The microscope : its history, construction, and applications : being a familiar introduction to the use of the instrument and the study of microscopical science

Front matter

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Introduction

and further amplifications of truths already discerned, have been made; but they have been either scattered in serials, or, more usually, devoted to the pages of class publications. 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 gratification ; the many not having, however, participated, to any general extent, in the instruction and entertainment which follow in its train.

The manifold and varied uses and advantages of the Microscope crowd upon us in such profusion, that we can only attempt to enume¬ rate them in the briefest and most rapid manner in these few passages of introduction. 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 appreciated, even by those who formerly were slow to see its benefits. Knowledge which could not be obtained by the minutest dissection is acquired by the aid of the Microscope, which has become as essential to the anato¬ mist and pathologist as the scalpel to the one and bedside observation to the other. 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 many days’ examination of the gross masses of disease in the ordinary method. Microscopic agency, in thus assisting 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 Sur¬ geons 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 collection of preparations of the elementary tissues of both animals and vegetables, healthy and morbid, adapted to illustrate the uses and results of microscopical investigations. From that time histological anatomy deservedly became an important branch of the education of the medical student.

The Microscope, as an ally of Chemistry, enables us to discover very minutely and completely the changes of form and colour effected by test-fluids upon solids ; it dissects for us, so to speak, the most

INTRODUCTION.

YU multiplex compounds ; it opens to tlie mind an extended and vast panorama, opulent in wonders, rich in beauties, and boundless in extent.

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 detecting the adulteration of our articles of food, as has been veri¬ fied by the exposures — which must have done great good — that have from time to time appeared in the Lancet. In the examination of sus¬ pected flour, an article of so much importance to all persons, the Micro¬ scope enables us to judge of the size and shape of the starch-grains, the markings of them, and their isolation and agglomeration, and thus to distinguish the s,tarch-grains of one meal from those of another. In the necessarily limited space of this work, the author has only been able to glance at the subject of the minute structure of vegetables ; but the remarks and reflections which he has devoted to it will, he hopes, lead the microscopist to a full investigation of one of the most beauti¬ ful departments of nature.

By “ conducting the eve to the confines of the visible form,"' the Microscope proves an effective auxiliary in defining the geometric pro¬ perties 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 has enabled us to detect the smallest structural difference, heretofore inappreciable ; and in our analysis to define positively the structure of tissues, beyond the capa¬ bility of the greatest magnifying power to change or modify.

The Zoologist finds in the Microscope a necessary co-operator. To the Geologist it reveals, among a multiplicity of other facts, “ that our large coal-beds are the ruins of a gigantic vegetation ; and the vast limestone rocks, which are so abundant on the earth’s surface, are the catacombs of myriads of animal tribes, that are too minute to be per¬ ceived by the unaided vision.”

In medico-legal investigations the Microscope has been frequently called into use; and in some cases human life has been pending upon its accuracy of decision.

Vlll

INTRODUCTION.

The Microscope not only assists studies, and develops objects of profound interest, but it also opens up innumerable sources of entertain¬ ment and amusement, in the ordinary conventional acceptation of these terms. It discloses 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. It enables us to bring to light latent beauties which the eye could never trace. “ Nature,” it has been well observed, “is not loquacious, although filled with inexhaustible stores ; she presents enough at first sight to attract the thoughtful ; but mankind must interrogate and study her, in order to come at all, perhaps to a thousandth part, of that which she has to communicate.”

It is not necessary, "in this place, to recapitulate further the various ways in which the Microscope is of paramount importance to the seeker after truth, the student, and the man of scientific acquirements ; and, it may be added, of infinite charm and interest to all.

The author has briefly explained the considerations which prompted him, and the objects which he hoped to advance, in undertaking the present work. He begs now to conclude these preliminary 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 has endeavoured to touch upon most of the interesting subjects for microscopic observation as fully as the restrictions of a limited space, and tlie 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, there¬ fore, 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 sections of the work devoted to the very minute portion of creation, he has ventured to dwell somewhat more upon the subject, in the belief that that department is more espe-

INTRODUCTION.

IX cially the province of the microscopist than many others. He has arranged his topics under special headings, and in separate chapters, for the sake of greater clearness and precision ; and has brought the everwelcome aid of illustrations to convey his explanatory remarks more vividly to the minds of his readers. To his friend, Mr. J. G. Kelly, he has to express his thanks for valuable assistance afforded in making drawings of the objects from the Microscope and camera lucida ; as also to Mr. George Pearson, for the admirable manner in which he has engraved them. The author has minutely described the use and mani¬ pulations of the Microscope, so as to render its management, he ven¬ tures to hope, intelligible to all ; and his marked and warm acknow¬ ledgments are due to the eminent maker and improver of the Micro¬ scope, Mr. Eoss, for his revision of this portion of the work. He is peculiarly indebted to Professor John Quekett, whose very valuable lectures, delivered in the Eoyal College of Surgeons, and other multi¬ farious and successful researches, have pre-eminently distinguished him as the microscopist of the day. From notes made at 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 the ready acquiescence that he gave to the writer's wish to cull from the choicest of the mass of contributions with which he has enriched the micro¬ scopical science of the day. The author has also freely availed him¬ self of the researches of other scientific investigators, — Ehrenberg, Carpenter, Johnston, Ealfs, Busk, Gosse, Huxley, and the members of the Microscopical Society of London, to the foundation of which ex¬ cellent society we may ascribe the rise and progress of many improve¬ ments in the instrument ; and it has tended, moreover, to stimulate discoveries, and induce greater accuracy of observation. In the ex¬ tracts which he has made from these various sources, the author’s ob¬ ject has been to give his readers, in addition to the results of his own original investigations, some adequate notion of the leading theories, the most characteristic views, and the most useful instructions, of these distinguished authorities, whose names are intimately identified with the studies of the Microscope.

Finally, it is the author's hope that, by the instrumentality of this

X

INTRODUCTION.

volume, he may possibly assist in bringing the Microscope, and its most valuable and delightful studies, before the general public in a more familiar, compendious, 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 publications, submit some further food for the exercise of the mental and intellectual faculties, — contribute to the additional 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 con¬ fided with great diffidence to the consideration of the public, to suc¬ ceed, in however slight a degree, in furthering this design, the author will feel sincerely happy; and will be fully repaid for the attention, time, and labour, that he has expended in writing, arranging, and com¬ piling it.

6 Gower Street , Bedford Square , May 1854.

DIRECTIONS FOR PLACING THE PLATES.

Frontispiece and Engraved Title to face each other, and precede the printed title-page.

PLATE

H. Eossil Infusoria .

III. Zoophytes, & c. .

IV. Sponges, Spicula from V. Echino-dermata, &c. .

VI. Proboscis of Ely VII. Tongue, &c. of Horse-fly VIII. Antenna, &c. of Silkworm Moth IX. Scales from Wings of Butterflies, &c X. Parasites .

XI. Animal Structures XII. Animal Tissues .

XIII. Human Skin and Hair

XIV. Vegetable Cells, &c. .

XV. Vegetable Structures to face page

*

Part I. History Of The Invention And Improvements Of The Microscope

CHAPTER L.

Derivation, and period of invention. Known before Christ. Single microscopes of Leeuwenhoek, Swammerdam, Lyonet, and' Ellis. Lieberkuhn’s reflecting instruments in the Royal College of Surgeons. Leeuwenhoek’s microscopes ; Dr. Hooke’s. Stephen Gray’s glass globules. Dr. Hooke’s compound instru¬ ment. Divini’s object-glasses. Bonnani’s, Martin’s, Sir Isaac Rewton’s, Dr. Barker’s, and Dr. Smith’s discoveries. Lieberkuhn’s solar microscope. Baker’s, Dr. Hill’s, M. Delabarre’s, and Mr. G. Adams’ published accounts of the microscope. Prauenhofer’s, Pelligne’s, Chevalier’s, and Amici’s experi¬ ments. Tulley’s, Dr. Goring’ s, and Lister’s experiments. Amici's achromatic glasses.. Sir John Herschel’s, Airy’s, Barlow’s, Coddington’s, and Lister’s combinations. Mr. Ross’s improvements. Dr. Goring’s,. Solly’s, Bowerbank’s, and Valentine’s improvements and assistance. Reports of the Juries of the Great Exhibition on the instruments manufactured by the London makers _ . . . Page 1

CHAPTER H.

Mechanical and optical principles involved in. the construction of the modern microscope. Mode of estimating the magnifying power. Rays of light passing through transparent media. Eorms of lenses. Refraction of light through lenses, and method of tracing the progress of rays. Spherical aberration of lenses. Sir John Herschel’s and Coddington’s lenses. Chro¬ matic and achromatic aberration of lenses. Limit of distinct vision through lenses. Dr. Wollaston’s doublet. Holland’s eye -piece. Wollaston and Coddington’s lenses. Stanhope lens. Ross’s simple microscope. Valen¬ tine’s simple microscope. The compound microscope, Air. Ross’s remarks upon. Lister’s, Powell’s, Tulley’s, and Boss’s improvements. Mr. Ross’s correction of object-glasses, Mr. Varley’s corrections of the eye-piece. Huyghenian and micrometer eye-pieces. Mr. Lister’s stage-micrometer. Mr. Jackson’s eye-piece micrometer. Dr. Hannover’s table of comparative mea¬ sures. M. Robert’s glass micrometers. Sets of eye-pieces, their value.

XIV

CONTENTS.

Magnifying power of Ross’s object-glasses. Mechanical arrangements. Mr. Ross’s smaller instrument. Ross’s large compound microscope. Baker’s student’s microscope. Useful Remarks in Juries’ report on the microscopes at the Great Exhibition . Page 12

Chapter III

Directions for using the instrument. Sir David Brewster’s directions for viewing objects. Mr. J. Smith on the management of light in viewing objects. Mr. Ross upon illumination. The diaphragm. Varley’s white cloud. Wollas¬ ton’s condenser. The achromatic illuminator. Wenham’s parabolic reflector. Mr. Shadbolt’s improvement. Condensing lenses. Mr. Brooke’s double re¬ flector. Lieberkuhn’s condenser. Mr.Highley’s achromatic lamp. Eorceps. Dipping tubes. Varley’s forceps. Collecting net. Animalcule box. Mr. Williamson’s collecting-stick. Varley’s animalculse cages. Smith and Beck’s chara troughs. Dissecting knives and needles ; mode of using the same. Dissections under the microscope. Valentin’s knife, improved. Dissecting scissors. Cutting glass covers. Mounting and preserving objects. Mr. Gorham’s “ cementing pencil.” Object-holder. Air-pump. Mr. Boys’ direc¬ tions for mounting. Mr. Deane’s mounting fluid. Mr. Warrington’s fluid, and directions for using it. Mr. Goadby’s solutions. Mode of injecting. Ruysch’s and Breschet’s method. Goadby’s process. Dr. Schacht’s re¬ agents and preservative fluids. Dr. Parkes’ caution. Mr. Ralfs’ directions for preserving algae, &c. Mr. Topping’s fluid. Mr. Shadbolt on collectingobjects. Localities of microscopic plants, &c. The camera-lucida. Quekett’s directions for using the camera-lucida. Polarised light. Nicol’s prisms. On using polarising prisms ; Mr. Woodward’s explanations ; Mr. J. King’s remarks on. Dr. Herepath’s new salt of quinine. Mr. Legg’s phenomena of polarised light. Crystals of urinary salts. Crystals of ice. Starch grains under polarised light. Binocularity applied to the microscope. Application of photography to the microscope .... Page 50

Part II. Chapter L

A glance at the microscopic world. History of animalculse. Professor Quekett on viewing ciliary movement. Infusorial animalculse. Baker’s account of wheel-animalculse. The marine animalculse. Phosphorescent appearance of the sea, described by Darwin, Captain Scoresby, and Mr. Gosse. Polygastric animalculse. Monads. Ehrenberg’s description of monads. The Breast-plate monad. The Proteus. Enchelis. Vibrio. Astatia. Bacillaria. Navicula. Vorticella. Stentors. Rotifera. Brachiona. Notommata aurita ; described by Mr. Gosse. Eloscularia. Melicerta ringens ; described by Mr. Gosse. Berg-mehl. Professor Owen on the uses of animalculse. Polypifera. Dr. John-

CONTEXTS.

XV ston on British zoophytes. Hydra ; first described by Leeuwenhoek. Trembley’s description of hydra. Marine animal plants. Alcyonidae ; Professor Grant’s description of their ova. Asteroidee. Corallidse. Jukes’ and Cap¬ tain Basil Hall’s descriptions of coral-reefs. Gorgoniae. Crisiadae. Mr. Busk’s description of the Shepherd’s Purse and Snake Corallines. Celleporidae. Madreporidae. Tubiporidae. Pennatulidae. Sertulariadae. Actiniadae. Lucernariae. Bryozoae. Plustrae. Escharae. Yesiculariadae. Zoo¬ logical Society’s marine vivaria, Mr. Gosse on. Porifera. Sponges ; described by Professor Dr. Grant. Dr. Lankaster’s definition of vegetable and animal life. Alcyonellae. Limniades. Clionae. Skeletons of zoophytes. Spicules of sponges. Mr. Carter on the seed-like bodies in sponges. Dr. Golding Bird on preserving the polypidoms of zoophytes. Possil infusoria. Professor Quekett on mounting infusoria. Professor Eedfern on isolating Naviculse, &c. Diatomaceae. Professor Bailey’s contributions to the Museum of the College of Surgeons. The value of studying fossil infusoria to artists. Mr. Shadbolt on the Arachnoidiscus. Eev. W. Smith on collecting and preserving Diatomaceae. Xanthidiae. Rev, J. B. Read on animals found in chalk. Poraminiferae. Echinidae. Acalephae. Annelidae. Crustaceae. Mollusca. Pearls. Gasteropoda. Lymnaea stagnalis, development of. Mode of show¬ ing the structure of shell ; Dr. Harvey on collecting salt-water specimens. List of interesting specimens . Page 120

CHAPTER HE.

INSECTS.

Their beauty of organisation. The Crane-fly. The Wasp’s tongue, feelers, &c. Eye of fly. Sucker on foot. Breathing-aperture. Peet and legs. Tongue and piercing-apparatus of fly. Mr. G. Xewport on the tongue of the fly. The wings of insects. The Dragon-fly. The Gnat. The Water-beetle. The Cricket. The common beetle. The Death-watch. The Bacon-beetle. The Cockchafer. Butterflies and moths ; Mr. G. Newport on the structure of their tongues ; scales from their wings. Mr. Topping’s test- objects. Eggs of insects. Saws of Saw-fly. The Proth-fly. Aphides. The Ant. The Ant-lion. Stings of insects. The Honey-bee. The Spider. Parasites. The Plea, The Louse. The Itch-insect, Entozoa folliculorum. The Guinea-worm. Echinococci. Dr. J. Leidy on vegetable parasites. Dr. Burnett on parasites, and their relation to other animals ; — found on the sparrow and snail, by Dr. Robins. On the water-snail. Swammerdam’s dissections of insects. Parts of insects for examination. Insect changes ; their distribution, their commercial value, and general uses .......... Page 272

CHAPTER III.

ANIMAT STRUCTURE.

Histological history of animal life ; the value of the microscope in the study of. Dr. Carpenter’s definition of a cell. The metamorphoses of the cell. Mr.

XY1

CONTENTS.

Huxley’s review of the unicellular theory. Mr. Huxley’s sketch of a general theory of the structure of animals and plants. The vital or molecular forces. The elementary tissues ; Professor Quekett’s classification of. Simple mem¬ brane. Blood-cells. Crystallisation of blood. Capillaries. Fibrous tissue, elastic and non-elastic. Muscular fibre. Structure of the skin. Structure of hair. Nerves. Cartilage. Consolidated tissues ; teeth. Bone. Professor Quekett on the structure of bone. Dr. Bennett on mode of investigating animal structures. To view the circulation of the blood in the frog. The newt. Structure of fishes ... _ Page 334

CHAP TEE IV..

VEGETABLE STRUCTURE.

Vital and chemical characteristics of plants. Mulder’s description of the vege¬ table cell. The Yeast-plant. Fungoid growths. The red snow of the northern regions. Scotch Siller-cups. Confervse. Algse. Mosses. The Volvox globata. Desmidiese. Balfs on the British Desmidiese, The cell-trans¬ formation.. Cellular tissue. Woody fibre. Lactiferous tissue. Crystals in plants. Baphides. Siliceous cuticle of grasses. Starch. Adulterations of wheat-flour. Potato disease. Hairs on the stinging-nettle. Chlorophyl. Nitella. Vallisneria. Chara. Mr. Varley on the cultivation of Chara. Vascular tissue ; Mr. Edwin Quekett upon the development of. Preparation of vegetable tissues. Machine for cutting sections. Method of making sections. Concluding remarks . . Page 386

THE MICROSCOPE.

PART I.

HISTORY OE THE INVENTION AND IMPROVEMENTS OE THE

MICROSCOPE.

Chapter I. History Of The Microscope

HE instrument known as tlie Microscope derives its name from two Greek words, fiiKpoQ small, and cko—eoj to view ; that is, to see or view such minute objects as with¬ out its aid would be invisible.

The invention of the microscope cannot traced with any degree of certainty before the year 1660, a period fruitful in discoveries. 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 with the rest of the world, and to hand down the name of its inventor to succeeding ages.

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 into public view. Zacharias Jansens and his son are said to have made microscopes before the year 1590 :

HISTORY OF THE MICROSCOPE.

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 was 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 loug and one inch in diameter, sup¬ ported 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 one person having made the same invention nearly simultaneously, 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.

If we consider the microscope as an instrument consisting 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 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 convexities, 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.

Aristophanes, who lived five centuries before Christ, speaks of a “ burning-sphere.” Seneca, who wrote in the first half-century of the Christian era, says that small and indistinct objects become larger and more distinct in form when seen through a globe of glass filled with water. Pliny also mentions the burning property of lenses made of glass.

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 extraordinary 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 im¬ provement 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 cultivated 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.

HISTORY OF THE MICROSCOPE.

We shall first speak of the single microscope, that, as we have already observed, 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 very 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, laid open some of her hidden recesses, and by their example stimulated others to the same pursuit.

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 additions to its apparatus. The greatest im¬ provement this instrument has received was made by Dr. Lieber¬ kuhn,** 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 ob- « ject, 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 and satisfaction ; for the dark side of the object being next the eye, and also overshadowed by the proximity of 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 anatomical preparations, twelve of which, with their microscopes, are deposited in the Museum of the Royal College of Sur¬ geons.

Lieberkuhn’s instrument, fig. 1, is thus described by Professor Quekett:f ab 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 - convex lens of half an inch in focal length, whilst the one at b carries a condensing lens three-quarters of an inch in diameter.

* Dr. Nathaniel Lieberkuhn of Berlin.

f Practical Treatise on the Microscope, p. 16.

fig. 1.

HISTORY OF THE MICROSCOPE.

LIEBERKUHN S MICROSCOPE.

A vertical section of one of these instruments is seen in fig. 2 : a represents the magnifier, which is lodged in a cavity formed partly by

the cap a and by the silver cup or speculum l. In front of the lens is the speculum l, which is a quar¬ ter of an inch thick at its edge, and whose focus is about half an inch ; in front of this again there is a disc of metal c, three-eighths of an inch in diameter, connected by a wire with the small knob d ; upon this disc the injected object is fastened, and is co¬ vered 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 disc 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 condens¬ ing lens b serves to concentrate the light on the speculum. To the 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 instru¬ ment 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 l] this, again, further condenses them on the object and the disc 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 Leeuwenhoek’s microscopes, which were rendered famous throughout all Europe, on account of the numerous discoveries he had made with them, as well as from his afterwards bequeathing a part of them to the Boyal Society.

The microscopes he used were all single, and fitted up in a con¬ venient and simple manner : each consisted of a very small double-convex 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 convenient examina¬ tion 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 exceedingly thin-blown glass, which he afterwards glued to the needle in the same manner as his other objects. The glasses

HISTORY OF THE MICROSCOPE.

were all exceedingly clear, and of different magnifying powers, pro¬ portioned to the nature of the object and the parts designed to be examined.

He observed, in his letter to the Royal Society, that, from upwards of forty years’ experience, he had found the most considerable dis¬ coveries were to be made with glasses of moderate magnifying power, which exhibited the object with the most perfect brightness and dis¬ tinctness. Each instrument was devoted to one or two objects; hence he had always some hundreds by him.

About the year 1665, small glass globules began to be occasion¬ ally applied to the single microscope, instead of convex lenses. By these globules an immense magnifying 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, published in the year 1656.

Mr. Stephen Gray* having observed some irregular particles within a glass globule, and finding that they appeared distinct and prodi¬ giously 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 animalculse; 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 animalcule most enormously magnified ; for those which were scarcely discernible with his glass globules, with this appeared as large as ordinary-sized peas. Montucla observes, that; when any ob¬ jects are enclosed within this transparent globule, the hinder part of the globule acts like a concave mirror, provided the objects be situated between that surface and the focus ; and that by this means they are magnified three times and a half more than they would be in the usual way.