i » r^> > ^»> •••> ^»> >>•> >^> >!&> >ift> ,>"&> -•> ;>., .?>>> ^Jv\^ Vv%^ <>tm^ >^ >» .^ ^ -» Jk.3^^5 v> > H ' THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID ,' > » > > ' > > > I THE AQUAHIUM.. THE MICEOSCOPE: HISTOKY, CONSTRUCTION, AND APPLICATION, BEING A FAMILIAR INTRODUCTION TO THE USE OF THE INSTRUMENT AND THE STUDY OF MICROSCOPICAL SCIENCE. BY JABEZ HOGG, MEMBER Or THE ROYAL COLLEGE OF SURGEONS OF ENGLAND, ETC. $U«sfratefr fcrrtlj npfoarbs 0f Jifre F OURTH EDITION. LONDON : KOUTLEDGE, WARNES, AND KOUTLEDGE, FARRINGDON STREET. NEW YORK : 5fi, WALKER STREET. 1859. V Hfc PEEFACE TO THE THIRD AND EDITIONS. Y endeavour to produce a cheap and popular guide to the use of the Micro- scope has been rewarded by a success which has far exceeded my expecta- tions. In a short space of time, three editions, each of Jive thousand copies, have been sold ; and I am now called upon for a Fourth. My efforts on this occasion have been directed to a revision of the whole work ; much new matter, and a large number of additional cuts to illustrate the text, have been added ; and I trust I have succeeded in making the present Edition more free from blemish and more generally useful to the student and the public than those which pre- ceded it. I have at all times freely and fully availed myself of the judicious suggestions and criticisms of kind friends ; and I wish to take this opportunity of acknow- ledging the obligations I am under to those who have so generously accorded to this small work, whatever its defects, their meed of approbation. J. H. 1859. S3S9403 PREFACE TO THE FIRST EDITION. jj,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 of studies — the study that belongs to the domain 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 inatten- tion were mainly attributable to the want of some concise, yet sufficiently comprehensive, popular account of the Microscope, both as regards the management and mani- pulation 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 destined for circulation amongst a special class of readers, were necessarily, from the nature of their contents and the style of their production, published at a price that renders thein practically unattainable by the great bulk of the public. PREFACE. They constitute careful and beautiful contributions to the purposes of science, but they cannot adequately serve to bring the value and charm of microscopic studies home, so to speak, to the firesides of the people. Repeatedly, day after day, new and interesting discoveries, and further amplifications of truth already discerned, have been made, but they have been either scattered 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 it a rich store of enlightenment and grati- fication : the many not having, however, participated, to any great extent, in the instruction and entertainment which always follow in the train of microscopical studies.1 The manifold and various 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 see its benefits; and knowledge which could not be obtained even by the minutest dissection is acquired readily by the aid of the Microscope, which has become equally as essen- tial to the anatomist and pathologist as the scalpel and bedside observation. The smallest portion of a diseased structure, placed under a Microscope, will tell more in one minute to the experienced eye, than could be ascertained by days of examination of the gross mass of disease in the ordinary method j and microscopic agency, in thus assist- (1) At the time this work was written; scarcely a book of the kind had been published at a price within the reach of the working classes. PREFACE. IX ing the medical man, materially contributes to the allevia- tion of those multiplied " ills which flesh is heir to." So fully impressed were the Council of the Royal College of Surgeons with the importance 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 col- lection 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, histolo- gical anatomy deservedly became an important branch of the education of the medical student. In prosecuting the study of Vegetable Physiology, the Microscope is an indispensable instrument ; it empowers 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 suspected flour, an article of the greatest importance to all, the Microscope enables us to judge of the size and shape of the starch-grains, their markings, their isolation and agglomeration ; and thus to distinguish the starch- grains of one meal from those of another. It detects these and other " invisible ingredients, whether precipitated in atoms or aggregated in crystals, which adulterate our food, our drink, and our medicines. It displays 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" The Zoologist finds in the Microscope a necessary co- operator. To the Geologist it reveals, among a multiplicity X PREFACE. of other facts, " that our large coal-beds are the ruins of a gigautic vegetation ; and the vast limestone rocks, which are so abundant on the earth's surface, are the catacombs of myriads of animal tribes, too minute to be perceived by the unaided vision." By " conducting the eye to the confines of che visible form," the Microscope proves an effective auxiliary in denning the geometric properties of bodies. Its influence as an instrument of research upon the structure of bodies has been compared to that of the galvanic battery, in the hands of Davy, upon Chemistry. It detects the smallest structural difference, heretofore inappreciable, and, as an ally of Chemistry, enables us to discover the very small changes of form and colour effected by test-fluids upon solids ; it dissects for us, so to speak, the most multiplex compounds ; it opens out to the mind an extended and vast tract, opulent in wonders, rich in beauties, and bound- less in extent. The Microscope not only assists studies, and develops objects of profound interest, but it also opens up innumer- able sources of entertainment and amusement, in the ordinary conventional acceptation of these terms ; — it dis- closes to us peculiarities and attractions in abundance ; — it impresses us with the wonderful and beautifully-skilful adaptation of all parts of creation, and fills our minds with additional reverence and admiration for the beneficent and Almighty Creator. The Author begs to conclude these prefatory observa- tions with a few words in explanation of his arrangements, and by way of acknowledgment to those to whom he is indebted. He has sought, in the volume that he now lays before the public, to point out and elucidate, at once in a practical manner and in a popular style, the vast fund of utility and amusement which the Microscope affords, and PREFACE. XI has endeavoured to touch upon most of the interesting subjects for microscopic observation as fully as the restric- tions of a limited space, and the nature of a succinct summary, would permit. To have dwelt upon each in complete detail would have necessitated volume upon volume, — expensive books must have resulted, — and this would have entirely frustrated the aim which the writer had in view ; he has, therefore, contented himself with the humble, but, he trusts, not useless, task of setting up a finger-post, so to say, to direct the inquirer into the wider road. In the section of the work devoted to the minuter po-riion of creation, he has ventured to dwell somewhat longer here, in the belief that that department is more especially the province of the microscopist. He has arranged his topics under special headings, and in separate chapters, for the sake of perspicuity and precision ; and has brought the ever-welcome aid of illustrations to convey his explanatory remarks more vividly to the minds of his readers. He is peculiarly indebted to Professor Quekett, whose valuable lectures, delivered annually in the Royal College of Surgeons, and other multifarious and successful researches, have pre-eminently distinguished him as the microscopist of the day. From notes made during the lectures spoken of, and from the many admirable papers which this gentleman has published, much sound information has been gleaned ; and the author has to thank him, in the most sincere and cordial manner, for placing at his disposal the mass of contributions with which he has enriched microscopical science. A free use has beeu made of the researches of scientific investigators generally — Leeuwenhoek, Ehrenberg, Carpenter, Johnston, Ralfs, Busk, Gosse, Hassell, Lobb, and other members of the Microscopical Society of London. His acknowledg- ments are likewise due to Mr. George Pearson, for the xii PREFACE. great care he has bestowed upon the engravings which adorn 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 publica- tions, submit some further food for the exercise of the mental and intellectual faculties, — contribute to the addi- tional amusement and instruction of the family circle around the domestic hearth, — and aid the student of nature in investigating the wonderful and exquisite works of the Almighty Hand. If it shall be the good fortune of this work, which is now confided with great diffidence to the consideration of the public, to succeed, in however slight a degree, in furthering this design, the Author will feel sincerely happy, and will be fully repaid for the atten- tion, time, and labour that he has expended in writing, arranging, and compiling it. 6, GOWER STREET, BEDFORD SauARE, May, 1854. PART I. HISTORY OF THE INVENTION AND IMPROVEMENTS OF THE MICROSCOPE. CHAPTER I. PAGE HISTORY OP THE MICROSCOPE 1 CHAPTER II. MECHANICAL AND OPTICAL PRINCIPLES INVOLVED IN THE CON- STRUCTION OP THE MICROSCOPE — LENSES — MODE OF ESTI- MATING THEIR POWER, ETC. — ACHROMATIC LENSES — MAGNI- FYING POWER — WOLLASTON'S DOUBLET — CODDINGTON'S LENS — ROSS'S SIMPLE AND COMPOUND MICROSCOPES — QUEKETT?S, WARINGTON'S, BAKER'S, AND POWELL'S MICROSCOPES — MICROMETERS, ETC 15 CHAPTER III. PRELIMINARY DIRECTIONS — ILLUMINATION — ACCESSORY APPA- RATUS— ACHROMATIC ILLUMINATOR— GILLET'S CONDENSER — COLLECTING OBJECTS — MODE OF INJECTING — PREPARING AND MOUNTING OBJECTS — POLARISED LIGHT — CAMERA LUCIDA — BINOCULAR INSTRUMENT — PHOTOGRAPHIC DRAW- ING, ETC 69 XiV CONTENTS. PART II. CHAPTER I. PAGE VEGETABLE STRUCTURE — VITAL AND CHEMICAL CHARACTERISTICS — MICROSCOPIC FORMS OF VEGETABLE LIFE — THE VEGE- TABLE CELL — FUNGI — FUNGOID DISEASES — MOSSES— ALG^J — CONFERVA — DESMIDIACE^J — STRUCTURE OF PLANTS — ADULTERATION OF ARTICLES USED FOR FOOD — PREPARA- TION FOR MICROSCOPIC EXAMINATION, ETC 174 CHAPTER II. DIVISION OF ANIMAL KINGDOM. PROTOZOA — HISTORY OF INFUSORIAL ANIMALCULES — RHIZO- PODA — MONADS — DIATOMACE-E — FOSSIL INFUSORIA — ROTI- . FERA — VORTICELLA — STENTORS — SPONGES — HYDRA — ZOOPHYTES — ETC 260 CHAPTER III. SUB-KINGDOM ANNULOSA — ARTHROPOD A — ANN ULATA — ANNU- LOIDA — MOLLUSCA — CONCHIFER A — GASTEROPODA — PTERO- PODA — • TUNICATA — CEPHALOPODA — CRUSTACEA— ANNELIDA — ARACHNIDA— SUCTORIA, ETC 411 CHAPTER IV. BUB-KINGDOM ARTICULATA — INSECTA 469 CHAPTER V. VERTEBRATA.— ANIMAL STRUCTURE. PHYSIOLOGY — HISTOLOGY — CELL THEORY — GROWTH OF TISSUES — SPECIAL TISSUES — SKIN, CARTILAGE, TEETH, BONE, ETC. . 523 CORRIGENDA ET ADDENDA . 601 THE MICEOSCOPE. THE MICEOSCOPE. PART I. HISTORY OF THE INVENTION AND IMPROVEMENTS OF THE MICROSCOPE. CHAPTER I. HISTORY OF THE MICROSCOPE. HE instrument known as the Mi- croscope derives its name from two Greek words, /UK/OOS, small, and o-KOTreo), to view; that is, to see or view such minute objects as without its aid would be in- visible. The honour of the invention is claimed by the Italians and the Dutch ; the name of the inventor, however, is lost. Probably the discovery did not at first appear sufficiently important to engage the attention of those men who, by their reputation in science, were able to establish an opinion of its merit, and to hand down the name of its inventor to succeeding ages. If we consider the microscope as an instrument con- sisting of one lens only, it is not at all improbable that it was known at a very early period, nay even in a degree to B 2 HISTORY OP THE MICROSCOPE. the Greeks and Romans ; at any rate, it is tolerably certain that spectacles were used as early as the thirteenth century. Now as the glasses of these were made of different con- vexities, and consequently of different magnifying powers, it is natural to suppose that smaller and more convex lenses were made, and applied to the examination of minute objects. Many among the learned refuse to the ancients a knowledge of magnifying lenses, and d, fortiori that of refracting tebscopes, since, according to them, the Greeks and Romans had only very imperfect notions with respect to the fabrication of glass. From a passage in Aristophanes it is plain that globules of glass were sold at the shops of the grocers of Athens, in the time of that comic author. He speaks of them as " burning spheres." Pliny states that the immense theatre (it was capable of containing eighty thousand persons) erected at Rome by Scaurus, son-in-law of Sylla, was three stories in height, and that the second of these stories was entirely inlaid with a mosaic of glass. Ptolemy, in his " Optics," has inserted a table of the refractions which light experiences under different angles of incidence, in passing from air into glass. The values of these angles, which differ only in a slight degree from those obtained in the present day by means of similar ex- periments, prove that the glass of the ancients differed very little from that manufactured in our own times. There, is in the French Cabinet of Medals a seal, said to have belonged to Michael Aiigelo, the fabrication of which, it is believed, ascends to a very remote epoch, and upon which fifteen figures have been engraven in a circular space of fourteen millimetres in diameter. These figures are not all visible to the naked eye. Cicero makes mention of an Iliad of Homer written upon parchment, which was comprised in a nutshell. Pliny relates that. Myrrnecides, a Milesian, executed in ivory a square figure which a fly covered with its wings. Unless it be maintained that the powers of vision of our ancestors surpassed those of the most skilful modern artists, these facts establish that the magnifying property of lenses was known to the Greeks and Romans nearly HISTORY OP THE MICROSCOPE. 6 two thousand years ago. We may besides advance a step further, and borrow from Seneca a passage whence the same truth will emerge in a manner still more direct and decisive. In the " Natural Questions" we read : " How- ever small and obscure the writing may be, it appears larger and clearer when viewed through a globule of glass filled with water." Dutens has seen in the Museum of Portici ancient lenses which had a focal length of only nine millimetres. Ho actually possessed one of these lenses, but of a longer focus, which was extracted from the ruins of Hercu- laneurn. At the meeting of the British Association, held at Belfast in the year 1852, Sir David Brewster showed a plate of rock-crystal worked into the form of a lens, which was recently found among the ruins of Nineveh. Sir David Brewster, so competent a judge in a question of this kind, maintained that this lens had been destined for optical purposes, and that it never was an article of dress. It is not difficult to fix the period when the microscope first began to be generally known, and to be used for the purpose of examining minute objects ; for though we are ignorant of the name of the first inventor, we are acquainted with the names of those who introduced it to public view. Zacharias Jansens and his son are said to have made micro- scopes before the year 1590 : about that time the ingenious Cornelius Drebell brought one made by them with him to England, and showed it to William Borrell and others. It is possible this instrument of Drebell's wTas not strictly what is now called a microscope, but was rather a kind of microscopic telescope, something similar in principle to that lately described by M. Aepinus in a letter to the Academy of Sciences at St. Petersburg. It was formed of a copper tube six feet long and one inch in diameter, supported by three brass pillars in the shape of dolphins; these were fixed to a base of ebony, on which the objects to be viewed by the microscope were placed. Fontana, in a work which he published in 1646, says that he had made microscopes in the year 1618 : this may be perfectly true, without derogating from the merit of the Jansens \ for we have many instances in our own times of more than B 2 4 HISTORY OF THE MICROSCOPE. one person having made the same invention nearly simul- taneously, without any communication from one to the other. In 1685 Stelluti published a description of the parts of a bee, which he had examined with a microscope. The history of the microscope, like that of nations and arts, has had its brilliant periods, in which it shone with uncommon splendour, and was cultivated with extraordi- nary ardour ; and these have been succeeded by intervals marked with no discovery, and in which the science seemed to fade away, or at least to lie dormant, till some favour- able circumstance — the discovery of a new object, or some new improvement in the instruments of observation — awakened the attention of the curious, and reanimated their researches. Thus, soon after the invention of the microscope, the field it presented to observation was culti- vated by men of the first rank in science, who enriched almost every branch of natural history by the discoveries they made by means of this instrument. The Single, or Simple Microscope. — We shall first speak of the single microscope, that having been invented and used long before the double or compound microscope. When the lenses of the single microscope are very convex, and consequently the magnifying power great, the field of view is small ; and it is so difficult to adjust with accuracy their focal distance, that it requires some practice to render the use of them familiar. It was with an instrument of this kind that Leeuwenhoek and Swammerdam, Lyonet and Ellis, examined the invisible forms of nature, and by their example stimulated others to the same pursuit. About the year 1665, small glass globules began to be occasionally applied to the single microscope, instead of convex lenses ; and by these globules an immense magni- fying power was obtained. Their invention has been generally attributed to M. Hartsoeker ; though it appears that we are really indebted to the celebrated Dr. Hooke for this discovery, for he described the manner of making them in the preface to his Micrographia Illustrata, pub- lished in the year 1656. Mr. Stephen Gray1 having observed some irregular particles within a glass globule, and finding that they (1) Philosophical Transactions, 1696. HISTORY OF THE MICROSCOPE. 5 appeared distinct and prodigiously magnified when held close to his eye, concluded, that if he placed a globule of water in which there were any particles more opaque than the water near his eye, he should see those particles dis- tinctly and highly magnified. The result of this idea far exceeded his expectation. His method was, to take on a pin a small portion of water which he knew contained some minute animalcules ; this he laid on the end of a small piece of brass wire, till there was formed somewhat more than a hemisphere of water ; on applying it then to the eye, he found the animalcules enormously magnified ; for those which were scarcely discernible with his glass globules, with this appeared as large as ordinary-sized peas. Dr. Hooke thus describes the method of using this water-microscope : " If you are desirous," he says, " of obtaining a microscope with one single refraction, and consequently capable of procuring the greatest clearness and brightness any one kind of microscope is susceptible of, spread a little of the fluid you intend to examine on a glass plate ; bring this under one of your globules, then move it gently upwards till the fluid touches the globule, to which it will soon adhere, and that so firmly as to bear being moved a little backwards or forwards. By looking through the globule, you will then have a perfect view of the animalcules in the drop." The construction of the single microscope is so simple, that it is susceptible of but little improvement, and has therefore undergone few alterations ; and these have been chiefly confined to the mode of mounting it, or to addi- tions to its apparatus. The greatest improvement this instrument has received was made by Lieberkuhn,1 about the year 1740 : it consists in placing the small lens in the centre of a highly-polished concave speculum of silver, by which means a strong light is reflected upon the upper surface of an object, which is thus examined with great ease and pleasure. Before this contrivance, it was almost impossible to examine small opaque objects with any degree of exactness ; for the dark side of the object being next the eye, and also overshadowed by the proximity of (1) Dr. Nathaniel Lieberkuhn of Berlin. 6 HISTORY OF THE MICROSCOPE. the instrument, its appearance was necessarily obscure and indistinct. Lieberkuhn adapted a separate microscope to every object : but all this labour was not bestowed on trifling objects ; his were generally the most curious ana- tomical preparations, twelve of which, with their microscopes, are deposited in the Museum of the Royal College of Surgeons. Lieberkuhn's instrument, fig. 1, is thus described by Professor Quekett -,1 a b represents a piece of brass tube, about an inch long and an inch in diameter, which is provided with a cap at each extremity ; the one at a carries a small double -con vex lens of half an inch in focal length, whilst the one at b carries a condensing lens three- quarters of an inch in diameter. A vertical section of one of these instru- ments is seen in fig. 2 : a represents the mag- nifier, which is lodged in a cavity formed partly by the cap a, and by the silver cup or speculum I. In front of the lens is the speculum Z, which is a quarter of an inch thick at its edge, and whose focus is about half an inch ; in front of this again there is a disk of metal c, three-eighths of an inch in diameter, connected by a wire with the small Fis- J- knob d ; upon this disk the injected object is fastened, and is covered over with some kind of varnish which has dried of a hemispherical figure. Between this knob and the inside and outside of the tube there are two slips of thin brass, which act as springs to keep the wire and disk steady. When the knob is moved, the injected object is carried to or from the lens, so as to be in its focus, and to be seen distinctly, whilst the condensing lens b serves to concen- trate the light on the speculum. To tho Micro- icope. (1) Practical Treatise on the Microscope, p. 16. HISTORY OF THE MICROSCOPE. \ lower part of the tube a handle of ebony, about three inches in length, is attached by a brass ferrule and two screws. The use of this instrument is obvious : it is, held in the hand in such a position that the rays of light from a lamp or white cloud may fall on the condenser b, by which they are concentrated on the speculum I ; this, again, further condenses them on the object and the disk c, which object, when so illuminated, can readily be adjusted by the little knob d, so as to be in the focus of the small magnifier at a. We must not omit in this place some account of Leeu- wenhoek's microscopes, which were rendered famous throughout all Europe, on account of the numerous dis- coveries he had made with them. At his death he be- queathed a part of them to the Royal Society. The microscopes he used were all single, and fitted up in a convenient and simple manner : each consisted of a very small double- con vex lens, let into a socket between two plates riveted together, and pierced with a small hole ; the object was placed on a silver point or needle, which, by means of screws adapted for that purpose, might be turned about, raised or depressed at pleasure, and thus be brought nearer to, or be removed farther from the glass, as the eye of the observer, the nature of the object, and the conve- nient examination of its parts required. Leeuwenhoek fixed his objects, if they were solid, to these points with glue ; if they were fluid, he fitted them on a little plate of talc, or thin-blown glass, which he afterwards glued to the needle in the same manner as his other objects. The glasses were all exceedingly clear, and of different magnifying powers, proportioned to the nature of the object and the parts designed to be examined. He observed, in his letter to the Royal Society, that te from upwards of forty years' experience, he had found the most considerable discoveries were to be made with glasses of moderate magnifying power, which exhibited the object with the most perfect brightness and distinctness." Each instrument was devoted to one or two objects; hence he had always some hundreds by him. The three first compound microscopes that attract our notice are those of Dr. Hooke, Eustachio Divini, and Philip tf HISTORY OF THE MICROSCOPE. Bonnani. Dr. Hooke gives us an account of his in the preface to his Micrographia, published in the year 1667 : it was about three inches in diameter, seven inches long, and furnished with four draw-out tubes, by which it might be lengthened as occasion required ; it had three glasses — a small object-glass, a middle glass, and a deep eye-glass. Dr. Hooke used all the glasses when he wanted to take in a considerable part of an object at once, as by the middle glass a number of radiating pencils were conveyed to the eye which would otherwise have been lost ; but when he wanted to examine with accuracy the small parts of any substance, he took out the middle glass, and only made use of the eye and object lenses ; " for," he writes, " the fewer the refractions are, the clearer and brighter the object appears." Dr. Hooke also gave us the first and most simple method of finding how much any compound microscope magnifies an object. He placed an accurate scale, divided into very minute parts of an inch, on the stage of the microscope ; adjusted the microscope till the divisions appeared distinct, and then observed with the other eye how many divisions of a rule similarly divided and laid on the stage were included in one of the magnified divisions ; " for if one division, as seen with one eye through the microscope, extends to thirty divisions on the rule, which is seen by the naked eye, it is evident that the diameter of the object is increased or magnified thirty times." An account of Eustachio Divini's microscope was read at the Eoyal Society in 1668. It consisted of an object- lens, a middle glass, and two eye-glasses, which were plano- convex lenses, and were placed so that they touched each other in the centre of their convex surfaces. The tube in which the glasses were enclosed was as large as a man's leg, and the eye-glasses as broad as the palm of the hand. It had four several lengths : when shut up was 1 6 inches long, and magnified the diameter of an object 41 times, at the second length 90, at the third length 111, and at the fourth length 143 times. It does not appear that Divini varied the object-glasses. Philip Bonnani published an account of his two micro- scopes in 1698. Both were compound. The first was HISTORY OF THE MICROSCOPE. 9 similar to that which Mr. Martin published as new, in his Micrographia Nova, in 1712. His second was like the former, composed of three glasses, one for the eye, a middle glass, and an object lens; they were mounted in a cylindrical tube, which was placed in a horizontal position; behind the stage was a small tube with a convex lens at each end; beyond this was a lamp; the whole capable of various adjustments, and regulated by a pinion and rack. The small tube was used to condense the light on to the object A short time before this, Sir Isaac Newton having dis- covered his celebrated theory of light and colours, was led to improve the telescope, and apply his principles most successfully to the construction of a compound reflecting microscope. On the 6th of February, 1672, he communi- cated to the Royal Society his " design of a microscope by reflection." It consisted of a concave spherical speculum of metal, and an eye-glass which magnified the reflected image of any object placed between them in the conjugate focus of the speculum. He also pointed out the proper mode of illuminating objects by artificial light, as he describes it, "of any convenient colour not too much compounded," memo-chromatic. We find other two plans of this kind; the first that of Dr. Robert Barker, and the second that of Dr. Smith. In the latter there were two reflecting mirrors, one concave, and the other convex : the image was viewed by a lens. This microscope, though far from being executed in the best manner, performed, says Dr. Smith, very well, so that he did not doubt it would have excelled others, had it been properly finished. In 1738, Lieberkuhn's invention of the solar microscope was communicated to the public. The vast magnifying power obtained by this instrument, the colossal grandeur with which it exhibited the " minutiae of nature/' the plea- sure which arose from being able to display the same object to a number of observers at the same time, by affording a new source of rational amusement, increased the number of microscopic observers, who were further stimulated to the same pursuits by Mr. Trembley's famous discovery of the polype. The discovery of the wonderful properties of this little animal, together with the works of Mr. Trembley, 10 HISTORY OF THE MICROSCOPE. Mr. Baker, and Mr. Adams, combined to spread the repu- tation of the instrument. In 1742, Mr. Henry Baker, F.R.S., published an ad- mirable treatise on the microscope. He also read several papers before the Royal Society on the subject of his microscopic discoveries. In the wood-cut (fig. 3) at tho end of this chapter we have represented an elegant scroll " pocket microscope with a speculum/' described by him as a new invention. In 1770, Dr. Hill published a treatise, in which he endeavours by means of the microscope to explain the construction of timber, and to show the number, the nature, and office of its several parts, their various arrangements and proportions in the different kinds ; and lie points out a way of judging, from the structure of trees, the uses they will best serve in the affairs of life. M. L. F. Delabarre published an account of his micro- scope in 1777. It does not appear that it was superior in any respect to those that were then made in England. It was inferior to some; for those made by Mr. Adams, in 1771, possessed all the advantages of Delabarre's in a higher degree, except that of changing the eye-glasses. In 1774, Mr. George Adams, the son of the above, im- proved his father's invention, and rendered it useful for viewing opaque as well as transparent objects. This in- strument, made and described by him,1 continued in use up to the time of the invention of the achromatic im- provement, proposed and made in 1815' for Amici, who subsequently gave so much time to the investigation of polarised light, and the adaptation of a polarising apparatus to the microscope. In 1812, Dr. Wollaston proposed a doublet in which the glasses were in contact, under the name of a " Periscopic Microscope." And he says, "with this doublet I have seen the finest strise and serratures on the scales of th3 lepisma and podura, and the scales on a gnat's wing." In the year 1816, Frauenhofer, a celebrated optician of Munich, constructed object-glasses for the microscope of a single achromatic lens, in which the two glasses, although in juxtaposition, were not cemented together: these glasses (1) Microscopical Essays, 1787. HISTORY OF THE MICROSCOPE. 11 were very thick, and of long focus. Although such con- siderable improvements had taken place in the making of achromatic object-glasses since their first discovery by Euler in 1776, we find, even at so late a period as 1821, M. Biot writing, "that opticians regarded as impossible the construction of a good achromatic microscope." Dr. AVollaston also was of the same opinion, " that the com- pound instrument would never rival the single." In 1823, experiments were commenced in France by M. Selligues, which were followed up by Frauenhofer in. Munich, by Amici in Modena, by M. Chevalier in Paris, and by the late Dr. Goring and 'Mr. Tulley in London. To M. Selligues we are indebted for the first plan of making an object-glass composed of four achromatic compound lenses, each consisting of two lenses. The focal length of each object-glass was eighteen lines, its diameter six lines, and its thickness in the centre six lines, the aperture only one line. They could be used combined or separated. A microscope constructed on this principle, by M. Che- valier, was presented by M. Selligues to the Academic de$ Sciences on the 5th of April, 1824. In the same year, and without a knowledge of what had been done on the Con- tinent, the late Mr. Tulley, at the suggestion of Dr. Goring, constructed an achromatic object-glass for a compound microscope of nine-tenths of an inch focal length, com- posed of three lenses, and transmitting a pencil of eighteen degrees ; this was the first that had been made in England. Sir David Brewster first pointed out in 1813, the value of precious stones, the diamond, ruby, garnet, &c., for the construction of microscopes. " The durability," he says, " of lenses made of precious stones is one of their greatest recommendations. Lenses of glass undergo decomposition, and lose their polish in course of time. Mr. Baker found the glass lenses of Leeuwenhoek utterly useless after they became the property of the .Royal Society. The glass articles found in Nimroud were decomposed, while the rock crystal lens was uninjured." Mr. Pritchard at one time made two plano-convex lenses from a very perfect diamond, one the twentieth of an inch focus, which v/aa 12 HISTORY OF THE MICROSCOPE. purchased by the late Duke of Buckingham, and another the thirtieth of an inch focus. In March 1 825, M. Chevalier presented to the Society for the Encouragement of the Sciences an achromatic lens of four lines focus, two lines in diameter, and one line in thickness in the centre. This lens was greatly superior to the one before noticed, which had been made by him for M. Selligues. In 1826, Professor Amici, of Modena, who from the year 1815 to 1824 had abandoned his experiments on the achromatic object-glass, was induced, after the report of Fresnel to the Academy of Science, to resume them; and in 1827 he brought to this country and to Paris a hori- zontal microscope, in which the object-glass was composed of three lenses superposed, each having a focus of six lines and a large aperture. This microscope had also extra eye- pieces, by which the magnifying power could be increased. A microscope constructed on Amici's plan by Chevalier, during the stay of that physician in Paris, was exhibited at the Louvre, and a silver medal was awarded to its maker.1 " While these practical investigations were in progress," says Mr. Boss, "the subject of achromatism engaged the attention of some of the most profound mathematicians in England. Sir John Herschel, Professors Airy and Barlow, Mr. Coddington, and others, contributed largely to the theoretical examination of the subject; and though the results of their labours were not immediately applicable to the microscope, they essentially promoted its im- provement." Mr. Jackson Lister, in 1829, succeeded in forming a combination of lenses upon the theory propounded by these gentlemen, and effected one of the greatest improve- ments in the manufacture of object-glasses, by joining together a plano-concave flint lens and a convex, by means of a transparent cement, Canada balsam. This is desirable (I) In 1855, when the Jury on Microscopes at the Paris Exposition were com- paring the rival instruments, Proi'essor Amici brought a compound achromatic microscope, comparatively of small dimensions, which exhibited certain striae in test objects better than any of the instruments under examination. This superiority was produced by the introduction of a drop of water between the object and the object-glass. HISTORY OF THE MICROSCOPE. 13 to be taken as a basis for the microscopic object-glass : it diminishes very nearly half the loss of light from reflec- tion, which is considerable at the numerous surfaces of a combination ; the clearness of the field and brightness of the picture is evidently increased by doing this; and it prevents any dewiness or vegetation from forming on the inner surfaces. Since this time, Mr. Ross has been con- stantly employed in bringing the manufacture of object- glasses to their greatest perfection, and at length they have attained to their present improved manufacture. Having applied Mr. Lister's principles with a degree of success never anticipated, so perfect were the corrections given to the achromatic object-glass, so completely were the errors of sphericity and dispersion balanced or de- stroyed, that the circumstance of covering the object with a plate of the thinnest glass or talc disturbed the corrections, if they had been adapted to an uncovered object, and rendered an object-glass which was perfect under one condition sensibly defective under the other. Here was another and unexpected difficulty to be over- come, but which was finally accomplished; for in a com- munication made to the Society of Arts in 1837, Mr. Ross stated, that by separating the anterior lens in the combi- nation from the other two, he had been completely suc- cessful. The construction of this object-glass will be illustrated and explained in a future chapter. The rapid improvement in the manufacture of the achromatic compound microscope in this country has been greatly furthered by the spirit of liberality evinced by Sir David Brewster, the late Dr. Goring, Mr. R. H. Solly, and Mr. Bowerbank. To the patronage of Dr. Goring we owe the construction of the first triplet achromatic object-glass, of the diamond lens, and of the improved reflecting instrument of Amici by Cuthbert. The achromatic microscopes now manufactured by our London makers, Mr. Ross, Messrs. Powell and Lealand, and Messrs. Smith and Beck, are unequalled in any part of the world. This opinion is confirmed by the reports of the juries on the Exhibition of Works of Industry of all Nations, 1851; at that time the instruments exhibited by Mr. Ross and Messrs. Smith and Beck, by far excelled 14- HISTORY OF THE MICROSCOPE. those of all other countries. Messrs. Powell and Lealand's microscope, with object-glasses, was selected by the Royal Society as the best against all competitors. See Juries' Reports for much interesting matter on this subject; article " Microscope" Penny Cyclopaedia, by Mr. Ross ; Practical Treatise on the Microscope, by Professor Quekett; Sir David Brewster's Treatise on the Microscope; Dujardin's Observateur; Maudl, Traite pratique du Microscope: Dr Robin-, Du Microscope) &c. Fig. 3.— Baker's Scroll or Pocket Microscope, and the Modern Compound Microscope of W. Ladd's manufacture. CHAPTER II. MECHANICAL AND OPTICAL PRINCIPLES INVOLVED IN THE CONSTRUCTION OF THE MICROSCOPE— LENSES— MODE OF ESTIMATING- THEIR POWER, ETC. — ACHROMATIC LENSES — MAGNIFYING POWER — WOLLASTON'S DOUBLET — CODDINGTON S LENS — ROSS'S SIMPLE AND COMPOUND MICROSCOPES — MICROMETERS, ETC. *i N the construction of the modern mi- croscope, optical and mechanical prin- ciples of some importance are involved. These principles we shall briefly ex- plain, together with the more recent improvements effected in the instru- ment generally.1 The microscope depends for its utility and operation upon concave and convex lenses, and the course of the rays of light passing through them. Lenses are usually defined as pieces of glass, or other transparent substances, having their two surfaces so formed that the rays of light, in passing through them, have their direction changed, and are made to converge or diverge from their original parallelism, or to become parallel after converging or diverging. When a ray of light passes in an oblique direction from one transparent medium to another of a different density, the direction of the ray is changed both on entering and leaving; this influence is the result of the well-known law of refraction, — that a ray of light passing from a rare into a dense medium is refracted towards the perpendicular, and vice versd. (1) For a full explanation of the laws of optics, and their application to the construction of lenses, the reader is referred to Dr. Bird and Mr. Brooke's i; Manual of Natural Philosophy," Professor Potter's "Elementary Treatise on Optics," Sir David Brewster's " Optics," &c. 16 CONSTRUCTION OP THE MICROSCOPE. Dr. Arnott remarks : " But for this fact, which to many persons might at first appear a subject of regret, as pre- venting the distinct vision of objects through all trans- parent media, light could have been of little utility to man. There could have been neither lenses, as now ; nor any optical instruments, as telescopes and microscopes, of which lenses form a part ; nor even the eye itself. Rays of light falling perpendicularly upon a surface of glass or other transparent substance, pass through without being bent from the original line of their direction. Thus, if a ray pass from k perpendicularly to the surface of the piece of glass at e (fig. 4), it will go on to h in the right line k e o g h. But if the same ray be directed to the surface e obliquely, as from a, instead of passing through in a direct line to b in the direction aemb, it will be refracted to d, in a direction approaching nearer to the perpendicular line k h. The ray a e is termed the ray of incidence, or the incident ray ; and the angle a e Jc which it makes with the perpendicular k h is called the angle of incidence. That part of the ray from e to d passing through the transparent medium is called the ray of refraction, or the refracted ray; and the angle deg which it makes with the CONSTRUCTION OF THE MICROSCOPE. 17 perpendicular is called the angle of refraction. The ray projected from a to e and refracted to d, in passing out of the transparent medium as at d, is as much bent from the line of the refracted ray e d as that was from the line of the original ray a e b ; the ray then passes from d to c, parallel to the line of the original ray a e b. It follows, then, that any ray passing through a transparent medium, whose two surfaces, the one at which the ray enters, and the one at which it passes out, are parallel planes, is first refracted from its original course j but in passing out is bent into a line parallel to, and running in the same direc- tion as the original line, the only difference being, that its course at this stage is shifted a little to one side of that of the original. If from the centre e a circle be described with any radius, as d e, the arc a a' measures the angle of incidence a e Jc, and the arc g' d the angle of refraction g e d. A line a Jc drawn from the point a perpendicular co k h is called the sine of the angle of incidence; and the line d g drawn from the point d perpendicular to Jc h is called the sine of the angle of refraction. From the con- clusions drawn from the principles of geometry, it has been found, that in any particular transparent substance the sine of the angle of incidence a k has always the same ratio to the sine d g of the angle of refraction, no matter what be the degree of obliquity with which the ray of inci- dence a e is projected to the surface of the transparent medium. If the ray of incidence passes from air obliquely into water, the sine of incidence is to that of refraction as 4 to 3 ; if it passes from air into glass, the proportion is as 3 to 2 j and if from air into diamond, it is as 5 to 2. By the help of glasses of certain forms, we unite in the same sensible point a great number of rays proceeding from one point of an object ; and as each ray carries with it the image of the point from whence it proceeded, and all the rays united must form an image of the object from whence they were emitted, this image is brighter in pro- portion as there are more rays united, and more distinct in proportion as the order in which they proceeded is better preserved in their union. The point at which parallel rays meet after converging through a lens is called the principal focus, and its distance from the middle of 18 CONSTRUCTION OF THE MICROSCOPE. the lens the focal length. The radiant point and its image after refraction are called conjugate foci. These foci vary according to the distance of the radiant points. In every lens the right line perpendicular to the two surfaces is called the axis of the lens, and is seen in the annexed figure ; the point where the axis cuts the surface is called the vertex of the lens. Fig. 5 is intended to represent the different forms of lenses in use : a is a plane glass of equal thickness Fig. 5. throughout ; b, a meniscus, concave on one side, convex on the other j c} a double-concave ; d, a plano-concave ; e, a double-convex ; /, a plano-convex. The lenses employed in the construction of microscopes are chiefly convex; concave lenses being only used to make certain modifications in the course of the rays passing through those of a convex form, whereby their perform- ance is rendered more exact. In accordance with the laws of refraction, when a pencil of parallel rays, passing through the air, impinges upon a convex surface of glass, the raj^s are made to converge ; for they will be bent towards the centre of the circle, the radius being perpen- dicular to each point of curvature. Parallel rays, falling on a plano-convex lens, are brought to a focus at the dis- tance of its diameter ; and conversely, rays diverging from that point are rendered parallel. Plano-convex lenses pos- sess properties which render them valuable in the con- struction of microscopes. Parallel rays, falling on a double-convex lens are brought to a focus in the centre of its diameter ; conversely, rays CONSTRUCTION OF THE MICROSCOPE. 19 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 refraction) 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 refracting power than the former. For all ordinary practical purposes, we may consider the principal focus — as the focus 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. Rays already converging are brought together at a point nearer than the principal focus : whereas rays diverging from a point within the principal focus are ren- dered still more diverging, though in a diminished degree. Rays diverging from points more distant than the prin- cipal 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 pro- duct by half the sum of the radii. The refracting influence of concave lenses will be pre- cisely the opposite of that of convex. Rays 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 plano-concave lens, at the distance of the diameter of the sphere of curvature ; and for a double-concave, in the centre of that sphere. If a lens be convex on one side and concave on the other, c 2 20 CONSTRUCTION OF THE MICROSCOPE. forming what is termed a meniscus, its effect will depend upon the proportion between the two curvatures. The rules by which the foci of all lenses may be found, will be more advantageously studied in works on Optics. As each ray carries with it the image of the object from whence it proceeded, it follows, that if those rays, after intersecting each other, and having formed an image at their intersection, are again united by refraction or re- flection, they will form a new image, and that repeatedly, so long as their order is not disturbed. It follows, also, that when the course of the luminous ray through several lenses is under consideration, we may look on the image first produced as an object in reference to the second lens, and may consider the second image as produced by this object, and so on successively. This is, indeed, a principle involved in the adaptation of lenses to magnifying objects ; and in fig. 6, it is seen that if the point of light be situated Fig. 6. above the line of the axis, the focus will then be below it, and vice versd; but the surface of every luminous body may be regarded as comprehending an infinite number of such points, from all of which a pencil of light- rays proceeds, and is refracted according to the general law ; so that a perfect but inverted image or picture of the object is formed upon any surface placed in the focus, and adapted to receive the rays. If any object be placed at twice the CONSTRUCTION OP THE MICROSCOPE. 21 distance of the principal focus, the image being formed at an equal distance on the other side of the lens, will be of the same dimensions with the object, as in fig. 7 ; but if Fig. 7. the object be placed nearer to the lens, the image will be farther from it, and of larger dimensions, as in fig. 8 ; and, Fig. 8. on the other hand, if the object be farther from the lens, the image will be nearer to it, and smaller than itself. But it is to be observed, that the larger the image is in proportion to the object, the less bright it will be, because the same amount of light has to be spread over a greater surface ; whilst a smaller image will be much more brilliant. Aberration of Lenses. — Although the image of an object produced by the convex lens, fig. 8, appears at first view to be an exact reproduction of the object, it is found, when submitted to rigorous examination, to be more or less confused and indistinct : which is augmented when viewed in a microscope. This indistinctness and confusion arises from two causes, one depending on form, and the other on the material of the lens. That which depends on the form of the lens we shall now proceed to explain. 22 CONSTRUCTION OF THE MICROSCOPE. In optical instruments the curvature of the lenses employed is spherical, that being the only form which can be given by grinding with the requisite degree of truth. But convergent lenses, 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. 9. The rays a a, Fig. 9. which pass through the lens near its circumference, it is seen to be more refracted, or come to a focus at a shorter distance behind it than the rays 6 6, which pass through near its centre or axis, and are less refracted. The conse- quence of this defect of lenses with spherical curvatures, which is called spherical aberration, is that a well defined image or picture is not formed by them, for when the object is focused, for the circumferential rays, the picture projected to the eye is rendered indistinct 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, now fall in a state of divergence, forming a circle of dissipation. The grosser effects of this 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 diaphragm, so that the central rays, b b, only are concerned in the formation of the picture. This defect is reduced to a minimum, by CONSTRUCTION OF THE MICROSCOPE. 23 using the meniscus form of lens, which is the segment of an ellipsoid instead of a sphere. The ellipse and the hyperbola are curves of this kind, in which the curvature diminishes from the central ray, or axis, to the circumference b ; and mathematicians have shown how spherical aberration may be entirely removed by lenses whose sections are ellipses or hyperbolas. For this curious discovery we are indebted to Descartes. If a I, a I', for example, fig. 10, be part of an ellipse Fig. 10. whose greater axis is to the distance between its foci // as the index of refraction is to unity, then parallel rays r I', r" I incident upon the elliptical surface I', 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 a I' round / as a centre, for the second surface of the lens I ' I. As all the rays refracted at the surface I a I' converge accurately to /, and as the circular surface I a V is perpen- dicular to every one of the refracted rays, all these rays will go on to/ with out suffering any refraction 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. 24 CONSTRUCTION OF THE MICROSCOPE. In like manner, a concavo-convex lens, fig. 11, ll\ whose concave surface I a I' is a circle described round the farther Fig. 11. focus of the ellipse, will cause parallel rays b I, b lf, to diverge in directions I r, I1 V, which, when continued back- wards, will meet exactly in the focus/ which will be its virtual focus. If a plano-convex lens, fig. 12, has its convex surface I a lf part of a hyperboloid, formed by the revolution of a Fig. 12. hyperbola whose greater axis is to the distance between the foci as unity is to the index of refraction, then parallel rays rl, r" I1 falling perpendicularly on the plane surface, will be refracted without aberration to the further focus of the hyperboloid. The same property belongs to a plano-concave lens having a similar hyperbolic surface, and receiving parallel rays on its plane surface.1 (1) It must be borne in mind, that in none of those lenses would the object be correctly seen in focus, except at the one point known as the mathematical or geometrical axis of the lens. CONSTRUCTION OF THE MICROSCOPE. 25 When the convex side of a plano-convex lens is exposed to parallel rays, the distance of the focus from the plane side will be equal to twice the radius of its convex surface diminished by two-thirds of the thickness of the lens ; but when the plane is exposed to parallel rays, the distance of the focus from the convex side will be equal to twice the radius. A meniscus with spherical surfaces, fig. 13, has the property of refracting all converging rays to its focus, if Fig. 13. its first surface is convex, provided the distance of the point of convergence or divergence from the centre of the first surface is to the radius of the first surface as the index of refraction is to unity. Thus, if m I, V n is a meni- scus, and r I, r I' rays converging to the point e, whose distance e c from the centre of the first surface la I' of the meniscus is to the radius c a, or c Z, as the index of refrac- tion is to unity, that is as 1-500 to 1 in glass ; then if /is the focus of the first surface, describe, with any radius less than / a, a circle m a' n for the second surface of the lens. Now it will be found by projection, that the rays r I, r I', whether near the axis a e or remote from it, will be re- fracted accurately to the focus/; and as all these rays fall perpendicularly on the second surface m n, they will still pass on, without refraction, to the focus/. In like man- ner, it is obvious that rays /£,/£', diverging from /will 26 CONSTRUCTION OF THE MICROSCOPE. be refracted into r I, r l'} which diverge accurately from the virtual focus.1 Spherical aberration is not so much connected with the focal length of the lens as depending on the relative con- vexity of its surfaces, and is much reduced by observing a certain ratio between the radii of its anterior and pos- terior surfaces ; thus the spherical aberration of a lens, the radius of one surface of which is six or seven times greater than that of the other, as in fig. 14, is very much Fig. 14. less when its more convex surface is turned forward to receive parallel rays, than when its less con vox surface is turned forwards. This is still better effected, or even got rid of altogether, by using combinations of lenses, so disposed that their opposite aberrations shall correct each other, whilst mag- nifying power is gained. For it is seen that, as the aber- ration of a concave lens is just the opposite of that of a convex lens, the aberration of a convex lens placed in its most favourable position may be corrected by a concave lens of much less power in its most favourable position. This is the principle of a combination proposed by Sir John F. W. Herschel, fig. 15, an " aplanatic doublet," consisting of a double-convex lens and a meniscus ; a doublet of this kind is found extremely useful and available for micro- scopic purposes : it affords a large field, like the Coddiugton lens. Chromatic aberration. — Another and serious difficulty arises, in the unequal refrangibility of the different fl) Brewster's "Treatise on Optics." Fig. 15. CONSTRUCTION OF THE MICROSCOPE. 27 coloured rays which together make up white light, so that they are not all brought to the same focus, even by a lens free from spherical aberration. It is, indeed, this differ- ence in their refrangibility which causes their complete separation by the prism into a spectrum. The correction of chromatic with spherical aberration is effected in a most ingenious manner, by combining a con- vex 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 common lens, we see that it is bordered with the colours of the rainbow ; or if we look through a common magnifying-glass at the letters on the title-page of a book, we see them slightly coloured at their edges in the same manner. The cause of this iridescent border is that the primitive rays — red, yellow, and blue,— of which a colourless ray of light is com- posed, are not all equally refrangible. Hence they are not all brought to one point or focus, but the blue rays being the most refrangible, come to a focus nearer the lens than the yellow ones, which are less refrangible, and the yellow rays than the red, which are the least refrangible. Thus, in fig. 16, chromatic aberration proves still more BLUE YELLOW detrimental to the distinct definition of images formed by a lens, than spherical aberration. This arises more from the size of the circles of dissipation, than from the iri- descent border, and it may still exist, although the spherical aberration of the lens be altogether corrected. Chromatic aberration is, as before stated, corrected by combining, in the construction of lenses, two media of opposite form, and differing from each other in the proportion in which they 28 CONSTRUCTION OP THE MICROSCOPE. respectively refract and disperse the rays of light ; so that the one medium may, by equal and contrary dispersion, neutralize the dispersion caused by the other, without, at the same time, wholly neutralizing its refraction. Remark- able enough, the media found the most valuable for such a purpose should be the combination of pieces of crown and flint glass, of crown-glass whose index of refraction is 1-519, and dispersive power 0-036, and of flint-glass whose index of refraction is 1-589, and dispersive power 0-0393. The focal length of the convex crown-glass lens must be 4^ inches, and that of the concave flint-glass lens 7f inches, the combined focal length of which is 10 inches. The following fig. 17 will serve us to explain how a ray of light is brought to a single focus, free from colour. Fig. 17. In this diagram, L L is a convex lens of crown-glass, and I I a concave one of flint-glass. A ray of light (s) falling at F on a convex lens, will refract it exactly in the same manner as the prism ABC, whose faces touch the two surfaces 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 (P T) on a screen or wall, had there been no other lens, the violet ray (F v) crossing the axis of the lens at v, and going to the upper end (P) of the spectrum ; and the red ray (F R) going to the lower end (T). But, as the flint-glass lens (I I) or the prism A a c, which receives the rays F v, F R, at the same points, is interposed, these rays will be united at /, and form a small circle of white light, the ray (s F) being now refracted without colour from its primitive direction CONSTRUCTION OF THE MICROSCOPE. 29 (s F Y) into the new direction (F/). In like manner, the corresponding ray (s' F') will be refracted to /, and a white and colourless image there formed by the two lenses. The Magnifying Power of Lenses. — To assist us in gain- ing a clearer notion of the mode in which a single lens serves to magnify minute objects, it is necessary to take a passing glance at the ordinary phenomena of vision. The human eye is so constituted, that it can only have distinct vision when the rays falling upon it are parallel or slightly divergent ; because the retina, on which the image im- pinges, requires the intervention of the crystalline lens to bring the rays to an accurate focus upon its surface. The limit of distinct vision is generally estimated at from six to ten inches; objects viewed nearer, to most persons, become indistinct, although they may be larger. The apparent size of an object is, indeed, the angle it sub- A tends to the eye, or the angle formed by two lines drawn from the centre of the eye to the extremity of the object. This will be understood upon reference to fig. 1 8. The lines drawn from the eye to A and R form an angle, which, when the distance is small, is nearly twice as great as the angle from the eye to o w, formed by lines drawn at twice the distance. The arrow at A R will therefore appear nearly twice as long as o W, being seen under twice the angle ; and in the same proportion for any greater or lesser differ- ence in distance. This, then, is called the angle of vision^ or the visual angle. Now the utility of a convex lens interposed between a near object and the eye consists in its reducing the divergence of the rays forming the several pencils issuing from it ; so that they enter the eye in a state of moderate divergence, as if they had issued from an object beyond the nearest limit of distinct vision ; and a well-defined image is consequently formed upon the retina. In fig. 19, a double-convex lens is placed before 30 CONSTRUCTION OF THE MICROSCOPE. the eye, near which is a small arrow, to represent the object under examination ; and the cones drawn from it Fig. 19. are portions of the rays of light diverging from those points and falling upon the lens. These rays, if permitted to fall at once upon the pupil, would be too divergent to allow of their being brought to a focus upon the retina by the dioptric media of the eye. But being first passed through the lens, they are bent into nearly parallel lines, or into lines diverging from some points within the limits of distinct vision. Thus altered, the eye receives them precisely as if they had emanated directly from a larger aiTow placed at ten inches from the eye. The difference between the real and the imaginary arrow is called the magnifying power of the lens. The object, when thus seen, appears to be magnified nearly in the proportion which the focal distance of the lens bears to the distance of the object when viewed by the unassisted eye ; and is entirely owing to the object being distinctly viewed so much nearer to the eye than it could be without the lens.1 With these preliminary remarks as to the medium by which microscopic power is obtained, we shall proceed to apply them to the construction of a perfect instrument. The Microscope. — A microscope, as we have before ex- plained, may be either a single, simple, or a compound (1) " The Magnifying Power of Short Spaces " has been most ably elucidated by John Gorham, Esq. M^R.C.S. See Journal of Microscopical Society, October. 1894. CONSTRUCTION OF THE MICROSCOPE. 31 instrument. The simple microscope may consist of one, as seen in fig. 1 9, or of two or three lenses ; but these latter are so arranged as to have the effect only of a single lens. In the compound microscope, not less than two lenses must be employed : one to form an inverted image of the object, which, being the nearest to the object, is called the object-glass; and the other to magnify this image, and from being next the eye of the observer, called the eye-glass. Both these may be formed out of a com- bination of lenses, as will be hereafter seen. We have hitherto considered a lens only in reference to its enlargement of the object, or 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 we have before pointed out, by the combination of two or more lenses instead of one, thus reducing the angles of incidence and refraction. The first satisfactory arrange- ment for this purpose was the invention of the celebrated Dr. Wollastou. His doublet (fig. 20) consisted of two plano-convex lenses having their focal lengths in the pro- portion of one to three, or nearly so, and placed at a distance which can be ascertained best by actual expe- riment. Their plane sides are placed towards the object, and the lens of shortest focal length next the object. It appears that Dr. Wollaston was led to this invention by considering that the achromatic Huyghenean eye- piece, which will be presently described, would, if reversed, possess similar good properties as a simple microscope. But it will be evident, when the eye-piece is understood, that the circumstances which render it achromatic are very imperfectly applicable to the simple microscope, and that the doublet, without a nice adjustment of the stop, vrould be valueless. Dr. Wollaston makes no allusion to a stop, nor is it certain that he contemplated its intro- duction ; although his illness, which terminated fatally soon after the presentation of his paper to the Eoyal Society, may account for the omission. The nature of the corrections which take place in the doublet is explained in the annexed diagram, where lo I' is 32 CONSTRUCTION OF THE MICROSCOPE. the object, p a portion of the cornea of the eye, and d d the stop, or limiting aperture. Fig. 20. Now it will be observed that each of the pencils of light from the extremities 1 11 of the object is rendered excentri- cal by the stop ; consequently, each passes through the two lenses on opposite sides of their common axis op; thus each becomes affectedly opposite errors, which to some ex- tent balance and correct each other. To take the pencil Z, for instance, which enters the eye at r b, r b: 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 rays more than the red, and moreover as the blue rays fall CONSTRUCTION OF THE MICROSCOPE. 33 nearer the margin of the second lens, where the refraction, being more powerful than near the centre, compensates in some degree for the greater focal length of the second lens, the blue rays will emerge very nearly parallel, and of consequence colourless to the eye. At the same time, the spherical aberration has been diminished by the circumstance that the side of the pencil which passes one lens nearest the axis passes the other nearest the margin. This explanation applies only to the pencils near the extremities of the object. The central pencils, it is obvious, would pass both lenses symmetrically, the same portions of light occupying nearly the same relative places on both lenses. The blue light 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 compensa- tion would take place, quite different in principle, and inferior in degree, to that which is produced in the excen- trical 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° without any very sensible errors. It exhibits, therefore, many of the usual test-objects in a very beautiful manner. The next step in the improvement of the simple micro- scope bears more relation to the eye-piece ; this was effected by Mr. Holland : it consists in substituting two lenses for the first in the doublet, and retaining the stop between them and the third. The first bending being thus effected by two lenses instead of one, is accompanied by smaller aberrations, which are, therefore, more com- pletely balanced or corrected at the second bending, in the opposite direction, by the third lens. Hand Magnifiers, — Before we proceed further, it will be as well to bestow a passing notice on the simple hand magnifier, so often employed by microscopists in the pre- liminary examinations of objects. A very good form of lens was proposed by Dr. Wollaston^ and called by him the Periscopic lens : which consisted of 34 CONSTRUCTION OF THE MICROSCOPE. two hemispherical lenses cemented together by their plane faces, having a stop between them to limit the aperture. A similar proposal was made by Sir David Brewster in 1820, who, however, executed the project in a better man- ner, by cutting a groove in a whole sphere, and filling the groove with opaque matter. His lens, which is better known as the Coddington lens,1 is shown at fig. 21 : it gives a large field of view, which is equally good in all directions, as it is evident that the pencils a b and b a pass Fig. 21. ' Fig. 22 through under precisely the same circumstances. Its spherical form has the further advantage of rendering the position in which it is held of comparatively little conse- quence. It is therefore very convenient as a hand magni- fier ; but its definition is, of course, not so good as that of a well-made doublet or achromatic lens. It is generally set in a folding case, as represented in the figure, and so contrived that it is admirably adapted for the waistcoat- pocket; which, together with the small holder, fig. 22, for (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." CONSTRUCTION OF TUB MICROSCOPE. 35 securing small objects and holding them during examina- tion, are all that is required for afield instrument during a day's ramble. This useful little holder may be purchased in a case at Mr. Weedon's, 41, Hart-street, Bloomsbury. The Stanhope lens is similarly constructed, although not so good and convenient as the former, and is but seldom to be purchased properly made. When the magnifying power of a lens is considerable, 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 accuracy and steadiness by the unassisted hand, but 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 microscope ; and it is furnished, accord- ing to circumstances, with lenses and mirrors to collect and reflect the light upon the object, with other conve- niences. Fig. 23. — Ross's Simple Microscope. The best of the kind was that contrived by Mr. Roes, represented in fig. 23 ; and consists of a circular foot e, from which rises a short tubular stem d} into which D2 36 CONSTRUCTION OF THE MICROSCOPE. 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 allowing 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 diminished. 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. Another highly-useful and more complete simple micro- scope was contrived by Mr. W. Valentine, and made for him by Mr. Eoss in 1831. It is thus described by the latter gentleman, and is represented in fig. 24. It is sup- ported on a firm tripod, made of bell-metal, the feet of which, a a a, are made to close up for the purpose of pack- ing it in a box. The firm pillar b rises from the tripod, and carries the stage e\ this is further strengthened by the two supports rr. From the pillar a triangular bar d, and a triangular tube c, is moved up and down by a screw, having fifty threads in the inch, and turned by a large milled head v, which is situated at the base of the pillar : this is the fine adjustment. The small triangular base d is moved up and down within the triangular bar c, by turning the milled head t, forming the coarse adjustment : this bar carries the lens-holder mnop. The stage e con- sists of three plates ; the lowest one is firmly attached to the pillar, and upon this the other two work. The upper one carries a small elevated stage #, on which the objects are placed • this stage is mounted on a tube f, and has a spring clip h, for holding, if necessary, the objects under examination. By means of two screws placed diagonally, one of which is seen at s, this elevated stage can be moved CONSTRUCTION OF THE MICROSCOPE. 37 in two directions, at right angles to one another; and thus different parts of objects can be brought successively into Fig- 24.— Valentine's Microscope. the field of view. The arm np, for carrying the lenses, is attached to the triangular bar d by a conical pin, on which it is made to turn horizontally, and the arm itself can be lengthened or shortened by means of the rack and pinion m o ; hence the lens q can be applied to every part of an object without moving the stage. The mirror / is fitted into the largest of the three legs, and consists of a concave and plane glass reflector. To the under side of the stage is fitted a Wollaston's condenser k ; and the lens is made to slide up and down by means of two small handles projecting from the cell in which the lens is set. Two small tubes i, with either a condensing lens for opaque objects, or a pair of forceps, may be attached to this side of the stage. The magnifiers are 38 CONSTRUCTION OF THE MICROSCOPE. either simple lenses or doublets ; or it could be easily con- verted into a compound microscope by inserting a com- pound body, supported on a bent arm, in the place of the one carrying the single lenses. An arrangement devised by Professor Quekett, for a dissecting microscope, represented in fig. 25, is one of value Fig. 25.—Quekett's Simple Microscope, and convenience. The instrument is made by Mr. Ladd of Chancery Lane, and is furnished by him with three magnifiers, namely, an inch, and half-inch, ordinary lenses, and a quarter-inch Coddington ; these will be found to be the powers most useful for the purposes to which this instrument is specially adapted. The lenses, mirror, con- denser, vertical stem, &c., all fit into hollows cut for their reception on the under side of the stage, and are then covered and kept in place by the side flaps : so that, when packed together, and the flaps kept secure by an India rubber band, the instrument is very conveniently portable. The size and firmness of the stage afford great facilities for dissection, and other scientific investigations. THE COMPOUND MICROSCOPE. — The compound microscope may, as before stated, consist of only two lenses, while a simple microscope has been shown to contain sometimes three. In the triplet for the simple microscope, however, it was explained that the object of the first two lenses was CONSTRUCTION OF THE MICROSCOPE. 39 / to do what might have been accomplished, though not so well, by one ; and the third merely effected certain modi- fications in the light before it en- tered the eye. But in the com- ^ pound microscope the two lenses have totally different functions : the first receives the rays from the ob- ject, and bringing them to new foci, forms an image, which the second lens treats as an original object, and magnifies it just as the single mi- croscope magnified the object itself. Fig. 26 shows the earliest form of the compound microscope, with the magnified image of a fly, as given by Adams, which he describes as consisting of an object-glass, In, a field glass de, and an eye-glass, / wards and the other downwards ; and even though the aberrations should be perfectly effaced, the superposition of such displaced images would effectually destroy the efficiency of the instru- ment. It should also be so accurate, that the optical axis of the instrument should not be in the least altered by movement in a vertical direction ; so that, if an object be brought into the centre of the field with a low power, and a higher power be then substituted, it should be found in the centre of its field, notwithstanding the great alteration in focus. Fig. 31 represents the body of one of Mr. Boss's compound microscopes with the triple object-glass, where o is an object; and above it is seen the triple achromatic object-glass, in con- nection with the eye-piece e e, ff the plano-convex lens ; e e being the eye- glass, and // the field-glass, and be- tween them, at b 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 b, would form an image at a a; but as they meet with the lens // in their passage, they are CONSTRUCTION OF THE MICROSCOPE. 47 converged by it and meet at b b, where the diaphragm is placed to intercept all the light except that required for the formation of a perfect image ; the image at b b is farther magnified by the lens e ey as if it were an original object. The triple achromatic combination constructed on Mr. Lister's improved plan, although capable of trans- mitting large angular pencils, and corrected as to its own errors of spherical and chromatic aberration, would, never- theless, be of little service without an eye -piece of peculiar construction. The eye-piece, which up to this time is considered to be the best to employ with achromatic object-glasses, to the performance of which it is desired to give the greatest possible effect, is described by Mr. Cornelius Varley, in the fifty-first volume of the Transactions of the Society of Arts. The eye-piece in question was invented by Huyghens for telescopes, with no other view than that of diminishing the spherical aberration by producing the refractions at two glasses instead of one, and of increasing the field of view. It consists of two plano-convex lenses, with their plane sides towards the eye, and placed at a distance apart equal to half the sum of their focal lengths, with a stop or diaphragm placed midway between the lenses. Huyghens was not aware of the value of his eye-piece ; it was reserved for Boscovich to point out that he had, by this important arrangement, accidentally corrected a great part of the achromatic aberration. Let fig. 32 represent the Huyghenian eye-piece of a microscope, // 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, corres- ponding with the colours of the prismatic spectrum. This order of colours is the reverse of that of the common com- pound microscope, in which the single object-glass projects the red image beyond the blue. 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 48 CONSTRUCTION OF THE MICROSCOPE. 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 dis- tinctly seen by a convex eye-lens, and this is a further defect of the compound microscope of two lenses. But Fig. 32. the field-glass, at the same time that it bends the rays and converges them to foci at b' b1 and / r', also reverses the curvature of the images as there shown, and gives them the form best adapted for distinct vision by the eye-glass e e. The field- glass has at the same time brought the blue and red images closer together, so that they are adapted to CONSTRUCTION OF THE MICROSCOPE. 49 pass uncoloured through the eye-glass. To render this important point more intelligible, let it be supposed that the object-glass had not been over-corrected, that it had been perfectly achromatic ; the rays would then have become coloured as soon as they had passed the field-glass ; the blue rays, to take the central pencil, for example, would converge at 6", and the red rays at r, which is just the reverse of what the eye-lens requires ; for as its blue focus is also shorter than its red, it would demand rather that the blue image should be at r", and the red at b". This effect we have shown to be produced by the over- correction of the object-glass, which protrudes the blue foci b b as much beyond the red foci r r as the sum of the distances between the red and the blue foci of the field-lens and eye-lens; so that the separation b r is exactly taken up in passing through those two lenses, and the whole of the colours coincide as to focal distance as soon as the rays have passed the eye-lens. But while they coincide as to distance, they differ in another respect, — the blue images are rendered smaller than the red by the superior refractive power of the field-glass upon the blue rays. In tracing the pencil I, for instance, it will be noticed that, after passing the field-glass, two sets of lines are drawn, one whole and one dotted, the former repre- senting the red, and the latter the blue rays. This is the accidental effect in the Huyghenian eye-piece pointed out by Boscovich. The separation into colours of the field- glass is like the over-correction of the object-glass, — it leads to a subsequent complete correction. For if the differently coloured rays were kept together till they reached the eye-glass, they would then become coloured, and present coloured images to the eye ; but fortunately, and most beautifully, the separation effected by the field- glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to the spherical figure, the refractive power is less than at the margin, that that spherical error of the eye-lens constitutes a nearly perfect balance to the chromatic dispersion of the field-lens, and the blue and red rays I' and I" emerge sensibly parallel, presenting, in consequence, the perfect defini- tion of a single point to the eye. The same reasoning E CONSTRUCTION OP THE MICROSCOPE. isv- true of the intermediate colours and of the other pencils. The eye-glass e e not only brings together the images b' b', r r, but it likewise has the most important effect of rendering them flat, thus at once correcting both the chromatic and spherical aberration. The Huyghenian eye-piece, which we have described, is the best for merely optical purposes ; but when it is required to measure the magnified image, i we use the eye-piece invented by Mr. Ramsden, and called by him the micrometer eye-piece. The arrangement may be readily understood upon reference to fig. 33. The field-glass has now its plane face turned towards the object ; the rays from the ob- ject are made to converge immediately in front of the field-glass; and here is \ laced a plane-glass, on which are engraved divisions of 1- 100th of an inch or less. The markings of these divisions come into focus, therefore, at the same time as the image of the object, and both are distinctly seen together. The glass with its divisions is shown in fig. 34, on which, at A, are seen some magnified grains of starch. Thus the measure of the magnified image is given by mere inspection ; and the value of such measurements, Fig. 34. in reference to the real object, when once obtained, is con- stant for the same object-glass. It is affirmed by Mr. Ross, that if the achromatic prin- ciple were applied to the construction of eye-pieces, the latter is the form with which the greatest perfection would be obtained. That such an adaptation might be produc- tive of valuable results, appears from Mr. Brooke's state- ment, that he has employed as an eye-piece, a triplet objective of one inch focus, the definition obtained by it being superior to that afforded by the ordinary Huyghe- iiian eye-piece. Some of the lowest French achromatic CONSTRUCTION OF THE Mn/rfOSCOPE. 51 object-glasses answer extremely well for this purpose ; and as the sets are usually made removable, the front pair can be readily separated for the experiment. Mr. Lister places on the stage of his microscope a divided scale, the value of which is known ; and viewing the scale as the microscopic object, observes how many of the divisions on the scale attached to the eye-piece corre- spond with one of those in the magnified image. If, for instance, ten of those in the eye-piece correspond with one of those in the image, and if the divisions are known to be equal, then the image is ten times larger than the object, and the dimensions of the object are ten times less than indicated by the micrometer. If the divisions on the micrometer and on the magnified scale are not equal, it becomes a mere rule-of-three sum ; but in general this trouble is taken by the maker of the instrument, who furnishes a table showing the value of each division of the micrometer for every object-glass with which it may be used. Mr. Jackson invented the simple and cheap form of micrometer, represented in fig. 35, which he described in the Microscopical Society's Transactions, 1840. It consists of a slip of glass placed in the focus of the eye- glass, with the divisions sufficiently fine to have the value of the ten-thousandth of an inch with the quarter-inch object-glass, and the twenty-thousandth with the eighth ; at the same time the half, or even the quarter of a division may be estimated, thus affording the means of attaining all the accuracy that is really available. It may therefore entirely supersede the more complicated and expensive screw-micrometer, being much handier to use, and not liable to derangement in inexperienced hands. The positive eye-piece gives the best view of the micro- meter, the negative of the object. The former is quite free from distortion, even to the edges of the field ; but the object is slightly coloured. The latter is free from colour, but is slightly distorted at the edges. In the centre of the field, however, to the extent of half its diameter, there is no perceptible distortion ; and the clearness of the definition gives a precision to the measure- E 2 52 CONSTRUCTION OF THE MICROSCOPE. ment which is very satisfactory. For this reason Mr, Jackson gives it the preference. Fig. 35. — Mr. Jackson's Micrometer eye-piece. Short bold lines are ruled on a piece of glass, a, fig. 35 ; and to facilitate counting, the fifth is drawn longer, and the tenth still longer, as in the common rule. Very finely levigated plumbago is rubbed into the lines, to render them visible ; and they are covered with a piece of thin glass, cemented by Canada balsam, to secure the plumbago from being wiped out. The slip of glass thus prepared is placed in a thin brass frame, so that it may slide freely ; and is acted on at one end by a pushing-screw, and at the other by a slight spring. Slips are cut in the negative eye-piece on each side, b, so that the brass frame may be pressed across the field in the focus of the eye-glass, as at m; the cell of which should have a longer screw than usual, to admit of adjust- CONSTRUCTION OF THE MICROSCOPE. 53 ment for different eyes. The brass frame is retained in its place by a spring within the tube of the eye-piece ; and in using it the object is brought to the centre of the field by the stage movements ; and the coincidence between one side of it and one of the long lines is made with great accuracy by means of the small pushing-screw that moves the slip of glass. The divisions are then read off as easily as the inches and tenths on a common rule. The operation, indeed, is nothing more than the laying a rule across the body to be measured; and it matters not whether the object be transparent or opaque, mounted or not mounted, if its edges can be distinctly seen, its diameter can be taken. Previously, however, to using the micrometer, the value of the divisions should be ascertained with each object- glass ; the mode of doing which is best performed as follows : — " Lay a slip of ruled glass on the stage ; and having turned the eye-piece so that the lines on the two glasses are parallel, read off the number of divisions in the eye- piece which cover one on the stage. Eepeat this process with different portions of the stage-micrometer, and if there be any difference, take the mean. Suppose the hundredth of an inch on the stage requires eighteen divi- sions in the eye-piece to cover it ; it is quite plain that an inch would require eighteen hundred, and an object which occupied nine of these divisions would measure the two- hundredth of an inch. This is the common mode of expressing microscopical measurements ; but I am of opinion that a decimal notation would be preferable, if universally adopted. " Take the instance supposed, and let the microscope be furnished with a draw-tube, marked on the side with inches and tenths. By drawing this out a short distance, the image of the stage micrometer may be expanded until one division is covered by twenty in the eye-piece. These will then have the value of two-thousandths of an inch, and the object which before measured nine will then mea- sure ten ; which, divided by 2,000, gives the decimal fraction '005. " Enter in a table the length to which the tube is drawn 54 CONSTRUCTION OF THE MICROSCOPE. out, and the number of divisions on the eye-piece micro- meter equivalent to an inch on the stage ; and any measurements afterwards taken with that micrometer and object-glass may, by a short process of mental arithmetic, be reduced to the decimal parts of an inch, if not actually observed in them. " In ascertaining the value of the micrometer with a deep object-glass, the hundredth of an inch on the stage will occupy too much of the field ; the two-hundredth or five -hundredth should then be used, and the number of divisions corresponding to that quantity be multiplied by two hundred or five hundred, as the case may be. " The micrometer should not be fitted into too deep an eye-piece, for it is essential to preserve clear definition. The middle eye-piece is for most purposes the best, pro- vided 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, as at £, fig. 35." We subjoin the following comparative micrometrical measures given by Dr. Hannover, as a reference-table. Millemetre. Paris lines. Vienna lines. Rhenish lines. English inch. 1 2-255829 2-195149 2-179538 25-39954 0-443296 1 0-973101 0-!,'6fil81 11-25952 0-4555550 1 027643 1 0-992888 11-57076 0-458813 1-035003 1T071625 1 11-65364 0-0393708 0-0888138 0-0864248 0-0858101 1 The wonderful tracing on glass executed by M. Nobert, of Earth, in Prussia, deserves attention. The plan adopted by him is, to trace on glass ten separate bands at equal distances from each other, each band being composed of parallel lines of some fraction 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 : — CONSTRUCTION OF THE MICROSCOPE. 0-001000 lines. 0-000857 ., 0-000735 „! 0-000630 „ ' 0-000540 „ 0-000463 lines. 0-000397 ,, 0-000340 .„ 0-000292 ,, 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 1-4 7 00th 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. These tracings have tended to confirm the undulating theory of light, the different colours of the spectrum being exhibited in the ruled spaces according to the separation of the lines ; and in those cases where the distances between the lines are smaller than the length of the violet-coloured waves, no colour is perceived ; and it is stated, that if inequalities amounting to '000002 line occur in some of the systems, stripes of another colour would appear in them. Achromatic object-glasses for microscopes are of various foci, differing from 2 inches to 1-1 6th of an inch. Magnifying Power of Mr. floss's Object-Glasses with his various Eye-Pieces. EYE GLASSES, OBJECT GLASSES. or EYE-PIECES. 2-inch. 1-inch. |-inch. ^-inch. i-inch. Tyhich. A 20 GO 100 220 420 600 B 30 80 130 350 670 870 C 40 100 180 500 900 1200 Value of each space in the Mi- crometer eye- 4^"d •efo T&S TaVo 9000 13500 glass, with the various object- •0025 •001031 •0005263 •0002325 •0001111 •000074 glasses. CONSTRUCTION OF THE MICROSCOPE. Magnifying Power of Messrs. Powell and Lealantfs Achromatic Object-Glasses. Object 1 Angular Glasses. | Aperture. Magnifying Power with the various Eye-Pieces. Price. No. 1. No. 2. No. 3. No. 4. No. 5. Inches. Deg. £ s. 2 14 25 37 50 100 150 2 15 1 28 50 74 100 200 300 3 0 i 70 100 148 200 400 600 5 0 i 95 200 296 400 800 1200 5 5 i 125 400 592 800 160C 2400 8 8 TV 145 600 888 1200 2400 3600 10 10 ft 175 800 1184 1600 3200 4800 16 16 Schmidt's goniometer positive eye-piece, for measuring 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, and care being taken that they lie perfectly flat, 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. To the most complete instruments a set of eye-pieces, consisting of not less than three, is usually made. These differ in power j the longest is always the lowest power, and is marked A. Its angular aperture, which determines the size of the field of view, is generally less than that of the others (if constructed on the Huyghenean plan), being limited by the diameter of the body. It is usually about 20 degrees. The next eye- piece, or middle power, marked B, and the deepest, c, have more than 30 degrees of angular aperture. 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 or DEFINING AND PENETRATING POWER. 57 the circulation in the frog, &c., the eye-piece A is best adapted. For examining the details of any of the above objects, it will be advisable to substitute the eye-piece B, which also should be used in the observation of crystals when illuminated by polarised light, the pollen of flowers, minute dissection of insects, the vascular and cellular tissues of plants, the Haversian canals and lacunae of bone, and the serrated laminae of the crystalline lens in the eyes of birds and fishes. The eye-piece c is of use when it is requisite to investi- gate the intimate structure of delicate tissues ; and also in observations upon fossil infusoria, volvox, scales from moths' wings, raphides, &c. The employment of this eye- piece, when a higher power is required, obviates the neces- sity of using a deeper object-glass, which always occasions a fresh arrangement of the illumination and focus. It must be borne in mind, that the more powerful the eye- piece, the more apparent will the imperfections of the object-glass become ; hence less confidence should be placed in the observations made under a powerful eye- piece than when a similar degree of amplification is obtained with a shallow one and a deeper object-glass. The degree of perfection in the construction of the optical part of a microscope is judged of by the distinct- ness and comfort with which it exhibits certain objects, the details of which can only be made visible by combi- nations of lenses of high magnifying power, and a near approach to correctness. Such are termed by the micro- scopist test-objects. Mr. C. Brooke, F.R.S., whose labours have been devoted to the correction of errors which have crept into this part of philosophical research, says: — In order to arrive at any satisfactory conclusions regarding the action of any transparent medium on light, it is neces- sary to form some definite conceptions regarding the ex- ternal form and internal structure of the medium. This observation appears to apply in full force to microscopic test-objects; and for the purposes of the present inquiry, it will suffice to limit our observations to the structure of two well-known test-objects, — the scales of Podura plumbea, and the siliceous loricse, or valves of the genus Pleuro&igma, 58 CONSTRUCTION OF THE . MICROSCOPE. freed from organic matter : the former of these is com- monly adopted as the test of the defining power of an achromatic object-glass, and the several species of the latter as the tests of the penetrating or separating power, as it has been termed. The denning power depends only on the due correction of chromatic and spherical aberra- tions, so that the image of any point of an object formed on the retina may not overlap and confuse the images of adjacent points. This correction is never theoretically perfect, since there will always be residual terms in the general expression for the aberration, whatever practicable number of surfaces we may introduce as arbitrary con- stants; but it is practically perfect when the residual error is a quantity less than that which the eye can appre- ciate. The separation of the markings of the Pleurosig- mata and other analogous objects is found to depend on good defining power associated with large angle of aperture. The Podura scale appears to be a compound structure, consisting of a very delicate transparent lamina or mem- brane, covered with an imbricated arrangement of epi- thelial plates, the length of which is six or eight times their breadth, somewhat resembling the tiles on a roof, or the long pile of some kinds of plush. This structure may be readily shown by putting a live Podura into a small test-tube, and inverting it on a glass-slide ; the insect should then be allowecl for some time to leap and run about in the confined space. By this means the scales will be freely deposited on the glass ; and being subse- quently 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 recognised. This structure appears to be rendered most evident by mounting the scales thus obtained in Canada balsam, and illuminating them by means of Wenham's parabolic re- flector. The structure may also be very clearly recognised when the scale is seen as an opaque object under a Boss's -J^th (specially adjusted for uncovered objects), illuminated by a combination of the parabola and a flat Lieberkuhn. The under-side of the scale thus appears as a smooth DEFINING AND PENETRATING POWER. 59 glistening 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 will readily account for their being a good test of definition, while their prominence renders them independent of the separating power due to large angle of aperture. The structure of the second class of test-objects above mentioned diifers entirely from that above described; it will suffice for the present purpose to notice the valves of three species only of the genus Pleurosigma ; which, as arranged in the order of easy visibility, are, P. formosum, P. hippocampus, P. angulatum. These appear to consist of a lamina of homogeneous transparent silex, studded with rounded knobs of protuberances, which, in P. formosum and P. angulatum, are arranged like a tier of round shot in a triangular pile, and in hippocampus like a similar tier in a quadrangular pile, as has frequently been described ; and the visibility of these projections is probably propro- tional to their convexity. The " dots" have by some been supposed to be depressions; this, however, is clearly not the case, as fracture is invariably observed to take place betiveen the rows of dots, and not through them, as would naturally occur if the dots were depressions, and conse- quently 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 kingdoms; as, for example, the Isthmia, which possesses 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 -other objects, have been by some writers sup- posed to be made up of two substances possessing different degrees of refractive power; but this hypothesis is purely gra- tuitous, 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 60 CONSTRUCTION OF THE MICROSCOPE. lens, it is difficult to conceive why the oblique rays only should be visibly affected. When P. hippocampus or P. for- mosum is illuminated by a Gillett's condenser, with a cen- tral stop placed under the lenses, and viewed by a quarter- inch object-glass of 70° aperture, 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 series of dark dots replacing these ; and thirdly, the latter are again replaced by bright dots, not, however, as 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. 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 surfaces 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 accumu- lation of the rays transmitted at those points. Moreover, as all rays capable of entering the object-glass are concerned in the formation of the second series of bright focal points, whereas the first series are 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. If the supposed lenses were of small convexity, it is evident that the course of the more oblique rays only "would be sensibly influenced ; hence probably the structure of P. angulatum is recognised only by object-glasses of large angular apertures, which are capable of admitting very oblique rays, It does not appear to be desirable that objects should be MECHANICAL ARRANGEMENTS. 61 illuminated by an entire, or, as it may be termed, a solid cone of light of much larger angle than that of the object- glass. The extinction of an object by excess of illumina- tion may be well illustrated by viewing with a one-inch object-glass the Isthmia, illuminated by'Gillett's condenser. When this is in focus, and its full aperture open, the markings above described are wholly invisible ; but as the aperture is successively diminished by the revolving dia- phragm, the object becomes more and more distinct, and is perfectly defined when the aperture of the illuminating pencil is reduced to about 20°. The same point may be attained, although with much sacrifice of definition, by gradually depressing the condenser, so that the rays may diverge before they reach the object; and it may be remarked, generally, that the definition of objects is always most perfect when an illuminating pencil of suit- able form is accurately adjusted to. focus, that is, so that the source of light and the plane of vision may be conju- gate foei of the illuminator. If a condenser of 120° aperture, or upwards, be used as an illuminator, the mark- ings of Diatomacese will be scarcely distinguishable with the best object-glass, the glare of the central rays over- powering the structure of those that are more oblique.1 MECHANICAL ARRANGEMENTS. Having now explained the more important optical principles of the achromatic compound microscope, it remains for us to notice the mechanical and accessory arrangements, for giving those principles their full effect. The mechanism of a microscope is of much more import- ance than might be imagined by those who have not studied the subject. In the first place, steadiness, or freedom from vibrations not equally communicated to the object under examination and to the lenses by which it is viewed, is a point of the utmost consequence. One of the best modes of mounting a compound micro- scope is that shown, fig. 36, which, although it does not (1) Object-finder.— It is a great saving of time to use an object-finder, when very minute objects are not distinguishable by the naked eye. Many forms have been suggested and described by Mr. Tyrrel. Mr. Bridgman, and Mr. T. E. Amyot in the "Microscopical Journal" for 1855 and 1856. 62 CONSTRUCTION OF THE MICROSCOPE. exhibit all the details, will serve to explain the chief features of the arrangement. In this and larger instruments, two uprights are Fig. 36. — Baker's Compound Microscope. strengthened by two internal buttresses mounted on a strong tripod; at the upper part, and between the uprights, we have an axis, upon which the whole of the THE COMPOUND MICROSCOPE. 63 upper part of the instrument turns, so as to enable it to take a horizontal or vertical position, or any intermediate inclination, — such, for instance, as that shown in the draw- ing. This movable part is fixed to the axis near its centre of gravity, and consists of the stage, the arm screwed into the triangular bar which carries the micro- scope tube or body, at the upper end of this is the eye- piece, and at the lower the object-glasses. The stage has Fig. 37.— Baker's Student's Microscope. rectangular movements one inch in extent on the racket- cylinders, moved by the pinions connected with the milled- heads. The triangular bar, together with the arm and 64 THE MICROSCOPE. microscope-tube, is moved by the larger milled-heads ; a,nd a more delicate adjustment of this optical part is effected by the small milled-head above the bar. The other milled-head fixes the arm to the triangular bar. The mirror slides up or down the tube to which it is attached. A smaller compound achromatic microscope, fig. 37, is particularly adapted for students : this is packed into a neat mahogany case, with excellent object-glasses, for the small sum of 51. 15s., by Mr. Baker, 244, Holborn, who likewise furnishes all the requisites for microscopical pur- poses, and well-selected specimens of mounted objects, very cheap. Fig. 38. — Powell and Lealand's Microscope, with Amid prism, arranged for the oblique illumination of test-objects. Messrs. Powell and Lealand's improved microscope is represented in fig. 38. The three legs are considerably stouter and more inclined than in their former instrument, THE STUDENT'S MICROSCOPE. 65 which gives additional stability ; they support, at their upper part, the trunnions to which the tube and the stage are attached. From out the tube a triangular bar is raised by a rack and pinion connected with the milled head. 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 com- pound body may be moved aside from the stage when necessary ; by a mechanical arrangement it stops when central. The stage is of an entirely new construction, having vertical, horizontal, and circular movements, and graduated for the purpose of registering objects so as to be found at pleasure ; and in order to do this effectually a clamping piece is provided against which the object slide rests, and the circular motion of the stage is stopped. It is an exceedingly effectual method of finding any favourite object. The stage is remarkably strong, and at the same time so thin, that the utmost obliquity of illumination is attainable, the under portion being entirely turned out : it has a dove-tailed sliding bar moveable by rack and pinion, on turning the milled head into this bar slides the under stage, having vertical and horizontal motions for centering, and also a circular motion ; into the stage are affixed the various appliances for underneath illumination. The achromatic condenser, if of 100° of aperture, with nine apertures and five central stops, the apertures and stops having independent movements, the manipulator can regulate at will ; this is considered to be a great improve- ment. There is an appliance provided for the dark- wells, which is put into the dove- tailed sliding bar instead of the underneath stage. The mirror is attached to a quadrant of brass and two arms, in order to obtain greater obliquity of illumination ; the whole fits into a short piece of tube made to slide either up or down the long tube attached to the bottom of the stage by which the mirror is connected with the other part of the stand; the re- flectors themselves are both plane and concave, as in other instruments. The achromatic prism for oblique light ia F 6G THE MICROSCOPE. very useful in bringing out the fine markings of the most difficult Diatomacese with the high-power object-glasses. This instrument combines extreme steadiness with re- markable simplicity, together with every motion and Fig. 39. — Warington's Microscope. appliance that has ever been discovered for the microscope; the compound body is supplied with a draw tube. WARINGTON'S MICROSCOPE-STAND. 67 Messrs. Powell and Lealand's microscopes are sold at prices suitable to the wants and means of most persons ; their No. 1, such as we have represented, but without object-glass, can be purchased for 221. A smaller instru- ment, fit for the student, with f inch of motion to the stage by means of a lever, coarse and fine adjustments to body, plane and concave mirrors, revolving diaphragm, Lister's dark-wells, and two eye-pieces, SI. We wish to call attention to the movement made by the Society of Arts, with the object of supplying the public, at low prices, with approved and warranted micro- scopes. The Society offered prizes to all manufacturers for the best simple microscope, to be called a School Microscope, and to be sold at 10$. 6d. ; and for the best compound microscope, to be called the Student's Micro- scope, and to be sold at 31. 85. Both prizes were awarded to Messrs. Field and Son, of Birmingham. Whilst alluding to cheap microscopes, we would men- tion Warington's Travelling Microscope, made by W. J. Salmon, 100, Fenchurch Street. It has a simple, firm, wooden stand, whereby the cost is greatly diminished ; and an arrangement of its parts, which enables it to be used for viewing objects in aquaria, and under other cir- cumstances where any ordinary form of instrument could not be made available. It is altogether a useful student's microscope, besides having the recommendation of folding up into a smaller compass than any instrument of its size, and of not being liable to much injury from chemical or marine investigations. For 31. this microscope is furnished Fig. 40. — Warington's Microscope packed. complete, with one eye-piece, quite sufficient for all ordi- nary investigations. Fig. 39 is a representation of Warington's microscope, F 2 68 THE MICROSCOPE. as it appears when put together, and ready for use ; and fig. 40 for packing in a small wood-case. The draw-tube itself is the coarse adjustment ; whilst a finer is secured by a well-made union-joint, into which the object-glass is made to screw. With an additional arm for the reception of a single lens, ifc may be converted into a dissecting microscope. Mr. I, Newton Tomkins has constructed a very ingenious and cheap moveable stage to his Warington microscope, by simply having a small horse-shoe magnet let into a piece of oak cut into the usual stage form, which he uses in the place of the brass one. A drawing and description of the contrivance is given in the Micros. Journal, July, 1857. M. Dujardin found that to reflect the rays of light truly parallel, a prism of glass should be used in place of the mirror. This prism, which is one of total reflection, must be so arranged as to slide upon the end of the condenser, and turn upon it in such a manner, that in whatever posi- tion the lamp or light may be, the prism may be adjusted to it. The quantity of light passing through it is less than with the mirror ; and those test objects in which delicate markings exist, are seen to much greater advan- tage, in consequence of all the rays being reflected from the same surface, which is not so with a silvered glass mirror. Space will not permit us to enter into details respecting the prices charged by the different manufacturers in London and elsewhere. We can, therefore, only add that the leading makers supply small instruments at from 10Z. to 151. each, and that other manufacturers supply very good instruments at from 51. to 10£. each. The micro- scopes made by Messrs. Smith and Beck, Mr. Matthews, Mr. Pillischer, Mr. Ladd, Mr. Dancer, M. Nachet, &c., will be found to meet all the requirements of the amateur or student. Mr. Ladd substitutes a chain-movement for the rack-and-pinion, in the construction of his instruments, which, « whilst it gives delicacy and smoothness, admits of an exact adjustment being made by its means alone." CHAPTER III. PRELIMINARY DIRECTIONS — ILLUMINATION — ACCESSOEY APPARATUS — ACHROMATIC ILLUMINATOR — GILLETT'S CONDENSER — PREPARING AND MOUNTING OBJECTS — POLARISED LIGHT — BINOCULAR INSTRUMENT — PHOTOGRAPHIC DRAWING, ETC. AYING 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 it is used, otherwise the perspiration from the eye will condense on the eye-glass, and greatly impede vision. When you clean the eye-glasses, do not remove more than one at a time, and replace it before you touch another ; by so doing you will preserve the component glasses in their proper places : recollect that if intermingled they are useless. Keep a piece of well-dusted and very dry chamois leather, slightly impregnated with the finest tripoli or rotten-stone powder, in a small box, to wipe the glasses. 70 THE MICROSCOPE. When you look through the instrument, be sure ta place your eye quite close to the eye-piece, otherwise the whole field of view will not be visible ; and observe, more- over, if you see a round disc of light, at least when the object is not on the slider-holder : if you do not, it is a sign that something is wrong; perhaps the body is not placed directly before the stage aperture, or may not be properly directed towards the light. Use the smallest amount of light possible, if you work for any length of time. Choose a steady light, with a shade to protect the eyes, one of the old-fashioned fan-shades will be found useful for this purpose : use the eyes alternately. Sit in a comfortable position, and bring the instrument to the proper angle, which will prevent congestion of the eyes ; this is indicated if the microscopist is annoyed with little moving objects apparently floating before them : if the eye-lashes are reflected from the eye-glass, you are looking upon the eye-glass instead of through it. Take care that the mirror is properly arranged. The following are Sir David Brewster's excellent direc- tions for viewing objects : — " First. Protect the eye from all surrounding light, let- ting only the rays which proceed from the illuminated centre of the object fall upon it. " Secondly. Delicate observations should not be made when the fluid which lubricates the cornea is in a viscid state, or there is any irritation or inflammation about any part of the eye. " Thirdly. The best position for microscopic observations is with the microscope bent to such an angle with the body, that the head may always remain in a natural and easy attitude ; consequently, the worst position would be that which compels us to look downwards vertically. " Fourthly. If we lie horizontally on the back, parallel markings and lines on objects will be seen more perfectly when their direction is vertical, or in a contrary direction to that in which the lubricating fluid descends over the cornea of the eye. " Fifthly. Only a portion of the object should be viewed at one time, and every other part excluded. The light which illuminates that part should be admitted through a ILLUMINATING THE OBJECTS. 71 small diaphragm : at night, from the concentrated light of a sperm-oil or gas lamp, having a faint blue-tinted chim- ney-glass, to correct the yellow colour which predominates in all our artificial illumination. If in the day-time, close a portion of the window-shutters. " Sixthly. In all cases when high powers are used, the intensity of the illumination should be increased by optical contrivances below the object and stage : this is generally effected by using achromatic condensers beneath the stage. The apparatus for illumination should be as perfect as the magnifying power." If these directions are strictly followed, no injury to the eyes from using a microscope need be feared. Mr. Boss very properly remarks, that the manner in which an object is lighted is second in importance only to the excellence of the glass through which it is seen. When investigating any new or unknown specimen, it should be viewed in turns by every description of light, direct and oblique, as a transparent object and as an opaque object, with strong and with faint light, with large angular pencils thrown in all possible directions. Every change will pro- bably develope some new fact in reference to the structure of the object, which should itself be varied in the mode of mounting in every possible way. It should be seen both wet and dry, and immersed in fluids of various qualities and densities ; such as water, alcohol, oil, and Canada balsam ; which last has a refrac- tive power nearly equal to that of glass. If the object be delicate vegetable tissue, it will be, in some respects, rendered more visible by gentle heating or scorching before a clear fire, between two plates of glass. In this way the spiral vessels of asparagus and other similar vegetables will be beautifully displayed. Dyeing the objects in tincture of iodine, or some one of the dye- woods, will, in some cases, answer the purpose better. But the principal question in regard to illumination is the magnitude of the illuminating pencil, particularly in reference to transparent objects. Generally speaking, the illuminating pencil should be not quite so large as can be received by the lens : any light beyond this produces indistinctness and glare. The superfluous light from the 72 THE MICROSCOPE. mirror may be cut off by a screen, having various-sized apertures placed below the stage. The Diaphragm, fig. 41, is the instrument used for effecting this purpose. It consists of two plates of brass, one of which is perforated with four or five holes of dif- Fig. 41.— The Diaphragm. ferent sizes ; this plate is of a circular figure, and is made to revolve upon another plate by a central pin or axis ; this last plate is also provided with a hole as large as the largest in the diaphragm-plate, and corresponds in situa- tion to the axis of the compound body. To ascertain when either of the holes in the diaphragm-plate is in the centre, a bent spring is fitted into the second plate, and rubs against the edge of the diaphragm-plate, which is provided with notches. The space between the smallest and largest is great enough to use for the purpose of shut- ting off all the light from the mirror. GILLETT'S ILLUMINATOR, OR CONDENSER. — The advan- tages of employing an achromatic condenser were first pointed out by Dujardin, since which time an object-glass has been frequently but inconveniently employed ; and more recently achromatic illuminators have been con- structed by most of our instrument makers. Some years since, Mr. Gillett was led by observation to appreciate the importance of controlling not merely the quantity of light which may be effected by a diaphragm placed anywhere between the source of light and the object, but the angle of aperture of the illuminating pencil, which can be effected only by a diaphragm placed immediately behind the achromatic illuminating combination. Such a diaphragm is represented in fig. 42, manufactured by Mr. Ross : it consists of an achromatic illuminating lens c, which is GILLETT'S ILLUMINATOR. 73 about equal to an object-glass of one-quarter of an inch focal length, having an angular aperture of 80°. This lens is placed on the top of a brass tube, intersecting Fig. 42.—Gillett's Condenser. which, at an angle of about 25°, is a circular rotating brass plate a b, provided with a conical diaphragm, having a series of circular apertures of different sizes h g, each of which in succession, as the diaphragm is rotated, propor- tionally limits the light transmitted through the illumi- nating lens. The circular plate in which the conical dia- phragm is fixed is provided with a spring and catch ef, the latter indicating when an aperture is central with the illuminating lens, also the number of the aperture as marked on the graduated circular plate. Three of these apertures have central discs, for circularly oblique illumi- nation, allowing only the passage of a hollow cone of light to illuminate the object. The illuminator above described is placed in the secondary stage i i, which is situated below the general stage of the microscope, and consists of a cylindrical tube having a rotatory motion, also a rect- angular adjustment, which is effected by means of two screws I m, one in front, and the other on the left side of its frame. This tube receives and supports all the various 74 THE MICROSCOPE. illuminating and polarising apparatus, and other auxiliaries which are placed underneath the object. The tube and its frame are affixed to a dovetailed sliding bar &, which can be easily moved up or down, or taken off for conveniently attaching the various apparatus. This sliding bar fits into a second sliding bar, which, by means of a milled-head screw, moving a rack and pinion, regulates the distance of the apparatus from the stage. Directions for Use by Day or Lamplight. — In the adjust- ment of the compound body of the microscope with the illuminator above described, two important results are to be sought — first, their centricity, and secondly, the fittest condensation of the light to be employed. With regard to the first, place the illuminator in the cylindrical tube, and press upwards the sliding bar 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 sliding bar, bring the illu- minating lens to a level with the upper surface of the object-stage ; then move the arm which holds the micro- scope 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 object, and take off the cap of the eye-piece, and in its place fix on the eye- glass called the " centering eye-glass," described below, which will be found greatly to facilitate the adjustment now under consideration, namely, the centering of the compound body of the microscope with the illuminating apparatus of whatever description.1 The centering-glass, being thus affixed to the top of the eye-piece, is then to (1) This centering-glass consists of a tubular cap containing two plano-convex lenses, which are applied and adjusted so that the image of the aperture in the object-glass, and the images of the apertures at the lenses and in the diaphragms contained in the tube which holds the illuminating combination, may all be in focus at the same time, as with the same adjustment they may be brought suffi- ciently near in focus to recognise their centricity. GILLETT'S ILLUMINATOR. 75 be adjusted by its sliding- tube (without disturbing the microscope-tube) until the images of the diaphragms in the object-glass and centering lens are distinctly seen. The illuminator should now be moved by means of the left-hand screw on the secondary stage, while looking through the microscope, to enable the observer to recog- nise the diaphragm belonging to the illuminator, and by means of the two adjusting screws, to place this diaphragm central with the others ; thus, the first condition, that of centricity, will be accomplished. Remove the white paper from the mirror, and also the centering-glass, and replace the cap on the eye- piece, also the object on the stage, of which distinct vision should then be obtained by the rack and pinion, or fine screw adjustment, should it have become deranged. The second process is to ascertain that the fittest con- centration of light is obtained. For this purpose the mirror should now be so inclined that the image of some intercepting distant object, such as a house-top, or chim- ney, tree, window-frame, or (if lamp -light be employed) the lamp's flame may be brought into the field of view ; these, though not distinctly seen, may be recognised by partially darkening or otherwise occupying the field; then distinct vision of such object must be obtained by means of the rack and pinion moving the secondary stage to and from the object. Excepting the case of the lamp's flame, the above objects are considered as the representatives of the source of light ; for when daylight is employed — as, for example, a white cloud — its motion prevents the image being easily produced : then it is convenient to employ a distant object, such as the above, — the difference of the focal length of the illuminating lens for such an object, and for the white cloud, being almost insensible. This last adjustment being effected by the movement of the secondary stage alone, the microscope tube remaining un- disturbed, also the object on the object-stage uninterrupted in focus, the source of the illuminating light and the object to be examined will both be distinctly seen at the same time. These adjustments, whether for daylight or lamplight, being completed, the mirror may be turned so as wholly to reflect the light either of the sky or of the 76 THE MICROSCOPE. lamp; and the eye-piece and object-glass suitable for examining the object may be employed, and the focus adjusted accordingly. The conical diaphragm with its various apertures may now be rotated, until that quality of illumination is obtained which gives a cool, distinct, and definite view of the object. Upon changing the object-glass, the centering eye-glass should always be employed to ascertain that the centricity of the illumi- nating condenser and microscope body has not been deranged. It has been stated that the image of a white cloud oppo- site the sun is the best for illuminating transparent objects when viewed by transmitted light. Mr. Gillett has success- fully imitated this natural surface by an apparatus consist- ing of a large parabolic reflector, with a small camphine lamp on an adjustable stand, having its flame nearly in the focus ; also of two other reflectors of hyperbolic figure, which are employed according to the object-glasses used on the microscope. The parabolic mirror and one of these are attached opposite to each other on the bent arm by which they are supported, having their axes coincident, and the enamel disc placed between them. The small hyperbolic reflector receives the light reflected from the large paro- bolic reflector, and concentrates the rays on the small enamel disc. The surface of this disc is roughened, so that the forms of all the incident pencils are broken up, and the effect of a white cloud produced. A good mode of imitating artificially the light of a white cloud opposite the sun has been proposed by Mr. Varley : he covers the surface of the mirror under the stage with carbonate of soda, or any similar material, and then concentrates the sun's light upon its surface by a large condensing lens. Boss's Achromatic Illuminator, or Condenser. — When employing this apparatus, the general practice is to insert in it, as an illuminating lens, the object-glass next lowest in power to that which is intended to be attached to the microscope; so that when the one-eighth is used on the microscope, the one-fourth is screwed into the illuminating apparatus; and so, in like manner, with the rest. But when economy is not regarded, a system of three achro- ROSS'S ACHROMATIC ILLUMINATOR. 77 matic combinations is supplied, adapted for the illumina- tion of the whole range of the powers of the microscope : the whole syotem being employed for the highest powers; two of such combinations with the middle powers ; and the largest combination by itself for the lowest powers. This illumination is not required for objects when viewed with object-glasses transmitting small pencils of rays, or whose angular aperture is less than thirty degrees ; that is, where the object-glass is of greater focal length than half an inch. The apparatus is fixed to the under side of the stage of the microscope, in the place of the diaphragm-plate; and before fixing, the proper ob- ject-glass, as an illuminating lens, must be screwed on to it. In fig. 43, two tubes are seen sliding one within the other; to the outer one, 6, is ' attached a flat plate a, which slides underneath the stage, and is adjusted for distance by the screw /; at c the milled- head is connected to a pinion; and by means of a rack at- tached, the inner tube, carry- ing the achromatic combina- Fig' «•-*««'' condenser. tion dj is raised or lowered : the upper part of the outer tube is larger than that where the milled-head is seen, for the purpose of allowing the milled ridge of the achromatic to pass up and down. For the ^ or £ inch, the combina- tion d is used; and for the higher power, 1 or -J^, the second e is slipped over d. Place the object to be viewed upon the stage of the microscope; and when the instru- ment is not directed at once to the source of light, such as the flame of a lamp, or a white cloud, arrange the reflector (having the plane mirror upwards) so as to throw the light up the tube of the apparatus; which may be ascer- tained by turning aside the microscope tube, and observing when the spot of light appears on the object placed on the stage. The microscope-tube is then to be replaced as nearly over the spot of light as possible, and vision of the 78 THE MICROSCOPE. object obtained, disregarding the precise quality of the light. Then proceed for perfect adjustment, as directed in using Gillett's condenser. The Parabolic Reflector. — F. H. Wenham, Esq., (Micros. Trans. 1851) proposed a new illuminator, for the purpose of obtaining perfect definition under high powers. Those who have experimented on the subject, may have observed that there is something in the nature of oblique light reflected from a metallic surface particularly favourable for the purpose of bringing out minute markings, which may, in some measure, be attributed to the circumstance of light so reflected being purely achromatic. In order to render this property available, Mr. Wenham contrived a very ingenious metallic reflector, by which the condensa- tion of lateral light may be effected.1 Fig. 44.— Wenham' s Parabolic Reflector. " The apparatus is shown in section in fig. 44 : a a is a parabolic reflector, of a tenth of an inch focus, with a (1) In Vol. IV. 1856, Microscopical Society's Transactions, p. 55, will be found another very instructive and scientific paper, " On the Method of Illumi- nating Opaque Objects under the Highest Powers of the Microscope," by Mr. Wenham. The principle of operation consists in causing rays of light to pass through the under side of the glass slip upon which the object is mounted, at the proper angle for causing total -internal reflection from the upper surface of the thin cover, which is thus made to act the part of a speculum, for throwing the light down upon the under-lying objects, immersed in the balsam on fluid. THE PARABOLIC REFLECTOR. 79 polished silver surface, having the apex so far cut away as to bring the focal point at such a distance above the top of the apparatus (which is closed with a screw-cap when not in use) as may allow the rays to pass through the thickest glass commonly used for mounting objects upon before coming to a focus. " At the base of the parabola is placed a disc of thin glass I) b, in the centre of which is cemented a dark well, with a flange equal in diameter to the aperture at the top of the reflector, for the purpose of preventing the direct rays from the source of light passing through the apparatus. "The reflector is moved to and from the object by means of the rack and pinion c, and has similar adjust- ments for centering, and is fixed under the stage of the microscope in the same way as the ordinary achromatic condenser: in addition, there is a revolving diaphragm d, made to slide on the bottom tube of the apparatus ; it has two apertures e e, placed diametrically, for the purpose of obtaining two pencils of oblique light in opposite direc- tions. The effects of the chromatic and spherical aber- rations, in the shape of fog and colour about the objects, caused by the glass slides upon which they are mounted, frequently require compensation; for as the parabola has the property of throwing parallel rays tincoloured to a point, when used alone, it is most suitable for objects without glass underneath. " By the addition of a meniscus, this compensation is obtained, and also greater purity and intensity of illumina- tion is procured ; and as the silver reflector is now closed with glass, it is hermetically sealed, and permanently pro- tected from dust and damp, and will therefore retain its polish. The light most suitable for this method of illumi- nation is lamp or candle light, the rays of which must in all cases be rendered parallel by means of a large plano- convex lens, or condenser; the light may then be used direct, or reflected from the plane mirror. The object having been adjusted, the illuminator is moved to and fro till the best effect is produced. For the purpose of viewing some objects, such as naviculse, the circular diaphragm should be slid on the extremity of the apparatus, and 80 THE MICROSCOPE. revolved till the two pencils of light are thrown most suitably across the object. " As the method of illuminating microscopic objects by means of a large angular pencil of light, having the central rays obscured, is of recent introduction, I shall mention a few instances where transparent objects are shown, under similar circumstances, with perfect or improved definition. The lateral mode of illumination will be found to possess peculiar advantages in the examination of test-objects and the internal mechanism of infusoria. The markings on most of the test-objects are either depressions or projections by direct light: all parts of an object are illuminated with equal intensity, and delicate colours are in great part destroyed, consequently there is a want of contrast. The effect of an angular pencil of rays of 175°, with the central ones stopped, is, that there is a greater relative amount of light thrown on these prominences, as they intercept the largest portion of the marginal rays near the apex of the reflector, leaving the base of the prominence in compara- tive shadow, consequently the markings we wish to see are the most strongly lighted. The different organs in the interior of an animalcule may be much of the same colour and transparency, and yet possess a different refraction, according to their density. Direct light will pass through these transparent membranes in straight lines without being affected by their various refractive powers. The effect of lateral or oblique light on such tissues is, that the rays are more refracted according to their inclination, and proportionate to the various densities of the medium, the most refractive structure transmitting the greatest quantity of light, and being in consequence the most illuminated ; and this reason is somewhat confirmed by the circumstance of lateral illumination showing the structure of some objects which, from slight variation in density, were invisible, except by the use of polarised light." Mr. Shadbolt has since modified this reflector, which he deno- minates "a sphero -annular condenser:" it has superior reflecting arrangements, with less liability of derangement, and is constructed of a solid cylinder of glass terminating above in a solid cone, the surface of which has the form of a parabola, and replaces the silver reflecting surface. CONDENSERS. 81 It is due to Mr. Lister to mention that in his paper on the "Achromatic Object- Glass," published in the 120th vol. of the Transactions of ike Royal Society, he makes mention " of some objects being better seen when the central rays are obscured." This observation has been carried out in many ways. Mr. Reade's "back-ground illuminator" is one in which the light is thrown under the object in such a direction as to avoid or pass by the aper- ture of the object-glass, and give a black field. The structure under view, if large, must have sufficient trans- parency to allow the light to enter into its substance, and to be diffused or radiated therefrom in all directions. This illuminator is very suitable for objects requiring only a low power to view them. Mr. John Furze directed the attention of microscopists to a beautiful arrangement for the "illumination of objects by polarised light on a dark field, in such a manner as to give the object a stereoscopic effect by a due contrast of light and shade." To obtain this result, he uses a plano- convex lens, three-fourths of an inch in diameter. This, when fitted, is of so small a size, that it can be adapted to any instrument. Such an illuminating lens should be arranged with a system of both central and external stops, each revolving on a separate axis; and an adjustable cap to slide over the top of the lens, containing a crystal of Herapathite mounted between thin glass; a plate of selenite, mounted in the same 'way, should be used on the stage above it. Objects of too great density for transmitted light will appear under this mode of illumination as if in relief; and the definition of the various parts will be so accurately displayed as to constitute a most perfect method of viewing them. Condensing lenses, fig. 45, are used either for opaque objects, or to condense the light upon the mirror attached to the microscope. A bull's-eye, or plano-convex lens, of three inches focal length, is best suited for the purpose. In fig. 46 the bull's-eye lens c slides up and down a brass rod, screwed into a steady foot; or it may be fixed into the stage of the microscope, through which the light is finally concentrated upon the object from the table gas- lamp d. Mr. Brooke's method of viewing opaque objects a 82 THE MICROSCOPE. under the highest powers of the microscope (the 1 and ^ inch object-glass) is effected by two reflections. The rays- from a lamp rendered pa- rallel by a condensing lens are received on an elliptic reflector, the end of which is cut off a little beyond the focus ; the rays of light converging from this surface are reflected down on the object by a plane mirror attached to the object-glass, and on a level with the outer surface. By such means the structure of the scale of the Podura, and the different characters of its inner and outer surfaces, are rendered distinctly visible. Silver specula, known as Lieber- kuhn's, are much employed, and preferred by some mi- croscopists. The Lieberkuhn is concave, and attached to the object-glasses, from the two-inch to the half-inch, in the manner represented at fig. 47, where a exhibits the lower part of the compound body ; 6 the object-glass, over which is slid a tube and the Lieberkuhn c attached to it ; the rays of light reflected from the mirror are brought to a focus upon an object d, placed between it and the mirror. The object may either be mounted on a slip of glass, or else held in the forceps/; and when too small to fill up the entire field of view, or when trans- parent, it is necessary to place behind it the dark- well e. Each Lieberkuhn being mounted on a short piece of tube, can be slid up and down on the outside of the object- glass, so that the maximum of illumination may be readily obtained. In the higher powers the end of the object-glass is turned small enough to pass through the aperture in the Fig. 45. LIEBERKUHN'S CONDENSER. 13 83 Fig. 46.— Condenser and Table-lamp. centre of the Lieberkuhn ; but in the lower powers, where a great amount of reflecting sur- face would be lost on account of the large size of the glasses employed if this plan were adopted, the aperture in the centre of the Lieberkuhn is made to admit as many rays as will fill the field of view, and no more. Lamps. — The achromatic gas- lamp, fig. 48, is the best and most economical.1 Gas, as a source of light, presents great advantages over oil and spirit, • .on account of cleanliness, being (1) Matthews, of Portugal Street, supplies lamps, condensers, and other articles useful to the raicroscopist. G 2 Fig. 47. 84 THE MICROSCOPE. ever ready for use, and affording a perfect control over the flame; but when the ordinary gas-lamps are used Fig. 48.— Gas-lamp, 8fC. for the purpose of illuminating the field of the micro- scope, a yellow glaring light is given, alike injurious to the eye and the definition of the object under examina- tion. To correct these evils, this lamp was arranged, which is also otherwise useful to the microscopist. It consists of a stage A, supported by a tube and socket, sliding on an upright rod rising from the stand. This carries an ar- gand burner B ; a metal cone c rises to the level of the burner, and is about one- eighth of an inch from its outer margin. This arrangement gives a bright cylindrical flame. The bottom, of the stage A is covered with wire-gauze, to cut FORCEPS. 85 off irregular currents of air, and thus secures a steady flame. Over the burner is placed a Leblond's blue glass chimney D. This corrects the colour of the flame to a certain extent ; but it is still further rectified by a disc of bluish-black neutral-tint glass E, fitted in a tube F, attached obliquely to the shield G. G is a half- cylinder of metal, which serves to shield the eyes from all extraneous light, but may be rotated on the stage A by aid of the ivory knot H, when the full light from the flame is desired. A metallic reflector i, fixed on its supports, so as to be parallel to E, concentrates the light. By the combination of the two glasses D and E, the yellow rays of the flame are absorbed, and the arrangement affords a soft white light, which may be still further improved by receiving the rays on a concave mirror, backed with plaster-of- Paris L ; and where a very strong light is required, a condensing lens should be interposed, as shown in the cut, between the lamp and the mirror of the microscope. By removing the shield G, and bringing the shade M over the burner, it may be used as a reading-lamp. A retort ring N supports a water-bath o, or a wrought- iron plate P, 6 inches by 2| inches, both used in mounting objects. The stop-cock Q gives the means of regulating the flame. The screw R clamps the lamp-head at any height desired. The lamp may be attached to any gas-supply by vulcanised India- rubber tubing. Price, complete, thirty-five shillings. Forceps. — For holding minute objects, such as parts of plants or insects, to be examined either as transparent or opaque objects, the most useful is represented by fig. 49, Fig. 49. and consists of a piece of steel wire, about three inches long, which slides through a small tube, connected to a stout pin by means of a cradle-joint ; to one end of the wire is attached a pair of blades, fitting closely together by their own elasticity, but which, for the reception of any 86 THE MICROSCOPE. object, may be separated by pressing the two projecting studs ; to the opposite end of the wire is adapted a small brass cup, filled with cork, into which pins, passed through discs of cork, card-board, or other material, having objects mounted on them, may be stuck. Dipping -tubes for talcing up Animalcules are tubes of glass, fig. 50, about nine inches in length, open at both d, Fig. 50. Fig. 51. ends, and from one-eighth to one-fourth of an inch in diameter. The ends should be nicely rounded off in the flame of a blow-pipe ; some of them may be straight, whilst others should be drawn out to a fine point, and made of either of the shapes represented. Mr. Yarley thus describes the method of using them in volume 48 of the Transactions of the Society of Arts. " Supposing the animalcules that are about to be examined to be COLLECTING ANIMALCULES. 87 contained in a phial or glass jar, as in fig. 51, having observed where they are most numerous, — either with the naked eye, if they are large, or with a pocket-magnifier, if they are small, — either of the glass-tubes, having one end previously closed by the thumb or fore-finger, wetted for the purpose, is introduced into the phial in the manner represented by the figure, — this prevents the water from entering the tube; and when the end is near to the object which it is wished to obtain, the finger is to be quickly removed and as quickly replaced. The moment the finger is taken off, the atmospheric pressure will force the water, and with it, in all probability, the desired objects, up the tube. When the finger has been replaced, the tube con- taining the fluid may be withdrawn from the phial ; and as the tube is almost certain to contain much more fluid than is requisite, the entire quantity must be dropped into a watch-glass, which spreads it, and the insect may be again caught by putting the tube over it, when a small quantity of fluid is sure to run in by capillary attraction. This small quantity is to be placed upon the tablet ; but should there be still too much for the tablet, if it be touched with the tube again, it will be diminished accordingly." If we wish to place several individuals together on the tablet, it is necessary that each should be taken up with the smallest amount of water : to effect this, Mr. Varley suggests that the tube should be emptied on a slip of glass, in separate drops ; and with one of the capillary tubes, but just large enough to catch them, they may be lifted out one by one, and placed on the tablet. Generally speaking, it is neces- sary to add a small quantity of vegetable matter to ani- malcules, to keep them alive; and as many species are found on confervas and duck-weed, some instrument is required to take small portions of such plants out of the Fig. 52. jar in which they are growing. For this purpose Mr. Varley uses the forceps fig. 52, made of brass ; the points 88 THE MICROSCOPE. are a little curved, to keep them accurately together, and the blades are provided with a hole and steadying-pin. This instrument is also useful for picking up minute objects, &c. Fig. 53.— Collecting Net. Collecting Animalcules. — For collecting the water ani- malcules, the cambric-muslin net, made similar to a land- ing-net, fig. 53, will be found to answer the purpose; this should be secured to a brass ring a, and fitted into a socket 6, by which it can be attached to the end of a walking-stick, or, when not in use, the socket may be carried in the pocket ; and the net, by contracting the diameter of the ring (which the construction admits of) may be put inside the hat. " For l the purpose of collecting aquatic animal- cules, I use, in preference to any kind of net, stout tin hoops, about four inches dia- meter, and one and a half deep, nested for stowage. Mus- lin of different degrees of fineness is strained over one - 54- opening of the hoop, and a screw is attached by its head to the rim, fig. 54. The net is thus portable, and is screwed into a hole in the end of a walking-stick, or, what is better, a fishing-rod. I find that for most purposes the fabric called bobbinet answers very well, and catches creatures much smaller than * its own meshes, while the free escape of water through the open- ings, prevents their being washed out, as they frequently are in withdrawing the net from the surface. If the stick have a pike at the other end, it may be stuck in the ground, and those animals that are visible to the naked U> Communicated by Mr. Gibbons of Australia, to the author. COLLECTING ANIMALCULES. 89 eye, leisurely picked out with a small thin spoon or palette knife, and transferred to bottles, care being taken that the more voracious ones be separated from their prey ; while the thick residuum, containing infusoriae, &c., may be ladled up, or strained off into its appropriate vessel. On arriving home, the contents of the bottles are poured into one of the finer nets, which is placed in a saucer of water ; the drafting-net is then lifted up out of the water, and a final classification must be made. To catch individual creatures that are too large for a fishing-tube, a small spoon-net, made of slips of thin metal, bent into the form of a spoon, with a large hole punched out of the bowl, and muslin cemented to the rim, will be found convenient. This form of net is free from the inconvenience of loose parts of material, in which choice specimens may be con- fused and lost." Fig. 55 is a box containing six bottles for holding the animalcules when caught. These bottles should be filled with the water when you collect the animalcules, and the larger put by themselves. When collecting from different locali- ties, take care not to mix the animals from one brook with those from another, otherwise serious conflicts may take place, and on reaching home you will find the greater part of your stock either dead or dying. Al- ways separate the various sizes and races as speedily as possible. This can be done most easily by emptying each bottle in its turn into a soup-plate ; then with the feather of a pen first lift out the smaller ones, and with the quill-end cut like a scoop lift out the larger, classifying and allotting each species to its separate "fish-pond." The best localities in the neighbourhood of London for collecting, are Epping Forest, Hampstead Heath, and Blackheath. Mr. Williamson uses a cheap and simple contrivance for converting the end of a walking-stick or umbrella into Pig. 55. IW THE MICROSCOPE. what he terms a "collecting-stick." In fig. 56, a repre- sents a piece of whalebone, about 18 inches long, bent round the end of the stick or umbrella, &, and made fast in that position by one or two rings, c, of gutta-percha, india-rubber, or of brass, d. A small wide- mouthed bottle, having a rim which will prevent its falling through, is now inserted in the loop thus formed, and is held tightly there by the ends of the whalebone being drawn further through the ring, and thus di- minishing the size of the loop. The bottle thus fixed may be used for dipping out the animal- cules. Whalebone can be moulded to any form by placing it for a short time near the fire. Animalcule Cage. — Mr. Varley, in the year 1831, greatly im- proved the form of this instru- ment by making a channel all round the object-plate, so that the fluid and the animalcules in it were retained at the top of the object-plate by capillary attraction ; and they then bear turning about in all directions without leaving the top, provided the cage be not suddenly shaken. The cover is made to slide down upon the object-plate. The plate of brass to which the tube supporting the tablet and cover is attached, is of a circular form, slightly flattened on two opposite sides fot convenience of package. One of these instruments is seen in elevation and in section in fig. 57. A B, in both figures, is the flat plate of brass to which the short tube carrying the object-plate or tablet is fixed ; d, the piece of brass into which the tablet c is fastened ; 6, the tubular part of the cover, into the rim of which the thin plate of glass a is cemented. Many microscopists make use of a compressorium, an Fig. 56. ANIMALCULE CAGE. 91 instrument in which an object may be submitted to gra- duated pressure between two plates of glass, the parallelism Fig. 57. — Farley's Animalcule-cage. of which is perfectly maintained. The class of investiga- tions in which the compressorium is valuable, is that in which such structures as the minute ovum need be closely scrutinised, without any further change in their shape than may render their contents more distinctly visible. For such purposes, a steady hand and a well-made animal- cule cage, such as the one previously described, will answer very well. Smith and Beck's troughs for chara and polypes, a sec- tional view of which is shown at fig. 58, are made of three pieces of glass, the bottom being a thick strip, and the front a of thinner glass than the back b ; the whole is cemented together with Jeffery's marine-glue. The method adopted for confining objects near to the front glass varies according b to circumstances. One of the most convenient plans is to place in the trough a piece of glass that will stand across it diagonally, as at c ; then if the object be heavier than water, it will sink, until stopped by this plate of glass. At other times, when used to view chara, the diagonal plate may be made to press it close to the front by means of thin strips of glass, a wedge of glass or cork, or even a folded spring. When using the trough, it is necessary that the microscope Fig- 58- should be in a position nearly horizontal. » Dissecting Knives, &c. — Knives and needles of various THE MICROSCOPE. kinds and sizes are required for microscopic dissection : the best for the purpose are represented in figs. 59 and 60, being, Fig. 59. in fact, the very delicately made knives used by surgeons in operations upon the eye. Dissecting needles may be either straight or curved. They may be fixed, or made to take in and out of their handles. The most convenient Fig. 60. Fig. 61. are shown in fig. 61 ; those made of Palladium by Mr. Weedon, Hart Street, are very much the best. In the preparation of objects, no microscopist was ever more successful than Swammerdam : " His chief delight seems to have been in constructing very fine scissors, and giving them an extreme sharpness ; these he made use of DISSECTING TISSUES. 93 to cut very minute objects, because they dissected them equally, whereas knives, if ever so fine and sharp, are apt to disorder delicate substances. His knives, lancets, and styles were so fine, that he could not see to sharpen them without a magnifying glass." (Fig. 65.) The mode adopted for breaking up tissues into very small pieces is usually conducted, as represented at fig. 62, Fig. 62. — Teasing-out membi one. with a pair of the small needles held firmly between the fore-finger and thumb. The structure must be teased out ; an operation which requires care and perseverance, as most of the animal tissues are very difficult of separation. All substances should be carefully separated from dust and other impurities which renders their structure indistinct or confusing. With very delicate membranes, and with those of the nervous system of the smaller animals, in- sects, &c., it becomes necessary that the investigation should be carried on under water, or in fluid of some sort, in a glass cell, and having a strong light thrown down upon it by the aid of the condensing lens, as represented in fig. 63. A certain amount of change of structure must be expected and allowed for ; as nearly all membranes imbibe some portion of the fluid. Delicate structures are often advantageously wetted with dilute solutions of sugar or common salt, to prevent the changes from endosmosis, which result from the use of pure water. The contents of bodies are frequently rendered more distinct by the addi- tion of re-agents referred to further on. Cells or troughs are made out of pieces of stout plate- glass, their edges being accurately ground, and cemented 94 THE MICROSCOPE. together with marine-glue or sealing-wax : the size of the trough should be about three inches square and one deep. Fig. 63. — Dissecting under water. If thought desirable to dissect under the microscope itself, the instrument must be brought over the trough, and the subject adjusted to the focus of an inch or a two-inch magnifier, as it is difficult to employ a higher power. The simple microscope is that generally em- ployed for the purpose. If the object be a portion of an injected animal, it is better to pin it out on a piece of cork, covered with white wax, and then immerse it in the water-trough j the more delicate the structure, the sooner after death should it be examined, especially animal tissues. With most vegetable structures, the dissection should be carried on under water. The separation of the woody and vascular tissues, and the spiral vessels, is best effected by maceration and tearing with fine needles. Valentin's Knife. — For making fine sections of large substances, or those soft in structure, such as the liver, VALENTIN S DISSECTING KNIFE. Ito spleen, kidney, &c., the double-bladed knife, the invention of Professor Valentin, may be used with advantage. An- improved construction of this knife, by Professor Quekett, is represented in fig. 64. It consists of two blades, one of which is pro- longed by a flat piece of steel to form a handle, and has two pieces of wood riveted to it, for the purpose of its being held more steadily; to this blade another one is attached by a screw; this last is also lengthened by a shorter piece of steel, and both it and the preceding have slits cut out in them exactly opposite to each other, up and down which slit a rivet with two heads is made to slide, for the purpose either of allow- ing the blades to be widely separated or brought so closely together as to touch. One head of this rivet, being smaller than the hole in the end of the slit, can be drawn through it ; so that the blade seen in the front of the figure may be turned away from the other in order to be sharpened, or allow of the section made by it being taken away from between the blades. The blades are so con- structed that their opposed surfaces are either flat or very slightly con- cave, that they may fit accurately to each other, which is effected more completely by a steadying pin, seen at the base of the front blade. When the instrument is required to be used, Fis- 64- the thickness of the section about to be made will depend upon the distance the blades are apart : and this is regu- lated by sliding up or down the rivet, as the blades, by their own elasticity, will always spring open and keep the rivet in place ; a cut is then to be made by it, as with an ordinary knife, ind the part cut will be found between 96 THE MICROSCOPE. the blades, from which it may be separated either by open- ing them as wide as possible by the rivet, or by turning them apart in the manner before described, and floating the section out in water. Dissecting Scissors. — In addition to the forceps and knives, scissors will be necessary for the purposes of dissec- tion : of these the most useful are shown in fig. 65. They Fig. 65. — Dissecting Scissors. are made both straight and curved ; of the first kind, two pairs will be required, one having the extremities broad, and the other sharp-pointed ; if large dissections be under- taken, a still stronger pair, with the extremities broad, and made rough like a file, will be necessary. In dis- secting under the microscope, the curved-pointed pair shown at / are the most convenient. In all of these instruments the points should fit accurately together : sometimes those that are very sharp are apt to cross ; this may in a great measure be prevented by having the branches wide at the base where they are riveted. The points can be sharpened on a hone, and a magnifier employed to examine if they fit closely together. Fig. 66 represents Mr. Gibbon's " Section-cutting Ma- chine." It consists of a stout brass frame, A A, having an opening in the top plate, for a tube B, half an inch in diameter, and in depth one and a half inches. In this tube a loose piston, c, works freely, and is steadied by the slot seen in it. To a female screw z>, motion is given by the toothed wheel ; and the teeth of which, E, answer the triple purposes of thumb-milling, ratchet- stop, and SECTION CUTTING. 97 graduation. This is screwed to a block of wood, F, having a rabbet cut in for the purpose of securing it to the table. Fig. 66. — Section Cutting Machine. The machine is self-regulating, and is capable of being worked as rapidly as the skill of the operator may dictate. Sections of woods, when cut from hard woods containing gum, resin, &c., should be soaked in essential oil, alcohol, or ether, before they are mounted as transparent objects. A razor may be fixed to the bench for the purpose of cutting these fine sections, or a fine plane will answer very well. The instrument used by Mr. Topping, fig. 67, con- sists of a b, a flat piece of mahogany, seven inches long and four wide, to the under surface of which is attached, at right angles, a piece g of same size as a b. d is a flat plate of brass, four inches long and three wide, screwed to the upper surface of a b ; to the middle of this plate is attached a tube of the same metal e i, three inches long and half an inch in diameter, and provided at its lower end with a screw/, working in a nut, and having a disk k exactly adapted to the bore of the tube j this disk is con- nected with the upper end of the screw, and is moved up or down by it. c is another screw connected with a curved piece of brass A, which is capable of being carried to the opposite side of the tube by it. The piece of wood about 98 THE MICROSCOPE. to be cut is put into the tube e, and is raised or depressed by the screw/; whilst, before cutting, the curved piece of Fig. 67. — Topping's Section Cutting Machine. metal h should be firmly pressed against it by the screw c. This instrument, if fastened to the edge of a bench or table, is always ready for use. The knife employed may be one constructed for the purpose ; or a razor ground flat on one side. Method of making /Sections. — If the wood is green, it should be cut to the required length, and be immersed for a few days in strong alcohol, to get rid of all resinous matters. When this is accomplished, it may be soaked in water for a week or ten days ; it will then be ready for cutting. If the wood be dry, it should be first soaked in water and afterwards immersed in spirit, and before cutting placed in water again, as in the case of the green wood. The wood, if too large, should be cut so as to fit tightly into the square hole, and be driven into it by a wooden mallet; if, on the contrary, it be round, and at the same time too small for the hole, wedges of deal or other soft wood may be employed to fix it firmly : these will have the advantage of affording support, an,d if necessary, may be cut with the specimen, from which they may afterwards be easily separated. The process of cutting consists in raising the wood by the micrometer screw, so that the SECTION-CUTTING. 99 thinnest possible slice may be taken off by the knife ; after a few thick slices have been removed to make the surface level, a small quantity of water or spirit may be placed upon it; the screw is then to be turned one or more divisions, and the knife passed over the wood until a slice is removed; this, if well wetted, will not curl up, but will adhere to the knife, from which it may be removed by pressing blotting-paper upon it, or by sliding it off upon a piece of glass by means of a wetted finger. The plan generally adopted is to have a vessel of water by the side of the machine, and to place every section in it : those that are thin can then be easily separated from the thick by their floating more readily in the water; and all that are good, and not immediately wanted, may be put away in bottles with spirit and water, and preserved for future examination. If the entire structure of any exogenous wood is required to be examined, the sections must be made in at least three different ways; these may be termed the transverse, the longitudinal, and the oblique, or, as they are sometimes called, the horizontal, ver- tical, and tangental : each of these will exhibit different Fig. 68.— Sections of Wood. appearances, as may be seen upon reference to fig. 68 : b is a vertical section through the pith of a coniferous plant : this exhibits the medullary rays, which are known H2 100 THE MICROSCOPE. to the cabinet-maker as the silver grain; and at e is a magnified view of a part of the same: the woody fibres are seen with their dots I, and the horizontal lines k indi- cating the tiaedullary rays cut lengthwise ; whilst at c is a tangental section, and /a portion of the same magnified: the openings of the medullary rays m ra, and the woody fibres with vertical slices of the dots, are seen. Very instructive preparations may be made by cutting oblique sections of the stem, especially when large vessels are present, as then the internal structure of the walls of some of them may oftentimes be examined. The diagram above given refers only to sections of a pine ; all exogenous stems, however, will exhibit three different appearances, according to the direction in which the cut is made ; but in order to arrive at a true understanding of the arrangement of the woody and vascular bundles in endogens, horizontal and vertical sections only will be required. Many specimens of wood that are very hard and brittle may be much softened by boiling in water ; and as the cutting-machine will answer other structures besides wood, it may here be stated, that all horny tissues may also be considerably softened by boiling, and can then be cut very readily.1 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 lacunse and canaliculi show up very much better. When we wish 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, (1) See Professor Quekett on the Microscope. SECTIONS OF TEETH. 101 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. 69, cuts sections very quickly — then rub down, first by the aid of the corundtim-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 ; when well polished and dried, it may be preserved under thin glass, and cemented down with gold-size or varnish. Such polished sections are preferable to many others which, on account of their irregular surface, require to be covered with fluids, as Canada-balsam, turpentine. &c., in order to fit them for examination with high powers. It almost always happens, that some portion of these fluids enters the dentine, which then becomes indistinct, and almost invisible in its ramifications. Two sections made perpendicularly to one another through the middle of the crown and fang of a tooth, from before backwards, and from right to left, are suffi- cient to exhibit the more important features of the teeth ; but sections ought also to be prepared, showing the surface of the pulp cavity and that of the enamel ; and likewise various oblique and transverse sections through the dentine Fig. C9.— Small Lathe for polishing. 102 THE MICROSCOPE. in the fangs, to exhibit the anastomoses of their branches. The dental cartilage is easily shown by maceration in hydrochloric- acid, a process which requires a longer or 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, and lime ; it is much used by dentists as a polishing material. PREPARING SECTIONS. 103 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 lacunae 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 104 THE MICROSCOPE. into its interior, by adopting the following method : — A small quantity of balsam, proportioned to the size of the specimen, is to be spread upon a slip of glass, and to be rendered stiffer by boiling, until it becomes nearly solid when cold ; the same is to be done to the thin glass cover ; next, the specimen being placed on the balsamed surface, and being overlaid by the balsamed cover, such a degree of warmth is to be applied as will suffice to liquefy the balsam, without causing it to flow freely ; and the glass cover is then to be quickly pressed down, and the slide to be rapidly cooled, so as to give as little time as possible for the penetration of the liquefied balsam. Circular Disc. — For the purpose of cutting glass covers or making shallow cells with japanners' gold-size for mounting objects, fig. 70 will be found useful ; it is made Fig. 70.— Circular Disc Machine. of two circular wheels of wood, these being let into a solid block of wood, and secured there by central screws. A handle of wood is fixed into the upper part of one, for the purpose of turning it round, the motion being com- municated to the other by an endless band of catgut running in the grooved edge of each. On the upper sur- face of the wheel, under the right hand, are fixed, by means of screws, two strips of brass, which serve as springs for securing the glass slip ; a camel' s-hair pencil, previously dipped in japanners' gold- size, is then taken between the ON MOUNTING OBJECTS. 105 finger and thumb, and held as represented in the woodcut, when the wheel is put in motion, and a perfect circle is rapidly formed ; the cell is then removed and put aside to dry. In the same way, by securing a sheet of thin glass under the brass springs, and substituting for the pencil a cutting diamond, a circular cover may be readily cut out. A cutting diamond is not only useful to the micro- scopist for the above purpose, but also for writing the names of mounted objects on the ends of the glass slides. A glazier's diamond for cutting glass slides is both conve- nient and economical : the mode of using it may be acquired in any glazier's shop. Mr. Brooke used a small brass press for the purpose of cutting out thin glass for cells. This does its work so quickly and so well, that it is likely to supersede all other methods. On mounting and preserving Objects. — Microscopic ob- jects are usually mounted on slips of glass three inches by one inch, either dry or immersed in some fluid. The minute structures, such as the tissues of animals, parts of insects, vessels of plants, &c., must be preserved in thin cells, made as directed above, with a small amount of fluid.1 Clean the glass with a weak solution of ammonia or potash, from all grease, and wipe it dry with a piece of chamois leather or cotton velvet; cloth generally leaves behind it small filaments, which are always unsightly when seen near the object. Let fall a drop of the preserving fluid or Canada balsam on the centre of the glass ; then place the object in. it with a small pair of forceps, and Fig. 71. spread it out very carefully with the point of one of your fine needles. Select a thin glass cover, previously cleaned, (1) Cells for microscopic purposes may be purchased of Mr. Bender, 6 Bruns- wick Place, City Road, or of Mr. Baker, 244, High Holborn. 106 THE MICROSCOPE, touch its edges with cement, and let it fall gently and gradually down upon the object, as represented in fig. 71 ; press it lightly to exclude any excess of fluid, which re- move with strips of blotting-paper, being careful to do all this with a light hand, that small bubbles of air may not insinuate themselves to replace any lost fluid : air-bubbles are at all times unsightly, and liable to spoil an object when allowed to remain. Lastly, cement the edges of the cover to the bottom glass with japanners' gold-size, or sealing-wax varnish, carefully drawn around the edges with a camel's-hair pencil. To accomplish this more effectually, Mr. J. Gorham invented the " Brass cementing Pencil" fig. 72. It is a brass tube, six inches long, with a conical bore, having a lid to screw OD. A small portion of the cement, crumbled into fragments, is shot into the tube, which is then ready for use. In using this instru- ment, the extremity is gently heated in the flame of a spirit-lamp; and when the cement begins to ooze out, holding the pencil like a pen, the point is traced along each side of the cover, leaving a line of cement in the angle. The cement recommended by Mr. Quekett for cementing deep cells, is made by melting together two ounces of black resin, one ounce of bees'- wax, and one of vermilion. Mr. Hett prefers dark-coloured, and old, japanners' gold-size for securing the cells of his beautifully delicate in- jected preparations.1 Mr. Brooke uses Brunswick black, to which has been previously added a little India-rubber dissolved in mineral naphtha, to pre- Fi ?9 vent its cracking when dry. A very useful cement for fine work will be made by heating Canada balsam until it becomes a hard resin: dissolve it in ether, and it is ready for use. It possesses the advantage of drying as rapidly as collodion, and, if kept carefully corked, remains fluid for any length of time. Mr. Gorham's drawing and description of a "holder" is similar to one long used by the author, fig. 73, for the purpose of pressing together objects mounted in the dry (1) For methods of making #ood cements, consult Ure's Dictionary of Arts and Manufactures. ON MOUNTING OBJECTS. 107 way, and during the drying process, after maceration, which facilitates removal of objects from time to time, for the purpose of examining them. It is made of two strips of whalebone three or four inches in length, held together at each end with square pieces of brass ; these can be moved at pleasure towards the centre, and thus made to exert considerable pressure upon the pieces of glass and the object, which are placed crossways between the strips of whalebone. The small spring-nip sold under the name of the American clothes-peg, when filed down, makes an excellent holder. LIE TrmmirmTmiinnmniu.u Fig. 74.—Air-Pump. For the purpose of more effectually removing bubbles of air from the cells before the objects are cemented down, 108 THE MICROSCOPE. the small air-pump, fig. 74, is very serviceable. This is made by Baker, of Holborn, at a moderate price. The mode of using it, after the object has been placed under the bell-glass c, is by drawing up and pushing down the handle B, to pump out the air from around the object to be secured and preserved. By turning the small screw at D, you let in sufficient air to remove the bell-glass : it is better to allow the object to remain under for several hours until the cement around the edge of the glass cover becomes perfectly dried and secure ; then, upon exposure to the external air, it will no longer affect it. It will be also found useful in withdrawing the air from the cells of woods. The pump itself, A, when unscrewed, can be used as an injecting syringe for fine anatomical injections. JPig. 75. — Steam Bath, by Mr. Gibbons. The accompanying is a drawing of a simple form of Steam Bath, for mounting, and other uses. A is a conical tin boiler, five inches in diameter, just large enough to enable the operator to vaporize a small quantity of water when placed over a lamp. B is a cage of perforated metal, made to hold one or more objects, which should fit tightly at the collar D ; into this a small escape-pipe is stopped with cork or luting, so that the steam shall pass around the object, and escape through it. This will also serve for the immersion of objects, such as parts of insects, &c., in turpentine, previous to mount- ing them in balsam. The plan recommended is, to immerse ON COUNTING OBJECTS. 109 the objects in a bath of turpentine, and then exhaust it under the air-pump before applying the Canada balsam. The limpidity of the turpentine allows of the free escape of air, and when the object is removed from the bath to be mounted, the balsam blends with the turpentine, and fol- lows it into the minute cavities whither it could not alone have penetrated. This turpentine bath is most useful for killing insects, and possesses the advantage of making them protrude their probosces, lancets, &c. ; they are rendered more transparent than after killing by other means. If it is intended to dissect out any part, then Swammerdam's plan of suffocating in spirits will perhaps be more suitable. But the best plan is to touch the mouth and spiracles of the insect with a pencil dipped in creosote ; this has also a preservative effect, and equally with spirit possesses the advantage of hardening the viscera, which tends materially to aid in their dissection, at least so long as the albuminous portions are not very much coagulated, so as to cause the delicate organs to adhere together. Mr. Boys says : — " For mounting objects in Canada balsam, the first thing to be considered is the apparatus required. " 1st. A small single-wick oil-lamp, having a glass chimney about four inches long; the name to be about the size of that in a hand-lantern : a spirit lamp will do even better. " 2d. Slips of glass of required size, and small pieces of thin glass to cover the object, all well cleaned. " 3d. A pair of nippers to hold the glass slips. " 4th. A pointed iron piercer in a wooden handle. " 5th. A bottle containing the clearest Canada balsam, diluted with the best spirits of turpentine to a consistency allowing it to drop readily from one end of the iron piercer, or Mr. Gorham's cementing pencil. The preceding articles being spread before you ready for use, and the object to be displayed well examined for choice of position, and cleaned if necessary, fix the glass slip in the nippers, dip the piercer into the balsam, and withdraw a full -sized drop to place upon the slide. The centre of the slide should now be rested across the chimney of the lamp until the balsam begins to spread, when it must be immediately withdrawn. THE MICROSCOPE. The object is then to be placed on this balsam, and at once covered with a second drop, applied in the same way as the first ; in this state the slide should remain (covered to exclude the dust) for two or three minutes, that the balsam may have time to penetrate ; the thin glass is then to be taken up between the finger and thumb, and placed gently upon the balsam covering the object. The slide being now neld by the nippers at one end, place the other extremity over the lamp-chimney, so that the heat may be gradually extended towards the object. The proof of its having done so sufficiently will be that the balsam flows to the edge of the thin glass, taking with it all air-bubbles from that part nearest the heat. The slide is now to be turned, the heated end being placed in the nippers, and the process repeated. The slide should remain flat till nearly cool, when pressure should be made perpendicularly with a small piece of wood on the upper glass ; this Avill expel all superfluous balsam, and with it all extraneous matter. Should any air-bubbles remain, they generally disappear in a few days. If the balsam requires hardening, place the slide on the mantel- piece, the gentle heat here will prove sufficient." Mr. Deane recommends a composition of gelatine for mounting dry or moist animal or vegetable structures, in place of Canada balsam; his formula for which is as follows : — " Take of White's patent size, 6 ounces by weight ; honey, 9 ounces by weight ; add a little spirits of wine and a few drops of creosote; mix and filter whilst hot, to render perfectly clear. Or take of pure glycerine, 4 fluid ounces; distilled water, 2 fluid ounces; gelatine, 1 ounce by weight ; dissolve the gelatine in the water made hot, then add the glycerine and mix with care; this need not be filtered. " It is probable that some animalcules may be better shown if some moisture be allowed to remain in the medium, the evaporation from which may be stopped at any stage by filling round the edge of the cover with some gold- size varnish, or even boiled linseed-oil. For many delicate objects this has a great advantage over Canada balsam, in not possessing the high refractive power of that substance ; and the minute hairs and other parts of insects, ON MOUNTING OBJECTS. Ill that are quite obliterated with the balsam, are beautifully shown in this medium." Glycerine was introduced by Mr. Warington, as a pre- servative fluid for mounting organic substances in. The method adopted by him in the employment of glycerine, is simply to mount the object in the manner it is usually performed when spirit of wine or creosote-water is used as a medium, and having covered the immersed object with the thin glass, and removed all excess of liquid, to cement the margin with a coating of shell-lac varnish; the one usually employed consists of the ordinary black sealing- wax dissolved in rectified spirit of wine. Care must be taken during this operation to maintain the slider in a flat position, until the varnish has become dry from the eva- poration of the spirit, and also until a sufficient number of coatings or layers of the varnish have been applied to render the subject perfectly secure, and prevent any escape of the fluid. Gold-size or copal dissolved in oil of lavender may be employed to effect the same purpose ; and the second and third coatings may, with advantage, consist of either of these, which yield a tough varnish above the lac, which is otherwise liable to become brittle. The glycerine may be used in its concentrated form or treacly state, or it may be diluted with distilled water to any required extent, according to the object of the operator, and the subject to be mounted : if there be extremely fine markings on the subject, it is better to add about four or five times its volume of water, as otherwise the thick fluid may prevent these from being so sharply defined as may be desired. " I have," adds Mr. Warington, " a number of slides of Desmidiece, which have been mounted from four to ten months by this means, and they have kept excellently. The glycerine may also be used with the addition of a small portion of culinary salt, corrosive sublimate, creosote, or spirit of wine, if considered desirable." Glycerine and gelatine combined in certain proportions makes an excellent mounting material; it should be so mixed as to permit of its complete solidification, without liability to crack when cold and dry. Castor-oil may be used as a medium for mounting 112 THE MICROSCOPE. crystals of salts and other objects. The object required to be mounted is placed on the slider in its dry state, or de- posited wet and allowed to dry; or if in solution, a drop of the liquid is to be placed on the slip of glass, and allowed to crystallise by spontaneous evaporation ; in the latter case, take a drop of a warm saturated solution of the salt required, and when a good group of well-defined crystals has been obtained, break through the marginal ring of crystalline deposit with a small point of wood, and carefully conduct off the uncrystallised portion or mother-liquor to the extremity of the slide, at the same time placing it in a vertical position to drain until it is dry. A small quantity of castor-oil is to be next carefully dropped on the subject, and guided over the field with the point of a needle; in this way it readily displaces the air and occupies the most minute cavities. After a short time the upper glass is to be placed on the surface, taking care to lower it gradually so as to exclude the air; if the field is too full of oil, the excess may be removed by a small piece of bibulous paper; and if, on the contrary, sufficient oil has not been used, an additional portion can be readily introduced by the capillary action between the glasses. The shell-lac varnish is then to be used as the cementing medium in the same way as has been described, and with the same precautions. This varnish cannot be replaced by either of the others, as it is actually necessary (and this should always be borne in mind) that there should exist no affinity between the fluid in the cell and the varnish used to seal it permanently. Hundreds of excellent objects have been lost from this cause, and much valuable time and labour thrown away. A moderately strong solution of Arsenious Acid is found to answer well for the preservation of animal substances ; it is prepared as follows : — Boil some arsenious acid in distilled water ; when saturated, dilute with four to six parts of distilled water, and filter it for use. Keep in a stopped bottle. Mr. Goadby' s fluids are cheap and most effectual for preserving and mounting animal structures in. The fol- lowing are his formulae : — Take for No. 1 solution, bay salt, 4 oz. ; alum, 2 oz. ; corrosive sublimate, 2 grains ; boiling water, 1 quart : mix. PRESERVATION OF AIXLE. 113 For No. 2 solution, bay salt, 4 oz. ; alum, 2 oz. ; corrosive sublimate, 4 grs. ; boiling-water, 2 quarts : mix. The No. 1 is too strong for most purposes, and should only be employed where great astringency is needed to give form and support to very delicate structures. No. 2 is best adapted for permanent preparation ; but neither should be used in the preservation of animals containing carbonate of lime (all the mollusca), as the alum becomes decomposed, sulphate of lime is precipitated, and the pre- paration spoiled. For such use the following : — Bay salt, 8 oz. ; corrosive sublimate, 2 grs.; water, 1 quart : mix. The corrosive sublimate is used to prevent the growth of vegetation in the fluid ; but as this salt possesses the property of coagulating albumen, these solutions cannot be used in the preservation of ova, or when it is desired to maintain the transparency of certain tissues, such as the cellular tissue, the white corpuscles of the blood, &c. Mr. Goadby's method of making marine-glue for cement- ing cells is as follows : dissolve separately equal parts of shell-lac and India-rubber in coal or mineral naphtha, and afterwards mix the solutions carefully by the application of heat. It may be rendered thinner by the addition of more naphtha, and is always readily dissolved by naphtha, ether, or solution of potash, when it becomes hard or dry in our stock-pots. Preparation and Preservation of Algce, &c. — Mr. Ralfs gives excellent directions for making preparations of algse for microscopic investigations : — " The fluid found to answer best is made in the following way : to sixteen parts of distilled water add one part of rectified spirits of wine, and a few drops of creosote, sufficient to saturate it ; stir in a small quantity of prepared chalk, and then filter ; with this fluid mix an equal measure of camphor • water (water saturated with camphor); and before using, strain off through a piece of fine linen. " This fluid I do not find to alter the appearance of the endochrome of algse more than distilled water alone does after some time ; there is certainly less probability of confervoid filaments making their appearance in the pre- parations ; and there would seem to be nothing to prevent I 114 THE MICROSCOPE. such a growth from taking place, when the object is mounted in water only, provided a germ of one of these minute plants happen to be present, as well as a small quantity of free carbonic acid. " My method of making cells in which to mount pre- parations of algae is as follows : some objects require very shallow, and others somewhat deeper cells. The former may be made with a mixture of japanners' gold-size and litharge, to which (if a dark colour is preferred) a small quantity of lamp-black can be added. These materials should be rubbed up together with a painter's muller, and the mixture laid on the slips of glass with a camel-hair pencil as expeditiously as possible, since it quickly becomes hard ; so that it is expedient to make but a small quantity at a time. For the deeper cells marine-glue answers ex- tremely well, provided it is not too soft. It must be melted and dropped upon the slip of glass ; then flattened, whilst warm, with a piece of wet glass, and what is super- fluous cut away with a knife, so as to leave only the walls of the cell ; these, if they have become loosened, may be made firm again by warming the under surface of the slip of glass. The surface of the cells must be made quite flat ; which can be easily done by rubbing them upon a wet piece of smooth marble, covered with the finest emery- powder. " When about to mount a preparation, a very thin layer of gold-size must be put upon the wall of the cell, as well as on the edge of the piece of thin glass which is to cover it ; before this is quite dry, the fluid with the object is te be put into the cell, and the cover of thin glass slowly laid upon it, beginning at one end ; gentle pressure must then be used to squeeze out the superfluous fluid ; and, after carefully wiping the slide dry, a thin coat of gold-size should be applied round the edge of the cell, and a second coat so soon as the first is dry; a thin coat or two of black sealing-wax varnish may then be put on with advantage, in order to prevent effectually the admission of air into the cell, or the escape of fluid out of it. " I would remark, that the gold-size employed should be of the consistence of treacle ; when purchased, it is usually too fluid, and should be exposed for some time in PRESERVATION OF ALG^l. 115 an open vessel ; a process which renders it fit for use. In mounting the Desmidiece, great attention is necessary to exclude air-bubbles, which cannot be avoided unless the fluid completely fills the cells; and also not to use too much fluid, as in this case the smaller species will often be washed away on the escape of the superfluous portion. As the cells cannot be sealed whilst any moisture remains on their edge, it should be removed by blotting-paper, in preference to any other mode. A thin description of glass is manufactured expressly for the purpose of covering specimens when mounted. " The rare species of Desmidiece are frequently scattered amongst decayed vegetable matter, so that it is difficult to procure good specimens for mounting. In such cases, a small portion of the mass should be mixed with a little of the creosote fluid, and stirred briskly with a needle. After this has been done, the Desmidiece will sink to the bottom, when the refuse should be carefully removed. Successive portions having been thus treated, specimens will at length be procured sufficiently free from foreign matter. Even in ordinary circumstances, if a small extra quantity of fluid be placed in the cell, and the slide gently inclined, most of the dirt may be removed by a needle before the cell is closed; which process will materially increase the beauty of the preparation. " If the cells are insufficiently baked, the japan occa- sionally peels off the glass after the specimen has been mounted for some time. To obviate this inconvenience, Mr. Jenner previously heats the cell, with much caution, over a rushlight, until the japan becomes of a dark colour, and vapour ceases to arise from it. When gold-size is used for closing the cell, the intrusion of some of it frequently destroys valuable specimens, whatever care may be taken. Mr. Jenner has therefore relinquished it, and now employs a varnish made of coarsely comminuted purified shell-lac or translucent sealing-wax, to which is added rectified spirits of wine, in sufficient quantity to cover it. This varnish will be ready for use in about twelve hours : when it is too thick, a little more spirit should be added. Mr. Jenner applies three coats of this varnish, and about a week afterwards a fourth, composed of japan varnish or gold- i 2 116 THE MICROSCOPE. size.1 To preserve the brush in a fit state, it should always be cleaned with spirits of wine whenever it has been used." Mr. Topping's fluid for mounting consists of one ounce of rectified spirits to five ounces of distilled water \ this he thinks superior to any other combination. To preserve delicate colours, however, he prefers to use a solution of acetate of alumina, one ounce of the acetate to four ounces of distilled water : of other solutions he says, that they tend to destroy the colouring matter of delicate objects, and ultimately spoil them by rendering them opaque. Injecting Minute Vessels. — For minute injections, the most essential instrument is a proper syringe. This is usually made of brass, of such a size that the top of the thumb may press on the button at the top of the piston-rod when drawn out, while the body is sup- ported between the two fingers. Fig. 76 represents the syringe : a is a cylin- drical brass body, with a screw at the top for the purpose of firmly screwing down the cover 6, after the piston c is introduced into it ; this is rendered air-tight with leather ; the bottom of the syringe d also unscrews for the con- venience of cleaning ; e is a stop-cock, on the end of which another stop-cock / fits accurately ; and on the end of this either of the small pipes g, which are of different sizes, may be fixed. The Fig. 76. transverse wires across the pipes are in- tended to secure them more tightly to the vessels, into which they may be inserted with thread, so that they may not slip out. In addition to the syringe, a large tin vessel, to contain hot water, with two or three lesser ones fixed in it, for the injections, will be found useful. To prepare the material for injecting : — Take of the finest and most transparent glue one pound, break it into small pieces, put it into an earthen pot, and pour on it (1) " Coachmaker's Black" is an excellent varnish. INJECTING SMALL VESSELS. 117 three pints of cold water ; let it stand twenty-four hours, stirring it now and then with a stick; then set it over a slow fire for half an hour, or until all the pieces are per- fectly dissolved ; skim off the froth from the surface, and strain through a flannel for use. Isinglass and cuttings of parchment make an excellent size, and are preferable for very particular injections. If gelatine is employed it must be soaked for some hours in cold water before it is warmed. About an ounce of gelatine to a pint of water will be suf- ficiently strong, but in very hot weather it is necessary to add a little more gelatine. It must be soaked in part of the cold water until it swells up and becomes soft, when the rest of the water, made hot, is to be added. Good gelatine for injecting purposes may be obtained for two shillings a pound. The size thus prepared may be coloured with any of the following : — For Red. To a pint of size, add 2 oz. of Chinese vermilion. ,, Yellow. ,, ,, 2| oz. of chrome-yellow. „ White. „ ,, 3£ oz. of flake-white. ,, Blue. ,, ,, 6 oz. of fine blue smalts. It is necessary to remember, that whatever colouring matter is employed must be very finely levigated before it is mixed with the injection. This is a matter of great im- portance: for a small lump or mass of colour, dirt, &c. will clog the minute vessels, so that the injection will not pass into them, and the object will be defeated. The mix- ture of size and colour should be frequently stirred, or the colouring matter will sink to the bottom. Respecting the choice of a proper subject for injecting, it may be remarked, that the injection will usually go furthest in young subjects ; and the more the fluids have been exhausted during life, the greater will be the success of the injection. To prepare the subject, the principal points to be aimed at are, to dissolve the fluids, empty the vessels of them, relax the solids, and prevent the injection from coagulating too soon. For this purpose it is necessary to place the animal, or part to be injected, in warm water, as hot as the operator's hand will bear. This should be kept at nearly the same temperature for some time by occasionally adding hot water. The length of time required is in pro- portion to the size of the part and the amount of its 118 THE MICROSCOPE. rigidity. Ruysch (from whom the art of injecting has been called the Ruyschian art) recommends a previous maceration for a day or two in cold water. The size must always be kept hot with the aid of warm water; for if a naked fire be used, there is danger of burning it. The size may be placed in a vessel which can be heated by standing it in a common tin saucepan of hot water. A convenient form of apparatus for melting the size, and afterwards keeping it at a proper temperature, is Fig. "if.— Melting Vessel. shown in fig. 77. It consists merely of a water-bath, in which the cans containing the injecting fluid can be kept hot, and their contents protected from dust by means of their covers. A small apparatus of this kind could be made by any tin-worker, and fitted over a gas jet to stand on the table. The operator should be provided with several pairs of - strong forceps, for seizing the vessel or stopping the escape Fig. 78. of injection. A small needle, fig. 78, will be found useful for passing the thread round the vessel into which the INJECTING SMALL VESSELS. 119 injecting pipe is to be inserted. Where the vessels are large, a needle commonly known as an aneurism needle answers the purpose very well. The thickness of thread must vary according to the size of the vessel. The silk used by surgeons will be found the best adapted for the purpose, and not too thin, or it may cut through the vessel. When the size and the subject have both been properly prepared, have the injection as hot as the fingers can well bear. One of the pipes g, fig. 76, must then be placed in the largest artery of the part, and made secure by tying. Put the stop-cock / into the open end of the pipe e, ai.d it is then ready to receive the injection from successive applications to the syringe a, leaving sufficient space only for the piston c. The injection should be thrown in by a very steady and gentle pressure on the end of the piston- rod. The resistance of the vessels, when nearly full, is often considerable; but it must not be overcome by violent pressure with the syringe. When as much injection is passed as may be thought advisable, the preparation may be left (with the stop-cock closed in the pipe) for twenty- four hours, when more material may be thrown in. As the method of injecting the minute capillaries with coloured size is often attended with doubtful success, various other plans have been proposed. Kuysch's method, according to Rigerius, was to employ melted tallow coloured with vermilion, to which in the summer a little white wax was added. Monro recommended coloured oil of turpen- tine for the small vessels ; after the use of which, he threw in the common coarse injection. This is made of tallow and red lead, well mixed and heated before it is used. The cold paint injection succeeds well i-f thrown carefully into the minute arteries; but its tendency to become brown by age is an objection to its use. Professor Breschet frequently employed with success milk, isinglass, the alcoholic solution of gum-lac, spirit varnish, and spirit of turpentine ; but he highly recommends the colouring matter extracted from Campeachy, Fernambone, or Sandal woods. He says : " The colouring matter of Campeachy wood easily dissolves in water and in alcohol ; it is so penetrating that it becomes rapidly spread through 120 THE MICROSCOPE. the vascular net-works. The sole inconvenience of this kind of injection is, that it cannot be made to distend any except most delicate vessels, and that its ready penetra- tion does not admit of distinguishing between arteries, veins, and lymphatics." He also recommends a solution of caoutchouc. Another process, which may be termed the chemical process, was published in the Comptes Rendus, 1841, as the invention of M. Doyers. According to this, an aqueous solution of bichromate of potass, 1,048 grains to two pints of water, is thrown into the vessels; and after a short time, in the same manner and in the same vessels, an aqueous solution of acetate of lead, 2,000 grains to a pint of water, is injected. This is an excellent method, as the material is quite fluid, and the precipitation of the chromate of lead which takes place in the vessels themselves gives a fine sulphur-yellow colour. Mr. Topping prepares many fine injections in this way. Mr. Goadby has improved upon the process last named by uniting to the chemical solutions a portion of gelatine, as follows: Saturated solution of bichromate of potash, 8 fluid ounces; water, 8 ounces; gelatine, 2 ounces. Saturated |olution of acetate of lead, 8 fluid ounces; water, 8 ounces; gelatine, 2 ounces. The majority of preparations thus injected require to be dried and mounted in Canada balsam. Each preparation, when placed on a slip of glass, will necessarily possess more or less of the coloured infiltrated gelatine, (by this is meant the gelatine coloured by the blood, which, together with the acetate of potash resulting from the chemical decomposition, may have transuded through the coats of the vessel,) which, when dry, forms, together with the different shades of the chromate of lead, beautiful objects, possessing depth and richness of colour. The gelatine also separates and defines the different layers of vessels : the arteries are always readily distinguishable by the purity and brightness of the chromate of lead within them, while the veins are detected by the altered colour imparted by the blood. Those preparations which require to be kept wet can be INJECTING SMALL VESSELS. 121 preserved perfectly in Mr. Goadby's No. 2 fluid, specific gravity 1*100; the No. 1 fluid destroys them. " I would recommend that the slips of glass employed for the dry preparation be instantly inscribed with the name of the preparation, written with a diamond ; for, when dry, it is difficult to recognise one preparation from the other, until the operator's eye be educated to the effects of this chemico-gelatinous injection. Where so much wet abounds, gummed paper is apt to come off. When dry, it is sufficient, for the purpose of brief exami- nation by the microscope, to wet the surface of a prepara- tion with clean oil of turpentine ; immediately after examination, it should be put away carefully in a box, to keep it from the dust, until it can be mounted in Canada balsam. " The bichromate of potash is greatly superior in colour to the chromate, which yields too pale a yellow; and sub- sequent experience has proved that the acetate of potash frequently effects its liberation by destruction of the capillaries, and this even long after the preparations have been mounted in Canada balsam; perhaps this may be owing to some chemical action of the acetate of potash upon them. I would suggest the substitution of the nitrate for the acetate of lead, as we should then have, in the liberated nitrate of potash, a valuable auxiliary in the process of preservation. Although highly desirable, as the demonstrator of the capillaries of normal tissues, I do not think this kind of injection fitted for morbid prepara- tions ; the infiltrated gelatine producing appearances of a puzzling kind, and calculated to mislead the pathologist. In preparing portions of dried well-injected skin for examination by the microscope, I have tried the effect of dilute nitric acid as a corroder with very good results. But, probably, liquor potassa would have answered this purpose better. " When size-injection is to be employed, coloured either with vermilion or the chromate of lead, the animal should be previously prepared by bleeding, to empty the vessels ; for if they be filled with coagulated blood, it is quite im- possible to transmit even size, to say nothing of the colour- ing matter. Hence the difficulty of procuring good 122 THE MICROSCOPE. injections of the human subject. But with the chemico- gelatinous injections no such preparation is necessary; and success should always be certain, for the potash liquefies the blood, while constant and long-continued pressure by the syringe drives it through the parietes of the vessel into the cellular tissue." Transparent Injections. — " Much more strongly," writes Dr. Beale, "can I recommend to you the use of transparent fluids for making injections. It is true, that these are not likely to be so much admired by general observers as opaque injections. Indeed, it is not easy to find any object which will rival in beauty many tissues which have been freely injected with vermilion or chromate of lead; although it must be confessed that from such preparations we learn but little save the general arrangement of the capillary vessels of the part, their capacity, and the mag- nitude of the meshes of the network. Of the relations which these vessels bear to the elementary structures which give to the texture under examination its peculiar properties, such preparations tell us nothing. Opaque injections are for the most part only adapted for examina- tion with low powers, while the tissues to which the vessels are distributed can only be seen with the help of very high magnifying powers. Transparent injections, on the other hand, though they fail to excite the wonder of the un- initiated, show us not only the general arrangement of the capillary network, but the precise relation which each little tube bears to the tissue with which it is in contact. " In order to make injected preparations for examination by transmitted light, several different substances may be used as injecting fluids. " Injection with Plain Size. — A tissue which has been in- jected with plain size, when cold is of a good consistence for obtaining thin sections, and many important points may be learned from a specimen prepared in this manner which would not be detected by other modes of prepara- tion. A mixture of equal parts of gelatine and glycerine is, however, much to be preferred for this purpose, and the specimen thus prepared is sure to keep well. " Colouring Matters for Transparent Injections. — The chief INJECTING TISSUES. 123 colouring matters used for making transparent injections are carmine and Prussian Hue. The former is prepared by adding a little solution of ammonia (liquor ammonia) to the carmine, and diluting the mixture until the proper colour is obtained, or it may be diluted with size. " The Prussian Blue consists of an insoluble precipitate, so minutely divided, that it appears like a solution to the eye. The particles of freshly prepared Prussian blue are very much smaller than those of any of the colouring matters employed for making opaque injections. "Advantages of Employing Prussian Blue. — I have lately been employing Prussian blue very much, and according to my experience it possesses advantages over every other colouring matter. It is inexpensive, — may be injected cold, — the preparation does not require to be warmed, — no size is required — it penetrates the capillaries without the necessity of applying much force, — it does not run out when a section is made for examination, — neither do any particles which may escape from the larger vessels divided in making the section, adhere to it and thus render the section obscure, — a structure may be well injected with it in the course of a few minutes. Specimens prepared in this manner may be preserved in any of the ordinary pre- servative solutions, or may be dried and mounted in Canada balsam, (but I give the preference to glycerine, or glycerine jelly,) and they may be examined with the highest magni- fying powers. After having tried very many methods of making this preparation I have found the following one to succeed best. " Composition of the Prussian Blue Fluid for Making Transparent Injections : — Glycerine 1 oz. Wood, naphtha, or pyroacetic spirit . , . 1^ drachms. Spirits of wine 1 oz. Ferrocyanide of potassium 12 grs. Tincture of sesquichloride of iron ... 1 drachm. Water 4 ozs. " The ferrocyanide of potassium is to be dissolved in one ounce of the water, and the tincture of sesquichloride of iron added to another ounce. These solutions should be mixed together very gradually, and well shaken in a bottle. 124 THE MICROSCOPE. The iron 'being added to the solution of the ferrocyanide of potassium. When thoroughly mixed, these solutions should produce a dark blue mixture, in which no precipitate or flocculi are observable. Next, the naphtha is to be mixed with the spirit, and the glycerine and the remaining two ounces of the water added. This colourless fluid is, lastly, to be slowly mixed with the Prussian blue, the whole being well shaken in a large bottle during the admixture. The tincture of sesquichloride of iron is recommended because it can always be obtained of uniform strength. It is generally called the muriated tincture of iron, and may always be purchased of druggists. " Permit me, then, most earnestly to recommend all who are fond of injecting, to employ transparent injections, and to endeavour, by trying various transparent colouring matters, to discover several which may be employed for the purpose ; for I feel sure that by the use of carefully prepared transparent injections, many new points in the anatomy of tissues will be made out. " Of Injecting Different Systems of Vessels with Different Colours. — It is often desirable to inject different systems of vessels distributed to a part with different colours, in order to ascertain the arrangement of each set of vessels and their relation to each other. A portion of the gall- bladder in which the veins have been injected with white lead, and the arteries with vermilion forms a beautiful preparation. Each artery, even to its smallest branches, is seen to be accompanied by two small veins, one lying on either side of it. "In this injection of the liver, four sets of tubes have been injected as follows : — The artery with vermilion, the portal vein with white lead, the duct with Prussian blue, and the hepatic vein with lake. There are many opaque colouring matters which may be employed for double injections, but I am acquainted with very few transparent ones, the employment of which affords very satisfactory results. " Mercurial Injections are not much used for micro- scopical purposes, although mercury was much employed formerly for injecting lymphatic vessels and the ducts of glandular organs. The pressure of the column of mercury INJECTING TISSUES. 125 supersedes the necessity of any other kind of force for driving it into the vessels. The mercurial injecting apparatus consists of a glass tube, about half an inch in diameter and twelve inches in length, to one end of which has been fitted a steel screw to which a steel injecting pipe may be attached. The pipes and stopcocks must be made of steel, for otherwise they would be destroyed by the action of the mercury. " Injecting the Lower Animals. — The vessels of fishes are exceedingly tender, and require great caution in filling them. It is often difficult or quite impossible to tie the pipe in the vessel of a fish, and it will generally be found a much easier process to cut off the tail of the fish, and put the pipe into the divided vessel which lies imme- diately beneath the spinal column. In this simple manner beautiful injections of fish may be made, "Mollusca. — (Slug, snail, oyster, &c.) The tenuity of the vessels of the mollusca often renders it impossible to tie the pipe in the usual manner. The capillaries are, how- ever, usually very large, so that the injection runs very readily. In different parts of the bodies of these animals are numerous lacunae or spaces, which communicate directly with the vessels. Now, if an opening be made through the integument of the muscular foot of the animal, a pipe may be inserted, and thus the vessels may be injected from these lacunae with comparative facility. "Insects. — Injections of insects may be made by forcing the injection into the general abdominal cavity, when it passes into the dorsal vessel and is afterwards distributed to the system. The superfluous injection is then washed away, and such parts of the body as may be required, removed for examination. " Of the Practical Operation of Injecting. — I propose now to inject a frog and the eye of an ox, in order that you may see the several steps of the process. We must bear in mind that a successful injection cannot be made until the muscular rigidity which comes on shortly after death, and which affects the muscular fibres of the arteries as well as those of the muscles themselves, has passed off. In some few instances in which the fluid does not neces- sarily pass through arterial trunks before it reaches the 126 THE MICROSCOPE. capillaries (as in the liver), the injection may be effected satisfactorily immediately after the death of the animal. " The steps of the process are very similar in making the opaque injections, except that when size is employed, the specimen must be placed in warm water until warm through, otherwise the size will solidify in the smaller vessels, and the further flow of the injecting fluid will be prevented. Soaking for many hours is sometimes neces- sary for warming a large preparation thoroughly, and it is desirable to change the water frequently. The size must also be kept warm, strained immediately before use, and well stirred up each time the syringe is filled. " In the first place, the following instruments must be conveniently arranged : — " The syringe thoroughly clean and in working order, with pipes, stopcock, and corks. " One or two scalpels. " Two or three pair of sharp scissors. " Dissecting forceps. " Bull's-nose forceps, for stopping up any vessel through which the injection may escape accidentally. " Curved needle, threaded with silk or thread, the thick- ness of the latter depending upon the size of the vessel to be tied. " Wash-bottle. Injecting fluid in a small vessel. " I will commence with the frog. An incision is made through the skin, and the sternum divided in the middle line with a pair of strong scissors ; the two sides may easily be separated, and the heart is exposed. Next the sac in which the heart is contained (pericardium) is opened with scissors and the fleshy part of the heart seized with the forceps ; a small opening is made near its lower part, and a considerable quantity of blood escapes from the wound — this is washed away carefully by the wash-bottle. Into the opening — the tip of the heart being still held firmly by the forceps, a pipe is inserted and directed upwards towards the base of the heart to the point where the artery is seen to be connected with the muscular sub- stance. Before I insert the pipe, however, I draw up a little of the injecting fluid so as to fill it, for if this were not done, when I began to inject, the air contained in the INJECTING TISSUES. 127 pipe would necessarily be forced into the vessels, and the injection would fail. " The point of the pipe can with very little trouble be made to enter the artery. The needle with the thread is next carried round the vessel and the thread seized with forceps, the needle unthreaded and withdrawn, or one end of the thread may be held firmly, while the needle is with- drawn over it in the opposite direction. The thread is now tied over the vessel, so as to include the tip of the pipe only, for if the pipe be tied too far up, there is greater danger of its point passing through the delicate coats of the vessel. " The nozzle of the syringe, which has been well washed in warm water, is now plunged beneath the surface of the fluid, the piston moved up and down two or three times, so as to force out the air completely, and the syringe filled with fluid. It is then connected with the pipe, which is firmly held by the finger and thumb of the left hand, with a screwing movement, a little of the injection being first forced into the wide part of the pipe so as to prevent the possibility of any air being included. " The pipe and syringe being still held with the left hand, the piston is slowly and gently forced down with a slightly screwing movement with the right, care being taken not to distend the vessel so as to endanger rupture of its coats. The handle of the syringe is to be kept uppermost, and the syringe should never be completely emptied, in case of a little air remaining, which would thus be forced into the vessels. The injection is now observed running into the smaller vessels in different parts of the organism. " I will now proceed to inject the ox-eye in the same manner. The pipe is inserted into this branch of artery close to the nerve. Two minutes will probably be suffi- cient to ensure a complete injection. In making an injection of the eye, if the globe becomes very much distended by the entrance of the injecting fluid, an opening may be made in the cornea to allow the escape of the aqueous fluid which will leave room for the entrance of the injection, and permit the complete distension of the vessels. 128 THE MICROSCOPE. " We will now examine these injections. A portion of the intestine of the frog may be removed with scissors, opened, and the mucous surface washed with the aid of the wash-bottle. It may be allowed to soak in glycerine for a short time, and then examined. " This portion of the delicate choroidal membrane which has been carefully removed in the same manner shows the vessels perfectly injected, and in this preparation of the ciliary processes you will not fail to observe that all the capillary vessels are fully distended with fluid, although the injection was made so quickly. " Of Injecting the Ducts of Glands. — The modes of inject- ing which we have just considered, although applicable to the injection of vessels, are not adapted for injecting the ducts and glandular structure of glands ; for as these ducts usually contain a certain quantity of the secretion, and are always lined with epithelium, it follows that when we attempt to force fluid into the duct, the epithelium and secretion must be driven towards the secreting struc- ture of the gland, which is thus effectually plugged up with a colourless material, and there is no possibility of making out the arrangement of the parts. In such a case it is obviously useless to introduce an injecting fluid, for the greatest force which could be employed would be insuf- ficient to drive the contents through the basement mem- brane, and the only possible result of the attempt would be rupture of the thin walls of the secreting structure and extravasation of the contents. As I have before mentioned, partial success has been obtained by employing mercury, but the preparations thus made are not adapted for micro- scopical observation. " After death the minute ducts of the liver always contain a little bile. No force which can be employed is sufficient to force this bile through the basement membrane, for it will not permeate it in this direction. When any attempt is made to inject the ducts, the epithelium and mucus, in their interior, and the bile, form an insurmountable barrier to the onward course of the injection. Hence it was ob- viously necessary to remove the bile from the ducts before .one could hope to make a successful injection. It occurred to me, that any accumulation of fluid in the smallest INJECTING TISSUES. 129 branches of the portal vein or in the capillaries, must ne- cessarily compress the ducts and the secreting structure of the liver which fill up the intervals between them. The result of such a pressure would be to drive the bile towards the large ducts and to promote its escape. Tepid water was, therefore, injected into the portal vein. The liver became greatly distended, and bile with much ductal epithelium flowed by drops from the divided extremity of the duct. The bile soon became thinner, owing to its dilution with water, which permeated the intervening mem- brane, and entered the ducts. These long, narrow, highly- tortuous channels were thus effectually washed out from the point where they commenced as tubes not more than l-300th of an inch in diameter, to their termination in the common duct, and much of the thick layer of epithelium lining their interior was washed out at the same time. The water was removed by placing the liver in cloths with sponges under pressure for twenty-four hours or longer. All the vessels and the duct were then perfectly empty and in a very favourable state for receiving injection. The duct was first injected with a coloured material. Freshly precipitated chromate of lead, white lead, vermilion, or other colouring matter may be used, but for many reasons to which I have alluded, the Prussian blue injection is the one best adapted for this purpose. It is the only material which furnishes good results when the injected prepara- tions are required to be submitted to high magnifying powers. Preparations injected in this manner should be examined as transparent objects." l (i Of Preparing Portions of Injected Preparations for Mi- croscojrical Examination. — When thin tissues, such as the mucous membrane of the intestines or other parts, have been injected, it is necessary to lay them perfectly flat, and wash the mucus and epithelium from the free sur- face, either by forcing a current of water from the wash- bottle, or by placing them in water and brushing the surface gently with a camel's hair brush. Pieces of a convenient size may then be removed and mounted in solution of naphtha and creosote, in dilute alcohol, in (1) Dr. L. Beale, "On the Anatomy of the Liver of Man and Vertebrate Animals." 130 THE MICROSCOPE. glycerine, or in gelatine and glycerine. The most im- portant points in any such injections are shown .if the preparation be dried and mounted in Canada balsam. The specimen must, in the first place, be well washed and floated upon a glass slide with a considerable quantity of water, which must be allowed to flow off the slide very gradually. The specimen may then be allowed to dry under a glass shade, in order that it may be protected from dust. The drying should be effected at the ordinary temperature of the air, but it is much expedited if a shallow basin filled with sulphuric acid be placed with it under a bell-jar." Chemical Re-agents. — The following chemical re-agents and preservative fluids are recommended for microscopic uses:1 1. Alcohol, principally for the removal of air from sections of wood and other preparations ; also as a solvent for certain colouring matters. 2. jfflther, chiefly as a solvent for resins, fatty and other essential oils, &c. ; also useful for the removal of air. 3. Solution of Caustic Potass, as a solvent for fatty matters; also of use occasionally in consequence of its action upon the rest of the cell-contents and thickening layers. This solution acts best upon being heated. 4. Solution of Iodine (iodine one grain, iodide of potas- sium three grains, distilled water one ounce) for the coloration of the cell-membrane and of the cell-contents. 5. Concentrated Sulphuric Acid, employed chiefly in the examination of pollen and spores. 6. Diluted Sulphuric Acid (three parts acid, one part water), for the coloration of cells previously immersed in the iodine solution. The preparation is first moistened with the iodine solution, which is afterwards removed with a hair pencil, and a drop of sulphuric acid added by means of a glass rod ; the preparation is then immediately covered with a piece of glass. The action of the sulphuric acid and iodine, as well as that of the iodised chloride of zinc solution, is not always uniform throughout the whole (1) A set of 12 test-bottles, packed in a small box, is supplied by Mr. Matthews of Portugal Street, Lincoln's Inn Fields, the price of which is only a few shillings. CHEMICAL RE- AGENTS. 131 surface of the preparation. The colour is more intense where the mixture is more concentrated; it frequently happens that many spots remain uncoloured. The colour changes after some time, the blue being frequently changed into red after twenty-four hours. 7. A Solution of Chloride of Zinc, Iodine, and Iodide of Potassium. A drop of this compound solution, added to a preparation placed in a little water, produces the same colour as iodine and sulphuric acid. This solution, which was first proposed and employed by Professor Schultz, is more convenient in its application than iodine and sulphuric acid, and performs nearly the same services, while it does not, like the sulphuric acid, destroy the tissues to which it is applied. It is prepared by dissolving zinc in hydro- chloric acid; the solution is then saturated with iodide of potassium : more iodine is to be added if necessary, and the solution diluted with water. 8. Nitric Acid, or what is better, chlorate of potass and nitric acid, as an agent to effect the isolation of cells. The mode of employing this agent, also discovered by Professor Schultz, is as follows : — The object, a thin section of wood, for instance, is intro- duced, with an equal bulk of chloride, or chlorate of potass, into a long and moderately wide tube, and as much nitric acid as will at least cover the whole. The tube is then warmed over a spirit-lamp ; a copious evohition of gas takes place, upon which the tube is re- moved from the flame, and the action of the oxidising agent allowed to continue for two or three minutes. The con- tents of the tube are then poured into a watch glass with water, from which the slightly cohering particles are col- lected and placed in a tube, and again boiled in alcohol as long as any colour is communicated. They are again boiled in a little water. The cells may now be isolated under the simple microscope by means of needles. The boiling with nitric acid and chlorate of potass should never be carried on in the same room with the microscope, the glasses of which may suffer injury from the vapours. The same remark applies to all chemical vapours. Thin sections of vegetable tissue are warmed for half a K 2 132 THE MICROSCOPE. minute, or a minute, in a watch-glass: boiling is here un- necessary. The section is taken out, and treated with water in another watch-glass. 9. Oil of Lemons, or any other essential oil, a drop of which will be found of value in the investigation of pollen and spores. Lastly may be enumerated a pretty strong solution of Carbonate of Soda and also of Acetic Acid; which latter, however, is more especially useful in the investigation of animal tissues. To the above may be added a test for protein compounds. This test is composed of sugar and sulphuric acid, and is thus employed: — A thin section or portion of the tissue to be examined is placed in a drop of simple syrup, this is Ihen removed by means of a hair pencil, and a drop of the diluted sulphuric acid added ; the red colour usually does not appear until after the lapse of about ten minutes. In making thin sections of tissues, it is recommended that, in those objects the consistence of which differs in different parts, the section should be carried from the harder into the softer portion; also, in making a thin section of a very minute yielding substance, to enclose it between two pieces of cork, and to slice the whole together. It is also useful sometimes to saturate the object with mucilage, which is to be allowed to dry slowly; in this way very delicate tissues may be sliced, or otherwise divided without injury, and with great facility. Some of the above re-agents must be used with caution, as it is not unusual for them to assume crystalline forms while under the microscope. Without a knowledge of this fact, and a perfect recognition of crystalline forms, errors in micro- chemical research must occur. For example, if a drop of liquor potassae be allowed to evaporate on a slip of glass, crystals appear, chiefly of six-sided tables, precisely like cystine ; when in quantity, they are often crowded together as the cystine plates are, and sometimes exhibit a similar nucleus-like body in their centres. This peculiarity of crystallization does not arise from the presence of impurities ; perfectly pure potash often exhibits the same phenomenon. The form of the crystals of acetate of potash varies according to the strength of the acid out of which it COLLECTING OBJECTS. crystallizes, and if formed out of strong acid, very much resembles that of the crystals of uric acid ; when mixed up with other forms, long dagger-like or lancet-shaped crystals are seen, which might well deceive. We may also notice in this place what Majendie pointed out, that in certain albuminous mixtures, iodine loses the property of colouring starch blue. This difficulty must be got rid of before iodine can be said to be an infallible test in micro-chemistry. Collecting Objects. — Mr. G. Shadbolt contributes the fol- lowing useful hints for collecting objects for microscopical examination : — " Rivers, brooks, springs, fountains, ponds, marshes, bogs, and rocks by the sea-side, are all localities that may be expected to be productive; some being more prolific than others, and the species obtained differing, of course, in genera], to a certain extent, according to the habitat. On considering the nature of some of the places indicated, it will be apparent that, in order to spend a day in col- lecting with any comfort, it will be necessary to make some provision for keeping the feet dry, for which a pair of India-rubber goloshes will answer, or better still, a pair of waterproof fishing-boots ; but without one or other the work is by no means pleasant. " A dozen or two of small bottles made of glass-tubing, about half an inch in diameter, and without necks, and from one to two inches in length, are the most convenient depositories for the specimens, if intended ultimately for mounting; and it is advisable also to take two or three wide-mouthed bottles of a larger size, holding from one to two fluid-ounces, an old iron spoon, a tin box, some pieces of linen or calico, two or three inches square, a piece of string, a slip or two of glass, with the edges ground, such as are used for mounting objects; and lastly, a good and pretty powerful hand-magnifier. Two Coddingtoii lenses, mounted in one frame of about half an inch, and one-tenth of an inch in focal power, are specially convenient. " Swanscombe Salt-marsh will be found well worth a visit; and it can be reached by steam-boat or railway from London-bridge to Northfleet. On quitting the rail- way station, make towards the almshouses on the top of 134 THE MICROSCOPE. the hill; and arriving at the road, turn to the left, descend the hill, and cross a sort of bridge over a somewhat insig- nificant stream. Continue along the main-road a little farther, to a point where it begins to ascend again, and diverges to the left towards the railway; here quit it, taking your course along an obscure road, nearly in a direct line with the main one ; passing a windmill on the right hand, and continuing until you arrive at another still more obscure road, turning off to the right ; which road appears as if made of the mud dredged from the bottom of the river, and partially hardened. This is Swanscombe Salt-marsh; and the road just described leads towards Broad Ness Beacon. On either side is a sort of ditch; one containing salt or very brackish water, the other filled with a sort of black mud, about the con- sistence of cream, the surface being in parts of a slaty grey, with little patches here and there of a most brilliant brown colour, glistening in the sunshine, and presenting a striking contrast to the sombre shade. By carefully in- sinuating the end of one of the slips of glass under this brilliant brown substance, and raising it gently, it can be examined with the Coddington; and it will probably be found to consist of myriads of specimens of Pleurosigma (navicula of Ehrenberg) angulatum, or balticum, or some other species of this genus. The iron spoon is now useful, as by its aid the brown stratum, with little or no mud, can be skimmed off and bottled for future examination. On the surface of the water in the other ditch may be noticed a floating mass of a dark olive colour, which to the touch feels not unlike a lump of the curd of milk, and consists of Cyclotella menighiniana, and a surirella or two em- bedded in a mass of Spirulina hutchinsia; and another mass of floating weed, which feels harsh to the touch, pro- ceeding from a quantity of a synedra, closely investing a filamentous alga; and elsewhere Melosira nummuloides (gallionella of Ehrenberg). " In a trench by the sea-wall, as it is termed, is a mass of brown matter of a shade somewhat different to any hitherto observed, adhering to some of the parts of the trench, being partially submerged, and having a some- what tremulous motion on agitating the water. This is a COLLECTING OBJECTS. 135 species of Schizonema; and it consists of a quantity of gelatinous hollow filaments filled with an immense number of bright-brown shuttle-shaped bodies, like very minute namculce. " It is not necessary to be particular about collecting the specimen free from mud, as the filaments are so tough that the mud can be readily washed away by shaking the whole violently in a bottle of water, and pouring off the mud, without at all injuring the specimen. The Amphi- porium alatum communicates a somewhat frothy appear- ance to the otherwise clear water, and to get any quantity of this requires a little management ; but by skimming the surface with the spoon, and using one of the larger bottles, an abundance may readily be obtained. Between the sea-wall and the river the marsh is intersected in every direction with a number of meandering creeks, being in some places eight to ten feet deep, though in others quite shallow; but it is exceedingly difficult to make one's way amongst them, and I have never found them so prolific any where, on the few occasions of my visiting the place, as in the parts more away from the influence of the tide. It will be observed, that the brilliant brown colour, of a deep but bright cinnamon tint, is one of the best indica- tions of the presence of diatomacece; and though this is by no means universal, the variation is most frequently de- pendent upon the presence of something which qualifies the tint. The peculiarity of the colour is due to the endochrome contained in the frustule ; and this must in general be got rid of before the beautiful and delicate marking can be made out. But it is highly advantageous and instinctive to view them in a living state; and this should be done as soon as possible after reaching home with all specimens procured from salt-water localities, as they rapidly putrefy in confinement, and emit a most disgusting odour, not unlike that arising from a box of inferior congreve-matches. " Washing in fresh water, and then immersing in creo- sote water, preserves many of the species in a very natural- looking manner; but they are killed by the fresh water, and the endochrome becomes much condensed in the Pleurosigmata and some other species. The addition of 136 THE MICROSCOPE. spirits quite spoils the appearance of the frustules, as it dissolves the endochrome. " There is another salt-marsh a little farther down the same railway, at Higham, which it would be well to explore. The most favourable months for procuring dia- tomacece, are April, May, September, and October; but some species are found in perfection as early as February, and many as late as November, and a few at all times of the year. There is a piece of boggy ground near Keston, beyond Bromley, in Kent, where the river Ravensbourne takes its rise, where many interesting species of desmidiece and other fresh-water algse may be procured. From Bromley, walk on towards Keston, passing near Hayes Common and Bromley Common on the right. Continue for about another half-mile along the road, and then turn to the right hand; pass the reservoirs, and approach au open space where there is a bog of about a quarter of a mile in extent; and tending towards the right, make your way amongst heaths, ferns, mosses, and the beautiful Drosera rotundifolia (sun- dew), to the lower part of the little stream rippling through a sort of narrow trench in the Sphagnum, &c. By working your way up the stream, you avoid the inconvenience which would otherwise be experienced of the water being rendered turbid, in con- sequence of having to tread in the boggy ground. In the centre of the little stream may be observed something of a pale pea-green colour flickering about in the current, which, on your attempting to grasp, most likely eludes you, and slips through the fingers, from being of a gelatin- ous natiire. It consists of a hyaline substance, with a comparatively small quantity of a bright green endo- chrome, disposed in little branches, and this is the Dra- parnaldia glomerata. Another object is a mass of green filaments, rather harsh to the touch, and very slippery. When viewed with a lens of moderate power, each filament is seen to be surrounded with several bands of green dots, looking like a ribbon twisted spirally, and may be recog- nised as a species of Zygnema. In various parts there are other Zygnemacece, as tyndaridea, mougeotia, meso- carpus, and many more. " Keeping up the stream, and occasionally diverging a COLLECTING OBJECTS. 137 little on either side of it, amongst the miniature bays and pools formed by the sphagnum, on looking straight down into the water we shall probably see at the bottom a little mass of jelly of a bright green, studded with numerous brilliant bubbles of oxygen-gas. This is the general appearance of most of the desmidiece, as Micrasterias, Eu- astrum, Closterium, Cosmarium, &c. The spoon is also a handy tool in this case, though, by practice, the finger will do nearly as well; the chief difficulty arises when the specimen is brought to the surface of the water, it not being easy to get it out without losing a considerable por- tion of it. Little pools in the bog, made by the footsteps of cattle, are particularly good spots to find desmidiece, many species being in a very contracted space. The most prolific bog is at Tunbridge Wells, near a house known as Fisher's Castle, not far from Hurst Wood. There is also a good one at Esher, at a spot called West-End. It must not be imagined that nothing can be obtained in this department of botany without going some distance from town ; but assuredly only commoner and fewer species can be met with nearer home. At the West India Docks are Synedra fasciculata, Gomphonema curvata, Diatoma elonga- tum, Diatoma milgare, Surirella ovata, &c. ; and at this same place a few objects, not of the botanical class, as Spongilla fluviatilis, Cordylophora lacustris, Alcyonella, stagnorum, Fig. si. a c d, are 101° 55' and 78° 6'. The line a x, called the axis of the rhomb, or of the crystal, is equally inclined to each of the six faces at an angle of 45° 23.' It is very transparent, and generally colourless. Its natural faces when it is split are commonly even and per- fectly polished ; but when they are not so, we may, by a new clevage, replace the imperfect face by a better one, or we may grind and polish an imperfect face. It is found that in all bodies where there seems to be an irregularity of structure,, as salts, crystallised minerals, &c., on light passing through them, it is divided into two distinct pencils. If we take a crystal of Iceland spar, and look at a black line or dot on a sheet of paper, there will appear to be two lines or dots; and on turning the spar round, these objects will seem to turn round also; and twice in the revolution they will fall upon each other, which occurs when the two positions of the spar are exactly opposite, that is, when turned one-half from the position (1) Brewster's " Optics." L 146 THE MICROSCOPE. where it is first observed. In the accompanying diagram, fig. 82, the line appears double, as a 6 and c d, or the dot, as e and/. Or allow a ray of light, g h, to fall thus on the crystal, it will in its passage through be separated into two rays, hf,he; and on coming to the opposite surface of the crystal, they will pass out at ef in the direction of i k, parallel to g h. The plane I m n o is designated the prin- cipal section of the crystal, and the line drawn from the solid angle I to the angle o is where the axis of the crystal is contained; it is also the optic axis of the mineral. Now when a ray of light passes along this axis, it is undivided, and there is only one image ; but in all other directions there are two. If two crystals of Iceland spar be 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 in the second, and so on vice versd. At the intermediate position of the two crystals there is a subdivision of each ray, and therefore four images are seen ; when the crystals are at an angle of 45° to each other, then the images are all seen of equal intensity. Mr. Nicol first succeeded in making rhombs of Iceland spar into single-image prisms, by dividing one into two equal portions. His mode of proceeding is thus described in the Edinburgh Philosophical Journal (vol. vi. p. 83) : POLARISED LIGHT. 147 " A rhomb of Iceland spar of one-fourth of an inch in length, and about four-eighths of an inch in breadth and thickness, is divided into two equal portions in a plane, passing through the acute lateral angle, and nearly touching the obtuse solid angle. The sectional plane of each of these halves must be carefully polished, and the portions cemented firmly with Canada balsam, .so as to form a rhomb similar to what it was before its division ; by this management the ordinary and extraordinary rays are so separated that only one of them is transmitted : the cause of this great divergence of the rays is considered to be owing to the action of the Canada balsam, the refractive index of which (1 -549) is that between the ordinary (1-6543) and the extraordinary (1-4833) refraction of calcareous spar, and which will change the direction of both rays in an opposite manner before they enter the posterior half of the combination." The direction of rays Fig. 83. passing through such a prism is indicated by the arrow, fig. 83, and the combination is shown mounted, one for Fig. 84. Fig. 85. use under the stage of the microscope, fig. 84, termed the polariser; another, fig. 85, screwed on to and above the L2 148 THE MICROSCOPE. object-glasses, is called the analyser. The definition 'is better if the analyser is placed at top of the A eye-piece, and it is more easily rotated than the polariser. Method of using the polarising Prism, fig. 84. — After having adapted it to slide into a groove on the under-surface of the stage, it is held in its place by turning the small milled-head screw at one end : the other prism, fig. 85, is screwed on above the object-glasses, and made to pass into the body of the microscope itself. The light having been reflected through them by the mirror, it becomes necessary to make the axes of the two prisms coincide ; this is done by regulating the milled-head screw, until by revolving the polarising prism, the field of view is entirely darkened twice during one revolution. This should be ascertained, and carefully corrected by the maker and adapter of the apparatus. If very minute salts or crystals are to be viewed, it is preferable to place the ana- lyser above the eye-piece; it will then require to be mounted as in fig. 86. Thus the polariscope consists of two parts ; one for polarising, the other for analysing or testing the light. There is no essen- tial difference between the two parts, except what convenience or economy may lead us to adopt ; and either part, there- fore, may be used as polariser or analyser; but whichever we use as the polariser, the other becomes the analyser. The tourmaline, a precious stone of a neutral or bluish tint, forms an excellent analyser; it should be cut about ^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 stone as perfect as possible, the size is of no importance when used with the microscope. In the illumination of objects by polarised light, when under view with high powers, for the purpose of obtaining Fig. 86. . POLARISED LIGHT. 149 the maximum effect, it is also requisite that the angle of aperture of the polariser should be the same as the object- glass, each ray of which should be directly opposed by a ray of polarised light. The Polarising Condenser is merely an ordinary achromatic condenser of large aperture, close cinder the bottom lens of which is placed a plate of tour- maline, used in combination with a superposed film of selenite or not. as required. The effect of this arrangement on some objects is very remarkable, bringing out strongly colours which are almost invisible by the usual mode. The production of colour by polarised light has been thus most clearly and comprehensively explained by Mr. Woodward, in his " Introduction to the Study of Polarised Light."1 Y Fig. 87 B. abed represent the rectangular vibrations by which a ray of common light is supposed to be propagated. e, a plate of tourmaline, called in this situation the polariser, and so turned that a b may vibrate in the plane of its crystallographical axis. (1) Mr. Woodward constructed a very available form of polariscope for most purposes ; the instrument is described in Elements of Natural Philosophy, by the author. 150 THE MICROSCOPE. /, light polarised by £, by stopping the vibrations c d, and transmitting those of a b. g, a piece of selenite of such a thickness as to produce red light, and its complementary colour green. h} the polarised light / bifurcated, or divided into ordi- nary and extraordinary rays, and thus said to be de- polarised by the double refractor g, and forming two planes of polarised light, o and e} vibrating at right angles to each other. i, a second plate of tourmaline, here called the analyser, with its axis in the same direction as that of e, through which the several systems of waves of the ordinary and extraordinary rays h, not being inclined at a greater angle to the axis of the analyser than that of 45 degrees, are transmitted and brought together under conditions that may produce interferences. k, the waves 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, 87 B, depolarised light, as fig. 87 A, i, the analyser turned one quarter of a circle, its axis being at right angles to that of i in fig. 87 A. k, the waves R o R e, for red light of the ordinary and extraordinary systems meeting together with a difference of an odd number of half undulations, and thus neutral- ising each other by interference. I m, the waves Y o Y e and B o B e, for yellow and blue of the two systems severally meeting together in the same state of vibration, occasioned by the difference of an even number of half undulations, and forming by their coin- cidences waves of doubled intensity for yellow and blue light, POLARISED LIGHT. 151 7i, 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 Ni col's prisms for the two plates of tourmaline, and by the addition of the object-glass and eye-piece, the diagrams would then represent the passage of polarised light through a microscope. For showing objects by polarised light under the micro- scope that are not in themselves doubly refractive, put upon the stage a film of selenite, which exhibits, under ordinary circumstances, the red ray in one position of the polarising prism, and the green ray in another, using a double-image prism over the eye-piece ; each arc will assume one of these complementary colours, whilst the centre of the field will remain colourless. Into this field introduce any microscopic object which in the usual arrangement of the polariscope undergoes no change in colour, when it will immediately display the most brilliant effects. Sections of wood, feathers, algse, and scales, are among the objects best suited for this kind of exhibition. The power suited for the purpose is a two-inch object- glass, the intensity of colour, as well as the separating power of the prism, being impaired under much higher amplification; although in some few instances, such as in viewing animalcules, the one-inch object-glass is perhaps to be preferred. Selenite is the native crystallised hydrated sulphate of lime. A beautiful fibrous variety called satin gypsum is found in Derbyshire. It is found also at Shotover Hill, near Oxford, where the labourers call it quarry-glass. Very large crystals of it are found at Montmartre, near Paris. The form of the crystal most frequently met with is that of an oblique rectangular prism, with ten rhomboidal faces, two of which are much larger than the rest. It is usually slit into thin laminae parallel to these large lateral faces ; the film having a thickness of from one- twentieth to the one-sixtieth of an inch. In the two rec- tangular directions they allow perpendicular rays of pola- rised light to traverse them unchanged ; these directions are called the neutral axes. In two other directions, however, which form respectively angles of 45° with the 152 THE MICROSCOPE. neutral axes, these films have the property of double refraction. These directions are known as the depolarising axes. The thickness of the film of selenite determines the particular tint. If, therefore, we use a film of irregular thickness, different colours are presented by the different thicknesses. These facts admit of very curious and beau- tiful illustration, when used under the object placed on the stage of the microscope. The films employed should be mounted between two glasses for protection. Some persons employ a large film mounted in this way between plates of glass, with a raised edge, to act as a stage for supporting the object, it is then called the " selenite stage." The best film for the microscope is that which gives blue, and its complementary colour yellow. Mr. Darker has constructed a very neat stage of brass for this purpose, producing a mixture of all the colours by superimposing three films, one on the other ; by a slight variation in their positions, produced by means of an endless-screw motion, all the colours of the spectrum are shown. When objects are thus exhibited, we must bear in mind that all the negative tints, as we term them, are diminished, and all the positive ones increased ; the effect of this plate is to mask the true character of the phenomena. Polarised structures should therefore never be drawn and coloured under such conditions. Dr. Herapath, of Bristol, described a salt of quinine, which is remarkable for its polarising properties. The salt was first accidentally observed by Mr. Phelps, a pupil of Dr. Herapath's, in a bottle which contained a solution of disulphate of quinine: the salt is formed by dissolving disulphate of quinine in concentrated acetic acid, then warming the solution, and dropping into it carefully, and by small quantities at a time, a spirituous solution of iodine. On placing this mixture aside for some hours, brilliant plates of the new salt will be formed. The crystals of this salt, when examined by reflected light, have a brilliant emerald-green colour, with almost a metallic lustre ; they appear like portions of the elytrse of cantha- rides, and are also very similar to murexide in appearance. When examined by transmitted light, they scarcely possess POLARISED LIGHT. 153 any colour, there is only a slightly olive-green tinge ; but if two crystals, crossing at right angles, be examined, the spot where they intersect appears perfectly black, even if the crystals are not one five-hundredth of an inch in thick- ness. If the light be in the slightest degree polarised — as by reflection from a cloud, or by the blue sky, or from the glass surface of the mirror of the microscope placed at the polarising angle 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 chocolate or deep chestnut-brown, instead of black. As the result of a series of very elaborate experiments, Dr. Herapath finds that this salt possesses the properties of tourmaline in a very exalted degree, as well as of a plate of selenite ; so that it combines the properties of polarising a ray and of depolarising it. Dr. Herapath has succeeded in making artificial tourmalines large enough to surmount the eye-piece of the microscope; so that all experiments with those crystals upon polarised light may be made without the tourmaline or Nicol's prism. The brilliancy of the colours is much more intense with the artificial crystal than 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 exten- sive field than can be had with the prism.1 These crystals frequently lose their polarising property. AVhen out of use they should be kept in a dark, dry place. Mr. Lobb has had one in use three years, it is as good at the present time as it was on the day he purchased it from Messrs. Home and Thornthwaite. A variety of interesting phenomena have been described by Mr. S. Legg, in the Transactions of the Microscopical Society. He observes: "The following experiments, if carefully performed, will illustrate the most striking phenomena of double refraction, and form a useful introduction to the practical application of this principle. 154 THE MICROSCOPE. "A plate of brass, fig. 88, three inches by one, perforated with a series of holes from about one-sixteenth to one- Fig. 88.— 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 POLARISED LIGHT. 155 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 is placed at an angle of 45° from the square part of the stage, no particular colour will be perceived; and it will then illustrate the phenomena of the neutral axis of the selenite, because when placed in that relative position no depolarisation takes place. The phenomena of polarised light may be further illustrated by the addition of a second double image prism, and a film of selenite adapted between the two. The systems of coloured rings in crystals cut perpendicularly to the principal axis of the crystal are best seen by employing the lowest object-glass." To show the phenomena of the rings reund the optic axes of the crystals, Mr. Lobb adopts the following plan, which is by far the best, and the rings are exhibited in the greatest perfection : — 1. The 6 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. 106 THE MICROSCOPE. the microscope to reach the polariser, that all extraneous light may be excluded. The concave mirror should be used with a bull's-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 AY hen the tourmaline is turned to the right, as the other does when turned to the left. Quartz. — Cut so as to exhibit straight lines. Cole 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. Roclielle 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 POLARISED LIGHT. 157 close as possible to the eye-lens, to see properly the pheno- mena in quartz and aragonite ; it must be placed at an intermediate distance for viewing topaz, borax, Rochelle salt, and carbonate of lead ; it must be drawn out to its full extent to view nitre and calc spar. The powers of the micro- polariscope cannot be better displayed than in the exhibition of the foregoing pheno- mena; there is nothing more beautiful, and few studies more interesting and enlarging to the mind than that of light, whether common or polarised, which must be entered upon if the phenomena are to be understood. The crystal eye-piece, with an artificial tourmaline as an analyser, will be found very useful for polariscope objects generally; there is some spherical aberration, but the largeness of the field far more than compensates for the same ; it does best for those objects that require the two- inch object glass. Mr. Darker, 6, Princes Street, Lambeth, is the only person in England who cuts the crystals properly ; and in Paris, M. Soliel, Rue de 1'Odeon. It was long believed that all crystals had only one axis of double refraction; but Brewster found that the great body of crystals, which are either formed by art, or which occur in the mineral kingdom, have two axes of double re- fraction, or rather axes around which the double refraction takes place; in the axes themselves there is no double refraction. Nitre crystallises in six-sided prisms with angles of about 120.° It has two axes of double refraction, along which a ray of light is not divided into two. These axes are each inclined about 2 1° to the axes of the prism, and 5° to each other. If, therefore, we cut off a piece from a prism of nitre with a knife driven by a smart blow of a hammer, and polish the two surfaces perpendicular to the axes of the prism, so as to leave the thickness of the sixth or eighth of an inch, and then transmit a ray of polarised light along the axes of the prism, we shall see the double system of rings shown in figs. 89 and 90. When the line connecting the two axes of the crystal is inclined 45° to the plane of primitive polarisation, a cross is seen as at fig. 89, on revolving the nitre, it gradually 158 THE MICROSCOPE. assumes the form of the two hyperbolic curves, fig. 90. But if the tourmaline be revolved, the black crossed lines will Fig, 90. ' be replaced by white spaces, and the red rings by green, the yellow by indigo, and so on. These systems of rings have, generally speaking, the same colours as those of thin plates, or as those of a system of rings round one axis. The orders of the colours commence at the centres of each system; but at a certain distance, which corre- sponds to the sixth ring, the rings, instead of returning and encircling each pole, encircle the two poles as an ellipse does its two foci. When we diminish or increase the thickness of the plate of nitre, the rings are diminishes 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. 91. Dr. W. B. Herapath contributed an interesting addi- tion to the uses of polarised light, by applying it to discover the salts of alkaloids, quinine, &c. in the urine of patients. POLARISED LIGHT. 159 He says : " It has long been a favourite subject of inquiry with the professional man to trace the course of remedies Fig. 91.— Urinary Salts. a, Uric acid; b, Oxalate of lime, octahedral crystals of; c, Oxalate of lime allowed to dry, forming a black cube; d, Oxalate of lime, as it occasionally appears, termed the dumb-bell crystal. in the system of the patient under his care, and to knew what has become of the various substances which he migut have administered during the treatment of the disease. " Having been struck with the facility of application, and the extreme delicacy of the reaction of polarised light, when going through the series of experiments upon the sulphate of iodo-quinine, I determined upon attempting to bring this method practically into use for the detection of minute quantities of quinine in organic fluids ; and after more or less success by different methods of experimenting, I have at length discovered a process by which it is possible to obtain demonstrative evidence of the presence of quinine, even if in quantities not exceeding the one-millionth part of a grain ; in fact, in quantities so exceedingly minute, that all other methods would fail in recognising its existence. Take for test-fluid a mixture of three drachms of pure acetic acid, with one fluid-drachm of rectified spirits-of- wine, to which add six drops of diluted sulphuric acid.1 " One drop of this test-fluid placed on a glass-slide, and the merest atom of the alkaloid added, in a short time i'l) A drop of a solution of Cinckonidine is better. See Addenda. 160 THE MICROSCOPE. solution will take place ; then, upon the tip of a very fine glass-rod let an extremely minute drop of the alcoholic solution of iodine be added. The first effect is the produc- tion of the yellow or cinnamon-coloured compound of iodine and quinine, which forms as a small circular spot ; the alcohol separates in little drops, which by a sort of repul- sive movement, drive the fluid away ; after a time, the acid liquid again flows over the spot, and the polarising crystals of sulphate of iodo-quinine are slowly produced in beautiful rosettes. This succeeds best without the aid of heat. t( 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, oeneach 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 that tint which an analysing-plate of tourmaline would produce when at right angles to the polarising-plate ; POLARISED LIGHT. 161 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. 92, a and b. Not only do these peculiar crystals act in the way just related, but they may be easily proved to possess the whole of the optical properties of that remark- able salt of quinine, the sulphate of iodo-quinine. " To test for quinidine, it is merely necessary to allow the drop of acid solution to evaporate to dry ness 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 cruss very vividly shown, start into existence, should quinidine be present even in very minute traces. These crystals are represented in fig. 93. Fig. 93. " If 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-microscope : the black cross at once disappears, and is replaced by one which consists of two colours, being divided into a cross THE MICROSCOPE. Ci g. 94.— 5?2ow Crystals. SNOW CRYSTALS. 163 having a red and green fringe, whilst the four intermediate sectors are of a gorgeous orange-yellow. These appear- ances alter upon the revolution of the analysing- plate of tourmaline ; when the blue stage is employed, the cross will assume a blue or yellow tint, according to the position of the analysing-plate. These phenomena are analogous to those exhibited 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 his better information on the subject. A great variety of animal, vegetable, and other sub- stances possess a doubly refracting or depolarising struc- ture, as : a quill cut and laid out flat on glass ; the cornea of a sheep's eye ; skin, hair, a thin section of a finger-nail ; sections of bone, teeth, horn, silk, cotton, whalebone ; stems of plants containing silica or flint ; barley, wheat, &c. The larger-grained starches form splendid objects; tons- les-mois, being the largest, may be taken as a type of all M 2 164 THE MICROSCOPE. the others. It presents a black cross, the arms of which meet at the hilum. On rotating the analyser, the black cross disappears, and at 90° is replaced by a white cross ; another, but much fainter black cross being per- ceived between the arms of the white cross. Hitherto, however, no colour is percep- tible. But if a thin plate of selenite be interposed between Flg' 95' the starch-grains and the po- Potato Starch, seen under polarised ,. ° , i i • j j light. lariser, most splendid and delicate colours appear. All the colours change by revolving the analyser, and become complementary at every quadrature 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 Treatise on the Use of the Microscope. THE BINOCULAR MICROSCOPE. 165 APPLICATION OP BINOCULARITY TO THE MICROSCOPE. The application of this principle to microscopic pur- poses seems to have been tried as early as 1677, by a French philosopher, le Pere Cherubin, of Orleans, a Capu- chin friar. The following is an extract from the description given by him of his instrument : " Some years ago I resolved to effect what I had long before premeditated, to make a microscope to see the smallest objects with the two eyes conjointly ; and this project has succeeded even beyond my expectation, -with advantages above the single instrument so extraordinary and so surprising, that every intelligent person to whom I have shown the effect, has assured me that inquiring philosophers will be highly pleased with the communication." This communication long slumbered and was forgotten, and nothing more was heard of the subject until Professor Wheatstone's very surprising invention of the stereoscope, when it again attracted the attention of this philosopher, who applied to both Eoss and Powell to make him a binocular instrument. But this was not done ; and during the year 1853 a notice appeared in Sillimaris American Journal of a binocular instrument constructed by Professor Riddell of America, who had contrived a binocular micro- scope in 1851, with the view "of rendering both eyes service- able in microscopic observations." ct Behind the objective," he writes, "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 first 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 2i inches distant. This even had disadvantages and inconveniences, which Mr. F. H. Wenham ingeniously succeeded in modifying and improving. 166 THE MICROSCOPE. In 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. 96 aft are enlarged drawings of a portion of \ the egg of the common bed-bug \ (Cimex lecticularis), the operculum /which covers the orifice having been forced off at the time the young was hatched. The figures exactly represent the two positions that the inclined orifice will oc- cupy when the right and left hand portions of the object-glass are stopped off. It was illumi- nated as an opaque object, and drawn under a two-thirds object-glass of about 28° of aperture. If this experiment is repeated, by holding the card over the eye-piece, and stopping off alternately the right and left half of the ultimate emergent pencil, exactly the same changes and appearances will be observed in the object under view. THE BINOCULAR MICROSCOPE. 1G7 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. 97, represents the methods that I have contrived for obtaining the effect of bringing the two eyes Fig. 97. 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, 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-J-0. I also find it is requisite to grind away the contact edges of the prisms, as represented, as it prevents the extreme margins 168 THE MICROSCOPE. of the reflecting surfaces from coming into operation, which can seldom be made very perfect. " The definition with these prisms is good ; but they are liable to objection on account of the extremely small portion of the field of view that they take in, and which arises from the distance that the eyes are of necessity placed beyond the focus of the eye-piece, where, the rays being divergent, the pupil of the eye is incapable of taking them all in ; also there is great nicety required in the length of the prisms, which must differ for nearly every different observer. " I have constructed an adjusting binocular eye-piece, not differing in principle from the last. The first reflection is performed by means of a triangular steel prism, with the two inclined facets very highly polished; this is repre- sented by the dotted outline g g. The rays, after having been reflected at right angles, are taken up by two rec- tangular glass prisms, shown by the dotted lines at //. The best effect that I have yet produced in the way of binocular vision applied to the microscope, is that next to be described, in which I have altogether dispensed with reflecting surfaces, merely using three refracting prisms, which, when placed together, are perfectly achromatic. a a, diagram 2, fig. 97, is a single prism of dense flint- glass, with the three surfaces well polished ; b b are two prisms- of crown-glass of half the length of the under flint- prism, to the upper inclines of which they are cemented with Canada balsam. " The angle of inclination to be given to the prisms must depend upon the dispersive power of the flint and crown glas& employed. In the combination that I have worked out, I have used, for the sake of simplicity, some flint and crown that Mr. Smith kindly furnished me with, in which the dispersive powers are exactly as two to one ; consequently, I have had to make the angle of the crown just double that of the flint, in order to obtain perfect achromatism. The refractive power of each must also be known, that we may determine the angles of the prisms suitable for refracting the rays from the object-glass into the two eyes, at a distance of nine inches, c, fig. 97, represents a ray of light incident at right angles upon the THE BINOCULAR MICROSCOPE. 169 under- surface of the flint-prism. On leaving the second surface, and entering the crown-prism, it is slightly bent inwards, and on finally emerging, it is refracted outwards in the direction required. The base of the compound prism should not be larger than is sufficient to cover the stop of the lowest object-glass, in order that they may be made very thin. " The method of applying the prism to the binocular microscope is shown in fig. 98 : a a is the object-glass ; 5, the prism placed as close be- y hind it as the fittings will admit. The prism is set in an aperture in a flat disc of brass, which has a horizontal play in every direc- tion, in order that it may be ad- justed and fixed in such a position that the junction of the prisms may bisect the rays from the ob- ject-glass, and at the same time be at right angles to the trans- verse centres of the eye-piece ; c c are the two bodies of the micro- scope, provided with the draw- tubes and the usual eye-pieces d d. The distance between them should be rather less than the average distance asunder of the eyes ; and in cases where these are very wide apart, we can pull out the draw- tubes, which will increase the distance between the eye-pieces. "With this apparatus I ob- tain the whole of the field of Fig- 98' view in each eye ; which circumstance I was not prepared to expect, as this must, in some measure, depend upon the correction of the oblique pencils of the object-glass, for we cannot expect to look obliquely through the objec- tive of a compound achromatic microscope in the same way as in the single lens arrangement, but can only avail ourselves of such oblique pencils of rays as are corrected for passing through the axis of the microscope." 170 THE MICROSCOPE. Mr. Wenham subsequently improved and simplified this arrangement, a detailed account of which will be found in the volume of the Journal of Microscopical Science for ISM. APPLICATION OF PHOTOGRAPHY TO THE MICROSCOPE. . At the time this book was projected, it was thought that if the objects so beautifully exhibited under the microscope could be drawn by light on the page of the book, or on the wood-blocks, so that the engraver might work directly from the drawings thus made, truthfulness would be in- sured, and we should present to the reader a valuable record of microscopic research never before seen or attempted. But in this we were doomed to disappoint- ment by the existence of a patent, which presented ob- stacles too great to be surmounted ; and the idea was abandoned, with the exception of a few drawings then prepared, and ready to hand : the patent restrictions having been since removed, we have embodied them in our pages. The eye and feet of fly, antenna of moth, paddles of whirli- gig, with a few others, were first taken on a film of collo- dion, then floated off the glass on to the surface of a block of wood, the wood having been previously and lightly inked with printer's ink or amber-varnish, and the film gently rubbed or smoothed down to an even surface, at the same time carefully pressing out all bubbles of air or fluid. For the purposes of photography the only necessary addition to the ordinary microscope is that of a dark chamber ; it should indeed form a camera obscura, having at one end an aperture for the insertion of the eye-piece end of the microscopic tube, and at the other a groove for carrying the crown-glass for focussing. This dark chamber must not exceed eighteen inches in length ; for if longer, the pencil of light transmitted by the object-glass is dif- fused over too large a surface, and a faint and unsatis- factory picture results therefrom. Another advantage is, that pictures at this distance are in size very nearly equal to the object seen in the microscope. In some instances, better pictures are produced by taking away the eye-piece APPLICATION OF PHOTOGRAPHY. 171 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. Wenbam gives direc- tions for improving " microscopic photography " in the Quarterly Journal of Microscopical Science for January, 1855. In this paper he has shown how to insure quick and accurate focussing ; or, in other words, the making of the actinic and visual foci of the objective coincident. The simplest and cheapest way of producing coincidence is to screw a biconvex lens into the place of the back-stop of the object-glass, which thus acts as part of its optical com- bination. An ordinary spectacle lens, carefully centred and turned down to the required size, answers the purpose exceedingly well. An excellent method has been proposed and adopted by Mr. Wenham, for exhibiting the form of certain very minute markings upon objects. A negative photographic impression of the object is first taken on collodion, in the ordinary way, with the highest power of the microscope that can be used. After this has been properly fixed, it is placed in the sliding frame of an ordinary camera, and the frame end of the latter adjusted into an opening cut in the shutter of a perfectly dark room. Parallel rays of sunlight are then thrown through the picture by means of a flat piece of looking-glass fixed outside the shutter at such an angle as to catch and reflect the rays through the camera. A screen standing in the room, opposite the lens of the camera, will now receive an image, exactly as from a magic lantern, and the size of the image will be propor- THE MICROSCOPE. 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. 99; a sec- tional view of his arrangement is here given, which is Fig. 99. — Highley'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 APPLICATION OF PHOTOGRAPHY. 173 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. Two exquisitely delineated negative objects obtained in this way by Mr. Delves were afterwards printed as positives, for the purpose of illustrating an excellent paper on the " Application of Photography to Microscopy," No. 3 of the Quarterly Journal of Microscopical Science. PART II. CHAPTER I. VEGETABLE STRUCTURE — VITAL AND CHEMICAL CHARACTERISTICS— MICRO- SCOPIC FORMS OF VEGETABLE" LIFE — THE VEGETABLE CELL — FUNGI — FUNGOID DISEASES — MOSSES— ALG.-E— CONFERV/E— DESMIDIACE.*:— STRUC- TURE OF PLANTS— ADULTERATION OF ARTICLES USED FOR FOOD — PREPARATION FOR MICROSCOPIC EXAMINATION, ETC. INCE the introduction of the achro- matic microscope, we have obtained nearly the whole of the valuable information which we now possess relative to the minute structure of vegetables. Before that time, al- though some progress had been made in vegetable physiology, yet the means of distinguishing one structure from another, with their several external characters, compre- hended the amount of our botanical knowledge. " The vegetation which everywhere adorns the surface of the globe, from the moss that covers the weather-worn stone, to the cedar that crowns the moun- tain, is replete with matter for reflection. Not a tree that lifts its branches aloft, not a flower or leaf that expands beneath the sunlight, but has something of habit, of structure, or of form, to arrest the attention." VITAL CHARACTERISTICS OF PLANTS. 175 The microscopist sees proof of a higher life in plants than he before conceived ; and he becomes convinced, after examining the functions which their organs are destined to perform, that animals and plants are only separate links in the great chain of organic nature. The vegetable kingdom is divided into three great classes — the Dicotyledonous, or Exogenous plants, the Monocoty- ledonous, or Endogenous plants, and the Acotyledonous, Plants of the first and second classes bear flowers, and are found in the temperate zones : those of the third do not flower, and include the simplest forms of vegetable life, being mostly found in warm climates. The characteristics of exogenous plants, are first the branched or reticu- lated veining of their leaves, next, the formation of their stems, which consist of central pith, wood, and bark : they increase in size by means of layers of new substance every year deposited between the two latter. This mode of growth gives to sections of their stems a ringed appear- ance, the number of rings corresponding to the number of years of growth. These rings are crossed at intervals by straight lines, — the medullary rays, — which diverge from the central pith, connecting them with the bark.1 Plants are organised beings ; that is, organisms com- posed of a number of essential and mutually dependent parts : in common, therefore, with animals, they possess a principle which is in continued action ; and which operates in such a manner, that the individual parts which it forms in the body, are adapted to the designs of the whole. Or, in more intelligible language, plants are living bodies; like animals, they are the offspring of other beings similar to themselves; they grow, are endowed with excita- bility, have their periods of infancy, adult age, decay, and death. Their affinity to animals is much closer than is commonly supposed. The vital or creative power exists already in the germ, in plants as well as in animals; and by its influence the essential parts of the future plant are formed. It might be supposed that the lateral generation of plants — namely, that renewal of the individual which is the result of budding or gemmation — is sufficient to dis- (I) See Dr. Lindley's Elements of Botany, for an excellent description of vegetable structure; or Henfrey's Elementary Course of Botany. 176 THE MICROSCOPE. tinguish them from animals ; but this opinion is erroneous, as we find that the formation of gems or buds is common in animals belonging to the class Protozoa. In the hydra we perceive the germs developed as small ovoid eleva- tions upon the cylindrical body of the animal, and are, when examined in this state, like the first formation of the buds in plants, mere masses of cells ; but as their growth proceeds, these cells undergo a special arrange- ment, so as to produce the different tissues of the body, and acquire the proper form of the polype: on the same principle, the bud in the plant is gradually developed, until it terminates and becomes a branch. Plants, like animals, possess excitability, or the faculty of being acted upon by external stimuli, impelling them to the exertion of their vegetable powers. Light acts on plants, directing the growth of the stem, vigour, and colour, the direction of the branches, position of leaves, the opening and shutting of flowers. Heat influences the protrusion of buds, and other stimulants affect vegetable irritability; as an instance of which, cut plants, when fading, revive if placed in water impregnated with certain chemicals. Besides the physical and physiological distinctions gene- rally pointed out as marking the line between animals and plants, chemistry furnishes many others. Thus, one of the great functions of a plant is to decompose water, and assi- milate its components to the vegetable tissues ; and it is equally a property of animal life constantly to reform itself from the same elements. The oxygen derived from the atmosphere, by whatever means it is introduced into the animal system, is expended in the production of carbonic acid and water, both of which are thrown off as excretions. It is true that water is exhaled in great quantities from the surfaces of plants ; but it is that fluid which has been taken into the system of the plant, and has not undergone decomposition; it is, therefore, not actually found in the body of the vegetable, as it is in that of the animal. During the process of vegetation, protein is formed from the constituents of water with carbonic acid and ammo- nia; protein is formed in the animal body, and enters largely into the blood and muscle. VITAL CHARACTERISTICS OF PLANTS. 177 There is the closest affinity in the chemical nature of the products between plants and animals. Vegetable albumen is identical in composition with that in blood and in eggs ; casein does not materially differ in milk and the juices of some plants : we have many other equally striking characteristics, which modern chemical investigations have unfolded. Plants in some characteristics differ most strik- ingly, in being almost destitute of voluntary sensation and motion : here we would not have sensibility confounded with instability, a principle which plants, in common with animals, possess. The simplest forms of animal life mani- fest both sensation and volition, even those that are fixed to rocks and other bodies presenting a ramified and vege- tative form ; for instance, in the compound polypes, each individual polype displays both sensation and voluntary motion. It is, nevertheless, difficult to attribute satisfac- torily the movement of some plants to irritability alone. Thus we find plants, in an apartment with light admitted on one side, not only turn the upper surface of their leaves to the light, but bend their stems and branches towards it. Many other instances might be cited ; but none of them, excepting the movements of the Oscillator ia,1 more closely resemble volition. Plants, again, differ from animals in having no nervous system. Another great distinction is connected with the function of digestion, which the simplest form of animals possess : those even which turn inside out, the hydra, have an internal cavity, into which their food is taken at intervals ; but vegetables are nourished from the surface, and by continual imbibition. It has been supposed, because the sap rises in plants, and in the- interior of the internodia and cells of some simple plants, a rotatory motion of fluid can be perceived, that plants, like animals, have a circulation of fluids This opinion is at least disputable, the sap of plants as- (1) Oscillatoria, a genus of confcrvoid algae, the filaments of which are en- closed in tubular cellulose sheets, open at the ends, from which the fragments emerge when they are broken across. It is the remarkable spontaneous move- ments of the Oscillator iucece, which make them objects of so much interest for the microscopist. They are found on damp ground, amongst mosses, rocks, stones, and in fresh and salt water. Another of the same family frequently covers over the surface of standing water, to which it imparts a green colour; it is called Aphanizomenon Flos-aquce, by Morr and Dr. Hassall. N 178 THE MICKOSCOPE. cending only once, — for that which is termed the descend- ing sap of the plant is the proper juice prepared in the leaf; and the fact of currents being observed in opposite directions, is no proof of the existence of a circulation. But it may be asked, is the motion in the Chara or the cells of the Vallisneria spiralis, or in the hairs of the radicle fibres of frog's-bit, any proof of a circulation ? It is certainly a proof of the motion of a fluid in the cells of a plant, and is very different from a general circulation of the sap ; which is the only answer that can be made to such an inquiry: and the true circulation in animals is derived from an internal impelling power, and not from external influences. A more distinctive character is obtained in the products of the respiratory function in plants : respiration is per- formed by the entire surface in most animals, as it is by all plants ; but the products are different. In plants, the process consists chiefly in the conversion of carbonic acid and water into vegetable matter ; hence oxygen is exhaled from the leaves, and carbonic acid absorbed by them from the atmosphere ; and it is by the decomposition of that acid in the leaf, that the greater part of the oxygen is restored to the air. And although plants exhale carbonic acid during the night and in the shade, yet the quantity is small ; and plants are, in reference to their respiration, a balance in the opposite scale to animals ; they remove from the air the carbonic acid exhaled from the lungs and spiracles of animals, and re-supply the oxygen requisite for their respiration. Without the vegetable tribes, the atmo- sphere would soon cease to be fitted for the present race of animals; without the carbonic acid formed by animal respiration, plants would lose the greater part of their nutriment; and by their reciprocal action the atmosphere is preserved very nearly unchanged. Therefore the most important difference between the two may be said to be essentially that pointed out by Dr. Lankester, in the nature of the distinctive character of the gases inhaled and exhaled "by animals and by plants. Dr. Carpenter accepts this as a sufficiently distinctive line of demarcation between the two kingdoms. In an address to the Microscopical Society, he says : " I wish to VITAL CHARACTERISTICS OF PLANTS. 179 stop for a moment, to notice how strongly the differences between the vegetable and animal kingdoms are marked out, even in the lowest and simplest forms of both. The Protophytes, like the most perfect plants, draw their nutri- ment from the inorganic compounds which are everywhere within their reach, — water, carbonic acid, and ammonia ; by decomposing carbonic acid, they give off oxygen ; and they form for themselves the starch and the chlorophyll, the cellulose and the albumen, which they apply to the augmentation of their own substance. On the other hand, even those humblest Protozoa, the Rhizopoda, can only exist (so far as we can see) upon organic materials pre- viously elaborated by other beings : these they receive ' bodily ' into their interior ; and though mouth, stomach, intestine, and anus, all have to be extemporized every time that the animal feeds, yet the digestion which the alimentary particles undergo in its interior, is not less complete than that which is performed by the most elabo- rate apparatus which we anywhere meet with; and the nutrient materials thus obtained seem to be appropriated, without any further conversion, to the augmentation of the substance of the body. Thus, notwithstanding the remarkable analogy which these two orders of beings exhibit, I cannot see that any difficulty need be experi- enced in separating them, when we are acquainted with their mode of nutrition. The Gregarina constitutes no real exception ; for although it imbibes its nutriment through its entire surface, like the Protopkyte, yet that nutriment has been previously digested and prepared for it by the animal whose body it inhabits ; and in the ab- sence of any oral orifice or digestive apparatus of its own, it corresponds with a far higher group of animals, the Cestoid Worms, which live under the same conditions. Some recent observations, it is true, would seem to invali- date this distinction, by showing that certain Ehizopods and Infusoria have their origin in undoubted plants ; but we must be permitted for the present to withhold our assent from conclusions so strange, and to question whether they may not be invalidated by some unsuspected fallacy. It has been well remarked, however, that l there is no limit to the possibilities of Nature;' and I should be the last to N 2 180 THE MICROSCOPE. attempt to set up as fixed laws what are merely the ex- pressions of the present state of our knowledge, or to wish to throw discredit on the observations of accomplished and careful microscopists, merely because they overthrow distinctions which I had imagined to be well founded. J would strongly recommend the observations of Professor Hartig (Quart. Journ. of Microsc. Science, vol. iv., p. 51) and of Mr. Carter (Ann. of Nat. Hist., Feb., 1856) to your attentive scrutiny." As we pass on to a more intimate examination of the various structures entering into a plant, it will be seen that we have objects of the deepest interest presented to our notice ; and strikingly differing as we find plants and animals in some essentials, we shall here, at our starting- point, find them gradually coalescing, until they meet in a common granule — "that of the simple and individual cell." Miilder, in describing this starting-point of life, says : " The cell is a concave globule. This concave globule is an individual; in the most simple form in which it can possibly exist (in the lowest moulds), it possesses all the powers of the molecules united into one whole, and thus reduced to a state of equilibrium. This state depends, not only on the nature of the substances and of their elements, carbon, hydrogen, oxygen, and nitrogen, but also on their form. The state of equilibrium, therefore, could not exist, unless this concave globular form existed. More- over, this hollow globule possesses the whole of these forces in a state of mutual combination, co-operating for one end ; this being a peculiarity which also apparently depends on the globular form." Cells from which plants are formed are very small deli- cate closed sacs, partaking of many forms, and enclosed in a perfectly transparent membrane, so excessively thin, that it is with difficulty detected, unless iodine or some colour- ing-matter be previously added. Dead and old cells form an exception, as they become thickened, and the broken surfaces are then readily detected. At one time the cells were said to be developed by an extrication of gaseous matter among mucus; but the double walls which separate cells are irreconcilable with such an origin. Mr. Thwaites VEGETABLE CELLS. 181 regards the original wall of the cell as a mere shell, having quite a subordinate office to perform in the growth of plants; and he ascribes all the vital powers of growth to the eytoblast and colouring-matter of the central nucleolus. He supposes the cell-membrane to arise from the action of electrical currents upon mucus, and that fissiparous division is caused by the presence of two centres of electrical force, each giving rise to a set of currents, and producing two cell-membranes instead of the original one.1 The first and most curious exemplification of the simple cell is the fungi known as the Yeast Plant: it consists of two parts, the cell-wall, composed of a matter termed cellulose, and the contents of the cells, resembling fat or oil. The notion that yeast was an organised living plant, was at first strongly opposed by even Berzelius and Liebig ; but by the microscope they have been convinced both of its organisation and vitality. The scientific name by which it is known is Fermentum cervisia, or Torula cervisia ; it Fig. 100.— The growth of the Yeast Plant. consists of globular or ovoidal transparent nucleated cells, represented in the accompanying fig. 100, and showing its (1) For further information on this very interesting subject, see Henfrey's translation of Mohl's Vegetable Cell ; Dr. J. B Sanderson on " Vegetable Repro- duction," Cyclopedia of Anatomy and Physialog^, &c. 182 THE MICROSCOPE. stages of growth as first observed by Turpin, who carefully watched the changes after mixing it with some newly-made beer. Fresh yeast has the appearance seen at No. 1 ; one hour after it had been added to the wort, germination commenced, and produced two buds or cells, as at No. 2. In three hours they were doubled, as at No. 3, and attained the size of the maternal cell. In eight hours the plants began to ramify, as at No. 4, and some to explode, emitting a fine powder; and in three days joined filaments with lateral branches were produced, as at No. 5. Yeast-cells occasionally form in the human body under certain states of disease, principally occurring in the urine of patients, hence the cell has been named Torula diabetica : for the sake of comparison, a few of those cells, highly magnified, are represented at No. 9, fig. 102. Mr. Busk met with a peculiar disease of the stomach, in some patients under his care, vomiting another form of this remarkable fungi, named by Professor Goodsir Sarcina ventrlculi; which under the microscope presents the appea'r- ance shown in fig. 101. Dr. John Ogle tells us that he Fig. 101. — Sarcina ventriculi. has met with Sarcina when disease was never previously suspected to exist, the average being one out of every five or six stomachs examined by him. Are not these Sarcina taken into the stomach with impure water 1 The Mycoderma cervisia of Desmazieres is another stage of growth of the same plant deposited in porter- vats. Its various stages are drawn in fig. 102, Nos. 6, 7, and 8 ; the perfect plant is seen, with its granular contents in the stem. One of the most remarkable of this tribe has been committing great devastation among our grape crops during the past two years. A section of the grape, mag- FUXGI. 185 nified 75 diameters, is represented in fig. 103 ; the fungi or mildew is growing from a section of the skin. 6 " • Fig. 102. — Fungoid disease, 1, A section of the Tomata, showing sporangia? growing from the spawn or root (mycelium). 2, A budding from the upper part of a branch. 3, Ver- tical and lateral views of sporangiae, with their granular contents turned out. fi, 7, and 8, Different stages of growth of Mycoderma cervisia. 9, To- rula diabetica. " Grapes," says Mr. Harris, " when blighted, are covered with what appears to be a white powder, like lime, a little darkened with brown or yellow. These fungi send forth laterally, in all directions, thread-like filaments, which become so completely interwoven with one another as entirely to cover and enclose the skin of the grape in a 184: THE MICROSCOPE. compact and firm network, and on each is seen the egg- shaped capsule or seed-pod." Fungoid diseases among our growing crops attracted but little attention until the mischief produced by them Fig. 103.— Section of a Grape. became serious ; and the microscope has enabled us to de- termine and grapple with the destroyer in its variety of forms ; thus, we have our corn-crops withering under the blighting influence of the Uredos and Pucinias, our vines, &c., under that of the Oidium, our esculents under the Botrytis infestans (potato blight), and the same disease in- fects the tomata, fig. 102. The microscope has revealed to us that many of the skin diseases attacking the human frame are but other forms of the same growth of parasitic fungi, Cryptogamia, a low form of plant presenting at first simple filaments, then ramified, consisting of a single elongated cell, or several cells placed end to end, as in those of the yeast-plant. The disease known as Ringworm, infesting the heads of children, is one out of forty- eight different species of Cryptogamia. The conditions of growth of this low form of vegetable life on the human^body are the same as in other situations. Dr. Gudden, who has lately published a work upon Cutaneous Diseases caused by " Parasitic Growths" describes Ringworm under the name of Porrigo- fungus ; the spores of which are round on the upper, and filamentous on the under surface. Whenever the healthy FUNGI. 185 chemical processes of nutrition are impaired, and the in- cessant changes between the solids and the fluids slacken, then the skin may furnish a proper soil for the fungi to take root in, should the sporules come in contact with it. That dreadful disease known as cancer will no doubt ulti- mately prove of vegetable growth, or a degeneration of the nutritive animal cell into that of a fungoid vegetable cell. The Rev. S. G. Osborne, during the cholera visitation of 1854, endeavoured to direct public attention to the very general distribution of fungi. He says, " Only those who have closely studied these fungi can be aware how very minute and yet how systematically formed they are. Preparations of a dozen different species, taken from the grape, potato, parsnip, bean, cucumber, cineraria, veronica, &c., many of which have been in fluid for more than a year, retain their form as perfectly as if only taken from the plant a day. No two are alike in form ; but all are alike in this — under the very high powers of the micro- scope, they show an external hyaline case, with a second utricle, or inner case, full of minute spores. If a few leaves of the infected haulm of the potato are taken and gently shaken over a piece of black paper, a quantity of very fine white powder is obtained: place a little of this in fluid, under a power of 500 linear; every atom of this powder will resolve itself into a distinct cell, somewhat of the form of an ace of spades, varying more or less in size from about 3-5000ths of an iuch in length. There will be seen a well-defined outline of an inner cell, in which are many hundred greenish-looking spores ; some of the cells will burst, and by using a still higher power it will be seen that these have all the shape and characteristics of the parent cell. Several of them lie easily between the lines on a microme- ter, which lines are just l-5000th of an inch apart. In one of our cuts the destructive ^effects upon the tuber are shown. There can scarcely be oiae spot of earth on which these fungi do not fall in their thousands. Insoluble in nature, they wait where they fall the growth of the particular plant for which each has its own affinity, that if that plant grows on that spot, its enemy is near, on the very soil 18G THE MICROSCOPE. from which it is to draw life. But I further believe that there must be some peculiar disposition yet to be developed in the plant before the fungus will act upon it, to its own rapid development, and the destruction of the said plant," Fig. 104. — Fungi. (Magnified 200 diameters.) I, Brachycladium penicillatum, found on the stem of a plant. 2, Aspergillus glaucus, found on cheese, &c. 3, Boirytis ; the common form of mould on decaying vegetable substances. 4, Fungi caught over a sewer (foul air). 5, Fungi growing on a pumpkin. 6, Fungi caught in the air at the time of the cholera. (Aerozoa?) Fig. 104, 4 and 6, represents forms of fungi taken in London by the author during the cholera visitation, Sep- tember 1854. Our limited knowledge of the matter does not forbid the supposition that there may be some, even among the purely vegetable fungi, which might, in certain conditions of the human body, when taken into the frame, produce immediate severe constitutional disturbance. The Sarcina may be cited as an instance of this fact. It strikes us, however, as far more probable, that from drains and cesspools — reservoirs as they are for excrementitious animal matter — may emanate specific fungi, the spores of which under certain conditions of atmosphere, wrould be given out in such quantities, and in such minute particles, as FUNGI. 187 easily to be carried about by every current of air. Persons in health may inhale and swallow these spores, and escape injury from them. Other persons, depressed physically from local or accidental causes, may afford to them just the pabulum which will develope their poisonous quality. Many animal organisms, such as infusorial animalcules and their ova, are frequently found floating about in the air, as well as the fungi spoken of. The upholders of the spontaneous hypothesis were con- siderably shaken by an experiment instituted by Schultze, and recorded in the Edinburgh New Philosophical Journal for 1837. He found that if the decomposing substances, which always generate infusoria and fungi when the atmo- spheric air is freely admitted to them, be shut up in vessels to which the air is admitted only after passing through a red-hot tube, or through strong sulphuric acid, no animal- cules or fungi appear. The experiment seemed conclusive. By destroying these germs — which the sulphuric acid did without altering the air — all development was pre- vented. All the fungi that constitute mouldiness are so small as to escape observation ; they clothe the surface of the body which they attack with light patches of yellow, blue, green, red, and various other colours ; they are all inter- esting objects for microscopic observation. The species of these plants are extremely numerous ; they chiefly belong to the Hyphomycetous division of the order; the peculiar cha- racteristic of which is, that the plants are flocculent, naked (that is, not enclosed in a case, or seated upon a peculiar receptacle), distinct, but interwoven into a general mass, which looks like a thin web, or a collection of cobwebs. One of the most common is the Ascophora mucedo, which forms a blue mould upon bread, paste, and sub- stances prepared from flour : this is even found to live under circumstances that would be fatal to any other form of vegetation ; that is, it flourishes in paste mixed up •with a solution of the well-known poison, bichloride of mercury. Their favourite soil is decaying animal or vegetable matter; but one species, the Botrytis bassiana, attacks the living silkworm just as it is about to enter the chrysalis 188 THE MICROSCOPE. state, and kills it : others destroy house-flies, which may be seen in the autumn glued by these parasites to the window, on which they have alighted in a semi-torpid state. Mother of vinegar (mater aceti) is a mould-plant which is developed in vinegar, and forms therein a thick leather- like coat, similar to the inflammatory crust which covers the crassarnentum of blood drawn from rheumatic patients. It is produced not merely in but from the vinegar, and as it forms the acetic acid diminishes, until ultimately water alone remains. This mould-plant belongs to the genus mycoderma of Persoon, or hygrocrocis of Agardh. It is one of the simplest vegetable formations, and belongs to the Fungi rather than to the Algae. It is formed in vinegar obtained both from wine and beer, but not in that procured from wood. It exists in unmixed vinegar, and also in vinegar in which organic substances are preserved. These substances, how- ever, contribute nothing towards the development of the plant, but merely promote the production of a germ or a cell from which the mould-plant is formed out of the elements of the acetic acid. In all cases, whether organic substances be or be not contained in the vinegar, the mycoderma has the same conformation and chemical composition. Mulder analyzed three plants formed in vinegars con- taining different vegetable substances. His results gave as the formula for the plant, CISG Hn5 Ns 096. The quan- tity of nitrogen contained in protein, is taken as the basis of the formula. By potash all the protein may be re- moved, and the residue is pure cellulose. The latter, according to Payen's analysis, and corroborated by experi- ment, has for its formula, €24 Ebi O^.1 Mycoderma aceti, or mother of vinegar, consists, there- fore, of protein and cellular tissue. Animals, birds, insects, and fishes, alike suffer from the ravages of fungi. One of the most prevalent observed among our domestic pets is that found growing over the upper surface of the gold-fish ; death is almost certain when this white fungoid disease once commences its ravages upon them. (1) Well manufactured paper is nearly, or wholly, composed of cellulose, and may be regarded as the microscopical standard of the cellulose contained in all vegetable tissues. FUNGI. 189 "VVe range by the side of these, the fungi known as mushrooms, toadstools, puff-balls ; and also a large num- ber of microscopic plants, forming those appearances which are referred to generally under the terms of mouldiness, mildew, blight, smut, dry-rot, &c. It is well known that fruit-preserves are very liable to be attacked by the com- mon bead-mould (No. 3, tig. 104); which no care employed in completely closing the mouths of the jars can prevent. It is to be remarked, however, that they are much less liable to suffer in this way, if not left open for a night before they are tied down: and this fact induces us to believe that the germs of the mould sow themselves before the jar is covered. Some kinds of cheese derive their flavour from the quantity of a fungous growth which spreads through the mass whilst it is yet soft. This appears to owe its origin to a damp atmosphere, with diminution of light ; which conditions are especially favourable to the development of these bodies. The power of reproduction of the vegetable mould-plant, mucor, is so great, that extensive tracts of snow are sud- denly reddened by the Gory-dew, Protococcus nivalis (red- snow) of the northern regions. That the Bed-snow plant consists of a cellular or filamentous tissue, may be easily ascertained by means of a microscope of even moderate powers ; and one of a higher power demonstrates that the filaments are nothing more than cells drawn out. Some- times, as in the genus Uredo, the cells are spheroidal, having little connection with each other; each cell con- taining propagating matter, and all separating from each other in the form of a fine powder when ripe. In plants of a more advanced organisation, as the genus Monilia, the constituent cells are connected in series which preserve their spherical, and also contain their own reproductive matter ; while in such plants as Aspergillus (fig. 104, No. 2 ), the cells partly combine into threads forming a stem, and partly preserve their spheroidal form for fructification. It is probable, however, that in all fungi, and certain that in most of them, the first development of the plant consists in what we here call a filamentous matter which radiates from the centre formed by the space or seeds ; and that all the cellular spheroidal appearances are subsequently deve- 190 THE MICROSCOPE. loped, more especially with a view to the dispersion of the species. One of the most remarkable of the lower forms of vegetable life is the Protococcus pluvialis, fig. 105, not uncommon in collections of rain-water, constituting the genus Chlamydomonas of Professor Ehrenberg, and the curious motile organs of which induced him to regard them as animalcules. Dr. Cohn describes the early form of the cell as a mass of endochrome, consisting of a colourless protoplasm, through which red or green-coloured granules are more or less uniformly diffused : on the sur- Fig. 105. — Vegetable Cell Development. (Protococcus pluvialis.) A, division of a simple cell into two, each primordial vesicle having developed a cellulose 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 ; p, division of an encysted cell into four, with vibratile filaments projecting; G, division of a cell into two. face of this endochrome the colourless protoplasm is con- densed into a more consistent layer, forming an imperfect " primordial utricle ; " and this is surrounded by a tolerably firm layer, which seems to consist of cellulose, or of some modification of it. Outside this again, as in fig. 105 E, when the still-cell is formed by a change in the condition of the cell that has been previously " motile/' we find another envelope, which seems to be of the same nature, but which is separated by the interposition of fluid. The multipli- cation of still- cells, by self-division, takes place as in the previous instance; the endochrome, enclosed in its pri- mordial utricle, first undergoes separation into two halves, PJIOTOCOCCUS. 191 as seen at G, and each of these again undergoing the same division in its turn. Sometimes the contents of the original cell subdivides at once into four, eight, or even thirty-two parts, many of which perish without any further change. The greater number, when set free, possess active powers of movement, and rank as Zoospores, fig. 105 B, which may either develope a loose cellulose investment or cyst, so as to attain the full dimensions of the original motile cell, or may become covered with a dense envelope, losing their vibratile cilia, and thus pass into the dill condition. All these changes, whose relation to each other has been clearly proved by competent observers, have been regarded as constituting, not merely distinct species, but distinct genera of animalcules; such as Cklamydomonas, Euglena, Trackelomonas, Gonium, Pandorina, Uvella, Monas, Astatia, and several others. The process of segmentation is often accomplished with great rapidity. Thus it is only necessary to pour the water containing those organisms from a smaller and deeper into a larger and shallower vessel, at once to deter- mine segmentation. The motile cells seem to be favour- ably affected by light, for they collect themselves at the surface of the water and at the edges of the vessel; but when about to undergo segmentation, or pass into the still condition, they sink to the bottom of the vessel, or retreat to that part in which they are least subjected to light: if kept in the dark, they lose most of their colouring matter, and remain stationary, and do not undergo segmentation. Mr. Busk kept his plants for observation in little glass vessels, having the form of a truncated cone, about two inches deep, and one inch and a quarter in diameter, with a flat bottom polished on both sides, aiid filled with water to the depth of from two to three lines. In vessels of this kind he was able to follow the development of a number of various cells throughout. The ordinary small cupping- glass, or glass cells, such as we have described at page 93, answer the purpose equally well. Another family of Protophytes, of singular beauty and interest to the microscopist, is the group known as Vol- vodneoBj — so closely allied to the former that they have been confounded by more than one observer. 192 THE MICROSCOPE. The cell is commonly known as the Volvox globator, or revolving-cell, represented in figs. 106 & 120, Nos. 1, 2, 3. These revolving globular bodies are found of various sizes, some large enough to be discernible by the naked eye, and for a very long time, were classed with the lower forms of animal life ; and there it remained for the micro- chemical investigators of the present time to settle the perplexing question, and assign to them a place amongst the lower order of plants. Leeuwenhoek first perceived the motion of what he termed globes, "not more than the 30th of an inch in diameter, through water ; and judged them to be ani- mated." These globes are studded with innumerable minute green spots at their surface, each of which is a cell about the 3500th part of an inch in size, with a vivid nu- cleus having many ever-active cilia, that bristle over their spherical home and are bound to each other by bands form- ing a beautiful net - work. Within this globe busy active nature is at work carefully providing a continuance of the species ; and from six to twenty little bright - green spheres have been found enclosed in the larger transparent case. Fig 106 As each one of these arrives #, just before the young burst at maturity, the parent cell forth, showing the vesicle which en- enlar^ea . then bursts J closes each. 2, Parent cell of Clos- - . teria. 3, Docidium clavatum. 4, to launch forth its offspring into a watery world. Both the older and younger spheres possess openings through which the water freely flows, affording food and air to the little organised being. Dr. Carpenter believes, " The Volvocinece, whose vegetable nature has been made known to us by observation of cer- VOLVOCINEJJ. 193 tain stages in the history of their lives, are but the motile forms (Zoospores) of some other plants, whose relation to them is at present unknown." Professor Williamson, having carefully examined the Volvox globator, says : — " That the increase of its internal cells is carried on in a manner precisely analogous to that of the algse; that between the outer integument and the primordial cell- wall of each cell, a hyaline membrane is secreted, causing the outer integument to expand; and as the primordial cell- wall is attached to it at various points, it causes the inter- nal colouring-matter, or endochrome, to assume a stellate form (see fig. 120, No. 3), the points of one cell being in contact with those of the neighbouring cell, these points forming at a subsequent period the lines of communication between the green spots generally seen within the full- grown Volvox." Cilia can be distinctly seen on the outer edge of the adult Volvox ; by compressing and rupturing one, they may even be counted, Professor Busk has been able to satisfy himself, by the addition of the chemical test iodine, of the presence of a very minute quantity of starch in the interior of the Volvox, which he considers as conclusive of their vegetable character. A singular provision is made in the structure of the gemmules, con- sisting of a slender elastic filament, by which each is at- tached to the parent cell- wall : at times it appears to thrust itself out, as if in search of food ; it is then seen quickly to recover its former nestling-place by contracting the tether. " Wonderful as it may appear, we have here an example of all the functions of vegetable life — namely, absorption, assimilation, exhalation, secretion, reproduction, smidium. 29, Pediastrum pecticuiii. 30, AnTtislrodesmus falcalus. 33, Penium margarilaceum. 34, Spirotamia. 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 y applying the highest power of the microscope, the peculiar colour of the purple iodide of starch will in general be perceived. DESM1DIACE.E. 199 proposed by Mr. Rainey, and adapted to a l-4th achro- matic condenser • with which must be used a l-8th ob- ject-glass. The Gillett's condenser, or parabolic reflector, will do equally well if used with a 1-Sth objective. In diagram A, fig. Ill, a specimen of the C. Lunula, as seen Fig. 111. — Ciosieria Lunula. with the above arrangement of microscopic power, and a deep eye-piece, the cilia is 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 branchiso 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 6 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 to which they are thus fastened ; from time to time they TBE MICROSCOPE. break away, 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 l-8th 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 dai-k 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 pot ; 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 endoehrome 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 ease and the endochrome. At B is given an enlarged sketch of one extremity of a C. Lunula. The arrows within the chamber pointing to 5, 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 endochrome. 201 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 cilia, causes strong eddies, which give a twisting motion to the free bodies. The line — a, in this diagram, denotes the outline of the mem- brane which encloses the endochrome ; on both sides of this cilia may be detected. The circulation exterior to it passes and repasses it in opposite directions, in three or four distinct courses of globules j 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 Mr. Osborne 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 feiv hours afterwards, runs quite round the obtuse end of the endochrome — a, by almost imperceptible degrees ; from the end of the endochrome symptoms of 202 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 contract! ag ; 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, the 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, Docidium, &c. The Arthrodesmus Incus has a very beautiful hyaline membrane stretching from point to point, cut at the edges, something like the Micrasterias. This is represented at fig. 112. 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. Ealfs, Mr. Jenner, 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 finger-like 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 DESM1DIACEJB. 203 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 througli 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 them, 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 ; but if it 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 rain-water be added occasionally, to replace what has been 204 THE MICROSCOPE. drawn off, and the bottle be exposed to the light of the sun, the Desmidiacece will remain unaltered for a long time. The Desmidiacece prefer an open country. They abound on moors and in exposed places, but are rarely found in shady woods or in deep ditches. To search for them in turbid waters is useless ; such situations are the haunts of animals not the habitats of the Desmidiacece; and the waters in which the latter are present are always clear to the very bottom. That very curious fungus, known in Scotland as Siller- cups (Nidularia campanulata), fig. 113, consists of a Fig. 113. — Siller-cups (Nidularia campanulata). curious leathery cup, in which are a number of small thecse, these contain the sporules ; and each plant looks like a bird's nest with several eggs in it. It generally grows on a twig, or a bit of rotten wood, and one has been found growing on a wooden tally, fixed in a pot containing a greenhouse plant. Several kinds of Agaricus have blue stems, others orange, yellow, and green, with caps of various colours, some of which are scarlet or crimson, and others have beautiful shades of purple or violet. MOSSES. 205 The little group of Hepaticce or Liverworts, which is 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 gemmae or buds. The gemmae of Marchantia polymorpliia 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. 114, 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 f^-.,-* *^A "U A 1 chantia poly morphia, expanding and rising formed by a develop- from the surf ace of a frond. rnent 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. Mwcacece, mosses, are another low form of vegetable life, Linnseus 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 .— Gemmiparous Conceptacle of Mar- 206 THE MICROSCOPE. winter's cold. The common, or Wall Screw-moss,, fig. 115, 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. 115. — Screw Moss. of which 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 ; from 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, fig. 136, 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 both antheridiaand pistillida, which Fig. 116. — Mouth of Capsule of i • , i /» t* Funaria, showing Peristome. are engaged in the process of fruc- tification. The fertilized cell be- comes gradually developed into a conical body elevated MOSSES. 207 upon a foot stalk ; and this at length tears across the •walls of the flask-shaped body, carrying the higher part upwards as a calyptra or hood upon its summit, while 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. 116 in mouth 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. Neclcera anti- pyrctica. fig 117, 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 is termed the columella j and at the Fig. 117.— Double Peristome of time of maturity, the interior of the Neekera An^rct'ca- 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 season, and consequently they are best studied in winter. One of the com- monest, Scale-moss, fig. 118 (Jungermannia biden- tata), grows in patches, in moist, shady situations, near the roots of trees, upon commons, and on T_ j i i rni i Fig. US. — Sca!e-Moss. hedge-banks. 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 of 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 208 THE MICROSCOPE. 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 ser- pents. The undulating Hair-moss ( Polytrichum undulatum), fig. 119, is found on moist shady banks, and in woods and thickets. The seed-vessel has a curious shaggy cap ; but in its construc- tion it is very similar to that of the Screw-moss, except that the fringe round its opening is not twisted. The Funaria hygro- metrica is a remarkable moss, differing widely in its powers of adaptation, and, consequently, in its greater geographical range, from most of its congeners. The Funaria is found in fruit, not only in London, but in every brick- field around it. Confervoidece — Algce. — The jointed Confervce and some Algce are met with in the smallest ac- cumulations of fresh water stand- ing for any length of time in the open air. They present the appearance of thread-like tubes, having joints differing in length, and the manner in which their contents are arranged. They multiply by means of little granules contained in their tubes, which are enclosed in tube after tube gradually added to the end of the previous one. Among these Confervce, the most remarkable are the Zygnema and Oscillatoria, both of which evince certain degrees of approach to the animal kingdom. The species of the latter genus form dark green and purple slimy patches in damp places, or in water, and are exceedingly remarkable for the power they possess of moving spontane- Fig. 119.— Hair-Moss in Fruit. CONFERVOIDE^:. 209 ously. When in an active state, their tubes are seen to unite and twist about, just as if they were vegetable worms ; but they grow like plants, and their manner of increase is also vegetable. Disjointed algce are extremely curious; they are characterised by their original or final spontaneous separation into distinct fragments, which have a common origin, but an individual life. They multiply by spontaneous division, as represented in fig. 120, Nos. 6 Fig. 120. 1, Voivox globator. 2, A section of volvox, showing the ciliated margin of the cell. 3, A portion more highly magnified, to show the young Volvicinite, with their nuclei and filamentary attachments. 4, Spirog>/ra quinina, near which , are spores in different stages of development. 5, Conferva floccosa, with cells breaking np. 6, Stigeoclonium protensum, showing germinating zoospores. 7. Staurocarpus gracilis, cells dividing. and 7 ; and are generally found attached to the stems of other plants immersed in water, or floating in pools or ditches. 210 THE MICROSCOPE. Algce , or marine sea- weeds, are usually classed by botanists in three great groups, each of which contains several families : these are again divided into genera ; and these, in their turn, are made up of one or more species. The species found on the British coasts number about 380. They are grouped into 105 genera. We must not enter into the niceties of classification, but confine ourselves to their general features. Taken in the order in which they present themselves to us on the shore, and limiting each by its most obvious characteristic, that of colour, we may observe, that the group of green sea-weeds (Chloro- spermece) abound near high- water mark, and in shallow tide-pools within the tidal limit ; that the olive-coloured (Melanospermece) cover all exposed rocks, feebly com- mencing at the margin of high-water mark, and increasing in luxuriance with increasing depth, but that the majority of them cease to grow soon after they reach a depth which is never laid bare to the influence of the atmosphere; on the contrary, the red sea- weeds (Ehodospermece) gradually increase in numbers and in purity of colour as they recede from high- water mark, and are never subjected to great changes of light or temperature. Dr. Harvey l writes of algce : " Some are so exceedingly minute as to be wholly invisible, except in masses, to the naked eye, and require the highest powers of our micro- scope to ascertain their form or structure. " Others, growing in the depths of the great Pacific Ocean, have stems which exceed in length (though not in diameter) the trunks of the tallest forest- trees ; and others have leaves that rival in expansion those of the palm. " Some are simple globules or spheres, consisting of a single cellule, or little bag of tissue, filled with a colouring matter ; some are mere strings of such cellules, cohering by their ends, as in Mesogloia, fig. 121; others, a little more perfect, exhibit the appearances of branched threads ; in others, again, the branches and stems are compound, consisting of several such threads joined together, and in others, the tissue expands into broad flat fronds, " Only the higher tribes show any distinction with (1) See Dr. Harvey's British Marine Algce, or Pfiyc. Britan. Fig. 12l.—Me. i i i nai shape of ceils. 2, A vertical lular tissue, 6. .Remarkable section of elongated cell. specimens of the filamentous tissue may be seen in fig. 145, No. 19, the fungiform elon- gated cells from the Mushroom; only another and more closely connected growth of mucedinous fungi, commonly called mushroom spawn. CELLULAR TISSUES. 229 Fig. 132.— Stellate tissue, from stein of a Rush. Fig. 132, in the stellate tissue cut from the stem of a j we have the forma- tive network dividing into ducts for the purpose of conveying the juices to the leaves 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. 155; 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 tracheae 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. 133.—^ section of stem of Clematis, r i v • /» VJith pores, hiqhly magnified, to show charge their contents from the line which passes rolnd them. one to the other, or into the cellular spaces. Such plants as have aromatic secretions are furnished with glands, — a circumstance which has led to the division of woody tissue into simple and glandular. A large central gland is seen in a section of a leaf from Ficus elastica, India-rubber-tree, fig. 134, 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 is 230 THE MICROSCOPE. a small circular or oval spot, fig. 156: the latter may occur singly, or be crossed by another at right angles, Fig. 134. 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. 161, 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." Plants are likewise furnished with lactiferous ducts or tissue, — the proper vessels of the old writers. These ducts convey a peculiar fluid, 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. 135.— Lactiferous tissue. CELLULAR TISSUES. 231 lated ducts seen in fig. 137. In a section from the India-rubber-tree, fig. 134, No. 2, a network of these lac- tiferous tubes will be found filled with a brownish or Fig. 136. 1, A portion of the leaf of Sphagnum, showing ducts, vascular tissue, and spiral fibre in the interior of its cells. 2, Porous cells, from the testa of Gourd- seed, communicating with each other, and resembling ducts. granular matter; that in fig. 135 is an enlarged view of this tissue from the wood of an exogen, taken near the root. _ Fig. 137. 1, Reticulated ducts. 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 232 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. 138, each surface has nearly 12,000 i. Fig. 138. 1, Portion of a vertical section of the Leaf of the Iris: a, a, elongated cells of the epidermis; 5, 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. Fig. 139. — A portion of the epidermis of the Sugar-cane, showing the two kinds oj cells of which is is composed. (Magnified 200 diameters.) CELLULAR TISSUE. 233 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-cortents of some plants, are beautiful crystals called Raphides : the term is derived from pa<£ig, 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. "Whether the result of chemical affinity, or of a vital process, cannot be decided ; but it is certain that they can be produced artificially in the tissue of plants." The French philosopher, Geoffrey St. Hilaire, endeavoured to prove that crystals are the possible transition of the inorganic to organic matter. Crystals have naturally been supposed to conceal the first beginnings of the phase named organic, because in crystals we first meet with determinate form as a constituent element. The matter named inorganic has no determinate form ; but a crystal is matter arranged in a particular and essential form. The differences, however, between the highest form of crystal and the lowest form of organic life known — a simple re- productive cell — are so manifold and striking, that the attempt to make crystals the bridge over which inorganic matter passes into the organic, is almost universally regarded as futile. 1 If we examine a portion of the layers of an onion, fig. 140, No. 1, or a thin section of the stem or root of the garden rhubarb, fig. 140, No. 4, we shall find many cells in which, either bundles of needle-shaped crystals, or masses of a stellate form occur. Raphides were first noticed by Malpighi in Opuntia, and subsequently described by Jurine 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 1-1 000th. They occur in all (1) See Addenda. 234: 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. 140. 1, A section from the outer layer of the bulb of an Onion, showing crystals of lime with raphides. 3, Cells of the Pear, showing Sclerogent 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 110 doubt act the part of a stay or prop to the internal soft pulp. In portions of the cuticle of the medicinal squill — Scilla maritima — several large cells will 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 (1) "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 be considered as an exceptional case, the vessels being regarded as elongated cells." — Quekett. CRYSTALS IN PLANTS. 235 crystals will be principally prismatic, and are arranged as 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 „ , , ° .,, -,, ,•£ Fig. 141.— Siliceous cuticle from under in COtton, With all the Care surface of leaf of Deutzia scabra. 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 (Gary a alba) may be ob- served masses of flattened prisms having both extre- mities pointed. In vertical sections from the stem of Elceagnus angwtifolia, 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. 141 and 142 we have other representations Fig. 142.— SiKc«>«« cuticle of Grass of the crystalline structure (PAanu cristatus). 236 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 Graminacece, 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. 142, we have beautifully-ar- ranged masses of silica with raphides. The leaves of Deutzia j fig. 141, 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, either with polarised or condensed light. Silica is found in all Ru- biacece ; both in the stem and 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 Composites, a plant popularly known as the "sneeze wort," (Archillce ptarmica') has a large amount of silica in the hairs found on the double serratures of its leaves ; commonly said to be the Fig. 143.— Portion of the husk of Wheat, showing siliceous crystals. CELL-CONTENTS. — STARCH. 237 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 cell-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. Being insoluble in cold water, it can be readily Fig. 144.— sectwnofaCane:withceii- washed away and separated jS^™^"*0"' 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. 140, from the garden-rhubarb. Starch-grains are variable in size ; the tous-les-mois, fig. 145, 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 : these are seen better under polarised light, fig. 95. Leeuwenhbek, 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 veiy carefully describes experiments made by him in order to investigate the structure of starch-granules. Dr. Reissek regards the 238 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 rugae, 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 off, 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 |-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 becoines 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 Microscopical Science, April, 1853. CELL-CONTENTS — STARCH. 239 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. J9 . SW ^iv* ^ %^As Fig. 145. Yeast-plant. 2, Stinging-nettle Hairs, Urtica Dioica. 3, Ciliated onferva. 4, Starch grains, broken by the application of heat. 5, -- spores of Conferva. 4, Starch grains, broken by the application of heat. 5, Starch from Tous-les-mois. 6, Starch from Rice. 7, Starch from Sago. 8, Imitation Sago-starch. 9, Wheat-starch. 10, Rhubarb- starch, in isolated cells. 11, Maize-starch. 12, Oat-starch. 13, Barley-starch. 14, Potato- early all starch-grains aosent. i», section 01 fotato, cens starch. (These starches are grouped for comparison.) spawn, elongated cells. 19, Mushroom 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. 240 THE MICROSCOPE. Pure wheat-flour is almost entirely dissolved in a strong solution of potash, containing twelve per cent, of the alkali j 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 Fig. 146. — Wheat-Flour Starch-granules, with a small portion of its 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. 241 starch will immediately swell up, and acquire three or 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. 147. — 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 R 242 THE MICROSCOPE. of an article very much more nutritious. This adulteration can be instantly detected by means of the microscope. The cells which contain the starch- corpuscles are, in the potato, very large, fig. 14-7 ; 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. 148. — 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-les- 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 OP FOOD. 243 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. 145. Hence it may be Fig. 149.— 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 B 2 244 THE MICROSCOPE. 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 heen 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. 150. — Sample of Coffee, adulterated with both Chicory and Roasted Wheat. (After Hassall.) a a a, small fragments of coffee ; b b b, portions of chicory ; c c c, starch-granules of wheat. Mitchell, Normandy, Chevalier, Jules Gamier, 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 (1) Professor Quekett's Histology of Vegetables. We would refer the reader to an admirable work on Fungi, by Arimini, an Italian botanist, 1759. ADULTERATION OF FOOD. 245 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 nearly 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. 151. — Tea adulterated with foreign leaves. (After Hassall.) c, upper surface of leaf ; 6, 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 ; h, 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 its Adulterations ; comprising the Reports of the Analytical Sani- tary Commission of the Lancet, for the years 1851 to 1854 inclusive. By Arthur Hill Hassall, M.D. 246 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. 152. 1, 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, &c., fig. 148. Now, these names are merely employed to show that they are compounds of sugar, starch, and other substances. Unfortunately, however, many of the preparations of the cocoa-nut sold under the names of chocolate and of cocoa flakes, consist of a most disgusting mixture of bad or musty cocoa-nuts, with their shells, coarse sugar of the very lowest quality, ground with potato-starch, old sea- biscuits, coarse branny flour, animal fat (generally tallow, or even greaves), &c." If we look into drugs and pharmaceutical preparations we shall find that nearly all the most useful and important articles of the Materia Medica are systematically adul- terated, often to an enormous extent ; so that it is impos- sible to estimate the strength of the different remedies WOODY TISSUE. 247 administered, or the extent and character of the effects produced. (See Reports of the Analytical Sanitary Com- mission, Lancet, 1853 and 1854.) We nevertheless believe that the growing intelligence and inquiring spirit of the masses, with the greatly in- creased facilities of detection so ably pointed out by Dr. Hassall, and afforded by modern science, will tend not merely to check the evil for the time being, but ultimately suppress such dangerous practices. Every man, and good housewife, should be able to ascertain the quality and general purity of the substances that form our daily food ; this may be done by taking such a book as Dr. HassaU's Adulterations Detected for our guide, and there learn to look for adulterations, and to detect them in- stantly. The next in order for investigation is the Woody tissue of plants, which consists of elongated transparent tubes of considerable strength : some are almost entirely made up of this tissue. It is by far the most useful, and supplies material for our linen, cordage, paper, and many other important articles in every branch of art. This tissue, re- markable for its toughness, is termed fibre, the outer membrane of which is usually structureless. In Flax and Hemp, in which the fibres are of great length, there are traces of transverse markings, and tubercles at short intervals. In the rough condition, in which it is imported into this country, the fibres have been separated, to a certain extent, by a process termed hackling. It is once more subjected to a repetition of hackling, maceration, and bleach- ing, before it can be reduced to the white silky condition required by the spinner and weaver, when it has the appearance of structureless tubes, fig. 153 A. China-grass, New Zealand flax, and some other plants, pro- duce a similar material, but are not so strong, in conse- Fig. 153. Fibres of Flax. B, Fibres of Cotton. ^45 THE MICROSCOPE. quence of the outer membrane containing more lignine. It is important to the manufacturer that he should be able to determine the true character of some of the textures of articles of clothing, and this he may readily do with the microscope. In linen we find each component thread made up of the longitudinal, rounded, unmarked fibres of flax j but if cotton has been mixed, we recognise a flattened, more or less twisted band, as in fig. 153, having a very striking resemblance to hair, which, in reality, it is ; since, in the condi- tion of elongated cells, it lines the inner surface of the pod. These, again, should be contrasted with the filaments of silk, fig. 154 B, and also of wool, fig. 154 A. The latter may be at once recognised by the zigzag transverse markings on its fibres. The surface of wool is covered with these furrowed and twisted fine cross lines, of which there are from 2,000 to 4,000 in an inch. On this structure de- pends its felting property. In judging of fleeces, attention should be paid to the fineness and elasti- city of the fibre, — the furrowed and scaly surface, as shown by the microscope, — the quantity of fibre in a given surface, the purity of the fleece, upon which depend the success of the scouring and subsequent operations. In the mummy-cloths of the Egyptians, flax only was used, whereas the Peruvians used cotton alone. By recent improvements introduced into the manufacturing pro- cesses, flax has been reduced to the fineness and texture of silk, and made to resemble other materials. Silk is secreted from a pair of long tubes ending in a pore of the under-lip of the silkworm. Each thread is made of two filaments coming from these, and they are glued together by a secretion from a small gland near. The quality of the silk depends on the character and difference of the two secretions. All woody fibre is made up of elongated cells, generally Fig. 154. , Wool of Sheep. ,B, Fila- ments of Silk. VASCULAR TISSUE. 249 more or less pointed at both extremities, and having their walls strengthened by internal deposits. Occasionally, however, the fibre is short, as in the Clematis, Elder, &c. j it is marked with pores or dots, from a deficiency of the internal deposits at these points. Vascular tissue consists of cells, more or less elongated, joined end to end, or over- lapping each other, in which either a spiral fibre, or a mo- dification of the same, has been deposited ; hence, if the spiral be perfect, it is called a true spiral vessel ; if inter- rupted, or the fibre breaks up into rings, it is termed annu- lar; if the rings are connected together by branching fibres, so that a network is pro- duced, the vessel is called reticulated; if the vertical fibres are short, and equidis- tant, the vessel is said to be scalariform, from its resemblance to a ladder. Spiral vessels have been also termed tracheae, from their resem- blance to the air-tubes of insects, as in fig. 155. Under this head other membranous tubes are included, in which the arrangement of the fibre has been consider- ably modified in its deposition. Elongated tubes or ducts, with porous walls, come under the head of vascular tissue; they somewhat differ from the spiral varieties, inasmuch as they cannot be unrolled without breaking. It is a curious fact, that mostly the spiral coils from right to left ; and it has been suggested that the direction of the fibre may determine that in which the plant coils round an upright pole. The Hop has left-handed spirals, and is a left-handed climber, which would therefore appear to support this theory. The nature of the fibre, and the development of the tissue, have been frequently the subject of dispute between botanists. The late Mr. Edwin Quekett gave much attention on the Fig. 155. — Spiral vessels , 250 THE MICROSCOPE. subject; and published an excellent paper in the Micro- scopical Society's Transactions) 1840, which contains the results of his observations. 1 Fig. 156. 1, Interior cast of the siliceous portion of spiral tubes of the Opuntia. 2, Vertical section of Elm, showing spiral fibre. In order to watch the development of the membranous tube of a vessel, no better example can be chosen than the ; " Fig. 157. 1, A transverse section of Taxus baccata (Yew), showing the woody fibre. 2, Vertical section of the Yew, exhibiting pores and spiral fibres. VASCULAR TISSUE. 251 young flower-stalk of the long-leek (Allium porrum), in the state in which this vegetable is usually sent to market; it is then most frequently found to be about an inch or Fig. 158. 1, Portion ot transverse section of stem of Cedar, showing pith, wood, and bark. 2, Portion of transverse section of stem of Clematis, showing medullary rays. more in length, and from a quarter to half an inch in diameter. This organ occurs very low down amidst the sheathing bases of the leaves ; and from having to lengthen to two or three feet, and containing large vessels, forms a very fit subject for ascertaining the early appearances of the vascular tissue. To examine the development of vessels, it is necessary to be very careful in making dissections of the recent plant ; and it will be found useful to macerate the specimen for a time in boiling water, which will render the tissues more easily separable. When the examination is directed in search of the larger vessels, it will be found that at this early stage they present merely the form of very elongated cells, arranged in distinct lines; amongst which some vessels, especially the annular, will be found matured, even before the cytoblasts have disappeared from the cells of the surrounding tissue. As development proceeds, the vessels rapidly increase in length, till they arrive at perfection. No increas ... Jn diameter is perceptible after their first formation. At this period, in the living plant the young vessels appear full of fluid, which is apparently, as remarked by Schleiden, of a thick character, and which he has designated vege- table jelly ; by boiling which, or by the addition of alcohol, the contents, or at least the albuminous portion, become coagulated. From this circumstance, every cell appears 252 THE MICROSCOPE. to enclose another in a shrivelled condition ; this state is sometimes so far extended, that a thick granular cord is all that can be seen of the contents. " The period of growth at which the laying down of fibre commences, determines the distance between the several coils; for instance, when it is first formed, the coils are quite close, scarcely any perceptible trace of mem- brane existing between them. In the annular vessel, the development of the cell and the adherence of the granules to each other are conducted in the same manner ; the deposit showing a tendency towards the spiral direction, by the presence of a spire connecting two rings, or by a ring being developed in the middle of a spiral fibre. The annular vessel is the first observed in the youngest ^^ parts of plants, and when found alone Fig. W.-A section from indicates a low degree of organisation; the stem of a coniferous as shown by its occurrence in Spha- S4vr^n^?oS gnum, Equisetum, and Lycopodium, the zones of annual which plants, in the ascending scale of growth, annual rings. *. ,. ° J vegetation, are almost the first that possess vascular tissue. " It will be found that spiral fibre occurring with rings marks a higher step in the scale of organising power; the true spiral more so; and the reticulated and dotted mark the highest ; this being the order in which these several vessels are placed in herbaceous exogens proceeding from within outwards, the differences of structure of the several vessels being indices of the vital energy of the plant at the several periods of its development. In those vessels in which the annular or spiral character of the fibre is departed from, some curious modifications of the above process are to be observed, as in the reticulated vessels met with in the common balsam (Balsamina hortensis). The primary formation of fibre in these vessels is marked by the tendency of the granules to take a spiral course, when it happens that some one of the granules becomes VASCULAR TISSUE. 253 enlarged by the deposition of new matter around it. This becomes a point originating another fibre or branch, which becomes developed by the successive attraction of granules into bead-like strings, taking a contrary direction to the original fibre, forming a cross-bar, or ramifying, thereby causing the appearance by which the vessel is recognised. " In the exogenic vessel, the development of fibre proceeds in the same manner as in the last example ; but the vessels will be seen to be dotted with a central mark, usually of a red colour, which, when viewed under high power, may be thought to resemble a minute garnet set in the centre of each dot. This red colour is owing to the dot being somewhat hollowed or cupped, and the centre only thin membrane. These vessels are best seen in the young shoots of the Willow. In the endogenic vessel the con- necting branches are given off beneath each other, so that the dots, which are rounded, are arranged in longitudinal rows ; but in the acrogenic, or scalariform, in which the vessels are generally angular, and present distinct facets, the branches come off in the same line, corresponding generally to the angles of the vessel; the spaces left between are linear instead of round." Mr. E. Quekett affirms, in opposition to the views enter- tained by Mirbel, Richard, and Bischoff, "that the dots left in these several vessels are not holes, neither do they consist of broken-up fibre, but are the membranous tubes, unsupported by internal deposit ; and on account of the extreme tenuity of the tissue, and the minute space between the fibres, the light in its transmission becomes decomposed, and appears of a greenish-red hue. The structure of the dot is best seen by examining the broken edge of any such vessels, when it will be found that the fracture has been caused by the vessel giving way from one dot to another, so that the torn edge of the membrane can be observed in each dot." PREPARATION OF VEGETABLE TISSUES. The proper mode of preparing and preserving vegetable tissues is a matter of some importance to the microscopist; we therefore propose to add a few general directions for the student's guidance. 254 THE MICROSCOPE. Vegetable tissues are best prepared for the microscope by making thin sections, either by maceration, by tearing be- tween the thumb and the blade of a knife, or by dissection. The spiral and other vessels of plants require to be dissected out under a simple magnifying-glass. Take, for instance, a piece of asparagus, and separate with the needle-points the vessels, which require to be finished under a magnifying-glass, in a single drop of distilled water. When properly done, keep in spirits of wine and water until moimted. Vascular tissue requires both maceration and dissection for its separation. The cuticle or external covering of plants is a highly interesting structure ; it is best seen in the pelargonium, oleander, &c.; and which may be mounted dry or in Canada balsam. Cellular tissue is best seen in fine sections from the pith of elder, pulp of peach, pear, &c. The petals of flowers are mostly composed of cellular tissue, and their brilliant colours arise from the fluid contained within the cells. In the petal of the anagallis, or scarlet chickweed, the spiral vessels diverging from the base, and the singular cellules which fringe the edge, are very interesting. The petal of the geranium is one of the most beautiful objects for microscopic examination. The usual way of preparing it is by immersing the leaf in sulphuric ether for a few seconds, allowing the fluid to evaporate, and then putting it up dry. Dr. Inman of Liverpool suggests the following method : " First peel off the epidermis from the petal, which may be readily done by making an incision through it at the end of the leaf, and then tearing it forwards by the forceps. This is then arranged on a slip of glass, and allowed to dry ; when dry, it adheres to the glass. Place on it a little Canada balsam diluted with turpentine, and boil it for an instant over the spirit-lamp ; this blisters it, but does not remove the colour ; then cover it with a thin slip of glass, to preserve it. Many cells will be found showing the mamilla very distinctly, and the hairs sur- rounding its base, each being slightly curved and pointed towards the apex of the mamilla. It is these hairs and the mamilla which give the velvety appearance to the petal." Fibro-cellular tissue is found readily in Sphagnum or PREPARATION OP TISSUES. 255 bog-moss, and in the elegant creeper Cobcea scandens. In some orchidaceous plants the leaves are almost entirely composed of it. A modification of this form of tissue is found in the testa of some seeds, as in those of Salvia, Collomia grandiflora, &c. The curious and interesting sporules of ferns, when ripe, burst, and are dispersed to a distance ; so that they should be gathered before they come to maturity, and mounted as opaque objects. The development of ferns may be observed by placing the seeds in moistened flannel, and keeping them at a warm temperature. At first a single cellule is produced, then a second ; after this the first divides into two, and then others follow ; by which a lateral increase takes place. Pollen-grains from most flowers are very interesting objects; the darker kinds show best when mounted in Canada balsam, and viewed as opaque ; the objects more Fig. 160. — Pollen grains and seeds. A, Seed of Clove-pink. B, Poppy seed, c, Pollen of Passion flower (Passiflora ccerulea). D, Pollen of Cobcea scandens. transparent in fluid, or even preserved dry, will show better. The prettiest and most delicate forms are found in Amarantacece, Circurbitacece, Malvacece, and Passi- florece; others are furnished from the Convolvulus, Gera- nium, Campanula, Hollyhock, and some other plants. The curious peculiarities of a few are shown in fig. 160. 256 THE MICROSCOPE. Many of the smaller kinds of seeds will reward the microscopist, seen under a low power ; that of Caryo- phyllum (clove-pink), is regularly covered with curiously- jagged divisions; every one of which has a small bright, black hemispherical knob in its middle, represented in % 160, A. The seeds of the carrot are remarkably formed, having some resemblance to a star-fish, with its long radiating processes. The seeds of umbelliferous plants have peculiar receptacles for essential oil, in their coats, termed vitce, various points of interest may be noted as occurring in the testae, envelopes of seeds, such as the fibre-cells of Cobcea, and the stellate cells of the Star-anise. All plants are provided with hairs; and a few, like insects, with weapons of a defensive character. Those in the Urtica dioica, commonly called the Stinging-nettle, are elongated hairs, developed from the cuticle, usually of a conical figure, and containing an irritating fluid ; in some of them a circulation is visible : when examined under the microscope, with a power of 100 diameters, they present the appearance seen at fig. 145, No. 2. At No. 3, same figure, are represented a few interesting ciliated spores from Confervce. The circulation of the fluid-contents of vegetable cells may be examined at the same time with the Chlorophyll globules, by selecting for the purpose the transparent water-plants Chara, Nitella, Anacharis, and Vallisneria, or the hairs of Groundsel and Tradescantia. The circula- tion of the sap in plants growing in water is termed by botanists Cyclosis. Fossil plants. — We detect in some of the primordial fossils a noticeable likeness to families familiar to the modern algoelogist. The cord-like plant, Chorda filium, known as ' dead men's ropes,' from its proving fatal at times to the too adventurous swimmer who gets entangled in its thick wreaths, had a Lower Silurian representative, known to the palseontologist as the Palceochorda, or ancient corda, which existed, apparently, in two species, — a larger and a smaller. The still better known Chondrus sj the Irish moss, or Carrageen moss, has, likewise, FOSSIL PLANTS. 257 its apparent, though more distant representative, in Chon- dritis, a Lower Silurian alga, of which there seems to exist at least three species. The fucoids, or kelp- weeds, appear to have also their representatives in such plants as Fucoides gracilis, of the Lower Silurians of the Malverns ; in short, the Thallogens of the first ages of vegetable life, seem to have resembled, in the group, and in at least their more prominent features, the Algce of the existing time. And with the first indications of land we pass direct from the Thallogens to the Acrogens, — from the sea- weeds to the fern -allies. The Lycopo- diacece, or club-mosses, bear in the axils of their leaves minute circular cases, which form the receptacles of their spore-like seeds. And when, high in the Upper Silurian lf Woody fibre7onT the root of the System, and just when pre- Elder, exhibiting small pores. paring to quit it for the Lower Old Eed Sandstone, we detect our earliest ter- restrial organisms, we find that they are composed exclu- sively of those little spore-receptacles. " The existing plants whence we derive our analogies in dealing with the vegetation of this early period, contribute but little, if at all, to the support of animal life. The ferns and their allies remain untouched by the grazing animals. Our native club-mosses, though once used in medicine, are positively deleterious ; horsetails (Equise- tacece), though harmless, so abound in silex, which wrap them round with a cuticle of stone, that they are rarely cropped by cattle ; while the thickets of fern which cover our hill and dell, and seem so temptingly rich and green Fig. 161. Woody fibre of fossil wood, showing large pores. 3, Woody fibre of fossil wood, bordered with pores and spiral fibres. 4, Fossil wood taken from coal. 258 THE MICROSCOPE. in their season, scarce support the existence of a single creature, and remain untouched, in stem and leaf, from their first appearance in spring, until they droop and wither under the frosts of early winter. " It is not until we enter into the earlier Tertiaries do we succeed in detecting a true dicotyledonous tree ; on such an amount of observation is this order determined, that when Dr. John Wilson, the Parsee Missionary, submitted to me specimens of fossil woods which he had picked up in the Egyptian Desert, in order that, if possible, I might determine their age, I told him that if they exhibited the coniferous structure, they might belong to any geologic period from the times of the Lower Old Red Sandstone downwards; but if they manifested in their tissue the dicotyledonous character, they could not be older than the times of the Tertiary. On submitting them in thin slices to the microscope, they were found to exhibit the peculiar dicotyledonous structure as strongly as the oak or chest- nut. And Lieutenant Newbold's researches in the deposit in which they occur has since demonstrated, on strati- graphical evidence, that it belongs to the comparatively modern formations of the Tertiary. " The flora of the coal measures was the richest and most luxuriant, in at least individual productions, with which the fossil botanist has formed any acquaintance. Never before or since did our planet bear so rank a vegetation as that of which the numerous coal seams and inflammable shales of the carboniferous period form but a portion of the remains, — the portion spared, in the first instance, by dissipation and decay, and in the second by the denuding agencies. Almost all our coal, — the stored-up fuel of a world, — forms but a comparatively small part of the pro- duce of this wonderful flora. Yet, with all this singularly profuse vegetation of the coal measures, it was a flora un- fitted, apparently, for the support of either graminiverous bird or herbivorous quadruped. Nor does the flora of the Oolite seem to have been in the least suited for the pur- poses of the shepherd or herdsman. Not until we enter on the Tertiary periods do we find floras amid which man might have profitably laboured : nay, there are whole orders and families of plants, of the very first importance to man, FOSSIL PLANTS. 259 which do not appear until late in even the Tertiary ages. The true grasses scarce appear in the fossil state at all. For the first time, amid the remains of a flora that seems to have had its few flowers, — the Oolitic ages, — do we detect, in a few broken fragments of the wings of butter- flies, decided traces of the flower-sucking insects. Not, however, until we enter into the great Tertiary division do these become numerous. The first bee makes its appear- ance in the amber of the Eocene, locked up hermetically in its gem-like tomb, — an embalmed corpse in a crystal coffin, — along with fragments of flower-bearing herbs and trees. Her tomb remains to testify to the gradual fitting up of our earth as a place of habitation for a creature destined to seek delight for the mind and eye, as certainly as for the proper senses, and in especial marks the intro- duction of the stately forest trees, and the arrival of the delicious flowers." l " Sweet flowers ! what living eye hath viewed Their myriads ? endlessly renewed ; Wherever strikes the sun's glad ray, Where'er the subtile waters stray, Wherever sportive zephyrs bend Their course, or genial showers descend." (1) Hugh Miller's Testimony of the Rocks. a '2 CHAPTER II. DIVISION OF ANIMAL KINGDOM. PROTOZOA— HISTORY OF INFUSORIAL ANIMALCULES— RHIZOPODA— MONADS— DIATOMACE^E — FOSSIL INFUSORIA, ROTIFERA, VORTICELLA, STENTORS, SPONGES, HYDRA, ZOOPHYTES, ETC. ^'IJSTCE our very limited space forbade more than a cursory glance at the many and varied points of beauty and arrangement dis- played in every part of the vege- table kingdom. ; so are we in the higher ranks of life, driven to be equally brief in noticing the wonders displayed by the help of the microscope, in the world of animal life. In the course of remarks made upon the early condition of vegetable life, we drew attention to the difficulties presented in all attempts to mark out the boundary line between vege- tables and animals, and to define where the one ends, and the other begins. After reviewing the different characters by which it has been attempted to distinguish the special subjects of the botanist and zoologist, we find that animals and plants are not two natural divisions, but are specialised members of one and the same group of organised beings. When a certain number of characters concur in the same organism, its title to be regarded as a "plant," or an " animal," may be readily recognised ; but c-here are very numerous living beings, especially those that retain the DIVISION OF ANIMAL KINGDOM. 261 form of nucleated cells, which manifest the common or- ganic characters, but are without the true distinctive superadditions of either kingdom. Our difficulties are yet far from a satisfactory adjust- ment ; only very lately it has been affirmed by Dr. Hartig that amoeba may be produced by the transforma- tion of the ' antherozoids* of Chara, Marchantia, or Mosses, and that, in their turn, they became metamorphosed, first into Protococci or other unicellular Algae, and then into articulated Algae. This consideration takes us back to the arguments adduced by Mr. Carter, in favour of the analogy between the nucleus of the cell of Chara and that of the Rhizopodous cell, which will be found given at some length in the preceding chapter. « If" — writes Mr. Lewes — " plants and animals present difficulties in our early attempts to distinguish them from each other, they are all distinguishable from minerals by a triple phenomenon— assimilation, reproduction, and death. The same elements are common to the animate and the inanimate kingdoms ; many forms are common to both : but no mineral assimilates — that is to say, grows by the intersusception of foreign material, which it con- verts into its own substance j no mineral dies, as the ine- vitable termination of a cycle of internal changes. " Nutrition belongs to all animals ; but although the final and fundamental act — assimilation — is the same in all, the preparatory and intermediate processes are singu- larly varied. Thus the Infusoria, or unicellular organisms, have no special organ whatever, the only distinction between the parts is that of ' envelope' and * contents ;' by its envelope the animal absorbs, feels, and moves; by its contents it assimilates. An Amoeba, for example, may be looked upon as an assimilating surface having the property of contractility : nothing more. Gradually we observe fresh distinctions of parts : a hole is formed, by way of mouth; then we have two holes, one for reception, the other for rejection of food. Then the mouth becomes fur- nished with jaws ; then with rudimentary teeth ; after- wards with actual teeth, but all of one type ; finally the teeth themselves become distinguished into incisors and molars ; a tongue is added to the mouth ; so that from a 262 THE MICROSCOPE. simple opening to a complicated mouth we trace a series of differentiations. The alimentary canal is at first a mass of cells, then a variety of assimilative sacks or spaces, then a simple canal, then a complicated canal, then a canal formed of oesophagus, stomach, small intestines and large intestines. With this increasing complication there is an accompaniment of accessory organs, liver, parotis, pan- creas, spleen, &c., secreting matters indispensable to the proper preparation of the food before it can be as- similated." Division of Animal Kingdom. — The Animal Kingdom is primarily divided into : — (Hcemastomata ..... 1. Vertebrata. ,, . . (2. Annulosa. Neurostomata . . . . { 3 Mollusca. 4. Ccelenterata. 5. Protozoa. Which are again divided into sub-kingdoms; each sub-kingdom being distinguished by its peculiar typical forms. Commencing from the lowest, we have, 1st. Protozoa, divided into : — 1. Gregarinida. 2. Rhizopoda. 3. Infusoria. The next sub-kingdom, that of the Ccelenterata, is divided into : — 1. Hydrazoa. 2. Actinozoa. The typical form of first — is the Hydra: its modifications, the Sertulariadce, Diphydce, Hydramedusce, &c. The typical form of second, Actinozoa, is Actinia ; others, Lucernaria, Beroe, &c., its modifications are exhibited in other Anthozoic polypes. Sub-kingdom Annulosa, presents difficulties of classifi- cation, owing to the great diversity of forms included among them ; but is primarily divided into three great groups. 1st. Arthropoda. 2d. Annulata. 3. Annuloida. The first, embracing annulose animals having articulated members; the second, those without articulated members, but having a ventral chain of ganglia; and the third, those whose nervous system is composed of cords with one or more ganglia, not disposed in a ventral chain. The whole are arranged in the following classes : — I. Arthropoda: I, Crustacea; 2,Arachnida; 3,Insecta; 4, Myriapoda. II. Annulata: I, Annelida; 2, Suctoria. III. Annuloida: 1, Scolecida ; 2, GepJiyrea ; 3, JZchino- dermata; 4, Rotifera. PROTOZOA. 263 Mollusca are usually included in one of these classes, although subdivided into groups of Mollusca propria, and Molluscoida, the latter consisting of Polyzoa and Tunicata. There is, indeed, a general relation between the Annulose and Molluscous sub-kingdoms. Cephalous Mollusca are divided into the following classes : — 1, Gasteropoda branchiata ; 2, Gasteropoda pulmonata; 3, Pteropoda ; 4, Cephalopoda. The last present us with modifications of organisation in the Nautilus, Ammonite, Belemnotenthis, afe-coloured specimens became green when they were exposed for a few days to the light and full rays of the sun ; while, on the contrary, green specimens were blanched by being made to grow in darkness or shade." The living sponge, when highly magnified, exhibits a cellular tissue, permeated by pores, which unite into cells or tubes, that ramify through the mass in every direction, and terminate in larger openings. In most (1) See Dr. Johnston's History of British Sponges, and Mr. Bowerbank's revision of the class, in the publications of the Ray Society. T2 276 THE MICROSCOPE. sponges the tissue is strengthened and supported by spines, spicula, of various forms ; and which, in some species, are siliceous, and in others calcareous. The minute pores, through which the water is imbibed, have a fine transverse gelatinous network and projecting spicula, for the purpose of excluding large animals or noxious par- ticles ; water incessantly enters into these pores, traverses the cells or tubes, and is finally ejected from the larger Fig. 168. 1, A portion of Sponge, Halichondria simulans, showing siliceous spicula imbedded in the sarcode matrix. 2, Spicula divested of its matrix. vents. But the pores of the sponge have not the power of contracting and expanding, as Ellis supposed ; the water is attracted to these openings by the action of instruments of a very extraordinary nature (cilia), by which currents are produced in the fluid, and propelled in the direction required by the economy of the animal. Mr. Bowerbank, in a paper on the "Structure and Vitality of Spongiadce" states that sponges consist princi- pally of sarcode, strengthened sometimes by a siliceous or calcareous skeleton, having remarkable reparative and digestive powers, and consequently a most tenacious vitality ; so much so, that having cut a living sponge into three segments, and reversed the position of the centre piece, after the lapse of a moderate interval, a complete junction of the parts became effected, so as to render the previous separation indistinguishable. Professor Grant's careful and laborious researches, have finally classed sponges in the animal series of the creation. &-o 5 S.'£3 278 THE MICROSCOPE. He ascertained that the water was perpetually sucked into' the substance of the sponge, through the minute pores that cover its surface, and again expelled through the larger orifices. His own account is so very interesting, that we cannot resist giving, in his own words, the results arrived at in these investigations : — " Having placed a portion of live sponge (Sponyia coalita, fig. 1, No. 170) in a watch- Fig. 170. 1, Spongia coalita. 2, Spongia panicea, highly magnified. glass with some sea-water, I beheld for the first time the splendid spectacle of this living fountain, better repre- sented in No. 2, vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along in rapid succession opaque masses, which it strewed every- where around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention ; but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish the rapidity of its course. In observing another species (Spongia panicea), I placed two entire portions of this together in a glass of sea-water, with their orifices opposite to each other at the distance of two inches ; they appeared to the naked eye like two living batteries, and soon covered each other with the materials they ejected. I placed one of them in a shallow vessel, and just covered SPONGES. 279 its surface and highest orifice with water. On strewing some powdered chalk on the surface of the water, the currents were visible to a great distance ; and on placing some pieces of cork or of dry paper over the apertures, I could perceive them moving, by the force of the currents, at the distance of ten feet from the table on which the specimen rested." Sponges grow attached to almost every thing which may serve them as a point of support, whether fixed or floating; some cover rocks, shells, and other submarine objects, with a close spongy incrustation; whilst others shoot up a branched stem into the water ; and others again hang freely from the sea- weeds floating in the ocean. Sometimes they select very unexpected objects on which to take up their abode. Thus, in one case recorded by Dr. Johnston in his Natural History of British Sponges, a specimen of the Halichondria oculata, a sponge not un- common on some parts of the British coasts, was found growing from the back of a small live crab, — " a burden," says the learned Doctor, " apparently as disproportionate as was that of Atlas, — and yet the creature has been seemingly little inconvenienced with its arboreous ex- crescence." In the next order, Hyppocrepia, all the members are inhabitants of fresh water; one of the most common spe- cies, and that which attracted the attention of Trembley as long ago as 1741, is the Alcyonella stagnorum. It- occurs in great abundance, attached to the leaves of aquatic plants, on floating logs of timber, in the West India Docks. When first taken out of the water it is of a lobulated form and brown colour ; the polypidom is soft and elastic, and feels very much like a sponge ; but, as Mr. Teale observes, this polype " is organically connected with the mass, the tube forming its tunic, from which the animated body issues by a process of evolution similar to that which developes the horn of a snail. When developed, the head projects a short way, and is crowned with a beautiful ex- pansion of tentacula, about fifty in number, arranged in an unbroken circle, which is, however, depressed into a deep concavity on one of its sides, so as to produce the appearance of a double row of tentacula, in a horse-shoe 280 THE MICROSCOPE. form. About 1,600 polypes are situated on a square inch of surface of the mass, consequently the number of polypes in one specimen, which weighed 17 ounces, and measured 14 \ inches in circumference, * may be computed at 106,000, and the tentacula at 5,320,000!'" This family is now classed with the Polyzoa : see Professor Allman's beauti- fully illustrated monograph of all the British species ; published by the Ray Society, 1857. Trembley gave an excellent and interesting account of the family of Alcyonella. Mr. J. Newton Tomkins has kindly furnished the following observations, on the de- velopment of a specimen of Alcyonella Stagnorum: — " The ova, now under examination, (^-inch obj. A. eye- piece — 100 lin. diain., Wollaston's condenser,) are the products of some healthy specimens of Alcyonella stag- norum, given me by Mr. Lloyd, and sketched in full activity in September, 1856, fig. 171. Soon after this period their movements de- creased in energy, numerous ova were detached, which floated to the surface of the water of the jar in which they were confined, and in the course of a very few weeks no trace remained of the parent animals, except a spongy mass of an almost gelatinous character, which still exists, though devoid of definite form> and aPPeail COmpOSed of a maSS Of i__._i_ori QTirq Hi