BIOL06Y LIBRARY -.- POLAEISCOPE ()J5JK( IS. 158 TuffenWeci.del Pi M K VITI. Edmund Kvans. THE MICROSCOPE: ITS HISTORY, CONSTRUCTION, AND APPLICATION: A FAMILIAR INTRODUCTION TO THE USE OF THE INSTRU- MENT, AND THE STUDY OF MICROSCOPICAL SCIENCE. BY JABEZ HOGG, M.R.C.S., F.R.M.S, V CONSULTING SURGEON TO THE ROYAL WESTMINSTER OPHTHALMIC HOSPITAL ; LATE PRESIDENT OF THE MEDICAL MICROSCOPICAL SOCIETY; LL.D. ; HONORARY FELLOW OF THE ACADEMY OF SCIENCES, PHILADELPHIA ; OF THE MEDICO-LEGAL SOCIETY, NEW YORK J OF THE BELGIAN MICRO- SCOPICAL SOCIETY, ETC. J AUTHOR OF " ELEMENTS OF NATURAL PHILO- SOPHY," "A MANUAL OF OPHTHALMOSCOPIC SURGERY," ETC. WITH UPWARDS OF FIVE HUNDRED ENGRAVINGS, AND COLOURED ILLUSTRATIONS BY TUFFEN WEST. Cbifion, LONDON: GEORGE ROUTLEDGE AND SONS, BROADWAY, LUDGATE HILL. NEW YORK: 9, LAFAYETTE PLACE. 1886. BIOLOGY LIBRARY PKEFACE TO THE TENTH EDITION. WENTY- EIGHT TEARS have passed away since the first edition of this book on the microscope was published. It was hailed as the pioneer of a more useful form of literature than had heretofore appeared on the history, construction and appli- cation of the instrument. In this period of time, nine large editions, exceeding in the aggregate 60,000 copies, have been sold out, and yet another edition is called for a proof, if it were wanting, that the book has not only maintained its ground, but its popularity. In issuing a tenth edition, the Author, by a careful revision of the text, and by an addition of no fewer than fifty new woodcuts, trusts that his book has been made more acceptable and useful to those for whom it was originally intended. Part I. has been almost wholly rewritten and rearranged. To Mr. Frank Crisp the Author is indebted for Professor Abbe's theory of microscopic vision. To the processes of hardening, section-cutting and staining much in vogue> and which have created a new era in physiological studies, have been assigned the space they seem to merit. Part II. has been carefully revised ; a chapter added on the application of the microscope to minera- logical and spectroscopical analysis, and the examina- tion of potable water. That his book may for some years to come be found a familiar introduction to the use of the microscope, and the study of microscopical science, is the earnest wish of the Author. 1, BEDFORD SQUARE, June, 1882. PREFACE TO THE EIGHTH EDITION. issuing the Eighth Edition of this Work on the MICROSCOPE, I may say that it has been thoroughly revised and for the most part rewritten. Eight carefully and beautifully executed Plates are added, which were drawn by Mr. Tuffen West from natural objects, engraved and printed by Mr. Edmund Evans in his usual excellent manner. The Author cannot but express his grateful surprise at the extraordinary popular reception which his book has met with : a sale of fifty thousand is an unpre- cedented event for a work of the kind. This circum- stance 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 Decent discoveries ; although, in a daily progressing field of science, it is almost im- possible to keep pace with the advance of knowledge in all its ramifications. 1, BEDFORD SQUARE, October, 1867. PREFACE TO THE WEST 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. 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 unongst 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 been, in a great measure, the province of the few only, who have derived from i\ 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 itv 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 beer published at a price within t>e reach of the working classes PREFACE. iA 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, ther 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." JU1 PREFACE. acknowledgments are likewise due to Mr. George Pearson, for the great care lie 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 rf 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, fcime, and labour that he has expended. I six spectra & g of fig. 11 (iden- tical with P. angulatum), giving the drawing copied in fig. 12. The special feature was the small markings between the hexagons, which had not before been noticed on P. angulatum. On more closely scruti- nizing a valve, stopping out the central beam and allowing the six spectra only to pass^, the small mark, ings were found actually to exist, though they were so faint that they had escaped observation until the result of the mathematical deduction had shown that they ought to be seen. These experiments prove that diffraction plays a 26 THE MICROSCOPE. most important part in the formation of microscopical images, since dissimilar structures give identical images when the difference of their diffractive effect is re- moved, and conversely similar structures may give dis- similar images when their diffractive images are made dissimilar. Whilst a purely dioptric image answers point for point to the object on the stage, and enables a safe inference to be drawn as to the actual nature of that object, the visible indications of minute struc- ture in a microscopical image are not always or neces- sarily conformable to the real nature of the object examined, so that nothing more can safely be inferred from the image as presented to the eye, than the presence in the object of such structural peculiarities as will produce the particular diffraction phenomena on which these images depend. It should be carefully noted that diffraction is not limited to lined objects, it applies to structures of all kinds. But lined objects give brighter and more dis- tinct diffraction spectra, and are best suited for experi- mental illustration. Nor, again, is diffraction limited to transparent or semi-transparent objects viewed by trans- mitted light. It equally applies to opaque objects, and is, in fact, universal whenever the strictly uniform propagation of the luminous waves is disturbed by the interposition either of opaque or semi-opaque elements, or of transparent elements of unequal refraction, which give rise to unequal retardations of the waves. The Simple Microscope. A single lens, or a sphere of glass or water, forms a simple microscope, or, as it is more familiarly called, a magnifying glass. Lenses are ground of various forms, as represented in fig. 13 ; a is a plane glass of equal thickness throughout ; &, a meniscus, concave on one side, convex on the other ; c, a double-concave : d, a plano-concave ; e, a double-convex ; /, a plano-convex. By a proper combination of certain forms of lenses, we unite on the same sensible point a number of rays, proceeding from the same point of an object, each THE SIMPLE MICROSCOPE. 27 ray carrying with, it the image of the point from whence it proceeds, and as all the rays unite to form an image of the object from whence they were emitted, this image is brighter in proportion as there are more rays united, and more distinct in proportion as the order in which they have proceeded is perfectly pre- served and in perfect union. The point at which parallel rays meet, after passing through a lens, is known as its principal focus, and its distance from the middle of the lens, the focal length. The radiant point and its image after refraction are known as the conjugate foci. In every lens the right line perpendi- cular to the two surfaces is the axis of the lens. This is indicated by the line drawn through the several lenses, as seen in the diagram. The point where the axis cuts the surface of the lens is termed the vertex. Parallel rays falling on a double-convex lens are brought to a focus in the centre of its diameter ; con- versely, rays diverging from that point are rendered parallel. Hence the focus of a double-convex lens will be at just half the distance, or half the length, of the focus of a plano-convex lens having the same curvature on one side. The distance of the focus from the lens will depend as much on the degree of curvature as upon the refracting power (called the index of refrac- tion) of the glass of which it may be formed. A lens of crown-glass will have a longer focus than a similar one of flint-glass ; since the latter has a greater refract- ing power than the former. For all ordinary practical purposes, we may consider the principal focus as the 28 THE MICROSCOPE. focns for parallel rays is termed of a double-convex lens to be at the distance of its radius, that is, in its centre of curvature ; and that of a plano-convex lens to be at the distance of twice its radius, that is, at the other end of the diameter of its sphere of curvature. The converse of all this occurs when divergent rays are made to fall on a convex lens. Kays already con- verging are brought together at a point nearer than the principal focus ; whereas rays diverging from a point within the principal focus are rendered still more diverging, though in a diminished degree. Bays diverging from points more distant than the principal focus on either side, are brought to a focus beyond it : if the point of divergence be within the circle of curva- ture, the focus of convergence will be beyond it ; and vice versa. The same principles apply equally to a plano-convex lens ; allowance being made for the double distance of its principal focus. They also apply to a lens whose surfaces have different curvatures ; the principal focus of such a lens is found by multiplying the radius of one surface by the radius of the other, and dividing this product by half the sum of the radii. The refracting influence of concave lenses will be precisely the opposite of that of convex. Bays which fall upon them in a parallel direction, will be made to diverge as if from the principal focus, which is here called the negative focus. This will be, for a piano- concave lens, at the distance of the diameter of the sphere of curvature ; and for a double-concave, in the centre of that sphere. A lens convex on one side and concave on the other, is known as a meniscus. In the construction of the microscope, either simple or compound, the curvature of the lenses employed is spherical. Convergent lenses, however, with spherical curvatures, have the defect of not bringing all the rays of light which pass through them to one and the same focus. Each circle of rays from the axis of the lens to its circumference has a different focus, as shown in fig. 14. The rays a a, which pass through the lens near its circumference, is seen to bo more refracted, or come to a focus at a shorter distance SIMPLE MICROSCOPICAL LENSES. 29 behind it than the rays b b, which pass through near its centre or axis, and are less refracted. The conse- quence of this defect of lenses with spherical curva- tures, which is called spherical aberration, is that a well-defined image or picture is not formed by them, for when the object is focussed, for the circumferential rays, the picture projected to the eye is rendered indis- Fio. 14. tinct by a halo or confusion produced by the central rays falling in a circle of dissipation, before they have come to a focus. On the other hand, when placed in the focus of the central rays, the picture formed by them is rendered indistinct by the halo produced by the circumferential rays, which have already come to a focus and crossed, and now fall in a state of diver- gence, forming a circle of dissipation. The grosser defects of spherical aberration are corrected by cutting off the passage of the rays a a, through the circumferences of the lens, by means of a stop dia- phragm, so that the central rays, b b, only are con- cerned in the formation of the picture. This defect is reduced to a minimum, by using the meniscus form of lens, which is the segment of an ellipsoid instead of a sphere. The ellipse and the hyperbola are forms of lenses in which the curvature diminishes from the central ray, or axis, to the circumference b ; and mathematicians have shown that spherical aberration may be practi- cally got rid of by employing lenses whose sections are ellipses or hyperbolas. The remarkable discovery of 30 THE MICROSCOPE. this fact was made by Descartes, who mathematically demonstrated it. If a I, a I', for example, fig. 15, be part of an ellipse whose greater axis is to the distance between its foci ff as the index of refraction is to unity, then parallel rays r I', r" I incident upon the elliptical surface V a I, will be refracted by the single action of that surface into lines which would meet exactly in the farther focus /, if there were no second surface intervening between I a I' and /. But as every useful lens must have two surfaces, we have only to describe a circle I of I' round f as a centre, for the second surface of the lens V I. As all the rays refracted at the surface I a V converge PIG. 15. accurately to /, and as the circular surface I of I' is perpendicular to every one of the refracted rays, all these rays will go on to / without suffering any refrac- tion at the circular surface. Hence it should follow, that a meniscus whose convex surface is part of an ellipsoid, and whose convex surface is part of any spherical surface whose centre is in the farther focus, will have no appreciable spherical aberration, and will refract parallel rays incident on its convex surface to the farther focus. It is almost impossible to give microscopical lenses other than the spherical form. The best made convex single lenses do not bring rays of light to an exact focus. If a true elliptical or hyperbolic curve could be SIMPLE MICROSCOPICAL LEN323. 31 got, lenses would not only be very nearly perfect, but spherical aberration would be nearly overcome. But even this serious defect can be considerably reduced in practice by observing a certain ratio between the radii of the anterior and posterior surfaces of lenses ; thus the spherical aberration of a lens, the radius of one FIG. 16. surface of which is six or seven times greater than that of the other, as in fig. 16, will be much reduced when its more convex surface is turned forward to receive parallel rays, than when its less convex surface is turned forwards. 1 Two forms of lenses may be so combined, that their opposite aberrations shall neutralize each other, and magnifying power be gained. The aberration of a concave lens is exactly the opposite of that of a convex lens, so that the aberration of a convex lens placed in its most favourable position may be cor- rected by a concave lens of much less power in its most favourable position. This prin- ciple of a combination was proposed by Sir John F. W. Herschel; his "aplanatic doublet," fig. 17, consists of a double-convex lens and a meniscus. A doublet of this kind is an extremely useful and avail- able one for microscopic purposes. By a skilful com- bination of crown and flint glass lenses with spherical curves assisted by the Lister adjusting collar, or, what is even more efficient, the homogeneous immersion (1) It must be borne in mind that in lenses having curvatures of the kind the object would only be correctly seen in focus at one point the mathe- matical or geometrical axis of the lens. 32 THE MICROSCOPE. system theoretical and practical difficulties have been overcome in the construction of the modern microscope, and which, until quite lately, were thought to be insur- mountable, thus greatly adding to the value of the instrument as a means of scientific research. Chromatic Aberration. A far greater difficulty arises from the unequal ref rangibility of the different coloured rays which together make up white light. It is this difference in refrangibility that produces a complete separation of rays by the prism, forming the spectrum. The correction of chromatic and spherical aberration is effected in a very ingenious manner, by combining a convex lens made of crown-glass, and a concave lens of flint-glass. If we examine closely the image projected on the table of a camera obscura provided with a com- mon lens, we see that it is fringed with the colours of the rainbow ; again, if we look through a common mag- FJG. 18. nifying-glass at the letters on the title-page of a book, we see them slightly coloured at their edges in a similar manner. The cause of this iridescent border is that the primitive rays red, yellow, and blue, of which a colourless ray of light is composed, are not equally re- frangible. Hence they are not simultaneously brought to one point or focus; the blue rays being the most refran- gible, come to a focus nearer the lens than the yellow, which are less refrangible, and the yellow rays than the red, which are the least refrangible. It is seen, in fig. 18, chromatic aberration proves still more detri- mental to the distinct definition of images formed by a lens, than spherical aberration. This arises more CHROMATIC ABERRATION. 33 from the sizes of the circles of dissipation, than from the iridescent border, and it may still exist, although the spherical aberration of the lens is quite corrected. Chromatic aberration is, as before stated, corrected by combining, in the construction of lenses, two media of opposite forms, differing from each other in the proportion in which they respectively refract and dis- perse the rays of light ; so that the one medium may, by equal and contrary dispersion, neutralize the disper- sion caused by the other, without, at the same time, wholly neutralizing its refraction. .It is a remarkable fact that the media found most valuable for the purpose should be a combination of pieces of crown and flint glass, of crown-glass whose index of refraction is T519, and dispersive power O036, and of flint-glass whose index of refraction is 1*589, and dispersive power FIG. 19. The focal length of the convex crown-glass lens must be 4J inches, and that of the concave flint-glass lens 7f inches, the combined focal length of which is 10 inches. The diagram, fig. 19, shows how rays of light are brought to a focus, free from colour. In this diagram, L L is a convex lens of crown-glass, and I I a concave one of flint-glass. A convex lens will! refract a ray of light (s) falling at P on it exactly in the same manner as the prism A B c, whose faces touch the two services of the lens at the points where the ray enters, and quits. The ray S F, thus refracted by the lens L L, or prism ABC, would have formed a spectrum (i j T) on a screen or wall, had there been no other lens, tmmtmg Item & aw mtioM of tike cm 0! fidbt - ^ - - * ; :,;; .? .---.- - , diwqp-ffcw.tfce ItMU *fce*e we shall proceed to applj them to its T%# Jfer&K^.--A Mienwcope. as I said, ma j be either a. JM^C, or JHM C 00 THE MICROSCOPE. instrument. The simple microscope may consist of one, as seen in fig. 21, or of two or three lenses; if the latter, then so arranged as to have the effect only of a single lens. In the compound microscope, not less than two lenses can be employed : one to form an in- verted image of the object, which, being the nearest to- the object, is called the object-glass ; the other to mag- nify this image, and from being near the eye of the observer, is called the eye-glass. I have so far considered a lens simply with reference to its enlargement of the object, the increase of the angle under which the object is seen. A further and equally important consideration is that of the number of rays or quantity of light by which every point of the object is rendered visible ; and much may be accomplished, as I have already pointed out, by the combination of two or more lenses, which w r ill at once reduce the angles of incidence and refraction. The first satisfactory combination for the purpose was the invention of the celebrated Dr. Wollaston. His doublet (fig. 22) consists of two plano-convex lenses having their focal lengths in the proportion of one to three, or nearly so, and mounted at a distance which is readily ascertained by experiment. The plane sides of the lenses should be towards the object, and the lens of shortest focal length next the object. It appears that Dr. Wollaston was led to this inven- tion by considering that the achromatic Huyghenian eye-piece, presently to be described, would, if reversed, possess a power equal to that of the simple microscope. But it will be evident, when the eye-piece is under- stood, that the circumstances which render it achro- matic are very imperfectly applicable to the simple microscope, and that the doublet, without a nice adjustment or a stop, would be valueless. The nature of the corrections which take place in the doublet is explained in the annexed diagram, where 1 o V is the object, p a portion of the cornea of the eye, and d d the stop, or limiting aperture (fig. 22). Now it will be observed that each pencil of light proceeding from I I' of the object is rendered THE DOUBLET. 37 excentrical by the diaphragm d, d ; consequently, they pass through the lenses on opposite sides of their common axis o p; thus each becomes affected by opposite errors, which to some extent balance and correct each other. To take the pencil Z, for instance, which enters the eye at r b, r I : it is bent to the right at the first lens, and to the left at the second ; and as each bending alters the direction of the blue ray more than the red, and, moreover, as the blue ray falls nearer the margin of the second lens, where the refraction being more powerful than nearer the centre, compensates in some degree for the greater focal length of the second lens, the blue rays will emerge very 38 THE MICROSCOPE. nearly parallel, and of consequence colourless to the eye. At the same time the spherical aberration has much diminished, "because the side of the pencil as it proceeds through one lens passes nearest the axis and in the other nearest the margin. This explanation applies to pencils farthest from the centre of the object. The central pencils, it is obvious, would pass both lenses symmetrically, tho same portions of rays occupying nearly the same relative places in both lenses. The blue ray would enter the second lens nearer to its axis than the red ; and being thus less refracted than the red by the second lens, a small amount of compensation would take place, quite different in principle, and inferior in degree, to that which is produced in the excentrical pencils. In the intermediate spaces the corrections are still more imperfect and uncertain ; and this explains the cause of the aberrations which must of necessity exist even in the best-made doublet. It is, however, infinitely superior to a single lens, and will transmit a pencil of an angle of from 35 to 50. The next step towards improving the simple micro- scope was in relation to the eye-piece ; this was effected by Mr. Holland. It consists in substituting two lenses for the first in the doublet, and placing a stop between them and the third. The first bending of the pencils of light being effected by two lenses instead of one, produces less aberration, and this is more completely balanced or corrected at the second bending, and in the opposite direction, by the third lens. A useful form of pocket lens was proposed by Dr. Wollaston, named by him " Periscopic." This combi- nation consists of two hemispherical lenses cemented together by their plane faces, with a stop between them to limit the aperture. A similar proposal, made by Sir David Brewster in 1820, is well known as the Coddington lens, 1 shown at fig. 23 : this gives a (1) The late Mr. Coddington, of Cambridge, who had a high opinion of the value of this lens, had one of these grooved spheres executed by Mr. Carey, who gave it the name of the Coddington Lens, supposing that it was invented by the person who employed him, whereas Mr. Coddington never laid claim to it, and the circumstance of his having one made was not until nine years after it was described by Sir David Brewster in the " Edinburgh Journal." THE CODDINGTON LENS. 39 larger field of view, and is equally good in all directions, as it is evident that the pencils a b and & a pass through under precisely the same circumstances. Its spherical form has the further advantage of render- ing the position in which it is held of comparatively little consequence. It is very generally used as a hand magnifier ; but its definition is certainly not so good as that of a well-made doublet or achromatic lens. It is usually set in a folding case, as represented in the figure, and so contrived as to be admirably adapted for the waistcoat-pocket. It is sold with the small holder, fig. 23#, for holding and securing small. objects Fio. 23. FIG. 23a. during examination. Browning's Platyscopic Pocket Lens is a useful form for botanists and mineralogists. Its focus is about three times farther from the object than the Coddington, and allows of opaque objects being easily examined ; it has also three degrees of magnifying power of 15, 20, and 30 diameters^ When the magnifying power of a lens is consider, able, or when its focal length is short, and its proper distance from the object equally short, it then becomes necessary to be placed at a proper distance with great precision; it cannot therefore be held with sufficient 40 THE MICROSCOPE. accuracy and steadiness bj the unassisted hand, bnt must be mounted in a frame, having a rack or screw to move it towards or from another frame or stage which holds the object. It is then called a micro- scope ; and it is furnished, according to circumstances, with lenses and mirrors to collect and reflect the light upon the object, with other conveniences. The best of the kind, contrived by the late Mr. Boss, |g represented in fig. 24; and consists of a circular foot e, from which rises a short tubular stem d, into which slides another short tube c, carrying at its top a joint /; to this joint is fixed a square tube a, through which a rod b slides ; this rod has at one end another but smaller joint g, to which is attached a collar h, for receiving the lens i. By means of the joint at /, the square rod can be moved up or down, so as to bring the lens close to the object ; and by the rod sliding through the square tube a, the distance between the stand and the lens may be either increased or diminished : the joint g, at the end of the rod, is for the purpose of allow- ing the lens to be brought either horizontally or at an angle to the subject to be investigated. By means of the sliding arm the distance between the table and the jointed arm can be increased or dimin- ished. This microscope is provided with lenses of one-inch and half-inch focal length, and is thereby most useful for the examination and dissection of objects. It is readily unscrewed and taken to pieces, and may be packed in a small case for the pocket. FIG. 24. Boss's Simple Microscope. THE COMPOUND MICROSCOPE. 41 THE COMPOUND MICROSCOPE. The compound micro- scope consists of two essential parts, the stanc optical arrangement ; the first image being further magnified by one or more lenses forming tHe eye- piece. The mechanical principles involved in tte con- struction of the compound instrument are feW and simple. The more finished form of microscope has assumed a degree of solidity and luxurious elegknce heretofore unknown, whilst its accessories have multi. plied to an almost unlimited extent. This has resulted from a desire to save time or overcome difficulties, as the practical skill and experience of the microscopisf or optician may have suggested, so that whilst the- wants of the amateur have been duly considered, tht- more modest demands of the student have in no wa^ been overlooked or forgotten. Fortunately for the student, no large and expensive form of instrument is absolutely necessary for the pursuit of microscopical science. A small and simple microscope is as well adapted to his wants indeed is all that he requires for the work he has to perform. And as for his cabinet of objects, these will grow day by day, and by the labour of his hands. It was with a very unpretending form of microscope that John Quekett worked ; that John Ralfs studied the " British DesmidiaceaD ; " John Denny the " Anoplura ; " Wil- liam Smith the " British Diatomaceee ; " George John- son the " British Zoophytes ; " and Dr. Bowerbank the " British Spongiidee." The microscope has its place in the educational movement of the age, and it is the more incumbent on opticians to manufacture an econo- mic form of instrument, adapted to the wants of a large and increasing class. In the selection of an instrument, it must be borne in mind that a firm stand and a well corrected object-glass are indispensably necessary. It may be of some advantage to those who are about to purchase an instrument if I were to shortly de- scribe its several parts the stand, body, stage, sub- stage, mirror, eye-piece and objective, seriatim. The etand, the bearings upon which the superstructure of the microscope rests, should be solid; and the foot 42 THE MICROSCOPE. or claw horseshoe shaped, that it may steadily grasp the table and assist in maintaining the centre of gravity in any position the instrument may be placed. To the foot should be attached two upright pillars, with trunnions, for making the attachments and securing the sliding bar which carries the tubular body and stage. The tubular body should be about eight or ten inches in length, with a second or inner tube, "the draw-tube," of five or six inches in length, sliding with- in it. The "draw-tube" assists in the magnification of the image, and is usually engraved with a scale of inches and parts of an inch, for measuring the dis- tance between the eye-piece and the front of the object-glass. The eye-piece surmounts the body, while to the other extremity is attached or screwed the object-glass. The focus is obtained by either a sliding or rack-and-pinioii motion, termed the " coarse-ad- justment," the fine-adjustment being obtained by a milled-head screw acting upon the long end of a lever, or by other mechanical means. Whatever the kind of motion adopted, neither jumping nor lateral movement of the body should take place, otherwise the object, when placed on the stage, will appear to change its position either to the right or left. A fine-adjustment is very necessary, as without it the highest magnifying powers can hardly be used without risk of damage being done either to the object or the objective. Pig. 25 represents the body of an ordinary com- pound microscope with triple object-glass ; o is an object, above it is seen the triple achromatic object- glass, in connection with the eye-piece e e, ff the plano- convex lens ; e e being the eye-glass, and // the field- glass, and between them, at b &, a dark spot or dia- phragm. The course of the light is shown by three rays drawn from the centre, and three from each end of the object o ; these rays, if not prevented by the lens / /, or the diaphragm at b &, would form an image at a a ; but as they meet with the lens f f in their passage, they are converged by it, and meeting at b b, where a diaphragm is placed to intercept all extraneous THE COMPOUND MICROSCOPE. 43 light, excepting that required for the formation of the image ; a further magnification of the image is effected by the eye-lens e e, and precisely as if it were an original object. The Stage. The stage of the microscope should be perfectly flat and rigid, without flexure, and as thin as may be consistent with these essential qualities. It should rotate on its axis, as a revolving stage possesses great advantages. It enables the observer to keep a diatom in view, while it is pre- sented in succession to rays of greater or less obliquity, and thus a better insight is obtained into structure. Supposing fig. 26 were an object marked by longitudi- nal strise, but too faint to be seen by ordinary direct light, the light most useful for bringing these into view will be that pro- ceeding obliquely in either of the directions C and D ; whilst rays of light falling upon it in the direc- tions A and B would tend to ob- scure the strias rather than disclose them. If the markings, however, are due to furrows or prominences having 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 seen best by that which produces the strongest shadow ; hence, if there be a projecting ridge, with an abrupt side looking towards c, they will be best seen by light from D ; if, however, there be a furrow with a steep bank on the side of c, they will be seen by light from the same side. It not unfrequently happens that longitudinal strise, or lines, 44 THE MICROSCOPE. are crossed by others, and then these transverse strise will be better seen by an illuminating pencil less favourable for longitudinal, so that, in order to bring them into distinct view, either the illuminating pencil or the object must be moved or rotated a quarter round. Swinging Sul-stage, or Tailpiece. The swinging sub- stage, although a revival of an invention contrived by Mr. T. Grubb some twenty years ago, has been very generally adopted, since it is thought by manufacturers to be an important and FIG. 27. THE MECHANICAL STAGE. 45 useful addition to the more perfected forms of instru- ments. This tailpiece, represented in sectional elevation fig. 27, consists of s, the limb carrying the body, with coarse and fine adjustments ; A, the stem carrying the sub-stage, B, and mirror. A is attached to s by the sleeve or socket I, clamped by the nut j, and on I A may be swung sideways in either direction to the right or left, either below or above the stage, the axis of revolution of which is the line x T ; that is, a line in the plane of the object to be viewed on the stage c, intersected by the optical axis of the instrument ; that is, the line N o, passing through the centre of the body and the object- glass of the microscope. The stage c is also attached to s by the pin C 1 , terminated by the screen C 2 , which pin passes through the centre of the socket I, and moves therein so that the stage C may readily turn in either direction in conjunction with or independent of A, the axis of its revolution being also the line x T. By this kind of arrangement the stage C and the stem A can be set at any angle to the axis of the microscope, either below or above x Y, intersecting the plane of the object to be viewed, and relatively to each other, and when so set the stage C can be clamped at the desired angle by the nut D on the screw C 2 acting on s and the collar K. A mechanical stage is one consisting of two or more plates, the rectangular motions of which are obtained by rack and pinion. It is considered a necessary appendage to the more finished instruments. The cheaper kinds are provided with a simple form of sliding plate, the lower part of which is a raised edge for the objects to rest against. It is often found con- venient to have a means of watching growing and other processes, which are either promoted or assisted by maintaining the object for a time at a certain tem- perature. For this purpose many forms of apparatus have been contrived. A very inexpensive form is that of Mr. Bartley's, fig. 28. The vessel E, three parts filled with water, and sup- ported on a ring-stand, is kept at any temperature by the spirit lamp c, a syphon-tube d conveys the Jbot 46 THE MICROSCOPE. water along /and through the bent tubing which sur- FIG. 28.Eartl#y's Warm Stage. rounds the object on the stage D, and passing off through the open end c into the receptacle B, placed to receive the overflow. Mr. Stephenson's safety-stage is one of those happy FIG. 29. Stephenson's Safety-Stage. contrivances by means of which an accident to either THE MIRROR, AND EYE-PIECE. 47 the object or objective will be prevented. When about to be used it is simply necessary to place it on the fixed stage of the microscope. The object about to be examined is supported and kept in place by a couple of clips or projecting springs. Should a tyro in the use of the instrument hastily rack down the body, all undue force is broken by the elasticity of the springs. Messrs. Watson and Son, of Holborn, manufacture a light and elegant form in ebonite of this accessory safety-stage. The Mirror. The mode in which an object is illu- minated is, in the words of Andrew Ross, " second only in importance to the excellence of the glass through which it is seen." To ensure good illumination the mirror should be in direct co-ordination with the objective and eye-piece; it must be regarded as a part of the same instrument, and tending by a com- bined series of acts to a common result. Illumination is spoken of as of three kinds or qualities reflected, transmitted, and refracted light. For the illumination of transparent objects, transmitted light is brought into use ; for opaque objects, reflected. The trans- mitted illuminating pencil should be as large as can well be received by the lens, and no larger. Any light beyond this is liable to produce confusion of image. In using the mirror the reflected light can be made brighter, more concentrated, by employing a bull's-eye condensing lens. The Eye-piece. The eye-piece of the compound microscope consists of two plano-convex lenses; that furthest from the eye, as I have already explained, is the field-glass, and that nearest the eye is the eye- glass. The former increases the field of vision, the latter magnifies, the enlarged inverted image. Combined together, the two materially assist in cor- recting residual imperfections of the objective. The magnifying power of the microscope depends in a measure, then, upon the eye-piece, but the limit of usefulness in this direction is soon reached, for, although the size of the image is thereby increased, this increase is achieved at the expense of per- 48 THE MICROSCOPE. feet definition; and it should be observed that only objectives of the finest construction will bear the deeper eye-pieces. Opticians furnish with most of their instruments two, three, or more oculars : A, B, and C ; these, together with a Kellner or orthoscopic eye-piece, D, the field-lens of which is bi-convex, and therefore gives a larger field, are all the microscopist will require. The Huyghenian eye-piece, which is still in use, consists of two plano-convex lenses, with their plane sides turned towards the eye, and at a distance apart equal to half the sum of their focal lengths, and having a stop or diaphragm midway between the lenses. Huyghens was not aware of the value of his eye-piece ; it was reserved for Boscovicli D c A a IG. 30. Eye-pieces. to point out that, by this important arrangement, he had accidentally corrected a portion of the chro- matic aberration incidental to the earlier forms. Let fig. 31 represent the Huyghenian eye-piece of a micro- scope, f f being the field glass, and e e the eye-glass, and I m n the two extreme rays of each of the three pencils emanating from the centre and ends of the object, of which, but for the field-glass, a series of coloured images would be formed from r r to b b ; those near r r being red, those near b b blue, and the intermediate ones green, yellow, and so on, correspond- ing with the colours of the prismatic spectrum. This order of colours is the reverse of that of the common compound microscope, in which the single object-glass projects the red image beyond the blue. HUYGHENIAN EYE-PIECE. 49 The effect just described, of projecting the blue image beyond the red, is purposely produced for reasons presently to be given, and is called over- correcting the object-glass as to colour. It is to be observed, also, that the images b b and r r are curved in the wrong direction to be distinctly seen by a convex eye -lens, and this is a further defect of the compound microscope of two lenses. But the field- glass, at the same time that it bends the rays and converges them to foci at &' V and r f /, also reverses the curvature of the images as here shown, giving E 50 THE MICROSCOPE. 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 pass uncoloured through the eye-glass. To render this important point more intel- ligible, 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 &", and the red rays at /', 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 W. This effect has already been referred to as due to over- correction of the object-glass, which re- moves the blue foci b 6 as much beyond the red foci r r as the sum of the distances between the red and the blue foci of the field-lens and eye-lens ; so that the separation b r is exactly taken up in passing through those two lenses, and the several colours coincide, so far as focal distance is concerned, as the rays pass 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 greater refractive power of the field-glass upon the blue rays. In tracing the pencil Z, 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 representing 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-cor- rection of the object-glass, and leads to 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 use- fully, 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 THE RAMSDEN EYE-PIECE. 51 FIG. 32. refractive power is less than at the margin, so 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 V and I" emerge sensibly parallel, presenting, in consequence, the perfect definition of a single point to the eye. The same reason- ing is true of the intermediate colours and of the other pencils. The eye-glass thus constructed not only brings together the images b' b', r r r', but it likewise has the most important effect of rendering them flat, and at once correcting both chromatic and spherical aberrations. The Huyghenian eye-piece described, has served the purpose of illustrating the optical effects of this part of the instrument ; but when it is required to measure the magnified image, we use the eye-piece invented by Bamsden, and called by him the micrometer eye-piece. The arrangement will be readily understood upon reference to fig. 32. The field- glass having its plane face turned to- wards the object, so that the rays from the object are made to converge immediately in front of the field-glass; and here is placed a plane-glass, on which is engraved divisions of l-100th of an inch or more. The markings of these divisions come into focus, therefore, at the same time as the image of the object, both being dis- tinctly seen together. The glass with its divisions is shown in fig. 33, on which, at A, are seen some magni- fied grains of starch. Thus the measure of the magni- fied image is given by mere inspection ; and the value of such measurements, in reference to the real object, when once obtained, is constant for the same object-glass. 1 (1) It was affirmed by Ross, that if the achromatic principle were applied E 2 FIG. 33. 52 THE MICROSCOPE. Mr. Lister proposed to place on the stage of tlie microscope a divided scale, of a certain value ; viewing the scale as a microscopic object, he observed how many of the divisions on the scale attached to the eye-pie.ce corresponded with one or more of a 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 ten times less than that 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 employed. Mr. Jackson invented the simple and cheap form of micrometer, represented in fig. 34, 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 afford- ing 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 derange. iDent in inexperienced hands. The positive eye-piece gives the best view of the micrometer, 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 to the construction of eye-pieces, the Ramsden is the form by which greater perfection should be obtained. That such an adaptation might be produc- tive of valuable results, appears from Mr. Brooke's statement, that he hai 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 Hu\ghet,ian eye-piece. An inch or half-inch achromatic object-glass answers extremely well as an eye-piece. MICROMETER EYE-PIECE. 53 extent of half its diameter, there is no perceptible distortion ; and the clearness of the definition gives a precision to the measurement which is very satisfactory. Short bold lines are ruled on a piece of glass, a, fig. 34; 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 prevent the plumbago from being wiped out. The slip of glasf thus prepared is secured in a thin brass frame, so thai FIG. 34. Jackson's Micrometer Eye-piece. it may slide freely ; it 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, #, 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 adjustment 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 54 THE MICROSCOPE. small screw acting upon the slip of glass. The divi- sions 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 its divisions should be ascertained with each object-glass ; the method of doing this is 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 numbrr of divisions in the eye-piece which cover one on tho stage. Repeat this process with different portions of the stage-micrometer, and if there be any difference, take the mean. Sup- pose the hundredth of an inch on the stage requires eighteen divisions in the eye-piece to cover it ; it is quite plain that an inch would require eighteen hun- dred, and an object which occupied nine of these divisions would measure the two-hundredth of an inch. Take the instance supposed, and let the micro- scope 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 measure ten ; which, divided by 2,000, gives the decimal fraction '005. Enter in a table the length to which the tube is drawn out, and the number of divisions on the eye- piece micrometer 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, if the hundredth of an inch on the stage occupies too much of the field, then the two- hundredth or five-hundredth should be used, and NOBERT'S LINES. 55 the number of divisions corresponding to that quantity "be multiplied by two hundred or five hundred, as the case may be. The micrometer should not be fitted into too deep an eye-piece, for it is essential to preserve clear defini- tion. A middle power or Kellner eye-piece is the best, provided the object-glass be of the first quality ; otherwise, use the eye-piece of lowest power. The lens above the micrometer should not be of shorter focus than three-quarters of an inch, even with the best object-glasses ; and the slit cut in the tube can be closed at any time by a small sliding bar, placed at Z, m, fig. 34. The wonderful tracings on glass executed by the late M. Nobert, of Earth, deserve attention. The plan adopted by him was to trace on glass ten or more separate bands at equal distances from each other, each band being composed of parallel lines of a frac- tional part of a Prussian inch apart ; in some they are l-1000th, and in others only 1 -4000th of a Prussian inch separated. The distance of these parallel lines forms part of a geometric series : O'OOIOOO lines. 0'000463 lines. 0-000857 C'000397 0-000735 0-000340 C'000630 0/000292 0-000540 0-000225 To see these lines at all, it is requisite to use a micro- scope with a magnifying power of 100 diameters ; the bands containing the fewest number of lines will then be visible. To distinguish the finer lines, it will be necessary to use a magnifying power of 300, and then the lines which are only 1 -4700th of an inch (Prussian) apart will be seen perfectly traced. Of all the tests yet found for object-glasses of high power, these would seem the most valuable. Robert's tracings have tended to confirm the undulating theory of light, the different colours of the spectrum being exhibited in the ruled spaces varying with the separation of the lines; in those cases where the distances between the lines are smaller- than the length of the violet-coloured waves, *io colour is perceived ; while, on the other hand, if 56 THE MICROSCOPE. Inequalities amounting to '000002 line occnr, stripes of another colour appear in them. Schmidt's goniometer positive eye-piece, for mea- suring the angles of crystals, is so arranged as to be easily rotated within a large and accurately graduated circle. In the focus of the eye-piece a single cobweb is drawn across, and to the upper part is attached a vernier. The crystals being placed in the field of the microscope, care being taken that they lie perfectly, fat, the vernier is brought to zero, and then the whole apparatus turned until the line is parallel with one face of the crystal; the frame- work bearing the cobweb, with the vernier, is now rotated until the cobweb becomes parallel with the next face of the crystal, and the number of degrees which it has traversed may then be accurately read off. Erector eye-pieces and erecting prisms are employed for the purpose of making the image presented to the eye correspond with the position of the object. They are most useful for minute dissections, but the loss of light occasioned by sending it through two addi- tional surfaces is a drawback impairs the sharpness of the image. Nachet designed an extremely ingenious arrangement whereby the inverted image became erect; he adapted a simple rectangular prism to the eye- piece. The obliquity which a prism gives to the visual rays, when the microscope is used in the erect position, as for dissecting, is an advantage, as it brings the image to the eye at an angle very nearly correspond- ing to the inclined position in which the microscope is ordinarily used. The Value of Eye-pieces. The magnifying power of Ross's lowest eye-piece A is about 5; that of B, 8 to 10;, C, 15; D, 20; and E, 25. For viewing thin sections of recent or fossil woods,, coal, the fructification of ferns and mosses, fossil-shells, seeds, small insects, or parts of larger ones ; molluscs, the circulation in the frog, etc., the A eye-piece is best adapted. For the examination of details of minuter objects, the B eye-piece is preferred; the pollen of flowers^ THE VALUE OF EYE-PIECES. 57 dissected insects, the vascular and cellular tissues of plants, the Haversian canals, the lacunas of bone, and the serrated laminaa of the crystalline lens of the eyes of birds and fishes require the B eye-piece. The C eye-piece is brought into use when it is neces- sary to investigate the structure of very delicate tissues ; and in observations upon minute diatoms and desmids, scales of moths, gnats, raphides, etc. The employment of a deep eye-piece sometimes obviates the necessity of using a deeper object-glass, and which always occasions a re-arrangement of the illumination generally. It must be borne in mind, that the more powerful the eye-piece, the more palpable will the imperfections of the object-glass become ; hence less confidence should be placed in observations made with a powerful eye- piece than when amplification is obtained with a shallow one and a deeper object-glass. 1 The Draw-tiibe is an intermediate tube, which when (1) AMPLIFICATION OF OBJECTIVES AND EYE-PIECES. OBJECTIVES. EYE-PIECES AND OBJECTIVES COMBINED. Focal Length. Magni- fying Power. With Beck's i, Powell's i, Boss's A. With Beck's 2, Powell's Boss's B. With Powell's 3- With Boss's C. With Beck's 3. With Beck's 4, Powell '34, Boss's B. With Beck's 5, Boss's E. Ins 5 2 10 15 20 25 30 40 50 4 2* 12* 18| 25 31J 37* 50 62* 3 3* 16 25 33* 4l| 50 66g 83* 4) 5 25 37* 50 62 75 100 125 1* 6 33* 50 66| 83* 100 133* 166 1 10 50 75 100 125 150 200 250 * * 13 62* 66 93| 100 125 133* 166| 187* 200 250 266 312* i 333* i 15 75 112* 150 187* 225 300 375 i 20 100 150 200 250 300 400 500 & 25 125 187*' 250 312* 375 500 625 * 30 150 225 300 375 450 600 750 TO 33* 160 250 333* 416-3- 500 666 833* 1 40 200 300 400 500 600 800 10CO i 50 250 375 500 625 750 1000 1250 \ 60 300 450 600 750 900 1200 1500 i 70 350 525 700 875 1050 1400 1750 | 80 400 600 800 1000 1200 1600 2000 1 g 90 450 675 900 1125 1350 1800 2250 T5 100 500 750 1000 1250 1500 2000 2500 Reduced from a comprehensive table of the magnifying power of eye- pieces, and the amplification of objectives and eye-pieces combined, issued in a separate form with the Journal of the Royal Microscopical Society. 58 THE MICROSCOPE. drawn out increases the magnification of the image, without having to change the eye-piece. When using the micrometer eye-piece, we are enabled by the aid of the draw-tube to fill the whole field of view and make a precise comparison between the divisions of the eye- piece and the stage micrometer. In Messrs. Beck's microscopes, the draw-tube is furnished with a rack- and- pinion movement for the purpose of facilitating adjustment. The Object-glass. The microscope depends so much for its effectiveness and general utility upon the perfec- tion of the object-glass or objective, that it is absolutely necessary for any one about to use the instrument to make himself perfectly familiar with its relative quali- ties. It will scarcely be possible to form any just estimate of the value of this or that maker's objective by a comparison of magnification ; indeed the propor- tional amplification of the object-glasses of the most conscientious opticians will on comparison be found to differ materially. To arrive at a literally correct judgment of the value of an objective to the microscopist, there are special qualifications by which it should be judged. These, for the sake of convenience, may be divided into : Its defining power; its penetrating power, or focal depth ; its resolving power ; its working distance and its flatness of field : all of which are qualifications of the greatest importance, especially when an objective is about to be employed in scientific researche. The Defining Power of the objective depends upon the perfection with which the corrections of its chro- matic and spherical aberrations are made. When these are nicely balanced, the image will be sharp, and the minutest details of an object seen with greater clear- ness. Whatever other qualities may be absent in the object-glass, fine definition must be secured. This quality may be tested by taking a known test-object, as it is termed, a blood corpuscle, a diatom mounted in balsam, or a Podura-scale, and comparing the sharp- ness and perfection of the image produced by one objective, against that of another. Penetrating Power, or focal depth, is a quality THE OBJECTIVE. 59 which affords the observer a deeper insight into struc- ture. The objective having the longest working dis- tance, as a rule, possesses the greatest amount of pene- tration. Theoretically, according to Professor Abbe, the penetration of an objective decreases as the square of the angular aperture increases. The botanist or phy- siologist, studying the minute anatomy of plant or animal, would gain a very imperfect idea of the structural elements entering into either, unless the objective possessed good penetration. It is, how- ever, somewhat unusual to find good penetrating or separating power combined with equally good definition in any objective. The latter quality is compatible only with the highest attainable aperture. Fio. 35. Wenham's Binocular Objective. Penetration is an indispensable quality for the bino- cular microscope, consequently opticians have been induced to furnish special forms of object-glasses for use with this form of instrument. Mr. Wenham carried a kind of speciality into the construction of high-power objectives for the binocular, by mounting a prism in a separate tube, and slipping it down the objective, and letting it almost touch the back lens. Fig. 35 represents a fth objective of the kind, full size, with correcting adjustment. D being the objective complete, and the tube with prism fixed in its place. The objective, it will be seen, is shorter than an ordinary Jth, and can be made to answer a double purpose. ^ It becomes more effective as a homogeneous-immersion 60 THE MICROSCOPE. objective, and if intended to be so used, the correcting adjustment will be unnecessary, since the body part can be made shorter, and the back lens brought into close contact with the binocular prism. Messrs. Swift have also constructed a series of object-glasses for a similar purpose. The taper front objectives (fig. 36) are for use with their erecting Stephenson's binocular instrument, and for the better illumination of opaque objects. From their peculiar construction, the illuminating rays from the bull's-eye condenser are made to impinge somewhat more verti- cally upon the object, thus avoiding deep shadows, which often give rise to false appearances when the light is thrown too obliquely on the object. FIG. 36. Swift's Taper Object-glasses. With regard to the binocular microscope, it should be understood, writes Professor Abbe, that fairly satis- factory stereoscopic observation cannot be extended beyond moderate amplification, not even when the binocular arrangement allows of the use of high powers. In fact, as soon as the use of higher powers is resorted to, stereoscopic vision is limited to objects of so little depth that a merely plastic view of them can hardly be productive of any scientific advantage, although effective images may still be obtained. Resolving Power is the power or capacity of the- objective to resolve the finest lines, striae or dots ; that is, separate and define them distinctly. Resolution increases with width of aperture, and may therefore be regarded as another expression for definition. The THE OBJECTIVE. 61 maximum attainable resolving power of an objective of 180 aperture,, according to Professor Abbe, is tlie sepa- ration or resolution of fine lines ruled 118000th of an inch apart. Resolution depends more or less upon the quality and quantity of the light admitted, the power of collecting the greatest number of rays, and the per- fection of centring. In other words, upon the co-ordi- nation of the illuminating system of the microscope mirror, achromatic condenser, objective and eye-piece. If diatoms are employed as test- objects, it should be borne in mind that there are great differences even in the same species, and in the distances their lines are apart. For this reason ruled lines of known value, as Robert's lines, are much to be preferred. The follow- ing example may be taken as a test of the value of a dry Jth objective of 120 in defining the rulings of a 19-band plate, which is equivalent to the l-67000th of an inch. This objective, with careful illumination, showed them all ; but when cut down by a diaphragm to 110, the eighteenth line was not separable ; further cut down to 100 the seventeenth was the limit, to 80 the four- teenth, and to 60 the tenth barely reached. Flatness of Field is a quality of some importance, and must be included in the general practical value of the objective, denoting its capacity to exhibit the peri- pheral portions of the field with the same degree of sharpness as the central. Flatness of field is much enhanced by the width of the opening or angular aper- ture. In all high -angled objectives the image should be sharp and quite free from colour to the very mar- ginal portions of the field. In experimenting on the comparative merits of two object-glasses as to flatness of field, an eye-piece of large aperture should be used. For testing flatness of field, Cole's exquisitely prepared double-stained sections of woods will be found in every way suitable objects. The proboscis of the fly is also recommended, and if its details have not been de- stroyed by being mounted in balsam, it is a good test. Glycerine is the proper medium for displaying its several structures. The cover-glass of the object, it must be remembered, as Amici pointed out, is not an nnimpor 62 THE MICROSCOPE. tant factor in the production of the image. An object viewed without a cover-glass is more clearly defined than one with. The late Andrew Ross explained the cause of this difference in a paper published in the " Transactions of the Society of Arts," vol. 41. Perfect definition is the quality most sought after by those engaged in histological pursuits ; whilst perfect resolution is more highly esteemed by those who take especial interest in the finest diatoms and test objects of a similar nature. Various modifications have taken place in the construction of the object-glass of the FIG. 37. Forms of Object-glasses. A, Double-convex lens ; B, Plano-concave ; C, Bi-convex and plano-concave united ; shown in their various combinations, as at D, form the 3-in., 2-in. or IJ-in. ; at E, 1-in. and jj-in. ; and at F, the -in. -in. J-in. and 5 i 3 -in. objectives. microscope : opticians, however, are quite agreed that the highest theoretical perfection will be obtained by an increase rather than a decrease in the number of lenses entering into its combination. Both at home and abroad first-class makers, such as Ross, Powell and Lealand, Beck, Dallmeyer, Tolles, Wales, Zeiss, &c., have been working on this principle. To a well- considered combination formula they have added a single front plano-convex lens of crown-glass, which gives increase of power with a longer working distance to the objective. THE OBJECTIVE. 63 The accompanying diagram is intended to show the several lenses that enter into the construction of the ordinary achromatic object-glass. A double convex lens and plano-convex lens of crown-glass, and a piano and donble concave lens and a miniscns lens of flint- glass, are ingeniously cemented with Canada balsam into a solid mass. Each objective, from the -|-inch to the -jL-inch and upwards, is thus made up of at least eight original lenses, the back combination of each being a triplet formed of two double convex lenses of crown-glass, with an intermediate double ccncave lens of flint-glass. I cannot bring these brief observations on the object- glass to a close without referring more directly to the great improvement effected in balancing its aberrations by the late Mr. Lister. This gentleman devised the very important screw-collar adjustment, by means of which the front lens of the objective is brought nearer to the back lens; this at once compensates for the disturbance produced by rays of light being made to pass through different thicknesses of glass covers. When an objective has its aberrations balanced for viewing an opaque object, and it is required to examine that object by transmitted light, the correction will remain ; but if it is necessary to immerse the object in a fluid, or to cover it with glass, an aberration arises from either circumstance which will disturb the pre- vious correction, and deteriorate the definition ; and this defect will be more obvious from the increase of distance between the object and objective. How this very important correction is effected may be further explained. If an object-glass be constructed as represented in fig. 38, where the posterior combina- tion p and the middle m have together an excess of negative aberration, and if this be corrected by the- anterior combination a having an excess of positive aberration, then this latter combination can be made to act more or less powerfully upon p and w, by making it approach to or recede from them ; for when the three act in close contact, the distance of the object from the object-glass is greatest, and conse- 64 THE MICROSCOPE. quently the rays from the object arc diverging from a point at a greater distance than when the combina- tions are separated ; and as a lens bends the rays more, or acts with greater effect, the more distant the object is from which the rays diverge, the effect of the anterior combination a upon the other two, p and m, will vary with its distance from thence. When, therefore, the correc- tion of the whole is effected for an opaque object, with a certain distance between the anterior and middle combination, if they are then put in contact, the dis- tance between the object and object-glass will be increased ; consequently, the anterior com- bination w r ill act more power- fully, and the whole will have an excess of positive aberration. Now the effect of the aberra- tion produced by a piece of flat and parallel glass being of a negative character, it is obvious that the above considerations suggest the means of correction, by moving the lenses nearer to- gether, and the positive aberration is made to balance the negative aberration caused by the medium. The preceding refers only to "the spherical aberra- tion ; but the effect of the chromatic is also seen when an object is covered with a piece of glass : it pro- duces chromatic thickening of the outline of Podura and other delicate scales ; and if diverging rays near the axis and at the margin are projected through a piece of flat parallel glass, with the various indices of refraction for the different colours, it will be seen that each ray will emerge, separated, into a beam con- sisting of the component colours of the ray, and that each beam is widely different in form. This difference, being magnified by the objective of the microscope, readily accounts for the chromatic thickening of the outline just mentioned. Therefore, to obtain the finest definition of extremely delicate and minute objects, THE CORRECTION OF THE OBJECTIVE. 65 they should be viewed without a covering ; if it be desirable to immerse them in a fluid, they should be covered with the thinnest possible film of talc, as, from the character of the chromatic aberration, it will be seen that varying the distances of the combinations will not sensibly affect the correction ; though object- lenses may be made to include a given fluid, or solid medium, in their correction for colour. The mechanism for applying these principles to the correction of an object-glass under the various circum- stances, is represented in fig. 39, where the anterior lens is set in the end of a tube a, which slides on the cylinder b, containing the remainder of the combination ; the tube a, holding the lens nearest the object, may then be moved upon the cylinder &, for the purpose of varying the dis- tance, according to the thick- ness of the glass covering the object, by turning the screwed ring c, or more simply by sliding the one on the other, and clamping them together when adjusted. An aperture is made in the tube a, within which is seen a mark engraved on the cylinder ; and on the edge of which are two marks, a longer and a shorter, engraved upon the tube. When the mark on the cylinder coincides with the longer mark on the tube, the adjustment is perfect for an uncovered object ; and when the coincidence is with the short mark, the proper distance is obtained to balance the aberrations produced by glass the hun- dredth of an inch thick, and such glass can be readily supplied. This adjustment should be tested experi- mentally by moving the milled edge, so as to separate or close together the combinations, and then bringing the object to distinct vision by the screw adjustment of the microscope. In this process the milled edge of FIG. 39. 66 THE MICROSCOPE. the object-glass will be employed to adjust for charac- ter of definition, and the fine screw movement of the microscope for correct focus. The graduations on the correction-collar are merely for convenience of registering the point of " best cor- rection " for particular objects, so that the objective may be set at the same correction if the observation has to be repeated. It is usual with amateurs, who have not practised themselves thoroughly in rapidly adjusting their objectives by inspection of the image, to note on their slides the best point of adjustment as well as the position of the object, either with reference to stage graduations or to Maltwood's finder. The registration of the position may save time in re- peating an observation ; but the registration of the best point of adjustment should, generally speaking, be regarded as an approximative process only, for the adjusting collar is seldom made so accurately that absolute reliance can be placed on the index. To obtain fine definition test the correction in both direc- tions, and take care to follow the image with the fine adjustment. With objectives of large aperture this process is of much importance, as the exact " distanc- ing " makes or mars the definition. Mr. Wenham recommends the following method of securing the most efficient performance of an object- glass. Select any dark speck or opaque portion of the object, and bring the outline into perfect focus ; then lay the finger on the milled head of the fine ad- justment, and move it briskly backwards and forwards in both directions from the first position. Observe the expansion of the dark outline of the object, both when within and when without- the focus. If the greater expansion, or coma, be when the object is without the focus, or farther from the objective, the lenses must be placed farther asunder, or towards the mark " un- covered." If the greater coma be when the object is within the focus, or nearer to the objective, the lenses must be brought closer together, or towards the mark "covered." When the object-glass is in proper ad- justment, the expansion of the outline is exactly the t TESTING THE OBJECTIVE. 67 same both within and without the focus. A different indication, however, is afforded by such test-objects as present (like the Podura-scale, the Diatoms, &c.) a set of distinct dots or other markings. If the dots have a tendency to run into lines when the object is without the focus, the glasses should be brought closer together; on the contrary, if the lines appear when the object is within the focal point, the glasses must be farther separated. The Podura-scale is an excellent test-object. The structure consists of a delicate transparent lamina or membrane, covered with an imbricated arrangement of epithelial plates, the length of which is six or eight times their breadth, and arranged like the tiles on a roof, or the long pile of some kinds of plush. The scales may be readily obtained by putting a live Podura into a small test-tube, and inverting it on a glass-slide; the insect should then be allowed for a time to leap and run about in the confined space. By this means the scales will be freely deposited on the glass ; and being subsequently trodden on by the insect, several will be found from which the epithelial plates have been partially rubbed off, and at the margin of the undisturbed portion the form and position of the plates may be readily recognized. Their structure ap- pears to be rendered more palpable by mounting the scales thus obtained in Canada balsam, and illuminating them by means of Wenham's parabolic reflector. The structure may also be very clearly recognized when the scale is seen as an opaque object under a Boss's -^ (spe- cially adjusted for uncovered objects), illuminated by a combination of the parabola and a flat Lieberkuhn. The under-side of the scale appears as a smooth glis- tening surface, with very slight markings, correspond- ing, probably, to the points of insertion of the plates on the contrary side. The minuteness and close proximity of the epithelial plates may account for their being found a erood test of definition, while their prominence renders them independent of the separating power due to larger aperture. The structure of another class of test-objects, the F 2 68 THE MICROSCOPE. diatomacene, differs entirely from that above described ; it will suffice for my present purpose to notice the valves of three species only of the genus Pleurosigma ; these, arranged in the order of easy visibility, are, P. formosum, P. hippocampus, P. angulatum. All appear to consist of laminae of homogeneous trans- parent silex, studded with dots, or rather protuberances, which in P. formosum and P. angulatum have a trian- gular arrangement, and in hippocampus a quadrangular. The " dots " have been described as depressions ; but if the frustule be broken the fracture is invariably observed to take place between the rows of dots, and not through them, as would naturally occur if the dots were depressions, for the substance is thinner there than elsewhere. This, in fact, is always observed to take place in the siliceous loricse of some of the border tribes that occupy a sort of neutral, and yet not undisputed, ground between the confines of the animal and vegetable king- doms ; as, for example, the Isthmia, which possess a reticulated structure, with depressions between the meshes, somewhat analogous to that which would result from pasting together bobbin-net and tissue- paper. The valves of P. angulatum, and similar objects, have been by some investigators supposed to be made up of two substances possessing different degrees of refractive power ; but this hypothesis is purely specula- tive, since the observed phenomena will naturally result from a series of rounded or lenticular protuberances of one homogeneous substance. Moreover, if the centres of the markings were centres of greatest density, if, in fact, the structure were at all analogous to that of the crystalline lens of the eye, it is difficult to conceive why oblique rays only should be visibly affected. When P. hippocampus or P. formosum is illuminated by a proper condenser, with a central stop placed under the lenses, and viewed by a quarter-inch object-glass of 70 aper- ture, both being accurately adjusted, we may observe in succession, as the object-glass approaches the object, first a series of well-defined bright dots ; secondly, a scries of dark dots replacing these ; and thirdly, the THE APEKTCRE OF THE OBJECTIVE. 60 latter again replaced by bright dots, not, however, so well defined as the first series. A similar succession of bright and dark points may be observed in the centre of the markings of some species of Coscinodiscus from Bermuda when viewed by transmitted light. These appearances would result if a thin plate of glass were studded with minute, equal, and equidistant plano-convex lenses, the foci of which would necessarily lie in the same plane. If the focal surface, or plane of vision, of the object-glass be made to coincide with this plane, a series of bright points would result from the accumulation of the light falling on each lens. If the plane of vision be next made to coincide with the sur- faces of the lenses, these points would appear dark, in consequence of the rays being refracted towards points now out of focus. Lastly, if the plane of vision be made to coincide with the plane beneath the lenses that contain their several foci, so that each lens may be, as it were, combined with the object-glass, then a second series of bright points will result from the accumula- tion of the rays transmitted at those points. Moreover, as all rays capable of entering the object-glass are con- cerned in the formation of the second series of bright focal points, whereas the first series being formed by the rays of a conical shell of light only, it is evident that the circle of least confusion must be much less, and therefore the bright points better defined in the first than in the last series. The Aperture of the Object-glass. The aperture of an objective has been, down to a comparatively recent period, the occasion of much controversy. It was contended that the aperture of a dry objective of 180 angle represented the largest aperture possible, that this could not be exceeded by any immersion objectives, the advantages of the latter resting solely upon the increase in light, through the absence of reflection at the surface of the front lens, and their greater working distance. The confusion into which the aperture question was brought by this contention, arose almost entirely from the fact that its supporters had not appreciated the 70 THE MICROSCOPE. proper definition of the term " aperture," bat had assumed it to be synonymous with what was known as " angular aperture." The angles of the pencils admit- ted by objectives cannot however serve as a measure of their apertures. When the medium in which they work is the same, as air, it is not the angles but the sines of those angles which enable the proper com- parison to be made, thus : if two dry objectives admit pencils of 60 and 180, their real apertures are not as 1 : 3, but as 1 : 2 only. When the media are different, as air, water and oil, the angles are still more misleading, as there may be three angles all with the same number of degrees, and yet representing entirely different aper tures. Whilst, however, those who insisted upon the increase of the apertures of objectives with the increase in the refractive index of the immersion fluid, were right in their contention, a somewhat similar lack of proper definition of the term aperture prevented the question being at that time effectually disposed of. The whole matter was, however, recently exhaustively dealt with in the course of a renewal of the " aperture question," before the Royal Microscopical Society, and in the papers of Professor Abbe (of Jena), and Mr. Crisp (Sec. R<. Micr. Soc.), printed in the journal of that society, 1 the subject of aperture will be found to be at last placed on a scientific basis. To follow the question in all its details, reference must be made to these papers, but a brief resume of the leading points will be found instructive and useful. The first essential step in the consideration of aper- ture is, as I have said, to understand clearly what is meant by the term. It will be at once recognized that its definition must necessarily refer to its primary meaning of " opening," and must, in the case of an optical instrument, define its capacity for receiving rays from the object, and transmitting them to the image. In the case of the telescope-objective, its capacity for receiving and transmitting rays is necessarily mea- (1) Journ. R. Micr. Soc., I. (1881), pp. 303-60 and pp. 388-123. THE APEKTUHE OF THE OBJECTIVE. 7] Eured by the expression of its absolute diameter or " opening." No such absolute measure can be applied in the case of microscope-objectives, the largest lenses having by no means the largest apertures, but being, in fact, found with the low powers, whose apertures are but small. The capacity of a microscope- objective for receiving and transmitting rays is, however, as will be seen, estimated by its relative opening, that is, its opening in relation to its focal length. When this relative opening has been ascertained, it may be regarded as synonymous with that denoted in the telescope by absolute opening. That this is so will be better appreciated by the following consideration : In a single lens, the rays admitted within one meri- dional plane evidently increase as the diameter of the lens (all other circumstances remaining the same), for in the microscope we have, at the back of the lens, the same circumstances as are in front in the case of the telescope. The larger or smaller number of emergent rays will therefore be measured by the clear diameter, and as no rays can emerge that have not first been admitted, this must also give the measure of the admit- ted rays. If the lenses compared have different focal lengths but the same clear " openings," they will transmit the same number of rays to equal areas of an image at a definite distance, because they would admit the same number if an object were substituted for the image ; that is, if the lens were used as a telescope-objective. But as the focal lengths are different, the amplification of the images is different also, and equal areas of these images correspond to different areas of the object from which the rays are collected. Therefore, the higher power lens with the same opening as the lower power, will admit a greater number of rays in all from the same object, because it admits the same number as the latter from a smaller portion of the object. Thus, if the focal lengths of two lenses are as 2 : 1, and the first ampli- fies N diameters, the second will amplify 2 N with the same distance of the image, so that the rays which are collected to a given field of I mm. diameter of the 72 THE MICROSCOPE. image are admitted from a field of N nim. in the first case, and of ^ mm. in the second. As the " opening " of the objective is estimated by the diameter (and not by the area) the higher power lens admits twice as many rays as the lower power, because it admits the same number from a field of half the diameter, and, in general, the admission of rays by the same opening, but different powers, must be in the inverse ratio of the focal lengths. In the case of the single lens, therefore, its aperture is determined by the ratio between the clear opening and the focal length. The same considerations apply to the case of a compound objective, substituting, however, for the clear opening of the single lens the diameter of the pencil at its emergence from the objective, that is, the clear utilized diameter of the back lens. All equally holds good whether the medium in which the objective is placed is the same in the case of the two objectives or different, as an alteration of the medium makes no difference in the power. Thus we arrive at a general proposition for all kinds of objectives: 1st, when the power is the same, the admission of rays (or aperture) varies with the diame- ter of the pencil at its emergence; 2nd, when the powers are different, the same aperture requires differ- ent openings in the ratio of the focal lengths, or conversely with the same opening the aperture is in inverse ratio to the focal lengths. We see, therefore, that just as in the telescope the absolute diameter of the object-glass defines its aperture, so in the micro- scope the ratio between the utilized diameter of the back lens and the focal length of the objective defines its aperture also, and this is clearly a definition of aperture in its primary and only legitimate meaning as " open- ing ; " that is, the capacity of the objective for admit- ting ravs from the object and transmitting them to the image. If, by way of illustration, we compare a series of dry and oil-immersion objectives, and commencing with small air angles, progress up to 180 air angle, and then take an oil -immersion of 82 and progress 180 Oil Angle. (Numerical Aperture T52.) NUMERICAL APERTURE. 73 again to 180 oil angle, the ratio of opening to power progresses also, and attains its maximum, not in the case of the air angle of 180 (when it is exactly equiva- lent to the oil angle of only 82), but is greatest at the oil angle of 180. If we assume the objectives to have the same power through- out we get rid of one of the factors of the ratio, and we have only to compare the dia- meters of the emergent beams, and can represent their relations by diagrams. Fig. 40 illus- trates five cases of different aper- tures of J - in. objectives, viz. : those of dry ob- jectives of 60, 97, and 180 air angle, a water- immersion of 180 water angle, and an oil-immersion of 180 oil angle. The inner dotted circles in the two latter cases are of the same size as ,i , j of various dry and mmerson that correspond- game power ( |. in-) from an air angi* of 60 to an ing to the 180 oil angle of 180. air angle. A dry objective of the maximum air angle of 180 is only able to utilize a diameter of back lens equal to twice the focal length, while an immersion lens of even only 100 utilizes a larger diameter, i.e., it is able to O 180 Water Anglo. (Numerical Aperture T33.) 180 Air Angle. 96 Water Angle. 82 Oil Angle. (Numerical Aperture I'OO.) 97 Air Angle. (Numerical Aperture 75.) 60 Air Angle. (Numerical Aperture '50.) FIG. 40. Relative diameters of the (utilized) back lenses of various dry and immersion objectives of the 74 THE MICROSCOPE. transmit more rays from the object to the image than any dry objective is capable of transmitting. When- ever the angle of an immersion lens exceeds twice the critical angle for the immersion fluid, i.e., 96 for water or 82 for oil, its aperture is in excess of that of a dry objective of 180. This excess will be seen if we take an oil-immersion objective of, say 122 balsam-angle, illuminating it so that the whole field is filled with the incident rays, and use it first on an object not mounted in balsam, but dry. We then have a dry objective of nearly 180 angular aper- ture, for, as will be seen by reference to fig. 41, the cover-glass is virtually the first surface of the objective, as the front lens, the immersion fluid, and the cover- glass are all approximately of the same index, and FRONTLENS ____^__ IMMERSION FLUID DBJUCTINMR __r^jr^ni COVE* CLASS SLIDE FIG. 41. form, therefore, a front lens of extra thickness. When the object is close to the cover-glass the pencil radiating from it will be very nearly 180, and the emergent pencil (observed by removing the eye-piece) will be seen to utilize as much of the back lens of the objective as is equal to twice the focal length, that is the inner of the two circles at the head of fig. 40. If now balsam is run in beneath the cover-glass so that the angle of the pencil taken up by the objective is no longer 180, but 122 only (that is, smaller), the diameter of the emergent pencil is larger than it was before, when the angle of the pencil was 180 in air, and will be approximately represented by the outer circle of fig. 40. . As the power remains the same in both cases, the larger diameter denotes the greater NUMERICAL APERTURE. 75 aperture of the immersion objective over a dry objective of even 180 angle, and the excess of aperture is made plainly visible. Having settled the principle, it is still necessary, however, to find a proper notation for comparing apertures. The astronomer can compare the apertures of his various objectives by simply expressing them in inches, but this is obviously not available to the micro- scopist, who has to deal with the ratio of two varying quantities. In consequence of a discovery made by Professor Abbe in 1873, that a general relation existed between the pencil admitted into the front of the objective, and that emerging from the back of the objective, he was able to show that the ratio of the semi -diameter of the emergent pencil to the focal length of the objective could be expressed by the formula n sin u> i.e., by the sine of half the angle of aperture (u) multiplied by the refractive index of the medium (n) in front of the objective (n being I'O for air, 1'33 for water, and 1*52 for oil or balsam). When, then, the values in any given cases of the expression n sin u (which is known as the " numerical aperture") has been ascertained, the objectives are instantly compared as regards their aperture, and, moreover, as 180 in air is equal to I'O (since %=1'0 and the sine of half 180=1'0) we see, with equal readiness, whether the aperture is smaller or larger than that corresponding to 180 in air. Thus, sup- pose we desire to compare the apertures of threo objectives, one a dry objective, the second a water immersion, and the third an oil immersion; these would be compared on the angular aperture view as, say 74 air angle, 85 water angle, and 118 oil angle, so that a calculation must be worked out to arrive at the actual relation between them. Applying, however, the " numerical " notation, which gives '60 for the dry objective, '90 for the water immersion, and 1'30 for the oil immersion, their relative apertures are immediately recognized, and it is seen, for instance, that the aperture of the water immersion is some- 'O TEE MICROSCOPE. what less than that of a dry objective of 180*, and that the aperture of the oil immersion exceeds thau of the latter by 30% The advantage of immersion, in comparison with dry objectives, is also at once apparent. Instead of consisting merely in a diminution of the loss of light by reflection or increased working distance, it is seen that a wide-angled immersion objective has a larger aperture than a dry objective of maximum angle, so that for any of the purposes for which aperture is essential an immersion must necessarily be preferred to a dry objective. That pencils of identical angular extension but in different media are different physically, will cease to appear in any way paradoxical if we recall the simple optical fact that rays, which in air are spread out over the whole hemisphere, are in a medium of higher refractive index such as oil compressed into a cone of 82 round the perpendicular, i.e., twice the critical angle. A cone exceeding twice the critical angle of the medium will therefore embrace a surplus of rays which do not exist even in the hemisphere when the object is in air. The whole aperture question, notwithstanding the in- numerable perplexities with which it has hitherto been surrounded, is in reality completely solved by these two simple considerations : First, that " aperture " is to be applied in its ordinary meaning as representing the greater or less capacity of the objective for receiving and transmitting rays ; and second, that when so applied the aperture of an objective is determined by the ratio between its opening and its focal length ; the objective that utilizes the larger back lens (or opening) relatively to its focal length having necessarily the larger aperture. It would hardly, therefore, serve any useful purpose if we were here to discuss the various erroneous ideas that gave rise to the contention that 180 in air must be the maximum aperture. Amongst these was the sugges- tion that the larger emergent beams of immersion objectives were due to the fact that the immersion THE IMMERSION APERTURE. 77 fluid abolished the refractive action of the first plane surface which, in the case of air, prevented there being any pencil exceeding 82 within the glass. Also the very curious mistake which arose from the assumption that a hemisphere did not magnify an object at its centre because the rays passed through without refraction. A further erroneous view has, however, been so wide- spread that it will be desirable to devote a few lines to it, especially as it always seems at first sight to be both simple and conclusive. If a dry objective is used upon an object in air as in fig. 42, the angle may approach 180, but when the object is mounted in balsam as in fig. 420, the angle at the object cannot exceed 82, all rays outside that limit (shown by dotted lines) being reflected back at the cover-glass and not emerging into air. On using an immersion objective, however, the immersion fluid which replaces the air above the cover-glass allows the rays former- ly reflected back to pass through to the objec- tive so that the angle at the object may again be nearly 180 as with the dry lens. The action of the immersion objective was, therefore, supposed to be simply that it repaired the loss in angle which was occasioned when the object was transferred from air to balsam, and merely restored the conditions existing in fig. 430 with the dry objective on a dry object. As the result of this erroneous supposition, it fol- lowed that an immersion objective could have no advantage over a dry objective, except in the case of the latter being used upon a balsam-mounted object, its aperture then being (as was supposed) " cut down." The error lies simply in overlooking the fact that the rays which are reflected back when the object is mounted in balsam (fig. 420) are not rays which are found when the object is in air (fig. 42), but are additional and different rays which do not exist in air, as they cannot be emitted in a substance of so low a refractive index. 78 THE MICROSCOPE. Lastly, it should also be noted that it is numerical and not angular aperture which measures the quantity of light admitted to the objective by different pencils. First take the case of the medium being the same. The popular notion of a pencil of light may be illus- trated by fig. 43, which assumes that there is equal intensity of emission in all directions, so that the quantity of light contained in any given pencils may be compared by simply comparing the contents of the solid cones. The Bouguer-Lambert law, however, shows that the quantity of light emitted by any bright point varies with the obliquity of the direction of emission, being greater in a perpendicular than in an oblique direction. The rays are less intense in proportion as they are more inclined to the surface FIG. 43. FIG. 43a. which emits them, so that a pencil is not correctly represented by fig. 43, but by fig. 43, the density of the rays decreasing continuously from the vertical to the horizontal, and the squares of the sines of the semi-angles (i.e., of the numerical aperture) constitut- ing the true measure of the quantity of light con- tained in any solid pencil. If, again, the media are of different refractive indices, as air (TO), water (T33), and oil (1'52), the total amount of light emitted over the whole 180 from radiant points in these media under a given illumination is not the same, but is greater in the case of the media of greater refractive indices in the ratio of the squares of those indices (i.e., as TO, 177 and 2'25). The quan- tity of light in pencils of different angle and in different media must therefore be compared by squaring the THE IMMERSION APERTURE. 79 product of the sines and the refractive indices, i.e. (n Sin w 2 ), for the square of the numerical aperture. The fact is therefore established that the aperture o{ a dry objective of 180 does not represent, as was sup. posed, a maximum, but that aperture increases with the increase in the refractive index of the immersion fluid ; and it should be borne in mind that this result has been arrived at in strict accordance with the ordinary propositions of geometrical optics, and without any reference to or deductions from the diffraction theory of Prof. Abbe. We have, however, still to determine the proper function of aperture, immersion objectives of large aperture excelling, as is well known, any dry objective in the delineation of minute structures. The old explanation of the increased power of vision obtained by increase of aperture was, that by the greater obliquity of the rays to the object " shadow effects " were produced, a view which overlooked the fact, first, that the utilization of increased aperture depends not on the obliquity of the rays to the object, but to bhe axis of the microscope ; and secondly, that just as there is no acoustic shadow produced by an obstacle, which is only a few multiples of the length of the sound waves, so there can be no shadow produced by minute objects which are only a few multiples of the light waves, the latter then passing completely round the object. The Abbe diffraction theory, however, supplies the true explanation, and shows that the increased performance of immersion objectives of large aperture is directly connected (as might havt been anticipated) with the larger "openings " in the proper sense of the term, which, as we have just seen, such objectives necessarily possess. It has also been shown, then, in order that the image should exactly correspond with the object, all the diffracted rays must be gathered up by the objective. If any ar^ lost we then get not an image of the real object but a spurious one. Now, if we have a coarse object, the diffracted rays are all comprised within a narrow cone round the direct beam, and an objective of small 80 THE MICROSCOPE. aperture will transmit them all. With a minute object, however, the diffracted rays are widely spread out so that a small aperture can admit only a fractional part to admit the whole or a very large part, and consequently to see the minute structure of the object, or to see it truly, a large aperture is necessary, and in this lies the value of aperture and of a wide-angled immersion-objective for the observation of minute structures. Object-glasses. With regard to the selection of object-glasses, this will depend on the work in which we may be about to engage. The amateur or student will be well advised to commence with moderate or even low powers, as a 3-inch, a 2-inch, a 1-inch, and a f -inch focus. These powers used with the A eye-piece will give a range of magnification of from 20 to 70 diameters ; and with the B eye-piece will be increased to 120 diameters. Zeiss, of Jena, has lately constructed a very useful adjustable objective; by an ingenious screw-collar arrangement the relative position of the front lens is changed, and a range of power, varying from 12 to 24, or 30 diameters, is obtained. This object-glass consists of a convex back lens and a con- cave front lens (both achromatic), the distance of which is changed by means of a screw acting in the manner of a correcting collar of wide range. When the collar is put to 10, the objective has its minimum focal length or maximum power, approximately corre- sponding to that of a single 2-inch lens. By closing the collar the equivalent focal length increases, the back lens is made to approach nearer the eye-piece, and the magnifying power is varied, so that when the collar is put to the actual power of the objective corresponds to that of a 4-inch lens. By a judicious use of the draw-tube of the microscope a further mag- nification of the image can be obtained, which is of value if botanical sections, opaque objects, and whole insects are under examination. With an erecting eye- piece the lower powers above mentioned are most useful for dissection purposes. Objectives of medium power are the |-inch, 4-10ths, THE SELECTION OF OBJECT-GLASSES. 81 J and Jth, with a magnification ranging from about 125 to 250 diameters with the A eye-piece, and increasing with the b eye-piece to 375 diameters. I have in my possession a fine J made for me by Dallmeyer, with an aperture of 120. It bears an extra deep eye-piece, and will then give a magnification of 1,000 diameters in every way satisfactory. It also works through almost any thickness of cover-glass ; its aberrations being equally well balanced for uncovered objects, no mean test of a good objective. These several points prove that its working aperture has been brought to the maximum of utility. On the whole, the power is one of considerable value for the investi- gation of organized structure and for viewing living action. Every one aiming at original observations upon the morphology of living creatures should become skilled in the use of high magnifying powers, as the -J, 7*05 T 1 2 > "iV an ^ "sV- -^ nave > however, always pointed to the futility of constructing higher power object-glasses, except with a proportionately increased width of aper- ture. As the maximum angle appears to be 180, or 160, for the odd 20 are not worth the having (compare the chords of 180 and 160 there is hardly any differ- ence), and as a T ^ can be made to transmit an angle of 160, I maintain that it will possess as much resolving power as any dry -fa or -g^th. 1 This is seen in the series of wonderful photographs of muscular tissue, blood corpuscles, etc., taken by Dr. Woodward, of Washington, U.S., and which certainly prove that the photographic eye sees after making every allowance for what is due to the nature and undulations of light what the human eye cannot see. The Immersion System. About fifty years ago Amici demonstrated the value of a drop of water in- serted as an adjustable film between the object and the objective, and showed that it materially assisted in (1) Professor Abbe has shown that no objective can possess at the same lime penetrating power and perfect definition. The practical outcome of this observation is, that neither penetrating objectives nor defining objectives Rie aloi.e sufficient for every kind of microscopical work. Both are neces- sary. If, however, the student is limited in the number of his objectives, then he should at least provide himself with a low-power defining lens, |-inch Ross, and high-power penetrating immersion. G 82 THE MICROSCOPE. balancing certain uncorrected aberrations. Soem- mering, writing of one of Arnici's microscopes, ob- serves : " The magnifying power, admirable precision, and clearness with which the object is seen, seems to me quite astonishing." It is not difficult to perceive that, this optician's method of connecting the objective with the cover-glass of the object by means of a drop of water should diminish the reflection which takes place in the incidence of oblique light when the dry objective is used. The limiting angle of refraction of water being nearly 48, it follows that whatever the degree of obliquity in the incident light falling on the object, the immersion lens can never have to deal with rays of greater obliquity than 48. To this circumstance, as well as to the greater number of parallel rays brought to a focus, and to the increased angle of aperture is due the greater clearness and pre- cision and sharpness of the image. The film of water not only furnishes increased angle of aperture, but it also collects the straying away peripheral rays of light, and sends them on to the eye-piece, to assist in render- ing the image more perfect ; becomes, indeed, an in- tegral part of the optical system, and very materially aids in the removal of residuary secondary aberrations. The water-immersion system was warmly advocated and fully developed by continental makers Hartnack, Merz, Nachet, and others long before English opticians could be persuaded to acknowledge its advantages. Messrs. Powell and Lealand were the first opticians who made a ^-inch and a |-th objective, which, by a mere change of the front lens, could be used either as a wet or dry lens. The immersion principle has recently been still further developed. The substitution of oil for water was first proposed by Amici, in 1844, who abandoned it as it seemed unmanageable and without correspond- ing advantages as compared with water, an opinion shared by Oberhauser and Wenham. At this time, however, it was supposed that the chief gain of the immersion consisted in a diminished loss by reflection at the front lens and an increase of working distance ; THE IMMERSION SYSTEM. 83 it was not recognized that additional aperture was also obtained. In 1877 Mr. J. W. Stephenson demonstrated that as the aperture of an objective increased with the increase in the refractive index of the immer- sion fluid great practical advantage would result from using instead of water a homogeneous fluid; that is, one not merely of the same refractive index, but also of the same dispersive power as the glass of the front lens of the objective. This sugges- tion was immediately acted upon by Professor Abbe, and in December, 1877, the, first objective on the new system was issued from Zeiss's workshop, giving an in- crease in aperture of upwards of 50 per cent, over a dry objective of equal angle. In addition to increase of aperture, the use of a homogeneous fluid gives a pre- viously unlooked-for advantage that it is possible to correct a "homogeneous immersion" objective with more facility than one which works in such media as air and water, both of which differ considerably in refrac- tive and dispersive power from the glass of the lenses. With air, or even water objectives, there is a large amount of aberration affecting the pencils on their passage from the radiant to the medium of the front lens, which bears a considerable ratio to the total spherical aberration within the objective, and in the case of wide angles increases disproportionately from the axis outwards. This can only be corrected by a rough method of balancing, that is, by introducing an excess of opposite aberration at the posterior lenses. An uncorrected residuum, rapidly increasing with larger apertures, is then left, and this appears in the image amplified by the total power of the objective, so that with a non-homogeneous medium there is a maximum angular aperture which cannot be surpassed without undergoing a perceptible loss of definition, provided working distance is required. If we abolish the an- terior aberration for all colours, by an immersion fluid which is equal to crown-glass in refractive and dis- persive power, the difficulty will be at once overcome. If, for instance, we have an objective of 140 in glass (= 1*25 N.A.) and water as the immersion fluid, the G 2 84 THE MICROSCOPE. aberration in front would affect a pencil of 140*. Sub. stituting a homogeneous medium, the same pencil, contracted to the equivalent angle in that medium of 112, will be admitted to the front lens without any aberration, and may be made to emerge from the curved surface also without any detrimental aberration, but contracted to an angle varying from 70 to 90. The first considerable spherical aberration of the pencil then occurs at the anterior surface of the second lens, where the maximum obliquity of the rays is already con- siderably diminished. Figs. 44 and 44# will serve to further elucidate this. If the objective of 140 works with water (fig. 44), there would be a cone of rays extending up to 70 on FIG. 44. FIG. 44a. both sides of the axis, and this large cone would be submitted to spherical aberration at the front surface a. But with homogeneous immersion (fig. 440) the whole cone of 112 is admitted to the front lens without any aberration, there being no refraction at the plane sur- face ; and as the spherical surface of the front lens is without notable spherical aberration, the incident pencil is brought from the focus P to the conjugate focus /, and contracted to an angle of divergence of 70 90 without having undergone any spherical aberration at all. Thus the problem of correcting a very wide-angled objective is reduced by the homogeneous- immersion THE IMMERSION SYSTEM. 85 system, both in theory and in practice, to the prob- lem of correcting an objective having a moderate air angle. 1 The adoption of the Homogeneons-immersion system is at present warmly advocated by all opticians. Messrs. Powell and Lealand take the lead with their new formula |, which, illuminated by their oil immer- sion-illuminator, will resolve the most difficult test- objects. This objective has an aperture, measured in crown-glass, of 150,= 1*47 N.A. "the widest aperture yet produced." The same firm have constructed a -^ 5 and a -fj on the homogeneous system, but the apertures are not so high as the ^th. " By the homogeneous- immersion formula adopted by Powell and Lealand 2 the focal distance is practically a constant quantity, and it follows that reduction of aperture by making the front lens thinner ensures a much greater working distance without affecting the aberrations ; for as the first refraction takes place at the posterior or curved surface of the front lens, the removal of any portion of thickness at the anterior or plane surface simply cuts off zones of peripheral rays without altering the dis- tance any space being filled up by the homogeneous- immersion fluid, or by an extra thickness of cover- glass. " By applying an extra front lens to the back construc- tion of such a -j^ th, the observer is enabled to view an object through a cover-glass that would be practically a maximum thickness for a -Jth (aperture =90) con- structed on the usual formula where the setting en- croaches on the active spherical refracting surfaces. A second front might give a high average aperture for a -j^- (115), while the thickest front representing the maximum aperture of the whole construction (142) would enable the observer to view an object with a greater aperture than has hitherto been obtained with any T V* n > owing to difficulties of construction, and through a thicker cover-glass than a y^th of the same (1) See Prof. Abbe " On Stephenson's System of Homogeneous Immersion for Microscope Objectives," Journ. R. Micr. Soc., II. (1S79), p. 256, and on 'The Essence of Homogeneous Immersion ." Ibid., I. (1881), p. 13L, (2) Jovial of the R. M. S., Vol. III., p. 1050 (1880;. 86 THE MICROSCOPE. aperture will admit of ; x hence three different fronts would give a great range of aperture with a correspond- ing working distance, which is practically what is sought by having objectives constructed of the threa different foci, |, T V, and -5^-." " There can be no doubt," adds the writer, " English Mechanic," "that the development of the homogeneous- immersion system, is the problem of the future as regards attaining the limit of resolution with the micro- scope." But to ensure the fullest advantages the system is capable of, a fluid is wanted which will meet all its requirements. Professor Abbe, Mr. Stephenson and others have experimented on various substances, and the con- clusion come to is that the essential oils of cedar wood, or fennel, which so nearly correspond to glass in their refractive and dispersive power, will be found to afford the best results. Oil of aniseed, chloral hydrate and glycerine, various turpentines, and lead solutions have been tried and ultimately abandoned. One precaution is required with regard to all essential oils, they can only be used with objectives having their fronts specially cemented. Very fine objectives constructed on the homogeneous immersion system by Zeiss, and requiring no adjusting collar, may be had of Baker, Holborn, the London agent of this optician. THE CONSTRUCTION OF THE MICROSCOPE. THE MICRO- SCOPE STAND. The Principal Forms of Microscopes. Having duly considered the essential parts of the compound achro- matic microscope, I shall proceed to offer a few re- marks on typical forms of English manufacture. I must not attempt a critical examination of the very numerous stands known to practical microscopists. This would be impossible in a limited space. Neither shall I attempt to institute invidious comparisons, as, in my opinion, most forms of instruments brought to notice possess some feature of a useful and praise- (1) Powell and Lealand have also constructed a V and 53 on the homo gcncons system, but the apertures are nots/> high as in the $ just mentioned THE ROSS-ZENTMAYER STAND. 87 FIG. 45. The Improved Ross-Zentmai/er Model. 00 THE MICROSCOPE. worthy character. My observations will therefore be almost exclusively confined to points of excellence in workmanship, to mechanical difficulties successfully overcome, and new forms introduced since the publica- tion of my ninth edition. The Improved Koss-Zentmayer Microscope is a thoroughly substantial and practical instrument, com- bining elegance of appearance with facility in the attainment of everything the microscope can at present be expected to accomplish. The stand is on the well-known Jackson model, with extra wide slides for the rack-and-pinion movement. The slow movement is obtained by a second slide close behind the first, but to avoid the friction of rub- bing surfaces, hardened steel rollers are inserted between them, which gives a frictionless fine motion, amenable to the slightest touch of the milled-head screw, situated conveniently at the back of the limb, through which a steel lever passes which actuates the slow motion slide. The body of the instrument is therefore not touched during the fine focussing, so that all lateral movement is avoided. The mechanical stage is made to rotate axially, and the outer edge of the lower plat? is divided into degrees, in order to register the angles, and a simple mode of adjustment is provided, for setting the centre of rotation exactly coincident with the focal point of the object-glass. As the plates of the stage have no screw or rack work between them (these being placed externally), they are brought close together, giving the advantage of a very thin sub- stantial stage, and ensuring rigidity where required ; phosphor bronze being used in its construction. The stage is attached to the limb by a conical stem, with a screw and clamp nut at the back, so that it can be easily removed for the substitution of a simple plate, or other stage, and by turning the stem in the socket the stage may be tilted sideways at any angle required. One special feature in the Ross-Zentmayer stand is a swinging sub- stage and bar, carrying the mirror, having its axis of rotation situated from an axial point in the plane of the object, which conse- THE ROSS-ZENTMAYER STAND. 89 quently receives the light without requiring altera- tion of focus in any position of the bar ; by this means facilities are afforded for the resolution of objects requiring oblique light and for the development of their structure. Rays are thus obtained in the readiest manner possible from any angle, which is indicated by a graduated circle round the eye or top of the swing- bar, and many troublesome and expensive pieces of sub-stage apparatus, before used as specialities for ob- taining oblique illumina- tion, are dispensed with. The value of this arrange- ment was recognized, as I have already stated, in Grubb's "Sector Stand," the movement of which was obtained in a far less efficient manner. Costly high - angled condensers may be dispensed with in Ross's microscope, and simple arrangements used in their place, as Wen- ham's immersion disc, or the hemispheric lens. A 1^-inch or 2-inch object- glass will generally suffice for a condenser. This and other lenses can also be used for opaque objects, by bringing the swing arm and holder round above the stage, which it clears in rotation. The base or stand of the Ross-Zentmayer instrument is sometimes made in one piece, but preference will, I believe, be given to the double pillar support, as this is very firm, and allows the sub-stage to swing free, while the microscope is in a vertical position, as in working with fluid preparations. The rim of the sub-stage is provided with set screws for centring the lenses used, and, when determined, can be secured by a clamping screw. FIG. 46. The Ross-Zentmayer Student's Microscope. -90 THE MICROSCOPE. The sub-stage, with its apparatus in place, can be instantly removed, by being drawn out sideways, so as to use the mirror alone, which is a great convenience. The mechanical movements of this instrument are perfect, and well adapted to their purpose, and the excellence of the workmanship is such as might have been expected from the long-established reputation of the house of Ross. The Ross-Zentmayer Student's Stand (fig. 46) is a useful instrument on a small scale, possessing all the advan- tages of a larger microscope. It has an excellent fine adjustment, the milled-head for working which is in as convenient a place as that of more expensive stands. It is not so costly as more pretentious instruments, .a consideration often of importance to the student of the collateral sciences. Messrs. Ross make a very good and cheap series of object-glasses for histological work, especially adapted for use with this instrument. The general plan of Powell and Lealand's Compound Microscope is represented in fig. 47. The tripod-stand gives a firm 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 move- ment set in action by the double- milled head, whereby the coarse adjustment of the focus is obtained. To the upper part of the triangular bar a broad arm is fixed, bearing the compound 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 triangular 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 .stops when central. The stage has been improved in construction, having vertical, horizontal, and circular movements, and graduated for the purpose of register- ing 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 POWELL AND LEALAND'S STAND. 91 stage is firm and 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 movable by rack and pinion ; into this bar slides the sub-stage, having vertical and horizontal motions for centring, and also a circular motion ; the sub- stage carries the various FIG. 47. Powell and LealancVs Microscope, with Amid prism arranged for oblique illumination. appliances for underneath illumination, removed in the woodcut. The achromatic condenser is seen detached. Powell and Lealand have another pattern, larger and more massive in its general arrangements. The construction of the stage and sub-stage differ some- what ; both resting 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 92 THE MICROSCOPE. 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 graduation of a circle of gun metal, which is borne on the upper surface of the ring. The rotatory action of the stage being effected beneath the traversing movement, the centring of an object brought into the axis of the microscope is not disturbed by it ; and the workmanship is so accu- FIG. 48. Poicell and Zealand's Microscope arranged for direct illumination. A. Secondary Stage racked up to bring the Achromatic Condenser close to the object. rate that the stage may be driven through its whole revolution without throwing out of the field an object viewed with the T Vth objective. The stage withal is made thin enough to admit of the most oblique light being thrown on the object. 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 Lealand ; a student's microscope, with |^-inch stage BECK'S POPULAR STAND. 93 movement, coarse and fine adjustments to body, plane and concave mirrors, revolving diaphragm, Lister's dark-wells, and two eye-pieces, for 8. An increasing demand for good, useful, and moder- ately priced instruments for students and general use, has had the effect of inducing makers to vie with each other in their endeavours to give a better class micro- scope for a small sum. Among those manufacturers, and to whom the microscope owes very many improve- ments, I may mention the firm of R. and J. Beck. Their Popular Microscope, fig. 49, is a fair example of their excellent workmanship. The body, A (in the illustration shown as binocular), is carried by a strong arm, B ; and this is attached to a square bar, c, which may be moved up or down by a rack- work and pinion in the lower part of the stand, where the stage, D, and the mirror, E, are attached. The base, F, is triangular ; and it is connected with the parts of the instrument already described by a broad stay, G, which moves on centres at the top and bottom, so as to allow the end of the tube, H, to fit by its projecting pin into various holes along the medial line of the base. With this arrangement, if the body of the microscope be required in a more or less inclined position as in the figure, four holes are provided near the extremity of the base for the pin of the tube to fit into. A hole near the stout pin, L, is used when a vertical position is wanted ; while to obtain the hori- zontal position, the pin of the tube is placed in a hole in the stud, K, the inner surface of the stay, G, resting at the same time on the top of the stout pin, L. This form of construction is novel, and possesses the fol- lowing advantages ; it is strong, firm, and yet light ; the instrument cannot alter from any particular incli- nation it is put into, which is not unfrequently the case when the ordinary joint works loose; and in every position the heavier part of the stand is brought over the centre of the base, to ensure an equality of balance. To adjust the focus of the object-glass, turn the milled-heads for a quick movement, or the milled- head p for a slow one. 94 THE MICROSCOPE. The stage, D, is circular, and upon it fits a plate, T ; FIG. 49.- Beck's Popular Microscope. this again carries the object-holder, u, which is pro- BECK'S " IDEAL" STAND. 95 vided with a ledge, v, and a light spring, w ; it is held on the plate T by a spring underneath, so that it can be moved about easily and smoothly by one or by both. FIG. 50. Beck's "Ideal" Microscope. hands. Messrs. Beck's latest improvement consists in a glass friction stage, and this is adapted to all their Students' microscopes. A polished glass plate is firmly embedded in the brass stage-plate, and there held in 96 THE MICROSCOPE. contact by a strong wire ring. A piece of velvet cemented to the under sur- face renders the stage move- ments extremely smooth and easy. The mirror, E, besides swinging in a rotating semi- circle, will slide up or down the tube, H, or it will turn on either side for oblique illumi- nation. Beck's "Ideal" Microscope. In this excellent and novel instrument (shown as a monocular in fig. 50) . 51. Beck's Double Nose-piece. FIG. 52. Baker's Student's Microscope. the stage is of very thin and stiff brass, with a large BAKER'S STUDENT'S STAND. 97 opening, and provided with reversible spring clips so as to attach an object to the underside if required. To the stage can be adapted either a circular stage- plate of thin sheet-brass revolving concentrically, or the glass stage-plate shown detached, with brass object-carrier, and allowing one inch of movement in all directions. The mirror and sub-stage slide FIG. 53. Baker's Model Histological Microscope, upon a swinging tailpiece, the latter being attached to a graduated circle, and allowing a wide range of motion above and below the stage. The body draws out to the standard length (10 inches), and takes a full- sized eye-piece. It has an adapter for the broad-gauge screw. When fully extended it is 15 inches in height, or can be reduced to 10 inches. 9b THE MICROSCOPE. The working microscopist will find Beck's double or triple nose-piece (fig. 51) a useful addition and an econo- mizer of time, as enabling him to find a minute object, which he may wish to submit to a more thorough examination under a high power. Mr. Baker (Holborn) has kept pace with most opti- cians, and his first-class microscopes are not inferior to those of any other manufacturer. One of his best, the FIG. 54. Baker's Stephenson's Binocular Dissecting Microscope. old Ross form, combines good workmanship with soli- dity and completeness in most of its details. The Student's Microscope (fig. 52) is a well-finished in- strument, with quick and slow motions, circular rotat- ing stage, live-box, stage and dissecting forceps, packed in mahogany case. It has a universal sub-stage fitting, capable of receiving all accessories, and being provided with good object-glasses and other apparatus, is cer- tainly a very complete and useful microscope. Baker's Model Histological Microscope (fig. 53), a highly-finished instrument, having sliding body, micro- BROWNING'S ROTATING STAND. 99 meter screw fine-adjustment, glass concentric rotating stage, double mirror, universal sub-stage fitting to carry FIG. 55. Broicning's Improved Rotating Microscope. all apparatus, and one eye-piece, packed in mahogany case, for the small price of 3 10s. An excellent form of binocular, in which the lowest H 2 100 THE MICROSCOPE. and the highest objective can be used with equal advantage, has been adapted by Mr. Baker to a micro- scope stand (fig. 54), well suited for laboratory use, and for many practical purposes. It is superior to the ordinary dissecting microscope; the erecting principle renders it generally serviceable, as, for example, in selecting and arranging shells, and in the proper dis- position of specimens in the process of mounting. The double bodies are attached by an arm to a rack- work of unusual length, suspended, as it were, over a stage 6 in. X 3 in. in the horizontal position ; it has the usual double mirror of large size, the whole being sup- ported by three solid uprights to a heavy base. The figure scarcely does the instrument justice ; it fails to show the second body. Browning's (63, Strand) Rotating Microscope (fig. 55) is well adapted for pathological work. It has a circu- lar glass sunk in to the stage, and is consequently not likely to be damaged by moist preparations. The utility of a turning-stage, as already explained, gives the com- mand of varying the position and illumination of the object to be examined. In the construction of this in- strument this fact has been kept in view; the stage and the eye-piece revolving together on thp same axis ; and the image remaining in the field of vision during the whole revolution. The microscope stand aims at com- bining, in the simplest and least expensive form, the high qualities of the best English models. Browning's Complete Binocular has a well-finished stand, with the latest improvements, mechanical mo- tions to stage, secondary stage, with removable fittings, etc., and is, in every respect, a complete instrument. Watson's Microscope Stand (fig. 56) presents points of novelty, the most notable among which is the inclining motion of the limb, carrying the optical body and stage on an axis in a line with the object on the stage. By the simple inclination of the limb, varying effects of oblique illumination can be obtained direct from the mirror, which can be attached for this purpose to the centre of the base, and is then independent ot the inclination of the limb. WATSON'S NEW STAND. 101 The base of the stand is circular, with three project- ing claws ; on this base a disc of metal, carrying the pillar-support (of the limb, stage, etc.), is made to FIG. 56. Watson and Son's New Microscope Stand. rotate on the perpendicular optic axis ; a graduated zone shows the angle of rotation. In the centre of the base a smaller disc (projecting slightly above the general plane) is made to rotate ; this disc has a groove into which the mirror-fitting 102 THE MICROSCOPE. slides, and a spring-notch shows the axial position. The sliding fitting allows the mirror to be placed some distance out of the axis radially, and then the rotation of the circular moving base plate gives a considerable range of obliquity of light in azimuth ; the light from the mirror remaining constantly directed upon the object, this facility obtains with all inclinations of the limb and stage, because the object itself forms the centre both of the azimuthal rotation and of the inclination in altitude. The limb is mounted in a " cradle " joint, at the top of the pillar, permitting inclination from the perpendicular. The angle of inclination is registered upon a gradu- ated ring against the clamping screw. The optical body is mounted, not as usual on the front of the " Jackson " limb, but on the side of it; thus converting the side of the limb into the front. The coarse-adjustment is by the ordinary rack and pinion ; the fine- adjustment lifts the optical body in a separate slide-fitting by means of a wedge-shaped block acted upon by the conical end of a fine micrometer- screw. The focal distance can be measured by the scale engraved on the slide-fitting. The stage is of the newest construction, and beneath which is the sub-stage arm, carrying the usual screw- centring and rack-adjusting sub-stage, so attached to a sector in the rear of the stage in which it traverses concentric with the object. The sub-stage bar also carries the usual centring fitting for condenser, etc., and swings forwards or backwards concentric with the object on the main stage, and the obliquity of the swing can be registered on a graduated ring imme- diately behind the stage. The construction is similar to that of the Ross-Zentmayer. An extra swinging bar is attached behind, into which the mirror can be slid for use in combination with the condenser, etc. It should be noticed that there is a third divided circle on the sub- stage sector giving the inclination of the sub-stage to the axis of the body. A strong clamp ou the other side of the cradle joint holds the body firmly at any inclination, and a graduation on the slide of WATSON'S COLLEGE STAND. 103 the coarse-adjustment enables the working distance of objectives to be measured and compared. Watson's Medical or College Microscope (fig. 57) i s an economical form of instrument, having a sliding body for coarse- adjustment, fine-adjustment, draw-tube*, wheel of diaphragms, tube -fitting for under-stage appa- ratus, plane and concave mirrors, with one eye-piece, FIG. 57. Watson's Medical or College Microscope. J-inch and 1-inch objectives, and stand condenser. It is well adapted for class or laboratory use. The maker strongly recommends it for the use of the brewer, and amateur, as a cheap and useful instrument. Mr. Pillischer (New Bond Street) is favourably known for the excellency of his instruments. His No. 1 Microscope (fig. 58) is of good workmanship, and 104 THE MICROSCOPE. somewhat novel in design. It is constructed on a plan Fio. 5S.PiUischer's Binocular Microscope. which may be described as intermediate between that of Smith and Beck and Ross's well-known pattern, and COLLINS'S STUDENT'S STAND. 105 in point of finish, is equal to the students' micro, scopes of first-class manufacturers. The semicircular form given to the arm carrying the body increases the strength and solidity of 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 parallel bar of solid brass, ploughed into which is a groove for the reception of the rack attached to the body, the groove being of such a form that the rack is held firmly while the pinion glides smoothly through it. A steady uni- form motion is thus obtained, and which almost renders the fine-adjustment unnecessary. The fine-adjust- ment 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 instru- ments ; nevertheless, the range of motion given to the eye-pieces by the rack and pinion, enables those whose eyes are widely separated to use the instrument with comfort. 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 milled- heads which effect these are placed on the same axis, instead of side by side, one of them the vertical one being repeated 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 plat- form 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 centring screws is made in the usual way to carry an achromatic condenser, diaphragm, polarising and other apparatus. Collins's Student's Microscope (fig. 59) has a 10-inch body and a draw- tube for increasing its length. The diameter of the tube is of full English size ; the field is consequently large. The fittings for objectives and 106 THE MICROSCOPE. accessory apparatus are all of standard size, and can, therefore, be applied to any of the larger and more elaborate instruments of English model. The fine- adjustment is ingeniously modified, very delicate, and quite free from lateral motion. Not only is the field of all the objectives supplied with it excellent, but their FIG. 59 Collins's Histological or Student's Microscope. penetration is of a high standard. The stand alone can be had at a lower price with eye-piece and case. On the whole, the instrument is of excellent workman- ship, and possesses all the advantages and conveniences which belong to students' microscopes. Collins's Binocular Microscope Stand (fig. 60), on the "Jackson" principle, is extremely steady and solid. COLLINS'S BINOCULAR STAND. 107 The limb that carries the body, stage, sub-stage, and mirror, is in one piece, with a machine-planed groove from end to end, thus ensuring considerable accuracy. A rack-work movement of 6 in. is given to the body, FIG. 60. Coliins's Binocular Microscope. allowing the use of low-power objectives, so much appreciated in binocular microscopes. The Wenham prism box is made on the Harleyplan, enabling the polariscope to be brought into action without removal of the objectives or putting the same out of focus. 108 THE MICROSCOPE. The stao^e is circular in form, with concentric rota- tion, horizontal and vertical mechanical movements, and top slide for holding the objects, trough, etc., while under examination. It has a clear aperture underneath of 3 in. when the apparatus plate is removed, and, in consequence of the improved plan of mounting the mirrors from the back, a great obliquity of illumination can be obtained as well as a considerable range of movement when a sub- stage is fitted. Collins's Dissecting Microscope, for plant or insect dissection (fig. 61), has a firm metal stand, sliding adjustment for focuss- ing, two simple lenses, to be used together or separate, with mirror for illuminating. Swift's Challenge Microscope (fig. 62), of the Jackson- Lister form, which experi- enced microscopists still believe, for many rea- sons, to be one of the best, is well finished in 'Microscope, on ^^ ^^ fa Q coarge . adjustment is sensi- tive; the focussing can be accurately done by it alone, whilst the fine-adjustment is so conveniently placed as to be within easy reach of one of the fingers of the hand which works the rack. By a somewhat novel arrangement, Messrs. Swift have succeeded in mounting the analyzer above the Weiiham's prism within the binocular body, so that it can be easily brought into use by pushing it into the optic axis of the instrument, without any screwing or unscrewing of the objective. To those in the habit of frequently employing the polariscope, this simple arrangement must commend itself, as the definition of the object- glass is much less interfered with by this method of mounting the analyzer than where the Wenharn prism intervenes between it and the eye-piece. The instru FIG. 61. Collins's Diasecti Prof. Henslow's SWIFT'S CHALLENGE STAND. 109 ment is furnished either with a simple rotating and FIG. 62. Swift's New Challenge Microscope. universal movement stage, or a very thin mechanical stage with rectangular as well as rotatory adjustments, 110 THE MICROSCOPE. and also with, a centring and focussing sub-stage, for the adaptation of the achromatic condenser, paraboloid, polariscope, etc. The mirror moves by a double elbow joint, and is arranged to be used at any angle. Swift's College or Student's Microscope is a solid, FIG. 63. Crouch's New Fine-Adjustment. well-made, handy instrument, designed for class use, and it may be expected to take the place of the con- tinental microscope hitherto much employed in our colleges and schools of medicine. Crouch's Microscope possesses certain advantages as a cheap microscope, since it combines perfect perform- CROUCH, AND HOw's STAND. 11] ance witli good workmanship in the construction of the stand. The chief point of novelty is the fine-adjustment, shown in detail in fig. 63. The solid bar A, carrying the optical body B, is suspended on the front ends of the two broad, flat, parallel tempered-steel springs c c, the other ends of which are attached to the limb D. The pressure of the focussing screw E, by the point at F on the solid bar, forces down this bar, the springs bending sufficiently to allow about J-in. range of motion Downwards from the normal position. The actual motion of focussing displaces the optic axis slightly ; but this displacement is attended with no inconvenience, except where the microscope is provided with a rotating stage. This mode of focussing must be regarded as practically free from friction, as there are no metal surfaces in contact ; the only friction is between the point of the screw at F, where it acts on the bar by pressure. The suspension of the optical body is strictly on the two springs c C. 1 How's (Farringdon Street) Student's instrument (fig. 64) is deserving of a place among microscopes designed for general use. The stand is of brass, firm and well finished ; the body is fitted with coarse and fine adjust- ments 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 movable, 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 up- wards, downwards, laterally, or in a slanting direction, 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. (1) Journal R. M. S , p. Ill 1S61. 112 THE MICROSCOPE. Murray and Heath's Student's Microscope (fig. 65) is a good solid form of instrument with a bent tripod- stand. The stand is remarkably firm, and, being FlG. 64. How's Student's Microscope. bronzed over, is well adapted for daily nse 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 MURRAY AND HEATH's STAND. 113 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 one. The objectives furnished with this microscope FIG. 65. Murray and Heath's Student's Microscope. are a -inch of 75 angular aperture, and a 1-inch of 15, both of excellent quality. Murray and Heath's Class Microscope, represented in fig. 66, is especially intended for the use of teachers in the demonstration of objects to a class of students. The instrument consists of the usual microscope body i 114 THE MICROSCOPE. (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 focus- sing, the 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 position, and fastened with a screw. The instrument can now be passed round a class-room FIG. 66. Murray and Heath's Class Microscope. without possibility of injury either to object or object- glass. The illumination is obtained either by direct, ing 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. In Mr. Ladd's student instrument, such as that repre- sented in fig. 67, he has taken great care to obtain a perfect balance in any position, even when placed in the horizontal axis ; no instrument can be better adapted than this to the wants of the microscopist j it is certainly one combining many of the advantages of the more expensive forms. LADD'S TRIPOD STAND. 115 Nachet and Hartnack of Paris, and Merz of Munich, hold an almost equal rank as makers of first-class FIG. 67. Ladd's Student's Microscope. microscopes, and in point of excellence of workman- ship fairly rival those of English makers. FIG. 68. Nacliefs Portable Demonstrating Microscope. i 2 116 THE MICROSCOPE. APPLICATION OF 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 f igo 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 Ross and Powell to make him a binocular microscope. But this was not done; and during the year 1853 a notice appeared in Sillimari* American Journal of a binocular instrument constructed by Professor Eiddel 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 n'rst 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. 1 J / 111 describing his improvements, he observes : " That in obtaining binocularity 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. 69 , b are enlarged drawings of a portion of the egg of the common bed-bug i {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- Flg< 69 - 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, exactly the same changes and appearances will be observed in the object under view 118 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. 70, represents the methods that I have contrived for obtaining the effect of bringing the two eyes sufficiently close to each other to enable them both to see through the same eye-piece together, a a a are rays con- verging from the field lens of the eye-piece ; after passing the eye-lens 6, 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 t 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 47. I also find it is requisite to grind away the contact edges of the prisms, as represented, as it prevents the extreme margins. ABBE'S STEREOSCOPIC EYE-PIECE. of the reflecting surfaces from coming into operation, \vhich can seldom be made very perfect. The purpose of the binocular microscope is to give a stereoscopic view of objects, whereby the form, distance and position of their various parts are simul- taneously seen ; the result is often as striking as if the minute object were placed in the hand as a model. To produce a stereoscopic effect there must be an equal division of the rays after they have passed through the object-glass, so that each eye may be furnished with an appropriate one-sided view of the object ; but the methods hitherto contrived to effect this not only materially injure the definition of the object-glasses, but also require expensive alterations in their adaptation, or, more frequently still, a separate stand ; whereas the arrangement contrived by Mr. Wenham is no obstacle to the use of the monocular instrument, and the definition even of the highest powers is scarcely impaired. Nachet of Paris has throughout endeavoured to vie with Wenham, and he substituted a double eye-piece for the binocular body. This idea was improved upon by Tolles, of Boston, U.S., and more recently it has received some improvement from Professor Abbe. Fig. 71 presents a sectional view of Abbe's stereoscopic eye-piece, and which consists of three prisms of crown glass, a t b and 5', placed below the field-glass of the two eye-pieces ; the tube C is slipped into the tube or body like an ordinary eye-piece. The two prisms a and b are united so as to form a thick plate with parallel sides, inclined to the axis at an angle of 38'5. The cone of rays from the objective is thus divided into two parts, one being transmitted and the other reflected ; that trans- mitted passing through a b and forming an image of the object in the axial eye-piece B. Adjustment for different distances between the eyes is effected by the screw placed to the right-hand side of the figure, which moves the eye-piece B', together with the prism &', in a parallel direction. The tubes can also be drawn out, if greater separation is required. The special feature of the instrument is an ingenious arrangement for 120 THE MICROSCOPE. halving the cones of rays above the eye-pieces, where, by simply turning the caps with the diaphragms, orthoscopic or pseudoscopic effects can be instanta- neously produced. This arrangement is particularly suitable for the cheaper forms of microscopes, and for those of foreign manufacture, which are usually shorter in the body than English -made instruments. FIG. 71. Professor Abbe's Stereoscopic Eye-pieces. The most important improvement effected by Wen- ham consists in splitting up or dividing the pencil of rays proceeding from the objective by the interposition of a prism of the form shown in fig. 72. This is placed in the body or tube of the microscope (fig. 72&, a) so as to interrupt only one-half (a. c) of the pencil, the other half (a &) going on continuously 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 perpen- WENHAM S BINOCULAR PRISM. 121 dicular 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 FIG. 72. received by the right eye is formed by the rays which have passed through the left half of the objec- tive; whilst the image received by the left eye is formed by the rays which have passed through the right half, and which have been subjec- tive to two reflections within the prism, passing through only two FlG< 72a 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 central line of the objective, and be 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. The binocular constructed as we have described performs satisfactorily up to the Jth inch; but for 122 THE MICROSCOPE. powers above this a special arrangement is needed for the prism, which must be set close behind the lens of the -|th or y^th inch, in order to obtain an entire field of view in each eye. A strong light should be avoided for the illumina- tion 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 excel- lent effect consists of three plano-convex lenses, so combined as to give a very large area of light, as well as great intensity. The improvement effected in Nachet's binocular eye- piece by Mr. Tolles, optician, of Boston, U.S., consists in mounting the prisms in a light material, vulcanite, which are made to fit into the monocular microscope body, taking 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 inserted beneath it. The second set of prisms are by a rack-and-pinion movement adjusted to suit any visual angle ; thus the illumination of both fields is of nearly equal brightness. Spectro-Microscopy* The application of the spectroscope to the microscope is one of the most beautiful additions the instrument has ever received. The honour of the invention appears to belong to H. C. Sorby, F.R.S., 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 imme- diately 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, for the purpose of viewing the spectra of opaque as well as transparent objects. The exact form since adopted is the Sorby-Browning Spectroscope. The first spectroscope made byMr.Browning(63 Strand) is represented in fig. 73. A prism is placed at P, which is enclosed in a box, so as to give a black field, by excluding THE BROWNING MICRO-SPECTROSCOPE. 123 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 3. Sectional uiuw uj tii,e JJrowuiug 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. 74. The Browning Huggin* Micro-spectroscope. The apparatus used bv Mr. Huggins (fig. 74) was a star THE MICROSCOPE. spectroscope, of which the collimativc-tube was inserted in the body of the microscope, instead of an eye-pieco. 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. 74. K represents the knife-edges, c the 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. 75. Kg. ira. Figs. 75 and 75 a represent prisms of the kind made by Mr. Browning, used in two different positions, i and i 1 being the same angle as I and i'. For 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 Friiun- hofer's lines in the solar spectrum, then the prism must be used as in fig 75 a . This answers well for liquids 01 transparent objects, but it is, of course, not applicable to opaque objects. THE DIRECT SPECTROSCOPE. 125 To combine both purposes, some form of direct vision- prisms which can be applied to the body of the micro- scope is required. Fig. 76 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 .\nd coincident with the incident ray R. Kg. 7(3. Fig. 7<3. Another very compact combination is shown in fig. 76a. 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 wilJ then be readily found by slightly altering the focus. This contrivance answers perfectly for opaque objects, 126 THE MICROSCOPE. without any preparation ; and, when desirable, the sam< 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. 77, 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. 17. The Sorby-Browning Micro-spectroscope. The prism is contained in a small tube, which can be. removed at pleasure. Mow 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 tht stage. A is a brass tube, carrying the compound direct vision prism. B, a milled head, with screw motion to adjust the focus of the achromatic eye lens, c, milled head, with screw motion to open or shut the slit vertically. Anothej THE MICRO-SPECTROSCOPE. 127 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^ Lieberkiihn, and dark well, by side reflector, or bull's-eye condenser if opaque. Remove A, and open the slit by means of the niilled-head, not shown in cut, but' which ia 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 B a small mirror is attached, which is omitted in the diagram to prevent confusion. It is like the mirror belov 1?3 THE MICROSCOPE. the 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 j^th 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'a line. An instrument that shows all these well is all that can be desired. " The objects most easily obtained, and which furnish as '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 TUB MICRO-SPECTROSCOPE. 129 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 bs 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 ^th of an inch in diameter, and an i ncn l n o> are very suitable for the examination of very small quan- tities ; but where plenty of material can be obtained, it IB far better to use cells cut out of strong tubes, having an interior diameter of about |-ths of an inch, cut wedge- ehape, so that the thickness of the solution may be JtJi 130 THE MICROSCOPE. of an inch, or more, on one side, and not above - 4 \jth 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 bulTs-ej condenser. A speck of blood on white paper shows th spectrum very well, provided it be fresh, and the colour 'fee 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 icoioooth of a grain, is then sufficient to show the characteristic absorption-bands. They are, however, far better seen in solution. About T&yth of a grain of liquid blood, in a cell of -^fth of an inch in diameter, and J 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 ABSORPTION-BANDS OF BLOOD. 131 changing tho cell held on the eye-piece for a tube con- taining an amnioniacal 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 thero is no chance of their being confounded when compared together side by side." l We have been usually taught that the red-blood corpuscles consisted of two substances, haematin and globulin; but later researches lead to the belief that they consist of one crystalline substance, termed globulin or luemato-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 for 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 a* the substance, which he has termed cruorine, is in a more or less oxidise! 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. 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- snatter 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. 132 THE MICROSCOPE. The Camera Lucida. The main point to be observed when using the Camera Lucida is that the microscope shall be placed in the horizontal position, and the object well lighted. The Ross-Wollaston Neutral Tint Camera Lucida con- sists of a metallic cylinder, cut at an angle of 45 to its axis, thus producing an elliptical opening, into which FIG. 78. The Ross-JFollaston Camera Lucida. a plate of neutral tint glass is fitted. Opposite to this is an opening about half an inch in diameter, through which the student may view, and trace or measure the object on drawing-paper, the microscope with the camera attached to the eye-piece having been previously brought into a horizontal position. MICEOSCOPICAL DRAWING. 133 Microscopical Drawing. The proper method of draw- ing microscopic objects is acquired by looking down the tube of the microscope with one eye (preferably the left) , and at the paper on which the drawing is to be made with the other. Place the microscope in the horizontal position, having first secured the object to be copied to the stage, focus it carefully, and take care not to place it too centrally, but as far towards the right as it will go without taking it out of the field of view. If the right eye is now opened, while the other is looking down the tube, the object will be seen projected on the paper, and can thus be easily traced in all its details. The Polarisation of Light. 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, 134 THE MICEOSCOPE. 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, a,nd 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. Oo 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 c d, fig. 79 ; and we shall distinguish the section of a beam POLAEISATION OF LIGHT. 13^ of common light by a circle with two diameters a 6, c d t 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, o E e, formed by double refraction, we shall 'find, 1st, that the beams 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 o differs from E e in nothing ex- ceptiug 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 c' d' for examplo Thesa two beams, 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 a b } c d ; or a' 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 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 a b c d 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 o be laid above E e, it will produce a figure like a b c d ; and we shall therefore represent polarised light by such figures. If we were to place 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 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 light, a b c d, into a beam or beams of polarised light. 1st. We may separate the beam of common light, a & c d, 136 THE MICROSCOPE. component parts 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 crystals or in masses, we can always cleave it or split it into shapes re- presented by fig. 80, 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, a c d, are 101 55' and 78 5'. 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 (1) Brewster's " Optics " POLARISATION OF LIGHT. 137 where it is first observed. In the accompanying diagram, fig. 81, the line appears double, as a b and c d, or the dot, as e and /. Or allow a ray of light, g h y 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 used, 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 m 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 Bpar 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) : 138 THE MICROSCOPE. "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. 82. passing through such a prism is indicated . by the arrow, fig. 82, and the combination is shown mounted, one for Fig. 83. Fig. 83. use under the stage of the microscope, fig. 83, termed the polariser; another, fig. 83ar. screwed on to and above the POLARISATION OF LIGHT. 139 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. 83. 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 small milled-head screw at one end : the other prism, fig. 83#. 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. 84. 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 plane. It is necessary to choose the etone 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. 84. 140 THE MICROSCOPE. the maximum effect, it is also requisite that the angle of aperture of the polariser should be the same a.s 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 Belenite 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 t( Introduction to the Study of Polarised Light." 1 Fig. 85. h 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 piano of its crystallographical axis. (U Mr Woodward constructed a very available form of polariscope for most yurposes'; the instrument is described in Elements of Natural Philosophy, by Jabez Hogg. POLARISATION OF LIGHT. Kl jr; 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. kj the waves R o and R e, 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. 85#, depolarised light, as fig. 85. i, the analyser turned one quarter of a circle, its axis being at right angles to that of i in fig. 85. 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. 142 THE MICROSCOPE. 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 laminse 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 HEEAPATHITE. 143 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 metallio lustre ; they appear like portions of the ely trss of cantha- rides, and are also very similar to murexide in appearance. When examined by transmitted light, they scarcely possess 144 THE MICROSCOPE. 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 per- fectly black, even if the crystals are not one five- hundredth of an inch in thickness. 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 56 45' these little prisms immediately assume complementary colours : one appears green, and the other pink, and the part at which they cross is a cho- colate 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 when 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 extensive field than can be had with the prism. 1 These crystals are liable to be injured by damp. " The following experiments, if carefully performed, will illustrate the most striking phenomena of double refraction, and form a useful introduction to the prac- tical application of this principle. (1) Dr. Herapath subsequently furnished a better process for the manu- facture of these artificial tourmalines, see Quarterly Journal of Microscopical Science for January, 1854. " These beautiful rosette crystals are 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 ft 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 of which will be found sufficient to stop further trystallization," POLARISATION OF LIGHT. 145 " A plate of brass, fig. 86, three inches by one, perforated with a series of holes from about one-sixteenth to one- Fig. S3. Red 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 146 THE MICROSCOPE. flaws in the film, which will be observed to run parallel with each other; these flaws 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 eye-piece 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 round the optic axes of the crystals, the following plan, which is by far the best, must be followed, and the rings will appear .in 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 POLAEISING CKYSTALS. 147 the microscope to reach the polariser, that all extraneous light may be excluded. The concave mirror should be used with a bull's-eye 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. Quartz. Cut so as to exhibit straight lines. Gale 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 L 2 148 THE MICROSCOPE. close as possible to the eye-lens, to see properly the phenomena 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. 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 refraction, 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 oj|!!' 'iiii'i. .; i|i " ~ ~ r ~~ ~ : FIG. 87. Darter's Selcnite Films and Stage. 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 2 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. 88 and S8a. 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. 88 ; on revolving the nitre, it gradually POLARISING CRYSTALS. 149 assumes the form of the two hyperbolic curves, fig 88a. But if the tourmaline be revolved, the black crossed lines will Fig. SSa. 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 few of which are shown at fig. 89. 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, &c. in the urine of patients. 150 THE MICROSCOPE. He says : " It has long been a favourita subject of inquiry with the professional man to trace the course of remedies Fig. 89. 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 acidL " One drop of this test-fluid placed on a glass-slide, and the merest atom of the alkaloid added, in a short time POLARISING CEYSTALS. 151 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; tho 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. 90. 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 ; 6, red stage of selenite employed. that tint which an analysing-plate of tourmaline would produce when at right angles to the polarising-plate ; 152 THE MICROSCOPE. 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, par excellence, for quinine : fig. 90, 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 necessary 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. 91. Fig. 91. ' If we employ the selenite stage in the examination of this object, we obtain one of the most gorgeous appear- ances in the whole domain of the polarising-microscopo : the black cross at once disappears, and is replaced by one which consists of two colours, being divided into a cross SNOW CRYSTALS. 153 Flff. 92. Sntic CryttaU. 154 THE MICROSCOPE. 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 aualysing-plate. These phenomena are analogous to those exhibited by certain circular crystals 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. Ice 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 hia 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 ; Btems of plants containing silica or flint ; barley, wheat, &c. The larger-grained starches form splendid objects; tous- Its-mois, being the largest, may be taken as a type of al] POTATOE STARCH. 155 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 colcur is percep- tible. But if a thin plate of selenite be interposed between Flg> 93 ' the starch-grains and the po- Potato Starch, seen under polarised , , , -.., * <* light. lariser, 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 fibrillse 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) Quekett's Practical Treatiteon the Use of the Microscope. 156 THE MICROSCOPE. Molecular Rotation. For the purpose of studying the various interesting phenomena of molecular rotation, a few necessary pieces of apparatus must be added to the microscope. First, an ordinary iron three-armed retort stand, to the lower arm of which must be attached either a polarising prism or a bundle of glass plates inclined at the polarising angle. In the upper an analysing prism. The fluid to be examined should be contained in a narrow glass tube about eight inches iii height, and this must be attached to the middle arm. If the prisms be crossed before inserting a fluid, possessing rotatory power, the light passing through the analyser will be coloured. If a solution of sugar be employed, and the light which passes through the second prism is seen to be red, but on rotating the ana- lyser towards the right, the colour changes to yellow, and passes through green to violet, it maybe concluded that the rotation is right-handed. If, on the contrary, the analyser requires to be turned towards the left hand, we conclude that the polarisation is left-handed. These phenomena are wholly distinct from those accompanying the action of doubly refracting sub- stances upon plane polarised light. It is not easy to explain in a limited space the course to be followed in ascertaining the amount of rotation produced by different substances. Monochromatic light should be used. If we are about to examine a sugar solution with the prisms crossed, the index attached to the analyser must first be made to point to zero. The sugar is then introduced, when it will be necessary to rotate the analyser 23 to the right, in order that the light may be extinguished. This is the amount of rotation for that particular fluid at a given density and that height of column. As the arc varies with increase or decrease of density and height of the fluid, it is needful to reduce it to a unit of height and density. The following formula is that given by Biot: P= quantity of matter in a unit of solution ; d = sp. gr. ; I = length of column ; a = arc of rotation ; m = mole- cular rotation. Then m = -= -, l d APPLICATION OF PHOTOGRAPHY. 15? 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 waa 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, better pictures are produced by taking away the eye-pieoe 158 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 shovn 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 euch 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 * magic lantern, and the size of the image will be proper, APPLICATION OP PHOTOGRAPHY, 139 tionate to the distance. On 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- gulatum. 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. 94; a sec- tional view of his arrangement is here given, which is Fig. 94. HigMey'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 p 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 160 THE MICROSCOPE. the stage-plate, after the manner 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 with 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. 95.Highley's Photo-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. MICROPHOTOGRAPHIC APPARATUS. 161 Professor Draper employed the following form of lantern for microphotography : a is a zirconia light rendered incandescent by the mixed gases ; b b a very short condensing lens ; c the stage or support carrying the object to be photographed ; d the projecting lens, formed of three sets of lenses, and giving a flat recti- linear field ; a, c, d are mounted on a base board, e, f, to the end of which the lantern box a b is attached, and which is freely opened above and below for perfect ventilation. The lateral grooves a, c, d slide and allow of an adjusting movement by the screw r, and by means of which the change of distance between a and c admits of a correct focus being obtained. By means of the hinge at Ji the whole can be adjusted at any angle. FIG. 95a. Draper's Microphotographic Apparatus. 162 THE MICROSCOPE. CHAPTER III. RELIMINART DIRECTIONS ILLUMINATION ACCESSORY APPARATUS GILLETT'S, ROSS'S, BECK'S, POWELL AND LEALAND'S, AND OTHER CONDENSERS THE LIEBERKUHN SIDE REFLECTOR LAMP- OBJECT FINDER COLLECTING STICK ANIMALCULE CAGE SECTION CUTTER PREPARING AND MOUNTING OBJECTS DOUBLE STARRING, ETC. AVINGr selected an apartment with a northern aspect, and, if possible,with only one window, 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 using it, otherwise the perspiration from the eye will condense on the eye-glass, and obscure vision. Management of the Microscope. Should the micro- scope not have been used for some time, dust and moisture will in all probability collect and settle on the eye-piece. The foggy atmosphere of large towns may insinuate itself into the interior of the objective. In such cases, dust and moisture can only be removed by gently wiping the glasses with a piece of soft, well-used chamois leather. When necessary to clean the eye-piece, unscrew one glass at a time and replace it before removing another. The objective can only ITS MANAGEMENT. 163 be unscrewed, or tampered with, at the risk of damaging the cement which binds the lenses together. If the objective be an immersion, carefully wipe off the fluid from the front lens as soon as it is done with, for even distilled water will leave a stain behind. When looking through the eye-piece be sure to place the eye close to the lens, otherwise the whole field will not be perfectly visible ; it should appear as an equally well-illuminated circular disc. The position of the observer should be easy and comfortable, and the microscope inclined to an agreeable working angle This will prevent fatigue and congestion of the eyes, the first indication of which is small bodies moving about or floating before them. If the eyelashes are reflected from the eye-glass, the observer is looking upon the eye-piece, and not through it. Eor the examination of transparent mounted objects, it is simply necessary to place them upon the stage of the microscope, and throw light from the concave mirror through them. The distance at which the mirror should be set depends upon the source whence the illumination is derived, and whether it be daylight or lamp-light. The stem which carries the mirror is generally so arranged as to be capable of elongation. The working focal distance of the mirror is that which brings the images of the window bars sharply out upon the glass slip or object resting upon the stage. In other words, the focus of the mirror is that which brings parallel rays to a correct focus on the object- glass. If employing artificial light, then the flame of the lamp should be distinguishable ; a slight change in the inclination of the mirror will be required to throw the image of the lamp-flame out of the field. A good illumination having been obtained, the diaphragm must be brought into use to regulate the amount of light. The more transparent the object, the less light will it require to display it properly. Some microscopists carefully tone down the light, by interposing a piece of monochromatic glass, or a fluid medium, a weak solution of sulphate of copper, between the light and the object. The best artificial source 16-4 THE MICROSCOPE. of illumination is the steady flame of a paraffin lamp, with a flat wick (fig. 96). Collins's Bocket Lamp, with bull's-eye condenser mounted on a stem, so as to be adjustable at any height, is a suitable form of lamp. Whatever be the source of light, the objects hould on no account be over-illuminated : a flood of light mars the image, and spoils the performance of the object- glass. For viewing opaque objects, or whole insects, the eletra of a beetle, etc., the light must be thrown down or condensed upon it, by the condensing or bull's- eye lens ; or by Beck's parabolic side- silver re- flector, placed at a pro- per angle to the source of illumination. For examining par- tially opaque minute ob- jects, as the Podura-scale, under high-power mag- nification, the vertical illuminator is useful. If the object is a small por- tion of a dissected animal or plant, or a patholo- gical specimen in a fluid medium, the microscope should be employed in the vertical or upright position. The object should be covered with a thin cover-glass, to prevent the escape of the fluid, which, should it run over, might damage the stage and its mechanical movements. Test for Illumination. Dr. C. Seiler recommends the human blood corpuscle as the best test of good illumina- tion. He prepares the object in the following manner : Take for the purpose a clean glass slide of the ordinary kind, and place near its extreme edge a drop of fresh blood drawn by pricking the finger with a needle. Then take another slide of the same size, with FIG. 96. Collins's Bocket Lamp. ERRORS OF INTERPRETATION. 165 ground edges, and bring one end in contact with the drop of blood, as shown in fig. 97, at an angle of 45 ; then draw it evenly and quickly across the uiider- slide, and the result will be to spread out the corpus- cles evenly throughout. The blood discs being lenti- cular bodies, with depressed centres, act like so many little glass-lenses, and show diffraction rings if the light is not properly adjusted. In the Journal of the Royal Microscopical Society, page 542, vol. iv., Dr. Seiler fully describes the arrangement of the lamp, condenser, and mirror. Errors of Interpretation. To be in a position to draw accurate conclusions of the nature and properties of the object under examination is a matter of the greatest importance to the microscopist. The viewing of objects by transmitted light is quite of an ex- ceptional charac- ter, much calcu- lated to mislead the judgment as well as the eye. It requires, there- FlG . ^.-Seller's lore, an unusual amount of care to avoid falling into errors of inter- pretation. There are perhaps no set of objects with which I have become acquainted, and which have given rise to more discussion as to the precise nature of their structural elements than those of certain of the diato- maceee. The minute scales of the Podura (Lepidocyr- tus cervicollis') and their congeners Lepisma saccharin are equally debatable. Mr. E. Beck, in an instructive paper published in the Transactions of the Royal Microscopical Society, says that the scales of the latter can be made to put on an appearance which bears little resemblance to their actual structure. On the more abundant kind of scales the prominent markings appear as a series of double lines, these run parallel and at considerable intervals from end to end of the scale, whilst other lines, generally much fainter, 166 THE MICROSCOPE. radiate from the quill, and take the same direction as the outline of the scale when near the fixed or quill end ; but there is, in addition, an interrupted appear- ance at the sides of the scale which is very different from the mere union, or ' cross-hatchings,' of the two sets of lines. (Fig, 98, Nos. 1 and 2, the upper portions.) The scales themselves are formed of some truly transparent substance, for water instantly and almost entirely obliterates their markings, but they reappear FIG. 98. Portions of Scales of Lepisma, after Beck. unaltered as the moisture leaves them ; therefore the fact of their being visible at all, under any circum- stances, is due to the refraction of light by superficial irregularities, and the following experiment establishes this fact, whilst it determines at the same time the structure of each side of the scale, a matter which it is impossible to do from the appearance of the mark- ings in their unaltered state : " Remove some of the scales by pressing a clean and dry slide against the body of the insect, and cover them with a piece of thin glass, which may be pre- IEPISMA SCALES. 167 vented from moving by a little paste at each corner. No. 3 may then be taken as an exaggerated section of the various parts. A B is the glass slide, with a scale, C, closely adherent to it, and D the thin glass-cover. If a very small drop of water be placed at the edge of the thin glass, it will run under by capillary attraction ; but when it reaches the scale, C, it will run first between it and the glass slide A B, because the attrac- tion there will be greater, and consequently the mark- ings on that side of the scale which is in contact with the slide will be obliterated, while those on the other side will, for some time at least, remain unaltered : when such is the case, the strongly marked vertical lines disappear, and the radiating ones become con- tinuous. (See No. 1, the lower left-hand portion.) To try the same experiment with the other, or inner surface of the scales, it is only requisite to transfer them, by pressing the first piece of glass, by which they were taken from the insect, upon another piece, and then the same process as before may be repeated with the scales that have adhered to the second slide ; the radiating lines will now disappear, and the vertical ones become continuous. (See No. 2, left portion.) These results, therefore, show that the interrupted appearance is produced by two sets of uninterrupted lines on different surfaces, the lines in each instance being caused by corrugations or folds on the external surfaces of the scales. Nos. 1 and 2 are parts of a camera lucida drawing of a scale which happened to have the opposite surfaces obliterated in different parts. No. 4 shows parts of a small scale in a dry and natural state ; at the upper part the interrupted appear- ance is not much unlike that seen at the sides of the larger scales, but lower down, where lines of equal strength cross nearly at right angles, the lines are entirely lost in a series of dots, and exactly the same appearance is shown in No. 5 to be produced by two scales at a part where they overlie each other, although each one separately shows only parallel vertical lines." Another very characteristic fallacy resulting from configuration is furnished in the supposed tubular 168 THE MICROSCOPE. structure of human hair. "When we view this object by transmitted light, it presents the appearance of a flattened band with a darkish centre ; this, however, is entirely due to the convergence of the rays of light produced by the convexity of the surface of the hair. That it is a solid structure is proved by making a transverse section of the hair-shaft, when it is seen quite filled by medullary substance, with the centre somewhat darker than the other part. It is, in fact, a spiral outgrowth of epithelial scales, overlapping each other like tiles on a house-top, which impart a striated appearance to the surface. A cylindrical thread of glass in balsam appears as a flattened, band-like streak, of little brilliancy. Another instance of fallacy arising from diversity in the refractive power of the internal parts of an object, is furnished by the mistakes for- merly made with regard to the true character of the lacunae and canaliculi of bone structure, which were long supposed to be solid corpuscles, with radiating opaque filaments proceeding from a dense centre ; on the contrary, they are minute chambers, with diverging passages, excavations in the solid osseous substance. That such is the case, is shown by the effects of Canada balsam, which infiltrates the osseous substance. The molecular movements of finely divided particles, seen in nearly all cases when certain objects are first suspended in water, or other fluids, is another source of embarrassment to beginners. If a minute portion of indigo or carmine be rubbed up with a little water, and a drop placed on a glass slide under the micro- scope, it will at once exhibit a peculiar perpetual motion appearance. This movement was first observed in the granular particles seen among pollen grains of plants, known as fovilla, and which are set free when the pollen is crushed. Important vital endowments were formerly attributed to these particles, but Dr. Robert Brown showed that such granules were com- mon enough both in organic and inorganic substances, and were in no way " indicative of life." Accessory Apparatus. In the more perfectly furnished instruments, a number of accessory pieces of apparatus THE DIAPHRAGM. 169 are usually included, many of which are essentially necessary for the prosecution of microscopical pursuits and for the perfect examination of most objects. The Diaphragm, fig. 99, is a circular plate with a series of circular apertures cut in it. In fact, there FIG. 99. The Diaphragm. are two plates of brass, one being perforated with four or five holes of different sizes, and arranged to revolve upon another plate by a central pin or axis, the last being also provided with a hole as large as the largest in the diaphragm-plate, and corresponding in situation to the axis of the compound body. The holes FIG. 100. Dr. Anthony's Stage Diaphragm. in the diaphragm-plate are centred and retained by a bent spring that fits into the second plate, which rubs against the edge of the diaphragm-plate and catches in a notch. The blank space shuts off the light from the mirror when condensed light is used. It is impossible to dispense with the use of the diaphragm, 170 THE MICROSCOPE. as without it the transmitted rays would in many cases produce confusion of the image. Dr. Anthony advo- cates the use of a stage- diaphragm, and which consists, as seen in fig. 100, of three slips of smooth blackened cardboard or vellum with perforations, any of which can be brought into the centre and clamped, and re- tained in its place under the glass slip. The larger perforated discs form an additional slide ; while various other forms, slits, slots, cat's-eyes, bars, &c., may be added at pleasure. The Iris Diaphragm, fig. 101, is an inexpensive and ingenious form of iris diaphragm designed by Wale, of America, for use with his "Working, or Student's," microscope. It consists of a piece of very thin cylindrical tube, A, about f of an inch in length and | of an inch in FIG. 101. Wale's Iris Diaphragm. diameter, the circumference of which is cut throughout with shears to nearly the whole length, and at intervals of about of an inch; by means of a screw collar B attached below, this cut tube is forced into a parabolic metal shsll, contained within c, whose apex is truncated to an aperture of about -| of an inch; the pressure of the screw causes the thin metal tongues to turn and to overlap in a spiral, which gradually dimin- ishes the aperture to the size of a pin-hole. On unscrew- ing the collar B, the spiral overlapping of the tongues is released, and by their elasticity causing the aperture gradually to expand. The whole device is fitted into the opening of the stage from beneath, so as to be flush with the upper surface, with one turn of a coarse screw on the edge of c. IRIS DIAPHRAGMS. 171 Beck's Iris Diaphragm (fig. 102) is very simple, and on that account preferred. By pressing the lever handle placed at the side of the brass box the aperture is gradually made to close up, and without for a moment losing sight of the object. FlG. 102. Beck-Brown's Iris Diaphragm, FIG. 102a. Collins- Davis' s Iris Nose- piece Diaphragm. Collins's Limiting Diaphragm, or Aperture Shutter. Fig. 102 shows the instrument as a nose-piece for screw- ing on to the lower end of the microscope tube. This form of aperture shutter enables the observer to adjust his objective to any aperture he wishes, and the closing of the shutter does not contract the absolute size of the field, but limits its brightness ; in this way the true value of penetration is observed without moving the eye from the tube. Mr. Nelson suggests the application of a series of diaphragms in connection with an ingenious cen- tring nose-piece devised likewise as a sub-stage. This piece of apparatus is recommended as a use- ful addition, and as a convenient and inexpen- sive centring sub- stage for small instruments. The optical part of a T 4 o objective forming the condenser, and which for the purpose should be fitted with the shortest possible adapter, so that the diaphragms may be brought close to the back lens. The sub-stage is seen in fig. 103. FIG. 103. Nelson's Sub-stage Condenser. 172 THE MICROSCOPE. Mr. Nelson recommends as the most useful of his diaphragms those represented in fig. 104, in which a may be regarded as a type shape for one pencil of light, and b for two, at right angles. The superposition of stops c will cut off more or less of the central light, d will stop out more or less of the peripheral zone ; while e is a combination intended to utilize the most FIG. 104. Nelson's Diaphragms. oblique pencil required for the resolution of fine lined objects. A variety of discs of the forms c and d may be used ; any of which, dropped into a metal holder with an inner ring made deep enough to receive two or three, which when in place can be rotated by a milled edge, or moved out of the axis by the handle. 1 Dark Field Illuminators. To Mr. F. H. Wenham's the microscope is deeply indebted for many valuable improvements ; not the least important being the dark- field or parabolic illuminator, invented in 1851. The operation of the para- bolic condenser (fig. 105) depends for its action on rays thrown on the object at an angle extending beyond that known as the aperture of the object- glass, and which otherwise would be lost ; consequently, as the source from which the light comes is without the range of the pencil of rays of the ob- jective, the field must be dark ; but if an object possessing a partial opacity is placed exactly in focus, it becomes brilliantly luminous by means of these rays. Dark-ground illumination is not suitable for very transparent objects that is, unless there is a considerable difference in their refraction, or they are pervaded by air-cells. One very remarkable example of this fact may be (1) Journal of the Royal Microscopical Society, Vol. IV., p. 126 (1881). THE PARABOLIC ILLUMINATOR. 173 seen in the tracheal system of insects. If any of the transparent larvae of the various kinds of gnat found about ponds in spring-time, be mounted in the elastic gelatine and glycerine jelly (which must be warmed only enough to run, and not kill the insect at the time), on about the third day afterwards all the water is absorbed from the tubes, and they become filled with air. Illuminated by the parabolic condenser, and viewed with the binocular microscope, and a low power, the gnat-larva is a superb object. The body of the insect is but faintly visible, but, in its place, is dis- played a marvellous tracheal skeleton, with each tube FIG. 106. A sectional vieio of Wenham's Parabolic Illuminator. standing out in perspective, shining brilliantly, like a structure of burnished silver. Unfortunately, such objects are not permanent, for when the whole of the free w r ater dries up, the tracheal tubes either collapse or become refilled with fluid. As the blackness of field, and luminosity of the object, depends upon the excess of light from the paraboloid received beyond the angle of aperture of the object-glass, it is found in practice that more and more of the inner annulus of rays from the paraboloid has to be stopped off, until, at last, with high-angled objectives, it is scarcely possible to obtain a black field. V 174 THE MICROSCOPE. The parabola answers quite well for objects in balsam or mounted dry, but its application scarcely extends to object-glasses higher than l-5th, unless of large aperture. 1 Wenham's parabolic reflector, seen in section, fig. 106, a a, of a tenth of an inch focus, has a polished silver surface, the apex of which is cut away so as to bring the focal point at a proper distance above the top of the apparatus (which is closed with a screw- cap when not in use), thus allowing the pencil ot light to pass through the thickest glass cover used for mounting. At the base of the parabola is a disc of thin glass 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 stopping all direct rays from passing. The reflector is moved to and from the object by means of the rack and pinion c, with a similar adjust- ment for centring, and is either fixed under the stage of the microscope or made to slide into the sub-stage ; in addition, there is a revolving diaphragm d, with two apertures e e, placed diametrically, for the purpose of obtaining two pencils of oblique light in opposite directions. In using the paraboloid, the plane mirror is so ad- justed that parallel rays enter it and impinge on the rabolic sides of the reflector, in such a manner as to totally reflected without suffering refraction, and meet in the centre of a spherical hollow made in the top of the paraboloid. The adjustable stop being either raised or lowered, will effectually arrest all superfluous rays. The light most suitable for this method of illumina- tion is lamp, the rays of which should in all cases be rendered more parallel by means of a large plano-convex lens, or condenser. The Immersion Illuminator. Mr. Wenham, in the year 1856, described various forms of oblique illumina- tors, one of which was an immersion ; a simple right- (1) See an excellent summary of the value of parabolic illumination and immersion illuminators by Mr. J. Mayall, junr., Vol. II., p. 27, Journal of the Royal Microscopical Society (1879). THE IMMERSION ILLUMINATOR, 175 angled prism, connected by a fluid medium of oil of turpentine, or oil of cloves. This, however, was aban- doned for a nearly hemispherical lens, connected with the slide, and which, although a great im- provement, did not reach the point of excellence Mr. Wenham was looking for. Ultimately he FIG. 107. FIG. io7a. adopted a semicircular disc of glass of the exact form and size represented in the drawing, fig. 107, being a side view, and fig. 10 7a, an edge view of the same, and having a quarter- inch radius, with a well-polished rounded edge, the sides being grasped by a simple kind of open clip attached to the sub-stage. The fluid medium used for Connecting the upper surface with the slide being either water, glycerine, or oil ; a certain increase of obliquity being obtained by swinging the ordinary mirror sideways. By means of an illuminator of the kind difficult objects mounted in balsam were resolyed. This simple piece of glass, in appearance somewhat resembling the half of a broken button half an inch in diameter, collects and concentrates light in a sur- prising way, and is by no means a bad substitute for the more costly forms of achromatic condenser. It can be used either in fluid contact with the slide, or dry, as an ordinary condenser. Mr. Wenham subsequently contrived a small trun- cated glass paraboloid, for use in fluid contact with the slide ; water, glycerine, gum, oil, or other substance being employed as a contact medium. The rays of light in this illuminator being internally reflected from a convex surface of glass, impinge very obliquely on the under surface of the slide, and are transmitted by the fluid uniting medium, and internally reflected from the upper surface of the cover-glass to the objective. To use the reflex illuminator efficiently it must be racked up to a level with the stage. The centre of rotation is then set true by a dot on the fitting, seen with a low power, a drop of water is then placed on the top, and upon this the slide is laid. Minute objects 176 THE MICROSCOPE. on the slide, found by the aid of a low power, and distinguished by their brilliancy, or by rotating the illuminator ; the effect on the Podura is superb, the whole scale appearing dotted with bright blue spots in a zig-zag direction. Objects for this illuminator should be specially selected or mounted on the slide. Mr. J. May all, Jun.'s, semi- cylinder or prism for oblique illumination (fig. 108) is a convenient form, as it permits of the semi-cylinder being tilted and placed excentrically ; in this manner, without immersion contact, and by suitable adjustment, a dry object can be viewed with any colour of monochromatic light. If placed in immersion contact with the slide, the utmost obliquity of in- cident light can be obtained. Objects in fluid may be placed on the plane- surface of the semi-cylinder, and illuminated by ordinary transmitted light, or rendered " self-luminous " in a dark field, as with the hemi- spherical illuminator or Wenham's immersion paraboloid. A concave mirror with a double arm is quite sufficient to direct the illuminating pencil. This semi- cylinder was originally made by Tolles, of Boston, for measuring apertures, but, at Mr. Mayall's suggestion, Messrs. Ross mounted it as an illuminator. The Achromatic Condenser. The aim of the microscopists in bringing the achro- matic condenser into use, is to secure a pencil of light that shall approximately fill the aperture of the objec- tive, and by the intervention of central stops, or slots, the various portions of the cone of condensed light, according to the kind of object under examination, shall fully utilize the same. The peculiar advantages of employing an achromatic condenser for the purpose indicated, were first pointed out by Dujardin, since which time an object-glass FIG. 108. Mayall's Semi-Cylinder Illuminator. GILLETT S ACHROMATIC CONDENSER. 177 has been frequently but inconveniently employed ; and more recently much, attention has been bestowed upon achromatic illuminators by most of our instrument makers. It is now some years since Mr. Gillett was led by observation to appreciate the importance of controlling and condensing the quantity of light by a diaphragm placed anywhere between the source of light and the object. This he found more fully effected by a diaphragm placed immediately behind the achro- matic illuminating combination. Such a diaphragm is represented in fig. 109, Ross's original Gillett. It con- sists of an achromatic illuminating lens c, which is about d. FIG. 109. The original form ofGrilletfs Achromatic Condenser. equal to an object-glass of one-quarter of an inch focal length, with an angular aperture of 80. This lens is screwed on to the top of a brass tube, and intersect- ing which, at an angle of about 25, is a circular rotat- ing 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, proportionally limits the light transmitted through the illuminating lens. The circular plate in which the conical diaphragm is fixed is provided with a spring and catch ef. the latter indicating when an aper- ture is central with the illuminating lens, also the num- 178 THE MICROSCOPE. ber of the aperture as marked on the graduated circular plate. Three of these apertures have central discs, for circularly oblique illumination, allowing only the pas- sage of a hollow cone of light to illuminate the object. The illuminator above described is placed in the second- ary 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 rectangular adjust- ment, which is effected by means of two screws I m, one in front, and the other on the left side of its frame. This tube receives and supports all the various illumi- nating and polarising apparatus, and other auxiliaries. Very many modifications of Gillett's condenser are known to microscopists, by far too numerous to de- scribe in detail. Boss's im- proved form is made to slip into the sub-stage in the same way as his Improved Achromatic Condenser (fig. 110), and when arranged for oblique illumination, is an extremely efficient instru- ment. The optical part is FIG. no. Ross's improved Achro- similar to a T 4 oths object- matic Gillett Condenser. glass. It has two Sets of revolving diaphragms with apertures and stops, for showing surface markings in a brilliant manner. Directions for Using Gillett's Condenser. In the ad- justment of the compound body of the microscope for using with Gillett's illuminator, one or two important points should be observed first, centricity, and second- ly, the fittest conpensation of the light to be employed. With regard to the first, place the illuminator in the cylindrical tube, and press upwards the sliding bar k in its place, until checked by the stop; move the microscope body either vertically or inclined for convenient use ; and with the rack and pinion which regulates the slid- ing bar, bring the illuminating lens to a level with the upper surface of the object-stage ; then move the 179 arm which, holds the microscope body to the right, until it meets the stop, whereby its central position is attained ; adjust the reflecting mirror so as to throw light up the illuminator, and place upon the mirror a piece of clean white paper to obtain a uniform disc of light. Then put on the low eye-piece, and a low power (the half -inch), as more convenient for the mere adjustment of the instrument ; place a transparent object on the stage, adjust the microscope-tube, until vision is obtained of the object ; then remove the ob- ject, and take off the cap of the eye-piece, and in its place fix on the eye-glass called the " centring eye- glass," described below, which will be found greatly to FIG. 111. Beck's New Achromatic FIG. Ilia. Beck's Dry Achromatic Condenser. Condenser. facilitate the adjustment now under consideration, namely, the centring of the compound body of the microscope with the illuminating apparatus of what- ever description. The centring-glass, being thus affixed to the top of the eye-piece, is then to be adjusted by its sliding- tube (without disturbing the microscope- tube) until the images of the diaphragms in the object- glass and centring lens are distinctly seen. The illu- minator 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 recognize the diaphragm belonging to the illuminator, and by means of the two adjusting screws, to place this diaphragm N 2 180 THE MICEOSCOPE. central with the others : thus, the first condition, that of centricity, will be accomplished. Remove the white paper from the mirror, and also the centring-glass, and replace the cap on the eye-piece, also the object on the stage, of which distinct vision should then be obtained by the rack and pinion, or fine screw adjust- ment, should it have become deranged. Beck's New "Wet and Dry" Condenser (fig. 111). In an earlier form of dry condenser (fig. Ilia) Messrs. teck made use of a revolving front, with the intention FIG. 112. Powell and Zealand's Condenser. of obtaining large angular aperture, and of rotating a series of lenses. They have more recently introduced a new form, and the advantages to be gained are First, That it is available for either dry or immersion object- glasses up to 1'3 numerical aperture on diatoms, &c., or dry ones on histological objects. Secondly, That the spherical form of the front, worked by a milled head, enables a series of lenses to be used, and yet avoids the inconvenience of having the connecting fluid drawn away from the one in use by capillary attraction, as POWELL AXD LEALAND'S IMMERSION. 181 would be the case if they were mounted on a flat surface. It also interferes less than the old form with the movements of the stage. Powell and Lealand's Immersion Condenser, or non- achromatic condenser (fig. 112), is constructed on a somewhat novel plan. It admits of a very large angle of light, about 130 degrees, and allows of the use of either central light, or one or two oblique pencils of 00 degrees apart. Two diaphragm slots (shown in the woodcut apart from the condenser), fit in at A and B ; FIG. 113. Swift's Achromatic Condenser. by means of which two beams of light at right angles can be used. The movement of these diaphragms is effected by means of an outer sliding tube b with a slot at the top, and into which the arm A fits; whilst another at B gives a ready command of the rotation of the two, either together or separately, thus producing consider- able modifications of light. Swift and Son's Achromatic Condenser is conveni- ently arranged to supply the place of a compound sub- stage, and to receive accessory diaphragms. The optical 182 THE MICROSCOPE. combination A is computed to be used as an effective spot-lens from a 3-inch objective up to a sixth, c C are two small milled heads by means of which the optical combination A is centred to the axis of the objective. The revolving diaphragm E has four apertures for the purpose of receiving central stops, oblique light discs, and selenite films. D is a frame carrying two revolv- ing cells, into one of which a mica film is placed, which can be revolved with ease over either of the selenites below, whereby changes of colour can be obtained in experimenting with polarised light. The darts and p A'S indicate the position of the positive axis of the mica and selenic films, and by this means results can be recorded, &c. Either of the revolving cells can be FIG. 114. Swift's Diaphragms and Central Stops. thrown into the centre of the condenser, and there stopped by means of a spring catch ; when so arranged the mica film, &c., may be revolved in its place by turning the cell D, asboth cells are geared together with fine racked teeth. F is a polarising prism mounted on an eccentric arm, rendered central when in use, or thrown out, as seen when out of use. G is the rack dove- tail slide for indicating focussing the condenser on the object. The advantages of this condenser consist in having the polarising prism, selenite films, dark ground and oblique light stops, so that they may be brought close under the optical combination. Collins's Webster's Universal Achromatic Condenser (fig. 115) is a mechanical contrivance provided with COLLINS'S UNIVERSAL CONDENSER. 183 a shutter diaphragm. This addition to the microscope can be used with any instrument and without a sub- stage, and is on this account easily adapted to the FIG. 115. Collinses Webster's Universal Condenser. Shutter Diaphragm seen separately. cheaper forms of microscopes. Its advantages are, that it is moderately cheap, is at once an achromatic condenser, parabolic illuminator, and graduating dia- phragm and polariser. By means of a lever, the central aperture can be gradually closed, and, pro- vided the object-glass has sufficient "resolving power," it facilitates the resolution of the more difficult test-objects. With a " spot-lens stop" the object is illu- minated on a dark ground, and when high powers are used in con- nection with the polariscope, the advantage derived by such an ad- dition to the ordinary mode of illumination is considerable. Mr. Hyde's " Condenser " is con- structed for use with immersion objectives, having apertures greater than correspond to 180 in air. The lens is a right-angled prism, having a plano-convex lens fitted in an up- right, and mounted in brass to slip into the sub- stage (fig. 116), will condense parallel rays to a focus on a balsam-mounted object, through a slide of average FIG. 116. Hyde's Illu- minator. 184 THE MICROSCOPE. thickness, \/hen the illuminator is brought into immer- sion contact. Its action is diagramatically shown in fig. 117. A is the first lens of an immersive objective in fluid contact with the cover-glass ; o the object in balsam ; p a right-angled prism in immersion contact with the base of the slide ; L a lens designed to focus FIG. 117. the illuminating rays on the object o. For oblique illumination, as seen in the figure, the apparatus must be thrown out of the axis of the microscope, and in this way and with any objective of less aperture than 90 in glass would give a dark field. If brought nearer the axial line it is evident that less oblique rays could be used. Mr. John Mayall devised a set of spiral diaphragms as a convenient mode of obtaining oblique illumination in con- nection with high-an- gled condensers. If a slot diaphragm, fig. 118, be fixed close be- Fio. 118. Mayall's Spiral Diaphragm. neath the larger lens, such as those of Powell .and Lealand, Zeiss and other makers, the rotation under it of a diaphragm having a spiral opening, CATOPTRIC ILLUMINATOR. 185 will give a pencil of light at varying degrees of ob- liquity throughout the range of the aperture of the condenser. The azimuthal direction of the incident pencil will be controlled either by rotating the object or the condenser carrying the diaphragms ; whilst the rotation of the spiral in the fixed slot will not change the direction in the azimuth but in altitude, so far as the aperture of the condenser will permit. Mr. J. W. Stephenson's " Catoptric Immersion Illumi- nator " attains its object in a simple way. Fig. 119 represents the form and size of the little piece of apparatus. It is a plano-convex lens worked on a 1-inch tool, and having a diameter of 1*2 inches, which is then edged down to 1 inch, as being more convenient in size, and as giving an aperture sufficient for the purpose. The upper or convex side of the lens is cut down or flattened, so as to give a surface of -A- of an inch in FIG. 119. catoptric im- diameter, with which the slide is to be brought into contact, by a drop of oil, glycerine, or water. The upper curved surface is silvered ; be- neath the lens a flat silvered plate gV of an inch thick, and corresponding in size and position with the upper flattened surface, is balsamed. The incident ray is thus rendered normal to the under surface, and is thrown back on the plane or under surface of the lens, whence the more oblique rays falling beyond the central angle are totally reflected and conveyed to a focus. A stop is placed about an -Jth of an inch or less below the condenser, and the opening used is of a lens- shaped form, which admits a broad beam of light with- out appreciable spherical aberration. The Iris dia- phragm greatly improves this illuminator. 1 The Oil Immersion Condenser. In this is combined the latest improvement in immersion condensers. In operation, an oil-medium possesses superior advantages in connection with high-angled objectives. The oil (1) Journal ofR. M. S., Vol. II., p. 36, 1879. 186 THE MICROSCOPE. immersion condenser of Powell and Lealand is an im- proved form, consisting of the truncation of the vertex of the upper lens of the condenser, and admits of the lower lens being brought into closer proximity, when the marginal rays become more effective. Its speciality is the conversion of axial light into con- densed obliquely incident light by the refraction of the condenser. For the illumination of opaque objects under high powers, Tolles of Boston, U.S.A., introduced a verti- cal illuminator into the body of the microscope close to and above the objective. The Vertical Illuminator consists of a small silver speculum (Professor Smith), or a movable disc of thin glass (Beck), or a small piece of parallel glass, placed at an angle of 45 (Powell and Lealand), and fixed in a short tube, with a side aperture, interposed between the objective and the body of the microscope ; by which means a pencil of light entering at the aperture, and striking against the speculum or inclined surface of the disc, is reflected downwards through the objec- tive and upon the object placed on the stage of the microscope. The object-glass is thus made its own achromatic condenser. When this form of illuminator was introduced, it was soon discarded on account of the halo or fog which surrounded the image, and which was caused, as Mr. Stephenson explained, by the reflec- tion, at the upper surface of the cover-glass, of the rays transmitted through the objective. With the introduction of the oil-immersion objective all this fogging disappeared ; the front lens of the objective, the intervening stratum of oil, and the cover-glass of the object all become optically continuous, so that the upper surface of the cover-glass virtually ceases to exist, the only reflection being from its under surface, when dry objects are used. " The explanation is that if the vertical illuminator be adjusted, and used with an immersion objective, having a numerical aperture greater than I'O, focussed on a plane glass slip, it is evident that (practically) all that part of the pencil comprised within the numerical aperture I'O will THE AMICI PEISM. 187 emerge at the plane base of the slip, and be lost to view ; but the peripheral zone of the pencil beyond the numerical aperture I'O will not emerge, but is totally- reflected at the internal surface of the base of the slip, and is seen as a luminous zone surrounding a nearly dark field (the field is not absolutely dark, because of the ordinary reflection of light that takes place before the emergence of the central pencil)." 1 Method of Using Condensers. Whatever the special form of apparatus, it should always subserve the pur- pose of condensing the light reflected by the mirror to a correct focus upon the object. The light reflected from either the plane or concave mirror should pass through the axis of the condenser, moving at the same time in all directions, and in the axis of the objective, body, and eye-piece of the microscope. The secondary stage should be made to admit of perfect centring and be provided with a racking adjustment. Upon changing the objective, Ross's centring eye-glass must be brought into use to ensure the centricity of the condenser and the body of the microscope. 3 I he Amici .Prism was originally designed for oblique illumination. It con- sists of a flattened triangular glass prism, the two narrower sides of which are slightly convex, while the third or broadest side forms the reflecting surface. When properly used, it is capable of transmitting a very oblique pencil of light. The prism is usually mounted, as in fig. 120, for slipping into the sub- stage. The LieberJcuhn. The concave speculum termed a (1) English Mechanic. (2) This centring-glass consists of a tubular cap with a minute aperture, containing two plano-convex lenses, so adjusted that the image of the aper- ture in the object-glass, and the images of the apertures of the lenses and the diaphragms contained in the tube which holds the illuminating com- bination, may all be in focus at the same time, so that by the same adjust- ment they may be brought sufficiently near to recognize their centricity. IS? THE MICKOSC01T. "Lieberkuhn," from its celebrated inventor, -was for- merly much in use as a re- tleetor, but is uow almost abandoned, or rather replaced by other and better contriv- ances. The Uoberkiilm is generally attached to the object -class, in the manner represented at tig. 1'Jl. Nvhoro a exhibits the lowor part of the compound body, /, the object-glass, over whieh is slid a tube and the Lieber- kiilin, ( \ attached to it ; the ra\ s of lii^-ht retleetevl fi'oin the mirror are brought to a focus upon an object ide, and is employed on this account. This is ei: into the stage of the microscope or used on a separate Maud, so that it may be turned in any direction towards the source of light The parabolic side- reflector (fig. 123y such an arrangement, the light produces the reverse appearances of the former mode of illumination, and is a valuable aid in determining the true condition of the object. The Bull's-eye Condenser. The Fio. 123. Beck's Silver FIG. 123a. Beck's Pmraljolic Reflector. bull's-eye condensing lens (fig. 124) is used for con- verging rays from a lamp upon the mirror; or for reducing the diverging rays of the lamp to parallelism, for use either with the parabolic illuminator, or silver side-reflector. A plano-convex lens of about three inches focal length, is the form generally 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 shortened. When used by daylight, its plane side should be turned towards the object, and the same position should be given when used for converging the rays from a lamp ; but when used with the parabolic 190 THE MICROSCOPE. or side-reflector, the plane side must be turned towards the lamp. FIG. 124. -Bull's-eye Condensing Lens. The Microscope Lamp. The introduction of paraffin into household use has somewhat modified our views with regard to the most suitable artificial source of illumination. Paraffin burns with a whiter and purer flame than either oil or gas, and consequently is less liable to pro- duce fatigue or injury to the eyes. The first cost of the lamp is trifling ; for a moderate sum a handy form of lamp can be pro- cured, mounted on an adjustable sliding-ring stand, and with a porcelain, metal, or paper shade, to protect the eyes from scat- tered rays of light (fig. 125). To give the increased effect of whiteness to the light ("white FIG. 125. Beck's Microscope cloud illumination" as it is Lamp. termed), take a piece of tissue paper, dip it into a hot FINDERS AND INDICATORS. 191 "bath of spermaceti, and, when nearly cold, cut out a circular piece and secure it over the largest opening in the diaphragm plate. This will materially moderate and soften the light. 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 particularly good specimen of a diatom, so that it may be referred to at a future time. The subject will be found fully discussed in the pages of the Journal of the Royal Microscopical Society. The traversing stage of FIG. 126. Amyot's Object finder. the microscope admits of such finders as those of Mr. Okeden, Mr. Tyrrell, Mr. Amyot (fig. 126), &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 relation to the fixed plate, while the second gives him the like power of setting the horizontally-sliding plate by the intermediate. For those microscopists whose instruments are with- 192 THE MICROSCOPE. out a traversing stage " Maltwood's finder " will be found an efficient substitute. It 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 contains two numbers marking its " latitude," or place in the vertical series, and its " longitude," or place in FIG, 127. -Dipping-tubes. FIG. 127a. Stock-bottle. 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. Messrs. Beck supply this finder with their microscopes. Dipping-tubes are tubes of glass (tig. 127) about nine COLLECTING- STICK. 193 inches in length, open at both ends, and from one- eighth to one-fourth of an inch in diameter. The ends must be nicely rounded off in the flame of a blow-pipe; some of them should be made perfectly straight, whiJe others should be bent or drawn out to a fine point, and made either of the shapes represented. FIG. 128. A. Trough for showing Circulation in Fisli-tail. B. Collecting-bottle and Stick. Fig. 128, at B, is represented a convenient and port- able " Collecting-bottle and Stick ;" an ordinary cane divided by a screw, or socket-joint, into two parts for the convenience of packing, and terminated by a brass ring, which is adapted to receive a wide-mouthed bottle. A small fine-gauze net and a hook can be screwed into the same stock, and the whole packed into a small compass. Fio. 129. Net for collecting Minute Animals. Compressorium. The purpose of this accessory 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. 130. Boss's Compressorium consists of a stout plate of brass A, about three inches long, having in its centre a 194 THE MICROSCOPE. 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 de- sirable. The upper movable part I) is attached to a screw- motion at C ; and at one end of the brass plate A, which forms the bed of the in- strument, is an up- right piece of brass C, grooved so as to receive a ver- tical plate, to which a downward motion is giyen by a FIG. 130. Ross's Compressorium. FIG. 131. Beck's Parallel-plate Compressor. single fine screw, surrounded by a spiral spring, which elevates the plate as soon as the screw-pressure is re- moved. Beck's Parallel-plate Compressor, fig. 131, affords a FIG. 132. Botterill's Live-trough. more exact means of regulating the pressure, and can be used for a variety of purposes. It is also easily cleaned. Lire-troughs are made to partake of a variety of GLASS TROUGHS AND CELL?. 195 forms and shapes. Botterill's (fig. 132) consists of two brass plates, screwed together by binding screws, and holding between them two plates of thin glass, and which, are maintained at a proper dis- tance by inserting half of a circular flat disc of india-rubber. Beck's glass trough, for chara and polypes, a sectional view of which is shown at fig. 133, is made of three pieces of glass, the bottom being a thick strip, and the front a of thinner glass than the back & ; the whole is cemented together with Jeffery's marine- glue. The method adopted for confining objects near to the front glass varies ac- cording 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 PIG 13 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, the microscope should be placed in a nearly horizontal position. Growing. cells. Considerable attention has been given FIG. 134. Weber's Slip with Convex FIG. 134a. Seek * Current-slide Live- Cell for use as a Live-trough. cell. to various forms of growing-cells for maintaining a con- tinuous supply of fresh water to objects under observa- tion, and for the purpose of sustaining their vital energy for a long period. The employment of live-cells is strongly commended to microscopists, as there is yet much to be discovered concerning the metamorphoses which some of the lower microscopic forms of plant o 2 196 THE MICROSCOPE. 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 alternate according to the varied Fro. 135. Holman's Life Slide. Full size. conditions of temperature and nutrition under which they are placed. Holman's life slide consists of a 3 x 1 inch glass slide, with a deep oval cavity in the middle to receive the material for observation. A shallow oval is ground and polished around the deep cavity, forming a bevel. FIG. 136. Holman's Moist Chamber. From this bevel a fine cut extends, to furnish fresh air to the living low forms of life which invariably seek the bevelled edge of the cavity, thus bringing them within the reach of the highest powers. Mr. Holman contrived a form of " moist chamber,'' HOLMAN'S SYPHON SLIDE. 197 or animalcule-cage, fig. 136, for the purpose of study- ing the growth of fungi and other delicate organisms, without in any way disturbing them for a lengthened period. This will also be found useful as a dry chamber for holding minute insects, and preserving them in a living condition for observation. Zentmayer's Holman Syphon Slide (fig. 137) is used either as a hot or cold water cell. It should be deep enough to hold a small fish or newt, and retain it with- out any undue pressure. When in use it is only necessary FIG. 137. Hotmail's Syphon Slide. to place the animal into the groove with some water, cover it with the glass cover, and immerse one of the rubber tubes in a jar of water, the other receiving it as it passes away. When the slide is on the stage of the microscope the jars should stand on a lower level, so that the slide be made the highest part of the syphon. The pressure of the atmosphere is sufficient to keep the cover-glass in its place. This apparatus is also adapted for the gas microscope. The examination of various kinds of infusorial life be greatly facilitated by the addition of the small- 198 THE MICROSCOPE. est particle of colouring matter, either carmine OP indigo. Mr. Thomas Bolton 1 directs a small quantity of either of these colours to be rubbed up in a little water in a watch-glass, and a portion taken up on the point of a brush, and the brush run along the top of the water in a trough; sufficient will be left behind to barely tinge the water with the colour, but this will gradually subside over the rotifers. Under the microscope this minute quantity will be seen like a rising cloud of dust, which as soon as it comes near a rotifer is whirled round in definite curves, showing at once the action of its wonderful coronary cilia. This colouring matter is greedily devoured by these creatures, and may be followed from the mouth to the digestive canal. If rotifers or infusoria are already in a cell and under a thin cover, a drop of the mixed colour may be placed at the edge of the cover- glass, and a piece of blotting paper touched at the other side will draw a current through the cell. The cilia and fine flagella on many of the small protophytes and infusoria, which are very difficult to see while they are in full activity, are easily seen when dying or after death from a drop of iodine. The effect of colour- ing matter on Volvox globator, Euglena viridis, and Protococcus pluvialis is very interesting ; besides show- ing the cilia, it brings out many histological specialities, which are otherwise invisible. Aniline dyes are occa- sionally useful for colouring. Osmic acid is used for killing infusoria quickly in their expanded condition, and they may afterwards be stained advantageously with picrate of carmine. The most useful aquaria for preserving and breeding minute organisms is the ordinary confectionery cake-glass inverted. A square block of wood (8 in. square) with a hollow turned in the centre is required to receive the knob. It should be covered with a round glass to exclude the dust. For finding or selecting minute animals, or dis- secting botanical specimens, the Houston-Browning (1) Mr. T. Bolton, of 57, Newhall Street, Birmingham, furnishes interest- ing tubes of living specimens for the microscope at a trifling wst to hi torrespondents. DISSECTING SPECIMENS. 199 Dissecting Microscope will be found a handy and use- ful form, fig. 138. This instrument consists of a duplex lens of three powers, magnifying 4, 6, and 10 diameters, screwed to the end of a brass focussing tube, and moving upon a brass pillar attached to a sliding bar at the bottom of the box. The dissecting stage is a cork slide, plain on one side for general work, and a shallow cell on the other, for the dissection of such objects as small glossy seeds which "fly" under the needles, whilst a pitted glass slide, for carrying on dissections under water, is also provided. Microscopic Dissection. The mode of using needles for teasing out tissue is very simple. With a pair FIG. 138. Browning's Houston Botanical Dissecting Microscope. of the small needles held firmly between the fore- finger and thumb, as shown in fig. 139, the structure must be teased out ; an operation which requires some care. All substances should be carefully separated from dust and other impurities which render their structure indistinct or confusing. With delicate mem- branes, those of the nervous system of the smaller animals, insects, etc., it is necessary to make the dissec- tion under water, or in fluid of some kind. For this purpose take a glass cell, and then throw a strong light down upon it by the aid of condensing lens, as repre- sented in fig. 140. Delicate structures will be better teased out in a dilute solution of sugar or common salt, to prevent change from endosmose. Should the object 200 THE MICROSCOPE. be a portion of an injected animal, it is better to pin it out on a leaded cork, covered with white wax, and im- mersed in a water-trough, as in fig. 140. The dissec- tion of delicate vegetable structures is better carried on under water. Dissecting Needles, Knives, and Scissors. In addition FIG. 139. Baker's Student's Dissecting Microscope. to forceps, needles, and knives, scissors are necessary for purposes of dissection. The most useful, straight and curved, are shown in fig. 141. In dissections under the microscope, the curved-pointed pair / will be convenient. In all the points should fit accurately together. SECTION CUTTING. 201 Section-cutting Instruments. Solid tissues are, as a rule, much too hard to admit of being cut either by FIG. 140. Dissecting in, a fluid Medium. scissors or Valentin's knife. As important information will be gained respecting the structure of such sub- Fio. 141.. "Dissecting Scissors and Forceps. stances as stems and roots of plants, horns, hoofs, car- tilages, and other firm parts of animals, by cutting 202 THE MICROSCOPE. thin sections, it would be quite impossible for the microscopist to get on without a section-cutting instrument. Hailes' Section- cutting Machine. A is a short tube of about 1J inches in diameter, provided with flanges B, B, at each end. The upper one of these flanges serves as a cutting bed or table. Inside the tube A is fitted, so as to slide freely up and down, a second tube c, in which is placed the material intended to be cut. This inner tube is provided with two clamping screws d, d, topped into a block which passes through a slot formed in the outer tube A, thereby preventing any PIG. 142. Dissecting Knives. FIG. 142a. Needles for teasing out Tissue. rotary movement of the inner tube. Inside the tube c, and at its lower end, is secured the nut or boss D, through which passes the micrometer screw E, pro- vided with a milled head e, and a divided collar /. This screw is carefully shouldered into a cock or bracket r, forming part of the lower flange B. In order to secure the machine firmly to the table, the upper flange B, is screwed to a transverse bar of wood G, which in its turn is secured to the table by the clamp H, thus avoiding all strain upon the machine itself. The material it is intended to cut may be packed in any convenient manner in the inner tube C, and secured by the clamping screws d, d. It will now be clearly seen HAILES' SECTION CUTTEE. 203 that by turning the micrometer screw E the inner tube C will be carried steadily upwards, and with it the mate- rial to be cut, without compression, and consequently without any jerking. The sections, however, may be cut with a chisel or with a razor in the usual manner. The upper flange B of the machine, which serves as a cutting table, is provided with two strips of hardened and polished steel a. These form convenient surfaces over which the cutting tool may be passed, when cutting wood or softer sections, and they also serve another important purpose. At the back of the cutting table is secured, by means of a spring and screw and steady-pins, a metal block fc. This block carries, fixed in it, two hard steel rods c, which overlie the strips a. By passing the blade of a fine saw between the rods c and the strips a, sections of bone or other material too hard to be cut with a knife, may be sawn off as thin as the nature of the material will permit, in some cases sufficiently thin to permit of their being at once mounted. The rods c and strips a, being of the hardest steel, receive no injury from the teeth of the saw. The machine is made by Baker, 144, Holborn. 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. FIG. 143. Hailes' Improved Stction-cutting Machine. 204 THE MICROSCOPE. With a little practice tlie finest and thinnest possible slices will be cut. It is usual to first slice off a few thicker slices to give a smooth and even surface to the specimen. Then turn the screw to raise it a little, sprinkle the surface with spirit and water, and cut with a light hand. Remove the cut sections with a fine camel's-hair brush or blotting-paper to a small vessel containing water, when the thinnest sections will float on the surface, and are more easily selected and removed to a bottle of spirit and water, where they should remain until they can be mounted. Sec- tions of hard woods, and of those containing gum, resin, and other insoluble materials, must first be soaked in alcohol or ether and then transferred to oil of cloves, to render them sufficiently transparent for mounting. I FIG. 143a. Sections of Wood. 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, vertical, and tangental : each of these will exhibit different appearances, as may be seen upon reference to fig. 143# : 6 is a vertical section through the pith of a coniferous plant : this exhibits the medullary rays, which are known PREPARATION OF HARD TISSUES. 205 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 fa, portion of the same magnified: the openings of the medullary rays mm, 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. Specimens of wood that are very hard and brittle should be first softened by boiling in water ; and as the cutting-machine- will answer for other structures besides wood, it should be understood, that 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 lacunas 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, 206 THE MICROSCOPE. 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 bone-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. 143. 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 ohlkiue and transverse sections through the dentine Fig 1436 Small Lathe for polishing. MAKING SECTIONS OP TEETH. 20? 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 shorter 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* Uld lime ; it is much used by dentists M & polishing material. 208 THE MICROSCOPE. hardness should be tested with the edge of the thumb nail, for it should be with difficulty indented by pressure, and yet 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 CIRCULAR DISC. 209 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 stiff er 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 mounting objects, Beck's Walmsley's turn-table (fig. 143c) is most useful. Fia. 143c. Beck's Walmsley's Cell-making Instrument. For making cells, take a camel's-hair pencil, pre- viously dipped in japanners' gold-size, hold it firmly between the finger and thumb, and set the wheel in motion, when a perfect circle will be rapidly formed ; it must be put aside to dry. To cut cover-glasses secure 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 microscopist for the above purpose, but also for writing the names of mounted objects on the ends of the glass slides. p 210 THE MICROSCOPE. a 5 5" r^ w aj 9 fco fco | bo " CJj _C Pi ~Z *n "3 u > 03 c 05 rr o cS 3 _fl tc &0 I s a -3 1 M c ]2 11 2 f ', ** o . i ** a 1 j Jj ^'^ ^ ETfl 9 n the point of a large needle or pointed pen-knife, and 214 THE MICROSCOPE. drop ifc 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 theJ 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. 143/*), to prevent the formation of bubbles from the sudden dis- placement of the air. It requires some little practice to keep the object in the centre of the circle. Notwithstanding very great care in manipulation, air- bubbles will appear. These may, however, be removed 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 ^)e 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 ia PREPARING AND MOUNTING. 215 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 Jeflerey'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 216 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 riount and preserve specimens o'f aiihnal tissues In shallow cells, to avoid undue pressure on the preparation. Cells intended to contain preparations 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. 143<7). The surface of the glass shoul(i 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, trachea, ovi- positors, stings, tongues, palates, cornece, &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 onpleasant proportions under the microscope, and not only MOUNTING AND PRESERVING OBJECTS. 217 mar the beauty, but posssiblj 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 open, the practice of closing the eye not in use being injurious to the sight of both. The beginner who is about to purchase a microscope, 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. In mounting objects in fluid, the glass cover should come nearly, but not quite, to the edge of the cell ; a slight margin being left for the cement, which ought to project slightly over the edge of the cover, in order to unite it securely to the cell. To preserve and mount diatomaceas in as nearly as possible a 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 various beautiful forms under the higher powers of the microscope, require more care in preparation. The guano, or in- fusorial earth containing them, should first be washed several times in water till the water is colourless, allowing sufficient time for precipitation between each washing. The deposit must then be put into nitro- hydrochloric acid (equal parts of nitric and hydro- chloric acids), when a violent effervescence will take place. When this has subsided, the whole should be subjected to heat, brought nearly up to the boiling- point, for six or eight hours. The acid must now be carefully 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 Borne of the lighter forms. The sediment must be 218 THE MICROSCOPE. examined under the microscope with an inch object- glass, and the siliceous valves of the diatoms picked out with a coarse hair or bristle. Dr. Rezner's Mechanical Finger (fig. 143/fc) for select- ing and arranging diatoms, adaptable to any micro- scope, is made to slip on to the objective far enough to have a firm bearing, and so that the bristle point can be brought into focus when depressed to its limit. It is clamped in its place by a small thumb-screw. The bristle holder slides into its place so that it can be brought into the centre of the field. When using the finger, the bristle is first raised by means of the micro- meter screw till so far within focus as to be nearly or quite invisible, then the objective is focussed on to the FIG. 1437t. Rentier's Mechanical Finger. slide, and the desired object sought for and brought into the centre of the field ; the bristle point is then lowered by the screw until it reaches the object, which usually adheres to it at once, and can then be exam- ined by rotating the bristle wire by means of the milled head. Professor H. L. Smith offers some useful hints which will facilitate the use of this finger. 1 The medium used for mounting diatomaceos is of very considerable importance, inasmuch as their visi' bility is either diminished or much increased there- by. Professor Abbe, experimenting with the more minute test-objects, diatoms, &c., found monobro- mide of naphthaline gave increased definition to most of them. This liquid is colourless, somewhat of an Journal of the Royal Microscopical Society, Vol. II., 1879, pp. 952-3. MOUNTING DIATOMS. 219 oleaginous nature, and is soluble in alcohol. Its density is 1*555, and refractive index 1'6. Its index of visibility is about twice that of Canada balsam. Mr. Stephenson, who first directed special attention to the subject, came to the conclusion that the visibility of lined objects depends upon the difference of the refractive indices of the object observed and the medium in which it is placed. Taking the refractive index of air as 1*0, and diato- maceous silex as 1'43, the visibility may be .expressed by the difference *43. The following table may be constructed : Refractive indices Visibility of sile.it (taken approximately). (Refr. index = T43). Water = 1'33 ... 10 Canada balsam = 1'54 ... 11 Bisulphide of carbon = 1'CS ... 25 Sol. of sulphur in bisulph. ... = 1'75 ... 32 phosphorus ,, ... = 2'11 ... 67 These data relating to visibility must be taken in connection with the numerical aperture 1 of the objec- tives and of the illuminating pencil. The effect pro- duced on diatoms is very remarkable, the markings on their siliceous frustules being visible under much lower powers. So that the visibility of the diatom mounted in phosphorus as compared with balsam is as sixty-seven to eleven ; in other words, the image is six times more visible. Mr. Stephenson's phosphorus medium is composed of a solution of solid or stick phosphorus dissolved in bisulphide of carbon. Great care is re- quired in preparing the solution owing to the very inflammable nature of the materials. So small a quantity of the bisulphide of carbon is required to dissolve the phosphorus that the diatom may be said to be mounted in nearly pure phosphorus. Remark- able enough, this medium has the reverse effect upon some other test-objects, as Podura and Lepisma scales, which lose their characteristic markings. F. M. Rimmington's Glycerine Jelly is especially (1) Professor Abbe introduced a new expression for apertnre (i.e., "numer- ical aperture"), by which the relative resolving power cf different objec- tives is se*n by the reading of their numerical apertures. 220 THE MICROSvOPE. adapted for mounting algse, fungi, vegetable and animal tissues, urinary deposits, casts, epithelium, crystals, starch granules, diatomaceas, &c. For certain delicate organisms, as the desmidiaceee, whose plasma may be- affected by too dense a medium, the jelly may be diluted one-quarter or one-third with camphor- water. Dr. E. Kaiser describes a process for preparing a pure glycerized gelatine : Take one part by weight of the finest French gelatine, steep for about two hours in six parts by weight of distilled water, and add seven parts of pure glycerine. Then to every 100 grams of the mixture 1 gram of concentrated carbolic acid. The whole must be warmed for ten or fifteen minutes, stirring all the while until the flakes produced by the carbolic acid have disappeared. Then filter while still warm through the finest spun glass, pre- viously washed in distilled water. When cold the preparation can be used like Canada balsam. This medium is also an excellent embedding substance for section-making. For this purpose the objects must bo placed in the glycerine-gelatine after again warming. When sections of objects have to be made so delicate that there is danger of their falling to pieces after cut- ting, the object must be left in the warmed glycerine- gelatine until it is thoroughly penetrated by the latter. The gelatine may be removed from the tissues by a fine jet of warm water after the section is made and placed on the slide. For imbedding hard tissues glycerine-gelatine is an excellent medium, for after it is set, any degree of hardness may be imparted to them by treating with absolute alcohol, the time required for this being from ten to thirty minutes. One special recommendation of this substance for imbedding is its- transparency, which enables the operator to see the precise position of the object. For mounting numerous minute objects, Half's Carbolic Acid Fluid is a very useful medium; it is more simple, cleanly and rapid than turpentine for insects, small crustaceans, moluscs, &c. The purest crystals of carbolic acid, with just sufficient water added to render them fluid, produces the best CEMENTING. 221 results. No more should be dissolved than can be used up, as after solution the light spoils it, and gives it colour. Vegetable tissues, foraminifera, the palates of moluscs (the latter, after boiling in liquid potash, and washing in water to remove all traces of alkali), may be immersed in carbolic acid. If it be wished to mount them foi-thwith, then place the specimen, after washing in a glass slip, and drop one or two drops of acid upon it. Should it appear to be thick or cloudy, warm the slide over a spirit lamp ; set it aside to get cool, and drain away the acid, or remove it with blotting- paper. If not perfectly clear add another drop or two of fresh carbolic acid and again warm it ; place a cover- glass over it, remove as much of the acid as possible, and then let a drop of fluid Canada balsam run under the cover to take the place of the acid. Gently warming the slide will facilitate this operation. A number of specimens may be put into a test tube with the carbolic acid solution and boiled for a few minutes, corked up tightly, and put aside for mounting at leisure, either in balsam or dammar. When the balsam becomes too thick, it can be rendered fluid by adding either benzoline or chloroform. Dammar varnish for cementing the cover-glass is prepared as follows : Take of gum dammar, 1 oz. ; spirits of turpentine, 1 oz. ; dissolve by gentle heat : then take gum mastic, 1 oz. ; chloroform, 2 oz. ; dissolve without heat, and, having filtered out all impurities, mix the two solutions together by shaking. Method of Cementing. After many years' experience, I have arrived at the conclusion that for cementing down the cover-glass, there is nothing better than either gold size or gum dammar varnish. The latter, for some preparations, will be improved by the addition of a small proportion of india-rubber dissolved in naphtha. Whichever is used, it should be applied with care and some skill. The brush should be held nearly in the upright position, and the turn-table spun round rapidly, so that the gold size may form a warm ring round the outside of the cover-glass. After the 222 THE MICROSCOPE. gold size has become dry and the slide cleaned off, it may be coloured by aniline mixed in a little cement, or by a coating of water- colour, over which a final thin coating of gold size should be applied. A good ringing medium for balsam mounts is dammar dissolved in chloroform, because if it is in- clined to run under the cover it will readily mix with the mounting material without leaving a visible trace behind. It is better to apply the brush to the edge of the cover almost dry, the slide on the turn-table being made to spin rapidly round, so as to create a track in which the dammar solution will readily flow. The second application is made immediately to follow the first, with the brush full, so that there will be a small drop of solution on the end, and this is allowed to touch the edge of the cover without letting the brush itself come in contact with the glass. This process must be repeated until the ring is built up to the proper size. In drying, however, the ring of dammar will shrink considerably, and thus it is necessary to make a subse- quent application in a few hours' time. Wash away all surplus glycerine by syringing, then apply a ring of a waterproof cement around the cover. Such a cement may be bought under the name of Bell's cement. A better and less expensive cement may, however, be made by dissolving 10 grs. of gum-ammo- niac in 1 oz. of acetic acid (No. 8) ; then add to this solution 2 drachms of Cox's gelatine. This liquid flows easily from the brush and is waterproof, rendered more so if subsequently brushed over with a solution of 10 grs. of bichromate of potash in 1 oz. of water. But what especially recommends this cement is its adhesive power to glass, even should there be a little glycerine left behind on the cover. After the gelatine ring is dry any kind of cement may be employed. When a considerable number of different objects are being prepared at the same time, write the name of each with pen and ink upon the glass slide. 1 .. Mounting Polyzoa. Mr. Morris, of Bath, has suc- [ ceeded in obtaining beautiful specimens of polyzoa and Mr. C. Seller, Microscopical Jourm.* PRESERVATIVE FLUIDS. 223 hydroid zoophytes, with, tentacles expanded, by adding \ / spirit of wine, drop by drop, to the salt-water cell in which they are confined. Animals should be killed in this way as soon as possible after capture. A plan of mounting objects in a mixture of balsam md chloroform is described by Mr. Wm. Henry Heys in the Microscopical Journal : Take a quantity of the oldest balsam procurable, and place it in an open ^lass 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 phials, and cork and set them aside for at least a month. The advantage of having the medium ^eady-made is, that there is no waste, and none of the usual and troublesome preparation required for putting up objects in Canada balsam; if it has stood for some time, it loses the yellow tinge which is observable ji most samples when first mixed, and, moreover, air- 3ubbles escape more readily. Groadby's fluids are cheap and efficient for preserving ind mounting animal structures. The following are lis formulae : Take for No. 1 solution, bay salt, 4 oz. ; alum, 2 oz. ; ;orrosive sublimate, 2 grains ; boiling water, 1 quart : nix. For No. 2 solution, bay salt, 4 oz. ; alum, 2 oz. ; corrosive sublimate, 4 grs. ; boiling-water, 2 quarts : nix. The No. 1 is too strong for most purposes, and should mly be employed where great astringency is needed to *ive form and support to very delicate structures, ^o. 2 is best adapted for permanent preparation ; but leither should be used in the preservation of animals containing carbonate of lime (the mollusca), as the ilum becomes decomposed, and sulphate of lime pre- cipitated. For the preservation of crustaceans use the ;ollowing : Bay salt, 8 oz. ; corrosive sublimate, 2 grs. ; water, L quart : mix. The corrosive sublimate is used to prevent the jroTvth of vegetation in the fluid; but it also pos- 224 THE MICEOSCOPE. sesses the property of coagulating albumen, and there- fore cannot be used iii the preservation of ova, cellular tissue, the white corpuscles of the blood, &c. Goadby's method of preparing -marine-glue for cementing 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 cf heat. It may be rendered thinner by the addition of more naphtha, and redissolved, when hard or dry, by adding naphtha, ether, or liquid potash. Multiple Sfaining, Animal and Vegetable. "Within a short period of time the staining of animal and vegetable tissues for microscopical exami- nation may be said to have almost superseded injec- tions. The results obtained lent a charm to the stain- ing process, and it was soon seen that other and far more important advantages could be gained by its adoption and further development. Formative tissue and structural differences, heretofore difficult to differ- entiate, were by a method of double staining, that is dying the tissues of two or more colours, made instruc- tive and palpable even by the aid of only moderate powers of the microscope. Various methods, mostly differing in details, have "been from time to time proposed for the combi- nation of colours and producing striking and perma- nent results. One of the most useful manuals of reference on the subject is Dr. Thin's, 1 whose experi- ence is founded on methods adopted by continental schools, and that of Ranvier's in particular. The student is recommended to provide himself for ordinary histological work, and which he should as far as possible become proficient in before he can expect to succeed in staining and preparing tissues, the following instru- ments : Two needles fixed in handles ; strong and fine scissors ; a scalpel ; a razor ; flat 011 the under surface and slightly grooved on the upper ; a section-lifter or (1) An Introduction to Practical Histology, by George Thin, M.D. Bail Iftre, Tindall r containing reagents and staining fluids ; glycerine ; cetic acid,osmic acid, alcohol, methylated spirit, distilled ater, &c. ; and lastly, a few camel's- hair brushes, watch- lasses, and two or three porcelain capsules. All spirits iould be kept in well-stoppered bottles. For dissec- ions and the examination of small animals, or portions f larger, and of tissues in general, a dissecting micro- 30pe fitted with two or three powers will be found lost convenient. The method of teasing out with eedles is shown in the annexed fig. 143^. All animal tissues should be examined in as fresh a iate as possible, and kept in blood serum, white of 2fg, or a very weak solution (1 part of salt in 200 FIG. 143i. Microscopic Dissection. Teasing out with Needles. arts of water) of salt. For hardening tissues, sec- ons, or organs, use the following reagents : Absolute Icohol, methylated spirit, solutions of chromic acid ;ome prefer a mixture of the latter and alcohol), E bichromate of potash, picric acid, chloride of palla- ium, and M tiller's fluid. All solutions of corrosive g^ents should be used weak. Chromic acid, a quarter or t most a 1 per cent, solution ; of bichromate of potash, or 2 per cent., picric acid, a saturated solution ; bloride of palladium a one-tenth per cent., with a few rops of hydrochloric acid added to prevent change, liiller's fluid is made by adding together equal parts f a 2 per cent, solution of bichromate of potash and 1 per cent, solution of sulphate of soda. A short mmersion in either of these solutions prepares tissues Q 226 THE MICROSCOPE. for teasing out ; for hardening they require to be kepi in either Miiller's fluid or chromic acid for a few days, and to complete the process by transferring to alcoho" or methylated spirit for from 12 to 24 hours. Wher employing chromic acid, bichromate of potash o] picric acid, immerse only a small portion of tissue in t large quantity of either fluid, always changing ii in 12 or 24 hours. Osmic acid not only hardens but stains tissues, and sections can be cut without sub. sequent immersion in alcohol. Sections of skin, anc" cornea, can also be made after reduction by th( chloride of gold in acidulated water ; a half per cent of either chloride of gold or of nitrate of silver wil be found strong enough for most purposes. A ] or a 1-|- per cent, of chloride of gold is recommendec for very special specimens. The most commonly usec colour stains are carmine, logwood, picro-carminatc of ammonia (which combines the action of picric anc carmine), hsematoxyline and the aniline colours, a$ magenta, commercially known as roseine or acetate o: rosaline, aniline red, eosin, aniline blue, violet, anc methyl - aniline. Purpurine, a dye extracted fron madder, is also highly spoken of as a stain by Banvier Dr. Thin strongly recommends it. It is prepared foi use as follows: "A solution of 1 part of alum in 20( parts of distilled w r ater is brought to the boiling poin in a porcelain capsule, and a small quantity of solic purpurine rubbed up in a little distilled water is addec to it. The purpurine quickly dissolves, but ther< should remain a small quantity undissolved, which in- dicates that the solution is concentrated. It must b< filtered whilst hot into rectified spirit. The alcoho should constitute a fourth part of the total volume o: the mixture. The fluid obtained is of a beautifu orange red by transmitted light : it is, in fact, fluores- cent. At the end of a month a slight precipitate ii observed, and it begins to lose some of its colouring matter. Tissues should remain in it from twenty-foul to forty-eight hours. Aniline blue-black is strongly recommended for staining the nerve-cells of the braii and spinal cord. STAINING FLUIDS. 227 To produce a third stain, Schwarz proposed picric acid in combination. A mixture of picro-carmine, he tells us, is a preferable stain for the unstriped muscle of the intestines, &c. Ranvier also employs a picro- carminate ; he discovered that a good green stain could be obtained by dissolving picric acid in glycerine, dilut- ing it with a decoction of logwood, and adding a small quantity of a solution of chromate of potash in the proportion of 1 part to 1,000. The solutions must be mixed together just before they are wanted for use, as they rapidly spoil. Dr. W. Stirling 1 f urnishes a brief but useful account of the methods he has employed with success for some time for double and treble staining. Osmic Acid and Picro-carmine. Mix on a glass slide a drop of the blood of newt or frog and a drop of a 1 per cent, aqueous solution of osmic acid, and allow the slide to stand by. This will fix the corpuscles with- out altering their shape. At the end of five minutes remove any excess of acid with blotting-paper, add a drop of a solution of picro-carmine, and a trace of glycerine to prevent evaporation, and set aside for three or four hours to see that no overstaming takes place. At the end of this time the nucleus will be found to be stained red, and the perinuclear part yellow. Picric Acid and Picro-carmine. Place a drop of the blood of a frog or newt on a glass slide, and add a drop of a saturated solution of picric acid : put the slide aside and allow it to remain for five minutes ; at the end of that time, when the acid has fixed the corpuscles (that is coagulated their contents), any excess of acid should be removed as before. A drop of a solution of picro-carmine should now be added, and a trace of glycerine, and the preparation set aside for an hour. At the end of that time remove the picro-carmine solution by means of a narrow slip of blotting-paper, and add a drop of Farrant's solution or glycerine and apply glass-cover. The perinuclear part of the cor- puscles will be seen to be highly granular and of a deep orange colour, whilst the nucleus is stained red. (1) Journal of Anat. and Physiol., xx. 1881, p. 349. Q 2 228 THE MICROSCOPE. Some of the corpuscles will appear of a delicate yellow colour, and threads are seen extending from the nucleus to the envelopes. The preparation should be preserved and mounted in glycerine. Picro-carmine and Aniline Dye. For glandular tissue, none of the aniline dyes answer so well as iodine green, used in the form of a 1 per cent, watery solution. Stain the tissue in picro-carmine, wash it in distilled water, acidulated with acetic acid, and stain it in a solution of iodine green. As it acts rapidly, care must be taken not to overstain. Wash the section in w r ater, and then transfer it to alcohol ; finally clear with oil of cloves. The washing should be done rapidly, as the spirit dissolves out the green dye. All prepara- tions stained with iodine green must be mounted in dammar. Picro-carmine and Iodine Green. Stain a section of the cancellated head of a very young bone (foetal bone) in picro-carmine, wash it in distilled water, and stain it with iodine green and mount in dammar. All newly- formed bone is stained red; that in the centre of the osseous trabeculse, the residue of the calcified car- tilage in which the bone is deposited, is stained green. Many of the bone corpuscles are also stained green. Ossifying cartilage, the back part of the tongue, Peyer's Patch, solitary-glands, trachea, and bronchus, may all be treated in the same way. In preparing the skin, take a vertical section from the sole of the foot of a foetus. The cuticle and superficial layers of the epithelium are dyed yellow, the rete Malpighii green, and the continuation of these cells can be traced into the ducts of the sweat-glands, which are green, and form a marked contrast to the red stained connective tissue of the cutis vera ; through which they have to ascend to reach the surface. The outer layer of the grey matter of the cerebellum with Purkinge's cells is, whon double stained, red, while the inner or granular I'ayer is green. Logwood and iodine green stains the mucous glands of the tongue (green), and the serous glands, lilac logwood stain. Eosin and Iodine Green. Eosin is used as the ground STAINING FLUIDS. 229 colour. Stain the tissue in an alcoholic solution of eosin, which will colour it very rapidly, usually in a few seconds. Wash the section thoroughly in water acidu- lated with acetic or hydrochloric acid, a 1 per cent, solu- tion, and stain with iodine green. This will double stain bone and cerebellum ; but if logwood is substituted for the latter, the cerebrum and general substance become stained by the eosin, while the logwood colours the nerve-cells a lilac. Gold Chloride and Aniline Dyes. The tissue must be impregnated with chloride of gold, and then stained with either aniline blue, iodine green, or rosein. The tail of a young rat, containing as it does so many dif- ferent structures, is an excellent material for experi- menting upon. Remove the skin from the tail, and place pieces half an inch long into the juice of a fresh lemon for five minutes, wash it to get rid of the acid. The fine tendons swell up under the action of the lemon acid, and permit of the more ready action of the chloride of gold solution. Place the piece for an hour or more in a 1 per cent, solution of gold, remove it and wash it thoroughly, and then place it in a 25 per cent, solution of formic acid for twenty-four hours. This reduces the gold : during the process of reduction the preparation must be kept in the dark. The osseous portion has then to be decalcified in the ordinary way, with a mixture of chromic and nitric acid. After decalcification preserve the whole in alcohol. Transverse sections of the decalcified tail are made, and may be stained with a red dye, as rosein, and afterwards with a watery solution of iodine green. Mounted in dammar. Dr. Taffani found that solutions of aniline blue and picric acid produce beautiful green- coloured preparations of the lymphatics, spinal cord, &c. The action of picric acid is not like that of chromic acid, which enters into combination with the sub- stances upon which it reacts, and which, after being hardened, will often part with all colour by repeated washings. The action of picric acid is decidedly less detrimental to most tissues than chromic acid. Dr. Seiler uses by preference the simple carmine 230 THE MICROSCOPE. solution of Dr. J. J. Woodward, made as follows : Best carmine, 15 grains ; borax, 1 drachm ; water, 5-- ounces; alcohol (95 per cent.), 11 ounces: mix and filter. Sections placed in this fluid will be stained evenly, in a few seconds, of a violet-red colour. Remove quickly and immerse them in a solution of hydrochloric acid 1 part, alcohol 4 parts. Let them remain until they assume a bright rose colour this will be accomplished in a few seconds. Wash the sections in distilled water and then transfer them to alcohol and finish off in the usual way. Specimens thus treated will have their nuclei and granules stained, while the cell contents and fibrous tissue remain uncoloured. The second solution is one composed of carmine and indigo. The sections stained with the carmine solu- tion must be immersed in a weak solution, 2 drops of sulphin-digotate of soda in one ounce of a 95 per cent, alcoholic solution, which should be filtered before using, and there left from 6 to 18 hours, according to the rapidity with which the elements take up the indigo. When sufficiently stained, the sections are immersed in strong alcohol ; they are then ready for mounting. The sulphin-digotate of soda as prepared by Bullock makes a solution of a deep greenish-blue colour, and the effect of the paint upon the section is to leave the nuclei bright red, while the fully- formed material of the cell is slightly tinged blue. The connective tissue fibres become stained deep blue, and the blood vessels are purplish and mapped out with distinctness. Epithe- lium cells and hair take the stain in a distinctive manner, thus affording a means of differentiation in epithelioma, the so-called pearls being brought out of a different colour from the rest of the cells. Mr. J. W. Groves, 1 in an instructive paper on stained sections of animal tissues, says the rule of almost uni- versal application is that the fluid should be weak and the quantity large in proportion to the number of sections, or to the mass. ,A section placed in a solution BO weak that 24 or 48 hours or more is required to give (1) Journal of the Quekett Microscopical Club, Nov., 1S79, p. 231. STAINING FLUIDS. 231 it the requisite tint is always better stained than one which has been a much shorter time, because the sur- face becomes stained before the colour has reached the deeper parts. The sections, for the same reason, should be as thin as possible, as they take the stain more per- fectly, and then the deeper portions are seen under the microscope as distinctly as the more superficial. Pro- vided the staining is perfect and sufficient to show all details, the paler it is the better, as it requires less light, and is less likely to fatigue the eye. The tints to be preferred are those that convey a cool and pleasant sensation to the eye. Intense reds and yellows are not nearly so pleasant as lilacs and pale blues. Stains which impart only a body-colour are of no value in differentiating structure. Distilled water should always be used for washing and all staining purposes. A five per cent, neutral aqueous solution of molybdate of ammonia produces a cool blue-grey stain in 24 hours. Eosin is a selective body-stain, which may be used either before or after the sections have been coloured with logwood. One part of eosin dissolved in a 1,000 parts of water is quite strong enough. But there is no more useful selective stain, or one more plea- Bant to work with, in Mr. Grroves's opinion, than log- wood. Kleinenburg's solution of logwood, modified by Oolding Bird, is prepared as follows : 1. Make saturated solutions of alum and calcium chloride, in proof spirit. 2. Mix in the proportions of eight of the former to one of the latter. 3. Pound a small piece of ext. hcematoxyli (the older the better) ; add it to the mixed solution, and agitate. After it has been allowed to stand two days, filter for use. A watch-glass should be filled with water, and a few drops of the mixed solutions added, till the fluid acquires a mauve tint. Into this the sections hould be placed, and allowed to remain for twenty-four hours or more. Another stain. S chafer's acid logwood solution is especially useful for certain structures, as ten- don, cells, &c. It is thus prepared : A one per cent. 232 THE MICROSCOPE. solution of acetic acid is coloured by the addition o 1*3 of its volume of logwood solution. The aniline dyes, whether in aqueous or alcoholic Solutions, give good results, and are prepared as follows r B/oseanilin or magenta (Igr. to loz. of alcohol), red ; Acetate of mauvein (4gr., alcohol loz., acid nitric 2 drops), blue; aniline black (2gr., water loz.), grey- black; Nicholson's soluble blue (l-6gr., alcohol loz~ and nitric 2m.), blue. These stains should be used weak; and specially observe that after sections are stained they should be passed through alcohol and oil of cloves as rapidly a& possible ; otherwise, the colour will dissolve out before- they can be mounted in balsam. Heidenhain, speaking of the use of aniline dyes r gays : " The sections, upon removal from alcohol, should remain for a day in a four per cent, neutral aqueous solution, in a moist place, and then be imme- diately mounted in glycerine and cemented." Some aniline dyes are but sparingly soluble in alco- hol, whereas they dissolve readily in water. Their colour is increased by acetic acid, and removed by am- monia. There are, however, exceptions. Use benzole- for clearing instead of clove oil ; this fixes the colours; better, but it has a tendency to produce shrinking in certain structures. The indigo carmine solution of Tiersch is a good and useful blue stain for sections of brain and spinal cord after they have been hardened in chromic acid; it possesses one convenient quality viz., that if the sections are too deeply stained, any excess of colour may be removed by the action of a saturated solution- of oxalic acid in alcohol. This reducing process should! be used with caution. Tiersch's fluid consists of: Oxalic acid, 1 part ; distilled water, 22 to 30 parts ; indigo carmine, as much as the solution will take up. A further dilution with alcohol may be necessary ; the sections should be immersed in it from 12 to 48 hours j the colour will determine the time. Beale's fluid is thus prepared : Carmine, lOgr. ; liq. amm. fort., 30m. ; glycerine, 2oz. ; distilled water,. STAINING FLUIDS. 233 2oz. ; Sp. vim rect., |oz. Dissolve the carmine in the ammonia, boil for a few seconds, add the water, filter, and finally add the glycerine and spirit, and keep in a stop- pered bottle. Beale says : " Let the excess of ammonia pass off ; " but this is unnecessary, as the excess is very slight. This solution reduced, with eleven times its bulk of water, produces good results in from 12 to 48 hours. Borax carmine, as follows : (1) carmine, |dr. ; (2) borax, 2dr. ; (3) distilled water, 4oz. Bub 1 and 2 together in a mortar and gradually add the water; let them stand in a warm place for 24 hours, after which pour off the supernatant fluid, and the solution is ready for use. There are stains which, being acted upon by light, get rapidly darker, and become opaque, and reach a stage when they are utterly useless. Nitrate of silver darkens by exposure ; it is used in a half per cent, watery solution. Specimens to be acted upon should be washed in distilled water to remove every trace of sodium chloride, and then steeped in the silver solution for some two or three minutes r after which they should be again washed until they cease to turn milky; then place them in gly- cerine and expose them to the action of light until they assume a dark brown colour, when they should be mounted in glycerine or glycerine jelly. By means of this stain the endothelial cells of the lymphatics, blood-vessels, &c., and the nodes of Ban- vier, in medullary nerves, are rendered visible. Sec- tions of any of these may subsequently be stained by logwood or carmine. Several methods have been adopted for staining with gold chloride. Dr. Klein's and Mr. Schafer's are among the best. 1. Dr. Klein's method of showing the nerves of the cornea is as follows ; Bemove the cornea within fifteen minutes of death ; place it in a half per cent, chloride of gold solution for half an hour or an hour ; wash in distilled water, and expose to the light for a few days ; in the meantime occasionally change the water. Then 234 THE MICROSCOPE. immerse it in glycerine and distilled water, in the proportion of one to two ; lastly, place it in water, and brush gently with a sable pencil to remove any precipitate, when it will be fit for mounting in glycerine. The colour of the cornea should be grey Tiolet. Mr. Schafer adopts another method a double chlo- ride of gold and potassium solution. Osmic acid, first used by Schultze, is useful for the demonstration of fatty matters, all of which it colours black ; it is also valuable for certain nerve preparations. Specimens should be allowed to remain in a 1-2 per cent, aqueous solution of the acid from a quarter to twenty-four hours, when the staining will be completed ; but if it is desired to harden specimens at the same time, they should remain in it for some few days. Osmic acid does not penetrate very deeply, therefore small portions should be selected for its action. Chloride of palladium, another of Schultze's staining fluids, is nsed to stain and harden the retina, crystalline lens, and other tissues of the eye ; the cornified fat and connective tissues remaining uncoloured. The solu- tion should be used very weak : Chloride of palladium, 1 part ; distilled water, 1,000 parts. Specimens should be mounted in glycerine at once, or further stained with carmine. Schafer also employs a silver nitrate and gelatine solution for demonstrating lung epithelium ; this is made as follows : Take of gelatine 10 grin., soak in cold water, dissolve, and add warm water to lOOc.c. Dissolve a decigramme of nitrate of silver in a little distilled water, and add to the gelatine solution. Inject this with a glass syringe into the lung until distension is pretty complete. Leave it to rest in a cool place until the gelatine has set ; then cut sections as thin as possible, place them on a slide with glycerine, and expose to light nntil ready for mounting. Of the double stains Mr. Groves refers only to those where the double colour is produced by a single pro- cess. Those in which one colour is first employed, and then another. Those used as a single fluid are Picro STAINING FLUIDS. 235 carmine, carmine and indigo carmine, aniline bine and aniline red. Picro-carmine is specially useful for staining sections hardened in picric acid. It is prepared in several ways : 1. Add to a saturated solution of picric acid in water a strong solution of carmine in ammonia to saturation. 2. Evaporate the mixture to one-fifth its bulk over a water bath, allow it to cool, filter from deposit, and evaporate to dryness, when picro-carmine is left as a crystalline powder of red-ochre colour. Sections can be stained in a 1 per cent, aqueous solution, requiring only ten minutes for the process ; wash well in distilled water, and transfer them to methylated alcohol, then to absolute alcohol, after which they can be made transparent by immersing in oil of cloves or benzole, before mounting in balsam or dammar. 1 The carmine and indigo fluids adopted by Merbel give a blue and a red stain, and are very selective. To prepare the red fluid, take Carmine, 2 dr. ; borax, 2dr. ; distilled water, 4oz. For the blue fluid, take Indigo carmine, 2dr. ; borax, 2dr. ; distilled water, 4oz. Mix each in a mortar, and allow it to stand, then pour off the supernatant fluid. If the sections have been hardened in chromic acid, picric acid, or a bichro- mate, they must be washed in water till no tinge appears. Place them in alcohol for fifteen or twenty minutes, then in the two fluids mixed in equal propor- tions, after which wash them in a saturated aqueous eolation of oxalic acid, where they should remain a rather shorter time than in the staining fluids. "When sufficiently bleached, wash them in water, to get rid of the acid, then pass them through spirit and oil of cloves, and mount in balsam or dammar. To summarize Mr. Groves' recommendations: 1. Let the material be quite fresh. 2. a. Take care that the hardening or softening fluid (1) See Rutherford, " Outlines of Practical Histology." Most of the stain- Ing agents mentioned may be obtained of W. Martindale. chemist .10, New W. 236 THE MICROSCOPE. is not too strong, b. Use a large bulk of fluid in portion to the material, c. Change the fluid frequently. d. If freezing be employed, take care that the speci- men is thoroughly frozen. 3. a. Always use a sharp razor, b. Take it with one diagonal sweep through the material, c. Make the sections as thin as possible ; and d. Remove each one as soon as cut, for if sections accumulate on the knife or razor they are sure to get torn. 4. a. Do not be in a hurry to stain, but b. Re- member that a weak colouring solution permeates the section better, and produces the best results; and c. That the thinner the section the better it will take the stains. 5. a. Always use glass slips and covers free from scratches and bubbles, and chemically clean, b. Never use any but extra thin circular covers, so that the speci- mens may be used with high powers, c. Always use cold preservatives, except in the case of glycerine jelly, and never use warmth to hasten the drying of balsam or dammar, but ran a ring of cement round the cover. 6. Label specimens correctly, keep them in a flat tray and in the dark. Dr. Cook pointed out that the results obtained by logwood were often unsatisfactory, and not fairly stable, because it must be understood that its colouring material consists of two substances, hsematoxylin and hoema- tein, differing from each other by two equivalents of hydrogen. The first named, containing the larger amount of hydrogen, is soluble in alum solution, while the latter, the heematein, is only slightly so, and is of no use for the colouring of animal tissues. Haematoxy- lin forms compounds with various metallic oxides ; and a solution of hoematoxylin, alum, and a metallic oxide, has a clear purple colour, becoming red on the addition of an acid. If an alkaline earth, hydrated earthy phosphate, be suspended in it, it will absorb the colour, and the solution w r ill become purple. If the solution be treated with a very small percentage of a chromate, the purple will be gradually replaced by a yellowish-brown colour; or if a tissue, stained with STAINING AND HARDENING. 237 itlum logwood, be immersed in an exceedingly dilute bichromate solution, the purple will be replaced by a yellow tint. It therefore follows that sections har- dened in chromic acid solutions, will not colour nearly so readily as if immersed in the fresh state. But it has been found that this objection may be overcome if the sections are well washed and immersed in a modi- fied solution of logwood. The most practical form is made as follows : Take of logwood extract 6 parts ; alum, 6 parts ; sulphate of copper, 1 part ; water, 40 parts. All ingredients must be free from iron. Grind up the powders together in a mortar, and when powdered add water sufficient to form a thin paste ; put them by and leave them for a day or two in this state, then add the rest of the water and filter the solution. The hoematein will be separated and left behind in the filter ; and a crystal of thymol may be added to preserve the solution from moulding. For chromic hardened tissues, dilute 8 drops of the fluid with 120 of water, and add one drop of a one-tenth per cent, solution of bichromate of potash just before using the solution. Wash the stained tissues as usual in water, and mount in glycerine, Warrant's solution, or dammar. In the former they keep best, in the last they are apt to fade, unless the sections be thoroughly freed from water by being immersed in absolute alcohol, before being brought into contact with oil of cloves. If any moisture be left behind, the preparations will be sure to spoil. A modified Warrant's solution may be prepared as follows : Take of gum arabic 5 parts ; water, 5 parts ; when the gum is fairly dissolved add 10 parts of a five per cent, solution of carbolic acid. Hardening, Preserving, and Section-cutting. What- ever be the hardening or softening fluid employed (for this is necessary when bone is the structure about to be examined), its bulk should be large in proportion to the size ; half a pint of fluid for a piece of about one cubic inch. The strength of the fluid must be made to suit the tissue about to be acted upon, and the fluid should be changed frequently, even though it 238 THE MICROSCOPE. be alcohol that is employed. It is better that any and every solution should be too iveak than too strong; for in the latter case the tissue is liable to become friable and break down under the cutting knife or razor, and the sections will not take the stain evenly. To recapitulate and enforce one or two points of impor- tance, note that the most useful strength for chromic acid is a | or i per cent, solution ; for the bichromate- of potash or ammonia 1% or 2 per cent, solution. When either of the latter solutions are used the material must be removed to an alcoholic solution in a week, or ten days at most, or it will become very brittle. Alco- hol is one of the best fluids for those to use who have not much time to devote to the subject ; tissues har- dened in alcohol afford, as a rule, the best staining results. The price of alcohol is much against its use,, but it may be used weak at first, and gradually increased in strength until the material is found hard enough to fcut with a knife. For cutting sections by hand the best substance for embedding is a mixture of equal / parts of olive oil and white wax ; or Cacao butter, or ( even soap dissolved in alcohol (Micros. Journal, vol. ii. \ p. 940, 1879) ; while in section-cutting machines with "hollow cylinders, either the pith of elder, carrot, or some other soft substance may be employed. If a razor is used, its surface must be kept moist with water when the freezing process is adopted, or with spirit w r hen the hardening process without freezing is used. By the freezing method we are enabled to cut and finish off specimens sooner and more expeditiously than by any other process. The material about to be frozen must be removed from the hardening; fluid and well washed in clean water before it is transferred to the machine. Zeiss's microtome, with its surface of glass, a practical and useful cutting and freezing machine, can be obtained at a moderate price, of Baker, Holborn. Swift's freezing microtome has the advantange of preserving the preparation for some hours unchanged in the frozen state. The method of using it is aa follows : Remove the lid of the box and fill the cham- SECTION-CUTTING MACHINE. 239 her ^ith equal parts of pulverized ice and salt, care being taken not to allow the mixture to touch the under side of the cover, which, when replaced, must be firmly- secured by the clamp screw. The substance to be cut must be placed on the surface of the central circular brass piece (there are three additional ones supplied with the instrument) and surrounded with a little common gum water, which readily congeals, and, as shown in the FIG. 143. Sioift's Micr.otome and Section-cutting Machine. woodcut, holds the specimen firmly in position, until solidified and frozen. The edge of the razor or knife must be raised by the three screws supporting the frame to the required height for cutting sections. After the first cut, each end of the razor must be again presented to the surface of the specimen, when either end of the blade must be adjusted by one of the back screws until its entire length is level. By turning the large screw in the frame it can also be lowered for each 240 THE MICROSCOPE. successive section required. One entire revolution of it produces a section T ^ of an inch in thickness, the screw-head being divided into sixths ; thus one division gives a section of ^-^ of an inch, but even thinner sec- tions can be cut by turning the screw. Substances that have been previously prepared in spirit or chromic acid should be steeped in syrup for 24 hours, otherwise they will not readily congeal. It is advisable to cover the apparatus with baize, to facilitate the freezing process. The brass cup (shown in the engraving) is used for holding substances embedded in cocoa-butter, or paraffin ; it also serves for securing hard wood, &c., when cements or sealing-wax are used. Vegetable Tissues. Sections of wood and vegetable tissues are susceptible of very fine double and even triple .staining dyes ; the best are atlas-scarlet, soluble blue, iodine, and malachite-green. Mr. Richardson secured success by steeping sections in spirits of wine for about a fortnight, and when not required for immediate investigation, storing them away in Price's glycerine for at least a couple of months. This renders them less liable to fold or break than when the staining is done immediately after the sections are cut. His method may be gathered from the following directions for pre- paring and staining sections of palm stem. After the sections are cut they should be bottled up in a toler- ably dark solution of atlas-scarlet and spirit of wine. Leave them in this solution, corked up tightly until they become of a uniform scarlet tint. Like sections of animal tissues, however, they may remain in the solution for many weeks without risk of spoiling or counteracting the energy of the green dyes. It is on the whole better to complete the process when the sec- tions seein to be of a deep scarlet colour. Remove them and wash them well in filtered water, repeatedly change the water until it ceases to be in the slightest degree coloured by the sections. Then transfer them to a white porcelain water, containing a solution of spirit of wine, coloured bluish-green by adding a couple of drops of an aqueous saturated solution of the green dyes ; a drop of each will be found sufficient. When STAINING VEGETABLE TISSUES. 241 the sections appear sufficiently coloured a dark blue, transfer them once more to a saucer of water to which a drop of an aqueous saturated solution of arsenious acid, or of oxalic acid (in the proportion of one grain of oxalic acid to the ounce of water) or glacial acetic acid has been added. Wash by rotating the saucer, then pour off the water and place the sections in a stoppered bottle containing absolute alcohol ; which should like- wise contain a drop of either of the before-mentioned acids. When all the water has been abstracted from the sections by the alcohol (which will be in about ten minutes) clear with oil of cloves, they are then quite readv for mounting in Klein's' or dammar solution. Clematis and other open sections take a very fine treble stain by this method. Buckthorn and sycamore seem to have a great affinity for the green stains, two minutes in staining fluids being usually sufficient to colour the walls of the central cells green and their contents of a light scarlet. The staining of thin sections of potato are equally effective, the starch granules being green, the loculi scarlet, the depth of colour depending upon the length of exposure to the atlas- scarlet, and mixed green dyes; always allowing the malachite to be in excess. 1 For double-stained vegetable tissues Mr. Barrett prefers some of the cheaper dyes. The sections must be first immersed in an aqueous one per cent, solution of Crawshaw's aniline blue; then removed into a strong acetic acid solution, which fixes the colour in certain tissues, and removes it from others, while it prepares the unstained portion for the reception of another colouring material. It must again be removed into a weak solution of magenta (Judson's dye), acidu- lated with acetic acid : then washed and mounted in glycerine jelly. By this process sections of burdock are stained, the pith, very pale magenta colour; cellular tissue, deep magenta ; spiral vessels of medullary sheath, deep blue ; pitted vessels, blue ; cambium, deep blue ; liber cells, dark magenta ; lactiferous vessels, deep blue ; cuticle, parenchyma, pale blue ; epidermis, deep (1) Transactio-M of the Uoral Microscopical Soe., page 870, 1881. 242 THE MICROSCOPE. blue ; Lairs, pale magenta. It is almost needless to add that both time and patience are required to attain perfection in double staining. Those, however, who, from want of time, cannot follow out the details of the several processes should pay a visit to the laboratory of the Messrs. Cole. 1 There they will find a large and choice selection of specimens of animal and vegetable tissues, and which, for perfection in staining, cutting and mounting, can- not be surpassed. The staining of vegetable tissues will give increased interest to the study of botanical histology ; the student in this way will obtain an insight into structure such as can be secured by no other means. The staining fluids most successfully employed by Mr. Gilburt 3 for staining sections of woods and plants blue and red by the aniline dyes are prepared as fol- lows : Magenta crystals . gr. J in ") -^ j Alcohol ... loz. ) Then Nicholson's soluble ~) pure blue . . . gr. in / Alcohol . . 1 oz., to f L which has been added acid nitric 4 drops ) Both solutions should be filtered. For use take 2 parts of the blue and add it to 7 parts of the magenta, and thoroughly mix. Place the section in the mixture for about a minute, then remove it to absolute alcohol, from that to oil of cloves or benzole, and finally mount in balsam and benzole. To fix the magenta it is necessary to pass the sections through benzole. As a preparation for staining, all tissues should be "bleached. This is effected in the case of soft vege- table stems in alcohol; the use of which, although (1) Arthur Cole and Son, 53, Oxford Gardens, Netting Hill, TV., are engaged in the publication of " Studies in Microscopical Science," that is, prepared specimens of typical objects beautifully stained, and drawings of the samfl, with directions for staining and preparing sections for the micro- scope. T commend these Studies to the notice of students and teachers. <2) Journal of the Quekett Microscopical Club. BLEACHING SECTIONS. 243 it discharges the natural colour, considerably abridges the process. It has a further advantage, as the cell- contents, starch, chlorophyll granules, &c., are pre- served intact, and the nucleus, when it exists, is ren- dered more palpable by staining. When the stem is hard and brown, a solution of chloride of lime should be used. A quarter of an ounce of chloride dissolved in a pint of water, well shaken and stood by to settle down, then pour off the clear fluid for use. For hard tissues this solution answers well, but it is not suitable for leaves, as they require not only bleaching, but the cell-contents should be dissolved out to render them transparent. A solu- tion of chlorinated soda answers well for both stems and leaves. It is prepared as follows : To one pint of water add two ounces of fresh chloride of lime, shake or stir it well two or three times, then allow it to stand till the lime has settled. Prepare meanwhile a saturated solution of carbonate of soda common washing soda. Now pour off the clear supernatant fluid from the chloride of lime, and add to it, by degrees, the soda solution, when a preci- pitate of carbonate of lime will be thrown down ; con- tinue to add the soda solution till no further precipitate is formed. Filter the solution, and keep it in a well- stoppered bottle in the dark, otherwise it speedily spoils. Sections bleached in chlorinated soda must, when white enough, be washed in distilled water and allowed to remain in it for twenty-four hours, changing the water four or five times, and adding a few drops of nitric acid, or at the rate of eight or ten drops to the half-pint, to the w r ater employed before the final wash- ing takes place. From water transfer them to alcohol, in which they must remain for an hour or more. Sections may be stained in two colours, either by alternate or single immersions. The first process is as follows : Transfer the section from alcohol to magenta dye for about twenty minutes, then remove and soak in alcohol till the colour is re- moved from the parenchyma ; next place it about a B 2 244 THE MICROSCOPE. minute in the blue dye, transfer it to alcohol for a few seconds, and to absolute alcohol for a few seconds ; remove it to oil of cloves, in which it should remain till quite clear. It is now ready for mounting in benzole balsam. For staining by a single immersion, add twelve drops of the blue dye to seven of the magenta, and thoroughly mix. Into this purple stain place the sections for about a minute, then remove them to alcohol ; shake well for a few seconds, and proceed as by the former method. The magenta dye stains the woody fibre and vascular tissue; the blue the parenchyma, cambium layer, and medullary rays; while the pith and bark remain neu- tral, or partake of both. In deciding upon which colour should be first em- ployed, this will depend upon the particular structure it is wished to bring out more forcibly than another. To show the structure of the lamina use the blue stain, because it displays the cell -walls far more dis- tinctly than magenta. There is some difficulty in fixing magenta, unless it is passed through benzole and not oil of cloves ; benzole may, however, produce an injurious effect upon the tissue. In using blue dye, no fixed time can be laid down for immersion, this so much depending upon the density or permeability of the tissue. Dr. Beatty recommends that two solutions should be prepared a- quarter and a half-grain solution, and that the leaf should be transferred to the stronger if the staining is not completed in the weaker in half an hour. There is, however, one objection to this, that far too much colour may be taken up in parts, and giving the sections a very mottled appearance. Experience proves that far better results are usually obtained by the use of weaker dyes, although a longer time may be required. As a general rule, sections should be left in the dye till equally stained throughout, then remove them into alcohol, brush the surfaces well with a camel-hair pencil, and transfer them to absolute alcohol for a few minutes, thence into oil of cloves till quite clear, and STAINING BACTERIA. 245 finally into clean oil of cloves, where they must remain ten or fifteen minutes before mounting in balsam and benzole. Preparations stained blue may be left in oil of cloves for a week or more without doing them any injury. The staining process has greatly facilitated the study of the minuter forms of life. For staining Bacilli employ the aniline reds as follows : Fuchsia in crystals, one centigram ; alcohol, from twenty to twenty-five drops ; distilled water, fifteen cubit centimetres : mix. The colour stain taken by the bacilli is less intense than that taken by the micrococci, and this serves to dis- tinguish the one from the other. The minute size of the bacilli renders their life history and study of their growth under artificial cultivation a work of great difficulty. Considerable importance, however, attaches to these organisms obtained by cultivation, from the fact that the resulting forms can be compared with those found in connection with disease. Blood cor- puscles are better studied under osmic acid staining fluid, and which shows that most of the white corpuscle may be divided into two or more kinds and forms. One set is stained black by osmic acid, and another, which contains granulous matter not fatty, is stained red by an eosine solution. The best mode of showing the three forms of corpuscles is to fix the blood in the network of the smaller blood vessels ; for instance, in the'choroid coat of the eye, by cutting the eye of the frog into two parts, subjecting the section to the vapour of osmic acid for twelve hours, then wash the segment in distilled water, and detach the capillary layer from the retina, spread it out on a glass slide and stain it with ha3matoxylate of eosine. The corpuscles will by this process be seen to be of three kinds the ordinary, granular, and fatty. Care must be taken, as the vapour of osmic acid is of a corrosive nature. M. Brandt finds hoematoxylin and Bismarck-brown suitable colours for staining living unicellular organ- isms. For amoebae and similar delicate bodies a dilute solution of hcematoxylin must be allowed to act for only a short time, not more than an hour, when 246 THE MICEOSCOPE. they must be transferred to pure water. The nuclei will be seen stained pale violet ; although at first no visible change is produced in the contractile vacuole, later on it assumes a yellowish tint, and finally becomes brown. Double staining may likewise be effected by first using Bismarck-brown for an hour, and then liEematoxylin for a shorter time ; the protoplasm alone remains uncoloured : the difference in colour showing which of the granules are fatty and which are nuclein. The strength of the Bismarck-brown stain should not exceed one in 3,000 or 5,000. A solution of safranine, one of the red aniline dyes, one or two grains to the ounce of water, is an excellent stain and test for amy- loid, starchy matters in unicellular plants. The starch is stained of a fine orange colour, and other portions of a rose colour. For bleaching sections before staining Mr. Marsh resorts to the direct action of free chlorine, generated in a pair of Woolf-bottles. Fill one of the bottles about two-thirds full of filtered water, and into this place the sections to be bleached. Into the other bottle put a sufficient quantity of crys- tals of chlorate of potash to slightly cover the bottom, and pour upon them a drachm or two of strong hydro- chloric acid. Connect the bottles by the glass tubes, and the yellow vapour of chlorine will be observed to pass into the water contained in the first bottle, and effectually and safely bleach the sections. The time required for bleaching will vary with the nature of the sections operated upon. Decoloration having been effected, the sections must be removed and thoroughly washed to eliminate all trace of chlorine before employ- ing a staining agent. Cementing. The following cements are recommended by Mr. Groves for mounting stained preparations : Cements. For balsam or glycerine jelly mounts al- most any varnish will do, but for fluids, glycerine, &c., it is necessary to have one tough and which will prevent leakage. The following will be found most efficient : ]. Mastic and Bismuth. Dissolve gum mastic in chloroform, and thicken with nitrate of bismuth. CEMENTING. 247 The solution of mastic should be nearly saturated. 2. Oxide of Zinc, Dammar, and Drying Oil. Bub up well-ground oxide of zinc, 2oz., with drying oil, to the consistence of thick paint. Then add an equal quantity of gum dammar, previously dissolved in benzoline, and of the thickness of syrup. Strain through close-meshed muslin. Keep in well-corked bottle, and, if necessary, thin with benzoline. 3. Kitton's Cement is made of white lead and red lead in powder and litharge powder in equal parts. Grind together with a little turpentine, until tho- roughly incorporated, and then mix with gold size. The mixture should be thin enough to use with a brush; in- using one coat should be allowed to dry before applying another ; no more cement should be mixed with the gold size than is required for imme- diate use, as it sets quickly, and becomes unworkable. Certain precautions are necessary to be observed in using varnishing fluid or glycerine preparations : 1. Use no more glycerine or fluid than is just neces- sary to fill up the space beneath the cover. 2. If the medium should escape beyond the cover- glass, soak it up with a piece of blotting-paper, and ba careful not to press the cover, or the cement will run into the cell. Of preservative mounting media, the most useful are balsam, glycerine, and glycerine jelly. Canada balsam should be exposed to heat until it becomes quite brittle when allowed to cool, then it should be dissolved in benzole till as thin as glycerine, and should always be used cold. Glycerine. Specimens which have been hardened in chromic acid or bichromates may be mounted in pure glycerine alone, but if they have been hardened in spirit, glycerine and carbolic acid, in the propor- tion of glycerine fifteen parts to carbolic acid one part, is better, as it is less refractive, and prevents tho sections becoming granular. For carmine stained preparations it is well to add a trace of acetic acid to the glycerine (2m., loz.). Glycerine jelly is a good medium, as it offers the advantages of glycerine with- 248 THE MICROSCOPE. out the chance of leaking, but it is rather difficult to prepare, and, therefore, had better be bought. A jelly composed of glycerine and gelatine equal parts is very useful ; the glycerine should be warmed, and the gelatine (Nelson's) be allowed to dissolve in it. Acetate of potash in a saturated solution is used for some prepara- tions, but is liable to leak. Injecting Small Animal Bodies. For making injections it is essential to have a proper syringe. One of brass is the best, and of such a size that the top of the thumb may cover the button at the top of the piston-rod when drawn out, while the body is supported between the two fingers. Fig. 143Z represents the syringe, a is the body, with a screw at the top for the purpose of firmly screwing down the cover I after the piston c FIG. 143L Injecting Syringe. FIG. 143;)i. Melting Vessel. is replaced ; e is a stop-cock, to the end of which either of the smaller pipes g can be fixed. The transverse INJECTING APPARATUS. 249 wires are for securing them tightly with thread to the vessels, into which they may be inserted. In addition to the syringe, two or three tinned vessels, to contain size, injecting fluid, and hot water, are necessary. The size must be kept hot by the aid of a water bath ; if a naked fire be used, there is danger of burning it. A convenient form of apparatus for melting the size, and afterwards keeping it at a proper temperature, is shown in fig. 143?;?.. A pair of strong forceps, for seizing the vessel, and a small needle, fig. 143#, is also necessary for passing the thread round the vessel into which the injecting pipe has been inserted, completes the list of apparatus. To prepare the material for opaque injections : 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 three pints of cold water ; let it stand twenty-four hours, stirring it now and then with a FIG. 143n,. Artery Needle. stick ; set it over a slow fire for half an hour, or until all the pieces are perfectly 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 particular injections. If gelatine be employed an ounce to a pint of water will be sufficiently strong, but in very hot weather it is necessary to add a little more gelatine. It must be first 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. The size thus prepared may be mixed with finely levigated vermilion, chrome- yellow, blue smalts, or flake white. 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 U50 THE MICROSCOPE. 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 proportion to the size of the part and the amount of its rigidity. Cold Injection-mass. A. Wikozemski describes a modification of Panseh's method. Thirty parts by weight of flour, and one of vermilion, are mixed while dry, and then added to fifteen parts by weight of glycerine, and subjected to a continuous stirring until of a homogeneous viscous consistency ; then two parts of carbolic acid (dissolved in a little spirit) are added to it, and finally thirty to forty parts of water. This injection-mass is specially adapted for subjects already injected with carbolic acid (in the proportion of one and a half part by weight each of carbolic acid, spirit, and glycerine, to twenty of water) : twenty-four hours are allowed to elapse between the two injections. Of Injecting Different Systems of Vessels witJi Different Colours. It is often desirable to inject different systems of vessels distributed to a part with different colours, in order to ascertain the arrange- ment 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. The structure of the kidney may be demonstrated as follows : Inject into the jugular vein of an animal, as soon as killed, say of a rabbit, a sufficient quantity of a one per cent, solution of the yellow prussiate of potash, and immediately afterwards inject through the renal artery a sufficient quantity of a weak solution of per- chloride of iron, to distend the capillaries of the kidney. INJECTING THE LOWER ANIMALS. 251 After the second injection has been made, pieces of kid- ney which have become of a bluish colour are cut off with a razor, and steeped in a one per cent, of osmic acid, in which the pieces must be left to harden from twelve to twenty-four hours. They should be small enough to allow the osmic acid to penetrate freely. After they are removed from the acid, they must be thrown into distilled water for half an hour, and finally kept for examination in alcohol. This method imparts a bluish tint, with a tinge of violet, to the protoplasm and nucleus, whilst the cells of the straight tubules receive no coloration. 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 immediately 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 mollusc often renders it impossible to tie the pipe in the usual manner. The capillaries are, however, usually very large, so that the injection runs very readily. In different parts of the bodies of these animals are numerous lacunas 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 lacunae with comparative facility. Insects. Injections of insects nid,y be made by forcing the injection into the general abdominal cavity, when it passes into the dorsal vessel and is after- wards distributed to the system. The superfluous injection is then washed away, and such parts of the body as may be required, removed for examination. Injection of Invertebrate Animals. Gr. Joseph uses filtered white of egg, diluted with 1 to 5 per cent, of carmine solution, for cold injections. This coagulates 252 THE MICROSCOPE. when immersed in dilute nitric, chromic or osmie acids, but remains transparent, and is sufficiently indif- ferent to reagents. A mass of similar properties is made of glue liquid when cold, coloured with the violet extract of logwood reduced with alum. Injec- tion is effected in the case of worms (leech and earth- worm) by way of the ventral or dorsal vessel, with large Crustaceans by the heart or the ventral vessel which lies in the sternal canal. In many cases, especially when lacunar spaces have to be filJed, use- ful preparations are obtained by natural injection (auto-injection, or autoplerosia). Natural injection of Medusa3 is effected without injuring the vessels ; in the case of Crustaceans, Insects, and Mollusca, through a slit with an opening at the side remote from it. Medusa3 are laid in a glass vessel, with the bell down- wards, and a bell- jar ending in a narrow tube above is placed over it and made air-tight; after the Medusa is covered with the injection-mass, the air in the glass is exhausted, and as the sea- water runs out by slits in the lower side of the annular canal, the coloured fluid runs in. In the case of leeches and large species of earthworms, the natural injection is made from the ventral sinus. In all cases a glass tube is nsed, with a finely drawn-out point. The injection is complete when the injection issues from the counter- opening. Animals to be injected alive are kept quiet by cold (upon ice). Besides the animals mentioned, large caterpillars, beetles, Libellulidse, Iarva3, locusts, &c., all serve as objects for injection ; the glass can- nula being introduced into the posterior end of the dorsal vessel, and the counter-opening made in the ventral vessel, and vice versa. Staining Living Protoplasm with Bismarck Drown. L. !\ Henneguy having treated Paramoecium aurelia, with an aqueous solution of aniline brown (known as " Bismarck Brown "), found that they assumed an intense yellow-brown colour. The colour first appears in the vacuoles of the protoplasm, and then in the protoplasm itself, the nucleus generally remaining colourless, and thus becoming more visible than in the STAINING LIYING BODIES. 253 normal state. Infusoria thus coloured were kept for nearly fifteen days. If a yellow-tinted Paramoecium is wounded or compressed so as to cause a small quantity of the protoplasm to exude, it is seen that it is really the protoplasmic substance which is coloured. All Infusoria may be equally stained with Bismarck brown, but no other aniline colours employed exhibit the same property they only stain the Infusoria after death, and some of them are in fact poisonous. As it is generally admitted that living protoplasm does not absorb colouring matters, and that Infusoria are essen- tially composed of protoplasm, an attempt has been made to ascertain whether protoplasm in general, of animal or vegetable origin, behaved in the same way in the presence of aniline brown. A tolerably strong- dose of Bismarck brown was injected under the skin of the back of several frogs. After some hours the tissues were uniformly tinted a deep yellow; the muscular substance especially had a very marked yellow tint. The frogs did not appear in the least incommoded. Small fry of trout placed in a solution stained rapidly and continued to swim about. Finally, a guinea-pig, under whose skin some powder of Bismarck brown had been introduced, soon presented a yellow staining of the buccal and anal mucous membranes and of the skin. Seeds of cress sown on cotton soaked with a concen- trated solution of the Bismarck brown sprouted, and the young plants were strongly stained brown; but on crushing the tissues and examining them under the microscope, it was ascertained that the protoplasm of the cells was very feebly coloured ; the vessels, on the contrary, showed a very deep brown staining up to their termination in the leaves. The mycelium of a mould which had been developed in a solution of Bismarck brown, was clearly stained after having been washed in water, whilst it is known that the mycelium, which frequently forms in coloured solutions, picro- carmine, haematoxylin, &c., remains perfectly colour- less. Other aniline colours injected under the skin of frogs stained the fundamental substance of the con- nective tissue as deeply as did the Bismarck brown ; 254 THE MICROSCOPE. but the cells of the muscular substance remained perfectly colourless. The author concludes, therefore, that Bismarck brown possesses the property of colouring living protoplasm both in plants and animals. 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 30s. Another box, more particularly adapted for anatomical purposes, includes a neat injecting apparatus. Fig. 14S0 Collins' Mounting Callnet. FUNGI, ALG.-K, LICIIKNS. ETC. 8 * : '* TuflVn West, del PART II. E VEGETABLE KINGDOM VITAL CHARACTERISTICS OF CELLS THE PRO- TOCOCCITS PLUVIALIS OSCILLATOR!^ FUNGI ALGJE DESMIDACE^E MOSSES FERNS STRUCTUEE OF PLANTS STABCH ADULTERATION OV ARTICLES USED FOR FOOD PREPARATION OF VEGETABLE STRUCTURE?-. ETC. I.NCE the introduction of the achro- matic microscope, we have obtained nearly the whole of the valuablo 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- blest tribes of plants have had for microscopists an attraction, 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 aa that of the higher orders of creation." 256 THE MICROSCOPE. Mirbel had shown that all the different forms of vege- table tissue are developed from cells which enter into the formation of the embryo plant. Schleiden followed Mir- bel in tracing out the development of the tissues of the fully formed plant from the nucleated cells composing the embryo ; and he also studied the mode of formation of the nucleated cell itself. On this point Schleiden came to the conclusion that the nucleus is the germ of the plant-cell, hence he named it the " cytoblast." Miiller subsequently contended that the spinal chord is com- posed of cells, resembling vegetable cells ; Schwann dis- covered a nucleus in these cells, and observed that the various forms of cells in animal structures is similar in every respect to those of plants. From his investigations he was led to the philosophical generalization, that the tissues of the animal body and those of plants were formed from cells. The various tissues, although formed from cells in different stages of their development, and not neces- sarily the formative element of all cells in their fully formed stage, for cells, when fully formed, in some cases do not undergo further development ; for example, in the parenchyma of glands when they break up, and are resolved into the secretive matter. The prevailing opinion now is and of which neither Schleiden nor Schwann appear to have had a true notion that nuclei and cells are propagated by the sub- division of pre-existing nuclei and cells. As to the particular endowments and potentialities of the dif- ferent kinds of cells whereby each is developed and converted into its own special tissue, and indifferently into any kind of tissue, I must refrain from discussing, as it would lead me into a region of speculation. " If nature," writes Humboldt, " had endowed us with microscopic powers of sight, and if the integu- ments of plants were transparent, the vegetable king- dom 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 creation, the muscles of animals very soon disappear as we descend in the CHARACTERISTICS OF CELLS. 257 scale to tlie simplest forms of life ; nevertheless, we cannot deny animality to those minute creatures as the Amoeba in which we are quite unable to dis- tinguish either muscle, or any other distinct organ. Hence there is danger of believing that to be simple which in reality only seems to be so. 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 wall, 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. t The inner layer has received the name of primordial utricle, and ; ts 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 y 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. 258 THF 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 exception 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 : 1st, 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 18 the protoplasm (protos, 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 also of different chemical composition, from the cell wall being allied in this respect to animal matter." Griffiths. 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 propagation also appears. Fig. 144. Cell Development. (Protococcus pluvialis.) Protococcus plnvialis, Kiitzing. Hcematococcus pluvialis, Flotow. Chlamido- coccus versatilis, A Braun. CJilamidococcus 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 ; ., 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 Palmoglcecb macrococca, of Kiitzing, which spreads itself as a green slime over damp stones, walls, &c. If a email portion be scraped off and placed on a slip of glass, and examined with a half or quarter-inch power, it will be Been 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 260 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 pluvialis, 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 Algce, 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 Eugkna, 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 >u much more generally distributed than has been supposed, among the lower Algae; occurring in many true brown ipores, such as of (Edogonium, Spirogyra, VaucJieria, &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 which, 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 layer 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 flagella arising from the pri- mordial 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 recognized by the currents produced in the water ; as death approaches motion slackens and they become evident enough. They are also rendered very distinct by iodine. They are always protruded about the extreme point of the conical elongation, at the anterior end of the primordial cell, and in such a manner as to appear to be mere continuations of its substance. Since these pro- cesses consist of protoplasm, it is evident that the flag- ella must be regarded as composed of the same sub- stance. They resemble, in some respects, the so-called proboscis of certain Infusoria, such as Euglena and Ifo- nads, 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 cumstance that when, passing from the cell form into the still condition, the flagella 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 by Flotow 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. Piopagation 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- division 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 still-colls 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 cellulose tissue, and become still-cells; but most of them probably perish without any further change. They would thus 264 THE MICROSCOPE. correspond with the smaller motile spores observed by Thuret and A. Braun in other Algae (the Fucoid, &c.), associated with the larger germinating spores, them- selves deprived of the germinative faculty. It appears that both longitudinal and transverse division of the primordial cell may take place ; but that the vibratile filaments of the parent cell retain almost to the last moment their function and their motion after the primordial cell inclosed by it has long been detached as a whole, and become transformed into the indepen- dent secondary cells (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 dia- 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 bo 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 provisional generation will have become free ; on the next, the bottom of the vessel will be found covered with a new generation of self-divid- ing 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 propagation is very re- markable. It is only necessary to pour water from a smaller into a larger and shallower vessel, or one of a dif- ferent kind, to at once induce the commencement of seg- mentation 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 con.- FRESH-WA/ER ALGJS. 265 ducive to tlie manifestation of vital action in the motile zoospores, and they always seek it, collecting themselves at the surface of the water, and at the edge of the vessel. But in the act of propagation, on the contrary, and whea about to pass into the still condition, the motile Pro- tococcus cell seems to shun tne 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.* StephanospJwera pluvialis is another variety of fresh- water alga3, 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 nagella, two of which are seen to pro- ceed 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 flagella, form a thicker membrane as at 5, for a time become motionless, and sink to the bottom of the vessel. If tlie vessel be permitted to become thorough- ly dry, and again water is poured into it, motile Stephan- osphsera reappear : from which circumstances it is proba- ble that the green globes are the resting spores of the plant. When in its condition of greatest activity its divi- sion 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 Protococcus pluvialis," by F. Conn, translated ly . Buk, F.R.S. for the Ray Society Zbb THE MICKOSCOPE. eight days, under favourable circumstances, 16,777,216 families may be formed from one resting-cell of Stephano- sphaera, 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 j 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 confervae consist of tmbranched filamentous delicate 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 ; Bometimes presenting a striated appearance, which has given rise to a supposition that confervoid filaments are spiral. They are indeed plentifully distributed both in fresh and salt-water. Oscillatoriacece. The study of the structure of the Oscillatorice is particularly interesting, from the fact that we may not unreasonably expect to find in it a key to the motive power 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 anything like a satis- factory 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 striae 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 P. Currey, F.K.S. Journal of Microscopiccfl Science, vol. vi. 1858, p. 131 ; also by Mr. Wm. Archer, vol. v. 1865, p. 116 OONPBRVOID ALG2E. 267 absent, or have not yet been observed ; when present, they will be found projecting on one or both sides of Fig. U5. Conferva:. I, Volvox globator. 2, A section of volvox, showing the flagellate margin of the cell. 3, A portion more highly magnified, to show the young volvo- eina, with their nuclei and thread-like attachments. 4, Spirogyra, near which are spores in different stages of development. 5, Conferva flocossa. 6, Stigeoclonium protensum, jointed filaments and single zoospores. . 7, Staurocarpus grnnlis, 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 con- sist wholly of protoplasm ; this view, however, is scarcely correct, since the protoplasm is enclosed in a cell- mem- brane. The cellulose always shows cross-markings corresponding to the strise when such are observable 268 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 its striae, will be found, as well 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. Mesogiiavermicujaris, most present a slight yellowish 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 Oscillatorice, and is de- posited within it in the form of circular bands or rings around the axis of the cylindrical filament ; iodine stains 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 those less exposed ; a 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 of 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 of Microscopical Scioice, voL iv. p. 245. 1856. composed of strings of cells co- hering end to end. MARINE ALG.E. 269 membrane. Their motion is not less lively than that of the Bacteria,, which Dujardin and Ehrenberg placed among infusional animalcules. To observe the movements of the filaments, the very uppermost surface ought to be brought into focus, leaving the margins rather undefined, bearing in mind that the filament is not a flat but a cylindrical body. Certainly, with regard to the movement, or the mechan- ism 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 Alga3 present a general appear- ance which might at first sight be mistaken for plants very much higher in the scale of organization. In the Ulvacea?, 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 Bryopsidece, 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 young branch of which, when destined to be- Fig 147 _ come a sporecase, instead of elongating drrhosa, with spores' ' i ! -j. 1 -L j> i j borne at the sides of tin indenmtely, begins, after having arrived irancuuts. 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 270 THE MICROSCOPE. division of the mass takes place, and a number of pyriform zoospores, each of which is furnished with a crown of ciliae, are set free. In many families of the olive-coloured Alg, reproduc- tion by zoospores is the general rule ; they differ, however, in the arrangement of their flagella. These organs, 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. JEctocarpus is one of the simplest formj of olive-coloured Algae, consisting of branching 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. 150), 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 the}- 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 ALGJE. 271 antheridia. In monoecious and dioecious Fuel, 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 j at first the spore appears simple, but soon after a series of changes take place, consisting in a splitting of the endo- chrome 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 ryopsidece, 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. 148. Development of Ulvce. 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 sporidiuni- bearing 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 tho 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 OP- and two rows of tetm- gan a difference of opinion exists as to S e eters Magni ' 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 MARINE ALG^!. 273 united in a bunch as in Grijfithsia, 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 epiral 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 011 the bifurcations of the terminal branches. Are not these spiral filaments closely allied to Oscillatoriacece ? The spores are simpler structures than the tetraspores, and mostly occupy a more important posi- 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 T conceptacle ; such a fruit is called a favellidium, and occurs in Haly- menia; the same name Flg - i50.-cflerio diploma. Section of is applied to the fruits oi 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 T lacinia of a frond, showing the stalked ijrW chambered oosporangeff, growing on tufts with intercalated filaments. Magnified 50 diameters. 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- midiuni 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 Red Sea- weeds is very varied. They are exquisite objects for the Micro- scope ; I 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 Florideae 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 Austr'alica" In the plant which results from the germination of the aggregate zoospores of Vaucheria, a genus of Siphortaceae (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 forjji; 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 Unger, Nageli, and Pringsheim on the process of their reproduction may be consulted with advantage. DESMIDTACB/K, DIATOMAC, ALGJK. Tuffen Weit, del. 1'LATK II. Edmund Kvar YOLVOX GLOBATOR. 275 A fresh- water alga of singular beauty and interest to fclie microscopist is the Volvox globator. This little cell BO well known to the older observers as the globe- animalcule, or revolving-cell, is represented in fig. 145, ISTos. 1, 2, 3, and Plate I. No. 15. These revolving globular bodies can be kept a long time alive if exposed in a .glass bottle to a rain-drip from a roof. In this way they maintain their activity and produce antheridia, which are distinguishable by their orange colour. 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.nuclens and two flagella attached. The whoje 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 , just before'the young burst ultimately bursts asunder, atMRSSBWfi: "^S *& its offspring tertum. 3, Doddium ciavatum. 4, to seek an independent exis- Staurastrum gracilis. Fig. 151. younger spheres possess openings through which the water freely flows, affording food and air to the wonderfully constructed little being. Dr. Carpenter believes, "The Volvocinece, whose vegetable nature has been made known to us by observation of cer- 276 THE MICROSCOPE. tain stages in the history of their lives, are but the motiU 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 algae; 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." Flagella can be distinctly seen on the outer edge of the adult Yolvox ; by compressing and rup- turing 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 quantitv 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 perticsum. Mr. Busk says that the body designated by Ehrenberg Splwerosira volvox is an ordinary volvox in a different phase of development; its only marked feature of dissimilarity being that a large proper- VOLVOCINE.fi. 277 tion of the green cells, instead of being single, are very commonly double or quadruple; and the groups of flagel- lated cells thus produced, form by their aggregation dis- coid bodies, each furnished with a single cilium. These clusters separate themselves from the primary sphere, and swim forth freely from under the forms which have been designated Uvella and Sfywcr^tabyEhrenberg. Accord- ing to Mr. Carter, however, Sphcerosira 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 au- tumn 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 macro-gonidium, it loses its colour and regularity of form, and becomes an irregular mass of colourless protoplasm, containing a number of brownish granules." (Plate I. No. 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, amaiboid 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 befoie 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 org^iic 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 thia 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 tnd reproduction are unknown, but which may possibly present an active!?' 278 THE MICROSCOPE. Desmidiacece. A remarkably beautiful family of confer- void algse, 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 filled 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 Diatomacese, 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 Astasisean Infusoria, suggest themselves ; and these must of course wait until their reproduction and history are better known before we can feel satisfied a& 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 vibratory movements of cilise, and drawing in of these, the circula- tory movements of the cell contents, as in the hairs of the Tradescantia, 8, Staurtutrum hirsutum. The compressed and deeply-constricted offer most favourable opportunities for ascertaining the manner of their division ; for although the frond is really a single cell, yet this cell in all its stages appears like two, the segments being always distinct, even from the com- mencement. As the connecting portion is so small, and necessarily produces the new segments, which cannot arise from a broader base than its opening, these are at first very minute ;' though they rapidly increase in size. The segments are separated by the elongation of the connecting tube, which is converted into two roundish hyaline lobules. These lobules increase in size, acquire colour, and gradually put on the appearance of the old portions. Of course, as they increase, the original segments are pushed further asunder, and at length are disconnected, each taking with it a new segment to supply the place of that from which it has separated. It is curious t? trace the progressive development of tLe new portions. At first they are devoid of colour, and 282 THE MICROSCOPE. have much the appearance of condensed gelatine ; but as they increase in size, the internal fluid acquires a green tint, which is at first very faint, but soon becomes darker ; Fig. 154. 7, Sphcerozosma vertebratum. 8, 9, XantMdiue. 10, X. armatum. 11, Cosmo- im crenatum. 13, 17, Sporangia of Cosmarium. 14, X. fasiculatum. 1, Arthrodcsmus convergens. 15, Sto-urastrumtumidum. IGfStaurastrumdilitatum. at length it assumes a granular state. At the same time the new segments increase in size, and obtain their normal figure ] the covering in some species shows the presence of puncta or granules. In Xanthidiwm and Staurastrum the spines and processes make their appearance last, beginning as mere tubercles, and then lengthening until they attain their perfect form and size, armed with seta3 ; but complete separation frequently occurs before the whole process is completed. This singular process is repeated again and again, so that the older segments are united successively, as it were, with many generations. When the cells approach maturity, molecular movements may be at times noticed in their contents, precisely similar to what has been described by Agardh and others as occurring in Confervce. This movement has been aptly termed a swarming. All the Desmidiacece are semi-gelatinous. In some the mucus is condensed into a distinct and well-defined DESMIDIACE^E. 286 hyaline sheath or covering, as in Didymoprium Grevillii and Staurastrum tumidum ; in others it is more attenu- ated, and the fact that it forms a covering is discerned Fig. 155. 21, Penium. 24, Pediastrum biradiatum. 25, Closterium, showing conjugation or self-division. 27, Penium Jenneri. 28, Aptogonum desmidium. 29, Pedias- trum pecticum. 30, AnTcistrodesmus falcatus. 33, Conjugation of Peniurr margaritaceum. 34. Spirotcemia. 35, Closterium. only by its preventing the contact of the coloured cells. In general its quantity is merely sufficient to hold the fronds together in a kind of filmy cloud, which is dispersed by the slightest touch. When they are left exposed by the evaporation of the water, this mucus becomes denser, and is apparently secreted in larger quantities, to protect them from the effects of drought. Meyen states, " that the large and small granules contain starch, and were some- times even entirely composed of it ; " and " in the month of May he observed many specimens of Closterium in which the whole interior was granulated; these grains gave with iodine the beautiful blue colour, indicative of the presence of starch." 1 (1) The test for starch can be easily applied, and so remove any doubt that nay exist. It is snly necessary to bear in mind that unless granular matter 284 THE MICROSCOPE " Did we trust solely to the eye, we should indeed be very liable to pronounce these variable and beautiful forms as belonging to animals rather than vegetables. All favours this supposition. Their symmetrical division into parts; the exquisite disc-form, finely cut and toothed Micrasterias ; the lobed Euastrum; the Cosmarium, glit- tering as it were with gems ; the Xanlhidium, armed with spines ; the scimitar-shaped Closterium, embellished with striae ; the Desmidium, resembling a tape- worm ; and the strangely insect-like Staurastrum, sometimes furnished with arms, as if for the purpose of seizing its prey ; all these characteristics appear to a superficial observer to belong rather to the lowest forms of animal, than vege- table life." Another indication Dr. Bailey adduced, by rendering apparent their power of motion; taking a por- tion of mud covered with Closteria, and placing it in water exposed to light ; after a time, it will be seen tha* if the Closteria are buried in the mud, they work their way to the surface, and cover it with a green stratum : this is no doubt owing to the stimulus light exerts upon all matter, although at first appearing very like a volun- tary effort. Another is afforded by their retiring beneath the surface when the pools dry up. Mr. Ealfs states that he has taken advantage of this circumstance to obtain specimens less mingled with foreign matter than they would otherwise have been. During the summmer of 1854 the Eev. Lord S. G. Osborne drew my attention to the economy of an interest- ing specimen of this family, the Closterium Lunula; after many careful investigations he came to the conclusion that the membrane of the endochrome, both on its inner and outer surface, is ciliated. In the Closterium Lunula, we have ascertained that the best view of its circulation is obtained by the use of strong daylight, or sunlight transmitted through coloured glass, or such a combination of tinted glass as that be seen in the interior of the cell, starch cannot be present. A small quantity ct diluted tincture of iodine may be applied, removing the free iodine by the aid of beat, occasionally adding a little water to facilitate its removal. This also will assist in the removal of the brownish stain which at first obscures the charac- teristic purple tint ; and then, by applying the highest power of the microscope, the peculiar colour of the purple iodide of starch will in general be perceived. DESMIDIACEJ2. 285 proposed by Mr. Rainey, and adapted to a l-4th achro- matic condenser ; with which must be used a 1-Sth ob- ject-glass. The Gillett's condenser, or parabolic reflector, will do equally well if used with a l-8th objective. In diagram A, fig. 156, a specimen of the C. Lunula, as seen Fig. 156.Closteria Lunula. with the above arrangement of microscopic power, and a deep eye-piece, the cilia are in full action along the edge of the membrane which encloses the endochrome ; and also, but not so distinctly, along the inside of the edges of the frond itself. Their action is precisely the same as that in the branchiae of the mussel : there is the same wavy motion; and as the water dries up between the glasses in which the specimen is enclosed, the circulation becomes fainter, and the cilia are seen with more distinctness. In diagram A, a line is drawn at b to a small oval mark; these exist at intervals, and more or less in number over the surface of the endochrome itself, beneath the mem- brane which invests it. These seem to be attached by small pedicles, and are usually seen in motion on the spot lo which they are thus fastened ; from time to time they 286 THE MICROSCOPE. break a way, and are carried by the circulation of the fluid, which works all over the endochrome, to the chambers at the extremities ; there they join a crowd of similar bodies, each in action within those chambers, when the specimen is a healthy one. The circulation, when made out over the centre of the frond, for instance at a, is in appearance of a wholly different nature from that seen at the edges. In the latter, the matter circulated is in globules, passing each other, in distinct lines, in opposite directions ; in the cir- culation as seen at a, the streams are broad, tortuous, of far greater body, and passing with much less rapidity. To see the centre circulation, use a Gillett's illuminator and the 1-Sth power; work the fine adjustment so as to bring the centre of the frond into focus, then almost lose it by raising the objective ; after this, with great care, work the milled head till the dark body of the endochrome is made out ; a hair's-breadth more adjustment gives this circula- tion with the utmost distinctness, if it is a good specimen. It will be clearly seen, by the same means, at all the points where the spaces are put ; and from them may be traced, with care, down to both extremities. The endochrome itself is evidently so constructed as to admit of contraction and expansion in every direction. At times the edges are in semi-lunar curves, leaving uninter- rupted clear spaces visible between the green matter and the investing membrane ; at other times, the endochrome is seen with a straight margin, but so contracted as to leave a well-defined transparent space along its whole edge, between itself and the exterior case. It is interesting to keep changing the focus, that at one moment we may see the globular circulation between the outer and inner case, and again the mere sluggish movement between the inner case and the endochrome. At B is given an enlarged sketch of one extremity of a G. Lunula. The arrows within the chamber pointing to b, denote the direction of a very strong current of fluid, which can be detected, and occasionally traced, most dis- tinctly ; it is acted upon by cilia at the edges of the chamber, but its chief force appears to come from some impulse given from the very centre of the endochromeu DESMIDIACEA 287 The fluid is here acting in positive jets, that is, with an almost arterial action ; and according to the strength with which it is acting at the time, the loose floating bodies are propelled to a greater or less distance from the end of the endochrome ; the fluid thus impelled from a centre, and kept in activity by the lateral flagella, causes strong eddies, which give a twisting motion to the free bodies. The line , in this diagram, denotes the outline of the mem- brane which encloses the endochrome ; on both sides of this flagella can be seen. The circulation exterior to it passes and repasses it in opposite directions, in three or four distinct courses of globules ; these, when they arrive at c, seem to encounter the fluid jetted through an aperture at the apex of the chamber ; which disperses them so much, that they appear to be driven, for the most part, back again on the precise course by which they had arrived. Some, however, do enter the chamber ; occasion- ally, but very rarely, one of the loose bodies may be seen to escape from within, and get into the outer current, it is then carried about until it becomes adherent to the side of the frond. With regard to the propagation of the C. Lunula, we have never seen anything like conjugation; but we have repeatedly seen what the reverend gentleman has so well described increase by self-division. Observe the diagram D ; but for the moment suppose the two halves of the frond, represented as separate, to just overlap each other. Having watched for some time, the one half may be seen to remain passive ; the other has a motion from side to side, as if moving on an axis at the point of juncture : the separation then becomes more and more evident, the motion more active, until at last with a jerk one segment leaves the other, and they are seen as drawn. It will be observed, that in each segment the endochrome has already a waist ; but there is only one chamber, which is the one belonging to the one extremity of the original entire frond. The globular circulation, for some hours previous to subdivision, and for some few hours afterwards, runs quite round the obtuse end of the endochrome a, by almost imperceptible degrees; from the end of the endochrome symptoms of 288 THE MICROSCOPE. an elongation of the membranous sac appear, giving a semi-lunar sort of chamber; this, as the endochrome elongates, becomes more denned, until it has the form and outline of the chamber at the perfect extremity. The obtuse end b of the frond is at the same time elongat- ing and contracting ; these processes go on ; in about five hours from the division of the one segment from the other, the appearance of each half is that of a nearly perfect specimen, the chamber at the new end is complete, tht globular circulation exterior to it becomes affected by the cir- culation from within the said chamber; and, in a few hours more, some of the free bodies descend, become exposed to, and tossed about in the eddies of the chamber, and the frond, under a l-6th power, shows itself in all its beau- tiful construction. E is a diagram, of one end of a C. didy- motocum, in which the same process was noticed. The Euastrum Didelta is well worthy of attention, as well as many other species, the Xanthidium Penium, Doddium, &c. The Arthrodesmus Incus has a very beautiful hyaline membrane stretching from point to point, cut at the edges, Flg * l something like the Micrasterias. This is represented at fig. 157. The Mode of finding and Taking Desmidiacece. As the difficulty of obtaining specimens is very great, it will materially assist the efforts of the microscopist to know the method adopted by Mr. Ralfs, Mr. Jeriner, and Mr. Thwaites. " In the water the filamentous species resemble the Zygnemata ; but their green colour is generally paler and more opaque. When they are much diffused in the water, take a piece of linen, about the size of a pocket handkerchief, lay it on the ground in the form of a bag, and then, by the aid of a tin box, scoop up the water and strain it through the bag, repeating the process as often as may be required. The larger species, Euastrum, Micrasterias, Closterium, &c., are generally situated at the bottom of the pool, either spread out as a thin gelatinous stratum, or collected into fiuger-lik<> tufts. If the finger be gently passed beneath them, they will rise to the sur- face in little masses, and with care may be removed and DESMIDIACE^. 289 strained through the linen as above described. At first nothing appears on the linen except a mere stain or a little dirt ; but by repeated fillings-up and strainings a consi- derable quantity will be obtained. If not very gelatinous, the water passes freely through the linen, from which the specimen can be scraped with a knife, and transferred to a smaller piece ; but in many species the fluid at length does not admit of being strained off without the employ- ment of such force as would cause the fronds also to pass through, and in this case it should be poured into bottles until they are quite full. But many species of Stauras- trum, Pediastrum, &c., usually form a greenish or dirty cloud upon the stems and leaves of the filiform aquatic plants; and to collect them requires more care than is necessary in the former instances. In this state the slightest touch will break up the whole mass, and disperse it through the water : for securing them, let the hand be passed very gently into the water and beneath the cloud, the palm upwards and the fingers apart, so that the leaves or stem of the inverted plant may lie between thera, and as near the palm as possible ; then close the fingers, and keeping the hand in the same position, but concave, draw it cautiously towards the surface ; when, if the plant has been allowed to slip easily and equably through the fingers, the Desmidiacece, in this way brushed off, will be found lying in the palm. The greatest difficulty is in withdraw- ing the hand from the surface of the water, and probably but little will be retained at first ; practice, however, will soon render the operation easy and successful. The con- tents of the hand should be at once transferred either to a bottle, or, in case much water has been taken up, into the box, which must be close at hand ; and when this is full, it can be emptied on the linen as before. But in this case the linen should be pressed gently, and a portion only of the water expelled, the remainder being poured into the bottle, and the process repeated as often as necessary." When carried home, the bottles will apparently contain only foul water ; if they remain undisturbed for a few hours, the Desmidiacece will sink to the bottom, and most of the water may then be poured off. If a little filtered pain-water be added occasionally, to replace what has been 290 THE MICROSCOPE. drawn off, and the bottle exposed to the light of the suj), the Desmidiacece will survive for a long time. Fungi. =-This interesting class of cellular nowerlesa plants are chiefly microscopic, many requiring a high magni- fying power to determine their peculiarities of structure, They abound in damp places, among decaying and decayed vegetable and animal matters, everywhere, and in almost every place. The structure of all Fungi exhibits a well de- fined separation into two parts, a mycelium (thallus) jointed and branched, forming a kind of cottony filamentous mass, and a reproductive spore or fruit, which, although exceed- ingly minute, differs somewhat in appearance under the microscope. The "spawn" used for planting mushroom beds is composed of mycelium, and may be readily obtained for examination (fig. 188, No. 19). The dust- like powder of any of the moulds or mildew when sprinkled on a slip of glass and kept under a bell glass over water, will soon throw out filaments and spores in all directions. De Bary's observations show that resting-spores are not peculiar to the algas ; for he found them in two genera ot fungi, and Tulasne ascertained their production in Per&no- sporce, many of which are parasitic, as P. parasitica, a species found on the cabbage and turnip leaf, as well as on the shepherd's-purse, Capsella bursapastoris. For the growth of P. infestans, the potato mould, the exclusion of light seems to be needful, and it is easy to conceive how the spores, washed down to the tuber during heavy rains, throw out germinating threads, which easily pene- trate the thick cuticle of the potato, and quickly produce a murrain. The Eev. M. J. Berkeley, the English authority on Fungi, says : " The genus Cystoptis comprises those para- sitic fungi amongst the Uredines which are remarkable for their white spores. Till the resting-spores of the different species were ascertained, it was almost impossible to find good distinctive characters : one species at least, Cystopus candidus, is to be found everywhere on the common shepherd's-purse, and often accompanied by Peronospora parasitica. It is also frequent on the crucifera : the aero- spores, or gonidia, which spring from the swollen threads of the mycelium, form necklaces, as in oidium, the joints PARASITIC FUNGI. 291 of which, give rise to zoospores, as first observed "by Pre- vost, in 1807. Like those of Peronospora, they move about in water by means of two lash-like appendages, and there germinate. When resting on the leaves of a plant, they make their way by means of a germinating thread into its subjacent tissues, and throw out little suckers. The branched mycelium gives off sporangia and antheridia, exactly as in Peronospora ; when ripe, the sporangia are strongly warted. They fall, doubtless, with the leaves to the ground, where they remain till a fitting season arrives for their development. The provision made for the rapid development of these parasites and for the preservation of their species is truly marvellous, and sufficiently accounts for the difficulty of extermination and their apparently sudden dispersion, especially in wet weather." De Bary's observations on the germination of Uromyces appendiculatus are interesting, inasmuch as they show that the sporidia produce a mycelium, from which springs in succession 1st, spermogonia; 2dly, peridia, producing chains of orange-coloured fruit, or, in other words, an JScidium ; and 3dly, the original fruit of Uromyces, ac- companied by the more simple fruit commonly called uredo, and now called wredo-stylospores. The germination of the fruit produced by the peridia, as well as that of the wredo-stylospores, produces, according to De Bary, 1st, 7rec?o-stylospores, and 2d, the original Uromyces- spores. Thus we see the Uromyces-spoies passing through the generations of promycelium, sporidia, and mycelium the latter producing successively the two different products, spermogonia and secidia, and ultimately the original fruit of Uromyces, accompanied by the Uredo. The sperrnatia, or contents of the spermogonia, never germinate ; but we find the fruit of the secidia, and also of the Uredo, repro- ducing first the Uredo itself, and subsequently the original fruit of Uromyces. Other interesting points, noticed by the same author, are, " that not only has each species a liking for certain special nutrient plants, but that in certain Uredines with multiple fruit and alternate generations each sort of reproductive organ buries its germ in a different nutrient plant ; and that the vegeta- tion of the parasite is the cause of the disease." THE MICROSCOPE. I)e Bary has also carried out a series of experiments which go far to satisfy him that the sporidia of Puccinia qraminis germinate on tha leaves of Herberts, and that the jEcidium of the Berberis (Plate I. No. 22) is a stage in the cycle of development of Puccinia. Thus, whilst in most Uredines the entire development is carried out upon one and the same nutrient plant, the alternate generations in Puccinia graminis require a change of host. This is a state of things well understood now in the animal kingdom in the Tsenise and Trematoda, but Puccinia graminis is, we believe, the first of the parasitic fungi in which it has %een particularly ascertained. Another point of interest is a confirmation of the supposed injurious effect of the proximity of Berberis to corn, which has been denied. De Bary further shows that Mucor mucedo (the common mould) has three, if not four, different forms of fruit; and "that the mould called Thamnidium by Link, or Ascophora elegans by Corda, and the mould described by Berkeley as Botrytis Jonesii, and made into a new genus by Fresenius, under the name of Chcetocladium, are only varieties of the 'fruit of Mucor mucedo. Also that yeast, Achy la, Sapro- -flegnia, and Entomophthora or Empusa, are identically the ame as Mucor mucedo, consequently that a large reduction is needed in the genera of the mucorini. The main interest, however, of De Bary's paper on the fructification of the Ascomycetes, consists in observations en Erysiphe Cichoracearum, &c., in which the author traces the origin of the perithecium, from its earliest state sip to the formation of the single ascus and spores. He notices two cells as being always present and visible from the earliest period, one of which he conjectures may be the female, and the other the antheridium or male organ. He says that the cell, by the division of which the ascus end its coating are formed, only develops itself when it iias been in contact with the antheridium ; and he con- eiders it very probable that impregration is effected by cuch contact, and that the perithecium of Erysiphe (ex- cepting the outer wall) is the product of such impreg mtion. De Bary's paper on parasitic -fungi was, it appears, asixlcrtakeu with a view to contribute to the solution of PARASITIC FUNGL the question as to their origin ; and he concludes that endophytes are not produced from the metamorphosed substance of diseased plants, but that they originate from germs which penetrate healthy plants and develop a mycelium. In the course of his investigations he notices- the occurrence in the genus Cystopus of organs similar to those long since discovered by Tulasne in Peronospora^ which have been called OogonicC. He observes that rami- fications perform the functions of antheridia, or male? organs; and he proceeds to describe the production by th oospores (or impregnated contents of the oogonia) of active zoospores, similar to those produced by the ordinary spores of Cystopus. Dr. De Bary states that these zoospores, after remaining active for three or four hours, lose their cilia and power of motion, assume a cellulose covering, and ger- minate. He adds that the germ-filaments enter readily by the stomates and leaves of the nutrient plant, but tha those filaments only become developed which enter ths stomates of cotyledons. In Peronospora the development of the antheridia, oogonia, and oospores is said by De Bary to be the same as in Cystopus; and he gives particulars of the mode of germination of the conidia, and remarks on. the growth of the parasite, which may be profitably studied in the paper itself. Parasitic fungi, vegetable blights as they are commonly, called, have of late years become objects of earnest atten- tion, on account both of the enormous damage done to our growing crops, and also of the many curious facts in their: history which have been brought to light. Corn-blights consist chiefly of mildew, Puccinia, smut, bunt, rust, 03? red-robin, Uredo. Oidium is a common mildew ; Bo- trytis another; jEcidiurn, forms a kind of rust infecting pear-trees, the peridia of which form a very pretty object for the microscope. (Plate I. No. 22, jEcidium Herberidis.)- In the full-grown condition they appear as little cups filled with reddish-brown powder (spores), and may be detected in their earliest stages by the deformities they produce hi the structure of the plants infested, or by pale or reddish spots on the green surface, arising from the presence of the fungus beneath. They are common on the coltsfoot, the berberry, gooseberry, buckthorn, nettle, &c. Plate I. No. 19^. 294 THE MICB080CPX. represents a vertical section of a leaf of black-currant, in- fested with JEcidium grossularice ; its sperm ogonia are seen on the surface, and the perithecia below. The family Sphceriacei (No. 3, Plate I.), common enough on most herbaceous stems, first seem to be little black spots, a; when examined more closely are found to resemble little brownish bottles, b, filled with rows of spores. Other in- structive specimens are * Cystopus candidws (Uredo olim), Crucifer White-rust; conidia equal, globose ; membrane equal, ochraceous ; oospores sub-globose, epispore yellowish-brown, with irre- gular obtuse warts : warts solid. On shepherd's purse, cabbage, and other Cruciferse : receptacle consisting of thick branched threads ; conidia concatenate, at length separating ; oospores deeply seated on the mycelium. Phyllactinia guttata (Olim Erysiphe). Plate I. No. 9. Hazel Blight ; amphigenous ; mycelium web-like, often evanescent; conceptacles large, scattered, hemispherical, at length depressed; appendages hyaline, rigid, simple; sporangia 4-20, containing 2-4 spores. On leaves of haw- thorn, hazel, ash, elm, &c. Aregma (Phragmidium) bul- bosum. Plate I. No. 20. Bramble Brand; hypogynous, with a dull red stain on the upper surface; spores in large tufts, 4-septate, terminal joint apiculate; peduncles incrassated, and bulbous at the base. Puccinia variabilis, Variable Brand ; sori amphigenous, minute, roundish, sur- rounded by the ruptured epidermis, nearly black ; spores variable, obtuse, cells often subdivided ; peduncle very short. On leaves of dandelion. Puccinia buxi, Box Brand. Plate I. No. 17. Sori sub-rotund, convex, and scattered; spores brown, oblong, rather strongly constricted, lower cell slightly attenuated; peduncle very long. On both surfaces of box leaves : spores uniseptate, supported on a distinct peduncle. Plate I. No. 18. Trichobasis (Uredo olim) senecionis, Groundsel-rust ; spots obliterated ; sori solitary or regularly crowded ; sub-rotund and oval, on the under surface, surrounded by the ruptured epidermis; spores sub-globose, orange. On various species of groundsel : spores free ; attached at first to a short peduncle, which at length falls away. It appears that at particular periods of the year the PARASITIC FUNGI. 295 atmosphere is, so to speak, more fully charged wiih the various spores of fungi than it is at others. The spores of the moulds aspcrgilbts, penicillium, and puccinia are per- haps the most widely distributed bodies, and towards the end of the hot weather, or about autumn time, they are very abundant. Among those who have taken them at this period of the year, we must ever associate the name of the Rev. Lord Godolphin Osborne, who first experimented in this direction during the cholera visitation of 1854. He exposed prepared slips of glass, slightly moistened with glycerine, over cesspools, gully-holes, &c., near the dwell- ings of those where the disease appeared, and caught what he termed aerozoa chiefly minute germs and spores of fungi. A drawing made from one of these glasses (Plate I. No. 13), exhibits spores almost identical with those found on the human skin, and eteewhere. From the year 1854 to the present time we have amused ourselves by catching these floating atoms, and, so far as we can judge, they are found everywhere, and in and on every conceivable thing, if we only look close enough for them. Even the open mouth is an excellent trap ; of this there is ample evidence, since we find on the delicate membrane lining the mouth of the sucking, crying infant, and on the diphtheritic sore throat of the adult, the de- structive plant Oidium albicans. The human or animal stomach is invaded, and in a certain deranged condition we find the Sarcina ventriculi, with its remarkable-looking quaternate spores, its torulae, &c., seriously interfering with the functions of this organ. 1 Torula diabetica is another 3f these destructive products found in the human bladder. Fig. 153. Sarcina ventriculi. (1) What part do tne fungi, or bacteria, play in the production of that fearful scourge of the human i - ace, cancer? is a question not unfrequently asked since 296 THE MICROSCOPE It is now more than a quarter of a century since Pro fessor Owen first pointed out the vegetable nature of a diseased growth found in the lungs of a Flamingo he was dissecting. Soon after, Bassi discovered the vege- table character of a disease which caused great devas- tation among silkworms ; and, about the same time, Schonlein, of Berlin, was led to the detection of certain cryptogamic vegetable formations in connexion with skin? diseases. The Favus fungus is perhaps best known from its having been the first to attract the attention of Schonlein. It is commonly called cupped ringworm, or honeycomb scall, but it is very rarely seen in this metropolis. The crust is of a dingy yellow colour, and almost entirely composed of the Aclwrion, mixed with epithelial scales and broken hairs. When the fungus once establishes itself, so fearful are its ravages, that in a very short space of time the whole of the cutaneous surface, with the ex- ception of the palms of the hands and soles of the feet, becomes covered with it. As the spores penetrate the hair-follicles they destroy the sheaths of the hairs, which shrivel up and lose their colouring matter, and then break off, leaving the surface bald. Upon comparing the fermentation of the achorion fungus with that of good healthy yeast, it will be seen to be almost identical. In the first place, it is as actively in the first edition of this book % (1854) I expressed a belief in "the fungoid origin of cancer." Subsequent examinations of diseased structure more or loss tend to confirm this view ; it appears that in this disease we have superadded to a fungoid growth " degraded germinal matter" which, by its eiirance into th circulation, produces a ferment and blood poisoning. The circular animal cell degenerates, is converted into the ovoid or elongated vegetable cell, and ulti- mately the structure, or some organ it may be, is changed into that remarkable- looking caudate body, the typical cancer cell. This in some respects bears the most perfect resemblance to certain spores of fungi, and to the yeast torulse. As might be expected, its form is modified and its character more or less changed by the peculiar kind of nourishment and condensed tissue in which it is deposited and grows ; its powers of growth are, so to speak, perverted and degraded, and then, as we see in other instances, it soon obtains a power of in- definite multiplication, and destroys, not only the vitality of the organ, but the individual. M. Davaiue believes he has traced splenic disease in sheep to the entrance into the blood of bacterium-like bodies, and fungi ; a zymotic disease is caused by the ferment, and by the rapid growth of the fungi the life of the animal is quickly sacrificed to the destroyer. To mount specimens of fungi, separate them, and add a drop or two of spirit : when this has evaporated, add a drop of glycerine solution, or balsam dissolved in chloroform, and put on a glass cover. If the balsam renders the asci tea transparent, use gelatine : no cells are required. YEAST DEVELOPMENT. 297 carried on by the former as by the latter. There is, how- ever, just a slight difference in the size of the spores or cells (Plate I. Nos. 7, 8, 11), those from yeast being the larger and more clearly spherical, with a greater number of reproductive spores, that is, cells with a single, clear, nucleated cell in their interior, while others are filled with a darker granular matter, having only a slight ten- dency to coalesce or become filamentous; those from achorion are for the most part ovoid, and very prone to coalesce and produce elongated cells or torulae. With re ference to the slight difference in size, -we must look upon this as a matter of very little importance ; for to the pre- sence of light in the one case, and its almost total exclu- sion in the other, this difference, no doubt, is almost en- tirely due. It would be more trustworthy if comparisons of this kind could be made at the same stage of develop- ment ; for be it remembered that yeast obtained from a brewery is in a more favourable state, inasmuch as it is stopped at a certain stage of growth or development, and then set to begin its fermentation over again in fresh sup- plies of a new pabulum, which give increased health an- vigour to the plant ; while, on the other hand, the achorion, or Favus fungus, is obtained and used in an ex- hausted state from an already ill-nourished or stai led-out soil. Neither can we attach much importance to differ- ences in size and form of the spores, for even this occurs in yeast ferment ; and although the ovoid is moet fre- quently seen in achorion, it is equally common to yeast when exhausted. This is strikingly exhibited in Plate I. No. 8, a drawing made from a drop of exhausted yeast taken from porter ; here we have oval and elongated cells with torulse. To ensure success in these and similar ex- periments., the fungus or yeast should be left floating on the surface of liquids ; the process is either carried on very slowly, or is entirely arrested by submersion. Turpin and others, in their experiments on yeast, noticed that the cells become oval and bud out in about an hour after being added to the wort (fig. 159) ; but this change depends as much upon temperature and density of the solution as upon the quality of the yeast. It is a well-ascertained fact that when yeast is added to distillery wa?h, which is 298 THE MICROSCOPE. worked at a higher temperature than brewers' wort, fer- mentation commences earlier, and the yeast-cell grows. to a much larger size. It is, indeed, forced in this way much as a plant in a hothouse is, and then obtains to greater perfection in a shorter time. It will, however, be seen that it sooner becomes exhausted ; and now, if we take a portion of this yeast and add it to barley wort, and at the same time keep it in a temperature of from 60 to 65 Fahr., it ferments languidly, and small yeast-cells are the product. If the yeast is allowed to stand in a warm place for a few days, it partially recovers its activity, but never quite. With such a yeast there is always a good deal of tomla3 mixed up with the degenerated cells, and sometimes a filamentous mass, which falls to the bottom of the vessel ; from this stage it readily passes to that of must and mildew, and then becomes a wasteful feeder or destroyer. With yeast already in a state of exhaustion, we have seen a crop of fungus produced in the head of a strumous boy, seven years of age, who was much out of health, and had suffered from eczema of the eyelids, with impetigo. On placing portions of the broken hairs on a glass slip, and moistening with a drop of liquor potassaB, spores and torulaB were seen in abundance; represented in Plate I, 'No. 14. In another experiment we took portions of penicillium and aspergillus moulds, and added these to sweetwort, and stood them by in a warm room. On the second day afterwards in one of the solutions, and the third in the other, fermentation had fairly set in ; the surface of the solution was covered with a film, which proved to be well-developed ovoid spores, filled with smaller granular spores (conidia) : Plate I. No. 8. On the sixth day the cells changed in form and were more spherical. Again removing these to another supply of fresh wort, the results obtained were quite characteristic of exhausted yeast ferment. Extreme simplicity of structure characterises all moulds or milojBws. Their reproductive organs are somewhat more complex, and both in penicillium and aspergillus the mycelium terminates in a club-shaped head, bearing upon YEAST PLANT. 299 it smaller filaments with small bead-like bodies upon the apex, piled one upon the other, or, more properly speak- ing, strung together; these, again, are surmounted by larger spores of a discoid shape filled with granular matter, and others which are quite empty. Those of the aspergillus are apparently without granular matter or nuclei, and are more highly refractive. - The puccinia are club-shaped, the very rapid growth of the spores and spawn of which appears to exert a specific and peculiarly exhaustive action over the tissues of the plant on which it feeds. Plate I. No. 12, represents a portion of the mould taken from a saccharine solution. The yeast plant, in its most perfect condition, is chiefly made up of globular vesicles, measuring, when fully grown, about the jj^nrth. of an inch in diameter. The older cells are filled with granular or nucleated matter ; the nucleus rapidly increases, and nearly fills up as to form a conceptacle like that of the Pyrenomy- oetes ; the form, however, of which is subject to mucK- variation. The reproductive organs of Lichens, as in Fungi, are of 4 five kinds . 1, Sporules, which are formed by the con- struction and subsequent separation of the extremity of a* simple cylindrical filament ; 2, Spermatia with their sup* porting pedicles ; 3, Stylospores with their styles ; 4 y Theca? or asci ; 5, Basidia with their basidiospores. As regards the complexity of their form and structure they, may be taken in the order in which they are here placecV; x 306 THE MICROSCOPE. but, of the last-mentioned, it should be stated that they are almost solely found in Fungi, which have really no other reproductive organ. The spores present many points of dif- fero.Dce, both in number and character, in different genera and species, and for this reason are most interesting micro- scopic objects. We would direct the reader's attention to an interesting and valuable paper, from the pen of Dr. Lauder Lindsay, in the Linmean Soc. Trans., vol. xxv. p. 493, " On the Lichens of New Zealand" (the country par excellence of certain Lichens). The paper is very beautifully illustrated, showing chiefly the minute or microscopic anatomy of the reproductive organs of the species examined, and more especially the character of their spores. A vertical section of Parmelia stellala is given in Plate I. No. 26: it belongs to an extensive genus of Gymnocarpous open-fruited Lichens, found growing upon trees, palings, stones, walls, &c. The emission of the ripe spores of the Lichens is a curious process, and not unlike that which is seen to take place in some of the Fungi, as in Pezizce, Sphcerice, &c. If a portion of the thallus be moistened and placed in a common phial, with the apothecia turned toward one side, in a few hours the opposite sur- face of the glass will be found covered with patches of spores, easily perceptible by their colour ; or if placed on a moistened surface, and one of the usual glass slips laid over it, the latter will be covered in a short time. As to the powers of dissemination of these lowly organized plants, Dr. Hicks's observations lead to the conclusion that the gonidia of Lichens have greater powers in this direction than has been generally supposed. He found by placing a clean sheet of glass in the open air during a fall of snow, and receiving the melting water in a tube or bottle, that he obtained large quantities of what has been looked upon as a " unicellular plant, commonly called ' ClilorococcusJ the cells of which may remain in a dor- mant condition for a long time during cold weather, but upon the return of warmth and moisture they begin to increase by a process of subdivision, into two, four or eight portions, which soon assume a rounded form and burst the parent cell-wall open; these secondary cells L10HEXS. 307 soon begin to divide and subdivide again, and tiis process may go on without much variation even for years. The phenomena described may also be watched by taking a portion of the bark of a tree on which the Chlorococcus has been deposited, and placing it under a glass to keep it in a moderately moist atmosphere ; the only difference being a change in colour, which is caused by the growth of the fibres, as may be seen on microscopical examination. And this," Dr. Hicks says, " is an instructive point, be- cause it will be found that the colour varies notably accord- ing to the Lichen prevalent in its neighbourhood." 1 He thinks there can be no doubt that what has been called Chlorococcus, is nothing more than the gonidia of some Lichen ; and that under suitable conditions, chiefly drought and warmth, the gonidium often throws out from its external envelope, a small fibre, which, adhering and branching, ultimately encases it and forms a " soridium." ' The soridia also remain dormant for a very long time, and do not develop into thalli unless in a favourable situation ; in some cases it may be for years. It will be easily perceived that the soridium contains all the elements of a thallus in miniature ; in fact, a thallus does frequently arise from one alone, yet, generally, the fibres of neigh bouring soridia interlace, and thus a thallus is matured more rapidly. This is one of the causes of the variation of appearance, so common in many species of Lichens, and is more readily seen towards the centre of the parent thallus. "When the gonidia remain attached to the parent thallus, the circumstances are, of course, generally very favourable, and then they develop into secondary thalli, attached more or less to the older one, which, in many instances, decays beneath them. This process being con- tinued year after year, gives an apparent thickness and spongy appearance to the Lichen, and is the principal cause of the various modifications in the external aspect of the Lichens which caused them formerly to be mis- classified." 2 (1) "For instance, where the yellow Parmelia is found, the Chlorococcu* will assumes yellow tinge in its soridial stage. Viewed by transmitted light, they are also opaque balls, with irregular outline." (2) " Contributions to the Knowledge of the Development of the Gonidia of Lichens." By J. Braxton Hicks, M.D. &c., Quarterly Journal of Micrcscopicai Science, vol. viii. 860, p. 239. x 2 308 THE MICROSCOPE. The little group of Ilepaticce or Liverworts, which if intermediate between Lichens and Mosses, presents nume- rous objects of interest for the microscopist. These plants are produced by dust-like grains called spores, and minute cellular nodules called gemmce or buds. The gemmae of Marchantia polymorphic^ are produced in elegant membranous cups, with a toothed margin growing on the upper surface of the frond, especially in very damp court yards between the stones, or near running water, where its lobed fronds are found covering extensive surfaces of moist soil. At the period of fructification, these fronds send up stalks, which carry at their summit round shield-like or radiating discs. Besides which, it generally bears upon its surface a number of little open basket-shaped " conceptacles " -which are borne upon the surface of the frond, as in fig. 162, and may be found in all stages of develop- ment. When mature it contains a number of little green round or ob- long discs, each com- posed of two or more layers of cells; the wall is surmounted by a glis- tening fringe of teeth, whose edges are them- selves regularly fringed with minute outgrowths. The cup seems to be formed by a develop- ment of the superior epidermis, which is raised up and finally bursts and spreads out, laying bare the seeds. The development of this structure presents much analogy to that of the sori of Ferns. Muscacece, Mosses, are an interesting form of vegetable life, Linnaeus called them servi, servants, or workmen, as they seem to labour to produce vegetation in newly- formed countries, where soil is not yet formed. They also fill and consolidate bogs, and form rich mould for the growth of larger plants, which they protect from the Fig. l62.Gemmiparous Conceptacle of Mar- chantia polymorphia, expanding and rising from the surface of a frond. MOSSES. 309 winter's cold. The common, or Wall Screw-moss, fig. 163, growing almost every where on old walls and other brick- work, if examined closely, will be found to have springing from its base numerous very slender stems, each Fig. 163. Screw Moss. of wh.ch terminates in a dark brown case, which encloses its fruit. If a patch of the moss is gathered when in this state, and the green part of the base is put into water, the threads of the fringe will uncoil and disentangle them- selves in a most curious and beautiful manner ; frcm this circumstance the plant takes its popular name of Screw-moss. The leaf usually consists of either a single or a double layer of cells, having flattened sides, by which they adhere one to another. The leaf-cells of the Sphagnum bog- moss, tig. 179, exhibit a very curi- ous departure from the ordinary type ; for instead of being small and polygonal, they are large and elon- gated, and contain spiral fibres loosely coiled in their interior. Mr. Huxley pointed out, that the young leaf does not differ from the older, and that both are evolved by a gradual process of" differentiation" Mosses, like liverworts, possess bothantheridiaand pistillida, which are engaged in the process of fruo- tification. The fertilized cell b&- eonies gradually developed into a conical body elevated 310 THE MICBOSCOPE. upon a foot stalk ; and this at length tears across the walls cf the flask-shaped body, carrying the higher part upwards as a calyptra or hood upon its summit, whila the lower part remains to form a kind of collar round the base. These spore-capsules are closed on their summit by opercula or lids, and their mouths when laid open are surrounded by a beautiful toothed fringe, termed the peristome. This fringe is shown in fig. 164 in centre of capsule of Funaria, with its peristome in situ. The fringes of teeth are variously constructed, and are of great service in discrimi- nating the genera. In Neckera anti- pyretica. fig. 165, the peristome is double, the inner being composed of teeth united by cross bars, forming a very pretty trellis. The seed spores are contained in the upper part of the capsule, where they are clus- tered round a central pillar, which Fig. lea. Double Peristome of is termed the columella ; and at the Neckera Antip.rctica. timQ Qf maturityj the int erior of the capsule is almost entirely occupied by spores. It may here be mentioned, that all mosses and lichens are more easily detached from the rocks and walls on which they grow in frosty weather than at any other period, and consequently they are best studied in- winter. One of the com- monest, Scale-moss, fig. 166 (Jungermannia biden- tata), grows in patches, in moist, shady situa- tions, near the roots of trees; see Plate II. Xos. Fig. iw.-Scaie.Mos,. 35 aa( j 36. The seed- vessels are little oval bodies, which if gathered when unexpanded, and brought into a warm room, burst under the eye with violence the moment a drop ot water is applied to them, the valves of the vessel taking the shape of a cross, and the seeds distending in a cloud of brown dust. If this dust be examined with the EQUISETACE^E. 31] microscope, a number of curious little chains, looking something like the spring of a watch, will be found among it, their use being to scatter the seeds ; and if the seed-vessel be examined while in the act of bursting, these little springs will be found twisting and writhing about like a nest of serpents. The undulating Hair-moss (Polytrichum undu- latum), fig. 167, is found on moist shady banks, and in woods and thickets. The seed- vessel has a curious shaggy cap ; but in its construction it is very similar to that of the Screw- moss, except that the fringe around its opening is not twisted. Hquisetacece. The history of the development of the Equise- tacese (horse-tails) corresponds in some respects with that of Ferns. The spore-case of this solitary genus is a most interesting object under the microscope ; they have apparently only one coat, for the outer coat splits up into four thread-like processes (elaters), clubbed at their free ends. While the spore remains on the sporange, these fibres are rolled round the spore, as in fig. 170, G; but by gently shaking the fruit spike, the spores are discharged, the coiled fibres immediately unroll, as at F, their elasticity causing them to spring about in a most curious manner. In a few minutes this motion appa- rently ceases, but if breathed upon they again unroll and .dart about with wonderful elasticity. Ferns. In the Ferns we have an intermediate state, somewhat between mosses and flowering plants; this would, not apply to the reproductive apparatus, which is formed upon the same type as that of Mosses ; and, furthermore, it is to be observed, that Ferns do not form Fig. 167. Hair-Moss in Fruit. THE MICROSCOPE. buds like other plants, but that their leaves, or fronds as they are properly called, when they first appear, are rolled *ip in a circinate form, and gradually unfold, as in fig. 168. Fig. 16S. Male Fern. A portion ol leal with BorL fi'erns have no visible flowers; and their seeds are produced in clusters, called sort, on the backs of the leaves. Each sorus contains numerous thecse, and each theca encloses ^almost innumerable sporanges, with spores or seeds. There are numerous kinds of ferns, all remarkable for some interesting peculiarity ; but it is their spores which are chiefly sought for by the microscopist. The first account of the true mode of development of .Ferns from their spores was published in 1844, by Nagoli, ia a memoir entitled Moving Spiral Filaments (spermatic filaments) in Ferns, wherein he announced the existence of the bodies now called antheridia; but, mistaking the archegonia for modified forms of the antheridia, he was led away from a minute investigation of them. If he had followed the development of the prothallia further, he would have detected the relations of the nascent embryo, which would probably have put him on the right track. As it was, the remarkable discovery of the moving spiral filaments occupied all his attention, and caused him to fall FERNS. 313 into an error in certain important respects ; for example, he has represented what is undoubtedly an archcgonium filled with cellules, sperm-cells, which, he states, " emerged from it as from the antheridia" This description is not quite correct. The reproduction of ferns had, until within. the last few years, been a vexed question among botanists. The riddle was at length solved by the labours of Count Suminski, who discovered that it is in the structures developed from the spores in germination that the pistillidia and autheridia of ferns are to be sought. The nature of the phenomena by which the propagation of ferns is effected, is as follows. In all the different species of ferns, the spores are contained in brown dots, on lines collected on the under surfaces, or along the edges of the fronds. Each of the spore-cases is surrounded by an elastic ring, which when the time arrives for the spores to be set free, makes an effort to straighten itself, and in so doing causes the spore- case to which it is attached to split open, and the spore dust to be dispersed. Very soon after these spores have begun to germinate, ^^ a flat plate-like expansion, p . , r . . * ig- 169. Sorus of Depana prohfera. somewhat resembling a heart in form, shows itself. This expansion gradually thickens, the tube from which it had sprung withering away. So far, observes Mr. Henfrey, there is nothing very remarkable in the development of these plants from their spores, but the succeeding phenomena are exceedingly curious. The main particulars are thus described by him : " At an early period of the expanding growth of the leaf- like product of the spore, termed the prothallium or -germ-frond, a number of little cellular bodies are found projecting from the lower surface, which, if placed in water when ripe, burst and discharge a quantity of micro- scopic filaments, curled like a corkscrew, and furnished with vibrating hair-like appendages, by the motion of 3ii THE MICROSCOPE. which they are rapidly propelled through the water. The cellular bodies from which these are discharged are termed the antheridia of the ferns, and are in their physiological nature the representatives of the pollen of the flowering plants. At a somewhat later period other cellular bodies of larger size and more complex structure are found in small numbers about the central part of the lower surface of the prothallium on the thickened portion, situated between the notch and the part where the radical filaments arise. These, the pistillidia or archegonia of the ferns, are analogous to the ovules or nascent seeds of flowering plants, and contain, like them, a germinal vesicle, which becomes fertilized through the agency of the spiral fila- ments mentioned above, and is then gradually developed into an embryo plant possessing a terminal bud. This bud begins at once to unfold and push out leaves with a circulate vernation, which are of a very simple form at first, and rise up to view beneath the prothallium, coming out at the notch; single fibrous roots are at the same time sent down into the earth, the delicate expanded pro- thallium withers away, and the foundation of the perfect fern plant is laid. As the bud unfolds new leaves, the root stock gradually acquires size and strength, and the leaves become larger and more developed ; but it is a long time before they assume the complete form characteristic of the species." These observations on Ferns have acquired vastly- increased interest from the subsequent investigations of Hoffmeister, Mettenius, and Suminski, on the allied Cryp- togams, and, above all, from Hoffmeister's observations on the processes occurring in the impregnation of the Coni- fers, Not only have these investigations given us a satis- factory interpretation of the archegonia and antheridia of the Mosses and Liverworts, but they have made known and co-ordinated the existence of analogous phenomena in the Equisetacece, Lycopodiacece, and RhizocArpece, and shown, moreover, that the bodies described by Mr. Brown in the Conifers, under the name of " corpuscles," are analogous to the archegonia of the Cryptogams ; so that a link is hereby formed between these groups and the higher flowering plants. CHARA. 315 The fruits, cr sori, of Ferns afford a very beautiful variety of objects for the microscopist, and they possess an advantage in requiring little or no preparation nothing more being necessary than that of taking a portion of a frond, place it on a glass-slip under the microscope, and throwing a condensed light upon it by the aid of the side reflector. Even germination may be watched by simply employing gentle heat and moisture. Take, as Hoff- meister directs, a frond of a Fern whose fructification is mature, lay it upon a piece of glass covered with fine paper, and place the spore-bearing surface downwards upon this ; in the course of a day or two this paper will be found to be covered with a fine brownish dust, which consists of the liberated spores. These must be carefully collected, and spread out upon the surface of a smooth fragment of porous sandstone, and then- placed in a saucer, the bottom of which is before covered with water ; a glass tumbler being inverted over it to ensure the requisite supply of moisture, and prevent rapid evaporization. Some of the prothallia soon germinate ; if the cup be kept only slightly moist for some time, and then suddenly watered, a large number of antheridia and archegonia quickly open, and in a few hours the surface of the larger prothallia will be covered with moving antherozoids. If sections of fhese be made, that is, the canals laid open, with a power of 200 or 300 diameters we may occasionally see antherozoids in motion, CHARACE.E. Ghara vulgaris is the plant in which the important fact of vegetable circulation was discovered ; Fig. 170, No. 1, is a portion of the plant of the natural size. Every knot or joint may produce roots ; but it is somewhat remarkable, that they always proceed from the upper surface of the knot, and then turn downwards ; so that it is not peculiar that the first roots also should rise upwards with- the plant, come out of the base of the branch, and then turn downwards. Mr. Varley noticed : " The ripe globules spontaneously open; the filaments expand and separate into clusters." "These tube-like filaments are divided into numerous compartments, in which are produced the most extra- ordinary objects ever observed of vegetable origin, Fig. 170 A. At first they are seen agitating and moving- in their 316 THE MICROSCOPE. cells, where they are coiled up in their confined spaces, every cell holding one. They gradually escape from their Fig. 170. 1, Branch of Chora vulgari*. 2, Magnified view : the arrows indicate the course taken by the granules in the tubes. 3, A limb of ditto, with buds at joints. 4. Portion of a leaf of ValHtneria tpiralii, with cells and granules. CHARA. 317 cells, and the whole field soon appears filled with life. They are generally spirals of two or three coils, and never become straight, though their agitated motion alters their shape in some degree. At their foremost end is a filament so fine as only to be seen by its motion, which is very rapid and vibratory, running along it in waves . and of a globule be forcibly opened before it is ripe, the fila- ments will give little or no indication of life." They swim about freely for a time, but gradually getting slower and slower ; in about an hour they become quite motionless. Unger described these moving filaments in Sphagnum (bog-moss) as Infusoria, under the name of Fig. 171. Anther idia of Chara fragilis, %c. A, Portion of filament dividing into Phytozoa, " anther aids." B, A valve, with its group of antheridial filaments, composed of a series of cells, within each of which an antherozoid is formed, c, The escape of the mature antherozoidt is shown. D, Antheridium, or globule, developed at the base of nucule. E, Nu- cule enlarged, and globule laid open by the separation of its valves. F, Spores and elatets of JZquisetum. G, Spores surrounded by elaters of Equlsetum. Spirillum ; and consequently they have been the cause of much controversy. Schleideri, very properly denying their animal nature, says : " They are nothing more than fibre 318 THE MICROSCOPE. in an early stage of development." The Characeoe are all aquatic plants of filamentous structure. Some authors have divided the species into two genera, Nitella (simple tubes) and Chara (cortical tubes). The circulation in the ordinary tubes or cells consists in the movement of the gelatinous protoplasmic sac, seen, as one mass, slowly passing up one side, across the ends, and down the opposite side, not perpendicularly, but in an oblique or spiral course, as indicated in the figure. The Characcoe multiply by gemmas, produced at the articulations of their stems. Mr. H. J. Carter, in a paper of great interest, published January, 1857, on the "Development of the Boot-cell and its Nucleus, in Chara Verticillata? describes a structureless cell-wall, and a protoplasm composed of many organs. " This," he says, " is surrounded by a cell, the l proto- plasmic sac,' which is divided into a fixed and rotatory portion ; these again respectively enclose the nucleus ' granules,' and axial fluid ; while small portions of irre- gular shaped granular bodies are common to both. If we take the simple root-cell about the eighteenth hour after germination, when it will be about half-an-inch long, and l-600th of an inch broad, and place it in water between two slips of glass for microscopic observation, under a magnifying power of about four hundred diameters, we shall find, if the circulation be active and the cell-wall strong and healthy, that the nucleus, which is globular, gradually becomes somewhat flattened, having several hyaline vacuoles of different sizes ; the change goes on gradually until it appears of more elongated form, growing fainter on its outline, and then entirely disappears, leaving a white space corresponding to its capsule or cell-wall, with a faint remnant of some structure on the centre. Subsequently, this space becomes filled up with the fixed protoplasm, and after an hour or two the nucleus re- appears a little behind its former situation, but now reduced in size, and with its nucleolus double, instead of single as before ; each nucleolus being about one-fourth part as large as the old nucleolus, and hardly perceptible. Meanwhile a faint septum is seen obliquely extending across the fixed protoplasm, a little beyond the nucleus ; DEVELOPMENT OF CHAEA. 31 V and if iodine be applied afc this time, the division is seen to be confined to the protoplasm, as the latter, from ccn- traction, withdraws itself from each side of the line where the septum appeared, ar>d leaves a free space which is bounded laterally by an uninterrupted continuation of the protoplasmic sac. In this way changes go on until its shape is altered and it becomes converted into a bunch of rootlets. Thus the new cells are never entirely with- out a nucleus, which would appear to exert some influ- ence upon their development, for as soon as the only two new- cells which the root-cell gives off are formed, the old nucleus becomes effete. " Now as to the office of the nucleus, nothing more is revealed to us in the developmejit of the roots of Chara, than that, so long as new cells are to be budded forth, the nucleus continues in active operation, but when this ceases it becomes effete ; while the rotation of the pro- toplasm and subsequent enlargement of the cell, &c., which are much better exemplified in the plant-stem than in the root-cell, go on after the nucleus ceases to exist. Hence the development of the root-cells of Chara affords us nothing positive respecting the functions of this organ ; and therefore, if we wish to assign to it any uses in par- ticular, they must be derived from analogy with some organism in which there is a similar nucleus whose office is known. If for this purpose we may be allowed to compare the nucleus of Chara with that of the rhizo- podous cell, which inhabits its protoplasm, we shall find the two identical in elementary composition j that is, both consist at first of a ' nuclear utricle,' respec- tively enclosing a structureless homogeneous nucleolus ; the latter, too, in both, is endowed with a low degree of movement. " After this, however, the nucleolus of the Rliizopod cell becomes granular and opaque ; and when, under circum- stances favourable for propagation, a new cell-wall is formed around the nuclear utricle, or this may be an enlargement of the nuclear utricle itself, I do not know which ; the granular substance of the nucleolus becomes circumscribed, and shows that it is surrounded by a sphe- rical, capsular cell ; the granules enlarge, separate, pass 320 THE MICROSCOPE. through the spherical capsule into the cavity of the nuclear utricle ; a mass of protoplasm makes its appearance, and this divides up into monads, or, as I first called them, * gonidia.' 1 "The movement of the rotating protoplasm in the Characece is also very slow ; for, when it is viewed in the long internodes of Nitella with a very low power, or even with the naked eye, it seems hardly to move faster thaJ* the foot of a Gasteropod ; still there is no positive evidence that it moves round the cell after the manner of the latter, although it would appear to possess the power of move- ment per sc. Hence the question remains undecided, viz. whether it moves round the cell hy itself, or by the aid of cilia disposed on the inner surface of the protoplasmic sac, in like manner to those which appear to exist in the abdominal cavity of Vaginicola crystallina, and which have been seen in Closterium lunula." 2 The stems and arms of Chara are tubular, and entirely covered with smaller tubes, the circulation can mostly be observed in these, as shown at Fig. 170. Any ordinary cut- ting to obtain sections would squeeze the tube flat, and spoil it and the lining ; it is, therefore, better to avoid this, by laying the Chara on smooth wood, just covered with water; then, with a sharp knife, make suddenly a num- ber of quick cuts across it, and w> obtain the various sections required. Wet a slip of glass, and turn the wood over so as just to touch the water, and the sections will fall from the wood on to the glass, ready for the microscope. "The Chara tribe is most abundant in still waters or ponds that never become quite dry ; if found in running water, it is mostly met with out of the current, in holes or side bays, where the stream has little effect, and never on any prominence exposed to the current. If the CJiara could bear a current, its fruit would mostly be carried on and be deposited in whorls ; but it sends out from its variouo joints very long roots into the water, and these would "by agitation be destroyed, and then the plant (1) Ann. and Mag. Nat. Hist. vol. xvii. 1856. (2) In Plate I. No. 27, is represented the Moss gonida, assuming the amoeboid loru as depcribed by Dr. HicKs iu the Linnean Tram. 1882. CHARA. 321 decays; for although it may grow long before roots ara formed, yet when they are produced their destruction involves the death of the plant. In order, therefore, to preserve Chara, every care must be taken to imitate the stillness of the water by never shaking or suddenly turning the vessel. It is also important that the Chara should be disturbed as little as possible ; and if requisite, it must be done in the most gentle manner, as, for instance, in cutting off a specimen, or causing it to descend in order to keep the summit of the plant below the surface of the water. Similar care is requisite forVallisneria; but the warmest and most equal temperature is better suited to this plant. It should be planted in the middle of the jar in about two inches deep of mould, which has been closely presse^ ; over this place two or three handfuls of leaves, then gently fill the jar with water. When the water requires to be changed, a small portion is sufficient to change at a time. It appears to thrive in proportion to the frequency of the changing of the water, taking care that the water added rather increases the temperature than lowers it. The natural habitat of the Frog-bit, another water- plant of great interest, is on the surface of ponds and ditches ; in the autumn its seeds fall, and become buried in the mud at the bottom during the winter; in the spring these plants rise to the surface, produce flowers, and grow to their full size during summer. Chara may be found in many places around London, the Isle of Dogs, and in ditches near the Thames bank. Anacharis alsinaslrum. This remarkable plant is so unlike any other water-plant, that it may be at once recognised by its leaves growing in threes round a slender stringy stem. The watermen on the river have already named it " Water-thyme," from a faint general resemblance which it bears to that plant. In 1851 the Anacharis was noticed by Mr. Marshall and others in the river at Ely, but not in great quantities. Next year it had increased so much, that the river might be said to be full of it. The colour of the plant is deep green ; the leaves are y 322 THE MICROSCOPE. nearly half an inch long, by an eighth wide, egg-shaped at the point, and beset with minute teeth, which cause them to cling. The stems are very brittle, so that whenever the plant is disturbed, fragments are broken off. Its powers of increase are prodigious, as every fragment is capable of becoming an independent plant, producing roots .and stems, and extending itself indefinitely in every direction. Most of our water-plants require, in order to their increase, to be rooted in the bottom or sides of the river or drain i which they are found ; but this is independent altogether of that condition, and actually grows as it travels slowly down the stream after being cut. The specific gravity of it is so nearly that of water, that it is more disposed to sink than float. A small branch of the plant is represented, with a Hydra attached to it, in a subsequent chapter. Mr. Lawson pointed out the particular cells in which the current or circulation will be most readily seen viz. the elongated cells around the margin of the leaf and those of the midrib. On examining the leaf with polarised light, these cells, and these alone, are found to contain a large proportion of silica, and present a very interesting appearance. A bright band of light encircles the leaf, and traverses its centre. In fact, the leaf is set, as it were, in a framework of silica. By boiling the leaf for a short time in equal parts of nitric acid and water, a portion of the vegetable tissue is destroyed, and the silica rendered more distinct, without changing the form of the leaf. 1 It is necessary to make a thin section or strip from the leaf of VaUisneria, for the purpose of exhibiting the circu- lation in the cells, as shown in fig. 170, No. 4. Among the cells granules, a few of a more transparent character than the rest, may be seen, having a nucleolus within. The phenomenon of cell rotation is seen in other plants besides those growing in water. The leaf of the common plantain or dock, Plantago, furnishes a good example ; the movement being seen both in the cells of the plant, and hairs of the cuticle torn from the midribs. The Spider- wort will be noticed further on. (1) See also a paper in Vol. IV. Microscopical Journal of Science, on the Ck culation in the Leaf of Anacharis, by Mr F H. WeUiaro STRUCTURE OP PLANTS. 323 Structure of Plants. The development of cells in plants takes place in all cases in essentially the same "way, but the form of the result is subject to a number of im- portant modifications. Most elementary works on Botany enter so fully into this interesting question, that it is quite unnecessary to do more than refer very briefly to the more important structural peculiarities of plants. Cell- division, as we have seen, is the universal formative pro- cess by which vegetative growth is effected, and free-cell- formation occurs only in the production of cells connected with reproduction. In the lower classes of plants, espe- cially in aquatic genera, we can observe the process of cell- division in all its details ; but in the higher, knowledge of this kind is only accessible by dissection. As long as the cell retains its primordial utricle, it is capable of producing new cells, and organized forms of assimilated matter, like starch, chlorophyll, &c. in its con- tents. This is the case in all nascent tissues, but it ceases to be so at various periods in different parts of the vege- table organization. In all woody tissues, in all pitted and spiral-fibrous cells it disappears early ; the secondary de- posits of the ligneous character being formed apparently from the watery cell-sap. In herbaceous organs, such, as leaves, in the cells of the cellular plants generally, the primordial utricle remains. " This explains why the power to form adventitious buds exists not only in the cambrium layer of the higher plants, but, under certain conditions, even in the leaves, and why germination or propagation by little cellular bulbels, or isolated cells detached from the vegetative organs, is so common among the cellular plants, and in the Mosses and Liverworts, where parenchymatous tissues so greatly predominate." (Renfrey.) The tissues of plants, properly so called, copsist of col- lections of cells of uniform character, permanently combined together by more or less complete union of their outer sur- faces. Tissues are of many kinds, according to the form of the cells, the character of the cell-membrane, and the manner in which the cells are connected together. Th* milk- vessels found in connexion with certain cells appear to be formed out of the intercellular passages, and not by Y2 824 THE MICROSCOPE. fusion of celis ; hence they do not constitute a true cellulai tissue. Vascular tissue is formed by the fusion of perpendicular TOWS of cells ; by the absorption of their contiguous walla they become converted into continuous tubes of more or less considerable length. Then we have a combination of tissues destined for particular purposes in the economy of the plant, divided into three primary systems the Cellular, the Fibro-vascular, and Cortical. The 1st, Cellular, forms the great mass of the living structure of plants ; and it is in this system that the vital processes of vegetation are chiefly carried on. The 2d, Eibro-vascular, forms all the woody structures, which in all cases are composed of a quantity of conjoined portions of cellular and vascular tissue arranged in a peculiar manner ; differing in their modes of growth in different classes of plants, and which in consequence present considerable differences in the structure of their mature stems. The 3d, Cortical, also termed the epidermal, exists in the form of a simple flat layer of cells united firmly together by their sides, and forming a continuous coat over the surface of a plant. Such a layer clothes all the organs of plants above the Mosses, and, as stems grow older, the epidermal layer gives place to the bark or rind. Stomata are orifices between the meeting angles of the epidermal cells ; most abundant usually on the lower surface of leaves, often wanting on the upper surface. On the leaves of aquatic plants they are only found on the upper surface, and are absent where the leaf touches the water. Hairs and scales of all kind depend on the development of the epidermal cells. Simple hairs are merely single epidermal cells produced in a tubular filament, and when cell-multiplication occurs in them they present a number of joints ; see hairs of nettle, fig. 188, No. 2. Thorns, such as those of the rose, are aborted branches, in which the cells become thickened by woody secondary deposits. In leathery or hard leaves, and in the thick tough leaves of succulent plants, such as the aloes, the secondary layers acquire great thickness. The aerial roots of the Orchi- daceaa exhibit a curious structure, the growing extremities being clothed by a whitish cellular tissue composed o^ several FORMATION OF WOODY FIBRE. 325 layers of cells with a delicate spiral fibrous deposit on their walls. This layer forms a kind of coat over the real epidermis of the root, and is known by the name of the Vela-men radicum. The young shoots of Dicotyledonous trees and shrubs are clothed with epidermis-like herbaceous plants ; but, before the close of the past season of growth, in most cases the green colour gives place to brown, which is owing to the formation of a layer of cork from the outer layers of cortical parenchyma. Cork is composed of tubular thin-walled cells containing only air ; and some- times these intercellular passages occupy a considerable space, and communicate in all directions, forming a system of air-spaces in the tissue. In addition there is the secre- tory system, consisting of glands, simple and compound, milk-vessels, and canals filled with resins, oils, &c. Much of the physiological history of plant life has yet to be made out : the mode in which the circulation and the formation of wood are carried on is by no means a settled question. Mr. Herbert Spencer observes i 1 "That the supposition that certain vessels and strings of partially united cells, lined with spiral, annular, reticulated, or other frameworks, are carriers of the plant juices, is objected to on the ground that they often contain air ; as the pressure of air arrests the movements of blood through arteries and veins, its presence on the ducts of stems and patioles is assumed to unfit them as channels for sap. On the other hand, that these structures have a respiratory office, as some have thought, is certainly not more tenable, since the presence of air in them negatives the belief that their function is to distribute liquid. The presence of liquid in them equally nega- tives the belief that their function is to distribute air. !Nor can any better defence be made for the hypothesis which I find propounded, that these parts serve 'to give strength to the parenchyma.' In the absence of any feasible alternative, the hypothesis that these vessels are distributors of sap claims reconsideration." To obtain data for an opinion on this vexed question, Mr. Spencer insti- tuted a series of experiments on the absorption of dyes by plants. His first experiments were failures, and it was only (]) LinneanSoc. Trans, vol. xxv. pape 405, 326 THE MICROSCOPE. after trying experiments with leaves of different ages and different characters, and with undeveloped axes, as well as with axes of special kinds, that it became manifest that the appearances presented by ordinary stems, when thus tested, are in a great degree misleading. " If an adult shoot of a tree or shrub be cut off, and have its lower end placed in an alumed decoction of logwood, or a dilute solution of magenta, 1 the dye will, in the course of a few hours, ascend to a distance, varying according to the rate of eva- poration from the leaves. On making longitudinal sections of the part traversed by it, the dye is found to have penetrated extensive tracts of the woody tissue ; and, on making transverse sections, the openings of the ducts appear as empty spaces in the midst of a deeply-coloured prosenchyma. It would thus seem that the liquid is carried up the denser parts of the vascular bundles, neglecting the cambium layer, the central pith, aud the spiral vessels of the medullary sheath." This, however, is found to be only partially true. " There are indications that while the layer of pitted cells next the cambium has served as a channel for part of the liquid, the rest has ascended the pitted ducts, and oozed out of these into the prosenchyma around. This is seen, if, instead of allowing the dye time for oozing through the prosenchyma, the end of the shoot be just dipped into the dye and taken out again, we find that, although it has become diffused to some distance round the ducts, it has left tracts of wood between the ducts mentioned. Again, if we use one dye after another, a shoot that has absorbed magenta for an hour and then placed for five minutes in the logwood decoction, transverse sections taken at a short distance from the end of the shoot, show the mouths of the ducts surrounded by dark stains in the midst of the much wider red stains. The behaviour of these corresponds perfectly with the expectation that a liquid will ascend capillary tubes in preference to simple cellular tissue, or tissue not differentiated into continuous (1) "These two dyes have affinities for different components of the tissues, and may be advantageously used in different cases. Magenta is rapidly taken up by woody Tnatter and secondary deposits, while logwood colours the cell-membranes, and takes but reluctantly to the substances seized by magenta. By trying both of them on the same structure, we may guard ourselves against any error arising froaj relative combination." STRUCTURE OP PLANTS. 327 canals. Experiments with leaves bring out parallel facts ; and this, then, is confirmed, that in ordinary stems the staining of the wood by an. ascending coloured liquid is due, not to the passage of the coloured liquid up the sub- Fig. 172. Termination of Vascular System (after Spencer). 1. Absorbent organ from the leaf of Euphorbia neriifolia. The cluster of fibrous cells forming one of the terminations of the vascular system is here embedded in a solid parenchyma. 2. A structure of analogous kind from the leaf of Ficus elastica. Here the expanded terminations of the vessels are embedded in the network paren- chyma, the cells of which unite to form envelopes for them. 3. End view of an absorbent organ from the root of a turnip. It is taken from the outermost layer of vessels. Its funnel-shaped interior is drawn as it presents itself when looked at from the outside of this layer, its narrow end being directed towards the centre of the turnip. 4. Shows on a larger scale one of these absorbents from the leaf of Panax Lessonii. In this figure is clearly seen the way in which the cells of the net- work parenchyma unite into a closely-fitting case for the spiral cells. 5. A less-developed absorbent, showing its approximate connexion with a duct. In their simplest forms, these structures consist of only two fenestratod cells, with their ends bent round so as to meet. Such types occur in the cen- 328 THE MICROSCOPE. tral mass of the turnip, where the vascular system is relatively imperfect Besides the comparatively regular forms of these absorbents, there are forma composed of amorphous masses of fenestrated cells. It should be added that both the regular and irregular kinds are very variable in their numbers : in some turnips they are abundant, and in others scarcely to be found. Possibly their presence depends on the age of the turnip. 6. Represents a much more massive absorbent from the same leaf, the sur- rounding tissues being omitted, 7. Similarly represents, without its sheath, an absorbent from the leaf of Clusia fiava. 8. A longitudinal section through the axis of another such organ, showing its annuli of reticulated cells when cut through. The cellular tissue which fill* the interior is supposed to be removed. stance of the wood, but to the permeability of its duct& and such of its pitted cells as are united into regular canals ; and the facts showing this at the same time in- dicate with tolerable clearness the process by which wood is formed, for what in these cases is seen to take place with dye may be fairly presumed to take place with sap." Taking it, then, as a fact that the vessels and ducts are the channels through which the sap is distributed, the- varying permeability of their Avails, and consequent for- mation of wood, is due to the exposure of the plant to intermittent mechanical strains, actual or potential, or both, in this way. "If a trunk, a bough, shoot, or a petule is bent by a gust of wind, the substance of its convex side is subject to longitudinal tension, the substance of its concave side being at the same time compressed. This is the primary mechanical effect. The secondary is when the fesues of the convex side are stretched, they also produce lateral compression of them. In short, that " the formation of wood is due to intermittent transverse strains, such as are produced in the aerial parts of upright plants by the action of the wind." Thus the subject is most ingeniously worked out, and the results of many very interesting and instructive experiments are recorded by the author of the paper. " In the course of experiments on the absorption of dyes by leaves, it happened that, in making sections parallel to the plane of a leaf, with the view of separating its middle layer, containing the vessels, I came upon some structures that were new to me. These structures, where they are present, form the terminations of the vascular ey stem. They are masses of irregular and imperfectly united fibrous cells, such as those out cf vrhich vessels STRUCTURE OF PLANTS. 329 are developed ; and they are sometimes slender, sometimes bulky usually, however, being more or less club-shaped. In transverse sections of leaves, their distinctive characters are not shown ; they are taken for the smaller veins. It is only by carefully slicing away the surface of a leaf, until we come down to that part which contains them, that we get any idea of their nature. Fig. 172, ISTo. 1, repre- sents a specimen taken from a leaf of Euphorbia neriifolia. Occupying one of the interspaces of the ultimate venous network, it consists of a spirally-lined duct or set of ducts, which connects with the neighbouring vein a cluster of half-reticulated, half-scalariform cells. These cells have projections, many of them tapering, which insert themselves into the adjacent intercellular spaces, thus producing an extensive surface of contact between the organ and the embedding tissues. A further trait is, that the ensheathing prosenchyma is either but little developed or wholly absent ; and consequently this expanded vascular structure, especially at its end, comes immediately in con- tact with the tissues concerned in assimilation. The leaf of Euphorbia neriifolia is a very fleshy one ; and in it these organs are distributed through a compact, though watery, cellular mass. But in any leaf, of the ordinary type, which possesses them, they lie in the network of parenchyma, composing its lower layer ; and wherever they occur in this layer its cells unite to enclose them. This arrangement is shown in No. 2, representing a sample from the Caoutchouc-leaf as seen with the upper part of its envelope removed ; and it is shown still more clearly in a sample from the leaf of Panax Lessonii, No. 4. ]S~os. 6 and 7 represent, without their sheaths, other such organs from the leaves of Panax Lessonii and Clusia flava. Some relation seems to exist between their forms and the thicknesses of the layers in which they lie. Certain very thick leaves, such as those of Clusia flava, have them less abundantly distributed than is usual, but more massive. When the parenchyma is developed not to so great an extreme, though still largely, as in the leaves of Holly, Aucuba, Camellia , they are not so bulky ; and in thinner leaves, like those of Privet, Elder, &c. they become longer and less conspicuously club-shaped. 330 THE MICROSCOPE. " Some adaptations to their respective positions seem implied by these modifications ; and we may naturally expect that in many thin leaves these free ends, becoming still narrower, lose the distinctive and suggestive characters possessed by those shown in the drawings. Eelations of this kind are not regular, however. In various other genera, members of which I have examined, as JRhus, Viburnum, Griselinia, JBrexia, Botryodendron, Pereskia, the variations in the bulk and form of these structures are not directly determined by the spaces which the leaves allow; obviously there are other modifying causes. It should be added that while these expanded free extre- mities graduate into tapering free extremities, not differing from ordinary vessels, they also pass insensibly into the ordinary inosculations. Occasionally, along with numerous free endings, there occur loops ; and from such loops there are transitions to the ultimate meshes of the veins. " These organs are by no means common to all leaves. In many that afford ample spaces for them they are not to be found. So far as I have observed, they are absent from the thick leaves of plants which form very little wood. In Sempervivum, in Echeveria, in Bryophyllum they do not appear to exist ; and I have been unable to discern them in Kalanchoe rotundifolia, in Kleinia ante-eupliorbium, and ficoides, in the several species of Crassula, and in other suc- culent plants. It may be added that they are not absolutely confined to leaves, but occur in stems that have assumed the functions of leaves. At least I have found, in the green parenchyma of Opuntia, organs that are analogous, though much more rudely and irregularly formed. In other parts, too, that have usurped the leaf-function, they occur, as' in the phyllodes of the Australian acacias. These have them abundantly developed ; and it is interest- ing to observe that here, where the two vertically-placed surfaces of the flattened-out petiole are equally adapted to the assimilative function, there exist two layers of these expanded vascular terminations, one applied to the inner surface of each layer of parenchyma. " Considering the structures and positions of these organs, as well as the natures of the plants possessing them, may we not form a shrewd suspicion respecting their function] STRUCTURE OF PLANTS. 331 Is- it not probable that they facilitate absorption of the juices carried back from the leaf for the nutrition of the stem. and roots ? They are admirably adapted for performing this office. Their component fibrous cells, having angles insinuated between the cells of the parenchyma, are shaped , just as they should be for taking up its contents, and the absence of sheathing tissue between them and the paren- chyma facilitates the passage of the elaborated liquids. Moreover, there is the fact that they are allied to organs which obviously have absorbent functions. I am indebted to Dr. Hooker for pointing out the figures of two such organs in the Icones Anatomicce of Link. One of them is from the end of a dicotyledonous root-fibre, and the other is from the prothallus of a young fern. In each case a cluster of fibrous cells, seated at a place from which liquid has to be drawn, is connected by vessels with the parts to which liquid has been carried. I have met with another such organ, more elaborately constructed, evidently adapted to the same office, in the common turnip-root. As shown by the end view and longitudinal section in Nos. 3, 5, and 8, this organ consists of rings of fenestrated cells, arranged with varying degrees of regularity into a funnel, ordinarily having its apex directed towards the central mass of the turnip, with which it has, in some cases at least, a traceable connexion by a canal. Presenting as it does an external porous surface terminating one of the branches of the vas- cular system, each of these organs is well fitted for taking up with rapidity the nutriment laid by in the turnip-root, and used by the plant when it sends up its flower-stalk. The cotyledons of young beans furnish other examples of such structures, exactly in the places where, if they be absorbents, we may expect to find them. Amid the branchings and inosculations of the vascular layer running through the mass of nutriment deposited in each coty- ledon, there are found conspicuous free terminations that are club-shaped^ and which prove to be composed, like those in leaves, of irregularly-formed and clustered fibrous cells, some of them diverging from the plane of the vascular layer, dipping down into the mass of starch and albumen which the young plant has to utilize, and which these structures can have no other function but to take up." 332 THE MICROSCOPE. To return to cell- development ; we found the cell chang- ing in its outward form, the transparent membranous cell wall becoming thickened, and spontaneous fissure taking place; and thus is formed a series of connected cells variously modified and arranged, according to the conditions under which they are developed and the functions which they are destined to exercise. The typical form, as we have Fig 173. o, elementary cells ; 6, branched cellular tissue. before observed, of the vegetable cell is spheroidal ; but when developed under pressure within walls, or denser tissues, it takes other shapes ; as the oblong, lobed, square, prismatical, cylindrical, fusi- form, muriform, stellate, fila- mentous, &c. : and is then termed Parenchyma, and the cells woven together are called cellular tissue. In pulpy fruits the cells may be easily separated one from the other : a thin trans- verse section of a strawberry is represented at fig. 188, No. 15 : within the cells are smaller cells, commonly known as the pulp. Fig. 173, a, is the ele- mentary form of oval cells or I, A transverse section of stom^of vesic l eS) passing on to the for- " mation of branched cellular tissue, b. Remarkable speci- mens of the filamentous tissue may be seen in fig. 188, No. 19, the circular elongated cells from the Mushroom; only another and more closely connected giowth of inuce- dinous fungi, commonly called mushroom spawn. Equisetum, showing nal shape of cells. 2, A vertical section of elongated cell. CELLULAR TISSUES. 333 Fig. 176. Stellate tisui,from ttem of a Rush. Fig. 175, in the stellate tissue cut from the stem of a rush, we have the forma- tive network dividing into ducts for the purpose of giving strength and light- ness to the stem of the plant. These ducts may undergo other transformations; the cell itself become gra- dually changed into a spiral continuous tube or duct, as seen in fig. 198 ; these are sometimes formed by the breaking down of the partitions ; in the centre of which we may have a com- pound spiral duct, resembling portions of trachese from the silkworm. Another important change occurs in the original cell, it is that of its conversion into woody fibre. Common woody fibre (Pleurenchyma) has its sides free from de- finite markings. In the coniferous plants, the tubes are furnished with circular discs ; these discs are thought to be contrivances to enable the tubules of the WOOdy tissues tO dis- Fig. 176. A section of stem of Clematis, i j> . r> vnlh pores, highly magnified, to shot* Charge their Contents from the line which passes round Mem. one to the other, or into the Cellular spaces. Plants having aromatic secretions are furnished with glands ; these form a series of interesting objects, and such as the sage-leaf should be mounted as opaque specimens. A large central gland is seen in a section of a leaf from Ficus elastica, India-rubber-tree, fig. 177, No. 2. Professor Quekett observes, " The nature of the pores, or discs, in conifers, has long been a subject for controversy ; it is now certain that the bordered pores are not peculiar to one fibre, but are formed between two contiguous to each other, and always exist in greatest numbers on those sides of the woody fibres parallel to the medullary rays. They are hollow ; their shape biconvex ; and in their centre ia 334 THE MICROSCOPE. a small circular or oval spot, fig. 176 : the latter may occur singly, or be crossed by another at right angles, Fig. ITT. 1, Vertical section of root of Alder, with outer wall. 2, A vertical section of a leaf of the India-rubber tree, exhibiting a central gland. which gives the appearance of a cross, as in fig. 204, Nos. 3, 4, a vertical section of fossil wood, remarkable for having three or four rows of woody tissue occupied by large pores without central markings." We now pass to the milk, lacticiferous ducts or tissue, the proper vessels of the old writers. These ducts con- vey a peculiar fluid, some- times called latex, usually turbid, and coloured red, white, or yellow ; often, however, colourless. It is supposed they carry latex to all the newly-formed organs, which are nourished by it. The fluid becomes darker after being mounted for specimens to be viewed under the microscope. This tissue is remarkable from its resemblance to the earliest aggregation of cells, the yeast- plant, and therefore has some claim to being considered the stage of development preceding that of the reticu- Fig. 178. Lact'iciferous tissue, CELLULAR TISSUES. 335 lated ducts seen in fig. 178. In a section from tho India-rubber-tree, fig. 177, No. 2, a network of these lac- tiferous tubes will be found filled with a brownish or Fig.l7. 1, A portion of the leaf of Sphagnum, showing ducts, vascular tissue, and spira fibre in the interior of its cells. 2, Porous cells, from the testa of GOUK seed, communicating with each other, and resembling ducts. granular matter ; that in fig. 178 is an enlarged view of this tissue from the wood of an exogen, taken near the root. TBittitiutaku'iriiTinwiWLuVi 1, Reticulated ducts. Fig. 180. 2, A vertical section of Fern-root. In many plants external to the cuticle, there exists a very delicate transparent pellicle, without any decided traces of organisation, though occasionally somewhat gra- nular in appearance, and marked by lines that seem to be impressions of the junction of the cells in contact with each other. In nearly all plants, the cuticle is perforated 336 THE MICROSCOPE. by minute openings termed Stomata, which are bordered by cells of a peculiar form, distinct from those of the cuticle. In Iris germanica, fig. 181, each surface has nearly 12,000 fl*tt 1. Fig. 181. 2. 1, Portion of a vertical section of the Leaf of the Iris: a, a, elongated cells of the epidermii ; b, stomata cut through longitudinally; c, c, cells of the parenchyma; d, d, colourless tissue of the interior of the leaf. 2, Portion of leaf of Iris germanica, torn from its surface; a, elongated cells of the cuticle; b, cells of the stomata; c, cells of the parenchyma: d, impressions on the epi- dermic cells ; e, lacunae in the parenchyma, stomata in every square inch ; and in Yucca each surface has about 40,000. The structure of the leaf of the common Iris shows a central portion, formed by thick-walled colourless tissue, very different from ordinary leaf-cells or from woody fibre. rtg. 182. A portion of the epidermii of the Sugar-cane, showing the two kinds a/ cells of tnhitk ii is composed, ( Magnified 200 diameters.) CELLULAR TISSUE. 337 Variously-cut sections of leaves should be made, and slices taken parallel to the surfaces at different distances, for the purpose of microscopic examination. Among the cell-contents of some plants, are beautiful crystals called Raphides : the term is derived from pa^i? a needle, from the resemblance of the crystal to a needle. They are composed of the phosphate and oxalate of lime ; there is a difference of opinion as to their use in the economy of the plant. Mr. Gulliver has insisted upon the value of Raphides as characteristic points in many families of plants. He observes that doubts as to the value of raphides as natural characters and as to their importance in the vege- table economy at all will be entertained by those who do not clearly distinguish between raphides and sphaeraphides. Schleiden asserts that the "needle-formed crystals, in bundles of from twenty to thirty in a cell, are present in almost all plants," and " that inorganic crystals are rarely met with in cells in a full state of vitality." He further states that so really practical is the presence or absence of raphides, that by noticing them he has been able to pick out pots of seedling OnagraceaB, which had been accidentally mixed with pots of other seedlings of the same age, and at that period of growth when no botanical character before in use would have been so readily sufficient for the diagnosis. If we examine a portion of the layers of an onion, fig. 183, No. 1, or a thin section of the stem or root of the garden rhubarb, fig. 183, No. 4, we shall find many cells. in which, either bundles of needle-shaped crystals, or masses of a stellate form occur, not strictly raphides. Raphides were first noticed by Malpighi in Opuntia. and subsequently described by Juriue and Raspail. According to the latter observer, the needle-shape or acicular are composed of phosphate, and the stellate of oxalate of lime. There are others having lime as a basis, in combination with tartaric, malic, or citric acid. These are easily destroyed by acetic acid, and are also very soluble in many of the fluids employed in the conservation of ob- jects; some of them are as large as the l-40th of an inch, others are as small as the 1 -1000th. They occur in all 2 338 THE MICROSCOPE. parts of the plant ; in the stem, bark, leaves, stipules, petals, fruit, root, and even in the pollen, with some exceptions. They are always situated in the interior of cells, and not, as stated by Raspail and others, in the Fig. 183. I, A section from the outer layer of the bulb of an Onion, showing crystals of carbonate of lime. 3, Cells of the Pear, showing Sclerogen, or gritty tissue. 4, Cells of garden Rhubarb, filled with raphides. 5, Cells from same, filled with starch-grains. intercellular passages. 1 Some of the containing cells be- come much elongated ; but still the cell- wall can be readily traced. In some species of Aloe, as, for instance, Aloe verrucosa, with the naked eye we are able to discern small silky filaments: when magnified, they are found to be bundles of the acicular form of raphides, which no doubt act the part of a stay or prop to the internal soft pulp. In portions of the cuticle of the medicinal squill SciUa maritima several large cells may be observed, full of bundles of needle-shaped crystal. These cells, however, do not lie in the same plane as the smaller ones belonging to the cuticle. In the cuticle of an onion every cell is oc- cupied either by an octahedral or a prismatic crystal of oxalate of lime : in some specimens the octahedral form predominates ; but in others from the same plant the (I) "As an exception, many years ago they were discovered in the interior of the spiral vessels in the stem of the grape-vine ; but with some botanists this vould not he considered as an exceptional case, the vessels being regarded as elongated cells." Quekett. CEYSTALS IN PLANTS. 339 yabra. crystals will be principally prismatic, and are arranged aa if they were beginning to assume a stellate form. Some plants, as many of the cactus tribe, are made up almost entirely of raphides. In some instances every cell of the cuticle contains a stel- late mass of crystals ; in others the whole interior is full of them, rendering the plant so exceedingly brittle, that the least touch will occasion a fracture; so much so, that some specimens of Cactus senilis, said to be a thousand years old, which were sent a few years since to Kew from South America, were obliged to be packed -^.i n .1 Fig. 184 Siliceous cuticle from in COtton, With all the Care surface of leaf of Deutzia scab of the most delicate jewel- lery, to preserve them during transport. Raphides, of peculiar figure, are common in the bark of many trees. In the Hiccory (Carya alba) may be ob- served masses of flattened prisms having both extre- mities pointed. In vertical sections from the stem of Elceagnus angustifolia, nu- merous raphides of large size are embedded in the pith. Raphides are also found in the bark of the apple-tree, and in the testa of the seeds of the elm ; every cell con- tains two or more very minute crystals. In figs. 184 and 185 we have other representations of the crystalline structure z 2 . 185. Siliceous cuticl", of Qrtut (Pharus cristatut), 340 THE MICROSCOPE. of plants, in sections taken from grass, and the leaf of Deutzia scabra. This insoluble material is called silica, and is abundantly distributed throughout certain orders of plants, leaving a skeleton after the soft vegetable matters have been destroyed : masses of it, having the appearance of irregularly-formed blackened glass, will always be found after the burning of hay or straw ; which is caused by the fusion of the silica contained in the cuticle combining with the potash in the vegetable tissue, thus forming a silicate of potash (glass). To display this siliceous structure, it is necessary to cut very thin slices from the cuticle, and mount them in fluid or Canada balsam. In the Oraminacece, especially the canes ; in the Equi- setum hyemale, or Dutch rush ; in the husk of the rice, wheat, and other grains, silica is abundantly found. In the Pharus cristatus, an exotic grass, fig. 185, we have beautifully-ar- ranged masses of silica with raphides. The leaves of Deutzia, fig. 184 are re- markable for their stel- late hairs developed from the cuticle, of both their upper and under surfaces ; forming most interesting arid attractive objects when examined under the micro- scope with polarised light. See Plate VIII. No. 173. Silica is found in all Ru- liacece; both in the stem ft,nd leaves, and if present in sufficient thickness, depolarises light. This is especially the case in the prickles, which all these plants have on the margin of the leaves and the angles of the stem. One of the order Compositce, a plant popularly known as the "sneezewort," (Archillce ptarmica) has a large amount of silica in the hairs found on the double serratures of its leaves ; commonly said to be tho Fig. 186. -Portion of the husk of Wheat, showing siliceous crystals. CELL-CONTENTS . STARCH. 341 cause of its errhine properties when powdered and used as snuff. It is in the underlying or true epidermis, that the silica occurs. This membrane is permeable by fluids, not by means of pores, but by endosmotic force. The most generally -distributed and conspicuous of the ^ell-contents is Starch; at the same time it is one of great value and interest, per- forming a similar office in the economy of plants as that of fat in animals. It occurs in all plants at some period of their existence, and is the chief and great mark of dis- tinction between the vege- table and animal kingdoms. Its presence is detected by testing with a solution of. iodine, which changes it to a characteristic blue or violet colour. 1 Being insoluble in COld Water, it Can be readily Fig- 187. Section of a Cnne; with ;ell- washed away and separated j$f e % wmgrTnu^mauJr!^ P T< from other matters contained in the cellular parts of full-grown plants. It is often found in small granular masses in the interior of cells, shown in fig. 183 from the garden-rhubarb. Starch-grains are variable in size: the tous-les-mois, fig. 188, No. 5, are very large ; in the potato, No. 14, they are smaller ; and in rice, No. 6, they are very small indeed. Nearly all pre- sent the appearance of concentric irregular circles; and most of the granules have a circular spot, termed the hilum, around which a large number of curved lines arrange them- selves : better seen under polarised light. Plate VIII. No. 1 67, Leeuwenhoek, to whom we are indebted for the earliest notice of starch-granules, enters with considerable minute- ness into a description of those of several plants such as wheat, barley, rye, oats, peas, beans, kidney-beans, buck- wheat, maize, and rice ; and very carefully describes experiments made by him in order to investigate the structure of starch-granules. Dr. Reissek regards tha (1) This is not a test for starch when combined with albuminous matters. 342 THE MICROSCOPE. granule as a perfect cell, from the phenomena presented during its decay or dissolution, when left for some time in water. Schleiden and others, after examining its expan- sion and alteration under the influence of heat and of sulphuric acid, considered it to be a solid homogeneous structure. Professor Busk agrees with M. Martin in believing the primary form of the starch-granule to be " a spherical or ovate vesicle, the appearance of which under the micro- scope, when submitted to the action of strong sulphuric- acid, conveys the idea of an unfolding of plaits or rugffi, which have, as it were, been tucked in towards the centre of the starch-grain." 1 The mode of applying the concen- trated sulphuric acid is thus described by Mr. Busk : " A small quantity of the starch to be examined is placed upon a slip of glass, and covered with five or six drops of water, in which it is well stirred about ; then with the point of a slender glass-rod the smallest possible quantity of solution of iodine is applied, which requires to be quickly and well mixed with the starch and water; as much of the latter as will must be allowed to drain on\ leaving the moistened starch behind, or a portion of it may be removed by an inclination of the glass, before it is covered with a piece of thin glass. The object must be placed on the field of the microscope, and the J-inch object-glass brought to a focus close to the upper edge of the thin glass. With a slender glass-rod a small drop of strong sulphuric acid must be carefully placed immediately upon, or rather above the edge of the cover, great care being necessary to prevent its running over. The acid quickly insinuates itself between the glasses, and its course may be traced by the rapid change in the appearance of the starch-granules as it comes in contact with them. The course of the acid is to be followed by moving the object gently upwards ; and when, from its diffusion, the re-agent begins to act slowly, the peculiar changes in the starch- granules can be more readily witnessed. In pressing or moving the glasses, the starch disc becomes torn, and is then distinctly seen, especially in those coloured blue, to (1) Professor G. Busk, F.R.S., on the Structure of the Starch-granule; Quar- terly Journal of Microtcoyical Science, April, 1853. CELL-CONTENTS STARCH. 343 consist of two layers, an upper and a lower one ; and the collapsed vesicular bodies of an extremely fine but strong and elastic membrane." Mr. Busk believes the hilum to be a central opening into the interior of the ovate vesicle. Fig. 188. 1, Nucleated Cells. 2, Stinging-nettle Hairs, Urtica Dioica. 3, Ciliated spores of Conferva;. 4, Starch grains, broken by the application of heat. 5, Starch from Toits-les-mois. 0. Starch from Rice. 7, Starch from Sago. 8, Imita- tion Sago-starch. 9, Wheat-starch. 10, Rhubarb-starch, in isolated cells. 11, Maize-starch. 12, Oat-starch. 13, Barley-starch. 14, Potato- starch. 15, Section of Strawberry, cells ovoid, containing granular matter. 16, Section of Potato, with starch destroyed by fungoid disease. 17, Potato, with nearly all starch-grains absent. 18, Section of Potato, cells filled with healthy starch. (These starches are grouped for comparison.) 19, Mushroom spawn, elongated cells. Nitric acid communicates to wheat-starch a fine orange- yellow colour; and recently-prepared tincture of guaiacum gives a blue colour to the starch of good wheat-flour. 344 THE MICROSCOPE. Pure wheat-flour is almost entirely dissolved in a strong solution of potash, containing twelve per cent, of the alkali ; but mineral substances used for the purpose of adultera- tion remain undissolved. Wheat-flour is frequently adulterated with various sub- stances ; and in the detection of these adulterations, the microscope, together with a slight knowledge of the action of chemical re-agents, lends important assistance. It enables us to judge of the size, shape, and markings on the starch grains, and thereby to distinguish the granules of Pig. 189. Wheat-Flour Starch-granules, with a small portion of Us cellulose. (Magnified 420 diameters.) one meal from that of another. In some cases the micro- scopic examination is aided by an application of a solu- tion of potash. Thus we may readily detect the mixture of wheat-flour with either potato-starch, meal of the pea or bean, by the addition of a little water to a small quantity of the flour, then, by adding a few drops of a solution of potash (made of the strength one part liquid potash to three parts of water), the granules of the potato- ADULTERATION OF WHEAT-FLOUR. 345 starch will immediately swell up, and acquire three 01 four times their natural size ; while those of the wheat-starch are scarcely affected by it ; if adulterated with pea or bean meal, the hexagonal tissue of the seed is at the same time rendered very obvious under the microscope. Polarised light will be of use as an additional aid ; wheat-starch presents a faint black cross proceeding from the central hilum, whereas the starch of the oat shows nothing of the kind. Fig. 190. Potato Starch-granules, sold under the name of British Arrow-root, used to adulterate flour and bread, (Magnified 240 diameters.) The diseases of wheat and corn are readily detected under the microscope ; some of which will be seen to be produced by a parasitic fungus, and by an animalcule re- presented in another place : all are more or less dangerous when mixed with articles of food. Adulteration of bread with boiled and mashed potatoes, next to that by alum, is, perhaps, the one which is most commonly resorted to. The great objection to the use of potatoes in bread, is, that they are made to take the place 346 THE MICROSCOPE. of an article very much more nutritious. This ad alteration can be instantly detected by means of the microscope. The cells which contain the starch-corpuscles are, in the potato, very large, fig. 190 ; in the raw potato they are adherent to each other, and form a reticulated structure, in the meshes of which the well-defined starch-granules are clearly seen ; in the boiled potato, however, the cells separate readily from each other, each forming a distinct article : the starch-corpuscles are less distinct and of an altered form. Fig. 181. Adulterated Cocoa, sold under the name of Homoeopathic Cocoa. (After Hassall.) a a a, granules and cells of cocoa; bbb, granules of Canna-starch, or Tous-lt*- mois; c, granules of Tapioca-starch. Adulteration with alum and "stuff." This adulteration is practised with a twofold object : first to render flour of a bad colour and inferior quality white and equal, in appearance only, to flour of superior quality ; and secondly to enable the flour to retain a larger proportion of water, ADULTERATION OF FOOD. 347 by which the loaf is made to weigh heavier. By dissolving out the alum in water and then re-crystallising it under the microscope, this adulteration is readily detected. Before leaving the subject of starch, allusion may be made to the prevalent and destructive epidemic amongst potatoes, which is a disease of the tuber, not of the haulm or leaves. " Examined in an early stage, such potatoes are found to be composed of cells of the usual size ; but they contain little or no starch : this will be seen upon reference to Nos. 16 and 17, fig. 188. Hence it may be Fig. 192. Structure and Character of genuine Ground Coffee. {After Hassall.) inferred, that the natural nutriment of the plant being deficient, the haulm dies, the cells of the tuber soon turn black and decompose; and fungi developed as in most other decaying vegetable substances. "This will undoubtedly explain the most prominent symptom of the potato-disease, the tendency to decom- position ; and is a point in which the microscope confirms the result of chemical experiment : for it has been found 348 THE MIQROSCOPE. that the diseased potatoes contain a larger proportion of water than those that are healthy. A want of organizing power is evidently the cause of this deficiency of starch ; but we fear the microscope will never tell us in what the want of this organising force consists." l The adulteration of articles of food and drink has long been a matter of uneasy interest, and of strong, though vague, misgiving. Accum's Death, in the Pot, between thirty and forty years ago, awoke attention to the subject; which has since been more or less accurately explored by Fig. 193. Sample of Coffee, adulterated with both Chicory and Roasted Wheat. (After Hassall.) < a a, small fragments of coffee ; b b b, portions of chicory ; ccc starcri-grai/uiea of wheat. Mitchell, Normandy, Chevalier, Jules Garnier, and Harel; and has at length derived a singularly lucid exposi- tion from Dr. Hassall's researches, whose report of these inquiries fills between 600 and 700 closely printed pages (I) Professor Quekett's Histology of Vegetables. We would refer the reader to a curious work on Fungi, by Arimini, an Italian botanist, 1759. ADULTERATION OP FOOD. 349 of a large octavo, replete with details of the fraudulent contaminations commonly practised by the people's pur- veyors, at the people's expense of health and pocket. 1 " In iiearly all articles," said Dr. Hassall, before a com- mittee appointed by the House of Commons to inquire into these adulterations, " whether food, drink, or drugs, my opinion is that adulteration prevails. And many of the substances employed in the adulterating process were not only injurious to health, but even poisonous." The microscope was the effective instrument in the work of Fig. 194. Tea adulterated with foreign leaves. (After Hassall.) a, upper surface of leaf ; b, lower surface, showing cells; c, chlorophyll cells; d, elongated cells found on the upper surface of the leaf in the course of the veins ; e, spiral vessel ; /, cell of turmeric ; g, fragment of Prussian blue ; /. particles of white powder, probably China clay. -detection. Less than five years ago, it would, we are told, have been impossible to detect the presence of chicory in coffee : in fact, the opinion of three distinguished chemists was actually quoted in the House of Commons to that effect ; (1) Food and Us Adulterations ; comprising the Reports of the Analytical Sani- tary Commission of the Lancet, for the years 1851 to 1854 inclusive. By Arthui dill Hassall, M.D. 360 THE MICROSCOPE. whereas by the use of the microscope the differences of structure in these two substances, as in many other cases, can be promptly discerned. Out of thirty-four samples of coffee purchased, chicory was discovered in thirty-one ; chicory itself being also adulterated with all manner of compounds. There is no falling back either upon tea or chocolate ; for these seem rather worse used' than coffee. Tea is adulterated, not only here, but still more in China ; while as to chocolate, the processes employed in corrupting the manufacture are described as " diabolical." " It is 2 Fig. 195. I, Radiating cells from the outer shell of the Ivory Nut. 2, Section of a Nut, * showing cells with small radiating pores. often mixed with brick-dust to the amount of ten per cent., ochre twelve per cent., and peroxide of iron twenty- two per cent*., and animal fats of the worst description. In this country, cocoa is sold under the names of flake, rock, granulated, soluble, dietetic, homoeopathic cocoa, r exposed surfaces. B B 370 THE MICROSCOPE. of tlio earthworm are developed from eggs, whence they emerge, not as Gregarinae, but as true nematodes. The transformation of two Gregarinae, after a process of encysta tion, into navicula-like bodies, has been fully described by Bruch ; but Leiberkiihn has more carefully illustrated the changes that go on, and has endeavoured to trace the ex- istence of the pseudo-naviculae after they have Leen expelled from the cyst. In the perivisceral cavity of the earthworm he found large numbers of small corpuscles, exhibiting amoeba-like movements, and likewise pseudo- navicula?, containing granules, formed from encysted Gre- garine. He imagines that these latter bodies burst, and that their contained granules develope into the amoeDiform bodies which subsequently become Gregarinae. M. Lei- berkiihn shortly afterwards published another paper, 'in which he adopts the same view, that the amoebiform corpuscles of the blood of fish are Gregarines. But few physiologists will feel disposed to agree with him, in considering these bodies as parasites. Mr. E. Eay Lankester has contributed a valuable and exhaustive paper on this subject ; x he observes : " I have made careful examination of more than a hundred worms, for the purpose of studying these questions, but have succeeded in arriving at no other conclusion than that certain forms of these may be the products of encysted Gregarinse. The G. Lumbrid is one of those forms which are unilocular, and are met with most frequently among Annelids. It consists of a transparent contractile sac (which has not hitherto been demonstrated to be formed by more than a single membrane), enclosing the charac- teristic granules and vesicle. The vesicle is not always very distinct, and is sometimes altogether absent ; occasionally it contains no granules, sometimes several, one of which is generally nucleated. In some of these cysts a number of nucleated cells may be seen, developing together from the enclosed Gregarina, which gradually become fused together and broken up, until the entire mass is converted into these nucleated bodies, which are then evident in different stages of development, assuming the form of a (1) E. Ray Lankester, " On the Gregarinidae found in the common Earth- worm," Micros. Trans. voL iii. p. 83. PROTOZOA. GREGARINJE. 37 1 double cone, like that presented by some species of Diatomaceae, whence their name pseudo-naviculae. At length the cyst contains nothing but pseud o-naviculae, sometimes enclosing granules, which gradually disappear, and finally the cyst bursts. Encystation seems to take place much more rarely among the bilocular forms of Gregarinae than in the unilocular species found in the earthworm and other Annelids." In the Gregarince the food is taken in indiscriminately at every point of the surface of the body by imbibition. The food most likely is in the fluid state. In Spongilla, also, this is probably the case. But it is generally agreed that in Amoeba, Actinophrys, and agastric Infusoria, only solid alimentary particles are taken as food. The simplest animal is indeed far more complex than is implied in the word unicellular, and it can be clearly proved that there are few points in common between a simple cell and a so- called unicellular protozoon. The system of contractile vesicles and dependent sinuses, so general in the least organized protozoon, is unknown in the history of cells. Fluid absoption by the surface is the normal method of feeding in these low types of animal life. This absorptive faculty is an inherent property of the substance of which they are composed. It attracts certain aliments, as gela- tine attracts water. Tissue, distinguished by the same character, prevails throughout the entire class of the Pro- tozoa. Although the Gregarince mostly inhabit the intes- tines of invertebrate animals, they are often found in the alimentary canal of the Vertebrata. In this class they appear to be represented, however, by very closely allied organisms, the Psorospermice. Muller gave this last- mentioned name to some very singular minute bodies he discovered within sacs upon the skin and gills, and in the internal organs, of many fishes. These animals are gene- rally of a cylindrical or somewhat elliptical form, although sometimes a sort of head appears to be produced by the constriction of the anterior extremity of the body, and this head-like portion is occasionally furnished with a curious 'soft process and lobes. They are very sluggish in their movements, although a few possess true cilia. Their curious mode of development, with other points in the BBS 372 THE MICROSCOPE. history of these minute parasites, are well worthy of investigation. The Rhizopoda appear as creatures of a low type of organization, and are considered, with the former, to hold a medium state between animals and vegetables. Almost all of them live in water ; it would be a fruitless search to look for distinct internal organs, as the small bladder- looking spaces enclosed within their substance, believeo by Ehrenberg to be stomachs, present only the appear- ance of transparent gelatinous cells, or rather moving spaces, within the sarcode envelope, and may be regarded as the earliest dawn of a circulatory system. The term Rhizopoda is derived from the Greek, and Fig. 205. Simple Rhizopods. A, Difflugia proteiformis. B, Difflugia oUonga. c, D, Arcella acuminata and dentata. means "root-footed," the body is composed entirely of gelatinous matter, sarcode, motion being effected by the extension of portions of the substance into processes, which, as in fig. 205, is seen to partake of various forms. Lobosa. In the deposit formed at the bottom of fresh- water ponds, we may often meet with a singular minute gelatinous body, which constantly changes its form even under our eyes ; and moves about by means of finger-liko processes, called pseudopodia, which it appears to have tho power of shooting out from any part of its substance. PROTOZOA. AMCEB A. 373 This shapeless mass is well known to microscopic observers under the name of the Proteus (Amoeba diffluens, fig. 206), which, from the continual changes of shape it presents, is Pig. 206. Amceba diffluent, or Proteus, in different forms. honoured with the name of a fabled god, who could be either animal, vegetable, or mineral in his nature. This curious animal presents us with the essential characters of the large class Rhizopoda in their simplest form. It ap- pears to be of an exceedingly voracious disposition, seizing upon any minute aquatic animals or plants that may come in its way, and appropriating them to the nutrition of its own gelatinous body. The mode in which this tender and apparently helpless creature effects this object is very remarkable. The gelatinous matter of which it is com- posed is capable, as we have seen, of extension in every direction ; accordingly, when the A moeba meets with any- thing that it regards as suitable for its support, the sub- stance of the creature, as it were, grows round the object until it is completely enclosed within its body. The sub- stances swallowed (if such a term be admissible) by this hungry mass of jelly are often so large, that the creature itself only seems to form a sort of gelatinous coat enclosing its prey. Professor Ecker believes in an exact similarity of con- tractile substance between that of the lower animal forms, such as the Rhizopoda, and that observed in the Hydra. He says : " The properties of this substance, in its simplest form, are seen in the Amoeba, the body of which, as is known, consists of a perfectly transparent albumen-like homogeneous substance, in which nothing but a few gran- lies are imbedded, and which presents no trace of 374 THE MICROSCOPE. further organization. This substance is in the highest degree extensible and contractile ; and from the main mass are given out, now in one part and now in another, per- fectly transparent rounded processes, which glide over the glass like oil, and are then again merged in a central mass. There is no external membrane. In the body of the Amoeba there occur, besides the granules, clear spaces with fluid contents, which are sometimes unchangeable in form, and sometimes exhibit rhythmical contractions." Belonging to the family is the very curious Acineta of Ehrenberg, Actinophrys sol, " sun-animalcule." This creature consists of a jelly-like contractile substance, or sarcode, with tentacular filaments radiating from the central mass, in such a manner as to have suggested the name for the species. It abounds in pools where Desmi- diacece are found in many parts around London ; they are ravenous feeders, not only upon the Desmidiacece, but also upon all kinds of minute spores and animalcules. (Plate III. fig. 66.) It was on examining some beautiful Desmidiacece that my attention was arrested by the curious appearance of two or three very small Actinophrys floating very lightly upon the surface of the water, in the form of a ball, with their delicate tentacular filaments perfectly erect all over their bodies ; in fact, they seemed to be floating upon these delicate filaments. 1 The most beautiful forms of the Rhizopoda are found among those possessing a calcareous covering, as the Polythalamia, Rosalina, Faujasina, &c. ; their systematic arrangement is founded upon their shells, which exhibit a very great diversity in form. Out of these forms, it would appear that the labours of various naturalists in the last hundred years have made us acquainted with nearly 2,000 recent and fossil Foraminifera ; and although the observations of Dr. Carpenter 2 tend to show the pro- bability that very many of these supposed species are merely varieties, still the number is sufficiently great fco prove the importance and interesting nature of the inquiry. (1) Western, Journ. Micros. Science, vol. iv. New series, p. 110; Clapa- rede, Ann. Nat. I/is. Second series, vol. xv. p. 211. (2) Carpenter's "Introduction to the Study of the Foraminifera," published by theRiySoc. 1862. FORAMINIFERA 375 Dr. Schultze acknowledges the difficulties attending the study of the Rhizopoda, and insists, very properly, upon the necessity of viewing them in all positions, and under different modes of illumination and of preparation, in order to arrive at a due conception of their astonishing conformation. When the shells of Foraminifera are dis- solved in dilute acid, an organic basis is always left after the removal of the calcareous matter, accurately retaining the form of the shell with all its openings and pores. The earthy constituent is mainly carbonate of lime ; but Dr. Schultze has satisfied himself of the presence of a minute amount of phosphate of lime in the shells of recent Orbiculina adunca from the Antilles and of Poly- stomella strigilata from the Adriatic. Fig. 20T. I, Separated prisms From outer layer of Pinna shell. .?.-Skeletons"of Forami- nifera from limestone. 3, Recent shell of Polystomella crispa ; viewed wit dark-ground illuminator. The solitary Rhizopoda, furnished with a horny shell or capsule, forming a case for the animal, is nearly the only representative of the Arcellidce. In the Arcella, from which the family derives its name, the shell is somewhat if a bell-shape, with a very large round opening. ID 376 THE MICROSCOPE. Englypha it is of an oval or flask-like form, with the opening at the smaller end, and the shell appears asr though formed of a sort of mosaic of small horny pieces. In Difflugia^ Fig. 205, A B, the shell is often globular. Rhizopods which never develop more than one chamber or loculus are classed as Monothalamia. 1 The Polythalamia, or Multilocular Rhizopods, in their earliest state, are unilocular ; but, as the animal increases,, successive chambers are added in a definite pattern for each family of the order. They all inhabit the sea, and frequently occur in such great numbers, that the fine calcareous sand which constitutes the sea-shore in many places consists almost entirely of their microscopic coats. At former periods of the earth's history, they existed in even greater profusion than at present ; and their fragile shells form the principal constituent of several very important geological formations. Thus the chalk appears to consist almost entirely of the shells of these animals, either in a perfect state, or worn and broken by the action of the waves ; they occur again in great quantities in the rnarly and sandy strata of the Tertiary epoch. In the Stichostegidce the chambers are placed end to- end in a row, so as form a straight or but slightly curved shell. In the second family, the Enallostegidce, the chambers are arranged alternately in two or three parallel lines ; and as the construction of the shell is always commenced with a single small chamber, the whole neces- sarily acquires a more or less pyramidal form. The third family, the Helicostegidce, presents us with some of the most beautiful forms that it is possible to meet with in shells. They, commence by a small central chamber ; and each of the subsequent chambers, which are arranged in a spiral form so as to give the entire shell much the aspect of a minute flattened snail, is larger than the one preceding it. It is in this family that we find the nearest approach, in external form, to the large chambered shells of the cepha- lopodous mollusca, of which the Nautilus pompilius is an example. The fourth family, the Entomostegidce, stand in the same relation to the preceding as the Enallostegidoe to the (1) Diffiugia and Ancella form a connecting link between the naked forms, Amoeba, Actinophrys, &c. and the shell-bearing Rhizopods, Lagena striata, &c. GRKGAIUNFDA, POIACVSTINA, FOKAMIMKF.KA, ROTIFKKA, KTC. PLATE III. Kduiund Kvans. FORAMINIFERA. 377 Stichostegidce ; that is to say, the chambers are also arranged in a spiral form, but in a double series. A fifth family includes those shells in which the chambers are arranged round a common perpendicular axis in such a manner that each chamber occupies the entire length of the shell. The orifices of the chambers are placed alternately at each end of the shell, and are furnished with a curious tooth-like process. The Miliola serve as an example of this family. Every handful of sea-sand, every shaking of a dried sponge, and the contents of the stomachs of most Lamellibranch molluscs, oyster and mussel, are pretty sure to exhibit a considerable admixture of these minute calcareous, or occasionally silicious, Foraminifera. It is considered that the fossil shells, termed Num- mulites, found in great quantities in the chalk and lowei tertiary strata, are also to be regarded as members of this class ; in a fossilized state, whole mountains consist almost entirely of their shells. The late Professor Quekett had an opportunity of examining a few living specimens, which, he says, " are composed of a sarcode element, built up into a series of chambers with calcareous material." The great Pyramid of Egypt, covering eleven acres of ground, is based on blocks of limestone consisting of Foraminifera, Nummulites, or stone coin, and other fossil animalcules. Nummulites vary in size from a very minute object to that of a crown-piece, and many appear like a snake coiled up in a round form. A chain of moun- tains in the United States, 300 feet high, seems wholly formed of one kind of these fossil-shells. The crystalline marble of the Pyrenees, and the limestone ranges at the head of the Adriatic gulf, are composed of small Nummulites. Vast deposits of Foraminifera have been traced in Egypt and the Holy Land, on the shores of the Red Sea, Arabia, and Hindostan, and, in fact, may be said to spread over thousands of square miles from the -Pyrenees to the Himalayas. The fossilized Foraminifera in the Poorbaudar lime- stone, although occasionally reaching the twenty-fifth, do not average more than the hundredth part of an inch in diameter ; so that more than a million of them msy be computed to exist in a cubic inch of the stone. They inay 378 THE MICROSCOPE. be separated into two divisions those in which the cells are large, the regularity of their arrangement visible, and their bond of union consisting of a single constructed por- tion between each; and those in which the cells are minute, not averaging more than the 900th part of an inch in diameter, the regularity of their arrangement not distinctly seen, and their bond of union consisting of many thread-like filaments. To ascertain the mineral composi- tion of the amber-coloured particles or casts, after having found that it was mostly carbonate of lime with which they were surrounded, they were placed for a few mo- ments in the reducing flame of a blow-pipe, and it was observed that on subsequently exposing them to the influ- ence of a magnet, they were all attracted by it. Hence, in a rough way, this rock may be said to be composed of carbonate of lime and oxide of iron. By far the greater number of Foraminifera are marine. They are found in most seas, and in those of the tropics they increase both in size and variety, forming extensive deposits. During the Canadian Geological Survey large masses of what appeared to be a fossil organism, the Eozoon Canadense, were discovered in rocks situated near the base of the Laurentian series of North America. Dr. Dawson, of Montreal, referred these remains to an animal of the foraminiferal type ; and specimens were sent by Sir "W". Logan to Dr. Carpenter, requesting him to subject them to a careful examination. As far back as 1858 Sir W. Logan had suspected the existence of organic remains in specimens from the Grand Calumet limestone, on the Ottawa river, but a microscopic examination of one of these specimens was not successful. Similar forms being seen by Sir William in blocks from the Grenville bed of the Laurentian limestone were in their turn tried, and ultimately revealed their true structure to Dr. Dawson. and Dr. Sterry Hunt. The masses ot which these fossils consist are composed of layers of serpentine alternating with calc-spar. It was found by these observers that the calcareous layers represented the original shell; and the siliceous layers the flesh, OT: sarcode y of the once living creature. These EOZOON CANADENSE. 379 results were arrived at through comparison of the appear- ance presented by the Eozoon with the microscopic struc- ture which Dr. Carpenter had previously shown to characterise certain members of the foraminifera. The Eozoon not only exceeded other known foraminifera in size, to an extent that might have easily led observers astray, but, from its apparently very irregular mode of growth, its general external form afforded no help in its identification, and it was only by careful examination of its minute structure that its true character could be ascertained. Dr. Carpenter says : " The minute struc- ture of Eozoon may be determined by the microscopic examination either of thin transparent sections, or of portions which have been subjected to the action of dilute acids, so as to remove the calcareous shell, leaving only the internal casts, or models, in silex, of the chambers and other cavities, originally occupied by the substance of one animal." Dr. Carpenter found the preservation of minute struc- ture so complete that he was able to detect " delicate pseudopodial threads, which were put forth through pores in the shell wall, of less than 3 ooogth of an inch in diameter " (see Plate III. figs. 64, 65) ; and in a paper read at the meeting of the Geological Society he stated that he had detected Eozoon in a specimen of ophicalcite from Cesha Lipa in Bohemia, in a specimen of gneiss from near Moldau, and in a specimen of serpentine limestone sent to Sir C. Lyell by Dr. Giimbel, of Bavaria, all these being parts of the great formation of "fundamental" gneiss, which is considered by Sir Eoderick Murchison as the equivalent of the Laurentian rocks of Canada. There can be little doubt that a rich field of research: is now opened to those who will undertake the examination of rocks of various ages, which present the appearance of analogous structure ; as it is, the microscope has been the means of demonstrating the existence of animal life at a very ancient geological date ; and, in the words of Sir W. Logan, " we are carried back to a period so far remote that the appearance of the so-called Primordial Fauna may be considered a comparatively modern event." Recent Foraminifera present symmetrical shells, of 380 THE MICROSCOPE. minute size for the most part, and consisting, as we have already seen, either of one, two, or more con- nected chambers. A jelly-like mass, or " sarcode," occu- pies the chambers and their connecting passages ; and, protruding itself both from the external aperture of Fig. 808. 1, Section of Faujasina . a a, radiating interseptal canals ; *>, their internal bifurcations ; c, a transverse branch ; d, tubular wall of the chambers. 2, Rosalina ornata, with its pscudopodia protruded. the last chamber, and in many cases from the sometimes numerous perforations in the shell- walls, extends itself not only over the surface of the shell, but also into radiating contractile threads or pseudopodia, and into gernniule-like masses, which latter become coated over with calcareous matter, and thus form additional segments of the animal. 1 " Foraminifera^ indeed, are to be compared with the other lowest orders of animals and of plants in the study of their specific relations. In these several low forms of creatures we have comparatively few species, but ex- tremely numerous individuals, with an enormous range of (1) Among the more important works on Foraminifera, reference may be made to D'Orbigny's Foraminifkres fossiles du Bassin Tertiaire de Vienna (Autrihe); Schultze, Ueber den Organismus der Polythalamien, 1854; Car- penter's and Williamson's Researches on the Foraminifera, Phil. Trans. 1856. Also an excellent paper by Mr. W. R. Parker, in the Annals of Natural History, April, 185T. Specimens of Foraminifera may be obtained for examination from the shaking of dried Sponges; but if required alive they must be dredged for, o picked off the fronda of living seaweeds, over the surface of which they are seen to move by the aid of a lens. FORAMINIFERA. 381 variety. In the higher orders of plants and animals the specific forms are more definite, there being a more com- plex organization, harmonizing with the special habits of each creature ; and the individuals of each species are less Fig. 209. Foraminifera taken in Deep-sea Sounding*. (Atlantic.) numerous than is the case in the Protozoans and Proto- phytes." These lowly organized Foraminifera, having great simplicity of structure, more easily adapt themselves to varying external conditions than the more complex and specialized higher animals. 382 THE MICROSCOPE. In the deep-sea soundings, portions of many beau- tiful Diatoms, figured and described by Professor W. Smith, in gatherings from the Bay of Biscay, near Biar- ritz, are Melosira pribrosa, marine, orbicular, cellulate, Fig. 210. Foraminifera taken in Deep-sea Soundings. (Atlantic.) cellules, all equal and hexagonal. He writes : " In De- cember, 1853, I received isolated frustules of this species, collected on the coast of Normandy, under the above name, from M. de Brebisson ; and I have since detected the same in a gathering from the Black Sea. In no case have I seen the frustules in a recent state, aud do not know FORAMINIFERA. 383 whether they ever form a lengthened filament. As this is the only circumstance that would justify their separation from Coscinodiscus, to which the separated valve would otherwise seem to belong (Synop. British Diatomacece, vol. i. p. 22), their position in Melosira must rest upon the authority of my accurate correspondent." In figs. 209 and 210 are represented many of the beau- tiful forms brought up with soundings made in 1856, for the purpose of ascertaining the depth of the Atlantic, prior to the laying down of the electric telegraph wire from England to America; these specimens were taken from a depth of 2,070 fathoms. Major S. R J. Owen, while dredging the surface oi mid-ocean Indian and Atlantic oceans found attached to his nets a few interesting forms of Ehizopods, belonging to the two genera Globigerina and Pulvinulina, which always make their appearance on the surface of the ocean after sunset. 1 "Many of the forms," writes this observer, "have hitherto been claimed by the geologist, but I have found' them enjoying life in this their true home, the siliceous shells filled with coloured sarcode, and sometimes this sarcode in a state of distension somewhat similar to that found projecting from the Foraminifera, but not in such slender threads. There are no objects in nature more brilliant in their colouring or more exquisitely delicate in their forms and structure. Some are of but one colour, crimson, yellow, or blue ; sometimes two colours are found on the same individual, but always separate, and rarely if ever mixed to form green or purple. In a globular species, whose shell is made up of the most delicate fret- work, the brilliant colours of the sarcode shine through the little perforations very prettily. In two specimens of the triangular and square forms (Plate III. figs. 44, 45, and 46), the respective tints of yellow and crimson are vivid and delicately shaded. In one the pink lines are concen- tric ; while another is of a stellate form (fig. 43), the points and uncoloured parts being bright clear crystal, while a beautiful crimson ring surrounds the central por- (1) Journ. Linn. Soc. vol. viii. p. 202 ; vol. ix. p. 147, 1866, and 1867. 384 TIIE MICROSCOPE. lion. One globular species appears like a specimen of the Chinese ball-cutting one sphere within another ; but it is of a marked and distinct kind. "The shells of some of the globular forms of these Polycystina, whose conjugation I believe I have witnessed, are composed of a fine fretwork, with one or more large circular holes ; and I suspect the junction to take place by the union of two such apertures. That the figures of these shells become elongated, lose their globular form after death, and present a disturbed surface is seen in some of the figures represented near the bottom part of Plate III." Major Owen proposes to make Orbulina a subgenus of Globigerina. The internal chambers of the former are in form remarkably like those of the latter, and like them also they present themselves with varying surfaces, some free from, while others are covered with, spines. Those without internal chambers have been known as Orbulina universa, fig. 78, Plate III. while figs. 75 and 76, although members of the same family, have been separated ; but he wishes to see all united under the name of " Globigerina universa" The minute siliceous shells of Polycystina present won- derful beauty and variety of form ; all are more or less perforated, and often prolonged into spines or other pro- jections, through which the sarcode body extends itself into pseudopodial prolongations resembling those of Acti- nophrys. When seen besporting themselves in all their living splendour, their brilliancy of colouring, says Major Owen, "renders them objects of unusual attraction." We have endeavoured to give some idea of the colour of the living forms in Plate III. Nos. 43 to 52. The same ob- server believes that they wish to avoid the light, " as they SU, "On the Development of the Spongilloe." 390 THE MICROSCOPE. found in it gemmules still retaining their brown colour and contents capable of development. Fig. 214. Geodia Barretti (Bowerbank). A. section at right angles to the surface, exhibiting the radial disposition of the fasciculi of the skeleton, and a portion of the dermal crust of the sponge, magnified 50 diameters, a, interrnarginal cavities ; b, the basal diaphragms of the intermarginal cavities ; c, imbedded ovaria forming the dermal crust of the sponge ; d, the large p'atentoternate spicula, the heads of which form the areas, for the valvular bases of the intermarginal cavities ; e, recurvo-ternate defensive and aggressive spicuia within the summits of the intercellular spaces of the sponge ; /, portions of the interstitial membranes of the sponge, crowded with minute stellate spicula ; g, portions of the secondary system of external defensive spicula. (1) The usual contents of the gemmules have been described by Meyen (Muller's Archiv. 1839, p. 83). In many in- (1) See Bowerbank's Monograph of the British Spougiadcc, Ray Soc. p. 169. SPONGES. 391 stances Lieberkiihn found that the globular arrangement no longer existed, the globules being replaced by granules exhibiting an active molecular motion. That the gem- niules are formed from agglomerations of sponge-cells may be readily proved in the branched sponge containing smooth gemmules. Lieberkiihn notices four kinds of gernmules characterised respectively by their cases or 8hells. 1. Those with smooth cases. 2. Those with stellate amphidiscs. 3. Those with amphidiscs, in which the discoid ex- tremities are entire, and not stellate. 4. Gemmules whose case, instead of amphidiscs, is fur- nished with minute, usually slightly curved siliceous spicules. It would appear, therefore, that the " globules " of Meyen are nothing more than altered sponge-cells. The autumn is the most favourable season for observing the process of their formation. In the journal of the Bombay branch of the Royal Asiatic Society for 1840, Surgeon H. J. Carter gives a very accurate account of fresh-water sponges found in the water tanks of Bombay. Of five species that he disco- vered, one was the Spongilla friabilis, the others he named Sp. cinerea, Sp. alba, Sp. Meyeni, Sp. plumosa. Spongilla cinerea is stated to present on its surface a dark rusty, copper colour, lighter towards the interior, and purplish under water. It throws up no processes, but extends horizontally in circular patches, over surfaces two or three feet in circumference, or accumulates on small objects ; and is seldom more than half an inch in thick- ness. It is found on the sides of fresh-water tanks, on rocks, stones, or gravel. The ova are spheroidal, about l-63d of an inch in diameter, presenting rough points ex ternally. Spicula of two kinds, large and small ; large epicula, slightly curved, smooth, pointed at both ends, about l-67th of an inch in length ; small spicula, slightly curved, thickly spiniferous, about 1-3 80th of an inch in length. Spongilla friabilis. Growing in circumscribed masses, dn fixed bodies, or enveloping floating objects ; seldom 392 THE MICRJSCOPE. attaining more than two inches in thickness. From tlie other sponges it is distinguished by the smooth spicula which surround its seed-like bodies, and the matted structure. Spongilla alba. Its texture is coarse and open ; struc- ture reticulated. The investing membrane abounds in minute spicula; has seed-like spheroidal bodies about l-30th of an inch in diameter, with rough points exter- nally. The large spicula are slightly curved, smooth, pointed at each end, about l-54th of an inch in length ; the small spicula are slightly curved, thickly spiniferous, or pointed at both ends; the former, pertaining to th7. 38 6 THE MICROSCOPE. factory conclusions upon the habits of an animal thai works so completely in the dark as the Cliona celata it will probably long remain so. Mr. Hancock, to whom we are indebted for a valuable memoir upon the boring sponges, published in the Annals and Magazine of Natural History, attributes their excavating power to the presence of a multitude of minute siliceous crystalline particles adhering to the surface of the sponge ; these he supposes to be set in motion by some means analogous to ciliary action. In whatever way this action may be produced, however, there can be no doubt that these sponges are constantly and silently effecting the disintegration of sub- marine calcareous bodies the shelly coverings, it may be, of animals far higher in organisation than they ; nay, in many instances they prove themselves formidable enemies even to living rnollusca, by boring completely through the shell. In this case the animal whose domicile is so unce- remoniously invaded, has no alternative but to raise a wall of new shelly matter between himself and his unwelcome guest ; and in this mariner generally succeeds at last in barring him out. Skeletons of Sponges. The skeletons of sponges, which give shape and substance to the mass of sarcode that con- stitutes the living animal, is best made out by cutting thin slices of sponge submitted to firm compression, and view- ing these slices mounted upon a dark ground, or backed up with black paper. The skeletons of sponges are composed principally of two materials, the one animal, the other mineral ; the first of a fibrous horny nature, the second either siliceous or calcareous. The fibrous portion consist of a network of smooth, and more or less cylindrical, threads of a light- yellow colour, and, with few exceptions, always solid ; t f hey frequently aiiastamose, and vary considerably in size ; when developed to a great extent, needle-shaped siliceous bodies termed spicula (little spines) are formed in their in- terior; in a few cases only one of these spicula is met tvdth, but most commonly they occur in bundles. In some sponges, as those belonging to the genus Halichondria, the name horny kind of material is present in greater or less abundance ; but its fibrous structure has become obscure : SPONGES. 397 the fibres, however, in these cases are represented ly sili- ceous needle-shaped spicula, and the horny matter serves the important office of binding them firmly together, as Fig. 215. 1. Transverse section of a branch of Myriapore. 2, A section of the stem of Virgularia miraUlis. 3, A. spiculum from the outer surface of a Sea-pen. 4, Spicala from crust of Isis hippuris. 5, Spicula from Gorgona elongate,. 6, Spicula from Alcyonium. 7, Spicula from Gorgonia, umbraculum. shown in fig. 213, No. 1. There are, however, some re- markable exceptions to this rule, one, Dictyochalix pumi- cens, described by Mr. S. Stutchbury, in which the fibrous skeleton is composed of threads of silex quite as trans- parent as glass ; another, the Hyalonema, Glass-rope. The mineral portion, as before stated, consists of spicula composed either of silica or carbonate of lime ; the first kind is the most common and likewise most variable in shape, and presents every gradation in form, from tho acuate or needle-shaped to that of a star. The calcareous spicula, on the contrary, are more simple in their form, 39ft THE MICROSCOPE. being principally acicular, but not unfrequently branched or even tri- or quad-radiate ; the two kinds, the sili- ceous and calcareous, according to Dr. Johnston, not having hitherto been detected co- existent in any native sponges. The spicula exhibit a more or less distinct trace of a central cavity or canal, the extremities of which are closed, or hermetically sealed ; in their natural situation they are invested by an animal membrane, sarcode, which is not confined to their external surface : but in many of the large kinds, as pointed out by Mr. Bowerbank. its presence may be detected in their central cavity, by exposing them for a short time to a red heat, when the animal matter will become carbonised, and appear as a black line in their interior. Many authors have described the spicula as being crys- talline, and of an angular figure, and have considered them analogous to the r aphides in plants ; but it requires no great magnifying power to prove that they are always round, and, according to their size, are made up of one or more concentric layers, as shown in fig. 212, No. 2. The spicula occupy certain definite situations in sponges ; some are peculiar to the crust, others to the sarcode, others to the margins of the large canals, others to the fibrous network of the skeleton, and others belong exclu- sively to the gemmules. Thus, for instance, in Pachyma- tisma Joknstonia, according to Mr. Bowerbank, the spicules of the crust are simple, minute, and fusiform, having their surfaces irregularly tuberculated, and their terminations very obtuse ; whilst those of the sarcode are of a stellate form, the rays varying in number from three to ten or twelve. Silica, however, may be found in one or more species of sponge of the genus Dysidea, not only in the form of spicula, but as grains of sand of irregular shape and size, evidently of extraneous origin, but so firmly surrounded by horny matter as to form, with a few short and slightly- curved spicula, the fibrous skeleton of the animal. In these sponges the spicula are of large size, and are disposed in lines parallel with the masses of sand. Most of the sponges of the earlier geological periods had SPONGES. 399 tubular fibres ; but in all existing species, with one or two exceptions, they are solid. These tubular fibres are very commonly filled with portions of iron, which accounts for the colour of many of the remains in flint. The Moss-agates, found among the pebbles at Brighton and elsewhere, are flints containing the fossilised remains of sponges. The coloured fibres seen in the Green-jaspers of the East are of the same character. There is reason to believe that most flints were originally sponges ; those from chalk even retain their original form. Recent sponges from the Sussex coast present forms precisely similar to some chalk flints, but it is from sections made sufficiently thin to be transparent, for examination under the microscope, that we learn their true nature and origin. Every horny sponge, whilst living, is invested with a coating of jelly-like substance, which can only be preserved by placing the sponge in spirit and water immediately after its removal from its place of growth. Spicula are not exclusively confined to the body of sponges, but occa- sionally form the skeleton of the gemmules, and are situated either on'the external or internal surface of these bodies. A good example of the former kind occurs in the common fresh-water sponge (Spongilla fluviatilis), represented in fig. 216, No. 1, and No. 3. The spicula are very minute in size, and are disposed in lines radiating from the centre to the circumference, the markings on the outer surface of the gemmules being the ends of spicula. In all the young gemmules the spicula project from i>he outer margin as so many spines ; but in process of growth the spines become more and more blunt, until at last they appear as so many angular tubercles. Turkey sponge (Spongia officinalis) is brought from the Mediterranean, has a horny network skeleton rather fine in the fibres, solid, small in size, and light in colour. In some larger specimens there is a single large fibre, or a bundle of smaller ones. In Halichondria simulans the skeleton is a framework of siliceous needle-shaped spicula, arranged in bundles kept together by a thick coat of horny matter. Other species of Halichondria have siliceous spicula pointed at both extremities aca-rate (fig. 212, No 2) ; 4CO THE MiCROSCOPia. whib the spicula of some are round at one end, and pointed at the other acuate ; some have spicula round at one end, the former being dilated into a knob spinulate. Fig. 216. t Gemmule of SpongW * '-/V//(V, enclosed in spicula. 2, Birotulate spicula, from Fluviatilis. 3, Gem mules of Spongilla fluviatilis, after having been im- mersed in acid, to show coating of birotulate spicula. Among the genus Grantia, Geodia, and Levant sponge, are found spicula of a large size, radiating in three direc- tions triradiate. In the Levant specimen, a central communicating cavity can be distinctly seen. Some Smyrna sponges, and species of Geodia, have four rays quadriradiate. Some spicula in P. Johnstonia and Geodia have as many as ten rays multiradiate. In some species