPE. out, and the number of divisions on the eye-piece micro- meter equivalent to an inch on the stage ; and any measurements afterwards taken with that micrometer and object-glass may, by a short process of mental arithmetic, be reduced to the decimal parts of an inch, if not actually observed in them. " In ascertaining the value of the micrometer with a deep object-glass, the hundredth of an inch on the stage will occupy too much of the field ; the two-hundredth or five -hundredth should then be used, and the number of divisions corresponding to that quantity be multiplied by two hundred or five hundred, as the case may be. " The micrometer shouid not be fitted into too deep an eye-piece, for it is essential to preserve clear definition. Th? middle eye-piece is for most purposes the best, pro- vided the object-glass be of the first quality ; otherwise, r is remarkable, and the flat appearance of the specimen as seen by the ^th gives place to that of con- siderable depth of tissue and perspective. The finest nerve-fibres, ramifying in the cornea of the eye, and in certain forms of connective tissue, are beautifully brought out ; and their relation to the delicate processes from the connective-tissue corpuscles can be more satisfactorily demonstrated than by the ^th. The advantage of the s\jth. in such investigations seems mainly due to its re- markable power of penetration." The one great drawback to the use of this objective, and a very serious one to most microscopists, is its costli- ness. The price of this power alone is almost more than many can afford to give for a complete instrument. In flatness of field, and in perfection of definition, both at the centre and margin of the field of view, few objec- tives have equalled the recent xV^n °f Mr. Ross, who appears to have inherited his late father's well-known skill PENETRATING POWER. 75 in all that appertains to the microscope. Mr. Baker has also made considerable advances in the same direction: his objectives have been made with a greater angle of aperture than formerly, and with more penetration. Much misconception, however, exists with regard to the available angle of aperture for optical requirements ; it will be as well to remark that an admirable method of determining this was proposed by Professor Govin, of Turin, which consists in placing the microscope perpendicularly to any plane dark non-reflecting surface (as a table covered with a green cloth), and having converted the instrument into a telescope, by placing above the eye-piece a suitable com- bination of two lenses (such as the Examining- Glass of Mr. Boss), and then examining and marking the greatest lateral distance on either side at which a clear image of some distinct object, such as a narrow strip of white card- board or paper, laid on the table can be perceived. " Half the distance between these two points, divided by the ver- tical distance of the focal point of the objective from the surface of the table, will, by reference to a table of natural tangents, give half the required angle of aperture. This will, in many cases, be found to be considerably less than what may be termed the angle of admission of diffused light. To illustrate this, supposing the focal point to be at a distance of 0*01 inch from the surface of the objective, a reference to a table of natural tangents will show that an angular aperture of 17° will necessitate a linear aperture of 0*22 inch : an aperture of 170° will require 0*28 inch, and one of 17-4°, of 0*38 inch, in order to admit the extreme rays, which, for objectives of |th mch focus, is manifestly impossible." In regard to angle of aperture, it may be stated once and for all, that large angle of aperture is necessarily incompatible with that far more generally useful quality of penetration. Penetrating power 1 is synonymous with (1) Penetrating Power. '* The origin of this term will be found in the Phil. Trans, for 1S00, in an article by Sir Wm. Herscuel, entitled ' On the Power of Penetration into space possessed by Telescopes.' In that article, we are told that when, owing to the darkness, a distant church steeple was invisible, a certain telescope described, clearly showed the time upon the clock. This, adds Sir William, was not owing to magnifying power alone, for the steeple could not be discerned by the naked eye. And he has shown in the same article, that trie 76 THE MICROSCOPE. depth of focus, that is, extreme distance of two planes, the points of which are at the same time sufficiently in focus for the purpose of distinct vision. This distance will manifestly increase as the angle of aperture diminishes, just as in a landscape camera the fore and hack grounds can be brought into sensible focus simultaneously only by the use of a small diaphragm, which greatly diminishes the angular aperture of the incident pencils. But, at the same time, it must be borne in mind, that illumination, costeris paribus, increases or diminishes with angle of aperture, and the best working glass will be that in which a compromise is effected between these two conflicting requisites. We entirely agree with Mr. Brooke, that "for all practical purposes, except developing the markings of diatoms, an objective of moderate aperture will be found most available. It may reasonably be doubted whether the development of the dottings of difficult diatoms is not an object rather of curiosity than of utility, and whether it is worth the labour that has been bestowed upon the production of glasses for that especial purpose ; the labour of construction being immensely augmented by the difficulty of duly balancing the aberrations of the more oblique pencils. So much is this the case, that in the best constructed objectives of the words 'penetrating power' have a definite meaning, and that the amount of this power possessed by a telescope, can be obtained by calculation. And of course, this must also be true of a microscope. This power, says Mr. Rykuid, In a very interesting paper, must not be confused with angular aperture, which has reference to the objective alone ; neither has it any connexion with either definition or thickness of field. In a word, as magnifying power expresses the angle subtended by an object or image at the eye of the observer, so penetrative power is the measure of the angle subtended by the eye at the object, or the •quivalent of that angle in the case of telescopic or microscopic vision. The one is the measure of size, the other of brightness. The latter, however, must not be confused with ' illumination.' The one power is neither less important nor less essential to distinct vision than the other. There required little mag- nifying power, and there was no illumination, in the case of the church steeple, still the hour could be read on the diaL" The third power, — the visual power of microscopes,— is one which has rarely been recognised as distinct. For an object to be magnified 100 times, that is wen at 100th part of the distance, it is necessary not only that the angle subtended by it at the eye — the magnifying power — but also the angle subtended by the eye at the object — the penetrating power— shall be increased one hundred-fold. When this is the case, the visual power will be 100 also. If we approach an object bodily, these angles naturally increase in the sam* Eroportion, but it is not so where optical instruments are employed. — R. G> lyland " On the Optical Powers of the Microscope." — Micros. Jour. Sciena^ voL vii. p. 27. MAGNIFYING POWER. 77 largest angles, the visual effect is sensibly impaired when the rays are transmitted through any other thickness of covering-glass than that for which they have been specially corrected." The value of penetrating power is especially felt when the binocular arrangement is employed, since the assistance which it is able to give in the estimation of the solid forms of objects, is altogether neutralized by the employ- ment of objectives of such wide angular aperture as not to show any part of the object, save what is precisely in focus; and, therefore, there cannot be a doubt of the supe- rior value of objectives of a moderate angle of aperture for ordinary working purposes. " Successful observation, with very high power, is mainly dependent upon illumination. Indeed, by ordinary means, it is not possible to obtain a light sufficiently in- tense to illustrate an object magnified 3,000 diameters. I have tried various plans, and have come to the conclu- sion, that the most satisfactory results are obtained by the use of Kelner's eye-piece as a condenser. By this means I can obtain a light sufficient for a magnifying power of 10,000 linear. " My method of increasing the size of the image, with- out altering the object-glass, is as follows : — " Supposing the limits of magnifying power of the object-glass to have been reached, I then increase the distance between it and the eye-piece, by lengthening the body by the aid of a draw-tube. The 3*5 th objective being applied when the tube is increased in length, so that from the lowest glass of the object-glass to the eye- glass of eye-piece, the distance measures 24 inches, the magnifying power corresponds to upwards of 10,000 dia- meters, 20 inches about 6,000, 15 inches about 2,600, 11 inches about 1,800. " If there is any reflection from the interior of the tube which renders the image indistinct, let the tube be lined with black velvet ; of course the practical utility of in- creasing the magnifying power entirely depends upon the character of the specimen, and the preparations beat. suited for examination by very high powers must be im- mersed in the strongest glycerine that can be procured, 78 THE MICROSCOPE. In this way, I have been able to see points which I have failed to bring out in any other way." x Magnifying power, as we have already explained, has to do with size only, and only expresses the magnitude of the angle subtended by the enlarged image, at the eye, as com- pared with that subtended by the object itself under the circumstances of ordinary standard vision; namely, at a distance of ten inches from the eye. The Kelner eye-piece, while it increases the magnifica- tion, detracts from the definition ; on this account, it is only used when desirous to show the whole of an object at once, as an insect, or in viewing the series of rings formed by diiferent crystals with polarized light. This differs from the ordinary Huyghenian eye-piece in having a double-convex field-glass, and an achromatic meniscus eye-glass. COMPOUND MICROSCOPES. It should be understood that the descriptions we are about to give of Compound Microscopes cannot embrace that of every manufacturer. Want of space forbids this, and therefore we particularly wish to disclaim any inten- tion of instituting invidious comparisons ; our desire is solely that of giving, as concisely as possible, a description of those instruments that have more immediately fallen under our notice, or have been the companions of our microscopical pursuits. At the head of our list stands — Ross's microscope, an instrument of which the aim is not simplicity, but perfection — not the production of the best effect compatible with limited means, but the attain- ment of everything that the microscope can accomplish, without regard to cost or complexity. Without any un- due preference, the first place may fairly be assigned to this large Compound Microscope of Mr. Ross, as the one which was earliest brought (in all essential features, at least) to its present form. The general plan of Mr. Ross's microscope will be seen to be essentially that which has been adopted in a simpler form by many other makers ; but it is carried out with the greatest attention to solidity (1) Beale, " How to Work with the Microscope." ROSS S MICROSCOPE. 79 of construction, in those parts especially which are most liable to tremor, and to the due balancing of the weight of the different parts upon the horizontal axis. The " coarse " adjustment is made by the large milled-head Fig. 39. — Ross's No. la Large Microscope, with Binocular arrangement and Object-glass. Scale $. B. Sub-stage of ditto, with new Achromatic Condenser. Scale \. situated just behind the summit of the uprights, which turns a pinion working into a rack cut on the back of a very strong flattened stem, that carries the transverse arm at its summit. A second milled-head (which is here con- 80 THE MICROSCOPE. cealed by the stage-fittings) is attached to the other end of the axis of the pinion (as in fig. 39), so as to be worked with the left hand. 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" ur tube pro- jecting below the arm, wherein the objectives are screwed. The other milled-head seen at the summit of the stem serves 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 side, being checked in the opposite by a " stop," which secures the coincidence of the axis of the body with the centre of the stage, and with the axis of the illu- minating apparatus beneath it. It is in the movements of the stage that the greatest contrivance is shown. These are three ; namely, a traversing movement from side to side, a traversing movement from before backwards, and a rotatory movement. The traversing movements, which allow the platform carrying the object to be shifted about an inch in each direction, are effected by the two milled- heads situated at the right of the stage; and these are placed side by side, in such a position that one may be conveniently acted on by the fore-finger, and the other by the middle-finger, the thumb being readily passed from one to the other. The traversing portion of the stage carries the platform whereon the object is laid, which has a ledge at the back for it to rest against ; and this platform has a sliding movement of its own from before backwards, by which the object is first brought near to the axis of the microscope, its perfect adjustment being then obtained by traversing movements. To this platform, and to the traversing slides which carry it, a rotatory movement is imparted by a milled-head placed underneath the stage on the left-hand side ; for this milled-head turns a pinion which works against the circular rack (seen in the figure), whereby the wThole 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 microscope. This rotatory movement is useful for twc purposes — first, in the examination of very delicate objects POWELL AND LEALAND'S MICROSCOPE. 81 by oblique light, in order that, without disturbing the illuminating apparatus, the effect of the light and shadow may be seen in every direction, whereby important addi- tional information is often gained ; and, secondly, in th6 examination of objects under polarized light — a class of appearances being produced by the rotation of the object between the prisms, which is not developed by the rota- tion of either of the prisms themselves. The graduation of the circular rack, moreover, enables it to be used as a goniometer. In the improved form of this instrument here represented, the whole stage-apparatus is made so thin, and the opening beneath so large, as to permit the employment of light of extreme obliquity ; and to enable the mirror to afford this, it is mounted upon an extending arm, the socket of which slides upon a cylindrical stem, Below the stage, and in front of the stem that carries the mirror, is a dovetail sliding-bar, which is moved up and down by the milled-head shown at its side. This sliding- bar carries what is termed by Mr. Ross the " secondary stage " (shown separately at b), which consists of a cylin- drical tube for the reception of the achromatic condenser, the polarizing prisms and other fittings as in Messrs. Powell and Lealand's arrangement; it is here shown as fitted with a condenser specially devised by Mr. T. Eoss for the illu- mination of a large field under low magnifying powers. To this secondary stage, also, a rotatory motion is com- municated by the turning of a milled-head ; and a tra- versing movement of limited extent is likewise given to it 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." The mechanical movements and general finish of the instrument is all that can be desired by the practised observer, and tends towards that saving of time and labour as essential in microscopical pursuits as in other branches of science. Although our description is confined to Mr. Ross's first-class microscope, other instruments without the accessory arrangements described may be had equally well made and suited to the means of the student, ranging in prices from 121. 1 2s. upwards. G 82 THE MICROSCOPE, The general plan of Powell and Lealand's Compound Microscope is represented in fig. 40. The tripod-stand gives a hrm support to the trunnions that carry the tube to which the stage is attached, and from which a triangular stem is raised by the rack-and-pinion movement set in Fig. 40. — Powell and Lealand's Microscope, with Amid prism arranged for oblique illumination. action by the double-milled head, whereby the coarse ad- justment of the focus is obtained. To the upper part of the triangular bar a broad arm is fixed, bearing the com- pound body ; this arm is hollow, and contains the mechanism for the fine adjustment, which is effected by turning the small milled-head. The arm is connected with the tri- angular bar by a strong conical pin, on which it turns, so that the compound body may be moved aside from the stage when necessary ; by a mechanical arrangement it ladd's MICKOSCOiE. 83 stops when central. The stage is of an entirely new con- struction, having vertical, horizontal, and circular move- ments, and graduated for the purpose of registering objects so as to be found at pleasure; and in order to do this effectually a clamping piece is provided against which the object slide rests, and the circular motion of the stage is stopped. It is an exceedingly effectual method of finding a favourite object. The stage is remarkably strong, and at the same time so thin, that the utmost obliquity of illumination is attainable, the under portion being entirely turned out : it has a dove-tailed sliding bar moveable by rack and pinion; into this bar slides the under stage, having vertical and horizontal motions for centering, and also a circular motion ; into the stage are affixed the various appliances for underneath illumination, removed in our woodcut. Their achromatic condenser is of 17(T aperture, with nine openings and five central stops. The openings and stops having independent movements, the manipulator can regulate these at will, which is admitted to be an improvement. There is an especial appliance orovided for the use of a set of dark- wells, with or with- out the condenser. The mirror is attached to a quadrant of brass and two arms, in order to obtain greater obliquity of illumination ; the whole fits into a short piece of tube made to slide either up or down the long tube attached to the bottom of the stage by which the mirror is con- nected with the other part of the stand ; the reflectors are both plane and concave, as in other instruments. Powell and Lealand have another pattern, somewhat resembling the instrument just described, but larger and more massive in its general arrangemeits. The construc- tion of the stage and sub- stage differ entirely : both of these rest on a large solid brass ring, firmly attached 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 motion is given by a milled-head, the amount of the movement which may be carried through an entire revolution being exactly measured by the gradua- tion of a circle of gun-metal, which is borne on the upper surface of the ring. The rotatory action of the stage taing thus effected beneath the traversing movement, the centex- 62 84 THE MICROSCOPE. ing of an object brought into the axis of the microscope is not disturbed by it ; and the workmanship is so accurate that the stage may be driven through its whole revolution without throwing out of the field an object viewed with the i^g- th objective. The stage withal is made thin enough to admit of the most oblique light being thrown on the object, such as that obtained by the use of the Amici prism, as shown arranged in the preceding figure. The sub-stage is also provided with rotatory, rectangular, and vertical movements, and is made in such a manner as to admit of the simultaneous use of the polarising prism and of the achromatic condenser. This instrument combines remarkable steadiness with great solidity, and is so well balanced on its horizontal axis that it requires no clamping in whatever position it may be placed. Cheaper instruments are furnished by Powell and Lea- land, such as a student's microscope, with |-inch stage movement, coarse and fine adjustments to body, plane and concave mirrors, revolving diaphragm, Lister's dark-wells, and two eye-pieces, for the price of 81. An improved form of microscope (fig. 41) is manu- factured by Mr. Ladd, of Beak Street, Eegent Street ; having a stand so simple and light in its construction as to render it very portable and useful. It is fitted with a magnetic stage, which facilitates the moving of the objects when placed on it by the unaided fingers; a point of some importance to such microscopists as desire to retain and cultivate delicacy of touch in preference to that growing dependence upon mechanical movements. The main features of this form of microscope are, that the bear- ings for the compound body, stage, and sub-stage are all fitted, while connected together into the dovetailed brass bar running from top to bottom of the instrument. The magnet is attached to the under part of the stage, and a gilt iron bar, ledge, or keeper, serves for an object-rest The sub-stage is constructed of three thin plates having rectangular movements, the top one having a tube attached, into which is fitted the Polariscope, spotted lens, &c, the focussing of which is by a rack placed below. The mirror, being provided with a double-jointed arm, can be used vrith any amount of obliquity. The stand forms a tripod BAKERS MICROSCOPE. 85 strengthened by cross bars ; the beauty of the chain move- ment (with which all Mr. Ladd's microscopes are furnished) is made apparent by a simple and effective fine adjustment Fig. 41. — LadcCs Students Microscope. attached to the milled head, thus making the one adjust- ment subsidiary to both purposes. The general appearance of the instrument is one of elegance, stability, lightness, and compactness. Mr. Baker (Holborn) has kept pace with our leading manufacturers, and his first-class microscopes fairly entitle him to take his place among the makers of superior in- struments. One of his best forms, shown in fig. 42, combines good workmanship with remarkable solidity and completeness in all its details. In this instrument, two uprights are strengthened by two internal buttresses mounted on a solid tripod. At the upper part, and between the uprights, is an axis upon which the whole of the upper part of the instrument turns, so as to enable it to take a horizontal, vertical, or 86 THE MICROSCOPE. any intermediate position — such, for instance, as that shown in the drawing. This moveable part is fixed to the axis near the centre of gravity, and consists of the stage and the arm screwed into the square bar which Fig. 42. — Baker's bo. 1 vunc^oaaa Mvcrvawpt. carries the tube or body, with object-glass screwed into its place. The upper or table- stage has rectangular rack- and-pinion movements, working one inch in each direction. A circular and sliding motion is also given to the top plate. BAKERS STUDENTS. 87 The square bar, together with the arm and niicroscope- tube, is moved by the larger milled-heads, and a more Fig. 43. — Baker's No. 1 Compound Micrvsmpz. delicate adjustment of this optical part is effected by the smaller milled-head seen behind the body. This milled- head is graduated, affording a means of measuring the 83 THE MICROSCOPF. thickness of an object or the thin covering-glass. The other milled-head fixes the arm to the square bar. Below the table-stage is the secondary or sub-stage, into which is fitted the diaphragm, polariscope, achromatic condenser, and other illuminating apparatus. It is supplied with centering screws, circular and focussing rack-work, giving perfect and accurate adjustments. Sliding upon the lower end of the instrument is a large double mirror, with double-jointed arm, and above this a full-size Amici prism for oblique illumination. The microscope is furnished with a draw-tube, divided into tenths of an inch, and is thus rendered as perfect as is necessary for all purposes. Pig. 43 represents Mr. Baker's smaller Compound Itficroscope, differing in some respects from that just Fig. 44. — Baker's Educutional Microscope- BAKER S BINOCULAR. 8'9 described. It is not fitted with sub-stage, but such an appliance may be readily attached, dovetailed grooves being left for the purpose. The motions to stage and body, and general finish, are similar to those of the best instruments, and it is altogether such a microscope as can be well re- commended for medical or other purposes where a good stand is required, and to which it is intended that further additions shall be hereafter made. A smaller compound achromatic microscope, called the " Educational," fig. 44, is made by Mr. Baker. This is peculiarly adapted for students, and is supplied in a neat mahogany case, with the necessary apparatus and excellent object-glasses, for the small price of il. 4s. If with iron stand and in portable case, 31. 3s. Mr. Baker, in his binocular instrument, fig. 46, has succeeded in reducing some of the difficulties to a mini- mum. The setting of the prism, with its necessary stops, is so contrived that it is contained in one piece or fitting, so that when th.9 monocular body is re- quired to be used, this piece can be removed, and an uninterrupted field obtained, the light enter- ing the tube at the utmost obli- quity for high powers. At fig. 45, the body and nose-piece is seen de- tached. It has this advantage, that the prism remains in perma- nent adjustment when the brass nose-piece B is screwed home. Another feature in connexion with the use of the binocular, and not the least important, is that of illumination ; we have been much pleased with the sim- ^%7*mSbZ£$S?*'* pie arrangement of a cheap COn- A. Bodies detached \from Stand. denser by the Same optician. It ^. Nose-piece containing Prum. consists of two plano-convex lenses, the lower one being hemispherical ; the upper lens (of a somewhat smaller diameter) is placed just within its focus ; the two are fitted into a sliding-tube which admits of easy adjustment, 90 THE MICROSCOPE. This condenser has the property of throwing a soft white-cloud with low powers, adding greatly to the defini- tion and stereoscopic effect of the binocular ; it is moreover useful in giving a full, clear field with the higher powers. Pig. 46. — Baker's Studenfs Binocular Microscope. We can also refer with some confidence to the Student's Binocular Microscope produced by Mr. Baker; it pos- sesses remarkable excellence for the small sum charged for it. This instrument is rather larger than the Educa- tional by the same maker, and is fitted with sliding-stage. It has the usual coarse and fine movements, and is supplied PILLISCHER S MICROSCOPE. 91 with a double mirror and one pair of eye-pieces, the latter having rack and pinion adjustment. It is seen in fig. 46, and sold as there represented for 61. — a less sum th&n is charged for altering the larger microscopes. Fiy. 47.— rilUscliefs No. 1 Binocular Microicoju. 92 THE MICROSCOPE. Mr. Pillischer (New Bond Street) is favourably known for the excellency of his instruments. His "No. 1 Micro- scope (fig. 47) is of good workmanship, and somewhat novel in design. It is constructed on a plan which may be described as intermediate between that of Smith and Beck and Boss's well-known pattern, and in point of finish is quite equal to the microscopes of the first- mentioned manufacturers. The bent form given to the arm carrying the body gives increased strength and soli- dity to the instrument, although it is doubtful whether it adds to its steadiness when placed in the horizontal position. The straight body rests for a great part of its length upon a straight bar of solid brass, ploughed into a groove for the reception of the rack which is attached to the body, the groove being of such a form that the rack is held firmly while it glides smoothly through it. This is so firm, and gives such a steady uniform motion, as almost to render the fine adjustment unnecessary. The fine adjustment screw is removed from the usual position and placed in front of the body, just above and in front of the Wenham prism. The binocular bodies are inclined at a smaller angle to one another than in most makers, which, with the range of motion given to the eye-pieces by the rack and pinion, enables observers whose eyes differ greatly in separation to use the instrument with equal facility. The prism is so well set that it illuminates both fields with equal intensity. The stage is provided with rectangular traversing movements to the extent of an inch and a quarter in each direction. The niilled-heads which effect these are placed on the same axis, instead of side by side, one of them — the vertical one — being re- peated on the left of the stage, so that the movements may be communicated either by the right hand alone or by both hands acting in concert. The stage -plate has the ordinary vertical and rotatory motions, but to a much greater extent than usual ; and the platform which carries the object is provided with a spring clip to secure the object when the stage is placed in the vertical position. A regularly fitted sub-stage with centering screws is made in the usual way to carry an achromatic condenser, dia- phragm, polarising, and other apparatus — in short, no FILLISCHERS STUDENT S. 93 instrument can be better adapted than this to all the ordinary wants of the pathologist or skilled microscopist. The object-glasses furnished with this instrument are thoroughly good working powers. Fig. 48. — Pillischer's larger Students Microscope. Fig. 48 is a compound microscope, made by Mr. Pilli- scher for the use of students. In every respect it is a compact, handy instrument, and well finished in its mechanical details. The body is furnished with a draw- tube, by which its length can be increased about six 94 THE MICROSCOPE. inches ; coarse and fine adjustments ; moveable mecha- nical stage as in the larger instrument; two eye-pieces; superior objectives of 1 inch and ^-inch focus, of 15° and 80° angular aperture ; condenser, polarising apparatus and selenite, parabolic reflector, diaphragm, &c, packed in a mahogany case, for the price of 151. 15s. This instru- ment takes its place among the best of its class. Fig. 49. — Pillischer's bl. Prize Medal Microscope. The instrument fig. 49 Mr. Pillischer designates his bl. Prize Medal Microscope, is an excellent student's in- strument, simple and novel in its construction, and well adapted to almost any description of work. The body is furnished with a draw-tube and fine adjustment ; the arm which supports this derives additional steadiness from the square solid form given to the box in which the rack and pinion move. A convenient and simple lever-stage, with a double joint, enables the right hand to move the object COLLINS's BINOCULAR. 95 in any direction with great facility. This can be readily detached when it is desired to clear the stage for a frog- plate, &c. The instrument is furnished with a dividing set of powers, working with a tolerably flat-field ; dia- phragm, condenser for opaque objects, live box, &c. The whole packs in a neat case, measuring only 8x6 inches ; forming one of the most portable microscopes for the use of the student we have seen. The Binocular manufactured by Mr. Collins (Great Titch- field Street, Portland Place), is constructed on the model suggested by Dr. Harley, and contains all the recent im- provements for combining rapidity of application with simplicity in manipulation. Indeed, so far as the saving of time is concerned, we scarcely know how a change for the better could be devised. The whole of the appliances of the instrument, prism, polariscope, stage condenser, objectives of both high and low powers, &c. are attached to the microscope itself, and that, too, in such a manner as to enable the observer to place them in exact position without the turn of a single screw or a moment's delay. Collins's Binocular, represented in fig. 50, is fitted into the bottom of a mahogany box, which forms at the same time the stand ; round it a groove is run to receive the lip of a glass shade. The instrument itself is made of polished brass, and is eighteen inches high. The eye- pieces are supplied with shades {a a) to protect the eyes. At the end of the transverse arm (/) is the box which contains both AYenham's binocular prism and the analyser of the polariscope ; and by merely drawing it a little out, or pushing it further in, the instrument can be instantly changed from a binocular to a monocular, and still further to a polarising microscope. Immediately beneath (/) are the two objectives, a quarter and an inch ; so that, in order to change the power, all that is necessary is to slide them backwards or forwards. Moreover, these are fitted with the universal screw, so that either of them may be detached, as in an ordinary instrument, and a quarter, or one-eighth, or other power put in its place, at the option of the observer. The instrument is fitted with a coarse and fine adjustment, and has the additional advantage of a magnetic stage, in the 96 THE MICROSCOPE. cross-bar (A) of which is a groove, in order that the ob- server may enjoy the luxury of applying a Maltwood's finder, as in larger instruments possessing moveable stages. « a Fig. bO.—Collins's Binocular Microscope. Beneath the stage is seen the polariser (p), fitted into the circular diaphragm. The double mirror (m) possesses a triple joint, so that it can be applied obliquely in all directions. Collins' s Student five-guinea Binocular Microscope con- sists of two eye-pieces, rack adjustment, top-sliding stage, wheel of diaphragms, concave mirrors with adjustment^ axes for inclining to any angle, tweezers and glass plate, 1 in. and J in. achromatic objectives, C series. Packed COLLINS S STUDENTS. 97 in polished mahogany cabinet complete (tig. 51) : an ex- ceedingly cheap instrument. Fig. 51. — Collins'c Student's Binocular Microscope-. Collins' s Lawson Binocular Dissecting Microscope. — This instrument is intended to supply a want, often felt in anatomical and botanical investigations, when only a moderate magnifying power is required. In consequence of using both eyes it can be worked with for a length of time with great comfort. A large range of field is obtained, and plenty of room for working. It consists of a neat oblong French-polished mahogany box, measuring, when closed, 6^ in. by 4 in., fig. 52. The top and front let down by hinges, and on the inside of them are fitted the scissors, needles, and knives necessary for dissecting. The two sides draw out about six inches, and are hollowed out so as to serve as rests for the hands. The magnification is obtained by two lenses mounted in the eye-pieces, as represented in the diagram, and may be adjusted to the focus by a sliding bar. These show the object beautifully in relief. Beneath is a gutta-percha trough or stage, to pin the object down to, which can be n 98 THE MICROSCOPE. filled with water if required. Under this is the mirror for transparent illumination, and the light from it is passed through a circle of glass in the centre of the trough. The ing in Fig. 52. — Lawson's Dissecting Microscope. instrument is admirably adapted for the wants of students in the preparation and dissection of microscopic objects, and also answers well for botanical investigations. A cheap form of Dissect- Microscope, represented the annexed woodcut (fig. 53), has been con- structed by Mr. Baker. The instrument consists of a solid circular foot of brass, from the border of which arises a firm pillar support- ing the stage — which is of ample dimensions — and a firm horizontal bar, into which the lenses are screwed. The latter is ele- vated and depressed by a rack and pinion movement, n* 63.- Baker's Dissiding Microscope the milled-head being situate HIGHLETS MICROSCOPE. 99 at a level a little "below tliat of tlie stage. In the centre of the foot is placed the mirror, which moves in an are of brass ; that, in its turn, works upon a pivot in the foot of the instrument. This handy microscope, with three powers and mahogany case, is sold at thirty-five shillings. Fig. 54. — HighUifs Professional Microscope. Highley's Professional Microscope, shown in fig. 54, is a useful and well-made instrument, mounted on a tripod, with coarse and fine adjustment, mechanical stage, &c. The mode in which the body is supported is very good in principle, and the milled-heads for the coarse adjust- ment are in a position which is easily reached by the left- hand when the elbow is resting on the table, whilst the right-hand finds the milled-heads of the traversing stage, &c. It is arranged for a secondary stage, and all the necessary apparatus to make a complete instrument. Al- though the tripod is a very good form for a microscope, it requires all the solidity and metal of a Powell and Lealand, or it will not remain (without clamping) perfectly steady and well balanced when placed in the horizontal n 2 100 THE MICROSCOPE. position. The new form of tripod by this manufacturer is, however, well constructed, and, in our opinion, possesses advantages both in weight and steadiness. Fig. 55. — Highley's Complete Students Microscope. Highley's Complete Microscope, of which the general plan is shown in the accompanying fig. 55, may be classed among the cheapest form of instruments manufactured for the use of the student. The stand is made in the tripod form, and the coarse adjustment given by the usual rack and pinion motion, whilst the fine adjustment, or slow motion, is given by a milled-head acting on the ob- jective. The roominess, flatness, and thinness of the stage how's microscope. 101 especially adapt it for the examination of anatomical and pathological specimens. The usual diaphragm-plate admits the application of illuminating apparatus, such as the Webster condenser, polarising prism, &c. The cost of the instrument, fitted with two good ordinary object- glasses of 1 inch and J inch focus, is 61. 16s. 6d. Fig. 56. — Hovfs Students Microscope. Mr. How (Foster Lane) is the manufacturer of the im- posing-looking instrument fig. 56. It is of fair workman- 102 THE MICROSCOPE. ship, suitable for all ordinary investigations, well deserving a place among cheap microscopes for the student. The stand is of brass, firm and well finished; the body is fitted with coarse and fine adjustments for focussing ; and a draw*-tube for increasing the magnifying power of the eye-piece. The stage has an arrangement, simple but novel in construction, by which a near approach to a universal movement is obtained. The moveable, or upper plate, is held to the fixed lower plate with springs, and, although offering a convenient resistance, allows of a smoothness of 'motion quite remarkable. It resembles the magnetic stage, but is far more reliable, and can be moved upwards, downwards, laterally, or in a slanting direc- tion, thus enabling the microscopist to follow living objects with great facility, superseding to some extent the more expensive mechanical stage. A dividing set of object- glasses is supplied with the b eye-piece, thus giving a range of power varying from 40 to about 200 diameters. His powers are of English workmanship, but differ from the higher-priced objectives in having smaller angular apertures, which is, perhaps, a legitimate mode of lessening the cost. The instrument being made with the universal screw, other objectives of a better class can be added at any time. There is also a condenser mounted on a brass stand for the illumination of opaque objects. The whole is fitted in a mahogany case, with drawer for objects, and sold at 51. 5s. Murray and Heath's (Jermyn Street) Student's Micro- scope (fig. 57) is a good solid form of instrument with a bent tripod-stand. The great object of furnishing a stand at a low price which shall be capable, if desired, of being adapted to the use of the higher objectives, and fitted for the addition of all accessory apparatus, has been very satisfactorily carried out and obtained in this microscope. The stand is remarkably firm, and, being bronzed over, is well adapted for daily use in the class-room or laboratory. The adjustment is effected by a chain-movement, which gives sufficient delicacy for powers up to the J inch. The stage is perfectly flat, and the slide-rest moves smoothly and freely over it. If the instrument is intended for use In the laboratory, a glass stage is made to replace the brass MURRAY AND HEATHS MICROSCOPE. 103 one. The objectives furnished with this microscope are a \ inch of 75° angular aperture, and a 1 inch of 15°, both of excellent quality. A binocular body, with fine adjust- Fig. 57. — Murray and Heath's 51. 5s. Students Microscope. ment, is added for a small additional sum, and the instru- ment then becomes all the student can desire. Murray and Heath's Class Microscope, represented in fig. 58, i3 especially intended for the use of teachers in the demonstration of objects to a class of students. It is but too well known to those who are engaged in teaching how liable the objects exhibited, and sometimes even tho object-glass itself are to be injured in the hands of those 104 THE MICROSCOPE. unaccustomed to use the microscope. In order to avoid this risk, Messrs. Murray and Heath have constructed an instrument intended to combine an ordinary with a de- monstrating or class microscope. It consists of the usual microscope body (a), which can be inclined at any angle, with a mirror (c) on a ball-and-socket joint ; and a stage- plate with universal movement. When about to be used as a class microscope, the slide is placed in a shallow box into which it is locked by means of a key. The same key locks this box firmly on the stage-plate. "When the object has been found, this latter can be secured firmly on the stage in the same manner. After focussing, the Fig. 58. — Murray and Heath's Class Microscope. body is also locked in its place with the same key, which is seen at d, the final adjustment being made with the eye-piece. The body is then placed in the horizontal posi- tion, and fastened with a screw. The instrument can now be passed round a class-room without possibility of injury either to object or object-glass. The illumination is ob- tained either by directing the instrument towards the window, or by means of a small lamp (b), similar to that employed by Dr. Beale, and which can be so adjusted as to be used either for opaque or transparent objects. This instrument appears to be particularly well adapted to the purposes for which it is intended, and, at the same time, if without the contrivance for locking, to be a useful portable form for general, professional, or sea-side pur- poses. beale's class microscope. 105 It should be added, that a novel and efficient form of achromatic condenser is supplied with the instrument; a series of small stops of various sizes are made to drop into a minute hole drilled in the centre of the anterior plano-convex lens, which convert it into a spot-glass, or dark-ground illuminator. The whole is packed in a mahogany case, and sold for 51. 5s. Fig. 59. — Beale'8 Clinical Microscope. Dr. Beale devised an exceedingly simple and con- venient form of microscope, for the purposes of clinical instruction and of class demonstration (fig. 59). Over the body of the microscope, which is of small dimensions, a tube is fitted with a bell-shaped mouth at the end. This tube slides freely over the body, but is capable of being fixed at will by means of a clamping-screw. The slide con- taining the object is placed across the bell-mouth, and held there by a spring pressing against the back of it, and is thus maintained perpendicularly to the axis of the instru- ment. When the focus is adjusted the clamping-screw is fixed, and the fine adjustment necessary for the differ- ences of vision in different individuals is effected by drawing out or pressing in the eye-piece. The object and object-glass are thus protected from mutual injury, an accident of by no means unfrequent occurrence in careless or unpractised hands. In this form the instrument is adapted to the clinical examination of secretions, &c. and must be directed by the hand towards day or artificial light. For demonstration to a class, this instrument is attached horizontally to a small wooden stand by means of a clamp, supported by two legs. To the stand a small 10G THE MICROSCOPE. oil-lamp is likewise attached ; and a stem proceeding from the lower edge of the bell-mouth carries any desired form of condensing or illuminating apparatus. This stand is capable of being freely handed round a large class without the focus becoming at all deranged, even when a very deep objective is employed. This instrument is manufactured by Mr. S. Highley.1 Fig. 60. — Warington's Universal Microscope. While alluding to cheap microscopes, we would men- tion Warington's Travelling Microscope, made by Salmon, 100, Fen church Street. It has a simple, firm, wooden stand, whereby the cost is greatly diminished; and an arrangement of its parts which enables it to be used for (1) A very simple instrument, contrived by Mr. T. C Archer for the purpose of being used either as a lecture-room or as an ordinary table microscope, is manufactured by Messrs. Parkes of Birmingham, and sold, with a set of achro- matic powers, for 21. 5s. in case complete. SEA-SIDE MICROSCOPES. 107 viewing objects in aquaria, and nnder other circumstances ■where any ordinary form of instrument could not be made available. It is altogether a useful student's microscope, having the recommendation of folding up into a small com- pass, and not liable to much injury either from chemical or marine investigations. For 3Z. this microscope is furnished Fig. 61. — IVarington's Microscope packed. complete, with one eye-piece, quite sufficient for all ordi- nary investigations. Fig. 60 is a representation of Warington's microscope, as it appears when put together, and ready for use ; and fig. 61 for packing in a small wood case. The draw-tube itself is the coarse adjustment; whilst a finer is secured by a well-made union-joint, into which the object-glass is made to screw. With an additional arm for the reception of a single lens, it can be converted into a dissecting microscope. Fig. 62.— Murray and Heath's Sea-side Pocket Microscope, 108 THE MICROSCOPE. Tig. 62 represents a small portable instrument by Murray and Heath, designated " The Sea-side Pocket Microscope," the chief recommendation of which is its simplicity of construction and its small compact form ; the whole packs into a case six inches long, and may be carried without incumbrance in the pocket of the field-naturalist. The body of the instrument is seen at c, supported on a tripod, which is removed and folded up at b. If desired, it can be used in the upright position, as at a, when a pair of short legs placed near the mirror must be turned down, and forms a tolerably steady support to the body. The adjustment is made by sliding the body through the outer tube, which carries a triple combination of achromatic objectives. A live-box, &c. is added ; and, when packed in a morocco case, is sold for 21. 5s. Fig. 63. — Baker's Traveller's Microscope. An instrument somewhat similar in appearance to the foregoing, but differing so materially in detail as almost to baker's traveller's microscope. 109 claim to be a new invention (fig. 63), has been introduced by Baker, and not inappropriately called "The Travel- ler's Microscope," from its obvious capabilities. The aim has been to combine steadiness with extreme portability. The compound body is permanently affixed to the fore-leg of the tripod-stand ; the two other legs are supported on capstan-bar joints, which can be tightened at pleasure, or folded up parallel with the former when not in use. The difficulty of using high powers with an instrument the body of which slides in cloth is well known; the tube becomes tarnished by continued use, and a firm adjust- ment, which shall be easy of access, is almost indispen- sable. To obtain an approximate focus, the inner tube is drawn out until the combined length of the tubes is eight inches; the body is then returned to its "jacket," and placed at a proper distance from the stage to suit the object-glass employed. The fine adjustment is effected by means of a tangent-screw (fifty threads to the inch) placed conveniently behind the body, and worked by a milled- head acting on a spring contained in the upright which supports the body. This part of the instrument is very satisfactory ; it is steady and works efficiently. A mechanical stage is not generally applied, but can be if required. Sufficient movement is obtained by a plain stage, with two springs to hold the live-box or glass slip. This microscope is carried in a leathern case 10 inches by 3, seen in the woodcut (similar to that made for deer- stalking telescopes), and fitted with object-glass, eye-piece, live-box, &c. ; the weight of the whole not exceeding two pounds in weight, and it therefore especially recommends itself to the field-naturalist. The whole merit of this invention is due to Mr. Moginie, of Mr. Baker's establishment, Holborn, who has devoted much time and thoughtfulness towards bringing it to its present state of perfection. Highley's Pocket Microscope (fig. 64), for botanical or field uses, consists of a short tube furnished with a sliding eye-tube, fitting into an outer tube. The coarse adjustment is made by sliding the body through the outer tube, which carries the object; the fine adjustment by sliding the eye- 110 THE MICROSCOPE. tube in or out. The object, if transparent, is illuminated either by holding up the microscope towards a white cloud, or other source of light, or by directing it towards a mirror laid upon the table at such an angle as to reflect the light. Fig. 64. — Highley's Pocket Microscope. If opaque, it is allowed to receive direct light through an aperture in the outer tube. The extreme simplicity and portability of the instrument — which is only six inches long — constitutes its chief recommendation. Norman's (178, City Road) Universal Educational Microscope consists of a well-finished stand with tripod foot and two uprights, with axis for giving inclination to the optical part. The body has quick and slow motions, one Huyghenian eye-piece, three achromatic object-glasses, viz. a ^-inch dividing into \ and 1 inch, all of fair de- fining power and English made. The stage has a large sliding-piece, and a revolving wheel of diaphragms ; the mirror has sliding and oblique motions for the better illu- mination of the object under examination. The following apparatus is also supplied with the instrument : — a stand condenser with adjustment, stage and hand forceps, live- box or animalculas cage, a frog-plate for viewing the circu- lation of the blood in the web of a frog's foot ; also three good objects to test the different object-glasses, one hol- lowed and two plain slips, some thin glass. The whole is packed in a mahogany or walnut cabinet, with a drawer for objects, lock and key, and sold for the small price of 31. 5a. 24 first-class objects, suited for the object-glasses, are supplied with this instrument for IV. Is. Mr. E. Wheeler's (Holloway) well-made instruments de- serve commendation and notice; they are carefully finished and quite up to the modern standard. The full assorted bets of objects which Mr. Wheeler supplies in a very neat SIMPLE MICROSCOPES. Ill book-folding case, the names on which can be read off at a glance, are particularly well selected. Many of the cases are so arranged as to form complete sets of certain classes of specimens admirably suited for educational purposes, and also for the elucidation of general and particular principles ; as, for instance, the various parts of an insect, to show peculiarities of structure for the illustration of entomo- logical lectures, the same for botanical, anatomical, &c. Many of our drawings have been made from Mr. Wheeler's excellent specimens. Mr. Piper, of the Old Change Microscopical Society, devised a convenient, cheap, and portable object-case. It is a compact oblong paste-board box, made to contain six, twelve, or twenty-four shallow trays, with six or twelve divisions just the size of the ordinary glass slides. The objects lie flat in these trays, which pack one above the other. For handiness, neat packing, facility of finding and reading off names of objects, this case cannot be surpassed. It is but right to add, that it is adapted to the use of those whose aim is the economically useful cabinet for storing and classifying objects. This " uni- versal" object-case is sold by Baker, Holborn. Several forms of simple microscopes have been devised for field use under various designations, such as " Diatom Finders," &c. One of the most useful little instruments of the class is that described by Mr. J. ST. Tomkins, in the Trans. Micros. Soc. vol. vii. p. 57, 1859. Another was invented by Dr. W. Gairdner of Edinburgh, and made by Mr. Bryson of that city, neatly packed in a case for the waistcoat-pocket. Want of space will not permit us to enter further into this department, nor can we go into a critical examination of the productions of numerous well-known makers of microscopes ; as, for instance, the Educational Microscopes of Messrs. Smith and Beck, of Cheapside ; Mr. Browning, 111, Minories ; Mr. Matthews of Portugal Street ; Mr. Dancer of Manchester ; Messrs. Abraham of Liverpool; Mr. King of Bristol, &c. — all of whom have obtained a deservedly high reputation for their convenient forms of educational and other well- manufactured instruments. 112 THE MICROSCOPE. APPLICATION OP BINOCULARITY TO THE MICROSCOPE. The application of this principle to microscopic pur- poses seems to have been tried as early as 1677, by a French philosopher, le Pere Cherubin, of Orleans, a Capu- chin friar. The following is an extract from the description given by him of his instrument: "Some years ago I resolved to effect what I had long before premeditated, to make a microscope to see the smallest objects with the two eyes conjointly; and this project has succeeded even beyond my expectation, with advantages above the single instrument so extraordinary and so surprising, that every intelligent person to whom I have shown the effect, has assured me that inquiring philosophers will be highly pleased with the communication." This communication long slumbered and was forgotten, and nothing more was heard of the subject until Professor "Wheatstone's very surprising invention of the stereoscope, which he evidently expected to apply to the microscope, for he applied to both Boss and Powell to make him a binocular microscope. But this was not done ; and during the year 1853 a notice appeared in Sillimaris American Journal of a binocular instrument constructed by Professor Biddel of America, who contrived a binocular microscope in 1851, with the view " of rendering both eyes serviceable in microscopic observations." "Behind the ob- jective/' he says, " and as near thereto as possible, the light is equally divided and bent at right angles, and made to travel in opposite directions, by means of two rectangular prisms, which are in contact by their edges somewhat ground away, the reflected rays are received, at a proper distance for binocular vision, upon two other rectangular prisms, and again bent at right angles, being thus either completely inverted for an inverted microscope, or restored to their nrst direction for the direct microscope." M. Nachet also constructed a binocular microscope, upon the same principle as his double microscope, with the tubes placed vertically and 2J inches distant. This had many disadvantages and inconveniences, which Mr. F. H. Wenham ingeniously succeeded in modifying and improving. THE BINOCULAR MICROSCOPE. 113 In describing his improvements, he observes : " That in obtaining binocnlarity with the compound achromatic mi- croscope, in its complete acting state, there are far greater practical difficulties to contend against, and which it is highly important to overcome, in order to correct some of the false appearances arising from what is considered the very perfection of the instrument. " All the object-glasses, from the one-inch upwards, are possessed of considerable angular aperture ; consequently, images of the object are obtained from a different point of view, with the two opposite extremes of the margin of the cone of rays; and the resulting effect is, that there are a number of dissimilar perspectives of the object all blended together upon the single retina at once. For this reason, if the object has any considerable bulk, we shall have a more accurate notion of its form by reducing the aperture of the object-glass. "Select any object lying in an inclined position, and place it in the centre of the field of view of the micro- scope; then, with a card held close to the object-glass, stop off alternately the right or left hand portion of the front lens : it will be seen that during each alternate change certain parts of the object will alter in their rela- tive position. " To illustrate this, fig. 65 a, b are enlarged drawings of a portion of the egg of the common bed-bug (Cimex lecticularis), the operculum which covers the orifice having been forced off at the time the young was hatched. The figures exactly represent the two positions that the inclined orifice will oc- cupy when the right and left hand portions of the object-glass are stopped off. It was illumi- nated as an opaque object, and drawn under a two-thirds object-glass of about 28° of aperture. If this experiment is repeated, by holding the card over the eye- piece, and stopping off alternately the right and left half of the ultimate emergent pencil, exnctly the same changes and appearances will be observed in the object under view. i Fig. Oo. 114 THE MICROSCOPE The two different images just produced are such as are required for obtaining stereoscopic vision. It is therefore evident that if, instead of bringing them confusedly toge- ther into one eye, we can separate them so as to bring fig. 96 a b into the left and right eye, in the combined effect of the two projections, we shall obtain all that is necessary to enable us to form a correct judgment of the solidity and distances of the various parts of the object. " Diagram 3, fig. 66, represents the methods that I have contrived for obtaining the effect of bringing the two eyes Fig. 66. sufficiently close to each other to enable them both to see through the same eye-piece together, aaa are rays con- verging from the field lens of the eye-piece ; after passing the eye-lens b, if not intercepted, they would come to a focus at c ; but they are arrested by the inclined surfaces, d d, of two solid glass prisms. From the refraction of the under incident surface of the prisms, the focus of the eye- piece becomes elongated, and falls within the substance of the glass at e. The rays then diverge, and after being reflected by the second inclined surface f, emerge from the upper side of the prism, when their course is rendered still more divergent, as shown by the figure. The reflecting angle that I have given to the prisms is 47i°. I also find it is requisite to grind away the contact edges of the prisms, as represented, as it prevents the extreme margins THE BINOCULAR MICROSCOPE. 115 of the reflecting surfaces from coming into operation which can seldom be made very perfect. " The definition with these prisms is good ; but they are liable to objection on account of the extremely small portion of the field of view that they take in, and which arises from the distance that the eyes are of necessity placed beyond the focus of the eye-piece, where, the rays being divergent, the pupil of the eye is incapable of taking them all in ; also there is great nicety required in the length of the prisms, which must differ for nearly every different observer." The great disadvantages of the first arrangement of the binocular microscope were the expensive alterations required in their adaptation : to most persons, the view that it gave of the object was pseudoscopic, and not that of solidity and roundness ; and the two bodies being united at a fixed angle of convergence, the distance be- tween their axes could not be adapted to the varying distances between the eyes of different individuals. At length, these, as well as other defects, have been com- pletely overcome by the improvements the instrument has recently received at the hands of the inventor; and we have no hesitation in saying that Wenham's prism is the most valuable addition the microscope has received since the perfection of the object-glasses. The adaptation does not at all interfere with the use of the instrument as a monocular microscope, and such additions as the microspectroscope can be as easily used with it as in the old form ; it also affords a ready comparison between the object as seen singly, and by natural double vision, and thus may be obtained an ever ready test, or sight analysis of the structure under examination. Besides, the relief afforded to the eyes is, to our sensation, something quite marvellous, and we believe, therefore, much less risk is run of doing injury to vision. Objects become easily distinguishable, more especially all opaque or semi-opaque ones ; and for the more transparent ones all that is required is to use care in their illumination — diffuse the light by placing a piece of ordinary tracing or tissue paper, or ground glass, between the light and the object ; or even polarised light for peculiar substances will i2 1 1 6 THE MICROSCOPE. enable us to bring them out in greater perfection and beauty. Another advantage gained by the last improvement is, that the ordinary single-bodied microscope can be con- verted into a binocular instrument by simply fixing an- other tube at the proper angle, and adding a small prism mounted in a brass box. The latter is made to slide into the lower part of the body immediately over the objective. By its aid the rays of light proceeding from the object are reflected in two directions, which, by means of the double body, are conveyed to both eyes, and thereby a stereoscopic view of the object under observation is ob- tained. The most important point to be observed, when using the binocular, is tbat each eye has a clear view of the object. This is readily ascertained by closing the eyes alternately without moving the head away from the in- strument, when, if it be found that the two images do not quite coincide, it must be corrected by racking out or in the draw-tubes, which should form a part of the bodies of all binoculars. If both fields be not equally illuminated, the object is not rendered stereoscopic. Mr. Wenham's most important improvement consists in splitting up and dividing the pencil of rays proceeding from the objective by the interposition of a prism of the form shown in fig. 67. This is so placed in the body or tube of the microscope (fig. 68, a) as only to interrupt one-half (a c) of the pencil, the other half (a b) going on con- tinuously to the field-glass, eye-piece, of the principal body. The interrupted half of the pencil, on its entrance into the prism, is subjected to very slight refraction, since its axial ray is perpendicular to the surface it meets. Within the prism it is subjected to two reflections at b and c, which send it forth again obliquely on the line b towards the eye-piece of the secondary body, to the left- hand side of the figure ; 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 re- ceived by the right eye is formed by the rays which have passed through the left half of the objective ; whilst the image received by the left eye is formed by the rays which have passed through the right half, and which have been WENHAM S BINOCULAR. 117 subjective to two reflections within the prism, passing through only two surfaces of glass. The prism is held by the ends only on the sides of a small brass drawer, so that all the four polished surfaces are accessible, and should slide in so far that its edge may just reach the Fig. 67. Fig. 68. central line of the objective, and oe drawn back against a stop, so as to clear the aperture of the same. In this case the straight tube acts as a single microscope. " Both the transmitting and reflecting surfaces of the prism should be accessible, for the purpose of wiping off particles of dust or mildew — particles of any kind ad- hering to the prism will prevent total reflection at the point of contact. If the prism is well made and polished, and of the smallest size possible for admitting the pencil, the difference between the direct and reflected image is scarcely appreciable. " The binocular constructed as we have described performs satisfactorily up to the £th inch; but for jDOwers above 118 THE MICROSCOPE. tliis a special arrangement is needed for the prism, -which must be set close behind the lens of the £tli or i^th inch, in order to obtain an entire field of view in each eye. This it is found to accomplish perfectly when placed in that position ; but still, for very delicate test- objects, requiring the utmost extent of aperture for their definition, it will not resolve them as clearly as with a single body, from the fact that the aperture is divided and half only effective in each eye. This difficulty has at length been nearly overcome by Messrs. Powell and Lealand, by means of an inclined disc of glass with parallel sides, the partial reflection from the under surface of which is again reflected into the second eye, by means of a rectangular prism. As the disc is made thinner, so do the images approximate and the distance between them diminish. Therefore, if the glass is made as thin as pos- sible, and a very slight angle given to the two sides, it may be so arranged that both images are ultimately com- bined at the eye-piece. There would be no difficulty in working the glass to a mean thickness of -g^th of an iuch. In this form the angle between the sides would be so exceedingly small that » the chromatic effect, considered as a prism, would be inappreciable in the direct eye-tube.1 " A strong light should be avoided for the illumination of objects observed with the binocular microscope, as direct rays tend to destroy the stereoscopic effect. The illuminator that has been found to give an excellent effect consists of three plano-convex lenses, so combined as to give a very large area of light, as well as great intensity. The final emergent pencil should have, if possible, an angular aperture of 170°. Just above the top lens should be placed a sliding-cap, the crown of which is covered with a diffusing film. For this, the best material is the beautiful snow-white powder obtained from turning glass with a diamond turning-tool. This may be obtained from the opticians, and should be well washed to free it from the larger particles. A thin film of this impalpable powder should be compressed between two discs of thin glass, and fixed in the top of the sliding-cap, which is to be raised or lowered till the most intense light is obtained (1) Quarterly Journal of Mioroscopieal Science, vol i. 1861, and vol. vL 1S66. SrECTRO-MICROSCOPY. 119 on the film. This illuminator is employed in the position of the achromatic condenser, and a disc of slightly coloured neutral tint-glass placed below the bottom lens increases the purity of the light, and gives greater distinctness to objects. The effect of this diffusing film is sometimes enhanced by condensing light down on the object from above as well as from below." An important improvement has been effected in Nachet's binocular by Professor Smith, of Kenyon College, U.S. It consists in the adaptation of Nachet's set of prisms for use as eye-pieces, with any monocular instrument. The prisms being mounted in a light material, vulcanite, are made to fit into the microscope body and take the place of the ordinary eye-piece. The image transmitted by the objective is brought to a focus on the face of the first equilateral triangular prism by the intervention of an erector eye-piece placed beneath it. The second set of prisms are by a rack-and-pinion movement adjusted to suit any visual angle; while the illumination of both fields is quite perfect, even with high powers. SPECTRO-MICROSCOPY. The application of the spectroscope to the microscope is one of the most beautiful additions the instrument has lately received. The honour of the invention appears to belong to H. C. Sorby, F.RS., whose first experiments were made with a simple triangular prism, arranged and fixed below the stage, so that a minute spectrum of any transparent object might be readily examined, when placed in position immediately before the slit. Shortly after the publication of Mr. Sorby's paper, Mr. Huggins proposed to adapt a direct vision spectroscope to the eye-piece, and so enable us to view the spectra of opaque as well as transparent objects. The exact form of prism finally adopted, and now in general use, is that known as the Sorby-Erowning Spectroscope. The first spectroscope made by Mr. Browning (Minories) is represented in fig. 69. A prism is placed at p, which is enclosed in a box, so as to give a black field, by excluding 120 THE MICROSCOPE. extraneous light. The rays of light, after passing between the knife-edges at k, are rendered parallel by means of the lens at L. Then passing through the prism and con- denser (c), they reach the object at o. The light is placed at w, and if it be proposed to examine a liquid, it can be placed in a small tube (t), closed at one end ; or a trans- parent object may be placed on the stage in the usual Fig. 69. — Sectional vitcu oj tne Browning Spectroscope. manner. By the addition of a small telescope, instead of a condenser, this contrivance can be applied to a micro- scope in place of the eye-piece, and it can then be used for the examination of opaque objects. The great objection to this form is its limited range, and the constant shifting of parts it requires for finding and focussing the object, and the awkward position of the microscope, whether it be used under the stage or as an eye-piece. Fig. 70.— T/w Browning Hugging MUro-spectrotuMpe. The apparatus used by Mr. "Muggins (fig. 70) was a star SPECTRO-MICROSCOPY. 121 spectroscope, of which the collimative-tube was inserted in the body of the microscope, instead of an eye-piece. With this apparatus he has succeeded in obtaining a spectrum showing the absorption-bands from a mere frag- ment of single blood- disc, when mounted as a transparent object. In fig. 70, k represents the knife-edges, cthe tube containing the collimating-lens, P the prisms, T the teles- cope, and m the micrometer ; the object is placed on the stages at o, and must be illuminated from below if trans- parent, or, if opaque, from above by any kind of con- denser. Mr. Sorby suggested that a prism might be made of dense flint-glass, of such a form that it could be used in two different positions, and that in one it should give twice the dispersion that it would in the other, but that the angle made by the incident and emergent rays should be the same in both positions. Fig. 71. Fig. 72. Figs. 71 and 72 represent prisms of the kind made by Mr. Browning, used in two different positions, i and i' being the same angle as I and i'. lor most absorption-bands, particularly if faint, the prism would be used in the first position, in which it gives the least dispersion ; but when greater dispersion is required, so as to separate some particular lines more widely, or to show the spectra of the metals, or Fraun- hofer's lines in the solar spectrum, then the prism must be used as in fig. 72. This answers well for liquids or transparent objects, but it is, of course, not applicable to opaque objects. 122 THE MICROSCOPE. To combine both, purposes, some form of direct vision- prisms which, can be applied to the body of the micro- scope is required. Fig. 73 represents the arrangement of direct vision-prisms, invented by A. Herschel. The line B r' shows the path of a ray of light through the prisms, where it would be seen that the emergent ray r' is parallel and coincident with the incident ray r. Pig. 73. Fig. 74. Another very compact combination is shown in fig. 74. Any number of these prisms (p p p) may be used, accord- ing to the amount of dispersion required. They are mounted in a similar way to a Nicols' prism, and are applied directly over the eye-piece of the microscope. The slit s s is placed in the focus of the first glass (p) if a negative, or below the second glass if a positive eye- piece be employed. One edge of the slit is moveable, and, in using the instrument, the slit is first opened wide, so that a clear view of the object is obtained. The part of the object of which the spectrum is to be examined is then made to coincide with the fixed edge of the slit, and the moveable edge is screwed up, until a brilliant coloured spectrum is produced. The absorption-bands will then be readily found by slightly altering the focus. This contrivance answers perfectly for opaque objects. SPECTRO-MICROSCOPT. 123 without any preparation ; and, when desirable, the same prism can be placed below the stage, and a micrometer used in the eye-piece of the microscope, thus avoiding a multiplication of apparatus. The latest improvement is that shown in fig. 75, also effected by Mr. Browning, who deserves great credit for the skill displayed in the invention and construction of this new and elegant micro-spectroscope. Fig. 75. — The Sorby -Br owning Micro-spectroscope. The prism is contained in a small tube, which can be removed at pleasure. Below the prism is an achromatic eye-piece, having an adjustable slit between the two lenses, the upper lens being furnished with a screw motion to focus the slit. A side slit, capable of adjust- ment, admits, when required, a second beam of light from any object whose spectrum it is desired to compare with that of the object placed on the stage of the microscope. This second beam of light strikes against a very small prism, suitably placed inside the apparatus, and is reflected up through the compound prism, forming a spectrum in the same field with that obtained from the object on the stage. A is a brass tube, carrying the compound direct vision prism, b, a milled head, with screw motion to adjust tli9 focus of the achromatic eye lens, c, milled head, with screw motion to open or shut the slit vertically. Another 124 THE MICROSCOPE. screw at right angles to c, but which from its position could not be shown in the cut, regulates the slit hori- zontally. This screw has a larger head, and when once recognised cannot be mistaken for the other, d d is an appa- ratus for holding a small tube, that the spectrum given by its contents may be compared with that from an object on the stage, e is a square-headed screw, opening and shut- ting a slit to admit the quantity of light required to form the second spectrum. A light entering the round hole near e, strikes against the right-angled prism, which we have mentioned as being placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object be placed in a suitable position with reference to the round hole, its spectrum will be obtained, f shows the position of the field lens of the eye-piece. G is a tube made to fit the microscope to which the instrument is applied. To use this instrument insert g, like an eye-piece in the microscope tube, taking care that the slit at the top of the eye-piece is in the same direction as the slit below the prism. Screw on to the microscope the object-glass required, and place the object whose spectrum is to be viewed on the stage. Illuminate with the stage mirror if it be transparent; with mirror, Lieberkuhn, and dark well, by side reflector, or bull's-eye condenser if opaque. Remove a, and open the slit by means of the milled-head, not shown in cut, but which is at right angles to d d. When the slit is sufficiently open the rest of the apparatus acts like an ordinary eye-piece, and any object can be focussed in the usual way. Having focussed the object, replace A, and gradually close the slit till a good spectrum is obtained. The spectrum will be much improved by throwing the object a little out of focus. Every part of the spectrum differs a little from adjacent parts in refrangibility, and delicate bands or lines can only be brought out by accurately focussing that particular part of the spectrum. This can be done by the milled head B. Disappointment will occur in any attempt at delicate in- vestigation if this direction be not carefully attended to. At E a small mirror is attached, which is omitted in the diagram to prevent confusion. It is like the mirror below SPECTRO- MICROSCOPY. 125 tlie stage of a microscope, and is mounted in a similar manner. By means of this mirror light may be reflected into the eye-piece, and in this way two spectra may be procured from one lamp. For observing the spectra of liquids in cells or tubes of considerable diameter, say not less than rVth of an inch, powers from 2 inch to 1 inch will be the most suitable, and of course low powers only can be used to investigate the spectra of opaque objects ; but when the spectra of very minute objects are to be viewed, powers of from half an inch to one-twentieth, or even higher, may be employed. Blood, madder, aniline red, permanganate of potash, in crystals or solution, are convenient substances to begin experiments with. Solutions when made too strong pro- duce dark clouds instead of absorption bands. Professor Church has recently pointed out that zircon, an almost colourless stone, gives well-defined absorption-bands. Mr. Sorby says of the correct performance of a spectrum adaptation, " The best tests are, first, that the absorption- bands in blood can be seen when they are very faint ; second, to well divide the bands in permanganate of potash ; and, third, to see distinctly the very fine line given in the red by a solution of chloride of cobalt dis- solved in a concentrated cold solution of chloride of calcium : there is a line so fine that it looks like a Fraunhofer's line. An instrument that shows all these well is all that can be desired. " The objects most easily obtained, and which furnish us with the greatest variety of spectra, are coloured crystals, coloured solutions, and coloured glasses. The spectrum microscope enables us to examine the spectra of very minute crystals, of very small quantities of material in solution, and of small blow-pipe beads. As previously named, the thickness of the object makes a very great difference in the spectrum. For example, an extremely thin crystal of ferricyanide of potassium cuts off all the blue rays, and leaves merely red, orange, yellow, and more or less green ; but on increasing the thickness, the green and yellow disappear ; and when very much thicker, little else but a bright red light is transmitted. In all such 126 THE MICROSCOPE. cases, the apparent magnitude of the effect of an increase in thickness is far greater when the object is thin than when thick, and past a certain thickness the change is comparatively very slight. If only small crystals can be obtained, it is well to mount a number of different thick- nesses ; but when it is possible to obtain crystals of suf- ficient size, it is far better to make them into wedge- shaped objects, since then the effect of gradual change in thickness can easily be observed. Different kinds of crystals require different treatment, but, as a general rule, I find that it is best to grind them on moderately soft Water-of-Ayr stone with a small quantity of water, which soon becomes a saturated solution, and then to polish them with a little rouge spread on paper laid over a flat surface ; or else, in some cases, to dissolve off a thin layer by carefully rubbing the crystal on moist blotting-paper until the scratches are removed. Then, whenever it is admissible, I mount the crystal on a glass, and also cover it with a piece of thin glass with Canada balsam. Strongly coloured solutions may be examined in test-tubes, or may be kept sealed up in small bottles made out of glass tubes, the light then examined being that which passes through the centre of the tube from side to side. (Most of these solutions require the addition of a little gum Arabic to make them keep.) Such tubes may be laid on the ordinary stage, or laid on the stage attached to the eye-piece. Smaller quantities may be examined in cells cut out of thick glass tubes, one side being fixed on. the ordinary glass with Canada balsam, like a microscopic object, and the other covered, with thin glass, which readily holds on by capillary attraction, or may be cemented fast with gold size or Canada balsam, if it be desirable to keep it as a permanent object. Such tubes may be made of any length that may be required for very slightly-coloured solutions. Cells made out of spirit thermometer tubes, so as to be about j^th of an inch in diameter, and £ an inch long, are very suitable for the examination of very small quan- tities ; but where plenty of material can be obtained, it is far better to use cells cut out of strong tubes, having an interior diameter of about £ths of an inch, cut wedge- shape, so that the thickness of the solution may be ith SPECTRO-MICROSCOPT. 127 of an inch, or more, on one side, and not above -/oth on the other ; and then the effect of different thicknesses can easily be ascertained. " Fortunately, the various modifications of the colouring matter of blood yield such well-marked and characteristic spectra, that there are few subjects to which the spectrum- microscope can be applied with greater advantage than the detection of blood-stains, even when perfectly dry. For this purpose condensed light may be used, provided a sufficiently bright light be thrown on the object by means of a parabolic reflector or bull's-eye condenser. A speck of blood on white paper shows the spectrum very well, provided it be fresh, and the colour be neither too dark nor too light, and the thickness of the colouring matter neither too great nor too little. A mere atom, invisible to the naked eye, which would not weigh above the nnrihnroth of a grain, is then sufficient to show the characteristic absorption-bands. They are, however, far better seen in solution. About yfeth of a grain of liquid blood, in a cell of j^th of an inch in diameter, and \ an inch long, gives a spectrum as well marked as could be desired. In exhibiting the instrument to a number of persons at a meeting, I have found that no object is more convenient, or excites more attention, than one in which a number of cells are fixed in a line, side by side, containing a solution of various red-colouring matters. In one I mount blood, which gives two well-marked absorption-bands in the green ; in another magenta, which gives only one distinct band in the green; and in another I place the juice of some red-coloured fruit, which shows no well-defined absorption-band. Keeping a larger cell containing blood on the stage attached to the eye-piece, these three objects can be passed one after another in front of the object-glass, and the total difference between the spectrum of blood and that of either fruit-juice or magenta, and the perfect iden- tity of the spectra when both are blood, can be seen at a glance. By holding coloured glasses, which cut off the red, but allow the green rays to pass, we can readily, show how the presence of any foreign colouring-matter, which entirely alters the general colour, might not in any degree disguise the characteristic part of the spectrum; and by 128 THE MICROSCOPE. changing the cell held on the eye-piece for a tube con- taining an ammoniacal solution of cochineal, it is easy to show that, though it yields a spectrum with two absorp- tion-bands, more like those due to blood than I have seen in any other substance, they differ so much in relation, size, and position, that there is no chance of their being confounded when compared together side by side." 1 We have been usually taught that the red-blood corpuscles consisted of two substances, hasmatin and globulin ; but later researches lead to the belief that they consist of one crystalline substance, termed globulin or hcemato-globu- lin. A solution of this substance, as well as of certain products of its decom- position, produces the absorption-bands referred to. Hoppe was the first to demonstrate this fact : he found that a very dilute solution of blood was suffi- cient fur the purpose. Professor Stokes proved that this colouring-matter is capable of existing in two states of oxidation, and that a very different spectrum is produced according as the substance, which he has termed cruorine, is in a more or less oxidised condition.2 Proto-sulphate of iron, or proto-chloride of tin, causes the reduction of the colouring-matter, and, by exposure to air, oxygen is absorbed, and the solution again exhibits the spectrum character- istic of the more oxidised state. The different sub- stances obtained from blood colouring - matter produce different bands. Thus, hcematin gives rise to a band in the red spec- trum ; hcemato - globulin produces two bands, the second twice the breadth of the first in the yellow portion of the spectrum between the lines d and e, No. 1. The absorption- bands differ according to the strength of the solu- tion employed, and the medium in which the blood- salt is dissolved ; but an exceedingly minute pro- portion dissolved in water is sufficient to bring out very distinct bands. No. 1. Arterial Blood, Scarlet Cruorine. No. 2. Venous Blood, Purple Cruorine. No. 3. Blood treated with Acetic Acid. II No. 4. Solution of Hcematin. ABSORPTION-BANDS, AFTER STOKES. (1) Popular Science Review, January, 1866. (2; Professor Stokes, "On the Reduction and Oxidation of the Colouring- matter of the Blood" (Proceed. Royal Soc. vol. xiii. p. 355). The oxidising solution is made as follows : — To a solution of proto-sulphate of iron, enough tartaric acid is added to prevent precipitation by alkalies. A small quantity of this solution, made slightly alkaline by ammonia or carbonate of soda, is to be added to the weak solution of blood in water. THE CAMERA LUCIDA. 129 THE CAMERA LUCIDA. The Camera Lucida, fig. 79, invented by Dr. Wollaston, in 1807, is a valuable addition to the microscope, for making drawings of structures, or for obtaining, with a micrometer, measurements of objects. It consists Fig. 79. of a four-sided prism of glass, set in a brass frame or case, us represented in the figure annexed ; and by means of a short tube it is slipped over the front part of either of the eye-pieces, its cap having been previously removed. Mr. Koss attaches the prism, by two short supports, to a circular piece of brass at the end of the tube ; on this it can be slightly rotated, whilst the prism itself can also be turned up or down, by means of two screws with milled K 130 TIIE MICROSCOPE. heads. So arranged, the camera may be adapted to the eye-piece, the microscope having been previously placed in a horizontal position ; if the light be then reflected up through the compound body, an eye placed over the square hole in the frame of the prism will see the image of any object on the stage upon a sheet of white paper placed on the table immediately below it. But should it happen that the whole of the field of view is not well illuminated, then, either by revolving the circular plate or turning the prism upon the screws, the desired object will be effected. The chief difficulty in the use of this instru- ment is for the artist to be able to see, at one and the same time, the pencil and the image. To facilitate this in some measure, the one or two lenses below the prism will cause the rays from the paper and pencil to diverge at the same angle as those received from the prism, whereby both object and pencil may be seen with the same degree of distinctness. The following is the method for employing the Camera Lucida with the microscope. The first step to be taken, after the object about to be drawn has been properly illuminated, adjusted, and brought into the centre of the field of view, is to place the compound body of the micro- scope in a horizontal position, and to fix it there. The cap of the eye-piece having been removed, the camera is to be slid on in its stead : if the prism be properly adjusted, a circle of white light, with the object within it, will be seen on a piece of white paper placed on the table immediately under the camera, when the eye of the observer is placed over the uncovered edge of the prism, and its axis directed towards the paper on the table. Should, however, the field of view be only in part illumi- nated, the prism must either be turned round on the eye- piece, or revolved on its axis, by the screws affixed to its frame-work, until the entire field is illuminated. The uext step is to procure a hard, sharp-pointed pencil, which, in order to be well seen, may be blackened with ink round the point ; the observer is then to bring his eye so near the edge of the prism that he may be able to see on the paper, at one and the same time, the pencil-point and the image of the object. When he has accomplished this, the THE CAMSRA LUCIDA. 131 pencil may be moved along the outline of the image, so as to trace it on the paper. However easy this may appear in description, it will be found very difficult in practice ; and the observer must not be foiled in his first attempts, but must persevere until he accomplishes his purpose. Sometimes he will find that he can see the pencil-point, and all at once it disappears : this happens from the movement of the axis of the eye. The plan then is to keep the pencil upon the paper, and to move about the eye until the pencil is again seen, when the eye is to be kept steadfastly fixed on the same position until the entire outline is traced. It will be found the best plan for the beginner to employ at first an inch object-glass, and some object, such as a piece of moss, that has a well- defined outline, and to make many tracings, and examine how nearly they agree with each other ; and when he has succeeded to his liking, he may then take a more compli- cated subject. If the operation is conducted by lamp- light, it will be found very advantageous not to illuminate the object too much, but rather to illuminate the paper on which the sketch is to be made, either by means of the lamp with the condensing lens, or a small taper placed near it. When the object is so complicated that too much time would be required for it to be completed at one sitting, the paper should be fixed to the table by a weight, or on a board by drawing-pins, An excellent plan to adopt is to fix the microscope on a piece of deal about two feet in length and one foot in breadth, and to pin the paper to the same ; there will then be no risk of the shift- ing of the paper, as, when the wood is moved, both micro- scope and paper will move with it. In all sketches made by the camera, certain things must be borne in mind ; the eye, when once applied to it, should be kept steadily fixed in one position; and if the sketches are to be reserved for comparison with others, the distance between the paper and the camera should be alwavs the same. A short rule or a piece of wood may be placed between the paper and the under-surface either of the compound body or the arm supporting it, in order to regulate the distance, as the size of the drawing made by the camera will depend upon the distance between it and the paper. It is also very desirable, k2 132 THE MICROSCOPE. before the camera is removed, to make a tracing in some part of the paper of two or more of the divisions of the stage micrometer, in order that they may form a guide to the measurement of all parts of the object. Some persons cover the whole of the drawing over with squares, to facili- tate, not only the measurement, but in order that a larger or smaller drawing may be made from it than that given by the camera. It must be recollected, that an accurate outline is the only thing the camera will give : the finishing of the picture must depend entirely upon the skill of the artist himself. ON THE POLARISATION OP LIGHT AS APPLIED TO THE ; MICROSCOPE. Common light moves in two planes at right angles to each other, polarised light moves only in one plane. Common light may be turned into polarised light either by transmission or reflection ; in the first instance, one of the planes of common light is got rid of by reflection, in the other, by absorption. Huyghens was among the first to notice that a ray of light has not the same properties in every part of its circumference, and he compared it to a magnet or a collection of magnets ; and supposed that the minute particles of which it was said to be composed had different poles, which, when acted on in certain ways, arranged themselves in particular positions ; and thence the term polarisation, a term having neither reference to cause nor effect. It is to Malus, however, who, in 1808, discovered polarisation by reflection, that we are indebted for the series of splendid phenomena which have since that period been developed ; phenomena of such surpassing beauty as far to exceed all ordinary objects presented to our eyes under the microscope. It has been truly observed by Sir David Brewster, that " the application of the principles of double refraction to the examination of structures is of the highest value. The chemist may per- form the most dexterous analysis ; the crystallographer may examine crystals by the nicest determination of their forms and cleavage : the anatomist or botanist may use the dissecting knife and microscope with the most exqui- site skill; but there are still structures in the mineral, POLARISED LIGHT. 133 vegetable, and animal kingdoms, which defy all such modes of examination, and which will yield only to the magical analysis of polarised light. A body which is quite trans- parent to the eye, and which might be judged as mono- tonous in structure as it is in aspect, will yet exhibit, under polarised light, the most exquisite organisation, and will display the result of new laws of combination which the imagination even could scarcely have conceived. In evidence of the utility of this agent in exploring mineral, vegetable, and animal structures, the extraordinary organi- sation of Apophyllite and Analcime may be referred to ; also the symmetrical and figurate depositions of siliceous crystals in the epidermis of equisetaceous plants, and the wonderful variations of density in the crystalline lenses of the eyes of animals. If we transmit a beam of the sun's light through a cir- cular aperture into a darkened room, and if we reflect it from any crystallised or uncrystallised body, or transmit it through a thin plate of either of them, it will be reflcted and transmitted in the very same manner, and with the same intensity, whether the surface of the body is held above or below the beam, or on the right side or left, pro- vided that in all cases it falls upon the surface in the same manner; or, what amounts to the same thing, the beam of solar light has the same properties on all its sides; and this is true, whether it is white light as directly emitted from the sun, or from a candle or any burning or self- luminous body; and all such light is called common light. A section of such a beam of light will be a circle, like a b cd, tig. 80; and we shall distinguish the section of a beam 134 THE MICROSCOPE. of common light by a circle with two diameters ah, cd, at right angles to each other. If we now allow the same beam of light to fall upon a rhomb of Iceland spar, and examine the two circular beams, Oo E e, formed by double refraction, we shall find, 1st, that the beams 0 o E e have different properties on different sides, so that each of them differs in this respect from the beam of common light. 2d. That the beam 0 o differs from E e in nothing ex- cepting that the former has the same properties at the sides a b' that the latter has at the sides c and d ; or in general that the diameters of the beam, at the extremities of which the beam has similar properties, are at right angles to each other, as a' b' and e' d' for example. Thesa two beams, 0 o, E e, are therefore said to be polarised, or to be beams of polarised light, because they have sides or poles of different properties and planes passing through the lines ah, cd) or d b' , c' d, are said to be the planes of polarisation of each beam, because they have the same property, and one which no other plane passing through the beam possesses. Now it is a curious fact, that if we cause the two polarised beams 0 o, E e to be united into one, or if we produce them by a thin plate of Iceland spar, which is not capable of separating them, we obtain a beam which has exactly the same properties as the beam abed of common light. Hence we infer that a beam of common light, a b c d, consists of two beams of polarised light, whose plane of polarisation, or whose diameters of similar properties, are at right angles to one another. If 0 o be laid above E e, it will produce a figure like abed; and we shall therefore represent polarised light by such figures. If we were to place 0 o above E e, so that the planes of polarisa- tion a! b' and c d coincide, then we should have a beam of polarised light twice as luminous as either 0 o or E e, and possessing exactly the same properties; for the lines of similar property in the one beam coincide with the lines of similar property in the other. Hence it follows that there are three ways of converting a beam of common lig] t, a b c d, into a beam or beams of polarised light. 1st. We may separate the beam of common light, abed, POLARISED LIGHT. 135 component parts 0 o and E e. 2d. We may turn round the planes of polarisation, abed, till they coincide or are parallel to each other. 3d. We may absorb or stop one of the beams, and leave the other, which will consequently be in a state of polarisation."1 The first of these methods of producing polarised light is that in which we employ a doubly refracting crystal, and was first discovered to exist in a transparent mineral substance called Iceland spar, calcareous spar, or carbonate of lime. This substance is admirably adapted for exhibit- ing this phenomenon, and is the one generally used by microscopists. Iceland spar is composed of fifty-six parts of lime and forty-four parts of carbonic acid ; it is found in various shapes in almost all countries; but whether found in ciystals or in masses, we can always cleave it or split it into shapes re- presented by fig. 81, which is called a rhomb of Iceland spar, a solid bounded by six equal and similar rhomboidal surfaces, whose sides & ^ are parallel, and whose angles b a c, Fig. 8i. a c d, are 101° 55' and 78° 6'. The line a x, called the axis of the rhomb, or of the crystal, is equally inclined to each of the six faces at an angle of 45° 23.' It is very transparent, and generally colourless. Its natural faces when it is split are commonly even and per- fectly polished ; but when they are not so, we may, by a new clevage, replace the imperfect face by a better one, or we may grind and polish an imperfect face. It is found that in all bodies where there seems to be an irregularity of structure, as salts, crystallised minerals, &c, on light passing through them, it is divided into two distinct pencils. If we take a crystal of Iceland spar, and look at a black line or dot on a sheet of paper, there will appear to be two lines or dots; and on turning the spar round, these objects will seem to turn round also; and twice in the revolution they will fall upon each other, which occurs when the two positions of the spar are exactly opposite, that is, when turned one-half from the position (l) Brewster's " Optics " / N 0^ Ay / >-' / / / / 136 THE MICROSCOPE. where it is first observed. In the accompanying diagram, fig. 82, the line appears double, as a 6 and c d, or the dot, as e and/. Or allow a ray of light, g h, to fall thus on the crystal, it will in its passage through be separated into two rays, hf,he; and on coming to the opposite surface of the crystal, they will pass out at ef in the direction of i k, parallel to g h. The plane I m n o is designated the prin- cipal section of the crystal, and the line drawn from the solid angle I to the angle o is where the axis of the crystal is contained; it is also the optic axis of the mineral. Now when a ray of light passes along this axis, it is undivided, and there is only one image; but in all other directions there are two. If two crystals of Iceland spar be u*ed, the only differ- ence will be, that the objects seem farther apart, from the increased thickness. But if two crystals be placed with their principal sections at right angles to each other, the ordinary ray refracted in the first will be the extraordinary in the second, and so on vice versd. At the intermediate position of the two crystals there is a subdivision of each ray, and therefore four images are seen ; when the crystals are at an angle of 45° to each other, then the images are all seen of equal intensity. Mr. Nicol first succeeded in making rhombs of Iceland spar into single-image prisms, by dividing one into two equal portions. His mode of proceeding is thus described in the Edinburgh Philosophical Journal (vol. vi. p. 83) : POLARISED LIGHT. 137 "A rhomb of Iceland spar of one-fourth of an inch in length, and about four-eighths of an inch in breadth and thickness, is divided into two equal portions in a plane, passing through the acute lateral angle, and nearly touching the obtuse solid angle. The sectional plane of each of these halves must be carefully polished, and the portions cemented firmly with Canada balsam, so as to form a rhomb similar to what it was before its division ; by this management the ordinary and extraordinary rays are so separated that only one of them is transmitted : the cause of this great divergence of the rays is considered to be owing to the action of the Canada balsam, the refractive index of which (1*549) is that between the ordinary (1-6543) and the extraordinary (1*4833) refraction of calcareous spar, and which will change the direction of both rays in an opposite manner before they enter the posterior half of the combination." The direction of rays Fig. 83. passing through such a prism is indicated by the arrow, fig. 83, and the combination is shown mounted, one for Fig. 84. Fig. 85. use under the stage of the microscope, fig. 84, termed the polariser; another, fig. 85, screwed on to and above the 1S8 THE MICROSCOPE. object-glasses, is called the analyser. The definition is better if the analyser be placed at top of the A eye-piece, and it is more easily rotated than the polariser. Method of using the polarising Prism, fig. 84. — After having adapted it to slide into a groove on the under-surface of the stage, it is held in its place by turning the smal] milled-head screw at one end : the other prism, fig. 85, is screwed on above the object-glasses, and made to pass into the body of the microscope itself. The light having been reflected through them by the mirror, it becomes necessary to make the axes of the two prisms coincide ; this is done by regulating the milled-head screw, until by revolving the polarising prism, the field of view is entirely darkened twice during one revolution. This should be ascertained, and carefully corrected by the maker and adapter of the apparatus. If very minute salts or crystals are to be viewed, it is preferable to place the ana- lyser above the eye-piece; it will then require to be mounted as in fig. 86. Thus the polariscope consists of two parts ; one for polarising, the other for analysing or testing the light. There is no essen- tial difference between the two parts, except what convenience or economy may lead us to adopt ; and either part, there- fore, may be used as polariser or analyser ; but whichever we use as the polariser, the other becomes the analyser. The tourmaline, a precious stone of a neutral or bluish tint, forms an excellent analyser; it should be cut about 2^th of an inch thick, and parallel to its axis. The great objection to it is, that the transmitted polarised beam is more or less coloured. The best tourmaline to choose is the one that stops the most light when its axis is at right angles to that of the polariser, and yet admits the most when in the same plaue. It is necessary to choose the stone as perfect as possible, the size is of no importance when used with the microscope. In the illumination of objects by polarised light, when under view with high powers, for the purpose of obtaining Fig. 86. POLARISED LIGHT. 130 the maximum effect, it is also requisite that the angle of aperture of the polariser should be the same as the object- glass, each ray of which should be directly opposed by a ray of polarised light. The Polarising Condenser is merely an ordinary achromatic condenser of large aperture, close under the bottom lens of which is placed a plate of tour- maline, used in combination with a superposed film of selenite or not, as required. The effect of this arrangement on some objects is very remarkable, bringing out strongly colours which are almost invisible by the usual mode. The production of colour by polarised light has been thus most clearly and comprehensively explained by Mr. Woodward, in his " Introduction to the Study of Polarised Light."1 Fig. 87 a. IB o llfy 7t- V* \> h ~r.o n Yo H^ LI Yi» h Fig. 87 b. abed represent the rectangular vibrations by which a ray of common light is supposed to be propagated. e, a plate of tourmaline, called in this situation the polariser, and so turned that a b may vibrate in the plane of its crystallographical axis. (1) Mr. Woodward constructed a very available form of polariscope for most purposes ; the instrument is described in Elements of Natural Philosophy, bj Jabez Hogg. 1 10 THE MICROSCOPE. /, light polarised by e, by stopping the vibrations c d, and transmitting those of a b. g, a piece of selenite of such a thickness as to produce red light, and its complementary colour green. h, the polarised light / bifurcated, or divided into ordi- nary and extraordinary rays, and thus said to be de- polarised by the double refractor g, and forming two planes of polarised light, o and e, vibrating at right angles to each other. i, a second plate of tourmaline, here called the analyser, with its axis in the same direction as that of e, through which the several systems of waves of the ordinary and extraordinary rays h. not being inclined at a greater angle to the axis of the analyser than that of 45 degrees, are transmitted and brought together under conditions that may produce interferences. k, the waves ro and Re, for red light of the ordinary and extraordinary systems meeting in the same state of vibration, occasioned by a difference of an even number of half undulations, and thus forming a wave of doubled intensity for red light. I m, the waves Y o and y e and B o and B e for yellow and blue of the ordinary and extraordinary systems respec- tively meeting together, with a difference of an odd number of half undulations, and thus neutralising each other by interferences. n, red light, the result of the coincidence of the waves for red light, and the neutralisation by interferences of those for yellow and blue respectively. h, fig. 87 b, depolarised light, as fig. 87 a. i, the analyser turned one quarter of a circle, its axis being at right angles to that of i in fig. 87 a. k, the waves r o r e, for red light of the ordinary and extraordinary systems meeting together with a difference of an odd number of half undulations, and thus neutral- ising each other by interference. I m, the waves Y o y e and b o b e, for yellow and blue of the two systems severally meeting together in the same state of vibration, occasioned by the difference of an even number of half undulations, and forming by their coin- cidences waves of doubled intensity for yellow and blue light. POLARISED LIGHT. 141 n, green light, the result of the coincidences of the waves for yellow and blue light respectively^ and the neutralisation by interference of those for red light. By substituting Nicol's prisms for the two plates of tourmaline, and by the addition of the object-glass and eye-piece, the diagrams would then represent the passage of polarised light through a microscope. For showing objects by polarised light under the micro- scope that are not in themselves doubly refractive, put upon the stage a film of selenite, which exhibits, under ordinary circumstances, the red ray in one position of the polarising prism, and the green ray in another, using a double-image prism over the eye-piece ; each arc will assume one of these complementary colours, whilst the centre of the field will remain colourless. Into this field introduce any microscopic object which in the usual arrangement of the polariscope undergoes no change in colour, when it will immediately display the most brilliant effects. Sections of wood, feathers, algse, and scales, are among the objects best suited for this kind of exhibition. The power suited for the purpose is a two-inch object- glass, the intensity of colour, as well as the separating power of the prism, being impaired under much higher amplification ; although in some few instances, such as in viewing animalcules, the one-inch object-glass is perhaps to be preferred. Selenite is the native crystallised hydrated sulphate of lime. A beautiful fibrous variety called satin gypsum is found in Derbyshire. It is found also at Shotover Hill, near Oxford, where the labourers call it quarry-glass. Very large crystals of it are found at Montmartre, near Paris. The form of the crystal most frequently met with is that of an oblique rectangular prism, with ten rhomboidal faces, two of which are much larger than the rest. It is usually slit into thin laminae parallel to these large lateral faces ; the film having a thickness of from one- twentieth to the one sixtieth of an inch. In the two rec- tangular directions they allow perpendicular rays of pola- rised light to traverse them unchanged ; these directions are called the neutral axes. In two other directions, however, which form respectively angles of 45° with the 142 THE MICROSCOPE. neutral axes, these films have the property of double refraction. These directions are known as the depolarising axes. The thickness of the film of selenite determines the particular tint. If, therefore, we use a film of irregular thickness, different colours are presented by the different thicknesses. These facts admit of very curious and beau- tiful illustration, when used under the object placed on the stage of the microscope. The films employed should be mounted between two glasses for protection. Some persons employ a large film mounted in this way between plates of glass, with a raised edge, to act as a stage for supporting the object, it is then called the " selenite stage." The best film for the microscope is that which gives blue, and its complementary colour yellow. Mr. Darker has constructed a very neat stage of brass for this purpose, producing a mixture of all the colours by superimposing three films, one on the other ; by a slight variation in their positions, produced by means of an endless-screw motion, all the colours of the spectrum are shown. When objects are thus exhibited, we must bear in mind that all the negative tints, as we term them, are diminished, and all the positive ones increased ; the effect of this plate is to mask the true character of the phenomena. Polarised structures should therefore never be drawn and coloured under such conditions. Dr. Herapath, of Bristol, described a salt of quinine, which is remarkable for its polarising properties. The salt was first accidentally observed by Mr. Phelps, a pupil of Dr. Herapath's, in a bottle which contained a solution of disulphate of quinine: the salt is formed by dissolving disulphate of quinine in concentrated acetic acid, then warming the solution, and dropping into it carefully, and by small quantities at a time, a spirituous solution of iodine. On placing this mixture aside for some hours, brilliant plates of the new salt will be formed. The crystals of this salt, when examined by reflected light, have a brilliant emerald-green colour, with almost a metallic lustre ; they appear like portions of the elytrse of cantha- rides, and are also very similar to murexide in appearance. When examined by transmitted light, they scarcely possess POLARISED LIGHT. 143 any colour, there is only a slightly olive-green tinge; but if two crystals, crossing at right angles, be examined, the spot where they intersect appears perfectly black, even if the crystals are not one five-hundredth of an inch in thick- ness. If the light be in the slightest degree polarised — as by reflection from a cloud, or by the blue sky, or from the glass surface of the mirror of the microscope placed at the polarising angle 5Q° 45' — these little prisms immediately assume complementary colours : one appears green, and the other pink, and the part at which they cross is a chocolate or deep chestnut-brown, instead of black. As the result of a series of very elaborate experiments, Dr. Herapath finds that this salt possesses the properties of tourmaline in a very exalted degree, as well as of a plate of selenite ; so that it combines the properties of polarising a ray and of depolarising it. Dr. Herapath has succeeded in making artificial tourmalines large enough to surmount the eye-piece of the microscope; so that all experiments with those crystals upon polarised light may be made without the tourmaline or Nicol's prism. The brilliancy of the colours is much more intense with the artificial crystal than wThen employing the natural tourmaline. As an analyser above the eye-piece, it offers some advantages over the Nicol's prism in the same position, as it gives a perfectly uniform tint of colour over a much more exten- sive field than can be had with the prism.1 As these crystals are liable to be injured by damp, and thus lose their polarising property, when out of use they should be kept in a dark dry place. " The following experiments, if carefully performed, will illustrate the most striking phenomena of double refraction, and form a useful introduction to the practical application of this principle. (1) Dr. Herapath lias given a later and better process for the manufacture of these artificial tourmalines in the Quarterly Journal of Microscopical Science for January, 1854. " These beautiful rosette crystals are best made as follows : — Take a moderately strong solution of Cinchonidine in Herapath's test-fluid (as already described). A little of this is dropped on the centre of a slide and laid down for a time, until the first crystals are observed to be formed near the margin. The slide should now be placed upon the stage of the microscope, and the progress of formation of the crystals closely watched. When these are seen to be large enough, and it is deemed necessary to stop their further development, the slide must be quickly transferred to the palm of the hand, the warmth o; which will be found sufficient to stop further crystallization." 144 THE MICROSCOPE. "A plate of brass, fig. 88, three inches by one, perforated with a series of holes from about one-sixteenth to one- Fig. 88. — Rrd is represented by perpendicular lines ; Green by oblique. fourth of an inch in diameter; the size of the smallest should be in accordance with the power of the object-glass, and the separating power of the double refraction. " Experiment 1. — Place the brass plate so that the smallest hole shall be in the centre of the stage of the instrument; employ a low power (1^ or 2 inch) object-glass, and adjust the focus as for an ordinary microscopic object; place the double image prism over the eye-piece, and there will appear two distinct images; then, by revolving the prism, these will describe a circle, the circumference of which cuts the centre of the field of view ; the one is called the ordinary, the other the extraordinary ray. By passing the slide along, that the larger orifices may appear in the field, the images will not be completely separated, but will overlap, as represented in the figure. " Experiment 2. — Screw the Nicol's prism into its place under the stage, still retaining the double image prism over the eye-piece ; then, by examining the object, there will appear in some positions two, but in others only one image; and it will be observed, that at 90° from the latter position this ray will be cut off, and that which was first observed will become visible; at 180°, or one-half the circle, an alternate change will take place; at 270°, another change; and at 360°, or the completion of the circle, the original appearance. " Before proceeding to the next experiment, it will be as well to observe the position of the Nicol's prism, which should be adjusted with its angles parallel to the square parts of the stage. In order to secure the greatest brilliancy in the experiment, the proper relative position of the selenite may be determined by noticing the natural POLARISED LIGHT. 145 flaws in the film, which will be observed to run parallel with each other; these flaw3 should be adjusted at about 46° from the square parts of the stage, to obtain the greatest amount of depolarisation. "Experiment 3. — If we now take the plate of selenite thus prepared, and place it under the piece of brass on the stage, we shall see, instead of the alternate black and white images, two coloured images composed of the con- stituents of white light, which will alternately change by revolving the eyepiece at every quarter of the circle ; then, by passing along the brass, the images will overlap ; and at the point at which they do so, white light will be pro- duced. If, by accident, the prism be placed at an angle of 45° from the square part of the stage, no particular colour will be perceived; and it will then illustrate the phenomena of the neutral axis of the selenite, because when placed in that relative position no depolarisation takes place. The phenomena of polarised light may be further illustrated by the addition of a second double image prism, and a film of selenite adapted between the two. The systems of coloured rings in crystals cut perpendicularly to the principal axis of the crystal are best seen by employing the lowest object-glass." To show the phenomena of the rings reund the optic axes of the crystals, Mr. Lobb adopts the following plan, which is by far the best, and the rings are exhibited in the greatest perfection : — 1. The b eye-piece without a diaphragm, and the lenses so adjusted that the field-lens may be brought nearer to, or farther from the eye-lens as occasion may require; thus giving different powers, and different fields, and when adjusted for the largest field it will be full 15 inches, and take in the widest separation of the axis of the aragonite. 2. A crystal stage to receive the crystals, and to be placed over the eye-piece, so constructed as to receive a tourmaline, and that to turn round. 3. A tourmaline of a blue tint. 4. A large Nicol's prism as a polariser. 5. A common two-inch lens, not achromatic; which must be set in a brass tube long enough when screwed into 1-iG THE MICROSCOPE. the microscope to reach the polariser, that all extraneous light may be excluded. The concave mirror should be used with a bull's-eve condenser by lamplight. The condenser may be dispensed with by daylight. The above apparatus is furnished by Messrs. Powell and Lealand. The crystals best adapted to show the phenomena of rings round the optic axes, are : — Quartz. — A uniaxial crystal, one system of rings, no entire cross of black, only the ends of it, the centre being coloured, and as the tourmaline is revolved, the colour gradually changing into all the colours of the spectrum, one colour only displayed at once. Quartz. — Cut so as to exhibit right-handed polarisation. Quartz. — Cut so as to exhibit left-handed polarisation ; that is, the one shows the same phenomena when the tourmaline is turned to the right, as the other does when turned to the left. Quaiiz. — Cut so as to exhibit straight lines. Calc Spar. — A uniaxial crystal, one system of rings, and a black cross, which changes into a white cross on revolving the tourmaline, and the colours of the rings into their complementary colours, Topaz. — A biaxial crystal, although it has two axes, only exhibits one system of rings with one fringe, owing to the wide separation of the axes. The fringe and colours change on revolving the tourmaline; this is the case in all the crystals. Borax. — A biaxial crystal; the colours more intense than in topaz, but the rings not so complete, — only one set of rings taken in, from the same cause as topaz. Rochelle Salt. — A biaxial crystal ; the colours more widely spread. Very beautiful. Only one set of rings taken in. Carbonate of Lead. — A biaxial crystal, axes not much separated, both systems of rings exhibited, far more widely spread than those of nitre. Aragonite. — A biaxial crystal, axes widely separated ; but both systems of rings exhibited, and decidedly the best crystal for displaying the phenomena of biaxial crystals. The field-lens of the eye-piece requires to be brought as 10LARISED LIGHT. H7 close as possible to the eye-lens, to see properly the pheno- mena in quartz and aragonite ; it must be placed at an intermediate distance for viewing topaz, borax, Rochelle salt, and carbonate of lead ; it must be drawn out to its full extent to view nitre and calc spar. The powers of the micro-polariscope cannot be better displayed than in the exhibition of the foregoing pheno- mena; there is nothing more beautiful, and few studies more interesting and enlarging to the mind than that of light, whether common or polarised, which must be entered upon if the phenomena are to be understood. The crystal eye-piece, with an artificial tourmaline as an analyser, will be found very useful for polariscope objects generally; there is some spherical aberration, but the largeness of the field far more than compensates for the same; it does best for those objects that require the two- inch object glass. Mr. Darker of Lambeth, Messrs. Elliott of the Strand, Messrs. Home and Co., Newgate St., and M. Soliel of Paris, supply properly cut crystals. It was long believed that all crystals had only one axis of double refraction; but Brewster found that the great body of crystals, which are either formed by art, or which occur in the mineral kingdom, have two axes of double re- fraction, or rather axes around which the double refraction takes place; in the axes themselves there is no double refraction. Nitre crystallises in six-sided prisms with angles of about 120.° It has two axes of double refraction, along which a ray of light is not divided into two. These axes are each inclined about 2i° to the axes of the prism, and 5° to each other. If, therefore, we cut off a piece from a prism of nitre with a knife driven by a smart blow of a hammer, and polish the two surfaces perpendicular to the axes of the prism, so as to leave the thickness of the sixth or eighth of an inch, and then transmit a ray of polarised light along the axes of the prism, we shall see the double system of rings shown in figs. 89 and 90. When the line connecting the two axes of the crystal is inclined 45° to the plane of primitive polarisation, a cross is seen as at fig. 89, on revolving the nitre, it gradually l2 148 THE MICROSCOPE. assumes the form of the two hyperbolic curves, fig. 90. But if the tourmaline be revolved, the black crossed lines will Fig. 89. Fig. 90. be replaced by white spaces, and the red rings by green, the yellow by indigo, and so on. These systems of rings have, generally speaking, the same colours as those of thin plates, or as those of a system of rings round one axis. The orders of the colours commence at the centres of each system ; but at a certain distance, which corre- sponds to the sixth ring, the rings, instead of returning and encircling each pole, encircle the two poles as an ellipse does its two foci. When we diminish or increase the thickness of the plate of nitre, the rings are diminished or increased accordingly. Small specimens of salts may also be crystallised and mounted in Canada balsam for viewing under the stage of the microscope ; by arresting the crystallisation at certain stages, a greater variety of forms and colours will be obtained : we may enumerate salicine, asparagine, acetate of copper, phospho-borate of soda, sugar, carbonate of lime, chlorate of potassa, oxalic acid, and all the oxalates found in urine, with the other salts from the same fluid, a lew of which are shown at fig 91. Dr. W. B. Herapath contributed an interesting addi- tion to the uses of polarised light, by applying it to discover the salts of alkaloids, quinine, *tc. in the urine of patients. TOLARISED LIGUT. no Tie says : " It has long been a favourite subject of inquiry with the professional man to trace the course of remedies Fig. 91.— Urinary Salts. a, Uric acid; b. Oxalate of lime, octahedral crystals of; c, Oxalate of lime allowed to dry, forming a black cube; d, Oxalate of lime, as it occasionally appears, termed the dumb-bell crystal. in the system of the patient under his care, and to know what has become of the various substances which he might have administered during the treatment of the disease. " Having been struck with the facility of application, and the extreme delicacy of the reaction of polarised light, when going through the series of experiments upon the sulphate of iodo-quinine, I determined upon attempting to bring this method practically into use for the detection of minute quantities of quinine in organic fluids; and after more or less success by different methods of experimenting, I have at length discovered a process by which it is possible to obtain demonstrative evidence of the presence of quinine, even if in quantities not exceeding the one-millionth part of a grain; in fact, in quantities so exceedingly minute, that all other methods would fail in recognising its existence. Take for test fluid a mixture of three drachms of pure acetic acid, with one fluid-drachm of rectified spirits-of- wine, to which add six drops of diluted sulphuric acid " One drop of this test-fluid placed on a glass-slide, and the merest atom of the alkaloid added, in a short time 150 THE MICROSCOPE. solution will take place ; then, upon the tip of a very fine glass-rod let an extremely minute drop of the alcoholic solution of iodine be added. The first effect is the produc- tion of the yellow or cinnamon-coloured compound of iodine and quinine, which forms as a small circular spot ; the alcohol separates in little drops, which by a sort of repul- sive movement, drive the fluid away ; after a time, the acid liquid again flows over the spot, and the polarising crystals of sulphate of iodo-quinine are slowly produced in beautiful rosettes. This succeeds best without the aid of heat. " To render these crystals evident, it merely remains to bring the glass-slide upon the field of the microscope, with the selenite stage and single tourmaline, or Nicol's prism, beneath it ; instantly the crystals assume the two comple- mentary colours of the stage ; red and green, supposing that the pink stage is employed, or blue and yellow, pro- vided the blue selenite is made use of. All those crystals at right angles to the plane of the tourmaline, producing Fig. 92. — In this figure heraldic lines are adopted to denote colour. The dotted parts indicate yellow, the straight lines red, the horizontal lines blue, and the diagonal, -or oblique lines, green. The arrows show the plane of the tourmaline, a, blue stage ; b, red stage of selenite employed. that tint which an analysing-plate of tourmaline would produce when at right angles to the polarising-nlate ; POLARISED LIGHT. 151 whilst those at 90° to these educe the complementary tint, as the analysing-plate would also have done if revolved through an arc of 90°. " This test is so ready of application, and so delicate, that it must become the test, far excellence, for quinine: fig. 92, a and b. Not only do these peculiar crystals act in the way just related, but they may be easily proved to possess the whole of the optical properties of that remark- able salt of quinine, the sulphate of iodo-quinine. " To test for quinidine, it is merely necessaiy to allow the drop of acid solution to evaporate to dryness upon the slide, and to examine the crystalline mass by two tourma- lines, crossed at right angles, and without the stage. Immediately little circular discs of white, with a well- defined black cross very vividly shown, start into existence, should quinidine be present even in very minute traces. These crystals are represented in fig. 93. Fig. 93. " If wt employ the selenite stage in the examination of this objee, we obtain one of the most gorgeous appear- ances in tie whole domain of the polarisiDg-microscope : the black ?s oJJi Fig. 94.— Snow Crystals. SNOW CRYSTALS. 153 having a red and green fringe, whilst the four intermediate sectors are of a gorgeous orange-yellow. These appear- ances alter upon the revolution of the analysing-plate of tourmaline ; when the blue stage is employed, the cross will assume a blue or yellow tint, according to the position of the analysing-plate. These phenomena are analogous to those exhibited bv certain circular crvstals of boracic acid, and to those circular discs of salicine (prepared by fusion) ; the difference being, that the salts of quinidine have more intense depolarising powers than either of the other substances ; besides which, the mode of preparation effectually excludes these from consideration. Quinine prepared in the same manner as quinidine has a very different mode of crystallisation ; but it occasionally pre- sents circular corneous plates, also exhibiting the black cross and white sectors, but not with one tenth part of the brilliancy, which of course enables us readily to discrimi- nate the two." Ice doubly refracts, while water singly refracts. Tee takes the rhomboidic form ; and snow in its crystalline form may be regarded as the skeleton crystals of this system. A sheet of clear ice, of about one inch thick, and slowly formed in still weather, will show the circular rings and cross if viewed by polarised light. It is probable that the conditions of snow formation are more complex than might be imagined, familiar as we are with the conditions relating to the crystallisation of water on the earth's surface. Dr. Smallwood, of Isle Jesus, Canada East, has traced an apparent connection between the form of the compound varieties of snow crystals and the electrical condition of the atmosphere, whether nega- tive or positive ; and is instituting experiments for his better information on the subject. A great variety of animal, vegetable, and other sub- stances possess a doubly refracting or depolarising struc- ture, as : a quill cut and laid out flat on glass ; the cornea of a sheep's eye ; skin, hair, a thin section of a finger-nail ; sections of bone, teeth, horn, silk, cotton, whalebone ; stems of plants containing silica or flint ; barley, wheat, &c. The larger-grained starches form splendid objects ; tous- les-mois, being the largest, may be taken as a type of all 154 THE MICROSCOPE. the others. It presents a black cross, the arms of which meet at the hilum. On rotating the analyser, the black cross disappears, and at 90° is replaced by a white cross ; another, but much fainter black cross being per- ceived between the arms of the white cross. Hitherto, however, no colour is percep- tible. But if a thin plate of selenite be interposed between n , , _. . lg' '' , . . the starch-grains and the po- Potato Starch, seen under polarised , ° ... - r , light. lanser, most splendid and delicate colours appear. All the colours change by revolving the analyser, and become complementary at every quadrant of the circle. West and East India arrow-root, sago, tapioca, and many other starch-grains, present a similar appearance ; but in pro- portion as the grains are smaller, so are their markings and colourings less distinct. " The application of this modification of light to the illumination of very minute structures has not yet been fully carried out ; but still there is no test of differences in density between any two or more parts of the same substance that can at all approach it in delicacy. All structures, therefore, belonging either to the animal, vege- table, or mineral kingdom, in which the power of unequal or double refraction is suspected to be present, are those that should especially be investigated by polarized light. Some of the most delicate of the elementary tissues of animal, such as the tubes of nerves, the ultimate fibrillar of muscles, &c., are amongst the most striking subjects that may be studied with advantage under this method of illu- mination. Every structure that the microscopist is investigating should be examined by this light, as well as by that either transmitted or reflected. Objects mounted in Canada balsam, that are far too delicate to exhibit any structure under transmitted, will often be well seen under polarised light ; its uses, therefore, are manifold."1 (1) Quskett's Practical Treatise on t'.t Use of the .Vicroscope. APPLICATION OF PHOTOGRAPHY. 155 A fine effect may be obtained by using Furze's spotted lens, with a Herapathite polariser; see Mic. Soc. Trans. 2d series, vol. iii. p. 63. APPLICATION OF PHOTOGRAPHY TO THE MICROSCOPE. At the time this book was projected, it was thought that if the objects so beautifully exhibited under the microscope could be drawn by light on the page of the book, or on the wood-blocks, so that the engraver might work directly from the drawings thus made, truthfulness would be in- sured, and we should present to the reader a valuable record of microscopic research never before seen or attempted. But in this we were doomed to disappoint- ment by the existence of a patent, which presented ob- stacles too great to be surmounted ; and the idea was abandoned, with the exception of a few drawings then prepared, and ready to hand : the patent restrictions having been since removed, we have embodied them in our pages. The eye and feet of fly, antenna of moth, paddles of whirli- gig, with a few others, were first taken on a film of collo- dion, then floated off the glass on to the surface of a block of wood, the wood having been previously and lightly inked with printer's ink or amber-varnish, and the film gently rubbed or smoothed down to an even surface, at the same time carefully pressing out all bubbles of air or fluid. For the purposes of photography the only necessary addition to the ordinary microscope is that of a dark chamber ; it should indeed form a camera obscura, having at one end an aperture for the insertion of the eye-piece end of the microscopic tube, and at the other a groove for carrying the crown-glass for focussing. This dark chamber must not exceed eighteen inches in length ; for if longer, the pencil of light transmitted by the object-glass is dif- fused over too large a surface, and a faint and unsatis- factory picture results therefrom. Another advantage is, that pictures at this distance are in size very nearly equal to the object seen in the microscope. In some instances, Detter pictures are produced by taking away the eye-piece loG THE MICROSCOPE. of the microscope altogether. The time of producing the picture varies from five to twenty seconds, with the strength of the daylight. A camphine lamp, light Cannel coal-gas, or the lime-light, will enable a good manipulator to pro- duce pictures nearly equal to those produced by sun-light. Collodion offers the best medium, as a strong negative can be made to produce any number of printed positives. The light is transmitted from the mirror through the object and lenses, and brought to a focus on the ground- glass, or prepared surface of collodion, in the usual manner. Care must be taken not to use the burning focus of the lenses. The gas microscope may be used to make an enlarged copy of an object, it is only necessary to pin up against the screen a piece of prepared calotype paper to receive the reflected image. Mr. Wenham gives direc- tions for improving "microscopic photography" in the Quarterly Journal of Microscopical Science for January, 1855. In this paper he has shown how to insure quick and accurate focussing ; or, in other words, the making of the actinic and visual foci of the objective coincident. The simplest and cheapest way of producing coincidence is to screw a biconvex lens into the place of the back-stop of the object-glass, which thus acts as part of its optical com- bination. An ordinary spectacle lens, carefully centred and turned down to the required size, answers the purpose exceedingly well. An excellent method has been proposed and adopted by Mr. Wenham, for exhibiting the form of certain very minute markings upon objects. A negative photographic impression of the object is first taken on collodion, in the ordinary way, with the highest power of the microscope that can be used. After this has been properly fixed, it is placed in the sliding frame of an ordinary camera, and the frame end of the latter adjusted into an opening cut in the shutter of a perfectly dark room. Parallel rays of sunlight are then thrown through the picture by means of a flat piece of looking-glass fixed outside the shutter at such an angle as to catch and reflect the rays through the camera. A screen standing in the room, opposite the lens of the camera, will now receive an image, exactly as from a magic lantern, and the size of the image will be propor- APPLICATION OF PHOTOGRAPHY. 157 tionate to the distance. Ou this screen is placed a sheet of photogenic paper intended to receive the magnified picture. We ought to add, however, that it requires con- siderable practice to avoid the distortion and error of definition occasioned by a want of coincidence in the chemical and visual foci. Imperfections are much in- creased when the highest powers of the microscope are employed ; false notions of structure are also given, which is the case in Mr. Wenham's photograph of P. An- gulation. Mr. S. Highley has a mode of adapting an object-glass to the ordinary camera, for the purpose of taking microscopic objects on collodion and other surfaces, fig. 96 ; a sec- tional view of his arrangement is here given, which is Fig. 91. — Hiyliley's Camera. very compact, steady, and ever ready for immediate use. The tube a screws into the flange of a camera which has a range of twenty-four inches; the front of this tube is closed, and into it screws the object-glass b. Over a slides another tube c; this is closed by a plate, d, which extends beyond the upper and lower circumference of c, and carries a small tube, e, on which the mirror f is adjusted. To the upper part of D the fine adjustment G is attached ; this consists of a spring-wire coil acting on an inner tube, to which the stage-plate h is fixed, and is regulated by a gra- duated head, k, acting on a fine screw, likewise attached to 158 THE MICROSCOPE. the stage-plate, after the maimer of Oberhauser's micro- scopes. An index L is affixed opposite the graduated head k. The stage and clamp slides vertically on h ; and by sliding this up or down, and the glass object-slide hori- zontally, the requisite amount of movement is obtained to bring the object into the field. The object being brought into view, the image is roughly adjusted on the focussing- glass by sliding c on a ; the focussing is completed by aid of the fine adjustments g k, and allowance then made for the amount of non-coincidence between the chemical and visual foci of the object-glass. The difference in each glass employed should be ascertained by experiment in the first instance, and then noted. By employing a finely-ground focussing-glass greased with oil, this arrangement forms an agreeable method of viewing microscopical objects wTith both eyes, and is less fatiguing. As a very large field is presented to the observer, this arrangement might be advantageously employed for class demonstration. Fig. 97. — Highley's PJioto-micrographic Arrangement. This arrangement combines the most recent improve- ments of Dr. Maddox, and consists of a lens-carrier with ordinary adjustments; stage with gymbal motions so as to bring any object parallel to the surface of the object- glass ; bright ground illuminator, graduating diaphragm ; and a speculum reflector for giving the light from a single surface. CHAPTER III. PRF.LIMINARY DIRECTIONS — ILLUMINATION — ACCESSORV APPARATUS — < GILLETT'S, ROSS'S, WEBSTER'S, AKD OTHER CONDENSERS — OBLIQUE ILLUMINATION'— DOUBLE PRISM ILLUMINATION— THE LIEBERKiJHN— SIDB REFLECTOR— GAS-LAMPS— OBJECT FINDERS — COLLECTING STICKS — ANI- MALCULE CAGES— SECTION CUTTERS— PREPARING AND MOUNTING OB- JECTS, ETC. AVING selected an apartment *c2$> "with a northern aspect, and, if ^C'C^X possible, with only one window, #V and that not overshadowed by trees or buildings : in such a room, on a firm, steady table, keep your instruments and ap- paratus open, and at all times ready for observation. A large bell-glass will be found most convenient for keeping dust from the microscope when set up for use. In winter it will be proper to slightly warm the instrument before it is used, otherwise the jjerspi ration from the eye will condense on the eye-glass, and greatly impede vision. When you clean the eye-glasses, do not remove more than one at a time, and replace it before you touch another ; by so doing you will preserve the component glasses in their proper places : recollect that if intermingled they are useless. Keep a piece of well-dusted and very dry chamois leather, slightly impregnated with the finest tripoli or rotten-stone powder, in a small box, to wipe the glasses. 1G0 THE MICROSCOPE. When you look through the instrument, be sure to place your eye quite close to the eye-piece, otherwise the whole field of view will not be visible ; and observe, more- over, if you see a round disc of light, at least when the object is not on the slider-holder : if you do not, it is a sign that something is wrong ; perhaps the body is not placed directly before the stage aperture, or may not be properly directed towards the light. Use the smallest amount of light possible, if you work for any length of time. Choose a steady light, with a shade to protect the eyes, one of the old-fashioned fan-shades will be found useful for this purpose : use the eyes alternately. Sit in a comfortable position, and bring the instrument to the proper angle, which will prevent congestion of the eyes ; this is indicated if the microscopist is annoyed with little moving objects apparently floating before them : if the eye-lashes are reflected from the eye-glass, you are looking upon the eye-glass instead of through it. Take care that the mirror is properly arranged. The following are Sir David Brewster's excellent direc- tions for viewing objects : — " First. Protect the eve from all surrounding light, let- ting only the rays which proceed from the illuminated centre of the object fall upon it. " Secondly. Delicate observations should not be made when the fluid which lubricates the cornea is in a viscid state, or there is any irritation or inflammation about any part of the eye. " Thirdly. The best position for microscopic observations is with the microscope bent to such an angle with the body, that the head may always remain in a natural and easy attitude ; consequently, the worst position would be that which compels us to look downwards vertically. " Fourthly. If we lie horizontally on the back, parallel markings and lines on objects will be seen more perfectly when their direction is vertical, or in a contrary direction to that in which the lubricating fluid descends over the cornea of the eye. " Fifthly. Only a portion of the object should be viewed at one time, and every other part excluded. The light which illuminates that part should be admitted through a ILLUMINATING THE OBJECTS. 101 small diaphragm ; at night, from the concentrated light of a sperm-oil or gas lamp, having a faint blue-tinted chim- ney-glass, to correct the yellow colour which predominates in all our artificial illumination. If in the day-time, close a portion of the window-shutters. " Sixthly. In all cases when high powei'S are used, the intensity of the illumination should be increased by optical contrivances below the object and stage : this is generally effected by using achromatic condensers beneath the stage. The apparatus for illumination should be as perfect as the magnifying power." If these directions are strictly followed, no injury to the eyes from using a microscope need be feared. Mr. Ross very properly remarks, that the manner in which an object is lighted is second in importance only to the excellence of the glass through which it is seen. When investigating any new or unknown specimen, it should be viewed in turns by every description of light, direct and oblique, as a transparent object and as an opaque object, with strong and with faint light, with large angular pencils thrown in all possible directions. Every change will pro- bably develope some new fact in reference to the structure of the object, which should itself be varied in the mode of mounting in every possible way. It should be seen both wet and dry, and immersed in fluids of various qualities and densities ; such as water, alcohol, oil, and Canada balsam ; the last having a refrac- tive power nearly equal to that of glass. If the object be a delicate vegetable tissue, it will be, in some respects, rendered more visible by gently heating 01 scorching before a clear fire, between two plates of glass. In this way the spiral vessels of asparagus and other similar vegetables will be beautifully displayed. Dyeing the objects in tincture of iodine, or some one of the dye° woods, will, in some cases, answer the purpose better. But the principal question in regard to illumination is the magnitude of the illuminating pencil, particularly in reference to transparent objects. Generally speaking, the illuminating pencil should be not quite so large as can be received by the lens : any light beyond this produces indistinctness and glare. The superfluous light from the IT) 2 THE MICROSCOPE. mirror may be cut off by a screen, having various-sized apertures placed below the stage. The Diaphragm, fig. 08, is the instrument used for effecting this purpose. It consists of two plates of brass, one of which is perforated with four or five holes of dif- Fig- 98.— The Diaphragm. ferent sizes ; this plate is of a circular figure, and is made to revolve upon another plate by a central pin or axis ; this last plate is also provided with a hole as large as the largest in the diaphragm-plate, and corresponds in situa- tion to the axis of the compound body. To ascertain when either of the holes in the diaphragm-plate is in the centre, a bent spring is fitted into the second plate, and rubs against the edge of the diaphragm-plate, which is provided with notches. The space between the smallest and largest is great enough to use for the purpose of shut- ting off all the light from the mirror. Gillett's Illuminator, or Condenser. — The advan- tages of employing an achromatic condenser were first pointed out by Dujardin, since which time an object-glass has been frequently but inconveniently employed ; and more recently achromatic illuminators have been con- structed by most of our instrument makers. Some years since, Mr. Gillett was led by observation to appreciate the importance of controlling not merely the quantity of light which may be effected by a diaphragm placed anywhere between the source of light and the object, but the angle of aperture of the illuminating pencil, which can be effected only by a diaphragm placed immediately behind the achromatic illuminating combination. Such a diaphragm is represented in fig. 99, manufactured by Mr. Ross : it consists of an achromatic illuminating lens c, which is GILLETTS ILLUMINATOR. 163 about equal to an object-glass of one-quarter of an inch focal length, having an angular aperture of 80°. This lens is placed on the top of a brass tube, intersecting Fig. 99.— GilleWa Condenser. which, at an angle of about 25°, is a circular rotating brass plate a b, provided with a conical diaphragm, having a series of circular apertures of different sizes h g, each of which in succession, as the diaphragm is rotated, propor- tionally limits the light transmitted through the illumi- nating lens. The circular plate in which the conical dia- phragm is fixed is provided with a spring and catch ef, the latter indicating when an aperture is central with the illuminating lens, also the number of the aperture as marked on the graduated circular plate. Three of these apertures have central discs, for circularly oblique illumi- nation, allowing only the passage of a hollow cone of light to illuminate the object. The illuminator above described is placed in the secondary stage i i, which is situated below the general stage of the microscope, and consists of a cylindrical tube having a rotatory motion, also a rect- angular adjustment, which is effected by means of two screws I m, one in front, and the other on the left side of :.ts frame. This tube receives and supports all the various " h are applied and adjusted so that the image of the aperture in the object-glass, and the images of the apertures at the lenses and in the diaphragms contained in the tube which holds the illuminating combination, may all be in focus at the same time, as with the same adjustment they may be brought suffi- ciently near in focus to recognise their ecutricity. gillett's illuminator. 155 be adjusted by its sliding-tube (without disturbing the microscope-tube) until the images of the diaphragms in the object-glass and centering lens are distinctly seen. The illuminator should now be moved by means of the left-hand screw on the secondary stage, while looking through the microscope, to enable the observer to recog- nise the diaphragm belonging to the illuminator, and by means of the two adjusting screws, to place this diaphragm central with the others; thus, the first condition, that of centricity, will be accomplished. Remove the white paper from the mirror, and also the centering-glass, and replace the cap on the eve-piece, also the object on the stage, of which distinct vision should then be obtained by the rack and pinion, or fine screw adjustment, should it have become deranged. The second process is to ascertain that the fittest con- centration of light is obtained. For this purpose the mirror should now be so inclined that the image of some intercepting distant object, such as a house-top, or chim- ney, tree, window-frame, or (if lamp-light be employed) the lamp's flame may be brought into the field of view; these, though not distinctly seen, may be recognised by partially darkening or otherwise occupying the field; then distinct vision of such object must be obtained by means of the rack and pinion moving the secondary stage to and from the object. Excepting the case of the lamp's flame, the above objects are considered as the representatives of the source of light ; for when daylight is employed — as, for example, a white cloud — its motion prevents the image being easily produced: then it is convenient to employ a distant object, such as the above, — the difference of the focal length of the illuminating lens for such an object, and for the white cloud, being almost insensible. This last adjustment being effected by the movement of the secondary stage alone, the microscope tube remaining un- disturbed, also the object on the object-stage uninterrupted in focus, the source of the illuminating light and the object to be examined will both be distinctly seen at the same time. These adjustments, whether for daylight or lamplight, being completed, the mirror may be turned so as wholly to reflect the light either of the sky or of the 168 THE MICROSCOPE. lamp; and the eye-piece and object-glass suitable for examining the object may be employed, and the focus adjusted accordingly. The conical diaphragm with its various apertures may now be rotated, until that quality of illumination is obtained which gives a cool, distinct, and definite view of the object. Upon changing the object-glass, the centering eye-glass should always be employed to ascertain that the centricity of the illumi- nating condenser and microscope body has not been deranged. It has been stated that the image of a white cloud oppo- site the sun is the best for illuminating transparent objects when viewed by transmitted light. Mr. Gillett has success- fully imitated this natural surface by an apparatus consist- ing of a large parabolic reflector, with a small camphine lamp on an adjustable stand, having its flame nearly in the focus ; also of two other reflectors of hyperbolic figure, which are employed according to the object-glasses used on the microscope. The parabolic mirror and one of these are attached opposite to each other on the bent arm by which they are supported, having their axes coincident, and ihe enamel disc placed between them. The small hyperbolic reflector receives the light reflected from the large paro- bolic reflector, and concentrates the rays on the small enamel disc. The surface of this disc is roughened, so that the forms of all the incident pencils are broken up, and the effect of a white cloud produced. Several important modilications and valuable improve- ments in condensers have been introduced by makers, and, therefore, deserve especial notice. Two things should be required of achromatic condensers intended for general use and for research : First, that the optical combination em- ployed should be adapted to a considerable range of power — say, from an inch, or Jds upwards to the highest ; and, secondly, that they should be capable of working with a large aperture through the ordinary glass slides. When low powers are employed, a pleasantly lit field can be obtained by using a condenser a little out of focus, so that the rays cross before reaching the object ; but a condenser for general use should send a sufficient quantity, and no more, of oblique rays, when in focus, through an object doss's condenser. 1G7 seen with very moderate magnification, and should be capable of giving a dark-ground illumination with a h inch or §ds object glass. In Mr. Eoss's T4oths condenser, represented at a, fig. 39, these desiderata are well provided for. The optical combination is exactly the same as in his large angled y^-ths objectives, and the front lens has a diameter of £th of an inch. In all good condensers the diaphragm should be brought close to the loM'er lens ; that is the case with this instrument, which is provided with two revolving wheels of diaphragms. The upper one is pierced with eight circular apertures ; one of which is filled in with a polarizing slice of tourmaline ; another is made with a little rim, so as to receive any experimental stops the microscopist may wish to try. Omitting this last- mentioned stop, we have seven others, marked respec- tively 109°, 95°, 82°, 70°, 59°, 49°, and 40°. These stops allow the lenses to work with the angles of aperture named. In the wheel of diaphragms, below them, is arranged another set of stops, for combination with the preceding. A single slot stop keeps out the central rays, and allows a radial beam, including its proportion of marginal rays, to illuminate the object. This can be used with the larger of the open stops. The two-slot stop gives passages to two such pencils of light, one at right angles to the other, a plan very effective with certain diatoms and other objects. The three-slot stop allows the transmission of three pencils, equidistant from each other. This gives the three readings of the P. wngulatum. The arrangement for working the diaphragms in the condenser is very con- venient, and the whole apparatus rotates with the sub-stage. The lenses are also capable of adjustment to suit different thicknesses of glass. The proportion which, from the size of the front lens, the marginal and central rays bear to each other, is such that, when the stop allowing 40° of aperture is employed, the two sets of lines, on Pleurosigma hippocampus and P. angulatum, are distinctly shown with a one-fifth objective and an A eye-piece. A slight change in the position of the flat mirror makes this stop work excellently with the 168 THE MICROSCOPE. Podura scale. Thus, this arrangement enables an experi- menter viewing a new object to see surface markings, and to obtain penetration with one and the same stop ; an important gain in original investigation. The slot stops have been found very useful in investigating unknown objects, as well as in displaying those that are known; and with the whole aperture and the two-slot stop, many diatoms with double sets of lines are well brought out. The microscopist will find that from 40° to 59° angle of aperture will, in many cases, give the best results, when a \ or £ objective is employed. An excellent condenser of very large angular aperture is made by Messrs. Powell and Lealand, in which every requisite modification of the illuminating pencil may be produced by two revolving discs, one containing apertures of various sizes, and the other various diaphragms for excluding the central portion, or for admitting only angular portions, of the pencil of light. These discs are placed immediately below the posterior lens of the illumi- nator. This method of modifying the illuminating pencil was first applied in Gillett's condenser, as constructed by Mr. Ross. The improved hemispherical condenser lately introduced by the Rev. J. B. Reade, answers its purpose remarkably well. The plane surface of the hemisphere is placed upwards, and is covered by a dia- phragm in which are marginal apertures, capable of adjust- ment either to an interval of 90° with each other, when the arrangement of the dots to be developed is quad- rangular, as in P. rhomboides, or P. hi2^pocampicsT or to one of 60° and 1 20°, when they are arranged in equilateral triangles, as in P. angulatum, &c. An ingenious plan of illuminating minute objects, mounted in Canada balsam, by reflected light, has been devised by Mr. Wenhani--: this consists of a small truncated glass paraboloid, which is temporarily attached to the under side of the slide containing the object, b}r a little gum, oil, or fluid Canada balsam. The rays internally reflected from the convex surface of the paraboloid, impinge very obliquely on the under-surface of the slide, are transmitted in consequence of the fluid-uniting medium, and then internally reflected from the upper surface of the covering glass on to the COLLINS S CONDENSER. 16S object. Very minute variations of surface contour may by these means be rendered evident. Collins's Webster's Universal Achromatic Condenser (fig. 100), a superior mechanical contrivance, is provided with a diaphragm which so gradually shuts off all excess of light, that any amount of illumination can be admitted Fig. 100.— Collins's Webster's Universal Condenser. Shutter Diaphragm seen separately with the greatest nicety. This valuable addition to the microscope can be fitted to any instrument with or without a sub-stage arrangement. Its chief advantages are that, while its performing qualities take rank with much more expensive forms, it is moderately cheap, and is at once an achromatic condenser, parabolic illuminator, and graduating diaphragm, with polariser added. By means of a lever, the central aperture can be gradually closed, without losing- sight of the object, and at the same time the alteration is effected with a great amount of delicacy. Provided an object-glass has sufficient "resolving power," its capa- bilities are greatly increased by this condenser, in bringing out the markings on the difficult test-objects ; when in connexion with the Polariscope, a great increase of light is obtained — so much so that, by using a polarising prism with the "spot lens stop," the object appears brilliantly illumined on a dark ground, with fine coloured effects. When high powers are used in connexion with the polari- scope, the advantage derived by such an addition to the ordinary mode of illumination will be at once apparent to every microscopist. 170 THE MICROSCOPE. Professor Smith, of Kenyon College, America, invented a condenser for the illumination of opaque objects under high powers. It has been felt that it would be of im- mense advantage to the physiologist if he could illuminate small bodies, such as blood-globules, and view them as opaque objects with the -|th or -j^th ; hitherto there have been great difficulties in the way of accomplishing this, but Professor Smith has hit upon a contrivance Avhich promises to be useful. In this instrument a pencil of light is admitted above the objective and thrown down through it on to the object, by means of a small silver mirror placed on one side, and cutting off a portion of the aperture. Powell and Lealand devised what they considered to be an improvement, and substituted for the small silver mirror, to which Professor Smith gave a preference, a flat piece of glass placed at an angle of 45° across the tube, interposed like an adapter between the objective and the microscope body. A pencil of light entering by a side aperture striking against this flat glass is partly reflected down through the objective and on to the object, the magnified image of which is viewed through the glass. If the flat glass be ground so as to have parallel surfaces, no great amount of error can be detected. Smith and Beck place a disc of thin covering glass, at an angle of about 45°, in the optic axis of the microscope body, close behind the setting of the object-glass in a special adapter, leaving a suitable aperture for admitting light from a lamp, rays from which are reflected downwards upon the object. The object-glass is thus made its own achromatic condenser. Put the idea of employing the object-glass as its ovni condenser was suggested by Mr. Hewitt live years ago, and then Mr. Wenham was induced to give the plan a trial. A concave speculum was fitted at an angle into the body of the microscope, having a central hole sufficiently large to admit the full pencil from the objective, through the back of which the rays from a lamp (the light from which was admitted through a hole in the side of the body) were reflected downwards. The object was strongly illuminated, but there was so much glare from the internal OBLIQUE ILLUMINATION. 171 fittings, and a reflection from the back of the object- glass-lenses, that an unfavourable opinion was formed of its practicability. It lias since been demonstrated that the light was too intense ; and the most useful, the central portion of rays, were wanting. A simple disc of thin glass and its partial reflection meets these objections. If such a disc be used with a little care, it is found to be quite as accurate as the other plan, and the natural surface of the glass is a better reflector than any artificial one. It has, however, the disadvantage of extreme fragility. By making the object-glass its own condenser, and examining diatoms as opaque objects under high powers, we may now hope to solve the much vexed question as to the true nature of their markings. Mr. Browning has employed the apparatus in a form much more nearly resembling that of the original inventor, only substituting a small glass reflecting-prism for the metallic reflector. Some advan- tages are gained by the adoption of this latter arrangement. Such modes of illumination bid fair to correct many errors of interpretation resulting from an exclusive use of trans parent illumination. Oblique Illumination is of the greatest importance iu the demonstration of structure, especially that of test- objects ; and much greater obliquity is often required than can be obtained by throwing the mirror out of the axis of the microscope. Various methods are described in the "Transactions " of the Microscopical Society of London, for almost every microscopist has devoted some attention to the subject in his attempts to resolve the more diffi- cult lined or dotted objects, as diatoms ; therefore, to give a full description of all is quite uncalled for, if not impossible, in our pages ; it is nevertheless most advisable, in doubtful cases, to have recourse to every method which shall present the object under a different aspect. One of the earliest methods devised for obtaining oblique light, was the eccentric prism of Nachet, which occupied the place of the present achromatic condenser, and, like it, received its light from the mirror ; by simply turning it in its socket, oblique rays were thrown upon the object from every side. This had, however, defects which Mr. Sollitt proposed to remedy, by employing an achro- 172 THE MICROSCOPE. matic condenser of a very long focus and large aperture, mounted in such a way as to enable its axis to be inclined to that of the microscope through a wide angular range ; a condenser of this construction, he says, is also suitable for all ordinary purposes.1 A combination of the reflect- ing and refracting powers of a prism is much preferred by some ; such, for instance, as that known as Amici's prism, which causes the rays to be reflected by a plane surface, and made to converge by a lenticular one, so that the prism answers the double purpose of mirror and condenser at the same time. "Abraham's Achromatic Lenticular Prism" is an excellent substitute for the achromatic con- denser, and answers the double purpose of mirror and con denser. This prism, represented half-size in fig. 101, is made Fig. 101. — Abraham's Achromatic Lenticular Prism. Half-size. up of two kinds of glass, set in a frame of brass ; the part employed as the reflector (a) is of flint glass, hollowed out at its upper surface, and into this is accurately fitted a double convex lens of crown glass (b), so contrived as to have a focus of about four inches. By the tube (g), the prism can be applied to the ordinary support of the mirror ; and by means of the flat semicircle (d), and a joint in the connect- ing piece (f), it can be turned in every possible direction, the semicircle sliding through a spring clip at e. By this (1) Quart. Journal Micros. Science, vol. iii. p. 87. OBLIQUE ILLUMINATION. 173 instrument, achromatic condensed light may be thrown upon any object on the stage. The prism has the usual swinging motion, accomplished by the frame turning on two screws, one of which is seen at c, at the end of the semicircle. A thin stage is of importance when a very oblique pencil is used, and, to a certain extent, the larger its lower aperture, the more oblique will be the rays we are enabled to transmit to the object. For this reason the thin stage introduced by such makers as Powell and Lealand, is in every way an improvement on the thicker stage of the older instruments. A revolving stage also possesses ad- vantages : it enables the observer to keep the object in the centre of the field of view, and while its various parts are thus presented in succession to rays of greater or less obliquity, an insight into the structure is often afforded which could not otherwise have been obtained. " Thus, suppose that an object be marked by longitudinal stria?, too faint to be seen by ordinary direct light, the oblique light most useful for bringing them into view will be that proceeding in either of the directions c and D ; that which falls upon it in the directions a and B tending to obscure the striae rather than a 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 brought into view indifferently by light from c, or from d, but will be shown best by that which makes the strongest shadow ; B 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 favourable for the longitudinal, so that, in order to bring them into distinct view, either the illumi- nating pencil or the object must be moved a quarter round." D 174 THE MICROSCOPE. For those who desire to obtain a very oblique illumi- nating pencil, with objectives of lower powers, but large angular aperture, and with an inexpensive arrangement, the Hemispherical Condenser {Kettledrum) of the Rev. J. B. Reade will be fouud serviceable.1 This gentleman now, however, uses a double hemispherical condenser for the examination of the most difficult tests ; and the great obliquity of the illuminating pencils gives every facility for dealing with the close fine lines of the Amician test and the Macrum. Diaphragms of tinfoil are j3laced between the two hemispheres. By looking down the body of the microscope when the eye-piece is removed, and examining the dimensions of the little discs of light, it is seen at a glance whether one or other of the aper- tures require to be more or less deeply cut. In the one case, the little lappet of tinfoil can be so doubled as to shorten the aperture, and, in the other, it may be cut deeper and thrown further back. To obtain these very slight but not unimportant variations, and in a moment, is an advantage which observers will readily recognise. Mr. Reade also uses more durable diaphragms of thin brass, and, by a new arrangement, the precise amount of light is obtained at once. A fixed diaphragm, with aper- tures at right angles to each other, and cut nearly to the centre, is placed on the lower hemisphere ; and another diaphragm, of which the outline is a right angle upon a semicircle, is placed in a slit of the tube between the condensers, so that the vertex of the right angle divides the space between the long V apertures of the lower diaphragm. When pushed home, it nearly shuts up these apertures of the fixed diaphragm ; but by gently drawing I it out, and moving it a little sideways, if necessary, we can obtain, with the utmost nicety, just that length of either aperture which the test-lines under examination require. The very perfection of oblique illumination for resolving the markings on test-objects, is the double prism made use of by Mr. J. Newton Tomkins. The method of employ- ing the two prisms we give in our friend's words : — (I) Rev. J. B. Reade, F.R.R. on a New Hem i spherical Condenser. Trans. :.'icros. Soc. 1861, p. 59, and vol. vii. page 3, 1bC>7. SOLLITT S ILLUMINATION. 175 " My first attempts at oblique prism-illumination were made, many years ago, with Abraham's (of Liverpool) Achromatic Lenticular Prism ; the high character given of the performance of this instrument by the late Professor Quekett and by Mr. Sollitt, of Hull, led me to believe that it would entirely supersede the necessity of using a condenser during the examination of the highest class test-objects. In my hands, however, it failed to do this ; the pencil of light transmitted proved to be too diffused, and the shadows too faint to enable me to bring out the markings of single-lined tests satisfactorily. I subjoin Mr. Sollitt's directions for using this prism, as communicated Fig. 102 — Sollitt's Single-Prism Illumination. to Mr. Abraham : — ' When I require the prism for defi- nition and direct light, I use it in place of the mirror under the stage ; but when required for oblique light, that is, for illuminating lined objects, or for black-groand illumination, I use it on a separate stand, as shown in fig. 102, the object being illuminated by the very oblique ray, c b. Let a b be a supposed ray of direct light, coin- cident with the axis of the microscope, then so long as the angle uba is less than half the angle of aperture of the object lenses, the field will remain illuminated by oblique light, and the lined objects (if placed in their proper position) will be brought out in the most beautiful manner. I place the prism about five inches in front of the object, and by this method, with my widest apertures, I bring 176 THE MICROSCOPE. out the lines on Navicula Angulata, Ceratoneis Fasciola, Aniician Test, Grammatophora Subtilissima, and even on the Navicula Acus, with the greatest ease. In viewing these delicate objects, the lines must be at right angles with the direction of the light. If the angle c b a be greater than half the angle of aperture of the object lenses, then you have the object beautifully illuminated on a black ground ; and when the light is properly managed, the result is very superior to that obtained by the parabolic reflector — the objects on the black-ground appear as though they were in full sunlight, bringing out the fine markings on delicate tissues most satisfactorily.' "My next trial was made with a small rectangular achromatic prism with a focus of two inches and a half — not of four or five inches, as in Mr. Sollitt's case — fur- nished by Messrs. Powell and Lealand ; and I then found it easy to resolve the markings of all test objects which, were disposed at right angles with the source of illumina- tion. With a view of resolving the crossed-lines of P. Formosum, N". Angulata, and the like, I employed the usual microscope mirror, so disposed by means of a double- jointed arm that the light from the lamp might be reflected as nearly as possible at right angles with the second set of lines, but met with but moderate success, since the double reflection, due to the primary reflection from the silvered surface of the mirror, and the secondary reflection from the glass of the mirror itself, caused a haze that t militated materially against true definition of the markings which characterise such delicate tests as iV. Rhomboides, P. Ma- crum, &c. " The next step was to substitute for the mirror a second lamp and a second prism, also mounted on a separate stand, with rack-work adjustment, and provided with a universal- joint arm. The success of this plan was undoubted ; I was now enabled to resolve the crossed lines of N. Rhom- boides (the Amician Test of the London, although, per- haps, not of the American microscopists) with a sharpness which was hitherto unattainable ; and even the difficult object P. Macrum came out in well-defined squares. The advantage of the universal-joint fitted to the second prism consists in the fact that the diagonal lines of such tests as DOUBLE PRISM ILLUMINATION. 177 P. Rigidum may "be brought out with ease by simply inclining the prism to the angle necessary to cause the shadows of this series of lines to fall directly opposite to the source of light. " The method of using this combination will be readily understood by referring to fig. 103. "The microscope (a) is placed at the usual angle of inclination ; a valve — say, of N. fihomboides — is selected which lies vertically on the stage ; the front lamp (c) is adjusted so that the centre of the flame shall be eleven inches above the table, and ten inches from the object ; an object glass of moderate angle, such as a Eoss's old J inch, with 60° of aperture, will be found convenient for this first step in the process ; the front prism (d), with its convex surface turned towards the lamp (c), is placed H — 2 inches from the stage, and so adjusted that the pencil of transmitted light shall fall with the utmost obliquity on the object ; now (referring to the former fig. 102), the angle c b a being about 40°, or greater than Fig. 103. — Mr. Newton Tomkins' Double Prism Illumination. half the angle of aperture of the lens, the valve under examination will be brightly illuminated on a black- N 178 THE MICROSCOPE. ground. An -|th, y^thj or a TS^h, *s ^nen substituted for the J inch lens ; and since the angle of aperture of either of these lenses exceeds 140°, the whole field will be illu- minated with a very oblique pencil of light. By careful manipulation of the prism, and the closest attention to the due adjustment of the object-glass itself, the transverse markings on the valve (which are by far the most difficult to resolve) should now come out with great clearness. "The reason for using the ^ inch lens in the first instance, is to find, by ' Maltwood,' or otherwise, the particular object desired, and to determine the exact position of lamp and prism ; by commencing to work with the ^th or ^-th these would be impracticable. The light of the lamp is now to be shut off, and a side prism' (b) inclined parallel to the plane of the stage-plate, and brought as near to the under-side of the object as the form of the fixed stage will admit, placed in position. The light of a second lamp (e) is then caused, as before, to fall very obliquely on the object, and the longitudinal markings will be readily shown. On restoring the light of the front lamp (c), and paying particular attention that neither light shall be in excess, the valve will be brought out in squares, with a sharpness and a delicacy of tracing which I venture to assert will bear comparison with the best results obtained by use of the expensive modern condenser. I would by no means undervalue the advan- tages to be derived from the employment of the condensers now furnished by our eminent makers ; but, seeing that these are manifestly articles of luxury, I incline to the opinion, that to those microscopists who are economically disposed, and already possess one rectangular prism, the trifling addition of a second prism and a second lamp will enable them to do good service to the cause of resolving many difficult test-objects hitherto insurmountable, amongst which I would place in the foremost rank Amphipleura pellucida (N. Acus), the markings on which I do not yet despair of seeing ' crossed ' by some enthusiastic observer." We have witnessed with surprise the ease with which difficult markings are brought out by this mode of illumi- nation. THE PARABOLIC REFLECTOR. 179 The Parabolic Reflector. — Mr. F. H. AVenham (Micros. Trans. 1851) proposed a parabolic illuminator, for the pur- pose of obtaining perfect definition under high powers. Those who have experimented on the subject, may have ob- served that there is something in the nature of oblique light reflected from a metallic surface particularly favourable for the purpose of bringing out minute markings, which may, in some measure, be attributed to the circumstance of light so reflected being purely achromatic. In order to render this property available, Mr. Wenbam contrived a very in- genious metallic reflector, by which the condensation of lateral light may be effected.1 M I cb \ ! I a I i 5 SSZS -*>"■ *~ g^ro, Fig. 104. — Wenluim's Parabolic Reflector. " The apparatus is shown in section in fig. 104 ; a a is a parabolic reflector, of a tenth of an inch focus, with a (1) Mr. Shadbolt contrived a modification of Wenham's Reflector, and called it a " Sphero-Annular Condenser ;" which consists of a ring of glass whose surface was so shaped as to present a prismatic section, the inclination of the outer side being such as to produce a total reflection of the rays impinging on it, and to direct these through the inner side of the ring, so as to fall at a very oblique angle upon the object from every azimuth of the circle. A combination of both methods is adopted in the parabolic illuminator now very generally used. This consists of a paraboloid of glass resembling a cast of the interim of Wenham's Parabolic Speculum, but reflecting the rays which fall upon tlio surface of the glass like Shadbolt's Annular Condens- r. o ISO THE MICROSCOPE. polished silver surface, having the apex so far cut away as to bring the focal point at such a distance above the top of the apparatus (which is closed with a screw-cap when not in use) as may allow the rays to pass through the thickest glass commonly used for mounting objects upon before coming to a focus. " At the base of the parabola is placed a disc of thin glass 6 b, in the centre of which is cemented a dark well, with a flange equal in diameter to the aperture at the top of the reflector, for the purpose of preventing the direct rays from the source of light passing through the apparatus. "The reflector is moved to and from the object by means of the rack and pinion a, and has similar adjust- ments for centering, and is fixed under the stage of the microscope in the same way as the ordinary achromatic condenser: in addition, there is a revolving diaphragm d, made to slide on the bottom tube of the apparatus ; it has two apertures e e, placed diametrically, for the purpose of obtaining two pencils of oblique light in opposite direc- tions. The effects of the chromatic and spherical aber- rations, in the shape of fog and colour about the objects, caused by the glass slides upon which they are mounted, frequently require compensation ; for as the parabola has the property of throwing parallel rays uncoloured to a point, when used alone, it is most suitable for objects without glass underneath. " By the addition of a meniscus, this compensation is obtained, and also greater purity and intensity of illumina- tion is procured ; and as the silver reflector is now closed with glass, it is hermetically sealed, and permanently pro- tected from dust and damp, and will therefore retain its polish. The light most suitable for this method of illumi- nation is lamp or candle light, the rays of which must in all cases be rendered parallel by means of a large plano- convex lens, or condenser; the light may then be used direct, or reflected from the plane mirror. The object having been adjusted, the illuminator is moved to and fro till the best effect is produced. For the purpose of viewing some objects, such as navicular, the circular diaphragm should be slid on the extremity of the apparatus, and THE LIEBERKUHN. 181 revolved till the two pencils of light are thrown most suitably across the object." The Lieberkuhn. — The concave speculum termed a " Lieberkuhn," from its celebrated inventor, was formerly much in use as a reflector, but is now almost abandoned, or rather replaced by other and better contrivances. The Lieberkuhn is generally attached to the object-glass, in the manner represented at fig. 105, where a exhibits the lower part of the compound body, 6, the object-glass, over which is slid a tube and the Lieberkuhn, c, attached to it ; the rays of light reflected from the mirror are brought to a focus upon an object d, placed between it and the mirror. The object may either be mounted on a slip of glass, or else held in the for- ceps, / ; and when too small to fill up the entire field of view, or when transparent, it is neces- sary to place behind it the dark well, e. Each Lieberkuhn being mounted on a short piece of tube, can be slid up and down on the outside of the object-glass, so that the maximum of illumination may be readily obtained. In the higher powers the end of the object-glass is turned small enough to pass through the aperture in the centre of the Lieberkuhn ; but in the lower powers, where a great amount of reflecting surface would be lost on account of the large size of the glasses employed if this plan were adopted, the aperture in the centre of the Lieberkuhn is made to admit as many rays as will fill the field of view, and no more. As the flood of light thrown back by the Lieberkuhn is sometimes so great as to obliterate much of the beauty and delicacy of structure of certain objects, it was proposed by Mr. Bridgeman, of Norwich, to cover up a portion of its reflecting surface by gumming a piece of dull-black paper over one half of the reflector; then, by rotating the Lieberkuhn upon the object-glass, any proportion of oblique. Fis. 105. i82 THE MICROSCOPE. light may be thrown upon the object, and in any particular direction. In this way points of structure have been brought out which were lost under the full illumination of the Lieberkiihn. Mr. Beck sought to effect an improve- ment upon the proposal of Mr. Bridgenian, and, by employing the segment of a Lieberkiihn, produced what he terms a "Parabolic reflector." This is made to slide in the ordinary way over the object-glass, and admits of being turned in any direction towards the source of light. The parabolic reflector can also be employed with high powers. Mr. Sorby discovered, while experimenting with a reflector of the kind, the value of studying the peculiarities of objects under every kind of illumination ; for, on viewing specimens of iron and steel with this reflector, he found that, owing to the obliquity of illumination, the more brilliantly polished parts reflected the light beyond the aperture of the objective, and he could not therefore distinguish them from those parts which merely absorbed the light. To throw the illumination more perpendicularly upon the specimen, he was obliged to place a small flat mirror immediately in front of the objective, and cover half its aperture, and at the same time stop-off, by means of a semi-cylindrical tube, the light from the parabolic reflector. By such an arrangement, Mr. Sorby found it produce the reverse appearances of the former mode of illumination, and it proved to be a valuable aid in ascertaining the true condition of the object. Mr. Baker has produced a cheaper and simpler form of this parabolic reflector, which answers its purpose remarkably well, giving increased facilities for the illumination of opaque objects with the microscope set in that direction most convenient for working. The brilliancy of the illumination is very much increased by placing a condensing-lens before the light, or by employing the Bockett lamp. It is, however, only just to Mr. Ross that we should add, that this " parabolic reflector " ia but a slight modification of his Side-rejlector, contrived several years ago, for the purpose of superseding the Lieberkiihn. The Bull's-eye Condenser. — All opaque objects require to be illuminated by rays which, being thrown upon their sur- face, shall be reflected back into the microscope. The same J3ULL-6-EYE CONDENSER. 183 mode of viewing objects is often applied to semi-opaque, or even some transparent ones, to demonstrate pec aliari ties of structure. A condensing lens is used for converging rays from a lamp upon the mirror ; or for reducing the diverging rays of the lamp to parallelism, for use either with the parabolic illumi- nator, or Eoss's side-reflector. A plano-convex lens (fig. 106), of about three inches focal length, is the form ge- nerally adopted ; it is borne upon a swivel-joint, which allows of its being turned in any direction, and placed at any angle; the tube is double, and thus admits of being lengthened or short- ened. When used by day- light, its plane side should be turned towards the ob- ject, and the same position should be given when used for converging the rays from a lamp ; but when used with the parabolic or side- reflector, the plane side must be turned towards the lamp. Sometimes a smaller, double convex lens is made use of in addition ; this is then often fixed into the stage of the microscope. In fig. 107 the bull's-eye lens, c, slides up and down a brass rod, screwed into a solid foot ; and made to con- centrate the light upon the object from the table gas- lamp, d. Mr. Brooke's method of viewing opaque objects under the highest powers of the microscope (the £ and -^ inch object-glass) is effected by two reflections. The rays from a lamp rendered parallel by a condensing lens are received on an elliptic reflector, the end of which is cut off a little beyond the focus; the rays of light converging Fig. 1&6. — Tlie. Condensing Lens. 184 THE MICROSCOPE. from this surface are reflected down on the object "by a piano mirror attached to the object-glass, and on a level Fig. 107. — Condenser and Table Gas-lamp. with the outer surface. By such means the structure of the scale of the Podura, and the different characters of its inner and outer surfaces, are rendered distinctly visible. Lamps. — It is of the utmost importance, both on account of the injury often done to the eyes, as well as for the per- fection of illumination when artificial light is employed, that its quality should be of the whitest and purest kind. The most useful and economical lamp is Collins' Bockett lamp ; it consists of a good paraffin lamp, reflector, and a 2^ condenser, mounted together on a brass-rod stand, which admits of being raised from 9 to 16 inches. The pale blue chimney supplied with it gives whiteness to the flame, and for all the requirements of the microscopist it is de- MICROSCOPE LAMPS. 185 Fig. 108. — Collins' Bockctt Lamp. cMedly the most convenient lamp we have seen. Those Fig. 109. — Highley's Gas-lamp, with Steam-bath attacliad. 1SG THE MICROSCOPE. among us who have gas at command will, no doubt, give it the preference on account of its convenience, and freedom from all kind of trouble. An excellent form of gas-lamp is Highley's (fig. 109). The mono-chromatic gas-lamp, fig. 1 10, is very useful and most economical. Gas, as a source of light, presents great advantages over oil and spirit, on account of cleanliness, being ever ready for use, and affording a perfect control over the flame ; but when the ordinary gas-lamps are used 0 D Fig. 110. — Gas-lamp arranged for use. \ U for the purpose of illuminating the field of the micro scope, a yellow glaring light is given, alike injurious to the eye and the definition of the object under examina- tion. To correct these evils, this lamp was arranged, which FINDERS AND INDICATORS. 187 is also otherwise useful to the microscopist. It consists of a stage a, supported by a tube and socket, sliding on an upright rod rising from the stand. This carries an argand burner b ; a metal cone c rises to the level of the burner, and is about one-eighth of an inch from its outer margin. This arrangement gives a bright cylindrical flame. The bottom of the stage a is covered with wire-gauze, to cut off irregular currents of air, and thus secures a steady flame. Over the burner is placed a Leblond's blue glass chimney D. This corrects the colour of the flame to a certain extent ; but it is still further rectified by a disc of bluish-black neutral-tint glass e, fitted in a tube f, attached obliquely to the shield G. G is a half-cylinder of metal, which serves to shield the eyes from all extraneous light, but may be rotated on the stage a by aid of the ivory knot H, when the full light from the flame is desired. A metallic reflector i, fixed on its supports, so as to be parallel to e, concentrates the light. By the combination of the two glasses D and e, the yellow rays of the flame are absorbed, and the arrangement affords a soft white light, which may be still further improved by receiving the rays on a concave mirror, backed with plaster-of- Paris l; and where a very strong light is required, a condensing lens should be interposed, as shown in the cut, between the lamp and the mirror of the microscope. By removing the shield G, and bringing the shade M over the burner, it may be used as a reading-lamp. A retort ring N supports a water-bath o, or a wrought- iron plate p, 6 inches by 2| inches, both used in mounting objects. The stop-cock q gives the means of regulating the flame. The screw R clamps the lamp-head at any height desired. The lamp may be attached to any gas-supply by vulcanised India- rubber tubing. Finders and Indicators. — A finder, as applied to the microscope, is the means of registering the position of any particular object in a slide : as, for instance, some particu- larly good specimen of a diatom, so that it may be referred to at a future time. The subject has been fully discussed in the pages of the Quarterly Journal of Microscoincal Science. The traversing stage admits of such finders as 18S THE MICROSCOPE. those of Mr. Okeden, Mr. Tyrrell, Mr. Amyot, &c., being used. The first named (Mr. Okeden's) finder consists of two graduated scales, one of them vertical, attached to the fixed stage-plate, and the other horizontal, attached to an arm carried by the intermediate plate ; the first of these scales enables the observer to " set '; the vertically-sliding plate to any determinate position in Fig. 111.- -Amyot's Object Finder. relation to the fixed plate, while the second gives him the like power of setting the horizontally-sliding plate by the intermediate. The finder the author has been in the habit of using for some years is a very ingenious and effective instrument described by Mr. Amyot. It consists of a box-wood plate, with a circular hole y^ths of an inch in diameter cut out of its centre, into which a disc of ivory, pierced in the exact centre with a very fine needle, fits ; this disc can be readily removed by seizing a little brass peg, attached to it, with the forceps. The box-wood is ruled with two axes, vertical and horizontal, which are graduated to 50ths or lOOths of an inch. The diagram (rig. Ill) represents the finder with bone centre-piece in situ. The dotted ring shows the rabbet on which the centre-piece rests ; a, brass pin attached to centre-piece ; b, b, two brass pins for steadying the instrument against the OBJECT FINDERS. ISO left side of the microscope stage. The scales may be ruled in "brass or bronze and inlaid, and horizontal lines might be ruled on the surface of the stage as a guide for placing the slide. This form of instrument possesses the advantage of not requiring the common object slides to be ruled, or in any way prepared. It is thus used : — 1st. Put the finder on the stage and find the central needle hole with the microscope. 2d. Re- move the centre-piece with a small pair of forceps. 3d. Place the slide on the wood, taking care that the stage screws be not used to move the object after the finder has once been centred. The position of any object then occupying the centre of the field will be shown by the sides and ends of the slide on the scales of the finder, and can be registered at sight. To find the object again : — Find the centre hole, remove the disk as before, then place the slide according to the letter and number marked upon it, when the object sought will occupy the centre of the field. For those microscopists whose instruments are without a traversing stage " Maltwood's finder " will be found an efficient substitute for the one just described. This consists of a glass slide, 3 x 1^ inches, on which is photographed a scale occupying a square inch ; this is divided by horizontal and vertical lines into 2,500 squares, each of which con- tains two numbers marking its "latitude," or place in the vertical series, and its "longitude," or place in the horizontal series. The scale is in each instance an exact distance from the bottom and left-hand end of the glass slide ; and the slide when in use should rest upon the ledge of the stage of the microscope, and be made to abut against a stop, a simple pin, about an inch and a half from the centre of the stage. To use this finder, the object- slide must be placed under the microscope with the same care as the finder ; and when some especial object, whose place it is desired to record, has been brought into the field of view, the object-slide being removed, the finder laid down in its place, the numbers of the square then in the field of view are to be read off and recorded. This should also be recorded upon the object-slide ; and at any future time, when it is wished to refer to the same object, 100 THE MICROSCOPE. the process will have to be reversed, by first finding this particular square on the finder, and then replacing it by the object-slide. A simple form of finder, suggested by Mr. W. K. Bridge- man, of ^Norwich, and sold by Baker, Holborn, is, from its ingenuity and comparative accuracy, worthy of notice. Fig. 112. — Bridgeman's Finder. This finder (fig. 112) consists of a curved pin of steel, which is attached to the stand by means of a circular fitting of brass. It is so contrived that when the mechanical stage is brought into the rectangular position, and an object laid thereon, it can be pressed down upon the slide. If the latter be covered with paper, the point of the pin leaves a slight puncture : all that is necessary, when we wish to examine the object in future, is to move the stage till the puncture on the slide comes under the point of the finder. Forceps. — For holding minute objects, such as parts of plants or insects, to be examined either as transparent or opaque objects, the most useful is represented by fig.l 13, s — 1 A«J Fig. 113. and consists of a piece of steel wire, about three inches long, which slides through a small tube, connected to a stout pin by means of a cradle-joint ; to one end of the wire is attached a pair of blades, fitting closely together by their own elasticity, but which, for the reception of any DIPPING TUBES. 191 object, may be separated by pressing the two projecting studs ; to the opposite end of the wire is adapted a small brass cup, filled with cork, into which pins, passed through discs of cork, card-board, or other material, having objects mounted on them, may be stuck. Dipping-tubes are tubes of glass, fig. 114, about nine inches in length, open at both ends, and from one-eighth to one- Fig. 114. — Dipping Tubes. Fig. 115. fourth of an inch in diameter. The ends must be nicely rounded off in the flame of a blow-pipe; some of them may be made perfectly straight, while others should be drawn out to a fine point, and made of either of the shapes a, b, c, d, e. The method of using them is as follows : — Supposing the animalcules are contained in a phial or glass jar (fig. 1 15). and having observed where they are most numerous, — either with the naked eye, or with a pocket-magnifier, — 192 THE MICROSCOPE. cither of the glass-tubes, having one end previously closed by the thumb or fore-finger, wetted for the purpose, is introduced into the phial in the manner represented by the figure : this prevents the water from entering the tube ; and when the end is near to the object which it is wished to obtain, the finger is to be quickly removed and as quickly replaced. The moment the finger is taken off, the atmospheric pressure will force the water, and with it, in all probability, the desired objects, up the tube. When the finger has been replaced, the tube containing the fluid should be withdrawn from the phial ; and as the tube is almost certain to contain much more fluid than is requisite, the entire quantity must be dropped into a watch-glass, and the particular insect can be caught by putting the tube over it, when a small quantity of fluid is sure to run up by capillary attraction. This small quantity is to be placed upon a glass-slide for examination. It is necessary to add a small quantity of vegetable matter to animalcules, if we wish to keep them alive for many days ; and as many species are found on confervas and duck-weed, some instrument is required to take small portions of such plants Fig. 116. out of the jar in which they are growing. For this pur- pose it will be necessary to make use of the forceps, fig. 116, made of brass; the points are a little curved, to keep them accurately together, and the blades are provided with a hole and a steady ing-pin. This instrument is also useful for picking up any minute object. Fig. 117, at b, is represented a convenient, cheap, and portable "Collecting-bottle and Stick;" simply an ordinary walking-cane divided by a screw, or socket-joint, into two parts for the convenience of packing, and terminated by a brass ring, which is either a fixture or made to screw on and off. This ring is adapted to receive the male-screw of the well-known wide-mouthed bottle, used by perfumers COLLECTING BOTTLE AND NET. 193 and chemists for pomade ; this is exactly adapted to fit into the screw of the brass ring. The facilities A. Trough for showing circulation in fish-tail. B. Collecting bottle and stick. afforded by such an arrangement are obvious enough ; broken bottles can be readily replaced at a small cost, and each bottle when filled corked up and replaced by another. A small fine-gauze net and a hook is made to screw into the same stock ; the whole packs into a very small compass. Fig. 118.— Collecting Net. Collecting Animalcules. — For collecting fresh water or marine animals, the small net, made similar to a landing- net, fig. 118, will be found useful : this should be securely fastened to the brass ring a, and fitted firmly into the socket b ; when not in use, the socket may be carried in the pocket ; and the net, by contracting the diameter of the ring (which the construction admits of) may be carried inside the hat. For collecting desmids, diatoms, and other small crea- tures, Mr. Williamson has a cheap and simple contrivance for converting the end of a walking-stick or umbrella into what he terms a "collecting-stick." In fig. 119, a repre- o 194 THE MICROSCOPE. Bents a piece of whalebone, about 18 inches long, bent round the end of the stick or umbrella, 6, and made fast in that position by one or two rings, c, of gutta-percha, india-rubber, or of brass, d. A small wide-mouthed bottle, having a rim which will prevent its falling through, is now inserted in the loop thus formed, and is held tightly there by the ends of the whalebone being drawn further through the ring, and thus diminishing the size of the loop. The bottle thus fixed may be used for dipping out the animalcules. Whalebone can be moulded to any form by plac- ing it for a short time near the fire. Fig. 120 is a box containing six bottles for holding the speci- mens when caught. These bottles should be filled up to the cork with water when and where col- lected, and care must be taken not to mix the specimens from one brook with those from another, otherwise serious damage may take place, and on reaching home we may find the greater part of our stock either dead or dying. Always separate the various species as speedily as possible. This can be done easily, by emptying each bottle in its turn into a soup-plate ; then with the feather of a pen first lift out the smaller ones, and with the quill-end cut like a scoop lift out the larger, classifying and allotting each to its separate bottle. Live-Box. — Mr. Varley im- proved the form of this instrument by making a channel Fig. 119. Fig. 120. ANIMALCULE-CAGE. 195 all round the object-plate, so that the fluid and animalcules in it were retained at the top of the object- plate by capil- lary attraction. The cover is made in the usual way to slide up and down over the object-plate. The plate of brass to which the tube supporting the tablet and cover is attached, is of a circular form, slightly flattened on its two opposite sides for convenience of package. The in- Fig. 121. — Varlci/s Animalcule-cage. strument is seen in elevation and in section in fig. 121 a, b, in both figures, is a flat plate of brass to which the short tube carrying the object-plate or tablet is fixed ; d, the piece of brass into which the tablet c is fastened ; 6, the tubular part of the cover, into the rim of which the thin plate of glass a is cemented. Compressorium. — The purpose of this instrument is to apply a gradual pressure to objects whose structure can only be made out when they are pressed or thinned out by extension. The general plan of the compressorium is shown in fig 122 Ross's Compressorium consists of a stout plate of brass A, about three inches long, having in its centre a piece of glass like the bottom of a live-box. This piece of glass is set in a frame B, which slides in and out so that it can be removed for the convenience of preparing any object upon it, under water if desirable. The upper moveable part D is attached to a screw-motion at G; and at one end of the brass-plate A, which forms the bed of the instru- ment, is an upright piece of brass C, grooved so as to o2 196 THE MICROSCOPE. receive a vertical plate, to which a downward motion is given by a single fine screw, surrounded by a spiral spring, which elevates the plate as soon as the screw-pressure is removed. The vertical plate carries an arm at right angles to its own plane, terminating in a square frame D, capable of receiving very thin, or somewhat thicker glass. The arm has likewise a horizontal motion, so that the upper Fig. 122. — Ross's Compressorium. plate D can be turned completely off the lower one B. Should the thin upper glass be broken, it can be instantly replaced, as no cement is required ; it is merely needful to remove the fragments and slip a fresh glass in. It often happens that on account of the trouble attendant upon the use of all ordinary Compressoriums, the microscopist simply uses a slide and a piece of covering-glass ; but if he wishes an exact means of regulating the pressure, some such compressorium as Eoss's should always be employed. CHARA AND POLYPE TROUGHS. 197 Smith and Beck's trough for chara and polypes, a sec- tional view of which is shown at fig. 123, is made of three pieces of glass, the bottom being a thick strip, j- and the front a of thinner glass than the back b ; the whole is cemented together with Jeffery's marine-glue. The method adopted for confining objects near to the front glass varies according b to circumstances. One of the most convenient plans is to place in the trough a piece of glass that will stand across it diagonally, as at c ; then if the object be heavier than water, it will sink, until stopped by this plate of glass. At other times, when used to view chara, the diagonal plate may be made to press it close to the front by means of thin strips of glass, a wedge of glass or cork, or even a folded spring. When using the trough, it is necessary that the microscope Kg* • should be in a position nearly horizontal. Mr. Walker's trough for exhibiting the circulation ot the blood in a fish's tail, &c. (fig. 117 a) consists of a piece of plate-glass about 6 inches long, and 2 inches wide ; upon this, three other pieces of plate-glass, about J- inch wide, are cemented with marine-glue. Three pieces of strong covering glass, about a \ inch wide, are also cemented on to the plate, and a piece of mode- rately strong covering glass on to the top of these thin slips ; a piece of plate-glass is then cemented to the top of the thin glass, abutting on the ends of the slips ; the whole forming an open trough, terminating in a thin cell, which is closed at all parts, except where it communicates with the bottom of the trough. The fish, wrapped in a little wet linen, is placed in the trough, and the tail is thrust flat into the thin cell ; a small quantity of water is placed in the large trough, and the fish kept in its place by one or two elastic bands or strips of thin sheet-lead passed round the glass and over the body of the fish. The advantages of this arrangement are : — 1st. The fish cannot throw up its tail and splash the object-glass with water. 2d. In consequence of the tail being kept flat in the cell, the view is perfect, and there is little risk 198 THE MICROSCOPE. of the object getting out of focus. 3d. If the tail he not pushed too far into the cell, the vessels at its root are not compressed, and the circulation goes on very freely. 4th. The fish may be kept on the stage of the microscope for two or three hours without injury. Mr. Macaulay has suggested, as an improvement, that the three sides of the cell should be made of a single strip of plate-glass, bent to the shape represented in our wood-cut. Growing-cells. — Considerable attention has been given to various forms of growing-cells for maintaining a con- tinuous supply of fresh water to objects under constant observation, fur the purpose of sustaining vital growth for a long period. The employment of such cells is strongly commended to microscopists, as there is yet much to be discovered concerning the metamorphoses which some of the lowei microscopic forms of plant and animal life pass through ; a patient investigation will probably show that many which are now classed as distinct species are merely different phases of the same type, which alter- nate in a higher or lower scale of development accord- ing to the varied conditions of temperature and nutrition under which they are grown. Professor Smith, of Kenyon College, furnished us with what he has called a growing slide, or trough. It con- sists of two pieces of thinnish glass cemented together; in one corner of the upper cover a small hole is bored, and through this a fresh supply of water is introduced without in any way disturbing the desmid, or living object under inspection. Mr. Beck has contrived an improved form of cell ; but, whilst the first-mentioned may be con- structed for a few pence, the latter cannot be had for a less price than ten shillings. Dissecting Knives, <&c. — Knives and needles of various kinds and sizes are required for microscopic dissection ; the best for the purpose are represented in tigs. 124, 125, and 126, being, in fact, the very delicately made knives used by surgeons in operations upon the eye. Dissecting needles may be either straight or curved. They may be fixed, or made to take in and out of their handles. The most con- venient are shown in fig. 125; those made of Palladium by Mr. Weedon, Hart Street, are very much the best. DISSECTING KNIVES. 199 HI. ■. ' _____Jjjl^— =Z^ Fig. 124. FU125 Fig. 126. The mode of using a pair of needles in breaking up tissues into very small pieces is represented in fig. 127. Fig. 127.— TeaMng-out Membrane. "With a pair of the small needles held firmly between the fore-finger and thumb, the structure must be teased out j 200 THE MICROSCOPE. an operation which requires care and perseverance, as most of the animal tissues are very difficult of separation. All substances should be carefully separated from dust and other impurities which renders their structure indistinct or confusing. "With very delicate membranes, and with those of the nervous system of the smaller animals, in- sects, &c, it becomes necessary that the investigation should be carried on under water, or in fluid of some sort, in a glass cell, and having a strong light thrown down upon it by the aid of the condensing lens, as represented in fig. 128. A certain amount of change of structure must be expected and allowed for ; as nearly all membranes imbibe some portion of the fluid. Delicate structures are often advantageously wetted with dilute solutions of sugar or common salt, to prevent the changes from endosmosis, which result from the use of pure water. The contents of bodies are frequently rendered more distinct by the addi- Fig. 128. — Dissecting underwater. tion of re-agents. If the object be a portion of an in- jected animal, it is better to pin it out on a leaded-cork, Valentin's knife. 201 c L covered with white wax, and then immerse it in the water- trough ; the more delicate the structure, the sooner after death should it be examined, especially animal tissues. With some vegetable structures, the dissection should be carried on under water. The sepa- ration of the woody and vascular tissues, and the spiral vessels, is best effected by maceration and tearing with fine needles. Valentin'' s Knife. — For making fine sections of large substances, or those soft in structure, such as the liver, spleen, kidney, &c, the double-bladed knife, the invention of Professor Valentin, may be used with advan- tage. An improved construction of this knife, by the late Mr. John Quekett, is represented in fig. 129.1 It consists of two blades, one of which is prolonged by a flat piece of steel to form a handle, and having two pieces of wood riveted to it, for the purpose of its being held more steadily ; to this blade another one is attached by a screw ; this last is also lengthened by a shorter piece of steel, and both it and the preceding have slots cut out in them exactly opposite to each other, up and down | which slot a rivet with two heads is made to slide, for the purpose either of allowing the blades to be widely separated or brought so closely to- gether as to touch. One head of this rivet, being smaller than the hole in the end of the slot, can be drawn through it ; so that the blade seen in the front of the figure may be turned away from the other in order to be sharpened, (1) Another fcim of this instrument is constructed by Mr. Matthews, the blades being made with a convex instead of a straight edge, their distances from each other being regulated by a milled-head screw, and their separation for cleaning being more readily accomplished. Fig. 129. 202 THE MICROSCOPE. or allow of the section made by it being tasen away from between the blades. The blades are so constructed that their opposed surfaces are either flat or very slightly concave, that they may fit accurately to each other, which is effected more completely by a steadying pin, seen at the base of the front blade. When the instrument is required to be used, the thickness of the section about to be made will depend upon the distance the blades are apart ; and this is regu- lated by sliding up and down the rivet, as the blades, by their own elasticity, will always spring open and keep the rivet in place ; a cut is then to be made by it, as with an ordinary knife, and the part cut will be found between the blades, from which it may be separated either by open- ing them as wide as possible by the rivet, or by turning them apart in the manner before described, and floating the section out in water. Dissecting Scissors. — In addition to the forceps and knives, scissors will be necessary for the purposes of dissec- tion: of these the most useful are shown in fig. 130. They Fig. 130. — Dissecting Scissors. are made both straight and curved ; of the first kind, two pairs will be required, one having the extremities broad, and the other sharp-pointed ; if large dissections be under- taken, a still stronger pair, with the extremities broad, and made rough like a file, will be necessary. In dis- secting under the microscope, the curved-pointed pair shown at / are the most convenient. In all of these instruments the points should fit accurately together : sometimes those that are very sharp are apt to cross ; this SMITHjS SECTION INSTRUMENT. 203 may in a great measure be prevented by having the branches wide at the "base where they are riveted. The points can be sharpened on a hone, and a magnifier em- ployed to examine if they fit closely together. Section-cutting Instruments. — There are a numerous class of substances much too hard to admit of beincr cut either by scissors or a Valentin's knife, more especially if we re- quire very thin and perfect sections. As most important information is to be gained respecting the structure of various substances, such as stems and roots of plants, horns, hoofs, cartilages, and other firm parts of animals, by cutting very thin sections for mounting as transparent objects, it would be quite impossible for the microscopist to get on without a section-cutting instrument. Among the many mechanical contrivances which have been de- vised for the purpose, the ingenious little machine in- vented by Mr. James Smith1 will be found to do its work with neatness and precision. Vertical or top view. Side view. Fig. 131. — Smith's Section Instrument. Smith's Section Instrument (fig. 131) consists of an outer tube a, a, the upper part of which screws into the lower, and has at the top a flat circular plate e, e, which forms the cutting-table. . Firmly fixed to the lower part of the tube a, and extending throughout its whole length, is the inner tube b, b, which forms, with the moveable bar d, a holding for the specimen to be cut, while, at the same time, it supports the upper part of the tube a, a, and gives it greater firmness in screwing up and down. The (1) " On a Section Instrument, by James Smith." (Micros. Soc Trans. voL viii. page 1, I860.) 204 THE MICROSCOPE. bar d moving backwards and forwards in the tube by means of the screws c, c, serves, in conjunction with the points f, F, to fix the specimen to be cut, which is effected as follows : — The cutting surface e, e, being slightly screwed up, as shown in the drawing, and the bar d being drawn back a sufficient distance, the specimen to be cut is placed in the instrument, and firmly fixed by turning the screws c, c ; it is then cut level with the surface by a proper knife or chisel, and the table being screwed down one or more divisions (as shown in the left-hand diagram), a section is cut, and. if found of sufficient thinness, a number may be cut by continuing to turn the table down a similar number of divisions, until it will screw no further, when the table must be again screwed up, and the specimen loosened and raised if more sections be required. The principal points of the instrument are : — 1st. Its portability — the tube being about two and a half inches long by one inch in diameter. 2d. The specimen to be cut is fixed once for all, and the cutting- surface screwed down to it, a feature that will render it peculiarly applicable to the cutting of soft substances. 3d. The ease with which a number of sections may be cut without disturbing the specimen when once properly fixed, while the size of the tube, and the facility with which it can be adapted to objects of various diameters, enable the operator to get sections of stems of plants, &c. whole. In cutting sections of hard woods, which require considerable purchase, the instrument may be placed in a semicircular opening in the edge of the working-table, so that the flat plate or cutting- surface may rest upon it, and the strain thrown on the table. When used for cutting soft substances, it can be held in the hand. Fig. 130 represents Mr. Gibbon's "Section-cutting Ma- chine." It consists of a stout brass frame, a a, having an opening in the top plate, for a tube b, half an inch in diameter, and in depth one and a half inches. In this tube a loose piston, c, works freely, and is steadied by the slot seen in it. To a female screw d, motion is given by the toothed wheel ; and the teeth of which, e, answer the triple purposes of thumb-milling, ratchet-stop, and SECTION CUTTING. 205 graduation. This is screwed to a block of wood, f, having a rabbet cut in for the purpose of securing it to the table. Fig. 132. —Gibbon.'* Section Cutting Machim. The machine is self-regulating, and is capable of being worked as rapidly as the skill of the operator may dictate. Sections of woods, when cut from hard woods containing gum, resin, &c, should be soaked in essential oil, alcohol, or ether, before they are mounted as transparent objects. A razor may be fixed to the bench for the purpose of cutting these fine sections, or a fine plane will answer very well. The instrument used by Mr. Topping, fig. 133, con- sists of a b, a flat piece of mahogany, seven inches long and four wide, to the under surface of which is attached, at right angles, a piece g of same size as a b. d is a flat plate of brass, four inches long and three wide, screwed to the upper surface of a b ; to the middle of this plate is attached a tube of the same metal e i, three inches long and half an inch in diameter, and provided at its lower end with a screw /, working in a nut, and having a disk k exactly adapted to the bore of the tube ; this disk is con- nected with the upper end of the screw, and is moved up or down by it. c is another screw connected with a curved piece of brass h, which is capable of being carried to the opposite side of the tube by it. The piece of wood about 206 THE MICROSCOPE. to be cut is put into the tube e, and is raised or depressed by the screw/; whilst, before cutting, the curved piece of Fig. 133.— Topping's Section Cutting Machine, metal h should be firmly pressed against it by the screw c. This instrument, if fastened to the edge of a bench or table, is always ready for use. The knife employed may be one constructed for the purpose ; or a razor ground flat on one side. Method of making Sections. — If the wood be green, it should be cut to the required length, and be immersed for a few days in strong alcohol, to get rid of all resinous matters. When this is accomplished, it may be soaked in water for a week or ten days ; it will then be ready for cutting. If the wood be dry, it should be first soaked in water and afterwards immersed in spirit, and before cutting placed in water again, as in the case of the green wood. The wood, if too large, should be cut so as to fit tightly into the square hole, and be driven into it by a wooden mallet; if, on the contrary, it be round, and at the same time too small for the hole, wedges of deal or other soft wood may be employed to fix it firmly : these will have the advantage of affording support, and if necessary, may be cut with the specimen, from which they may afterwards be easily separated. The process of cutting consists in raising the wood by the micrometer screw, so that the SECTION-CUTTING. 207 thinnest possible slice may be taken off by the knife ; after a few thick slices have been removed to make the surface level, a small quantity of water or spirit may be placed upon it j the screw is then to be turned one or more divisions, and the knife passed over the wood until a slice is removed; this, if well wetted, will not curl up, but will adhere to the knife, from which it may be removed by pressing blotting-paper upon it, or by sliding it off upon a piece of glass by means of a wetted finger. The plan generally adopted is to have a vessel of water by the side of the machine, and to place every section in it : those that are thin can then be easily separated from the thick by their floating more readily in the water; and all that are good, and not immediately wanted, may be put away in bottles with spirit and water, and preserved for future examination. If the entire structure of any exogenous wood is required to be examined, the sections must be made in at least three different ways; these may be termed the transverse, the longitudinal, and the oblique, or, as they are sometimes called, the horizontal, ver- tical, and tangental : each of these will exhibit different Fig. 134. — Sections of Wood. appearances, as may be seen upon reference to fig. 134: b is a vertical section through the pith of a coniferous plant: this exhibits the medullary rays, which are known 208 THE MICROSCOPE. to the cabinet-maker as the silver grain; and at e is a magnified view of a part of the same: the woody fibres are seen with their dots I, and the horizontal lines k indi- cating the medullary rays cut lengthwise ; whilst at c is a tangental section, and /a portion of the same magnified: the openings of the medullary rays m m, and the woody fibres with vertical slices of the dots, are seen. Very instructive preparations may be made by cutting oblique sections of the stem, especially when large vessels are present, as then the internal structure of the walls of some of them may oftentimes be examined. The diagram above given refers only to sections of a pine ; all exogenous stems, however, will exhibit three different appearances, according to the direction in which the cut is made ; but in order to arrive at a true understanding of the arrangement of the woody and vascular bundles in endogens, horizontal and vertical sections only will be required. Many specimens of wood that are very hard and brittle may be much softened by boiling in water ; and as the cutting-machine will answer for other structures besides wood, it should be stated, that all horny tissues will also be softened by boiling, and can then be cut very readily. Preparation of Hard Tissues. — All sections of recent and greasy bones should be soaked in ether for some time, and afterwards dried in the air, before they are fit for the saw, file, and hone ; by dissolving out the grease, the lacunae and canaliculi show up very much better. When it is wished to examine the bone-cells of fossil bone, chippings only are required; these may be procured by striking the bone with the sharp edge of a small mineralogical-hammer: carefully select the thinnest of the chips, and mount them at once, without grinding, in Canada balsam. If desirable to compare bone structures, it must be borne in mind that the specimens for comparison should be cut in one and the same direction; as the bone-cells, on which we rely for our determination, are always longest in the direction of the shaft of the bone, it follows that if one section were trans- verse, and the other longitudinal, there must be a vast difference in the measurement of the bone- cells, in conse- quence of their long diameter being seen in the one case, SECTIONS OP TEETH. 209 and their short diameter in the other. In all doubtful cases, the better plan is to examine a number of fragments, both transverse and longitudinal, taken from the same bone, and to form an opinion from the shape of bon/6-cell which most commonly prevails. The Teeth. — The best mode of examining teeth is by making fine sections. Specimens should be taken, both from young and old teeth, to note the changes. A longitudinal or transverse slice should be first taken off; a circular saw, fitted to the lathe, fig. 135, cuts sections very quickly — then rub down, first by the aid of the corundum-wheel, — which should also be fitted to the head-stock of the lathe, — then finish them off between two pieces of water-of-Ayr stone, and finally clean and polish between plates of glass, or on a polishing strap with putty powder. The section requires to be washed in ether, to remove all dirt and im- purities j when well polished and dried, it may be preserved under thin glass, and cemented down with gold-size or varnish. Such polished sections are preferable to many others which, on account of their irregular surface, require to be covered with fluids, as Canada-balsam, turpentine, &c, in order to fit them for examination with high powers. It almost always happens, that some portion of these fluids enters the dentine, which then becomes indistinct, and almost invisible in its ramifications. Two sections made perpendicularly to one another through the middle of the crown and fang of a tooth, from before backwards, and from right to left, are suffi- cient to exhibit the more important features of the teeth ; but sections ought also to be prepared, showing the surface of the pulp cavity and that of the enamel ; and likewise various oblique and transverse sections through the dentine Fig. 135 —Small Lathe for polishing. 210 THE MICROSCOPE. in the fangs, to exhibit the anastomoses of their branches. The dental cartilage is easily shown by maceration in hydrochloric-acid, a process which requires a longer or snorter time, according to the concentration of the acid. It is very instructive also to macerate thin sections in acid, and to examine them upon a slip of glass, at intervals, until they entirely break up. The enamel prisms are readily isolated in developing enamel in this way, and the transverse lines readily seen when the section is moistened with hydrochloric acid. The early develop- ment may be studied in embryos of two, three, or four months with the simple microscope ; and in transverse sections of parts hardened in spirits of wine. The pulp of mature teeth is obtained by breaking them in a vice, and the nerves can be made out without difficulty on the addi- tion of a dilute solution of caustic soda. To cut through the enamel of the tooth, it will be necessary to lessen the friction, by dropping water upon the saw as it is made to revolve. The section is after- wards very quickly ground down by holding it against the flat side of the corundum- wheel.1 A small handle, mounted with shell-lac, to fix the section in, forms a ready holder : polish, as before directed, between two pieces of the water-of-Ayr stone, or on a hone of Turkey-stone kept wet with water. As the flatness of the polished surface is a matter of the first importance, that of the stones them- selves should be tested from time to time ; and whenever they are found to have been rubbed down on one part more than another, they should be flattened on a paving stone with fine sand, or on a lead plate with emery. When this has been sufficiently accomplished, the section is to be secured, with Canada balsam, to a slip of thick well- annealed glass, in the following manner : — Some Canada balsam, previously rendered somewhat stiff by evaporation of part of its turpentine, is to be melted on the glass-slip, so as to form a thick drop, covering a space somewhat larger than the size of the section, and it should then be set aside to cool ; during which process, the bubbles that may have formed in it will usually burst. When cold, its (1) Corundum is a species of emery composition ; alumina, red oxide of iron, and lime; it is much used by dentists as a polishing material. PREPARING SECTIONS. 211 hardness should be tested with the edge of the thumb- nail, for it should be with difficulty indented by pressure, and vet should not be so resinous as to be brittle. If it be too soft, as indicated by its too ready yielding to the thumb-nail, it should be boiled a little more ; 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 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, care being taken to avoid the formation of bubbles, and the section is then to be gently pressed down upon the liquefied balsam in a sort of wave towards the side, and an equable pressure being finally made over the whole. When the section has been thus secured to the glass, it may be readily reduced in thickness by grinding. 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 section 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 that a margin be still left round the edge of the section. As the section approaches the degree of thinness which is most suitable for the display of its organization, great care must be taken that the grinding process is not carried too far ; and frequent recourse should be had to the microscope to examine it. The final polish must be given upon a leathern strap, or upon the surface of a board covered with buff-leather, sprinkled with putty-powder and water, until all marks and scratches have been rubbed out of the section. In mounting sections of bone, or teeth, it is important to avoid the penetration of the Canada balsam into the interior of the lacunce and canaliculi ; since, when these are filled by it, they become almost invisible. The benefit which is derived from covering the surfaces of the specimen with Canada balsam, may be obtained, without the injury resulting from the penetration of the balsam p2 212 THE MICROSCOPE. into its interior, by adopting the following method : — A small quantity of balsam, proportioned to the size of the specimen, is to be spread upon a slip of glass, and to be rendered stiffer by boiling, until it becomes nearly solid when cold ; the same is to be done to the thin glass cover ; next, the specimen being placed on the balsamed surface, and being overlaid by the balsamed cover, such a degree of warmth is to be applied as will suffice to liquefy the balsam, without causing it to flow freely ; and the glass cover is then to be quickly pressed down, and the slide to be rapidly cooled, so as to give as little time as possible for the penetration of the liquefied balsam. Circular Disc. — For the purpose of cutting glass covers or making shallow cells with japanners' gold-size for mount- ing objects,1 fig. 136 will be found useful; it is made Fig. 13(3. — Circular Disc Machine. of two circular wheels of wood, these being let into a solid block of wood, and secured there by central screws. A. handle of wood is fixed into the upper part of one, for the purpose of turning it round, the motion being com- municated to the other by an endless band of catgut running in the grooved edge of each. On the upper sur- face of the wheel, under the right hand, are fixed, by means of screws, two strips of brass, which serve as springs (1) Mr. Shadbolt's little turn-table, invented for the same purpose, is in every ray superior to this, and is now in very general use. GLASS CELL CUTTING. 213 for securing the glass slip; a camel's-hair pencil, previously- dipped in japanners' gold-size, is then taken between the finger and thumb, and held as represented in the woodcut, when the wheel is put in motion, and a perfect circle is rapidly formed j the cell is then removed and put aside to dry. In the same way, by securing a sheet of thin glass under the brass springs, and substituting for the pencil a cutting diamond, a circular cover may be readily cut out. A cutting diamond is not only useful to the micro- scopist for the above purpose, but also for writing the names of mounted objects on the ends of the glass slides. A glazier's diamond for cutting glass slides is both conve- nient and economical : the mode of using it may be acquired in any glazier's shop. Cutting glass cells. — Mr. H. H. Brown has also con- trived a brass plate for cutting out the centres of pieces of thin glass, to form very shallow cells. It consists of a thin plate of brass, perforated with holes corresponding to the diameter of the cell required, and counter-sunk to a suit- able thickness. To use it, it must be heated, and a little sealing-wax or shellac melted into the counter-sunk parts ; on this, while hot, the disc of thin glass is to be firmly pressed. When cold, the centres may be easily removed with a common rat's-tail file, the edges being afterwards smoothed over by a fine watch-maker's file. The plate must once more be heated and the glass rings removed, and finally be cleansed from adhesive wax by a little ether. Mr. Brooke has long made use of a small brass press for the purpose of cutting out thin circles of glass and con- verting the same into shallow cells. Cells for mounting objects may be made of vulcanite by slicing-tubing made of this material. Glycerine does not act upon it. Mr. Henry Lee uses cells cut from tubes of cardboard; these he finds very useful for mounting dry and opaque objects. Both have the great advantage of cheapness — the former are sold for about 3d. a dozen and the latter Is. a gross. Mr. Suffolk devised a cheap tin-cell, which must partially supersede the more expensive glass- cell; it appears to answer equally well for fluid or dry mounting. The metallic rings for making these cells are manufactured in various sizes and thicknesses by 214 THE MICROSCOPE. Collins, of Great Titchfield Street, and sold at 3d. per dozen. Dr. Guy brought under the notice of microscopists, a plan for preserving metallic sublimations, crystals, &c. in small round, or oval tubes, hermetically sealed at both ends. This plan may be made available for the display of insects, parts of flowers, as stamens, pollen, &c. See his paper in the Micros. Journal, New Series, vol. ii. p. 77. Fig. 137, full size. — Spring-clip for mounting. Dr. Maddox describes a Wire Spring-clip for mounting purposes, Micros. Soc. Trans. 1865, p. 84. To make the clip, take a straight piece of brass wire four and three- quarter inches in length, turn one end at six-eighths of an inch with a pair of wire-pliers at right angles ; this second portion, at half an inch, again bend at right angles in the same plane ; now, at three-quarters of an inch, turn the wire over on itself, leaving at the bend space sufficient to admit a thick slide. At one inch and five-eighths twist the wire completely on itself, and bring the now short ends at right angles to the longest part ; file this end quit* Hat. Give the first portion of the wire a slight curvature, so that the point and bend may act as a stiff spring against the under surface of the slide when applied. This, although a very simple and inexpensive^ clip to make, has been very much improved upon, as will be admitted by those who have used the little handy spring-clip shown full size in fig. 137. The Universal Clip, as it deserves to be called, may be purchased for Is. 6d. per doz. of Baker, Holborn, so that objects of great delicacy can be mounted and left to dry and harden for any necessary length of time. The little nipple which is seen to press upon the glass cover in fig. 138, is made of a piece of cork or thick leather. The clip simply requires pinching together be- tween the thumb and finger; as soon as it is released the MOUNTING OBJECTS. 215 little cork nipple comes down with considerable force upon the glass cover. In mounting algae, &c. with a gelatine medium, the specimen should be placed on the glass slide with a small quantity of water, and when properly arranged, the thin glass cover must be carefully put on, and a suffi- cient quantity of gelatine me- dium placed by its side ; gentle heat should be applied to drive ^SJKSKSSSSS.*""' out the water, when the gela- tine flows under to take its place ; then put the object aside to dry and harden. General Directions for the Preparation and Mounting of Objects. — The objects exhibited by the microscope are either opaque or transparent. The former, in the majority of instances, require little or no preparation beyond placing them in such a position as to show their external surface by reflected or condensed light, and covering with thin glass to exclude dust. Those objects, however, which it is intended to examine by transmitted light require, in most cases, to be prepared previously to mounting them, in whatevei vehicle may be found most suitable for exhibiting then structure. The medium most used for' mounting trans parent objects is Canada balsam. The pure balsam is, however, too thick for use, and it requires to be diluted with spirit of turpentine to render it sufficiently fluid to permeate the structure to be exhibited. As a general rule, it should be just fluid enough to drop readily from the point of a needle. Those who desire to avoid the trouble of mixing their own mounting medium, can procure it ready for use from any of the microscope makers. There are some few objects whose structure is so transparent that they must be mounted dry. Scales from the wings of butterflies and moths, of the podura and lepisma sac- charina, and some of the diatomaceae are of this class. All that is necessary in preparing objects for dry mounting, is to take care that they are free from extraneous matter, and to fix them permanently in that position in which their structure will show to the best advantage. Care 21 G THE MICROSCOPE. should be taken to have no draught of air through the room while handling very delicate objects; many a beauti- ful object has been wafted from under the hand of the microscopist in this way, sometimes, even by his own breath. The preparation of very minute objects which require particular chemical treatment before mounting, will be more fully described hereafter. To this class belong the diatomaceae, whose delicate structure forms one of the most beautiful objects which can be exhibited. In mounting entomological specimens, the first thing, of course, is the dissection of the insect. This is best accomplished by the aid of Collin s's Dissecting Microscope, a pair of small brass forceps, and very finely-pointed scissors ; the parts to be prepared and mounted should first be carefully detached from the insect with the scissors, then immersed in a solution of caustic alkali (Liquor Potassse) for a few days, to soften and dissolve out the fat and soft parts : the length of time it is necessary to immerse them can only be ascertained by experience, but, as a general rule, the objects assume a certain amount of transparency when they have been long enough in the alkali ; when this is ascertained to be the case, the object is to be placed in a flat receptacle (a shallow pomatum pot is as good a thing as can be used), and put to soak for two or three hours in soft or distilled water. It is then to be placed between two slips of glass, and gently pressed till the softer parts, &c. are removed. These will frequently adhere to the edge of the object ; it will, therefore, be necessary to wash the latter carefully in water to get rid of the superfluous matter, a process which will be much aided by delicate touches of a camel's- hair brush. Place the object now and then under the microscope to see that all extraneous matter is removed, and when this is accomplished take the specimen up care- fully with the camel's-hair brush, and lay it on a piece of very smooth paper (thick ivory note is very good for the purpose), arrange it, if necessary, to its natural appear- ance with the brush and a finely-pointed needle, place a second piece of paper over it, and press it flat between two slips of glass, and compress it by one of the American clips which may be bought for a few pence per dozen. PREPARING AND MOUNTING. 217 When thoroughly dry (which it will probably be in about twenty-four hours, if in a warm room), separate the glasses, and gently unfold the paper ; then, with a little careful manipulation, the object may be readily detached, and should be at once placed in a little spirit of turpentine, where it should remain for a few days till it is rendered transparent and fit for mounting. The time during which it should remain in this liquid will depend on the struc- ture ; some objects, such as wings of flies, will be quickly permeated, while horny and dense objects require an immersion of a fortnight or even longer. A pomatum pot with a concave bottom and well-fitting lid will be found to answer admirably for the soaking process, and it is well, in preparing several specimens at a time, to have two pots, one for large and medium, the other for very small objects, or the smaller ones will be found often to adhere to the larger. The glasses on which objects are mounted are usually slips of flatted crown or sheet glass cut to a size of three inches by one, and ground at the edges. The mode ot mounting the object is as follows: — Having chosen a glass slide, clean and polish it with a piece of chamois leather, ascertain the centre of the slide by means of a piece of paper or card of exactly the same size as itself, and in which a hole has been cut exactly in the centre, place the piece of paper under the slide, and, having removed the object to be mounted from the turpentine in Fig. 139.— Showing the mode of placing Glass Cover on the Object. which it has been soaked, lay it on the slide on the spot corresponding to the hole in the card underneath ; then takn up a small quantity of the prepared ^Canada balsam on the point of a large needle or pointed pen-knife, and '218 THE MICROSCOPE. drop it immediately over the object, slightly warm the under part of the slide over a spirit-lamp to diffuse the balsam and cause it thoroughly to penetrate the object, and immediately cover the latter with one of the small circles of thin glass, sold by opticians for the purpose. In laying the glass cover on the object, care should be taken to bring the edge of the circle down first, and let the other fall slowly on the object (see fig. 139), to prevent the formation of bubbles from the sudden displacement of the air. It requires some little practice to keep the object in the centre of the circle. Sometimes, notwithstanding all the care of the operator, bubbles will appear. These may generally be got rid of by gently warming the under part of the slide over the spirit-lamp, when the bubbles will usually leave the object, and travel towards the edge of the circle. In most cases they will entirely disappear as the balsam becomes firmer and drier. If it be desired to dry the balsam quickly, the slide may be placed in some warm situation where the heat does not much exceed 100°, and it must be maintained in a perfectly horizontal position, to prevent displacement, until the balsam has become dry. When this has been ascertained to be the case, the superfluous balsam which surrounds the edge of the circle may be scraped off by the point of a penknife ; and when the major part has been removed in this way, the remainder may be got rid of, and the edges of balsam rendered smooth by rubbing gently with an old silk handkerchief moistened with spirit of turpentine. The edge of the circle of balsam will probably appear white and dull, but it may be rendered transparent by gently warming the under part of the slide over a spirit-lamp, and again placing the object in a warm room till the balsam has a second time become hard and dry ; after which the name of the object should be written with a small writing diamond at one end of the slide. Some microscopists prefer to cover the slide with orna- mental paper, which may be procured very cheaply. In covering the slides with paper, their edges need not be ground, but may be rubbed with a fine file, which will prevent the sharp glass from damaging the paper cover, and cutting the fingers of the operator. The foregoing i3 PREPARING AND MOUNTING. 219 the method by which objects are mounted in balsam ; there are, however, some specimens, the mounting of which, in balsam, would render them almost invisible, in which case — if not suitable for dry-mounting — they should be placed in fluid in cells, the size and depth of which must be regulated by the proportions of the object. If it be the scale of a fish, or the pollen of a flower, a very shallow cell will suffice, and it may be formed of " Bruns- wick black" in the manner already described. When the cell is quite dry, take the object (which should have been some time previously soaked in the fluid in which it is to be mounted to dispel the air from its substance), place it in the middle of the circle, fill the space quite full of the mounting fluid, and cover it with a glass circle ; place the edge down first, and bring the whole surface of the circle very gradually upon the cell as pointed out in the former case. Some of the fluid will immediately escape under the edge ; this may be absorbed by a piece of filtering paper. Should too much escape, a bubble will make its appearance in the cell; in this case the process must be repeated. When this has been performed successfully, secure the glass circle in its place with a small spring-clip ; then take a camel' s-hair brush, charged with varnish, and carry it round, and slightly over the edge of the cover. Allow the first layer to dry before another is added, and continue to add more gradually until the cell is made perfectly air-tight. Glass or metal cells must be employed for those objects whose bulk renders the method just described inadmissible. Glass-cells may be fastened to the glass-slide either by Canada balsam, by Jefferey's marine glue, or Brunswick black ; the latter will be rendered very durable by mixing it with a small quantity of India-rubber varnish (made by dissolving small strips of caoutchouc in gas-tar). The pro- cess of mounting in glass-cells is similar to that employed in making varnish-cells, except that a somewhat larger quantity of cementing medium is required on account of the greater weight of the ceLL Objects mounted in this way should always be kept in the horizontal position, and a little fresh varnish applied now and then, if the cement show any tendency to crack. In mounting objects in balsam, great care should be 220 THE MICROSCOPE. taken to have the specimens quite dry before soaking them in turpentine. Objects mounted in cells, on the contrary should have become perfectly saturated with the mounting fluid before being finally secured. It is preferable to mount and preserve specimens of animal tissues in shallow cells, to avoid undue pressure on the preparation. Cells intended to contain prejDarations im- mersed in fluid must be made of a substance impervious to the fluid used; on the whole, the most useful are those made with circles of thin glass, cemented to the glass-slide with marine glue, such as we have here represented (fig. 140). The surface of the glass Fig" U0Mu!SZ'tlS f°r should be slightly roughened before applying the cement. Different modes of mounting may be employed with advantage, to show different structures ; entomological specimens, such as legs, wings, spiracles, tracheae, ovi- positors, stings, tongues, palates, corneae, &c. show best in balsam : the trachea of the house-cricket, however should be mounted dry. Sections of bone show bes*, when mounted dry, or in a cell with fluid. Scales of butterflies, moths, &c. should be mounted dry. Other objects, as sections of wood and stones of fruit, exhibit their structure best in a cell with fluid. There are some objects much more difficult to prepare than others, and which tax the patience of the beginner in a manner which can hardly be imagined by any one who has never made the attempt. The structure of many creatures is so delicate, as to require the very greatest care to prevent mutilation, and consequent spoliation of the spe- cimen. The beginner, therefore, must not be discouraged by a few failures in commencing, but should persevere in his attempts, and constant practice will soon teach him the best way of managing intricate and difficult objects. The room in which he operates should be free from dust, smoke, and intrusion, and everything used should be kept scrupulously clean, since a very small speck of dirt, which may be almost invisible by the naked eye, will assume unpleasant proportions under the microscope, and not only PREPARING AND MOUNTING. 221 mar the beauty, but possibly interrupt a clear view of a very splendid and delicate object. Then, again, if the microscopist prefers to cut and grind his own glass slips, he should be very careful that there are no sand-specks or air-bubbles in the centre of the slide, or of the glass- cover : many a good object has been spoiled from neglect of this precaution. A good light by which to work is also highly important. In using the ordinary microscope, the microscopist should keep both eyes ojoen, the practice of closing the eye not in use being injurious to the sight of both. The beginner who is about to purchase a micro- scope, will do well to procure a binocular, the price of which has been reduced so much as to bring it within the reach of those of even moderate means. The preservation of objects is a matter of much im- portance. They ought to be kept in a cool and dry place. Sections of bone, the pollen of plants, and the scales of the podura and butterflies, become spoiled if kept in a damp atmosphere. Objects mounted dry should be carefully protected from dust, which will find its way into crevices and corners where one might hardly suppose it could possibly enter. Some opaque objects require dry mounting; others show best in fluid ; the elytra of some beetles are mag- nificent objects when mounted in balsam. Cells punched out of thick mill-board, or turned in some hard wood, or Mr. Suffolk's metal-rings, are much lighter and more con- venient than glass for such objects, and also for those mounted dry. In mounting objects in fluid, the glass-cover should come nearly, but not quite, to the edge of the cell ; a slight margin must be left for the cement, which ought to project slightly over the edge of the cover, in order to unite it securely to the cell. In cells, however, that con- tain balsam, there is no necessity for cement, and the appearance of the slide will be improved by choosing a cover of exactly the same size as the cell. There are some objects which require considerable dex- terity and patience to prepare them for mounting. The petal of the pelargonium is one of these. It is necessary to split it in order to show its structure to perfection. This is effected bv dividing the base of the petal with a 252 TEE MICROSCOPE. sharp penknife, and then, with two small pairs of forceps, tearing asunder the two parts ; the upper surface, which is the part to be mounted, must then be dried and placed for a short time in spirit of turpentine, previously to being mounted in balsam. If well prepared, the colour of the petal will be almost as fresh as in its natural state. In mounting diatomaceae, to show them as nearly as possible in their natural condition, they should be first well washed in distilled water and mounted in a medium composed of one part of spirits of wine to seven parts of distilled water. The siliceous coverings of the diatoms, however, which show such beautiful forms under the higher powers of the microscope, require considerable preparation. The guano, or infusorial earth containing them, should first be washed several times in water till the water is colourless, allowing sufficient time for pre- cipitation between each washing. The deposit must then be put into nitro-hydrochloric acid (equal parts of nitric and hydrochloric acids). A violent effervescence takes place, and when this has subsided, the whole should be subjected to heat, nearly but not quite up to the boiling- point, for six or eight hours. The acid must now be care- fully poured off, and the precipitate washed in a large quantity of water, allowing some three or four hours between each washing, for the subsidence of some of the lighter forms ; the sediment must be examined under the microscope with an inch object-glass, and the siliceous valves of the diatoms picked out with a coarse hair or bristle. "When it is desired to exhibit one of the larger forms, it can be placed separately in the middle of a glass-slide, and mounted in Canada balsam ; or when it is wished to mount several forms at once, the fluid containing the skeletons should be gently shaken, and a drop spread over the centre of a slide, the glass being held over a spirit- lamp till the fluid has all evaporated, when the object may be either dry-covered or mounted in balsam in the usual way, according as its structure may require the one or the other treatment. Diatoms may be kept in distilled water in a small corked phial for any length of time. For vegetable preparations, distilled water in which GLYCERINE JELLY. 223 camphor lias been kept is a good mounting medium ; some preparers, however, add spirits of wine in the proportion of one draehm to eleven drachms of water ; others add to these saltpetre, two grains to the ounce of fluid. The mounting-fluid should be prepared and allowed to settle, when the major part can be poured off clear — a much better mode than filtering, as particles of the paper are apt to get mixed with the liquid. F. M. Kimmington's Glycerine Jelly has become an established preparation, and one found to possess many advantages over Canada balsam, and some other media used for the same purpose, in the facility with which objects can be mounted, and efficiently preserved, and the transparency it gives to many animal and vegetable struc- tures. Its antiseptic powers are sufficient to prevent changes in any object immersed in it, if not too bulky. It is especially adapted for mounting algse, fungi, vegetable and animal tissues, urinary deposits (as casts, epithelium, crystals), also anatomical specimens, starch granules, des- midiaceae, diatomaceae, &c. Objects to be mounted in this medium only require to be soaked in weak alcohol for a time, greater or less, as may be necessary, for the purpose of dissolving out any soluble matter, or for driving out air entangled in the cellular meshes. "When it is thought that this has been accomplished, the object is laid on a slide and freed from superfluous moisture, the slide gently warmed, and a quantity of the jelly, deemed sufficient for the purpose, laid by the side of it, and made to touch it. It will thus be immersed ; the cover, previously warmed, is to be laid upon it, and gently pressed. The slide may now be laid aside and finished by running it round with varnish. The jelly should be liquefied by holding the bottle at some distance above a lamp, or by dipping it into hot water. No more than what is necessary need be liquefied, and it should never be made hot. For certain delicate organisms, as the desmidiacese and diatomaceae, whose plasma may be affected by too dense a medium, the jelly may be diluted one-quarter or one- third with camphor-water. For mounting pathological or injected preparations, the following medium will be found to answer very well : — 224 THE MICROSCOPE. Bichloride of mercury, one grain ; chloride of sodium, ninety grains; distilled water, one pint: set aside, and, when perfectly clear, decant and preserve in a stoppered bottle. All preparations used in mounting, and objects themselves, should be carefully preserved from dust. Balsam should be kept in a wide-mouthed bottle, over which a small tumbler or cupping-glass must be inverted. It is ad- visable to avoid the use of a cork, as it will be broken in attempting to withdraw it, and portions will not only adhere to the neck of the bottle, but are apt to get into the balsam. Objects, and parts of objeets, already prepared and dry, may be kept neatly folded up in ivory-paper, as it is not well to have too many objects in the turpentine at the same time, in order to avoid eonfusion. Glass- slides which are soiled with balsam can be cleaned with turpentine and a piece of rag — the glass circles will require to be soaked in turpentine to get rid of any balsam they may happen to contract. When a greasy appearance is present, it may be removed by a piece of rag moistened with rectified spirit. Some remove it by rubbing the glass with caustic alkali ; but as this liquid acts on flint- glass, and sometimes makes it partially opaque or milky, its use should be entirely discarded — a weak solution of ammonia, or cyanide of potassium, answers the purpose best. In order to demonstrate the internal structure of animal tissues under very high magnifying powers, Dr. Beale l says the specimen must be placed in a viscid medium, and sub- jected to considerable pressure to make it sufficiently thin. Price's glycerine, having a specific gravity of 1,240, and a strong syrup, are the best adapted for preserving all soft tissues. In practice, the specimen is first immersed in a solution of weak glycerine or syrup, and the density ot the fluid is gradually increased. In this way, and in the course of1 two or three days, the softest and most delicate tissues swell out, and become more transparent; but no chemical change is produced in the specimens. The one thing to be observed is, that the strength of these fluids should be increased very gradually until the whole of the tissues are thoroughly penetrated and saturated by the strongest that can be obtained. Cerebral tissues, delicate (1) Beale, " How to Work with the Microscope." STAINING ANIMAL TISSUES. 22/5 nervous textures, like the retina, may be permeated by the strongest glycerine, and, when fully saturated with it, dissected with a.-degree of minuteness unattainable in any other way. " The smallest animalcules, entozoa, polyps, molluscs, insects, Crustacea, fungi, algae, and the delicate parts of vegetable tissues, are equally well preserved by it." All the various colouring-solutions, as carmine, aniline, &c, should be always prepared for use with glycerine or syrup as a basis. The staining or colouring fluid employed by Dr. Beale is made as follows : — Carmine, 10 grains ; strong liquid ammonia, J drachm ; Price's glycerine, 2 ounces ; dis- tilled water, 2 ounces ; alcohol, J ounce. The carmine is «to be placed in a test-tube, and the ammonia added to it. Upon applying the gentle heat of a spirit-lamp, it is dissolved. Boil it up for a few seconds, and allow it to cool before adding the glycerine and rest of the ingredients. Lastly, pass it through a filter, or allow it to stand by and decant off the clear solution. The solution should neither be too alkaline, nor perfectly neutral ; if the former, the colouring becomes too intense, and thus much of the soft or imperfectly formed tissue is destroyed; and, if the latter, the uniform staining of tissue and germinal matter equally mars the result. The permeating power of the solution may be increased by the addition of a little more water and alcohol. After the specimen has been properly stained, it should be washed in a solution, consisting of strong glycerine two parts, water one part ; and then transferred to the following acid fluid: — Strong glycerine, one ounce; strong acetic acid, five drops; where it must remain three or four days to regain the volume it occupied when fresh. To mount specimens so prepared, take a small portion and spread it out on the glass-slide ; add a drop of fresh glycerine, and cover with thin glass ; warm it gently, and press it down with a needle-point. Examine the specimen with a quarter power, and, if a good deal of granular matter appears to be mixed in with it, remove the glass cover, and wash it by adding drop after drop of the glycerine and acid solution. The slide fchould be inclined, and, at the same time, gently warmed. Q 226 THE MICROSCOPE. over the lamp. When a few drops of pure glycerine have been allowed to flow over it, the thin glass cover must be re-applied and pressed tightly down. If the preparation looks clear under an eighth or twelfth, the glass cover may be cemented on, and the specimen left to dry. The success of Dr. Beale's process, it appears, much depends upon the care and patience with which each stage of the soaking, washing, warming, and pressure is carried out. Each point of structural difference is gradually brought out by sub- jecting the specimen to a little firmer pressure, or by soaking it in a little fresh glycerine in a watch-glass, and then applying gentle heat. By the aid of needles, and a little careful manipulation, tissues may be laid out per- fectly smooth and flat. A word about making and preparing Transparent In- jections.— The great desideratum of a transparent injecting fluid is, that it shall not, by the action of osmosis, dye the tissue meant to be injected. This at once bars the use of soluble colours, and necessitates the use of insoluble colour- ing matter in an exceedingly fine state of subdivision. The following composition is stated to succeed admirably, showing vessels of ^njth of an inch diameter, with a clear outline even under a }ih objective, without a grain of extravasation of the colouring matter: — Take 180 grains best carmine ; |- fluid ounce of ammonia, common strength, sp. gr. 0-92 ; 3 to 4 ounces distilled water. Put these ingredients into a small flask, and allow them to digest without heat for 24 or 36 hours, or until the carmine ;.s dissolved. Then take a Winchester quart-bottle, and mark upon it the line to which 16 ounces of water extend. The coloured solution must then be filtered into the bottle, and pure water must be added until the whole is equal to 16 ounces. Next dissolve 600 grains of potash-alum in about 10 fluid ounces of water, and add to this, under constant boiling, a solution of carbonate of soda, until a slight permanent precipitate is produced. Filter and add water up to 16 fluid ounces. Boil, and add this solution, while boiling, to the cold ammoniacal solution of carmine in the Winchester quart, and shake vigorously for a few minutes. A drop now placed upon white filter-paper should show no coloured MOUNTING POLYZOA. 227 ring ; if it do, the whole must be rejected. Supposing the precipitation to be complete, or nearly so, shake vigorously for half an hour, and allow to stand till quite cold ; the shaking must then be Tenewed, and the bottle filled up with cold water. After allowing the precipitate to settle for a day, draw off the clear supernatant fluid with a syphon. Repeat the washing until the clear fluid gives no precipitate with chloride of barium. So much water must be left with the fluid that at last it may measure 40 fluid ounces. For the injection-fluid, take 24 ounces of the above coloured fluid, and 3 ounces of good gelatine; allow these to remain together all night, then dissolve by the aid of a water-bath, and strain through fine muslin. On injecting, the ordinary precautions for a gelatine injection are alone necessary. Mounting Polyzoa. — Mr. Morris, of Bath, has succeeded in obtaining beautiful specimens of polyzoa and hydroid zoophytes, with expanded tentacles, by adding spirit of wine, drop by drop, to the salt-water cell in which they have been confined. The polypidoms should be thus treated as soon as possible after capture. A plan of mounting objects in a mixture of balsam and chloroform is described by Mr. Wm. Henry Heys in the Microscojrical Journal thus : — Take a quantity of the oldest balsam procurable, and place it in an open glass cup, and mix with it as much chloroform as will make the whole quite fluid, so that a very small quantity will drop from the lip of the containing vessel. Then put this prepared balsam into long thin half-ounce vials, and cork and set them aside for at least a month. The advantage of having it ready-made is, that there is no waste, and none of the ► usual and troublesome preparation required for putting up objects in Canada balsam; and if it has stood for some time, it loses the yellow tinge which is observable in most samples when first mixed, and, moreover, air-bubbles escape more readily. Mr. Goadby's fluids are cheap and most effectual for preserving and mounting animal structures in. The fol- lowing are his formulae : — Take for No. 1 solution, bay salt, 4 oz. ; alum, 2 oz. ; corrosive sublimate, 2 grains; boiling water, 1 quart: mix. 228 THE MICROSCOPE. For No. 2 solution, bay salt, 4 oz. ; alum, 2 oz. ; corrosive sublimate, 4 grs. ; boiling-water, 2 quarts : mix. The No. 1 is too strong for most purposes, and should only be employed where great astringency is needed to give form and support to very delicate structures. No. 2 is best adapted for permanent preparation ; but neither should be used in the preservation of animals containing carbonate of lime (all the mollusca), as the alum becomes decomposed, sulphate of lime is precipitated, and the pre- paration spoiled. For such use the following : — Bay salt, 8 oz. ; corrosive sublimate, 2 grs.; water, 1 quart : mix. The corrosive sublimate is used to prevent the growth of vegetation in the fluid ; but as this salt possesses the property of coagulating albumen, these solutions cannot be used in the preservation of ova, or when it is desired to maintain the transparency of certain tissues, such as the cellular tissue, the white corpuscles of the blood, &c. Mr. Goadby's method of making marine -glue for cement- ing cells is as follows : dissolve separately equal parts of shell-lac and India-rubber in coal or mineral naphtha, and afterwards mix the solutions carefully by the application of heat. It may be rendered thinner by the addition of more naphtha, and is always readily dissolved by naphtha, ether, or solution of potash, when it becomes hard or dry in our stock-pots. Preparation and Preservation of Algae, &c. — Mr. Ralfs gives excellent directions for making preparations of algse for microscopic investigations : — " The fluid found to answer best is macvj in the following way : to sixteen parts of distilled water add one part of rectified spirits of wine, and a few drops of creosote, sufficient to saturate it ; stir in a small quantity of prepared chalk, and then filter; with this fluid mix an equal measure of camphor • water (water saturated with camphor); and before using, strain off through a piece of fine linen. " This fluid I do not find to alter the appearance of the endochrome of algae more than distilled water alone does after some time ; there is certainly less probability of confervoid filaments making their appearance in the pre- parations; and there would seem to be nothing to prevent PRESERVATION OF ALG.E. 229 such a growth from taking place, when the object is mounted in water only, provided a germ of one of these minute plants happen to be present, as well as a small quantity of free carbonic acid. " My method of making cells in which to mount pre- parations of algse is as follows : some objects require very shallow, and others somewhat deeper cells. The former may be made with a mixture of japanners' gold-size and litharge, to which (if a dark colour is preferred) a small quantity of lamp-black can be added. These materials should be rubbed up together with a painter's muller, and the mixture laid on the slips of glass with a camel-hair pencil as expeditiously as possible, since it quickly becomes hard ; so that it is expedient to make but a small quantity at a time. For the deeper cells marine-glue answers ex- tremely well, provided it is not too soft. It must be melted and dropped upon the slip of glass ; then flattened, whilst warm, with a piece of wet glass, and what is super- fluous cut away with a knife, so as to leave only the walls of the cell ; these, if they have become loosened, may be made firm again by warming the under surface of the slip of glass. The surface of the cells must be made quite flat ; which can be easily done by rubbing them upon a wet piece of smooth marble, covered with the finest emery- powder. " When about to mount a preparation, a very thin layer of gold-size must be put upon the wall of the cell, as well as on the edge of the piece of thin glass which is to cover it ; before this is quite dry, the fluid with the cbject is to be put into the cell, and the cover of thin glass slowly laid upon it, beginning at one end ; gentle pressure must then be used to squeeze out the superfluous fluid ; and, after carefully wiping the slide dry, a thin coat of gold-size should be applied round the edge of the cell, and a second coat so soon as the first is dry; a thin coat or two of black sealing-wax varnish may then be put on with advantage, in order to prevent effectually the admission of air into the cell, or the escape of fluid out of it. " I would remark, that the gold-size employed should be of the coi sistance of treacle ; when purchased, it is usually too fluid, and should be exposed for some time in 230 THE MICROSCOPE. an open vessel ; a process which renders it fit for use. In mounting the Desmidacece, great attention is necessary to exclude air-bubbles, which cannot be avoided unless the fluid completely fills the ceils ; and also not to use too much fluid, as in this case the smaller species will often be washed away on the escape of the superfluous portion. As the cells cannot be sealed whilst any moisture remains on their edge, it should be removed by blotting-paper, in preference to any other mode. A thin description of glass is manufactured expressly for the purpose of covering specimens when mounted. " The rare species of Desmidacece are frequently scattered amongst decayed vegetable matter, so that it is difficult to procure good specimens for mounting. In such cases, a small portion of the mass should be mixed with a little of the creosote fluid, and stirred briskly with a needle. After this has been done, the Desmidacece will sink to the bottom, when the refuse should be carefully removed. Successive portions having been thus treated, specimens will at length be procured sufficiently free from foreign matter. Even in ordinary circumstances, if a small extra quantity of fluid be placed in the cell, and the slide gently inclined, most of the dirt may be removed by a needle before the cell is closed ; which process will materially increase the beauty of the preparation. "If the cells are insufficiently baked, the japan occa- sionally peels off the glass after the specimen has been mounted for some time. To obviate this inconvenience, Mr. Jenner previously heats the cell, with much caution, over a rushlight, until the japan becomes of a dark colour, and vapour ceases to arise from it. When gold-size is used for closing the cell, the intrusion of some of it frequently destroys valuable specimens, whatever care may be taken. Mr. Jenner has therefore relinquished it, and now employs a varnish made of coarsely comminuted purified shell-lac or translucent sealing-wax, to which is added rectified spirits of wine, in sufficient quantity to cover it. This varnish will be ready for use in about twelve hours : when it is too thick, a little more spirit should be added. Mr. Jenner applies three coats of this varnish, and about a week afterwards a fourth, composed of japan varnish or gold- INJECTING ANIMAL BODIES. 231 size1 To preserve the brush in a fit state, it should always be cleaned with spirits of wine whenever it has been used." Mr. Topping's fluid for mounting consists of one ounce of rectified spirits to five ounces of distilled water ; this he thinks superior to any other combination. To preserve delicate colours, however, he prefers to use a solution of acetate of alumina, one ounce of the acetate to four ounces of distilled water : of other solutions he says, that they tend to destroy the colouring matter of delicate objects, and ultimately spoil them by rendering them opaque. Injecting Animal Bodies. — For minute injections, the most essential instrument is a proper syringe. This is usually made of brass, of such a size that the top of the thumb may press on the button at the top of the piston-rod when drawn out, while the body is sup- ported between the two fingers. Fig. 141 represents the syringe : a is a cylin- drical brass body, with a screw at the top for the purpose of firmly screwing down the cover b, after the piston c is introduced into it ; this is rendered air-tight with leather ; the bottom of the syringe cl also unscrews for the con- venience of cleaning ; e is a stop-cock, on the end of which another stop-cock / fits accurately ; and on the end of this either of the small pipes g, which are of different sizes, may be fixed. The transverse wires across the pipes are in- tended to secure them more tightly to the vessels, into which they may be inserted with thread, so that they may not slip out. In addition to the syringe, a large tin vessel, to contain hot water, with two or three smaller ones fixed in it, for the injections, will be found useful. To prepare the material for injecting : — Take of the finest and most transparent glue one pound, break it into small pieces, put it into an earthen pot, and pour on it (1) " Coachmaker's Black" is an excellent varnish. Fig. 141. 232 THE MICROSCOPE. three pints of cold water ; let it stand twenty-four hours, stirring it now and then with a stick; then set it over a slow fire for half an hour, or until all the pieces are per- fectly dissolved ; skim off the froth from the surface, and strain through a flannel for use. Isinglass and cuttings of parchment make an excellent size, and are preferable for very particular injections. If gelatine be employed it must be soaked for some hours in cold water before it is warmed. About an ounce of gelatine to a pint of water will be suf- ficiently strong, but in very hot weather it is necessary to add a little more gelatine. It must be soaked in part of the cold water until it swells up and becomes soft, when the rest of the water, made hot, is to be added. Good gelatine for injecting purposes may be obtained for two shillings a pound. The size thus prepared may be coloured with any of the following : — For Red. To a pint of size, add 2 oz. of Chinese vermilion. ,, Yellow. ,. „ 2| oz. of chrome-yellow. ,, White. ,, ,, 3| oz. of flake-white. ,, Blue. ,, ,,- 6 oz. of fine blue smalts. It is necessary to remember, that whatever colouring matter is employed must be very finely levigated before it is mixed with the injection. This is a matter of great im- portance: for a small lump or mass of colour, dirt, &c. will clog the minute vessels, so that the injection will not pass into them, and the object will be defeated. The mix- ture of size and colour should be frequently stirred, or the colouring matter will sink to the bottom. Respecting the choice of a proper subject for injecting, it may be remarked, that the injection will usually go furthest in young subjects ; and the more the fluids have been exhausted during life, the greater will be the success of the injection. To prepare the subject, the principal points to be aimed at are, to dissolve the fluids, empty the vessels of them, relax the solids, and prevent the injection from coagulating too soon. For this purpose it is necessary to place the animal, or part to be injected, in warm water, as hot as the operator's hand will bear. This should be kept at nearly the same temperature for some time by occasionally adding hot water. The length of time required is in pro- portion to the size of the part and the amount of its INJECTING ANIMAL BODIES. 233 rigidity. Ruysch (from whom the art of injecting has been called the Ruyschian art) recommends a previous maceration for a day or two in cold water. The size must always be kept hot with the aid of warm water ; for if a naked fire be used, there is danger of burning it. The size may be placed in a vessel which can be heated by standing it in a common tin saucepan of hot water. A convenient form of apparatus for melting the size, and afterwards keeping it at a proper temperature, is Fig. 142. — Melting Vessel. shown in fig. 142. It consists merely of a water-bath, in which the cans containing the injecting fluid can be kept hot, and their contents protected from dust by means of their covers. A small apparatus of this kind could be made by any tin-worker, and fitted over a gas jet to stand on the table. The operator should be provided with several pairs of strong forceps, for seizing the vessel or stopping the escape Fig. 143. of injection. A small needle, fig. 143, will be found useful for passing the thread round the vessel into which the 234 THE MICROSCOPE. injecting pipe is to b.e inserted. Where the vessels are large, a needle commonly known as an aneurism needle answers the purpose very well. The thickness of thread must vary according to the size of the vessel. The silk used by surgeons will be found the best adapted for the purpose, and not too thin, or it may cut through the vessel. When the size and the subject have both been properly prepared, have the injection as hot as the fingers can well bear. One of the pipes g, fig. 141, must then be placed in the largest artery of the part, and made secure by tying. Put the stop-cock / into the open end of the pipe e, and it is then ready to receive the injection from successive applications to the syringe a, leaving sufficient space only for the piston c. The injection should be thrown in by a very steady and gentle pressure on the end of the piston- rod. The resistance of the vessels, when nearly full, is often considerable; but it must not be overcome by violent pressure with the syringe. When as much injection is passed as may be thought advisable, the preparation may be left (with the stop-cock closed in the pipe) for twenty- four hours, when more material may be thrown in. As the method of injecting the minute capillaries with coloured size is often attended with doubtful success, various other plans have been proposed. Ruysch's method, according to Rigerius, was to employ melted tallow coloured with vermilion, to which in the summer a little white wax was added. Monro recommended coloured oil of turpen- tine for the small vessels ; after the use of which, he threw in the common coarse injection. This is made of tallow and red lead, well mixed and heated before it is used. The cold paint injection succeeds well if thrown carefully into the minute arteries; but its tendency to become brown by age is an objection to its use. Professor Breschet frequently employed with success milk, isinglass, the alcoholic solution of gum-lac, spirit varnish, and spirit of turpentine ; but he highly recommends the colouring matter extracted from Campeachy, Fernambone, or Sandal woods. He says : " The colouring matter of Campeachy wood easily dissolves in water and in alcohol ; it is so penetrating that it becomes rapidly spread through INJECTING ANIMAL BODIES. 235 the vascular net-works. The sole inconvenience of this kind of injection is, that it cannot be made to distend any except most delicate vessels, and that its ready penetra- tion does not admit of distinguishing between arteries, veins, and lymphatics." He also recommends a solution of caoutchouc. Another process, which may be termed the chemical process, was published in the Comptes Rendus, 1841, as the invention of M. Doyers. According to this, an aqueous solution of bichromate of potass, 1,048 grains to two pints of water, is thrown into the vessels; and after a short time, in the same manner and in the same vessels, an aqueous solution of acetate of lead, 2,000 grains to a pint of water, is injected. This is an excellent method, as the material is quite fluid, and the precipitation of the chromate of lead which takes place in the vessels themselves gives a fine sulphur-yellow colour. Mr. Topping prepares many fine injections in this way. Mr. Goadby has improved upon the process last named by uniting to the chemical solutions a portion of gelatine, as follows : Saturated solution of bichromate of potash, 8 fluid ounces; water, 8 ounces; gelatine, 2 ounces. Saturated solution of acetate of lead, 8 fluid ounces; water, 8 ounces; gelatine, 2 ounces. The majority of preparations thus injected require to be dried and mounted in Canada balsam. Each preparation, when placed on a slip of glass, will necessarily possess more or less of the coloured infiltrated gelatine, (by this is meant the gelatine coloured by the blood, which, together with the acetate of potash resulting from the chemical decomposition, may have transuded through the coats of the vessel,) which, when dry, forms, together with the different shades of the chromate of lead, beautiful objects, possessing depth and richness of colour. The gelatine also separates and defines the different layers of vessels : the arteries are always readily distinguishable by the purity and brightness of the chromate of lead within them, while the veins are detected by the altered colour imparted by the blood. Those preparations which require to be kept wet can be 236 THE MICROSCOPE. preserved perfectly in Mr. Goadby's No. 2 fluid, specific gravity I'lOO; the No. 1 fluid destroys them. " I would recommend that the slips of glass employed for the dry preparation be instantly iuscribed with the name of the preparation, written with a diamond ; for, when dry, it is difficult to recognise one preparation from the other, until the operator's eye be educated to the effects of this chemico-gelatinous injection. Where so much wet abounds, gummed paper is apt to come off. When dry, it is sufficient, for the purpose of brief exami- nation by the microscope, to wet the surface of a prepara- tion with clean oil of turpentine ; immediately after examination, it should be put away carefully in a box, to keep it from the dust, until it can be mounted in Canada balsam. " The bichromate of potash is greatly superior in colour to the chromate, which yields too pale a yellow; and sub- sequent experience has proved that the acetate of potash frequently effects its liberation by destruction of the capillaries, and this even long after the preparations have been mounted in Canada balsam; perhaps this may be owing to some chemical action of the acetate of potash upon them. I would suggest the substitution of the nitrate for the acetate of lead, as we should then have, in the liberated nitrate of potash, a valuable auxiliary in the process of preservation. Although highly desirable, as the demonstrator of the capillaries of normal tissues, I do not think this kind of injection fitted for morbid prepara- tions ; the infiltrated gelatine producing appearances of a puzzling kind, and calculated to mislead the pathologist. In preparing portions of dried well-injected skin for examination by the microscope, I have tried the effect of dilute nitric acid as a corroder with very good results. But, probably, liquor potassa would have auswered this purpose better. " When size-injection is to be employed, coloured either with vermilion or the chromate of lead, the animal should be previously prepared by bleeding, to empty the vessels ; for if they be filled with coagulated blood, it is quite im- possible to transmit even size, to say nothing of the colour- ing matter. Hence the difficulty of procuring good TRANSPARENT INJECTIONS. 237 rejections of the human subject. But with the chemico gelatinous injections no such preparation is necessary; and success should always be certain, for the potash liquefies the blood, while constant and long-continued pressure by the syringe drives it through the parietes of the vessel into the cellular tissue." Transparent Injections. — " Much more strongly," writes Dr. Beale, "can I recommend to you the use of transparent fluids for making injections. It is true, that these are not likely to be so much admired by general observers as opaque injections. Indeed, it is not easy to find any object which will rival in beauty many tissues that have been freely injected with vermilion or chromate of lead; although it must be confessed that from such preparations we learn but little save the general arrangement of the capillary vessels of the part, their capacity, and the mag- nitude of the meshes of the network. Of the relations which these vessels bear to the elementary structures which give to the texture under examination its peculiar properties, such preparations tell us nothing. Opaque injections are for the most part only adapted for examina- tion with low powers, while the tissues to which the vessels are distributed can only be seen with the help of very high magnifying powers. Transparent injections, on the other hand, though they fail to excite the wonder of the un- initiated, show us not only the general arrangement of the capillary network, but the precise relation which each little tube bears to the tissue with which it is in contact. " In order to make injected preparations for examination by transmitted light, several different substances may be u&ed as injecting fluids. " Injection with Plain Size. — A tissue which has been in- jected with plain size, when cold is of a good consistence for obtaining thin sections, and many important points may be learned from a specimen prepared in this manner which would not be detected by other modes of prepara- tion. A mixture of equal parts of gelatine and glycerine is, however, much to be preferred for this purpose, and the specimen thus prepared is sure to keep well. " Colouring Matters for Transparent Injections. — The chief 238 THE MICROSCOPE. colouring matters used for making transparent injections are carmine and Prussian blue. The former is prepared by adding a little solution of ammonia (liquor ammoniae) to the carmine, and diluting the mixture until the proper colour is obtained, or it may be diluted with size. " The Prussian Blue consists of an insoluble precipitate, so minutely divided, that it appears like a solution to the eye. The particles of freshly prepared Prussian blue are very much smaller than those of any of the colouring matters employed for making opaque injections. "Advantages of Employing Prussian Blue. — I have lately been employing Prussian blue very much, and according to my experience it possesses advantages over every other colouring matter. It is inexpensive, — may be injected cold, — the preparation does not require to be warmed, — no size is required — it penetrates the capillaries without the necessity of applying much force, — it does not run out when a section is made for examination, — neither do any particles which may escape from the larger vessels divided in making the section, adhere to it and thus render the section obscure, — a structure may be well injected with it in the course of a few minutes. Specimens prepared in this manner may be preserved in any of the ordinary pre- servative solutions, or may be dried and mounted in Canada balsam, (but I give the preference to glycerine, or glycerine jelly,) and they may be examined with the highest magni- fying powers. After having tried very many methods of making this preparation I have found the following one to succeed best. " Composition of the Prussian Blue Fluid for Making Transparent Injections : — ■ Glycerine 1 oz. Wood, naphtha, or pyroacetic spirit . . . 1| drachms. Spirits of wine 1 oz. Ferrocyanide of potassium 12 grs. Tincture of sesquichloride of iron . . . 1 drachm. Water 4 ozs. " The ferrocyanide of potassium is to be dissolved in one ounce of the water, and the tincture of sesquichloride of iron added to another ounce. These solutions should be mixed together very gradually, and well shaken in a bottle. INJECTING TISSUES. 239 The iron being added to the solution of the ferrocyanide of potassium. When thoroughly mixed, these solutions should produce a dark blue mixture, in which no precipitate or flocculi are observable. Next, the naphtha is to be mixed with the spirit, and the glycerine and the remaining two ounces of the water added. This colourless fluid is, lastly, to be slowly mixed with the Prussian blue, the whole being well shaken in a large bottle during the admixture. The tincture of sesquichloride of iron is recommended because it can always be obtained of uniform strength. It is generally called the muriated tincture of iron, and may always be purchased of druggists. " Permit me, then, most earnestly to recommend all who are fond of injecting, to employ transparent injections, and to endeavour, by trying various transparent colouring matters, to discover several which may be employed for the purpose ; for I feel sure that by the use of carefully prepared transparent injections, many new points in the anatomy of tissues will be made out. " Of Injecting Different Systems of Vessels with Different Colours. — It is often desirable to inject different systems of vessels distributed to a part with different colours, in order to ascertain the arrangement of each set of vessels and their relation to each other. A portion of the gall- bladder in which the veins have been injected with white lead, and the arteries with vermilion forms a beautiful preparation. Each artery, even to its smallest branches, is seen to be accompanied by two small veins, one lying on either side of it. " In this injection of the liver, four sets of tubes have been injected as follows : — The artery with vermilion, the portal vein with white lead, the duct with Prussian blue, and the hepatic vein with lake. There are many opaque colouring matters which may be employed for double injections, but I am acquainted with very few transparent ones, the employment of which affords very satisfactory results. " Mercurial Injections are not much used for micro- scopical purposes, although mercury was much employed formerly for injecting lymphatic vessels and the ducts of glandular organs. The pressure of the column of mercury 240 THE MICROSCOPE. supersedes the necessity of any other kind of force foi driving it into the vessels. The mercurial injecting apparatus consists of a glass tube, about half an inch in diameter and twelve inches in length, to one end of which has been fitted a steel screw to which a steel injecting pipe may be attached. The pipes and stopcocks must be made of steel, for otherwise they would be destroyed by the action of the mercury. " Injecting the Lower Animals. — The vessels of fishes are exceedingly tender, and require great caution in filling them. It is often difficult or quite impossible to tie the pipe in the vessel of a fish, and it will generally be found a much easier process to cut off the tail of the fish, and put the pipe into the divided vessel which lies imme- diately beneath the spinal column. In this simple manner beautiful injections of fish may be made. "Mollusca. — (Slug, snail, oyster, &c.) The tenuity of the vessels of the mollusca often renders it impossible to tie the pipe in the usual manner. The capillaries are, how- ever, usually very large, so that the injection runs very readily. In different parts of the bodies of these animals are numerous lacunae or spaces, which communicate directly with the vessels. Now, if an opening be made through the integument of the muscular foot of the animal, a pipe may be inserted, and thus the vessels may be injected from these lacunse with comparative facility. "Insects. — Injections of insects may be made by forcing the injection into the general abdominal cavity, when it passes into the dorsal vessel and is afterwards distributed to the system. The superfluous injection is then washed away, and such parts of the body as may be required, removed for examination. " Of the Practical Operation of Injecting. — I propose now to inject a frog and the eye of an ox, in order that you may see the several steps of the process. We must bear in mind that a successful injection cannot be made until the muscular rigidity which comes on shortly after death, and which affects the muscular fibres of the arteries as well as those of the muscles themselves, has passed off. In some few instances in which the fluid does not neces- sarily pass through arterial trunks before it reaches the INJECTING TISSUES. 2-11 capillaries (as in the liver), the injection may be effected satisfactorily immediately after the death of the animal. " The steps of the process are very similar in making the opaque injections, except that when size is employed, the specimen must be placed in warm water until warm through, otherwise the size will solidify in the smaller vessels, and the further flow of the injecting fluid will be prevented. Soaking for many hours is sometimes neces- sary for warming a large preparation thoroughly, and it is desirable to change the water frequently. The size must also be kept warm, strained immediately before use, and well stirred up each time the syringe is filled. " Iu the first place, the following instruments must be conveniently arranged : — " The syringe thoroughly clean and in working order, with pipes, stopcock, and corks. " One or two scalpels. " Two or three pair of sharp scissors. "Dissecting forceps. " Bull's-nose forceps, for stopping up any vessel through which the injection may escape accidentally. " Curved needle, threaded with silk or thread, the thick- ness of the latter depending upon the size of the vessel to be tied. " Wash-bottle. Injecting fluid in a small vessel. " I will commence with the frog. An incision is made through the skin, and the sternum divided in the middle line with a pair of strong scissors ; the two sides may easily be separated, and the heart is exposed. Next the sac in which the heart is contained (pericardium) is opened with scissors and the fleshy part of the heart seized with the forceps ; a small opening is made near its lower part, and a considerable quantity of blood escapes from the wound — this is washed away carefully by the wash-bottle. Into the opening — the tip of the heart being still held firmly by the forceps, a pipe is inserted and directed upwards towards the base of the heart to the point where the artery is seen to be connected with the muscular sub- stance. Before I insert the pipe, however, I draw up a little of the injecting fluid so as to fill it, for if this were not done, when I began to inject, the air contained in the 242 THE MICROSCOPE. pipe would necessarily be forced into the vessels, and the injection would fail. " The point of the pipe can with very little trouble be made to enter the artery. The needle with the thread is next carried round the vessel and the thread seized with forceps, the needle unthreaded and withdrawn, or one end of the thread may be held firmly, while the needle is with- drawn over it in the opposite direction. The thread is now tied over the vessel, so as to include the tip of the pipe only, for if the pipe be tied too far up, there is greater danger of its point passing through the delicate coats of the vessel. " The nozzle of the syringe, which has been well washed in warm water, is now plunged beneath the surface of the fluid, the piston moved up and down two or three times, so as to force out the air completely, and the syringe filled with fluid. It is then connected with the pipe, which is firmly held by the finger and thumb of the left hand, with a screwing movement, a little of the injection being first forced into the wide part of the pipe so as to prevent the possibility of any air being included. " The pipe and syringe being still held with the left hand, the piston is slowly and gently forced down with a slightly screwing movement with the right, care being taken not to distend the vessel so as to endanger rupture of its coats. The handle of the syringe is to be kept uppermost, and the syringe should never be completely emptied, in case of a little air remaining, which would thus be forced into the vessels. The injection is now observed running into the smaller vessels in different parts of the organism. " I will now proceed to inject the ox-eye in the same manner. The pipe is inserted into this branch of artery close to the nerve. Two minutes will probably be suffi- cient to ensure a complete injection. In making an injection of the eye, if the globe becomes very much distended by the entrance of the injecting fluid, an opening may be made in the cornea to allow the escape of the aqueous fluid which will leave room for the entrance of the injection, and permit the complete distension of the vessels. INJECTING TISSUES. 243 " We will now examine these injections. A portion of the intestine of the frog may be removed with scissors, opened, and the mucous surface washed with the aid of the wash-bottle. It may be allowed to soak in glycerine for a short time, and then examined. " This portion of the delicate choroidal membrane which has been carefully removed in the same manner shows the vessels perfectly injected, and in this preparation of the ciliary processes you will not fail to observe that all the capillary vessels are fully distended with fluid, although the injection was made so quickly. " Of Injecting the Ducts of Glands. — The modes of inject- ing which we have just considered, although applicable to the injection of vessels, are not adapted for injecting the ducts and glandular structure of glands ; for as these ducts usually contain a certain quantity of the secretion, and are always lined with epithelium, it follows that when we attempt to force fluid into the duct, the epithelium and secretion must be driven towards the secreting struc- ture of the gland, which is thus effectually plugged up with a colourless material, and there is no possibility of making out the arrangement of the parts. In such a case it is obviously useless to introduce an injecting fluid, for the greatest force which could be employed would be insuf- ficient to drive the contents through the basement mem- brane, and the only possible result of the attempt would be rupture of the thin walls of the secreting structure and extravasation of the contents. As I have before mentioned, partial success has been obtained by employing mercury, but the preparations thus made are not adapted for micro- scopical observation. " After death the minute ducts of the liver always contain a little bile. No force which can be employed is sufficient to force this bile through the basement membrane, for it will not permeate it in this direction. When any attempt is made to inject the ducts, the epithelium and mucus, in their interior, and the bile, form an insurmountable barrier to the onward course of the injection. Hence it was ob- viously necessary to remove the bile from the ducts before one could hope to make a successful injection. It occurred to me, that any accumulation of fluid in the smallest r2 244 THE MICROSCOPE. branches of the portal vein or in the capillaries, must ne- cessarily compress the ducts and the secreting structure of the liver which fill up the intervals between them. The result of such a pressure would be to drive the bile towards the large ducts and to promote its escape. Tepid water was, therefore, injected into the portal vein. The liver became greatly distended, and bile with much ductal epithelium flowed by drops from the divided extremity of the duct. The bile soon became thinner, owing to its dilution with water, which permeated the intervening mem- brane, and entered the ducts. These long, narrow, highly- tortuous channels were thus effectually washed out from the point where they commenced as tubes not more than 1 -300th of an inch in diameter, to their termination in the common duct, and much of the thick layer of epithelium lining their interior was washed out at the same time. The water was removed by placing the liver in cloths with sponges under pressure for twenty-four hours or longer. All the vessels and the duct were then perfectly empty and in a very favourable state for receiving injection. The duct was first injected with a coloured material. Freshly precipitated chromate of lead, white lead, vermilion, or other colouring matter may be used, but for many reasons to which I have alluded, the Prussian blue injection is the one best adapted for this purpose. It is the only material which furnishes good results when the injected prepara- tions are required to be submitted to high magnifying powers. Preparations injected in this manner should be examined as transparent objects." 1 Of Preparing Portions of Injected Preparations for Mi- croscopical Examination. — When thin tissues, such as the mucous membrane of the intestines or other parts, have been injected, it is necessary to lay them perfectly flat and wash the mucus and epithelium from the free sur- face, either by forcing a current of water from the wash- bottle, or by placing them in water and brushing the surface gently with a camel's hair brush. Pieces of a convenient size may then be removed and mounted in a solution of naphtha and creosote, In. dilute alcohol, in (1) Dr. L. Beale, "On the Anatomy of the Liver of Man and Vertebrate Animals." CHEMICAL RE- AGENTS. 245 glycerine, or in gelatine and glycerine. The most im- portant points in any snch injections are shown if the preparation be dried and mounted in Canada balsam. The specimen must, in the first place, be well washed and floated upon a glass slide with a considerable quantity of water, which must be allowed to flow off the slide very gradually. The specimen may then be allowed to dry under a glass shade, in order that it may be protected from dust. The drying should be effected at the ordinary temperature of the air, but it is much expedited if a shallow basin filled with sulphuric acid be placed with it under a bell-jar." Chemical Re-agents. — The following chemical re-agents and preservative fluids are recommended for microscopic uses:1 1. Alcohol, principally for the removal of air from sections of wood and other preparations ; also as a solvent for certain colouring matters. 2. jEther, chiefly as a solvent for resins, fatty and other essential oils, &c. ; also useful for the removal of air. 3. Solution of Caustic Potass, as a solvent for fatty matters; also of use occasionally in consequence of its action upon the rest of the cell-contents and thickening layers. This solution acts best upon being heated. 4. Solution of Iodine (iodine one grain, iodide of potas- sium three grains, distilled water one ounce) for the coloration of the cell-membrane and of the cell-contents. 5. Concentrated Sulphuric Acid, employed chiefly in the examination of pollen and spores. 6. Diluted Sulphuric Acid (three parts acid, one part water), for the coloration of cells previously immersed in the iodine solution. The preparation is first moistened with the iodine solution, which is afterwards removed with a hair pencil, and a drop of sulphuric acid added by means of a glass rod; the preparation is then immediately covered with a piece of glass. The action of the sulphuric acid and iodine, as well as that of the iodised chloride of zinc solution, is not always uniform throughout the whole (1) A set of 12 test-bottles, packed in a small box, is supplied by Mr. Matthews of Portugal Street, Lincoln's Inn Fields, the price of which is only a few shillings. 246 THE MICROSCOPE. surface of the preparation. The colour is more intense where the mixture is more concentrated; it frequently happens that many spots remain uncoloured. The colour changes after some time, the blue being frequently changed into red after twenty-four hours. 7. A Solution of Chloride of Zinc, Iodine, and Iodide of Potassium. A drop of this compound solution, added to a preparation placed in a little water, produces the same colour as iodine and sulphuric acid. This solution, which was first proposed and employed by Professor Schultz, is more convenient in its application than iodine and sulphuric acid, and performs nearly the same services, while it does not, like the sulphuric acid, destroy the tissues to which it is applied. It is prepared by dissolving zinc in hydro- chloric acid; the solution is then saturated with iodide of potassium : more iodine is to be added if necessary, and the solution diluted with water. 8. Nitric Acid, or what is better, chlorate of potass and nitric acid, as an agent to effect the isolation of cells. The mode of employing this agent, also discovered by Professor Schultz, is as follows : — The object, a thin section of wood, for instance, is intro- duced, with an equal bulk of chloride, or chlorate of potass, into a long and moderately wide tube, and as much nitric acid as will at least cover the whole. The tube is then warmed over a spirit-lamp; a copious evolution of gas takes place, upon which the tube is re- moved from the flame, and the action of the oxidising agent allowed to continue for two or three minutes. The con- tents of the tube are then poured into a watch glass with water, from which the slightly cohering particles are col- lected and placed in a tube, and again boiled in alcohol as long as any colour is communicated. They are again boiled in a little water. The cells may now be isolated under the simple microscope by means of needles. The boiling with nitric acid and chlorate of potass should never be carried on in the same room with the microscope, the glasses of which may suffer injury from the vapours. The same remark applies to all chemical vapours. Thin sections of vegetable tissue are warmed for half a CHEMICAL RE-AGENTS. 247 minute, or a minute, in a watch-glass: boiling is here un- necessary. The section is taken out, and treated with water in another watch-glass. 9. Oil of Lemons, or any other essential oil, a drop of which will be found of value in the investigation of pollen and spores. Lastly may be enumerated a pretty strong solution of Carbonate of Soda and also of Acetic Acid; which latter, however, is more especially useful in the investigation of animal tissues. To the above may be added a test for protein compounds. This test is composed of sugar and sulphuric acid, and is thus employed: — A thin section or portion of the tissue to be examined is placed in a drop of simple syrup, this is tfien removed by means of a hair pencil, and a drop of the diluted sulphuric acid added ; the red colour usually does not appear until after the lapse of about ten minutes. In making thin sections of tissues, it is recommended that, in those objects the consistence of which differs in different parts, the section should be carried from the harder into the softer portion; also, in making a thin section of a very minute yielding substance, to enclose it between two pieces of cork, and to slice the whole together. It is also useful sometimes to saturate the object with mucilage, which is to be allowed to dry slowly; in this way very delicate tissues may be sliced, or otherwise divided without injury, and with great facility. Some of the above re-agents must be used with caution, as it is not unusual for them to assume crystalline forms while under the microscope. Without a knowledge of this fact, and a perfect recognition of crystalline forms, errors in micro- chemical research must occur. For example, if a drop of liquor potassse be allowed to evaporate on a slip of glass, crystals appear, chiefly of six-sided tables, precisely like cystine ; when in quantity, they are often crowded together as the cystine plates are, and sometimes exhibit a similar nucleus-like body in their centres. This peculiarity of crystallization does not arise from the presence of impurities; perfectly pure potash often exhibits the same phenomenon. The form of the crystals of acetate of potash varies according to the strength of the acid out of which it 248 THE MICROSCOPE. crystallizes, and if formed out of strong acid, very much resembles that of the crystals of uric acid ; when mixed up with other forms, long dagger-like or lancet-shaped crystals are seen, which might well deceive. We may also notice in this place what Majendie pointed out, that in certain albuminous mixtures, iodine loses the property of colouring starch blue. This difficulty must be got rid of before iodine can be said to be an infallible test in micro-chemistry. Collecting Objects. — Mr. G. Shadbolt contributes the fol- lowing useful hints for collecting objects for microscopical examination : — " Rivers, brooks, springs, fountains, ponds, marshes, bogs,* and rocks by the sea-side, are all localities that may be expected to be productive; some being more prolific than others, and the species obtained differing, of course, in general, to a certain extent, according to the habitat. On considering the nature of some of the places indicated, it will be apparent that, in order to spend a day in col- lecting with any comfort, it will be necessary to make some provision for keeping the feet dry, for which a pair of India-rubber goloshes will answer, or better still, a pair of waterproof fishing-boots ; but without one or other the work is by no means pleasant. " A dozen or two of small bottles made of glass-tubing, about half an inch in diameter, and without necks, and from one to two inches in length, are the most convenient depositories for the specimens, if intended ultimately foi mounting; and it is advisable also to take two or three wide-mouthed bottles of a larger size, holding from one to two fluid-ounces, an old iron spoon, a tin box, some pieces of linen or calico, two or three inches square, a piece of string, a slip or two of glass, with the edges ground, such as are used for mounting objects; and lastly, a good and pretty powerful hand-magnifier. Two Coddington lenses, mounted in one frame of about half an inch, and one-tenth of an inch in focal power, are specially convenient. " Swanscombe Salt-marsh will be found well worth a visit; and it can be reached by steam-boat or railway from London-bridge to Northfleet. On quitting the rail- way station, make towards the almshouses on the top of COLLECTING OBJECTS. 249 the hill; and arriving at the road, turn to the left, descend the hill, and cross a sort of bridge over a somewhat insig- nificant stream. Continue along the main-road a little farther, to a point where it begins to ascend again, and diverges to the left towards the railway; here quit it, taking your course along an obscure road, nearly in a direct line with the main one ; passing a windmill on the right hand, and continuing until you arrive at another still more obscure road, turning off to the right; which road appears as if made of the mud dredged from the bottom of the river, and partially hardened. This is Swanscombe Salt-marsh; and the road just described leads towards Broad Ness Beacon. On either side is a sort of ditch ; one containing salt or very brackish wTater, the other filled with a sort of black mud, about the con- sistence of cream, the surface being in parts of a slaty grey, with little patches here and there of a most brilliant brown colour, glistening in the sunshine, and presenting a striking contrast to the sombre shade. By carefully in- sinuating the end of one of the slips of glass under this brilliant brown substance, and raising it gently, it can be examined with the Coddington; and it will probably be found to consist of myriads of specimens of Pleurosigma (navicula of Ehrenberg) angidatum, or balticum, or some other species of this genus. The iron spoon is now useful, as by its aid the brown stratum, with little or no mud, can be skimmed off and bottled for future examination. On the surface of the water in the other ditch may be noticed a floating mass of a dark olive colour, which to the touch feels not unlike a lump of the curd of milk, and consists of Cyclotella menighiniana, and a surirella or two em- bedded in a mass of Spirulina hutchinsia; and another mass of floating weed, which feels harsh to the touch, pro- ceeding from a quantity of a synedra, closely investing a filamentous alga; and elsewhere Melosira nummuloides {gallionella of Ehrenberg). " In a trench by the sea-wall, as it is termed, is a mass of brown matter of a shade somewhat different to any hitherto observed, adhering to some of the parts of the trench, being partially submerged, and having a some- what tremulous motion on agitating the water. This is a 250 THE MICROSCOPE. species of JSchizonema; and it consists of a quantity of gelatinous hollow filaments filled with an immense number of bright-brown shuttle-shaped bodies, like very minute naviculce. "It is not necessary to be particular about collecting the specimen free from mud, as the filaments are so tough that the mud can be readily washed away by shaking the whole violently in a bottle of water, and pouring off the mud, without at all injuring the specimen. The Amphi- porium alatum communicates a somewhat frothy appear- ance to the otherwise clear water, and to get any quantity of this requires a little management ; but by skimming the surface with the spoon, and using one of the larger bottles, an abundance may readily be obtained. Between the sea-wall and the river the marsh is intersected in every direction with a number of meandering creeks, being in some places eight to ten feet deep, though in others quite shallow; but it is exceedingly difficult to make one's way amongst them, and I have never found them so prolific any where, on the few occasions of my visiting the place, as in the parts more away from the influence of the tide. It will be observed, that the brilliant brown colour, of a deep but bright cinnamon tint, is one of the best indica- tions of the presence of diatomacece; and though this is by no means universal, the variation is most frequently de- pendent upon the presence of something which qualifies the tint. The peculiarity of the colour is due to the endochrome contained in the frustule ; and this must in general be got rid of before the beautiful and delicate marking can be made out. But it is highly advantageous and instructive to view them in a living state ; and this should be done as soon as possible after reaching home with all specimens procured from salt-water localities, as they rapidly putrefy in confinement, and emit a most disgusting odour, not unlike that arising from a box of inferior congreve-matches. " Washing in fresh water, and then immersing in creo- sote water, preserves many of the species in a very natural- looking manner; but they are killed by the fresh water, and the endochrome becomes much condensed in the Pleurosigmata and some other species. The addition of COLLECTING OBJECTS. 251 spirits quite spoils the appearance of the frustules, as it dissolves the endochrome. " There is another salt-marsh a little farther down the same railway, at Higham, which it would be well to explore. The most favourable months for procuring dia- tomacece, are April, May, September, and October; but some species are found in perfection as early as February, and many as late as November, and a few at all times of the year. There is a piece of boggy ground near Keston, beyond Bromley, in Kent, where the river Kavensbourne takes its rise, where many interesting species of desmi- dacece and other fresh- water algae may be procured. From Bromley, walk on towards Keston, passing near Hayes Common and Bromley Common on the right. Continue for about another half-mile along the road, and then turn to the right hand; pass the reservoirs, and approach an open space where there is a bog of about a quarter of a mile in extent ; and tending towards the right, make your way amongst heaths, ferns, mosses, and the beautiful Drosera rotundifolia (sun-dew), to the lower part of the little stream rippling through a sort of narrow trench in the Sphagnum, &c. By working your way up the stream, you avoid the inconvenience which would otherwise be experienced of the water being rendered turbid, in con- sequence of having to tread in the boggy ground. In the centre of the little stream may be observed something of a pale pea-green colour flickering about in the current, which, on your attempting to grasp, most likely eludes you, and slips through the fingers, from being of a gelatin- ous nature. It consists of a hyaline substance, with a comparatively small quantity of a bright green endo- chrome, disposed in little branches, and this is the Dra- parnaldia glomerata. Another object is a mass of green filaments, rather harsh to the touch, and veiy slippery. When viewed with a lens of moderate power, each filament is seen to be surrounded with several bands of green dots, looking like a ribbon twisted spirally, and may be recog- nised as a species of Zygnema. In various parts there are other Zygnemacece, as spirogyra, mougeotia, mesocarpus, and many more. " Keeping up the stream, and occasionally diver^in^ a 252 THE MICROSCOPE. little on either side of it, amongst the miniature bays and pools formed by the sphagnum, on looking straight down into the water we shall probably see at the bottom a little mass of jelly of a bright green, studded with numerous brilliant bubbles of oxygen-gas. This is the general appearance of most of the desmidacece, as Micrasterias, Euastrum, Closterium, Cosmarium, &c. The spoon is also a handy tool in this case, though, by practice, the finger will do nearly as well ; the ehief difficulty arises when the specimen is brought to the surface of the water, it not being easy to get it out without losing a considerable por- tion of it. Little pools in the bog, made by the footsteps of cattle, are particularly good spots to find desmidiece, many species being in a very contracted space. The most prolific bog is at Tunbridge Wells, near a house known as Fisher's Castle, not far from Hurst Wood. There is also a good one at Esher, at a spot called West-End. It must not be imagined that nothing can be obtained in this department of botany without going some distance from town ; but assuredly only commoner and fewer species can be met wTith nearer home. At the West India Docks are Synedra fasciculata, Gomphonema curvata, Diatoma elouga- tum, Diatoma vulgare, Surirella ovata, &c. ; and at this same place a few objects, not of the botanical class, as Spongilla jluviatilis, Cordylophora lacustris, Alcyonella, stagnorum, &c, are obtainable in abundance in the autumn. In the ornamental water in St. James's Park may be found Cocconema lanceolatum, and other sjjecies of this genus, Gomphonema cristatum, » Fossil wood (Exosen). „ (Endogen). Sections of coal. Simple cellular tissue. Stellate tissue. Fibro-cellular tissue. Spiral vessels. Hairs from leaf of Deutzia. Seeds of a fern. Sections of fir. ,, oak. ,, mahogany. ,, clematis. Petal of geranium. Leaf insect. Flea. Pirasite of peacock. Skin of caterpillar. Wing of a butterfly. Scales of ditto, t'roboscis of blow-fly. Stomach of ditto Foot of ditto. Spiracles of Dytiscus. Foot of Ophion. Proboscis of moth. Tran. sects, of human hairs. ,, hairs of elephant. ,, whalebone. Feather of bird. Trans, sects, of human bone. ,, bone of bird, i ,, h fish. „ „ reptile. Blood of bird. ,, fish. reptile, human. >> M Opaque : — Gold dust. Fossil shells. Pollens. Fern spores. Needle antimony. Avanturine. Polariscope : — Selenite. Starch. Hairs from leaf. Embryo oysters. Rhinoceros horn. Hoof of horse. Agate. Sandstone. Sulphate of Cadmium. Salicine. Tartaric acid. Carbonate of lime. Anatomical : — Section of cartilage, showing formation of the bone-cells. Muscle of mammalia. ,, reptile. ,, bird. ,, fish. It* Injections of: — Human lung. ,, intestine. skin, kidney, stomach, muscle, section of finger. 254 THE MICROSCOPE. Mr. Collins has introduced a very complete Mounting- Case, which will prove useful to microscopists, especially so to those who devote a good deal of attention to the preparation of specimens. A place is here found for everything : the little box contains : — Shadbolt's turn- table, brass table, spirit-lamp, pipettes, spring clips, wooden clips, tweezers, tin cells, balsam, marine glue, asphalte, turps, gold size, thin glass covers, glass slips, and five extra bottles. The price of this neat and convenient case is 305. Another box, more particularly adapted for anatomical purposes, includes a neat injecting apparatus. Fig. 143. — Collins' Mounting Cabi7iet. A modification of Dr. Beale's neutral-tint glass for drawing is made by Mr. Collins, who supplies three glasses of different shades, which drop into a dovetail, so that they can be more easily cleaned and efficiently adapted to the power used ; with his more complete instruments a prism is also supplied. - Fungi, Alg.e, Lichens, etc. •#1 t , s liiil'rii West dd 1 Kdmund Kv;m>. Plate I PAET II. TDK VEGETABLE KINGDOM— VITAL CHARACTERISTICS OF CELLS — THE PEO- TOCOCCUS PLUVIALIS — OSCILLATORIvE — FUNGI — ALG.E — DESMIBACEvE — MOSSES — FERNS— STRUCTURE OF PLANTS— STARCH— ADULTERATION OF ARTICLES USED FOR FOOD— PREPARATION OF VEGETABLE STRUCTURES, ETC. IXCE the introduction of the achro- ji matic microscope, we have obtained nearly the whole of the valuable information we possess of the mi- nute structure of plants. Indeed in no department of nature has microscopic investigation been more fertile of results than in that of the vegetable kingdom. The hum- §3 blest tribes of plants have had sAy? for microscopists an attraction, — jT unequalled by that of any other department of nature, — from the time of our countryman Eobert Brown, down to the present day. Although Brown had observed and recorded certain facts in the phy- siology of vegetable life, it was Professor Schleiden's labours that brought to light the great truth, " that the life-history of the individual cell is the first important and indispensable basis whereon to found a true physiology of the life-history of plants, as well as that of the higher orders of creation." The first problem 256 THE MICROSCOPE. which this observer set himself to solve, was, How is the cell originated ? It is not here desirable to go deeply into this most interesting inquiry ; the object proposed is to give a slight sketch of the formative processes of plant life, chiefly in its relation to the earliest, or cell condition. Almost every day brings forth a new discovery by which old land-marks established with a view to the separa- tion, arrangement, and classification of the vegetable and animal kingdoms become unsettled ; the lowest forms of life, vegetable and animal, approach so near to each other, that we cannot with certainty always discriminate them, and say where the one ends and the other begins. The boundary assigned to the vegetable kingdom is, perhaps, too limited, and our definition of a vegetable organism requires to be enlarged : of this any one who will be at the trouble to study the microscopic forms of life must feel perfectly convinced. At the present day, the only generally applicable rule that can be applied to distinguish, animals from vegetables is the dependence of the animal for nutriment upon organic compounds already formed, which it takes into the interior of its body; this at once distinguishes it from the plant, which only possesses the power of obtaining its alimentary matter, by absorp- tion, from the inorganic elements by which it is surrounded. Although there appear to be certain exceptions to this rule, yet it almost universally prevails. Kiitzing maintains, that every organic being is con- stituted of vegetable and animal elements, and according as the one or the other prevails, the being becomes an animal or a vegetable : in the first stages of the develop- ment of superior beings, and permanently in those of inferior rank, the two elements are equally balanced. " If nature," writes Humboldt, " had endowed us with micro- scopic powers of sight, and if the integuments of plants were transparent, the vegetable kingdom would by no means present that aspect of immobility and repose under which it appears to our senses." And so with regard to the instruments of motion in the higher classes of crea- tion, the muscles of animals very soon disappear as we descend in the scale to the simplest forms of life ; never- theless, we cannot deny animality to those minute crea- CHARACTERISTICS OF CELLS. 257 tures — as the Amoeba — in which we are quite unable to distinguish either muscles or any other distinct organs. Hence there is always some danger of believing that to be simple which in reality only seems to be so, and which the minuteness or transparency of organization may only conceal from our limited power of vision. Plants and animals, if seen at the earliest stage of existence, present themselves to our eyes as an aggregation of transparent cells. Everything prior to the appearance of the cell may, in the actual state of our microscopical knowledge, be considered as not fully and certainly demon- strated ; and therefore it is incumbent upon us to take our starting-point from the simple cell, which is the same, in re- spect to its principal characters, in animals and vegetables. The external coating of a cell is nearly or quite solid and transparent, and with no indication of structure; while in its interior is found a liquid or solid substance, with a nucleus either adhering to its wall or within its cavity. A nucleolus can sometimes be demonstrated within the nucleus ; and (a state common to all living cells) an in- cessant mutual interchange of materials is going on between the fluid contents and matter external to the cell, by a process termed osmose, or diffusion, which causes a per- petual variation in its relative condition. Chemical reagents give a manifestly different result in the animal and vege- table cell, hence we may conclude that there is an important difference in their chemical composition. The vegetable cell has an extremely fine delicate membrane lining the inner Avail, to perceive which we must have recourse to reagents, and then we find the apparently simple cell- wall made up of two layers, each differing in composition and properties. The inner layer has received the name of primordial utricle, and Hs composition has been shown to be albuminous ; agreeing in this respect with the form- ative substance of animal tissues. The external layer is regarded as the cell-wall, although it takes no part, essen- tially, in the formation of the cell ; it is composed of cellu- lin, a material allied to the cellulose of vegetable tissues. The contents are more or less coloured : the internal colouring substance is termed endochrome ; when green it is called chlorophyll. s 258 THE MICROSCOPE. The successive changes in the cell contents furnish other very important characteristics, such as the dis- appearance and re-absorption of the nucleus ; this occurs in every cell at some period of its existence ; in the cells of the higher plants, the inner membrane, or primordial utricle, entirely disappears. The Algae, and some few- unicellular plants, form an excej3tion to the rule. In the animal, the enlargement of the cell-wall takes place in a uniform manner, whereas in the plant this is effected by a deposition of successive layers on its inner surface, in the shape of continuous rings, spiral bands, or other inter- mediate forms. Then the wall not only increases in size, but appears to possess a power of separating and appro- priating certain substances, as lime, silica, lignine, &c, which form the so-called cuticle. In animals as well as in plants, new cells are formed within the old cells ; but in the former, this process of a new formation begins in the extracellular fluid, while in the latter it is mostly endogenous. Multiplication of vegetable cells is effected by three different modes : 1 st, Many nuclei appear in the maternal cell floating together with granular matter ; around each collects a minute vesicle, this gradually increasing fills the maternal cell, which is eventually absorbed. 2d, The internal substance of the cell divides into two or more portions, each being furnished with a nucleus. 3d, In the third mode of multiplication, the wall itself of the maternal cell becomes gradually con- stricted, and divides into two portions.* * "In most cells, especially when young, a minute, rounded, colourless body may be seen, either in the middle or on one side, called the nucleus. This is very distinct in a cell of the pulp of an apple : and within this nucleus is often to be seen another smaller body, frequently appearing as a mere dot, called the nucleolus. " The nucleus is imbedded in a soft substance, which fills up the entire cell ; this is the protoplasm fprotos, first, plasma, formative substance). As it is very transparent, it is readily overlooked ; but it may usually be shown distinctly by adding a little glycerine to the edge of the cover with a glass rod, when it contracts and separates from the cell-walls. The protoplasm in some cells is semi-solid, and of uniform consistence, while in others it is liquid in the centre, the outer portion being somewhat firmer, and immediately in contact with the cell-wall. In the latter case it forms an inner cell to the cell-wall, and is called the primordial utricle. The terms ' protoplasm ' and ' primordial utricle ' are however used by some authors synonymously. "The protoplasm is the essential portion of the cell, and it forms or secretes the cell-wall upon its outer surface in the process of formation of the cell, con- sidered as a whole. It is alao of different chemical composition, from the cell wall being allied in this respect to animal matter. "^Griffitlu. CELL-DEVELOPMENT. 259 Taking for our examination the more simple organisms among vegetables, we shall find numbers which present, in their earliest as well as in their permanent state, the cell in its simplest condition, and its reproduction a bare re- petition of the same thing. Unicellular plants, then, in the strictest sense, are represented only by those in which the whole cycle of life is completely shut up in the one cell ; the first reconstruction or division being at once the commencement of a new cycle, in which, consequently, the whole vegetative life is run through in the same cell where the prorogation also appears. ^**K / /■ Fig. 144.— Cell Development. (Protococcus pluvialis.) Protococcus pluvialis, Kiitzing. Hcematococcus pluvialis, Flotow. ChlamidO' encetM versatilis, A Braun. Chlamidococcws pluvialis. Flotow and Braun. a, division of a simple cell into two, each primordial vesicle having developed a cellulin envelope around itself; b, Zoospores, after their escape from the cells ; c, division of an encysted cell into segments ; d, division of another cell, with vibratile filaments projecting from cell- wall ; e, an encysted cell ; F, division of an encysted cell into four, with vibratile filaments projecting ; o, division of a young cell into two. The most widely distributed of these single-cell plants is the Palmoglcea macrococca, of Kiitzing, which spreads itself as a green slime over damp stones, walls, &c. If a small portion be scraped off and placed on a slip of glass, and examined with a half or quarter-inch power, it will be seen to consist of a number of ovoid cells, having a trans- parent structureless envelope, nearly filled by a granular matter of a greenish colour. ■ At certain periods this mass divides into two parts, and ultimately the cell becomes two. Sometimes the cells are united end to end, just as we see s 2 200 THE MICROSCOPE. them united in the actively-growing yeast plant ; but in this case the growth is accelerated, apparently, by cold and damp. Another plant belonging to the same species, the Protococcus phivialis, is found in every pool of water, the spores of which must be always floating in the air, since it appears after every shower of rain. Unicellular plants occur in the series of Fungi and Algoe, which have many and very varied correspondence in morphological respects. The unicellular Algae — that is to say, Algae, the contents of which, containing already organized particles, are inclosed in a single, semifluid envelope, and this again in a cell-membrane, often consisting of several layers of different kinds ; and many, moreover, possess the power of dividing into several secondary cells, for the most part equivalent to the primary cell. To this species of unicellular plant belongs Protococcus pluvialis. That this is the case is clearly seen in the still form of this plant, which is most distinctly characterised by its cell-membrane, a more or less thick though always colourless envelope. It never, however, secretes true thickening layers on the surface. Although this cell- membrane exhibits all the optical characters of one com- posed of cellulose, it is impossible to demonstrate the presence of that principle by means of iodine and sulphuric acid ; it is not coloured by those reagents even after the contents of the cell have been expressed. The contents vary much in consistence, colour, solid and fluid constituents ; the red and green portions of which appear to be of equal physiological importance. The green colour is removed by ether, on the evaporation of which solvent there remain green as well as colourless drops. Dilute sulphuric acid at first renders the colour paler; but its prolonged action produces a bright green hue, which gradually becomes more and more intense, and often almost a blue-green. Hydrochloric acid has a simi- lar effect ; a tinge of brown is produced by nitric acid. Carbonate of potash scarcely affects the green colour ; it is gradually but totally destroyed by caustic potash, the contents at the same time swelling and becoming transparent. The change of colour from green to red in Euglena UNICELLULAR PLANTS. 2G1 appears to be a process very nearly allied to that which takes place in Protococcus, if it be not identical with it. The red substance of Prot. pluvialis is not always of an oily aspect ; it only becomes so in more advanced age. And according to Cohn's researches, this oily material is much more generally distributed than has been supposed, among the lower Algae; occurring in many true brown spores, such as of (Edogonium, Spirogyra, Vaucheria, &c. When still or motile cells of Protococcus are brought in contact with a very weak solution of iodine, they become internally, in most parts, of an intense violet or blue colour. With respect to the solid constituents of the Protococcus cell contents, they may be distinguished into chlorophyll vesicles, colourless or green particles, amylaceous granules, and nucleus. The motile form of Protococcus consists, as it were, of two cells, one within the other, both of wdiich, however, differ essentially from the common vegetable cell : the external having a true cell-membrane and fluid con- tents ; the other, or internal one, with denser, muco-gela- tinous coloured contents, but without a true cell-wall. Cohn called the external transparent vesicle the " enve- loping" cell," and the internal coloured one the "primor- dial cell." The term "primordial sac, or utricle," can only be applied to its peripheral layer, and not to that together with the contents. The form of Protococcus (fig. 144) presents a perfect analogy between the primordial cell and the nucleus of the common plant-celL The filaments which proceed from the central mass to the peripheric cell-wall, are tubular, giving passage to the red molecules from the central mass. These filaments, however, which proceed from the outer wall of the primordial cell towards the inner surface of the enveloping cell, correspond morpho- logically to the so-termed mucous filaments by which the cytoblasts are commonly retained in the centre of their cells. That they also correspond chemically with these, is proved by the fact that they are rendered more distinct by iodine, and that they can be made to retract by means of reagents; and in fact they exhibit, in the course of development, peculiarities which characterise them as consisting of protoplasm. 262 THE MICROSCOPE. The existence of delicate threads passing from the central mass to the enveloping cells, and the appearance occasionally of little particles having molecular motion, serve to show that the contents of the enveloping cell are less of a gelatinous consistence, than of a fluid nature. And the continuity of the primordial cell- wall with the filaments proves it is surrounded only with a la}rer of protoplasm, and is not inclosed in a dense membrane of cellulose. The most distinctive characteristic of the primordial cell, and what appears to constitute its most essential importance in the life of the cell in general, but particularly in that of the zoospore, consists in its being the contractile element of the vegetable organism — that, is to say, that from an intrinsic activity it possesses the faculty of altering its figure, without any corresponding change in volume. The Protococcus pluvialis has true motile organs, namely, two long vibratile cilia arising from the primordial cell (fig. 144, b, a), which, passing through two openings in the enveloping cell, move about in the water. These organs, during the life of the cell, move so rapidly, that it is then difficult to perceive them ; they are only recognisable by the currents they produce in the water. But when the motion is slackened they are evident enough. They are also rendered very distinct by iodine. They are always placed upon the extreme point of the conical elongation, on the anterior end of the primordial cell, and in such a manner as to appear to be immediate continuations of its substance ; and as that process itself consists of protoplasm, it is evident that the cilia must be regarded also as com- posed of the same substance. They resemble, in some respects, the so-called proboscis of certain Infusoria, such as Euglena and Monads, and do not differ very materially from the non- vibratile, retractile filaments of Acineta and Actinophrys. It is only that portion of the vibratile filaments beyond the enveloping cell that exhibits any motion, the portion within the outer cell being always motionless, and in that part of their course the filaments appear to be surrounded with a sheath. This seems to be the case, not only from the greater thickness at that part, but also from the cir- UNICELLULAR PLANTS. 263 eumstance that when, passing from the cell form into the still condition, the cilia disappear, the V-shaped, or forked internal portions remain visible. And it is then, also, that the openings through the enveloping cell-wall become, for the first time, visible. Perhaps the most remarkable of all the numerous aspects presented by Protococcus pluvialis, is the form of naked zoospores named byFlotow Hcematococcus porphyro- cephalus. These are extraordinarily minute globules, con- sisting of a green, red, and colourless substance in unequal proportions. The colourless protoplasm in them, as in all primordial cells, constitutes the outermost delicate boun- dary ; the red substance is for the most part collected towards the anterior end in minute spherules ; the granular green substance occupies more the under part, while the middle is usually colourless. Propagation depends upon a division of the cell contents, particularly of the colourless or coloured proto- plasm, or of the primordial sac. This body, without any demonstrable influence of a nucleus, is capable of sub- dividing into a determinate number of portions. Each of these acquires a globular figure, and in the next place surrounds itself with an envelope of protoplasm, and then represents a visible organism, which after the reabsorption of the parent cell-membrane, is capable of existence as an independent reproductive individual. Besides these, which are the most usual modes of propagation — viz. that of the sfo'W-cells into two, and of the motile into four, secondary cells — there are a number of others which may be con- sidered as irregular, and in which forms are produced which do not re-enter the usual cycle until they have gone through a series of generations. Sometimes, under certain circumstances, the cell-contents of the still form separate into eight or more portions, which become naked zoospores of small size (fig. 144 B.) It is not quite clear what becomes of this form of motile zoospores, but there seems reason for believing that they occasionally develop an enveloping cyst, and thus become encysted zoospores, and at other times secrete a cellulin tissue, and become still-ceWs ; but most of them probably perish without any further change. They would thus correspond .-> 61: THE MICROSCOPE. ■with the smaller motile spores observed by Thuret and A. Braun in other Algae (the Tucoid, &c), associated with the larger germinating spores, themselves deprived of the germinative faculty. It appears that both longitudinal and transverse division of the primordial cell may take place ; but that the vibratile cilia of the parent cell retain almost to the last moment their function and their motion after the primor- dial cell inclosed by it has long been detached as a whole, and become transformed into the independent secondary ceils (fig. 144, g). The most striking of the vital phenomena presented by this organism is that of periodicity. Certain forms — for instance, encysted zoospores, of a certain colour, appear in a given infusion, at first exclusively, then they gradually diminish, become more and more rare, and finally dis- appear altogether. After some time their number again increases, and reaches as before to an incredible extent ; and this proceeding may be repeated several times. Thus, a glass which at one time presented only still forms, contained at another nothing but motile ones. The same thing may be observed with respect to segmen- tation. If a number of motile cells be transferred from a larger glass into a small vessel, it will be found, after the lapse of a few hours, that most of them have subsided to the bottom, and in the course of the day they will all be observed to be on the point of subdivision. On the following morning the divisional generation will have become free ; on the next, the bottom of the vessel will be found covered with a new generation of self- dividing cells, which again proceed to the formation of a new generation, and so on. This regularity, however, is not always observed. The influence of every change in the external conditions of life upon the propagation is very remarkable. It is only necessary to pour water from a smaller into a larger and shallower vessel, or one of a different kind, to at once induce the commencement of segmentation in numerous cells. The same thing occurs in other Algae ; thus the Vaucheria almost always develop zoospores, at whatever time of year they may be brought from their natural habitat into a room. Light is conducive FRESH-WATER ALGiE. 263 to the manifestation of vital action in the motile zoospores, and they always seek it, collecting themselves at the surface of the water, and at the edges of the vessel. But in the act of propagation, on the contrary, and when about to pass into the still condition, the motile Pro- tococcus cell seems to shun the light ; at all events it then seeks the bottom of the vessel, or that part of the drop of water in which it may be placed, furthest from the light. Too strong sunlight, as when it is concentrated by a lens, at once kills the zoospores. A temperature of undue elevation is injurious to the development of the more vigorous vital activity, that is to say, for the forma- tion of the zoospores ; whilst a more moderate warmth, particularly that of the vernal sun, is singularly favour- able to it. Frost destroys the motile, but not the still zoospores.* Stephanosphcera pluvialis is another variety of fresh- water algae, first observed by Cohn. It consists of a hyaline globe, containing eight green primordial cells, arranged in a circle (see Plate 1, No. 24 d). The globe rotates, somewhat in the same manner as the volvox, by the aid of projecting cilise, two of which are seen to proceed from each cell and pierce the transparent envelope. Every cell divides first into two, then four, and lastly eight young cells, each of which divides into a great number of microgonidia, and are seen to have a motion within the globe, and ultimately escape from it, Under certain cir- cumstances each of the eight cells is observed to move about in the interior of the mother-cell ; eventually they escape, lose their cilia, form a thicker membrane as at 6, for a time become motionless, and sink to the bottom of the vessel. If the vessel be permitted to become thoroughly dry, and again water is poured into it, motile Stephano- sphaera reappear : from which circumstances it is probable that the green globes are the resting spores of the plant. "When in its condition of greatest activity its division into eight is perfected during the night, and early in the morning the young family escapes from the cell, soon to pass through similar changes. It is calculated that in • On the "Natural History of Protococcu? pluvial is," by F. Cohn, translated by G. Busk, F.R.S. for the Ray Society 266 THE MICROSCOPE. eight days, under favourable circumstances, 16,777,216 families may be formed from one resting-cell of Stephano- spheera. In certain of the cells, and at particular periods, the remarkable amaboid bodies ('Plate 1, No. 24 c), have been noticed. There is a marked difference between Stephanosphsera and Chlamydococcus, " for, while in the latter the individual portions of a primordial cell separate entirely from one another, each developing its own enve- loping membrane, and ultimately escaping as a unicellular individual ; in the former, on the other hand, the eight portions remain for a time united as a family." * The simplest forms of vegetable life are met with in the Confervoids, which are as interesting as they are in- structive to the microscopist. The confervas consist of unbranched filaments composed of cylindrical cells, placed end to end ; their reproductive process is carried on by zoospores produced from the cell contents. The fresh- water genera are principally of a yellowish green colour ; sometimes presenting a striated appearance, which has given rise to a supposition that their filaments are spiral. They are indeed plentifully distributed both in fresh and salt-water. Oscillator iacece. — The study of the structure of the Oscil- latorice is particularly interesting, from the fact that we may not unreasonably expect to find in it a key to the singular motion from which they received their generic name, and which now, for more than a century, has formed an object of curiosity and interest to the micro- scopist, without having received, as yet, a satisfactory explanation. The following different tissues are observable in the true Oscillatorice : — 1, An outer inclosing sheath ; 2, A special cell-membrane, with its contents ; and 3, The axis, or pith, of the filament. " The filaments of certain species are inclosed in sheaths or continuous tubes, never showing any cross-markings corresponding to the stria?, of the filament ; they are clearly composed of a kind of cellulose, although they remain unaffected by iodine- In other species, these tubes are * See an interesting paper by F. Currey, F.R.S. Journal of Microscopic?1 Science, vol. vi. 1858, p. 131 : also by Mr. Win. Archer, vol. v. 1865, p. 116 OONFERVOID ALGiE. 267 absent, or have not yet been observed ; when present, they will be found projecting on one or both sides of Fig. 145. — Conferva. 1, Volvox gldbator. 2, A section of volvox, showing the ciliated margin of the cell 3, A portion more highly magnified, to show the young volvocina, with their nuclei and thread-like attachments. 4, Spirogyra, near which are spores in different stages of development. 5, Conferva floccosa. 6, Stigeoclonium protensum, jointed filaments and single zoospores. 7, Staurocarpus gracilis, conjugating filaments and spores. the filament, being somewhat longer than the latter. Filaments inclosed in sheaths never, or but slightly, ex- hibit their peculiar motion, although they may be seen sliding in them, backwards and forwards, or leaving them altogether. The filaments themselves have been supposed to consist wholly of protoplasm ; this view, however, is scarcely correct, since the protoplasm is enclosed in a cell-mem- brane. The cellulin coat always shows cross-markings corresponding to the striae when such are observable 2G5 THE MICROSCOPE. in the filament, and which divide it into distinct joints or cells. The presence of this cell-membrane may be best de- monstrated by breaking up the filaments, either by moving the thin glass cover, or by cutting through a mass of them in all directions with a fine dissecting knife. On now examining the slide, in most in- stances many detached empty pieces of this cell-membrane, with f its striae, will be found, as well ~;$y as filaments partly deprived of the protoplasm, showing in those places the empty, striated cel- lulin coat. On the application of iodine all these appearances become unmistakably evident ; the greater portions of the fila- ment turning brown or red, while the empty cells, with their striae, remain either unaffected, or at Fig. 146. —Mesoglia vermicular is, UlOSt present a slight yellowish Zr^gfndtoVnd.SOf CdlS C°' tint> as is frequently the case with cellulose when old. With regard to the contents of the cell, the protoplasm (or endochrome) is coloured in the Oscillator ice, and is de- posited within it in the form of circular bands or rings around the axis of the cylindrical filament ; iodine turns them brown or red, and syrup and dilute sulphuric acid produce a beautiful rose colour. As to their mode of pro- pagation, nothing positive is known. If kept for some time they gradually lose their green colour — those exposed to the sun, much sooner than others less exposed ; the stratum eventually becoming brown, sinks to the bottom of the vessel, and presents a granular layer, embodying great numbers of filaments in all stages of decay.* The movements ol the Oscillatorice are indeed very sin- gular, so much so that it is in vain to attempt to explain them as altogether dependent on physical causes, and equally so to show that they are due to a sarcode or animal * Dr. F. d'Alquen, "On the Structure of the OscillatorisE," Journal oj Microscopical Science, vol. iv. p. 245. 1S56. MARINE AXGiE. 269 membrane. Their motion is not less lively than that of the Bacteria?, which Dujardinand Ehrenberg placed among infusional animalcules. To observe the movements of the hlaments, the very uppermost surface ought to be brought into focus, leaving the margins rather undefined, bearing in mind that the filament is not a Hat but a cylindrical body. Certainly, with regard to its movement, or the mechanism by which it is effected, nothing positive is known. The Bacillaria paradoxa is by far the most interesting specimen of the genus ; the movements of which are very remarkable, and so little understood that it is rightly called paradoxical. The Marine Confervoid Algse present a general appear- ance which might at first sight be mistaken for plants very much higher in the scale of organization. In the Ulvacese, the frond has no longer the form of a filament, but assumes that of a membranous expansion of the cell. These cells, in which zoospores are found, have an in. creased quantity of green protoplasm accumulated towards one point of the cell-wall ; and the zoospores are ob- served to converge with their apices towards the same point. In some genera, which seem to be closely re- lated in form and structure to the Bryopsideos, we notice this important difference, that the zoospores are de- veloped in an organ specially destined to this purpose, which presents pecu- liarities of form, distinguishing it from every other part of the branching tubular frond. In the genus Derbesia, distinct spore cases are seen, a youDg branch of which, when destined to be- come a sporecase, instead of elongating indefinitely, begins, after having arrived at a certain length, to swell out into an ovoid -vesicle, in the cavity of which a rapid accumu- lation of protoplasma takes place. This is then separated from the rest of the plant, and becomes an opaque mass, surrounded by a distinct membrane. After a time a Fig, 147. — Sphacelaria cirrhosa, with spores borne at the sides of the branchlets. 270 THE MICROSCOPE. division of the mass takes place, and a number of pyriform zoospores, each of which is furnished with a crown of cilia?, are set free. In many families of the olive-coloured Algae, reproduc- tion by zoospores is the general rule ; they differ, however, in the arrangement of their cilia. These organs, which are always two in number, are usually of unequal length, and emanate not from the beak, but from a reddish- coloured point in its neighbourhood. The shortest is directed backwards, and seems to serve during the motion of the spoje as a rudder. The longest, directed forwards, is closely applied to the colourless beak. Ectocarpus is one of the simplest forms of olive-coloured Algae, consisting of bran clung filaments, the extremity of any of which is liable to become converted into a sporangium, by the ab- sorption of the septa of the terminal cells. The zoospores are arranged in regular horizontal layers. In many genera a peculiarity exists, the signification of which is not yet completely understood — namely, that of a double fructifi- cation. The ovoidal sporangia contain numerous zoospores. In the genus Cutleria (fig. 1 50), there is seen another feature of interest : the appearance of two kinds of organs, which seem to be opposed to each other as regards their repro- ductive functions. The sporangia not only differ from those of other genera, but the frond consists of olive- coloured irregularly-divided flabelli, on each side of which tufts (sori) consisting of the reproductive organs, inter- mixed with hair-like bodies, are scattered. The zoospores are divided by transverse partitions into four cavities, each of which is again bisected by a longitudinal median septum. When first thrown off they are in appearance so much like the spores of Puccinia, that they may be mis- taken for them ; they are, however, about three times larger than those of the other olive-coloured algae. The fruit of most olive-green Sea-weeds is enclosed in spherical cavities under the epidermis of the frond, termed conceptacles, and may be either male or female. The zoids are bottle-shaped, each possessing a pair of cilia ; the transparent vesicle in which they are contained is itself inclosed in a second of similar form, and we have no certain evidence of the function performed by the MARINE ALG.E. 271 antheridia. In monoecious and dioecious Fuci, the female conceptacles are distinguished from the male by their olive colour. The spores are developed in each in the interior of a perispore, which is borne on a pedicle emanating from the inner wall of the conceptacle. They rupture the perispore at the apex ; at first the spore appears simple, but soon after a series of changes tako place, consisting in a splitting of the endo- chronie into six or eight masses, which become spheroidal sporules. A budding-out occurs in a few hours' time, and ulti- mately elongates into a cylindrical tube. The Vaucherice present a dou- ble mode of reproduction, and their fronds consist of branched tubes, much resembling in general character that of the Bryopsidece, from which indeed they differ only in respect of the arrangement of their contents, chlorophyll. In that most remarkable plant Saprolegnia ferox, which is structually so closely allied to Vaucherice, though separated from them by the absence of green colouring matter, we find a correspond- ing analogy in the processes of its development. In the process of the formation of its zoospores, we have an intermediate step between that of the Algae and a class of plants usually placed among Fungi. Cohn has shown us that Pilobolus is structually more closely allied to the former class than to that of the latter. Pilobolus has a somewhat remarkable ephemeral existence ; the spore germinates about mid-day, the plant grows till evening, re-opens during the night, and in the morning the spore-case bursts and the whole disappears, leaving behind scarcely a trace of its former existence. Red Sea-weeds, Floridece, present great varieties of struc- ture, although comparatively little is known of their re- Fig. 14S.— Development of Vivas. A, isolated cells of spores, b and c, clus- tering of the same, d, cells in the fila- mentous stage. 272 THE MICROSCOPE. productive processes ; it will, however, be sufficient for our purpose to notice the three leading forms. The first form, to which the term polyspore has been applied, is that of a gelatinous or membranous pericarp or conceptacle, in which an indefinite number of sporidia are contained. This organ may be either at the summit or base of a branch, or it may be concealed in or below the cortical layer of the stem. In some cases a number of sporidium- bearins: filaments emanate from a kind of membrane at the base of a spheroidal cellular perisporangium, by the rupture of which the sporidia formed from the endochrome of the filaments make their escape. Other changes have been observed ; however, they all agree in one particular, namely, — that the sporidium is developed in the interior of a cell, the wall of which forms its perispore, and the internal protoplasmic membrane en- dochrome, the sporidium itself, for the escape of which the perispore rup- tures at its apex. The second form is more simple, and consists of a globular or ovoid cell, containing a central granular mass, which ultimately divides into four quadrate-shaped spores, which when at maturity escape by rupture of the cell- wall. This organ, called a tetraspore, takes its origin in the cortical layer. The tetraspores are arranged either in an isolated manner along the branches, or in numbers to- gether ; in some instances the branches which contain them are so modified in form that they look like special or- gans, and have been called stichidia ; as, for example, in Dasya (fig. 149). Of the third kind of reproductive or- gan a difference of opinion exists as to the signification of their antheridia; although always produced in precisely the same situations as the tetraspores and polyspores, they are " agglomerations of little colourless cells, either Fig. 149. — Dasya Kutz in g- iana, with seed vessel and two rows of tetra- spores. Magnified 50 diameters. MARINE ALG.E. 273 united in a bunch as in Griffithsia, or enclosed in a trans- parent cylinder, as in Polysiphonia, or covering a kind of ex- panded disc of peculiar form, as in Laurencia." According to competent observers, these cellules contain spermatozoids. Nageli describes the spermatozoid as a spiral fibre, which, as it escapes, lengthens itself in the form of a screw. Thuret does not coincide in this view ; on the contrary, he says that the contents are granular, and offer no trace of a spiral filament, but are expelled from the cells by a slow motion. The antheridia appear in their most simple form in Callithamnion, being reduced to a mass of cells com- posed of numerous little bunches which are sessile on the bifurcations of the terminal branches. Are not these spiral filaments closely allied to Oscillatoriacece 1 The spores are simpler structures than the tetraspores, and mostly occupy a more important jDOsi- tion. They are not scat- tered through the frond, but grouped in definite masses, and generally enclosed in a special capsule or conceptacle, which may be mistaken for a tetraspore case. The simplest form of the spore fruit consists of spherical masses of spores attached to the wall of the frond, or imbedded in its sub- stance, without a prope" conceptacle ; such a fruit is called a favellidium, and occurs in Haly- raenia ; the same name is applied to the fruits of similar structures not perfectly immersed,, as those of Gigartina, Gelidium, &c, where they form tuber- cular swellings on the lobes. In some, the tubercles pre- sent a pore at the summit, through which the spores find Fig. 150. — Cutleria dichotoma. Section of a lucinia of a frond, showing the stalked eight chambered oosporanges growing on tufts with intercalated hairs Magnified 50 dia- meters. 274 THE MICROSCOPE. exit ; when such a fruit is wholly external, as in Cera- mium (see Plate II. Nos. 27 and 37) and Callithamnion, it is called a favella. The characteristic of Delesseria, 'No. 39, the coccidium, either occurs on lateral "branches, or is sessile on the face of the frond, and consists of a case of angular spores attached to a central wall. The cera- niidium is the most complete form of the conceptacular fruit : this is enclosed in an ovate case, with an apical spore, containing a tuft of pear-shaped spores arising from the base of the cavity. The general external appearance of the Eed Sea-weeds is very varied. They are exquisite objects for the Micro- scope ; we have figured several interesting varieties in Plate II. , each showing peculiarities of fructification. Their beautiful leaf-like fronds are either simple, lobed, or curiously pinnate or feathered. The Floridese of warmer climates exhibit most elegantly formed reticulated fronds, as may be seen on reference to the late Dr. Harvey's last great work, " Phycologia Australica." In the plant which results from the germination of the aggregate zoospores of Vaucheria, a genus of Siphonaceae (Plate I. fig. 23), Kaisten has observed that on those filaments which come in contact with the atmosphere, are formed organs of a peculiar structure, which have the appearance of nipple or egg-shaped buddings-out of the cell-wall, distributed in pairs along the whole course of the older filaments ; one elongates and curves round to meet its fellow, which is seen to swell out into a globular form ; finally conjugation takes place, preceded, however, by the conversion of the green con- tents of the tubular organ into oil globules. If the fila- ments be gathered at a favourable period, and cultivated in a vessel of water well exposed to the light, the blind ends, or ramifications of the filaments, are found densely filled with green contents, appearing to be almost black ; if these ends be watched early in the morning, a remark- able series of changes is seen to occur in them when about to produce gonidia, and, ultimately, they escape in a peculiar way from the filament. The admirable essays of linger, Nageli, and Pringsheim on the process of their :• j reduction might be consulted with advantage. Desmidiace^e, Diatom acf.k, Alu.e. "c? IS ^ Tiiffen \\ ' I-Mmuiid Kvaus. i'LAH. il TOLVOX GLOBATOR* 275 A fresh-water alga of singular beauty and interest to the microscopist is the Volvox globator. This little cell so well known to the older observers as the globe- animalcule, or revolving-cell, is represented in fig. 145, Nos. 1, 2, 3, and Plate I. No. 15. These revolving globular bodies were for a very long time classed with the lower forms of animal life, and there remained for the micro- chemical investigator of the present time to settle the per- plexing question, and assign to them a place among plants. .Leeuwenhoek first perceived the motion of what he termed globes, "not more than the 30th of an inch in diameter, rolling through water ; and judged them to be animated." These globes are studded with innumerable minute green spots, each of which is seen to be a perfect cell, about the 3,500th part of an inch in size, with a nucleus and two active cilia attached. The whole are bound together by threads forming a beautiful net-work. "Within the globe busy active nature is at work carefully providing a continu- ance of the species ; and from six to twenty little bright- green spheres have been found enclosed in the larger trans- parent case. As each little cell arrives at maturity, the parent cell enlarges, and l.roivox, just before the young burst ultimately bursts asunder, forth, showing the vesicle which en- launching iorth its offspring closes each. 2, Parent cell of Clos- i_ • j i • ttrxum. 3, Docidium ciacutum. i, to seek an independent exis- Staurastrum gracilis. tenca Both 0jdep ^ younger spheres possess openings through which the water freely flows, affording food and air to the wonderfully constructed little being. Jjr. Carpenter believes, "The Volvocinece, whose vegetable nature has been made known to us by observation of cer- t 2 Fig. 151. 276 -THE MICROSCOPE. tain stages in the history of their lives, are but the motile forms {Zoospores) of some other plants, whose relation to them is at present unknown." Professor Williamson, having carefully examined the Volvox globator, says : — " That the increase of its internal cells is carried on in a manner precisely analogous to that of the algse; that between the outer integument and the primordial cell- wall of each cell, a hyaline membrane is secreted, causing the outer integument to expand; and as the primordial cell- wall is attached to it at various points, it causes the inter- nal colouring- matter, or endochrome, to assume a stellate form (see Plate I. No. 15), the points of one cell being in contact with those of the neighbouring cell, these points forming at a subsequent period the lines of communication between the green spots generally seen within the full- grown Volvox." Cilia can be distinctly seen on the outer edge of the adult Volvox ; by compressing and rupturing one, they may even be counted. Professor Busk has been able to satisfy himself, by the addition of the chemical test iodine, of the presence of a very minute quantity of starch in the interior of the Volvox, which he considers as conclusive of their vegetable character. A singular provision is made in the structure of the gemmules, con- sisting of a slender elastic filament, by which each is at- tached to the parent cell- wall : at times it appears to thrust itself out, as if in search of food ; it is then seen quickly to recover its former nestling-place by contracting the tether. It is impossible not to recognize the great similarity between the structure of Volvox, and that of the motile cell of Protococcus pluvialis. The influence of re-agents will sometimes cause the connecting processes of the young cells as in Protococcus, to be drawn back into the central mass, and the connecting threads are sometimes seen as double lines, which seem like tubular prolongations of a consistent membrane. At other times they appear to be con- nected by star-like prolongations to the parent cell, Plate I. No. 15, presenting an almost identical appearance with Pediastrum pertusum. Mr. Busk says that the body designated by Ehrenberg Sp/icerosira volvox is an ordinary volvox in a different phase of development; its only marked feature of dissimilarity being that a large proper- VOLVOCINE.E. • 277 fcion of the green cells, instead of being single, are very commonly double or quadruple ; and the groups of ciliated cells thus produced, form by their aggregation discoid bodies, each furnished with a single cilium. These clusters separate themselves from the primary sphere, and swim forth freely under the forms which have been designated Uvella and Syncrypta by Ehrenberg. According to Mr. Carter, however, Splwerosira is the male or spermatic form of Volvox globator. Dr. Braxton Hicks believes that he has seen the young volvox pass into an amoeboid state ; he observes : — "Towards the end of autumn the endochrome mass of the volvox increases to nearly double its ordinary size, but instead of undergoing the usual subdivision, so as to produce a mac?'o-gonidiuin, it loses its colour and regularity of form, and becomes an irregular mass of colourless protoplasum, containing a number of brownish granules." (Plate I. "So. 16.) The final change and ultimate destination of these curious amoeboid bodies have not as yet been made out ; but from Dr. Hick's previous observation, made on similar bodies developed from the protoplasmic contents of the cells of the roots of mosses, " which in the course of two hours become changed into ciliated bodies," he thinks it very probable that this is designedly the way in which these fragile structures are enabled to retain life, and to resist all the varied external conditions, such as damp, dryness, and rapid alternations of heat and cold.1 (1) We have had volvox under the microscope for several months, towards the end of summer and throughout the autumn, and made more than a hundred examinations, without having once seen the remarkable change described by Dr. Hicks in the Quarterly Jour. Micros. Science, vol. viii. p. 96, 1862. Never- theless, as Mr. Archer observes: — "If this reasoning be correct, then contrac- tility, amoeboid contractility — for I can find no more comprehensive and expressive single adjective — must be accepted as an inherent quality or characteristic, occasionally more or less vividly evinced, of the vegetable cell-contents, and this in common with the animal ; in other words, that the nature of the proto- plasm in each is similar, as has indeed, as is well known, been urged before on grounds not so strong; thus reserving Siebold's doctrine, that this very con- tractility formed the strongest distinction between animals and plants, as he assumed it to be present in the former and absent in the latter of the two kingdoms of the organic world. Therefore, an organism whose known structural affinities, and whose mode of growth and of ultimate fructification point it out as truly a plant, but of which, however, certain cells may for a time assume a contractile, even a locomotive, quasi-rhizopodous state, must not by any means on this latter account alone be assumed as even temporarily belonging to the animal kingdom, or as tending towards a mutation of its vegetable nature. And from this it of course follows that an organism whose structural affinities and reproduction are unknown, but which may possibly present an actively 278 THE MICROSCOPE. Desmidiacece. — A remarkably beautiful family of confer- void algae, the most distinctive characteristics of the species being their bilateral symmetry. Each frustule is, however, a perfect unicellular plant, with a homogeneous structureless membrane, enclosing a cellular skeleton rilled with chloro- phyll. Four modes of reproduction have been observed in the desmids, and many points still remain to be cleared up. Braun remarks of the products of conjugation, "that they do not pass, like the swarming- cells of the Palmellacece and the reproductive cells of the Diatoinaceae, directly and by uninterrupted growth into the primary generation of the new vegetative series, but persist for a long time in a condition of rest, during which, excepting as regards im- perceptible internal processes, they remain wholly un- changed. To distinguish these from the germ-cell (gonidia) I shall call them seed-cells (spores). Certain early condi- tions observed in Closterium and Euastrum, namely, families of unusually small individuals, enclosed in transparent, colourless vesicles, render it even probable that in certain genera of this family a number of individuals are produced from one spore, by a formation of transitory generations occurring already within the spore." 1 contractile, even locomotive power, need not on this latter account be assumed as therefore necessarily an animal. In the former category fall the Volvocinacese and Rhisidium ; in the latter category Euglena and its allies, the so-called Astasiajan Infusoria, suggest themselves ; and these must of course wait until their reproduction and history are better known before we can feel satisfied as to their true position ; yet it seems highly probable that these will presently, if they do not even now, take their place amongst admitted plants. " Several writers have, indeed, from time to time, put forward th& (now, I think, generally accepted) view that the protoplasm of the vegatable and the sarcode of the animal cell are identical in nature ; and, in seeking for analogies as regards contractility in the vegetable protoplasm as compared with the animal, and as demonstrative thereof, special attention has been directed to several of the now familiar phenomena displayed by certain vegetable cells. Such are the vibrator}- movements of cilise, and drawing in of these, the circula- tory movements of the cell contents, as in the hairs of theTradescantia,