mOSCOPY THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID Mailed Abroad Free. HENRY CROUCH, 66, Barbican, London, E.G. MEDALS AND HIGHEST AWARDS London, Paris, Philadelphia, l^BSfc. Antwerp, Sydney, &c^ ROSS' IMPROVED MICROSCOPES, WITH NEW PATENT FRICTIONLESS FINE- ADJUSTMENT, AND ZENTMAYEFS Patent Swinging SUBSTAGE ARRANGEMENT, FOR obtaining any degree of Oblique Illumination. ABOUT 25 per Cent. less Cost than former MODELS. PULL DESCEIPTION AND CATALOGUE ON APPLICATION TO :ROSS ^.ISTID oo., U2 (REMOVED PHOM 164), NEW BOND STREET, \ONDON. MICROSCOPES. R. & J. BECK, ESTABLISHED NEARLY 50 YEARS, HAVE DEVOTED GREAT CARE AND ATTENTION TO THE MANUFACTURE OF ACHROMATIC MICROSCOPES TO SUIT THE TASTE AND REQUIREMENTS OF PURCHASERS, WHETHER STUDENTS, PROFESSIONAL MEN, OR AMATEURS. THEIR STUDENTS' MICROSCOPES ARE THE CHEAPEST MADE, COMBINING LOW PRICE WITH EXCELLENT WORKMANSHIP. And the quality of the "Economic" Object Glasses is such thgt the higher powers will resolve most of the difficult Diatomaceous Tests, and meet the wants of Medical Students, By their Fine Correction, Superior Definition and Penetration. PRICE FROM 1 TO 10 10s. THEIR FIRST-CLASS STANDS ARE OF THE MOST COMPLETE CONSTRUCTION. "VIDE" INTERNATIONAL MICROSCOPE ILLUSTRATED IN THIS VOLUME. COMPLETE ILLUSTRATED CATALOGUE, Price 2s. 6d. OR POST-FREE TO INTENDING PURCHASERS. ILLUSTRATED DESCRIPTIONS OF THE "POPULAR" AND "ECONOMIC" MICROSCOPES UPON APPLICATION. R. & J. BECK, 68 CORNHILL, LONDON, E.C. 1016 CHESTNUT ST., PHILADELPHIA, U.S.A. Factory: LISTER WORKS, HOLLOWAY, LONDON, N. NTISPIEC ~f<*s EUCALYPTUS GLOBULES WOOD SECTIONS DOUBLE STAINED CHAP. XU. PRACTICAL MICROSCOPY. BY GEORGE E. DAVIS, F.R.M.S., F.I.C., F.C.S., ETC., ETC. ILLUSTRATED WITH TWO HUNDRED AND FIFTY-SEVEN WOODCUTS AND A COLOURED FRONTISPIECE. LONDON: DAVID BOGUE, 3 ST. MARTIN'S PLACE, TRAFALGAR SQUARE, W.C. 1882. PREFACE. THE Author's object in presenting this work to the student of microscopy is to supply at a reasonable cost a book written upon somewhat similar lines to Quekett's ' Prac- tical Treatise on the Use of the Microscope,' the second and last edition of which appeared in 1852. The necessity of a treatise bringing the science to the present day has been long felt, and it is hoped that a vacant place in micro- scopical literature is now filled up. Although essentially a practical work upon microscopy, the reader is treated to a little theory, in the hope that it may lead to a more minute study of the optical principles upon which the microscope is constructed. Some may object, perhaps, to so little theory being introduced, espe- cially on those subjects intimately connected with Professor Abbe's recent researches ; but it should be remembered that the addition of such matter would have added considerably to the cost of production and detracted somewhat from the practical character of the book : this, taken together with the fact that ample references have been given to Professor Abbe's papers, will, it is hoped, be sufficient apology. Moreover, the information lately put forth is novel to the general body of microscopists ; the views now held are so a 2 IV PREFACE. decidedly opposite to those promulgated but a few years ago, that it is evident any one writing on the subject now could only put forth an abstract of the learned Professor's views. This the Author did not wish to attempt, as such a proceeding would only have mystified the student, and not have furnished the advanced worker with as much informa- tion as he could obtain from the 'Journal of the Royal Microscopical Society,' where Professor Abbe's papers may be found in extenso. With regard to the selection of objectives, the Author hopes he will not be misunderstood : medium angles have been advised for students' use, for the simple reason that they can be employed without much previous knowledge or difficulty ; but for all purposes of scientific investigation, wide apertures, requiring often much skill in manipulation, will give the most satisfactory results. Our American brethren have for many years been im- pressed with the importance of wide apertures, and no doubt many of their arguments were sound, though it can- not be said that their case was ever well demonstrated and supported. Dr. G. E. Blackham, in a paper read before the Micro- scopical Congress at Indianapolis in 1878, argued ex- clusively for wide apertures, though he was singularly unfortunate in his selection of a theory to account for "penetration" in objectives; while this brochure has been supplemented by a volume from the pen of Professor J. E. Smith, ' How to See with the Microscope/ putting forward again the merits of wide angles. Microscopists who have read these works will do well PREFACE. to study what Professor Abbe has written, and then they will probably come to the conclusion that the high-angle school possess some forcible arguments, and may also find that with wide apertures their battery of objectives may be considerably smaller to accomplish work of a better quality than when low angles are employed. Under the head of objectives, the almost total absence of American productions will be remarked : it is only recently that American objectives of the widest aperture have found their way into the Author's hands. Their definition is marvellous, but it has not been thought advisable to include such objectives in a practical work until after they have been used in ordinary observation for a sufficient length of time. However, the reader may like to know that Spencer, of Geneva, U.S.A., produces a 3-inch of I3,*a 2-inch of 20, and a i-inch of 50; while Tolles, of Boston, U.S.A., makes similar objectives, a ^ of 145; both of these opticians pro- ducing the J-inch and all higher powers, of 180 air-angle. These wide apertures demonstrate clearly the accuracy of the statement made on page 5 1, that wide angles require more care in their correction, and are consequently more expensive. Spencer's i-inch of 50 costs forty-five dollars, or 9/. ; while the i-inch of 22 in their student's series costs but ten dollars, or 2/. The whole of the information in the work has been selected to aid the student as much as possible ; but it should be remembered that the microscope is useful, from a scientific point of view, only as an aid in research, unfolding to us objects either invisible, or but faintly to be distinguished vi PREFACE. by the unassisted eye. The Author has not pandered to the tastes of mere lovers of pretty objects. As to the preparation and mounting of objects much more might have been said ; indeed, volumes written on this subject alone ; but types have been selected to avoid repetition, and which the student will do well to follow. All the processes in practical microscopy must be carried out with intelligence, and the why and the wherefore of each well understood. If this be done the student will find no insuperable difficulties. Many of the illustrations found herein have been photo- graphed from nature by the Author, and cut in the en- graving department of 'Design and Work,' and thanks are here expressed to the proprietor for these blocks, which have enabled the work to be so fully illustrated at a moderate cost. To the publisher of 'Science-Gossip/ for similar reasons, thanks are likewise expressed. Many of the illus- trations have been lent from the ' Northern Microscopist.' In conclusion, the Author asks his readers to communi- cate to him, under care of the publisher, any ideas or suggestions in view of future editions, and hopes he has established a sufficient raison d'etre. GEORGE E. DAVIS. CONTENTS. CHAPTER I. PAGE INTRODUCTION .... i CHAPTER II. THE MICROSCOPE STAND CHAPTER III. EYE-PIECES AND OBJECTIVES 40 CHAPTER IV. ACCESSORIES 58 CHAPTER V. GENERAL REMARKS UPON OBJECTIVES TEST OBJECTS .. 97 CHAPTER VI. THE COLLECTION OF OBJECTS 127 CHAPTER VII. MICRO-DISSECTIONS ! CONTENTS. CHAPTER VIII. PAGE SECTION-CUTTING .. 182 CHAPTER IX. THE DELINEATION OF OBJECTS MICROSCOPIC MEASURE- MENTS 203 CHAPTER X. THE POLARISCOPE 238 CHAPTER XL THE MICROSPECTROSCOPE .. .. 249 CHAPTER XII. STAINING AND INJECTING ,.'->....).. .. 259 CHAPTER XIII. THE PREPARATION AND MOUNTING OF OBJECTS 271 CHAPTER XIV. REAGENTS RECIPES 312 INDEX 325 PRACTICAL MICROSCOPY. CHAPTER I. INTRODUCTION. DURING the past few years rapid strides have been made in the manufacture and supply of cheap and at the same time really good working microscopes ; and these having been extensively purchased, have extended and disseminated a taste for the study of minute things, in those to whom the possession of a microscope a few years ago seemed a luxury to be only dreamt of. Good works treating upon the practical use of the microscope and its accessories, have hitherto been of so expensive a character as to be certainly beyond the reach of most students, and many who might have been led into the paths of scientific research, have probably kept their instruments to gratify the curiosity of themselves and their friends. Such persons often think that the possession of a microscope must always make a microscopist, and in order to study objects, that it is only necessary to place them on the stage of the microscope when their hidden structure will manifest itself at once. No greater mistake could ever be made. It is very desirable that the student should be thoroughly acquainted with the physics and chemistry of the science, for when once these are mastered B PRACTICAL MICROSCOPY. he will see his way to carry on experiments in a rational manner, as often as fresh problems are presented to him. Again, no inexpensive work upon practical microscopy brings the subject down to the present time ; this and other minor details urged us to cut out a path for our- selves, and produce a treatise which might be of use to the student. In using the microscope as a means of gaining an insight into the anatomy or life-history of forms only faintly dis- tinguished by the unassisted eye, the student must be prepared to exercise a certain amount of patience ; skill in manipulation comes by practice only, and before com- mencing with any investigation, all preconceived notions should be cast aside, the observer being guided chiefly by the results of his own observations. Isolated observations should never be considered con- clusive ; experiments require frequent repetition under very varying conditions before we can accept the results as true interpretations of what we have actually seen. We should also be careful to take into account the degree of per- fection of our instruments as well as our own capability of vision ; what one observer will see, another will fail to detect, and if any preconceived notions exist, the observations are sure to be moulded after the same fashion. Expertness in microscopy is only to be attained by study and practice, and as it is necessary, in order to progress, that we take up the subject where our predecessors left off, it is necessary to become acquainted with what has been done previously, and this may be arrived at by the perusal of old standard works upon the science, such as those of Baker, Pritchard, and Goring, or the more recent work of Mr. Quekett's, ' The Practical Treatise on the Use of the Microscope.' These works, though old, are valuable, and INTRODUCTION. the executive of every microscopical society should en- deavour to secure a copy of each for its library. If microscopical students would only peruse these older works, and enter into communication and discussion with their brother microscopists, much time and work would be saved, and a great deal of that re-discovery which goes on at the present day averted ; there is no other way of avoiding repetitions except by the methods above stated. We might advance more rapidly, or more certainly at least, if we asked, before performing any microscopical operation, the reason why. We are too apt to do things in a certain manner because some one who instructed us in the art and mystery did so before us ; no inquiry was made, but we did likewise. If the principles upon which all operations are conducted were thoroughly understood by all those who intend working with the microscope, much labour would be saved, needless experiments avoided, slides rendered more permanent, and microscopical research brought more into favour on account of some of the barriers which now obstruct progress having been broken down. It is in this direction that a treatise may be useful to the student ; objects themselves, whether of animal, vegetable, or mineral origin, are best treated of in works entirely independent of practical microscopy; an organism, how- ever minute, has its life-history, and it should be the aim of each student to be useful in his generation by endeavour- ing to furnish an accurate account of the cycle of existence of some member of the animal or the vegetable world. Of course, before he can be expected to occupy himself with original research, he should be fairly acquainted with all that has been done before upon the subject ; but above all must he be familiar with the instrument with which he works, and the methods whereby certain results may be attained. B 2 PRACTICAL MICROSCOPY. Microscopical research does not always require the aid of expensive apparatus. It is very handy and often saves time, no doubt, to have ready all those accessories so ingeniously devised by the makers, for microscopists with long purses ; but apparatus to answer the same purposes may often be made by the ingenious worker, which, though not possessing such a good appearance, serve just as well as the more expensive articles. Many microscopists, after a few years' devotion to their favourite instrument, find themselves encumbered with a host of paraphernalia of no use to themselves or to any one else, the cost of which might have been saved by con- sidering beforehand the capabilities of the required appa- ratus. As to instruments, each individual taste has to be considered, some prefer one pattern and some another ; but, after all, these matters are easily arranged if the principle of construction is good. The main office of the microscope is that of enlargement ; but this amplification of the image of an object must be attained without distortion or the introduction of colours not in the original, and it is because single lenses give images blurred with spherical and chromatic aberration that double and triple combinations are used in the con- struction of all good microscopes. Single lenses are how- ever very useful for general purposes: as a pocket lens, it prevents the collection of much useless material during a naturalist's rambles, and upon reaching home a further use is found in its employment as an aid in dissecting or in mounting the objects culled. The most useful magnifiers are the ordinary watchmakers' or engravers' eye-glasses in the usual horn mount, but their amplifying power should not be too great or continued use may impair the eyesight. A combination of several single lenses, such as is shown in Fig. i, is much used as a pocket magnifier for the field BRO WNINGS PLA TVS CO PIC LENSES. 5 where a high power is not required ; they are cheap, but the lower powers only are generally useful. The Stanhope and Coddington lenses are also used by some collectors, though there is little doubt that they are going out of date on account of their not being so useful as newer forms of the simple microscope. The Stanhope magnifier is a double convex lens having two unequal curvatures. In observing, the deepest curve is placed towards the eye, the object adhering to the least convex side being just in focus. The Coddington lens is generally a sphere of glass, round the periphery of which a deep groove has been cut and filled up again with black cement. This lens focusses at a short distance from the object, and is much superior to the Stanhope form. Inferior p Coddingtons are now made from re- jected double convex lenses which do not act as well as the form described. Without doubt the best magnifiers for field use and such work generally are the platyscopic lenses of Mr. John Browning, which he makes of three degrees of power, amplifying 15, 20, and 30 diameters respectively. They are really achromatic triplets, are set in ebonite cells, and mounted in tortoise-shell frames. These lenses focus at about three times the distance from an object as a Coddington of the same power, and so allow of the easy examination of opaque objects. They are shown engraved full size in Fig. 2. Steinheil has produced similar lenses, which he terms " aplanatische loupen " ; they are of similar construction to the above, and are made to magnify 5j, 8, 12, 1 6, and 24 diameters. One of these lenses, or, preferably, two of them, carried PRACTICAL MICROSCOPY. in the pocket when field hunting, will prove of invaluable assistance to the student. The most useful powers will be found in those amplifying 1 5 and 30 diameters : the former serves well for the examination of mosses, ferns, lichens, algae, and such members of the animal world as can be FIG. 2. recognised by the aid of a 3-inch objective and the A eye-piece, to which combined, it is about equal in power ; whilst the smaller glass, giving greater amplification, answers admirably for micro-fungi, minute algae and lichens, and those forms of animal life for which a i-J-inch objective is desirable. If the reader refers to any work treating upon optics, he will find that convex lenses yield images in two distinct manners virtital images and real images. A double convex lens, when used as an ordinary magnifying glass, produces a virtual image which is erect and larger than the object, as may be seen by reference to Fig. 3. The greater the convexity given to the surfaces of the lens, the more will it amplify, and it may also be said VIRTUAL AND REAL IMAGES. that the nearer the object be placed to the principal focus of the lens (F) for parallel rays, the larger will the virtual image appear. To yield a virtual image, the object must be placed between the lens and its principal focus. FIG. 3. A real image is formed from a double convex lens, only when the object is outside, or in front of, the principal focus for parallel rays ; this image is inverted, and may be studied by receiving it upon a screen of ground glass. The action of lenses in the production of real images should be care- fully considered by the student, as upon it depends chiefly FIG. 4. the results of his manipulation, which may be perfect or defective in proportion as he understands the theory of this simple optical instrument. An inspection of Fig. 4 will show the two consequences of this action. If A B represents a lighted candle placed far distant from the principal focus of the lens, a small, real 8 PRACTICAL MICROSCOPY, but inverted image of the candle will be thrown upon the screen at a d, and the further A B is placed away from the principal focus for parallel rays, the smaller will be a b, and the nearer will it be formed to the opposite principal focus. On the other hand, if a b represents the lighted candle placed in front of the principal focus F but very near to it, the real image which is formed, A B, is much amplified, in proportion as a b is nearer to the principal focus, and the distance at which A B is formed is likewise regulated by the distance of the object from this point. The nearer a b is to F, the further will A B be formed away from the lens, and vice versd. The student will now be able to see the principle upon which the microscope is constructed, and, in closing this chapter, we do so by giving a table of the magnifying power of single convex lenses of varying focal lengths, for parallel rays at a distance of 12 J inches from the micro- meter to the screen, which has been constructed from a paper upon the subject by Dr. Woodward in 'Silliman's Journal' for June 1872. Focal Distance Amplification for in Parallel Rays. Diameters. i" .;. ...... 10-4 ...... 23-0 F ........ 48-0 F ...... .. 98-0 T y ........ 198-0 CHAPTER II. THE MICROSCOPE STAND. As we are supposed to be treating nearly exclusively of compound microscopes that is to say, of instruments in which the amplification of an object is produced by means of a combination of lenses called an " objective," the image being further magnified by another set called an " ocular " or " eye-piece " we will consider that our readers require no demonstration as to the necessity of some mechanical contrivance for holding these lenses in their correct positions. This is the office of the stand of a compound microscope, and when we notice the progress which has been made in the details of this instrument from the time the first was made for sale in England by Mr. John Marshall, we shall find that not to opticians only, properly so called, are we indebted for improvements, but to mechanicians equally. It was but of little use attempting to perfect the optical arrangements while the mechanical contrivances were imperfect, and when this was fully appreciated, real improvements were made by mechanics and optics working hand in hand. At the same time, we must not forget the aid which has been rendered by amateurs and micro- scopists with unlimited means at their disposal; it is certain that many of the early and even the more recent improvements would not have been executed, except at the instance of one possessing a well-lined purse and with great interest in the science. 10 PRACTICAL MICROSCOPY. As previously mentioned, the chief function of the microscope stand is to receive the eye-piece and objectives ; FIG. 5. but it has to do much more than this, and upon its form and general construction, together with the character of THE ROSS MODEL. 1 1 the workmanship, depends the excellence of the work it is able to perform. The two principal models, upon which nearly all stands are made, have been named the " Ross model " and the " Jackson model," both of which may be studied in the productions of Messrs. Ross & Co. Fig. 5 shows the Ross model, in which the body is attached by its base to a transverse arm, which latter is borne on the summit of a racked stem, to be seen in the figure. This is a very efficient and convenient form when the workmanship is good, and leaves nothing to be desired if the stem and transverse arm are sufficiently solid. At the same time, it is the worst model on which to make cheap instruments, as those who have used them will only be too ready to testify. A cheap stand made on the Ross model practically requires no fine adjustment, there being sufficient spring in the stem and arm to focus with, by slightly pressing upon the latter at its junction with the body. In the Jackson model, shown in Fig. 6, the body has the rackwork attached to it, and is supported for a great part of its length on a solid limb, as shown in the figure. It is, there- fore, much less liable to vibration than in the Ross model. In a paper read before the Royal Microscopical Society in March 1870, Dr. Carpenter detailed his experience of these two forms of instruments. He sums up as follows : " My own very decided conviction is that the adoption of the principles of the Jackson model would be decidedly advantageous alike for first-class instruments, in which the steadiness of the image when the highest powers are being employed ought to be of primary consideration ; for those second-class instruments which are intended, at a less cost, to do as much of the work of the first class as they can be made to perform, portability here being of essential im- portance; and for those third-class instruments in which 12 PRACTICAL MICROSCOPY. everything has to be reduced to its simplest form, so as to permit the greatest reduction in their cost." FIG. 6. This summary of Dr. Carpenter's is so complete in its apology for separating microscope stands into three classes, THE POWELL AND LE ALAND STAND. 13 that we will not add a word more. The first-class stands are usually made from harder metal than the cheaper instruments, in which reduction of cost is an important item : this enables the former to wear better, the screws and racks allowing of a great deal of work before they begin to exhibit " backlash." The productions of Messrs. Ross & Co., Messrs. R. and J. Beck, and Messrs. Powell and Lealand deserve to be placed in the highest rank, as microscopes of the first water, and we cannot do better than describe them. The two different patterns made by Messrs. Ross, and shown in Figs. 5 and 6, are furnished with an extremely solid foot, which is cast in one piece. The "coarse adjust- ment " may be easily understood from the illustration, and the "fine adjustment" is obtained by the action of a milled head upon a lever which moves the nose-piece, in either case. Both instruments are furnished with a centering and traversing substage, and also with a rotating movement which is worked by a rack and pinion. The stage itself is very complete, the object slide when laid upon it can be instantly secured in position, and the whole stage with the object in situ can be rotated round the optical axis as a centre. The circular motion is graduated, and thus answers many useful purposes. A rectangular motion in two directions can be imparted to the stage by means of two milled heads. In the most recent forms of instrument the stage is made very thin (so far as is consistent with steadiness), and the central opening large, so as to admit light of great obliquity, for which purpose the mirror is placed at the end of a jointed arm, so that it may be considerably extended. The large best microscope of Messrs. Powell and Lealand is very heavy and massive ; Dr. Lionel Beale speaks of it in high terms of praise. The focussing movement is upon FIG. 7. BECKYS INTERNATIONAL STAND. 15 the Ross model, but the pattern itself is quite unique, and though of different construction to other forms, responds satisfactorily to all the requirements of the microscopist. The stage has one inch of motion in two directions, by means of the milled heads, and is capable of rotation round the optical centre by means of a wheel and pinion. This is combined with a very thin stage for oblique illumination of objects, either by means of the mirror or the achromatic prism. There is also a graduated silver circle, which can be used as a goniometer or for a variety of purposes. The substage has rotary, vertical, and rectangular motions, and both stages have for their foundation a solid brass ring, which is firmly attached to the stem. This stand is a specimen of excellent workmanship produced regardless of cost, in order to give those who require and can afford it perfection. With the best stand of Messrs. Beck we encounter a difficulty. It was our intention at the outset to describe those instruments furnished with swinging substages, under a special heading, but the best or " International " stand of Messrs. Beck being furnished with these additions, we feel constrained to mention it here. See Fig. 7. All the instruments turned out from the factory of these opticians are made upon the Jackson model (by whom indeed it was first adopted) with the single exception of their "popular" microscope, which is focussed on the Ross principle. The " International " has a heavy brass tripod for its base, upon which is placed a revolving gradu- ated fitting, from which rise two pillars, together forming the foot. Between these pillars is hung the limb which carries the body at its upper end. In its centre the com- pound stage is fixed, beneath which is a circular plate carrying the swinging bar and substage. The stage itself is attached to the limb on a pivot, and FIG. 8. STANDS OF MEDIUM PRICE. 17 can be set to any angle of inclination, the plate being divided so that the angle may be recorded, and in this way also, light of any degree of obliquity may be used without interference from the thickness of the stage. Horizontal movements by two milled heads are capable of being imparted to the stage, which may also be revolved by means of a milled head, or when this is pulled out, by using the two ivory studs fixed to the top plate for that purpose. For a description of the substage, we must refer our readers to a later portion of this chapter, wherein special forms of instruments are described, and in concluding a notice of these microscopes, urge every one who is wishful to possess a first-class instrument to see the work of each of these three firms before deciding. One cannot be too careful in the selection of an instrument, and we have nearly always found that if any one has rushed blindly into the purchase of a microscope on the recommendation- of a friend, sooner or later he has been dissatisfied with it, and has finally purchased another of his own choice. Other opticians produce as specialities good working stands at a medium price. Swift's " Challenge Binocular " is shown at Fig. 8, and has been in especial favour during the past few years ; it is a good working instrument, especially when combined with his universal substage condenser. The general construction of the stand will be seen on reference to the figure ; it is not too tall or large, and is exceedingly well balanced. Collins* " Harley Binocular," though formerly made upon the Ross model, is now constructed upon the Jackson model, and has this advantage, that commencing with a simple stand, a very complete instrument, such as shown by Fig. 9, can be built up from it. The stage is made to rotate and also to move by means of milled heads c FIG. 9. ROYAL MICROSCOPICAL SOCIETY'S SCREW. 19 in two rectangular directions, so that the remarks made in Dr. Carpenter's 'Microscope' do not now apply to it. Crouch's "Premier Binocular" is also one of the first- class instruments. An illustration of it is given in Fig. 10. Messrs. Pillischer, Browning, Murray and Heath, Baker, Watson, and others make medium priced stands of con- siderable excellence, but seeing that all the principles of construction are exhibited in the foregoing, it would be mere repetition to give illustrations of each. After all, a microscope stand must satisfy certain con- ditions, and if these be fulfilled it scarcely matters to the owner who the maker has been. It must be made of good hard brass, be furnished with a heavy foot, and well balanced, so that it may be placed in any position without overturning. All the rackwork and screws must move easily, firmly but not stiffly, and without " loss of time " or " backlash." Stops should be placed so that the body may be set either horizontally or vertically as required, and plane and concave mirrors should be always provided, preferably on a jointed arm. The lower extremity of the body should be furnished with the Society's screw, and no microscope should be purchased which has not this thread. A mechanical stage is not absolutely necessary but is a great convenience, despite all opinions to the contrary, and some accessories cannot be used without it. A circular and rectangular motion should be capable of being imparted to the stage without the intervention of rackwork ; but a duplicate motion by means of rack and pinion should be supplied to the better instruments. Reasons for the rota- tion of the stage will be found in the chapter treating on the use of the polariscope. C 2 20 PRACTICAL MICROSCOPY. The medium and best stands should be furnished with an adjusting substage fitting, capable of being moved verti- FlG. 10. BROWNINGS ROTATING MICROSCOPE. 21 cally by means of rack and pinion ; but the cheaper kinds are usually provided with a removable tube, into which the polariscope, condenser, and other accessories are made to fit. These should be capable of taking accessories made to the one and a half inch gauge, even if purchased from different makers. A cheap but well-made microscope of special pattern has been devised by Mr. Browning, in which the entire body and stage rotates upon the same axis, perfectly concen- tric even with the highest powers. It has a large circular glass stage, re- movable stage -fitting for apparatus, and is a very good form for laboratory or general use. It is made both with monocular and binocular bodies, the for- mer of which is shown in Fig. II. A very good form of portable microscope is made by Mr. Browning and shown in Figs. 12 and 13. The stage is fitted with rectangular and cir- cular motions, has an ad- justable substage with cen- tering movements, and is fitted with plane and concave mirrors on jointed arm. When fitted up for use it is as steady as a stand of the ordinary make, as the author can testify, and is shown in this position in Fig. 12. By a novel arrangement the body of the microscope turns on a joint and packs away with- FlG. II. 22 PRACTICAL MICROSCOPY. out detaching any portion of the instrument, as shown in Fig. 13. The stand also folds in a novel manner and packs into a case, the outside dimensions of which are 6x6x9 inches. FIG. 12. It may now be as well to notice those forms of micro- scope to which swinging substages have been adapted, or MR. GRUBB'S IMPROVEMENTS. those forms whereby the same effects may be obtained by other means. In 1854 Mr. Grubb, of Dublin, in a Pro- visional Specification for Improvements in Microscopes, stated, " My third improvement consists in the addition of a graduated sectorial arc to the microscope concentric to the plane of the object in situ, on which either the afore- said prism or other suitable illuminator is made to slide, thereby producing every kind of illumination required for microscopic examination, and also the means of registering FIG. 13. or applying any definite angle of illumination at pleasure." This seems to be the embryo of the modern swinging sub- stage, which was not, however, developed in this country until Zentmayer, in America, produced stands with these modifications in the regular course of business. As an instance of the pattern and the price at which these stands are sold in the United States may be men- tioned the Biological stand sold by Bulloch of Chicago. The microscope stands I2j inches high and the stage 24 PRACTICAL MICROSCOPY. is 3J inches from the table. The body is 5 inches long and the draw-tube also 5 inches in length, marked with a ring to show the standard length. The diameter of the body-tube is 1*4 inch. The stage has a revolving concentric movement, the mirror and substage can be made to swing over the tube, and there are spring stops to indicate when they are in a line with the axis of the instrument. The lower end of the main tube is fitted with the Butterfield broad-gauge screw, forming an adapter which carries the Society thread. The price of this instru- ment in walnut case is, with one eye-piece $40, equal to about SI. Ss. of our money, or with j-inch and |-inch objectives I2/. It is shown by Fig. 14. After the introduction of these swinging substages by Zentmayer the manufacture of them was taken up in this country by Messrs. Ross and Co., of New Bond Street, so that those who desire to be furnished with the latest im- provements or modifications of the instrument can easily be supplied. The methods of construction are very similar both in the American and English instruments, though the superior workmanship of details in Fig. 15, over the Biological stand of Bulloch, is only to be expected owing to the difference in price. This instrument is constructed on the Jackson model, and is particularly free from tremor. The coarse adjust- ment is effected by means of the ordinary rack and pinion, the fine movement being worked by the action of a micro- meter screw acting on a lever, by means of which the body is not touched when using the fine adjustment, and as the length is not changed, the relative distance of object-glass, binocular prism, and eye-piece remains the same. The substage bar which carries the mirror, condenser, and other substage illuminating apparatus, swings from a pivot placed behind the stage, the axis passing through the BULLOCKS BIOLOGICAL MICROSCOPE. 2$ object when in situ, and remains in the focus of illumination in every position of obliquity of the bar. The angle may FIG. 14. be read off from a graduated circle on a sector at the upper end of the bar. 26 PRACTICAL MICROSCOPY. For oblique illumination a very thin stage has been devised by Mr. Wenham ; it has only one top-plate, with rectangular and concentric rotary movements worked by two milled heads upon the same axis. This stage has two set-screws by means of which it can be easily and effec- tually centered. It is bevelled on the edge and graduated to serve as a goniometer. By releasing a strong clamp-nut this stage can be at once removed and any other sub- stituted. The swinging substage attached to the " International " microscope stand of Messrs. R. and J. Beck may be seen in Fig. 7. It is mounted perfectly true with the body, and is moved up and down in its fitting by means of milled heads. In this fitting all the varied appliances for modi- fying the character and direction of the light are fitted. The bar into which the substage fits is itself attached to an arc working in a circular fitting, and is capable of rotation by means of a milled head, so that it may be carried round and above the stage if necessary. The amount of angular movement may be recorded from the graduated circle. The lower triangular bar carries the mirror when the illumi- nation is required to be concentric with the optical axis of the instrument ; when desired it can be made to slide on the substage bar, and then can be moved above or below the stage in the same manner as the substage, Messrs. Swift and Son have produced what they term a " Radial Traversing Substage Condenser," in order to produce the same results in illumination as can be obtained with swing- ing substages. Mention only is made here, as its construc- tion and use will be fully explained when treating of accessories. Messrs. Watson and Sons, of Holborn, have recently patented the arrangement, an illustration of which is shown in Fig. 1 6. In this the mirror is accurately centered, while THE ROSS-ZENTMA YER STAND. the body of the instrument, together with the stage, is made to incline in order to arrive at the same results as attained with the foregoing accessories. It will be seen that the new stand enables the observer to FIG. 15. 28 PRACTICAL MICROSCOPY. examine an object as he would if it were held in the hand, and viewed by the naked eye that is, to turn it about in every possible way towards a ray of light proceeding in a fixed direction and so, without once losing sight either of the light or the object, to observe its appearance when illuminated by light of every degree of obliquity. This is the fundamental idea underlying its construction, and in this consists the great difference between it and the old forms of stand (although it has all the uses of the latter), where the object remaining fixed, the only way in which its illumination can be varied is by moving the illuminating ray which, in the amount of the results it affords, and the amount of time it consumes, is stated by the inventors to be in every way inferior to the new one. An inspection of the engraving (Fig. 16) will show how this idea is worked out. On the top of a strong pillar, to which it is attached by a massive cradle-joint allowing of inclination in a vertical plane, is fixed the arm carrying the body, which latter is provided with rack adjustment, and a new and improved fine adjustment, rendering unnecessary the usual often unsatisfactory loose nose-piece. The stage is so fixed with regard to the arm that the object when lying upon it is in a line with the centre of the cradle-joint, so that upon inclining the body the object moves with it, and is presented at every possible vertical angle to a ray proceeding to it from a given direction. The stage is of a new and improved construction, being exceedingly thin in fact the thinnest mechanical stage yet devised and is capable of giving a complete rotation of the object. Beneath the stage swings the substage arm, concentric with the object, and carrying the usual screw centering and rack adjusting substage. Behind the substage arm is a strong bar, provided with a dovetailed groove, into which the mirror bar slides. This WA TSOWS NEW MICROSCOPE. 2Q is so pivoted to the substage arm as to allow the latter to be swung aside and the mirror used alone when requisite, without the trouble of taking the substage away altogether. This is a great advantage, as it permits the substage, and any apparatus it may be carrying, to be swung into or out of position in a moment with the mirror in the position here indicated. The stand has all the uses of the old forms of microscope, and can be employed in exactly the same way, but even then its peculiar motions round the object as a centre give it very great advantages in every class of investigation. But it is when the mirror occupies the position now to be described that the peculiar properties of the new stand are brought fully into play. The upright pillar, carrying the body and stage, is attached at its foot to a massive circular plate, carrying a graduated circle which rotates round a point exactly beneath the centre of the stage ; and moving independently and concentrically with this is another smaller circle, having a dovetailed groove ploughed across it, into which the mirror bar can be slid when withdrawn from the sub- stage arm. A spring catch attached to the dovetailed circle falls into a notch in the mirror bar when the centre of the mirror is exactly beneath the centre of the stage. This is the most useful position for the mirror, as a ray fall- ing from a source of light upon it may be reflected upwards perpendicularly upon the object, when the body of the microscope is vertical, then without interfering again either with the mirror or lamp, or interposing any accessory apparatus whatever, but simply by inclining the body, the light falls upon the object with a gradually increasing obliquity until, when the instrument is nearly horizontal, a perfect dark-ground illumination can be obtained even with the highest powers, while the gradual way in which the light becomes more and more oblique immediately FIG. 16. MR. WENHAM^S BINOCULAR. 31 under the eye, and the capability of arresting the incli- nation at that point where the most suitable illumination for the object under examination is obtained, give to the observer powers he has seldom before had at his command in any form of microscope yet produced. The horizontal rotation mentioned above allows the object to be directed to the light at every angle in azimuth to borrow a term from the astronomer as the cradle-joint on the top of the pillar gives every angle in altitude ; the object occupying the centre of both motions, by a combina- tion of the two it can of course be placed in every possible position. These angles are read, the latter by a graduated circle in the outer side of the cradle-joint, giving the incli- nation of the body to the vertical, the former by means of the graduated circle at the foot; readings of these circles being taken with the mirror placed as above described at any time by so fixing the instrument that these circles read the same. Any desired effect will be exactly repro- duced, wherever the lamp may be placed a point of the greatest importance to workers with high powers. - There is a third divided circle on the substage axis, giving the inclination of the substage to the axis of the body. A strong clamp on the outer side of the cradle-joint holds the body firmly at any inclination, and a graduation on the slide of the coarse adjustment enables the working distance of the objectives to be measured and compared. Time alone can show whether these present apparent advantages are lasting, or whether the instrument will wear as well as those of the old form of construction. The foregoing illustrations of microscope stands are seen to be both monocular and binocular that is to say, used with one or two eye-pieces. A binocular should always be purchased when the student's means are sufficient, as most objects can be studied with much more ease and comfort PRACTICAL MICROSCOPY. than with a monocular microscope. Binocular instruments are made in such a manner that the rays proceeding from an object lying upon the stage are split into two parts, each portion passing separately through a tube to the eye-pieces, as shown in Fig. 17. The form of prism used for dividing the rays (we may say universally) is that devised by Mr. Wenham many years ago. It is mounted in a small brass box, sliding into the lower end of the microscope body immediately over the objective, as shown in Fig. 17. A section of this prism is shown in Fig. 1 8, the dotted line indi- cating the direction of one por- tion of the divided ray. It should not be forgotten that a binocular of this form is FIG. 17. FIG. 18. not well suited for use with objectives of a higher numerical aperture than o 34 or 40 air angle ; a certain amount of dis- tortion occurs with higher apertures, which may be readily perceived on the examination of spherical pollen-grains and such like objects. Dr. Carpenter has pointed out this ex- aggerated relief years ago, yet there are some folks now THE STEPHENSON BINOCULAR. 33 who delight in showing this imperfection as one of the beauties of the binocular microscope. It will thus be seen that objectives of higher amplifica- tion than the half-inch cannot be satisfactorily used with the binocular, and even the half-inch requires making of specially small aperture to adapt it to this use. The prism is always made removable, so that at any time the microscope can be used as a monocular by simply with- drawing it. In order to enable observers to use binocular vision with high powers, Messrs. Powell and Lealand have devised a system of prisms which may be used with objectives as high as the -$ The images produced are not stereoscopic, but exactly similar. Mr. Wenham has also devised prisms for using high powers binocularly, which appear to act in a very successful manner. Tolles, of Canastota, U.S., makes a binocular eye-piece for the ordinary single body, which gives a large and well-illu- minated field with low and medium powers. Professor H. L. Smith remarks that he has used the ^ and ^ objectives with it. Another binocular microscope was devised some years ago by Mr. J. W. Stephenson, which has since been success- fully made by Mr. Browning. Within the last few years the cost of this instrument has been considerably reduced, Messrs. Swift and Son and Mr. Baker having each con- structed a Stephenson binocular for dissecting purposes at the moderate price of //. The Stephenson binocular, as made by Mr. Browning, is shown at Fig. 19, in which the change from binocular to monocular or vice versd can be effected without unscrewing any part or interfering with the object under examina- tion. In this form of binocular the rays are divided by two D FIG. 19. THE "STEPHMNSON* PRISMS. 35 prisms A A, Fig. 20, which, passing upwards, are reflected towards the eye-pieces by the triangular prism B, Fig. 21. The instrument is erecting that is to say, objects are pre- sented to the eye in a normal manner, and not inverted as in the ordinary form of instrument, which adapts it specially for use in dissecting. The bodies of the micro- scope are made to rotate, carrying the prisms with them, so that two observers may work with the instrument, FIG. 20. FIG. 21. observe the same object, and compare notes without alter- ing the conditions under which the object is being examined. Amongst the foregoing medium and best stands, the reader must be fastidious indeed if he is not able to find one to suit his choice. Price might be a deterrent, and therefore some illustrations are given of several microscopes which, though cheap, are good working instruments. Fig. 22 is the Economic monocular microscope of Messrs. R. and J. Beck, which with coarse and fine adjust- ment, I -inch and J-inch objectives, two eye-pieces, con- cave mirror, side condensing lens, diaphragm, forceps, pliers, and glass slip with ledge, in mahogany case, costs 61. I2s. 6d. A small cheap stand such as this is nearly always required by the practical microscopist, even if he possesses a first-rate instrument; for travelling, Society D 2 FIG. 22. THIRD CLASS MICROSCOPES. 37 demonstrations, dissecting, and many other purposes a small microscope is exceedingly useful. Another cheap instrument, which is furnished by several makers, is shown in Fig. 23 ; it has a sliding body for coarse adjustment, a fine adjustment, draw - tube, wheel of diaphragms, tube fitting for substage appa- ratus, plane and concave mirrors, one eye-piece, i- inch and J-inch objectives, and stand condenser, in mahogany case, for which is charged the moderate price of 5/. 12s. 6d., to which can be added after- wards a polariscope for 25^., a camera lucida 6s., and a stage micrometer for 5^-. 6d. Equipped with one of these instruments, which are remarkably steady, being hung on the Jack- son model, the student is ready for an immense amount of work, and it is certainly much better to invest in an instrument such as this than to waste money on a cheaper instrument which has not the Society's screw. In the purchase of a low- price stand, we would give the student the following advice : Never purchase a cheap instrument made upon the Ross model ; never purchase one which will not take objectives with the Society's thread without an adapter ; never purchase an instrument without a fine adjustment, FIG. 23. PRACTICAL MICROSCOPY. without it be for petrological studies merely; and above all do not purchase the extremely trashy foreign separating objectives. If you want foreign glasses, there are good ones to be bought buy them. After all, there are a few objections to the form of instrument shown at Fig. 23. The body tube is narrow and the field lens of the eye- piece small in propor- tion, so that the field is limited in size ; then, again, the absence of a coarse adjustment may for some purposes be found inconvenient, while the fitting below the stage should be made removable. On the other hand, the short tube gives a larger field when used for photo- micrography, and is also more convenient as a dissecting microscope, lengthening as it does the anterior conjugate focus and giving more room for the needles. Recognising these and several other advantages, the author had one specially constructed, as shown in Fig. 24. The draw tube is wide enough to take the full-sized eye- pieces, and when it is fully extended the whole forms a body of the ordinary length. The stage, together with the body, may be rotated round the optical axis in the same Fro. 24. A WORKING MICROSCOPE. 39 manner as has been already shown in Fig. n. There are coarse and fine adjustments, a diaphragm in the thickness of the stage, plane and concave mirrors, the whole costing the author the moderate sum of three guineas and a half. With this form one is not bound to bad or medium eye- pieces and objectives ; they can be bought separately and selected. We should like to see some enterprising manu- facturer making the instrument as shown in Fig. 24, and selling it for six guineas with an A eye-piece and a good inch objective, or without eye-piece and objective for 3/. los. The details of this instrument are as follow: When standing vertically and closed down, the top of the eye- piece is ii inches from the table, the collar in which the body slides being 3 inches in length, and lined with velvet. The body is 5 inches in length, and the draw-tube 4J inches, with an outside diameter of 1*3 inches ; the stage is 3 7 inches from the table, and has a diameter of 3 inches ; the mirrors are 2*2 inches in diameter, and the fine ad- justment raises or depresses the entire body T -J-g- of an inch for each complete revolution. The stage is less than of an inch in thickness. The author has found this a very convenient instrument for photomicrography and for general microscopical work. The substage fitting will take all accessories made to the usual I J-inch gauge. It is greatly to be deplored that there exists no universal gauge for eye-pieces and substage fittings, is it too much to ask the Council of the Royal Microscopical Society to take the matter in hand ? CHAPTER III. EYE-PIECES AND OBJECTIVES. Eye-pieces or Oculars. When the student purchases a microscope stand, he will generally find it supplied with the lowest power Huyghenian or negative eye-piece, usually designated by the letter A. At the same time, it may be stated that others, possessing greater degrees of amplifica- tion are often substituted or added at the wish of the purchaser ; and it should be remembered, in the selection of a microscope stand, that the eye-pieces of one maker will not always fit the tubes made by another. It is a thousand pities opticians have not yet learned that their time may be more profitably occupied than by making adapters for each other's instruments. The Huyghenian eye-pieces or " oculars " of low power are generally styled "shallow," to distinguish them from those which give greater amplification, which are called " deep " the terms deep and shallow being applied to the degree of curvature possessed by the lenses employed in their construction, and not to the distance between them, as some writers have stated. A full-size section of the Huyghenian A eye-piece is shown in Fig. 25, so that the student may understand the details of its construction. It consists of two plano-convex lenses, placed at a dis- tance from each other equal to half the sum of their focal lengths, the best proportion of relative radii being i to 3. THE HUYGHENIAN EYE-PIECE. The lower lens is called the field-glass, and the upper one the eye-glass, while a circular stop or diaphragm is placed nearly midway between the two. The practical optician Gundlach has stated that the correction afforded by the Huyghenian eye-piece is not a complete one ; for at the point where the spherical aberration is entirely corrected, the chro- matic has not completely disap- peared. This even at the most favourable interval between the two lenses. This eye-piece was first em- ployed by Huyghens for his telescopes, in order to diminish spherical aberration and to in- crease the size of the field. An elaborate dissertation upon it has been published by Mr. Varley in the ' Transactions of the Society of Arts,' vol. li., to which the student is referred. It is often called the negative eye-piece, on account of its correcting the positive aberrations of the objective. In Fig. 26 is shown a section of a deep eye-piece the Huyghenian C in which it will be seen that the lenses possess deeper curvature than in the A, while the diaphragm is more contracted, and the aperture in the cap covering the eye-glass is very small indeed ; the C eye-piece gives about double the amplification of the A. There is another kind of ocular in occasional use, called Ramsden's positive eye-piece ; it is formed of two plano- convex lenses, but the curvature of the field-glass is turned towards the eye instead of towards the object, as in the FIG. 25. PRACTICAL MICROSCOPY. Huyghenian. In this eye-piece the focus is obtained in front of the field-glass, while in the Huyghenian ocular the image is formed at the diaphragm, about midway between the field-lens and eye-glass. The Ramsden eye-piece was much in use at one time for purposes of micrometry, as it gave an excellent view of the micrometer, free from distortion even to the edges of the field, though the image was slightly coloured. It is still used by Messrs. Ross and Co. for their eye-piece micro- meters. FIG. 26. FIG. 27. Kellner's orthoscopic eye-piece is much employed where a large and flat field is required for use with low powers. A section is shown in Fig. 27, from which the student may gather that the field-glass is doubly convex, and the eye- glass a slightly under-corrected achromatic combination, while the diaphragm is dispensed with altogether. In the ordinary or Huyghenian ocular, English opticians designate their power by means of letters, A, B, C, D, E, and F, while some few call their productions by the numbers i, 2, 3, 4, and 5 ; they seem fairly agreed as to what should be the relative degrees of amplification of the A and B eye-pieces, and some with the A, B, and C ; but with higher powers there seems no uniformity, as the following RELATIVE POWER OF OCULARS. 43 table will show. The numbers have been calculated from the catalogues of the different makers : Objectives. A B c D E F Ross Powell an Beck Swift Browning Watson Crouch Collins Baker Pillischer Parkes Zeiss Hartnack dLe alanc 1 o o 1 'O o o o o 1 o o o 1 o o 1 o 1 6 '5 2 4 3 4 o 2 6 6 6 2 I 2-0* 2'6 2'0 2*0 2'0 3'3 3 2'5 3 ' 3 3 . 3 2'2 i-8 3 i-5 3 4*o 4-0* I'l 3'7 2'5 4-0 ft 2'6 4 2 . 3 4 5' 6'o 5 5'2 4'3 3'4 3' Si 3 . s 8-0 5'6 4'3 3'5 6 Our American brethren treat their oculars in a more rational manner, they style them as "2-inch," " i-inch," or otherwise, as the case may be, according to the degree of amplification they yield when compared with single lenses ; thus a " 2-inch ocular " would amplify the same as a single lens of 2-inch focus, and so on in like proportion. Now reference to Chapter I. will show us that a single lens of 2-inch focus (equal to the English A eye-piece) magnifies about five diameters at a distance of I2j inches from the micrometer to the screen, and the i-inch, 10 diameters (equal to the C eye-piece), so that it is well the student should as early as possible grasp the fact that the amplification of an object is arrived at by two stages, the objective producing an enlarged image of the object, which the ocular magnifies still further. Roughly it may be stated that if the inch objective be used with the i-inch ocular an amplification of 100 diameters is arrived at, the objective magnifying the object 10 diameters, the image of which is further magnified 10 diameters by the ocular ; and further, if the same objective 44 PRACTICAL MICROSCOPY. be used with the 2-inch ocular (the A eye-piece), the former will produce an enlarged image of 10 diameters, which the ocular will again magnify five times, producing a total amplification of fifty. This is not mathematically exact without every disturb- ing element be taken into consideration, but is quite near enough to illustrate the case in practice. Fifty diameters is the recognised amplification for the i-inch objective combined with the A ocular at a distance of 10 inches, and as the enlargement of the object takes place in two distinct stages, it will be seen that the optician is able to vary the powers of both ocular and objective and still obtain the standard result. An inch (so-called) objective magnifying 8 3 diameters when used with an A eye-piece magnifying the image six diameters, will give a normal result, as will an objective of the same designation magnifying 12 '5 diameters with an A ocular magnifying four. Similar cases to both of these have recently fallen under the notice of the author, and it is on account of like depar- tures, that abnormal amplifications are obtained when the ob- jectives of one maker are used with the oculars of another. It often happens with new oculars that particles of brass get detached, and fall upon the inner surface of the lenses, which must be removed by unscrewing, and then carefully wiping with a very soft wash-leather. Dust specks and bubbles may be easily detected by deflecting a dull light through the body of the instrument, when, by observation during the rotation of the eye-piece, they show very plainly. A good eye-piece should be perfectly transparent, and free from striae and markings and spots of any kind. The marginal circle of the field of vision should be sharp, clear, and intensely black. If these conditions are not fulfilled, the eye-piece cannot be considered as perfect, or fit for general use. TULLE Y'S A CHROMA TICS. 45 Before proceeding to treat of objectives, the student may be advised to commence with the 2-inch ocular (an A). If he wishes for more than one power, the i-inch ocular (C) is recommended, it being double the power of the A. An orthoscopic C would perhaps be useful. Object-glasses or Objectives. The history of the achro- matic objective is a curious one interesting certainly, but it should teach us the serious lesson not to be dogmatic in our assertions. Biot and Wollaston, the latter especially, were wedded to doublets, and they both predicted, on the faith of certain experiments which were then unsuccessful, that the compound microscope would never excel the simple. How far this prediction has been verified most of our readers will know; but it is certain that Wollaston never thought that within fifty years of his prediction the doublet would be a thing of the past, rarely heard of and never seen. Light seems to have dawned upon objective construction through the elder Dollond, who employed two different kinds of glass in the construction of his telescopes. Recognising this principle, several foreign opticians made partly corrected glasses as early as 1824, and at the same time Tulley of London produced the first achromatic objec- tive made in England : it was composed of three lenses and possessed an air angle of 18, which he soon after increased to 38 by placing another corrected combination in front of it. In the year 1829, Mr. Joseph Jackson Lister, in his cele- brated paper published in the < Philosophical Transactions of the Royal Society,' pointed out how many of the diffi- culties could be overcome, and exhibited an objective of 50 air angle which gave a large field and a correct image. This advance was so great that it astonished Dr. Goring, who wrote, in his 'Exordium to Microscopic 46 PRACTICAL MICROSCOPY. Illustrations,' that " microscopes are now placed completely on a level with telescopes and, like them, must remain stationary in their construction." Improvements, however, continued to be effected ; Mr. Thomas Ross, upon increasing the air angle, discovered that different thicknesses of covering glass disturbed the corrections for spherical and chromatic aberrations no matter how carefully made, and in 1837 he presented a paper to the Society of Arts upon the subject. In this paper he stated having made an improved combination, the focal length being one-eighth of an inch, with an air angle of 60. After this he announced obtaining an air angle of 135, and falling into a similar error of dogmatism as Goring, Wollaston, and Biot, stated that "135 is the largest angular pencil that can be passed through a microscope object-glass." In 1851, Chas. A. Spencer, of Canastota, N.Y., pro- duced objectives of 146 air angle, and in 1857 he con- structed a one-twelfth with an angle of 178. Since this, Mr. Tolles, of Boston, has made lenses claiming to be infinitely near 180, and this angle in air has been approached by several of the best English makers. But these are not the whole of the improvements which have been effected ; air lenses or dry objectives have been supplemented by water-immersion powers, and finally we have the homogeneous-immersion system, in which the trans- mitted ray pursues a rectilinear course from the under side of the object slide until it leaves the posterior surface of the front lens. These immersion lenses will be fully described later on ; all we wish to state here in reference to them may be said in the words of Prof. Abbe : " A wide angle immer- sion glass may therefore utilise rays from an object in a denser medium which are entirely lost for the image which in fact do not exist when the same object is in air, or is CONSTRUCTION OF OBJECTIVES. 47 observed through a film of air ; " and again, " Consequently we have a loss of aperture when an air angle of 180 is substituted for a balsam-angle of ioo," for " an immersion objective of balsam angle exceeding twice the critical angle (41) has a greater aperture than any dry lens can ever have." It was upon this subject that Mr. Wenham fell into error and so may be classed with Ross, Goring, Wollaston, and Biot. He denied that Tolles had produced, or could ever produce an objective of greater balsam angle than was equivalent to infinitely near 180 measured in air ; and those who remember the correspondence on this subject which appeared in the pages of the ' Monthly Microscopical Journal/ will now smile when they see the announcements in the 'Journal of the Royal Microscopical Society ' of the productions of Messrs. Powell and Lealand of 1 50 balsam angle. That memorable correspondence should teach us to remember how easy it is to fall into error, and also that it is quite easy to persuade ourselves we are well acquainted with our subject, while at the same time we may totally misunderstand it. Objectives which, by-the-by, are sometimes called " powers " being made from glass of varying density and also of varying refractive indices, it follows that they must differ also in construction in some degree. The various lenses of which each combination is constructed are ground to a series of curves, suitable to the glass employed, and the combinations are placed at different distances apart, so that we can only give a rough outline of their general construction. As a rule, objective mounts are turned out much too long. There is no apparent reason why the brasswork (of some opticians especially) should not be consider- ably reduced. When the posterior lens is too far away from the Wenham prism, in a binocular instrument, it is 48 PRACTICAL MICROSCOPY. extremely difficult to procure an equal illumination of the whole field in fact, the general performance of the instru- ment is seriously interfered with, and therefore, for use with the binocular, short mounts should be preferred and the longer ones rejected. Mr. Swift has recently issued a series of objectives in short mounts, especially for use with the Wenham binocular, each of which allows of perfect illumination, without the aid of an achromatic condenser. They range from i inch upwards. Objectives are generally spoken of in terms of the ampli- fication which they yield, the standard of comparison being the magnification given by a single lens of the nominal focus. The student must not, therefore, imagine that an objective stated to be i-inch, -inch, or so on, will focus at these distances from the object. Opticians have never used the term in that sense, though a few writers in public journals seem to have understood the nomenclature in that light. The degree of amplification, taken with the aperture, is the surest guide to the focal distance ; for as the aperture increases, the less will the working distance be, a point to be considered in the selection of an objective for students' use. The lower powers are often made to separate. An objective giving the amplification of a 2-inch when com- plete, is converted into a 4-inch by removing the anterior combination. Separating 2-inch and i-inch, i-inch and J-inch, J-inch and i-inch, are also made, with others ; but the student is advised not to invest much in these separating powers. The best low-power objectives for general work will be found to possess the following air angles : 9 for the 4-inch, 15 for the 2-inch, while the i-inch seems to per- form most satisfactorily at 30. THE SELECTION OF OBJECTIVES. 49 No objective with an air angle of more than 40 should be used with the Wenham binocular. Dr. Carpenter pointed out long ago the exaggerated effect of projection produced when pollen-grains of the Malvacece and other similar objects are examined binocularly with high-angle objectives ; perfectly spherical objects, instead of resembling a hemisphere, appearing like the small end of an egg. Powers yielding amplifications ranging between 50 and 200 diameters may be called medium, and are represented in our list by the J-inch and J-inch objectives. The former is a very handy glass, though it does not seem to be an easy one to construct, judging from many the author has seen. Makers of cheap but really good i-inch and J-inch powers seem to fail sometimes in producing cheap and perfect J-inch objectives. A good working J-inch may have an aperture of 60, though it is made of 35 by Mr. Browning, and 40 by Messrs. Powell and Lealand and Mr. Collins, for special use with the binocular. The medium and higher powers should be chosen with conical fronts, as shown in Fig. 30, for with them it is more easy to illuminate opaque objects satisfactorily. The illustration is that of Mr. Swift's short mounted -inch objective, with collar adjustment. Zentmayer, Wray, and Zeiss have produced low power objectives in which the two combinations composing them are separated or brought nearer to each other by means of a screw collar, the lens being nominally a 4-inch, a 2-inch, or any intermediate power at will. This glass defines well, and, moreover, possesses a flat field ; but, according to measurement, the amplification it yields more nearly approaches a 5-inch and 3-inch than an English 4-inch and 2-inch. For several years cheap foreign lenses -were in great E 50 PRACTICAL MICROSCOPY. request, and much work has been done with them. This has induced several English makers to offer a series of low- angle objectives at a cheap rate, in addition to the really good glasses which the same opticians turn out. Browning, Wray, Watson, Swift, Dancer, Collins, and Parkes make very excellent and cheap lenses for histological work. It sometimes happens that very good powers may be met with second-hand, or from the workshops of neglected makers ; but the selection of a good glass from several of average quality, though it can be done by an optician, or a microscopist who has mastered the elements of science, is not a very easy task, and one which certainly cannot be performed by a novice on his first purchase of an instru- ment. Be very careful in purchasing objectives. Don't buy trash, or any objective simply because it is cheap. Wait until you can meet with a really good glass at the price you are disposed to give, or be satisfied with the powers you already possess. For botanical and general work, especially if the student's means are limited, the inch and J-inch objectives will be found sufficient ; the 2-inch, and afterwards the J-inch, may be added, if required ; following, if necessary, with the ^-inch and ^-inch. A 4-inch power is used by some for the examination of wood sections, whole insects, etc. A cabinet will generally contain the following objectives by the time the owner considers it furnished : 4, 2, i, J, , ! -TO which yield degrees of amplification varying from 12 to 800 diameters with the A eye-piece, and 24 to 1600 with the C. The low powers are constructed in several ways, ac- cording to the aperture desired. Those of greater ampli- fication than the i-inch are made either as triplets or of two pairs of lenses placed at certain distances apart, as all THE ONE-INCH OBJECTIVE. IS the corrections required are easily made on this system. The triplet, used only for low angles, is the least to be commended, and should not be used with deep eye-pieces. The i -inch objective may be a triplet, as shown in Fig. 28, or a double combi- nation, as in Fig. 29. In the latter the front is a plano-con- vex of crown, with a meniscus of flint, being separated by a considerable inter- val from the posterior combination, which is composed of two double convex crown lenses, holding between them a double concave of flint. It will be seen from Figs. 28 and 29 FIG. 28. that there is more work in the construction of the higher angle. The cost is consequently greater ; but when we remember that the i-inch of 25 admits more light than the one of 1 6, that it defines better, resolves better, and proves to be a much superior working glass in every respect, the extra money will not perhaps be grudged for it. Half-inch objectives are made on two systems : the low angles for binocular use of a thick solid front, at the back of which are two pairs of partly cor- rected lenses, the aberrations being finally corrected by the thickness of the front. The higher angles are constructed of three pairs of lenses, the posterior combination being of considerable width. They are made of as high an aperture as o'66 or 82 air angle, while the A-ths of 100 air angle, or numerical aperture 0-76, is far from uncommon. E 2 FIG. 29. 52 PRACTICAL MICROSCOPY. The J-inch objective, yielding a power of 200 diameters with the A eye-piece, is a most useful glass to the student, when possessing an air angle of about 85. It is con- structed of a triple back lens, a double middle, and a single front. Messrs. Ross's system is believed to differ from this, inas- much as their objectives, from the J-inch upwards, are supposed to be constructed on Mr. Wenham's new formula, in which the "" flint concave of a triple middle is made to FIG. 30. correct the aberrations of the anterior and posterior crown lenses. An enlarged section of this formula is shown in Fig. 31. Quarter-inch objectives may be obtained of as high an air angle as 140, very good for the experienced micro- scopist but of limited use to the student, as such glasses focus very close to the object. All objectives possessing a numerical aperture of more than / "v. 0*42 (50) should be furnished with | a screw collar to adjust the lenses ' for varying thicknesses of covering FIG. 31. , , . . , , glass, and this is specially needed on Mr. Wenham's new formula. It is often a very tedious operation to find the exact adjustment requisite ; but it may be approximately performed in the following manner : Set the collar at zero, and focus the glass upon the object ; next turn the collar until the dust upon the cover- glass is in focus, when an approximate correction will have been applied ; if the observer again focusses by means of the fine adjustment, the object will be found more sharply defined than before. This may only be considered a rough method. The best way is to work at some well-known AMPLIFYING POWER. 53 object, under varying positions of the screw collar, such as Gyrosignia angulatum or the Podura scale, until the best definition has been arrived at. Under the denomination of high powers may be classed all those yielding greater amplification than the J-inch. They have become very common during the last few years, nearly every maker of objectives issuing an " eighth," and an immersion ^ or ^. It appears probable that most of the high powers now made are constructed upon a formula similar to that last described by Mr. Wenham, and illustrated in .Fig. 31 (though this is denied by some makers), as he states most distinctly that the formula is successful with all powers requiring adjustment for cover-glasses, " from the J-inch upwards." Eighths and higher powers used to be specialities of the first-class makers for which very high prices were charged ; but now, for a student's general work, very good eighths may be purchased for 50^., of an aperture of 0-82 (110) and good working distance. As to magnifying power, the annexed list has been ex- tracted from the catalogue of Messrs. Powell and Lealand, and will serve to show what degree of amplification is yielded by the various objectives, in combination with different oculars, at the recognised distance of 10 inches. Objectives. A. B. c. D. E. inch. 4 12 18 25 50 75 2 25 37 50 IOO 150 I 50 74 100 200 300 100 148 200 400 600 ZOO 296 400 800 1,200 400 592 800 1, 600 2,4OO 800 1,184 1, 600 3,200 4,800 ^ 1,250 1,850 2,500 5,000 7,500 *v 2,500 3,7oo 5,ooo 10,000 15,000 54 PRACTICAL MICROSCOPY. It may be useful also to add a list of the magnifying power of the oculars and objectives of Hartnack (now Prazmowski) and of Zeiss. They are used in this country to some extent, and the users always have the foible of omitting the number of diameters to which the object has been magnified, adding, however, the numbers of both ocular and objective, to which the student generally has no reference. It should not be forgotten that the amplifying powers of the foreign objectives are calculated for a tube of 153 millimetres (6 inches) in length, and most of these objectives lose in their performance if employed in the ordinary English length of tube. Prazmowski (Hartnack) gives the following values for his oculars and objectives : Objec- tives. Oculars. Equivalent Focus in Inches. 1 2 3 4 5 6 I 15 20 25 . , 2 2 3 4 25 g 70 45 80 90 120 140 I I I 100 150 "5 1 80 160 240 2 4 350 f I 200 250 240 300 300 400 450 600 600 800 700 1000 I 9 350 400 550 860 IIOO 1400 A IMMERSION OBJECTIVES WITH CORRECTION. 9 410 480 630 95 1300 ISOO A 10 520 600 750 IIOO 1500 I800 A 13 820 95 1170 1700 2370 3100 A 15 1040 1200 1500 2200 3000 3600 A 18 1560 I800 2250 3300 4500 5400 A Zeiss, whose objectives have been much in request during the past year or more, is noted for great working distance ; his angles are low and he designates his objectives by FOREIGN OBJECTIVES. 55 letters, but gives also the equivalent focal lengths in inches and millimetres ; they are as follow : O 1 Mark. Equivalent Focal Length. Angular Aperture. 1 2 3 4 5 inch. mm* o ' aa I 25 24 20 27 36 52 70 i A AA | 15 15 24 \ 36 / 4 55 75 105 140 ~o B BB | 10 10 40 \ 60 / 70 100 135 180 240 1 Beck. For these dark wells the author often uses small circles of black paper mounted on the ordinary 3-inch by i-inch slips, and for most purposes they answer admirably. We now come to several very simple but important additions to every microscope, the first of which is generally supplied with all instruments. Fig. 66 delineates the stage forceps, shown holding a fly for rough examination under FIG. 66. low powers. They are useful chiefly to beginners, with such objectives as the 3-inch or 2-inch, but are altogether unsuitable for high powers. In the investigation of minerals it is often necessary to examine small angular pieces which require to be viewed on G 82 PRACTICAL MICROSCOPY. all sides. In order that this may be done easily, Messrs. Beck make what is called a stage mineral-holder (Fig. 67), one of the jaws being movable in a right line, so that it may clamp any sized specimen, and by turning the milled FIG. 67. head of the jaw the mineral is made to revolve. Fig. 67 will perhaps show more clearly the action of this holder than any description can do. FIG. 68. Morris's rotating stage (Fig. 68) often serves the purpose of stage forceps. In its improved form it can be used for both opaque and transparent objects. Small flies, larvae, beetles, &c., can be affixed to the cork by means of a small pin, or with gum, and as the stage moves upon a secondary plate by means of a ball-and-socket joint, the object can be placed in a variety of positions hardly possible by any other means except the disc-holder of Messrs. Beck. Beck's disc-holder, shown at Fig. 69, is for the purpose of holding for examination under the microscope the small discs upon which objects have been temporarily or per- DISC HOLDER ROTIFER-TRAP. 83 manently mounted. The object is attached to the disc by the aid of gum, or any other suitable adhesive material, and when placed in the holder for observation, the disc can be rotated in both vertical and horizon- tal directions. These discs are very useful for many objects, especially those not needing a cover, and Messrs. Beck supply ^ * FIG. 69. boxes into which they are fitted when not in use, so that they may be kept excluded from the dust. With many objects, in order to examine them minutely, all that is necessary is to place them upon a glass slip, add a drop of the right medium, and cover with a thin covering glass. This method of working is all very well for organisms which are still, or comparatively so ; but when we come to examine Rotifers, the Entomostraca, and other active forms of life, we find means are required to keep them in one position. Suppose, for instance, we wish to examine an Entomostracon, which turns out to be Bosmina longirostris : during its rapid motions through the water we would be apt to come to false conclusions concerning it, but if we prevent the organism from moving it can then be studied in all its details. The simplest and least expensive way of examining the Infusoria and other moving micro-organisms is by the use of the Rotifer-trap of Mr. F. Bedwell. This consists of a few filaments of cotton wool placed upon the under glass of a " live-box." The organism contained in a drop of water is then run over it, and eventually becoming entangled G 2 8 4 PRACTICAL MICROSCOPY. amongst the fibres, is kept comparatively still, by which its form can be clearly made out. Two forms of cages or live-boxes are shown in Figs. 70 and 71. They are, however, not all that can be desired. Just so much pressure must be applied to the cap as is necessary to keep the animal still, and no more, or it will be crushed and distorted, so that the cap requires much dextrous manipulation. Often- times such objects as Entomostraca, &c., will FIG. 70. not display themselves to adVantage, so that the cap requires loosening, to be again squeezed down at an opportune moment. Moreover, it is generally found that some particular organism will get near the periphery of the cover, and in this all the interest may be concentrated, yet the objective will not reach it if it happens to be of the form of Fig. 71, and if we adopt the form of Fig. 70 we are often precluded from using the achromatic condenser to any portion but the centre of the slide. Instead of this we may use the compressorium as made by Messrs. Beck, Ross, Collins, and others, for use with high powers ; but Piper's form, made by Mr. Swift, and illustrated in Fig. 72, will be found more convenient. The most delicate pressure can be applied by means of 1 FIG. 71. COMPRESSORIUM ZOOPHYTE TROUGHS. these instruments, and all such intended for real use should be reversible, so that the objects maybe easily viewed from both sides, and this can be done with the form shown in Fig. 72. FIG. 72. A form of compressor differing from all others has been devised by Mr. Holman, U.S.A. ; the top or mica cover is fixed while the lower thicker plate of glass is raised or depressed by means of a screw-nut and spiral spring. The employment of a thin mica cover is certainly an im- provement, and one which English opticians would do well to follow. In this we imagine every practical microscopist will concur, as the breakage of a glass cover in the middle of an interesting observation is, to say the least, vexing. Zoophyte troughs may be easily constructed by the student ; the form shown in Fig. 73 is made FIG. 73. of a thick glass base plate and ends, while two pieces of thinner material furnish the front and back, which should be of the same thickness as the thinnest slips are cut from. This trough should measure T ^ of an inch in width, 86 PRACTICAL MICROSCOPY. 2 inches in length, I J deep at the front, and 2 inches at the back ; the base plate and ends being made from glass T % of an inch in thickness and cemented together with marine-glue. For higher powers, the form of trough shown in Fig. 74 is desirable. It consists of a glass slide, 3 x ij inches, upon which is cemented with marine glue an ebonite or glass semi-rectangular piece as shown in the figure ; the half of a flat indiarubber ring will answer admirably, the front being formed of a piece of thin glass. In use, nearly fill the trough with water, place it on the FlG stage of the microscope, and incline the body of the instru- ment so that the observations may be made with comfort ; adjust the lamp and concave mirror so that the most intense and central light is thrown through the instrument; and lastly, adjust the diaphragm until most of the marginal rays of light are cut off, there remaining only just sufficient light to work with. In all cases apply a larger aperture only when absolutely necessary. It may be here stated that the moderate use of the micro- scope, either monocular or binocular, when employed in the above manner, will not injure the eyesight of a healthy person. When, however, an excessive glare of light is con- stantly employed, the eye becomes less sensitive to ordinary light; excess of illumination is a common fault with beginners. The several forms of zoophyte troughs such as those shown in Figs. 73 and 74 may be obtained from any maker or dealer in microscopic apparatus. Fig. 73 is a wide trough, but it may be narrowed by means of the wedge and spring, which drives a thin glass BOTTERILLS TROUGH AND LIFE-SLIDE. %7 partition close to the front plate; Fig. 74 is not adjustable, but is easily made by the student. Another adjustable form of trough is Botterill's, which consists of two brass or ebonite plates bolted together, as shown in Fig. 75, the plates of glass being separated, according to the space required, by an ordinary indiarubber ring of the requisite thickness. The trough can thus be taken apart and the glasses cleaned, or a broken front replaced without the trouble of cementing, the glass sides being sufficiently thin to allow the use of high power objectives. FIG. 75. A microscopic life-slide also devised by Mr. Botterill is shown at Fig. 76 ; the advantages claimed for it are, the facility with which it can be used and cleaned ; its reversi- bility, allowing either side of the object to be examined FIG. 76. through thin glass ; the provision for renewing the supply of water without disturbing any part of the apparatus, thus enabling objects to be kept under examination for an indefinite period ; the same arrangement also allowing of 88 PRACTICAL MICROSCOPY. the introduction of colouring matters, as carmine and indigo ; and lastly, its moderate cost and durability. For Confervae, small Infusoria, and similar organisms it is sufficient to place the object on the bottom glass, with a drop of water, and apply the covering glass in same manner as when using a glass stage-plate. When a thicker layer of water is required, a narrow ring of vulcanite, cork, or other suitable material, of the requisite thickness, should be placed on the lower glass, and the object put in position, the cover- ing glass being finally applied as in mounting objects in a cell. The supply of water can be maintained by putting a drop occasionally in one of the side " wells," keeping the slide, when not under examination, in a small damp cham- ber, to prevent evaporation. To change the water, supply through one "well," and draw out through the other by means of a roll of blotting paper. Messrs. Thompson and Capper, of Liverpool, were the original makers of this slide, and also of BotterilFs zoophyte trough, illustrated at Fig. 75. The ordinary slip with a ledge of glass cemented to its lower edge, as shown in Fig. 77, is very useful, and saves the stage of the microscope from corrosion when marine . , . organisms are being ex- amined ; it may easily be constructed by the student. The glass for the construction of these troughs and slides may easily be cut with a glazier's diamond, and the edges ground parallel upon the flat face of an ordinary grindstone kept well wetted with water, or even by rubbing them upon a flat well-moistened piece of Yorkshire flagstone. An emery wheel will also answer the purpose. Pieces of superfluous glass may be removed by the use of a fine cut file lubricated with oil of turpentine, and holes made with STEPHENSON'S SAFETY STAGE. 8 9 an ordinary drill lubricated with the same material. Glass may be turned to any shape in the lathe by using a smooth cut file, kept moistened with oil of turpentine, as the turning tool ; but in order that this operation may be successful it is necessary that the piece for turning be not of too large diameter. FIG. 78. In working with high powers and expensive slides there is often a risk of either one or the other getting damaged, and this is especially the case with immersion objectives, where the kindly help of the dust on the cover-glass is not obtainable. It is never advisable to take high powers or rare slides to conversa- zione or other public meetings, on account of the miscellaneous character of observers ; but if such a proceeding is imperative, the exhibitor should certainly provide himself with one of Stephenson's new safety stages, shown in Figs. 78 and 79, which may be the means of saving him a few regrets. It is so constructed that if by chance the object-glass is racked down on the thin cover, FIG. 79. 90 PRACTICAL MICROSCOPY. no damage is done, on account of the object receding as soon as contact is made, the springs shown in the figure making that motion possible. The object is placed on the two short arms C C, and is held in its place by the spring, which is placed above and between them. In order to make safety doubly sure Mr. Stephenson has devised a second piece of apparatus to act with the former; it is shown in Fig. 79, and consists of a square rod of brass which FIG. 80. must be adjusted to suit the various objectives used ; it is held in its place by a pin passing through it, attached to a screw at the outer side of the socket in which the rod slides. This instrument is placed (in the Ross model) beneath the bar which carries the microscope body, and, when properly adjusted, allows the objective to touch the object upon the stage, but arrests all further progress, no matter with what degree of force the coarse adjustment MICROSCOPE LAMPS. may be pressed, a property of considerable value to public exhibitors. Another very useful accessory is the revolving table, several forms of which are now sold at a very cheap rate. At one time the cheapest which could be obtained was about 9/., and now they may be procured (with a slate top) for less than one-fourth of that sum. When two or more microscopists are pursuing any investigation together, the constant rising from chairs must often have been thought a nuisance, but a cheap revolving table enables mutual observations to be made with comfort. The author's revolving table is 2 feet 4 inches in diameter, the top of it is 2 feet 3 inches from the ground, and four or even six observers may com- fortably sit round it. And now a few words as to illuminating ap- paratus. The best light which can be obtained is that from a good white cloud on a sunny day, but unfortunately in our towns and crowded cities we get but little sun- light undiluted with smoke, and students generally are occupied the day through, so that it becomes necessary to use artificial light. When using light from the sky or from the sun, it should FIG. 81. 9 2 PRACTICAL MICROSCOPY. be remembered that the rays are, for all practical purposes, parallel, and thereby differ essentially from artificial light, the rays of which converge strongly from the luminous centre. For use at home there is nothing, perhaps, so convenient as an argand gas reading lamp, sliding up and down on a metal rod, with a shade over it to prevent extraneous light from reaching the eyes. Students are very apt to work with too much light, and thereby impair the sensitiveness of their eyes ; they should endeavour, however, to work with only just as much light as is necessary to bring out plainly the details of the object under examination, and no more. If an oil lamp is desired, a very com- mon one maybe made to answer almost every ordinary pur- pose, provided it is low enough, as when it is required to be raised, that may be readily accomplished by means of blocks of wood of varying thickness. The ordinary form of microscope lamp is shown in Fig. 8 1 ; it differs slightly in con- struction in the hands of different makers, but the student should eschew all forms in which the oil reservoir case is FIG. 82. MR. SWIFTS MICROSCOPE LAMP. 93 soldered to the sliding ring ; all the author has seen from different makers have come to pieces in a very short time. Browning's oil lamp, with bull's-eye condenser and silvered reflector, is shown at Fig. 82. It has a porcelain shade covering the chimney to protect the eyes from the excessive glare. This lamp (and in- deed most oil lamps) is used with paraffin oil, and the brilliancy of the light may be increased by dissolving a little camphor in it. The light is more intense when the edge of the flame is turned towards the object to be illuminated ; but if quantity of light is required rather than intensity, the flat side of the flame may be so disposed. A rather better lamp than the above for general work is that of Mr. Swift, and shown in Fig. 83, which, however, in the writer's estimation, would be much improved by a glazed porcelain chimney with two opposite perfora- tions, for in most examina- tions (perhaps all) it is a very important point to avoid a flood of extraneous light passing to the eyes. There are several other lamps which may be mentioned here : Collins's Bockett lamp and Fiddian's lamp, made by Messrs. Ross and Co. The Fiddian lamp is supported by a massive claw stand, from which rises a vertical support on a FIG. 83. 94 PRACTICAL MICROSCOPY. ball-and-socket joint. A brass tube slides on the vertical rod bearing the condenser and lamp with neutral tint shade and " white cloud " reflector having telescope and clamping screw adjustments. When these are placed in any desired relation to each other, the whole can be vertically adjusted by a rack and pinion with the greatest accuracy. Its price is five guineas. Another beautiful lamp has been made by Messrs. Wood, of Liverpool, for Messrs. Dallin- ger and Drysdale. These two observers, whilst work- ing at the life-history of the monads, appreciated the diffi- culty of accurately centering the image of the flame when working with the Jy and ^ objectives, and so devised this lamp, which is illustrated in the April number of the 'Monthly Microscopical Journal' for 1876, vol. xv. Parkes' microscope lamp with cooling evaporator may be seen in Figs. 84 and 85. C is a bronzed copper cylin- FIG. 84. , . . , , , ' dncal shade 3^ inches m diameter with a hood at the front to prevent the upward reflection of light. At the back is a parabolic reflector transmitting nearly parallel rays, made removable for the purpose of cleaning. At the front is a tinted " light-modi- fier," secured by a bayonet joint, and may be also removed when desirable. D is the " cooling evaporator " ; a layer of thick felt is placed inside for saturation. When the lamp is PARKED MICROSCOPE LAMP. 95 lighted, this vessel is filled with water, and so prevents the radiation of heat upon the observer's head. The felt requires moistening about once every five hours. The light of the sun, a white cloud, or the electric light, which the author has used, and will illustrate in a future chapter, each give a light of remarkable purity. This is not the case, however, with the light from gas or from oil lamps. These last, especially gas sources of illumination, give a very objectionable yellow tone, while some tints are nearly suppressed. This effect has been noticed by all observers, and in 1872 Mr. Collins produced a light-corrector, and exhibited the same at a soire'e at the Quekett Club. It consists of a brass stage-plate with a groove in which rotates a diaphragm of 4 apertures one open, one fitted with a finely ground glass, while the others are fitted with two different tints of blue. Rainey produced a light-modifier before this, but it was of such construction that it required fitting to each microscope; that of Mr. Collins, on the other hand, can be used with any instrument, and without fitting. The effect of the blue glass is to effectually correct the yellowness proceeding, from all artificial illumination, rendering the light soft and agreeable, as well as to im- prove the definition. To produce this effect, the writer uses a simple 3 in. x i in. slide of blue glass, such as is used by the chemist for the qualitative analysis of potash salts. It was obtained from Messrs. J. J. Griffin and Sons, Garrick Street, Covent Garden. FIG. 85. 96 PRACTICAL MICROSCOPY. Dr. Woodward prefers to use the ammonia-sulphate of copper cell, and then only for high power definition, and he says he has been able to resolve the markings on Amphi- pleura pellucida with objectives found incapable of doing it with white light. Professor Smith, of Ashtabula, also expresses his approval of the use of monochromatic light. He says that with its use, and an eighth dry objective, he has easily resolved the A. pellucida to beads, in balsam, with deep eye-pieces ; and with the lowest eye-piece the transverse and longitudinal striae were easily seen. The white cloud illuminator is a contrivance made in order to produce the same kind of illumination from artificial light as is obtainable from a white cloud. It is generally used with low powers only, and is made in several ways a concave surface of plaster of Paris, a mirror coated at the back with zinc-white paint, roughened enamel, and white paper have all been used to produce this effect, as well as the disc of ground glass found in Mr. Collins's light-modifier. Thus closes the chapter on accessories; but the student must not think we have exhausted the subject : there are many pieces of apparatus in occasional use which it has not been thought necessary to include here, and many others will be described in the subsequent chapters under the headings in which they are more intimately concerned. CHAPTER V. GENERAL REMARKS UPON OBJECTIVES TEST OBJECTS. WHEN we consider the many adjustments of apparatus needed ere a correct picture of an object can be placed before the eye, it will be readily seen how necessary it is to pay strict attention to details more especially of illumina- tion, this being one of the first and most important lessons the microscopist has to learn. When rays of light pass through media with parallel faces, such as the glass slips used by every worker with the microscope, the emerging rays are parallel with those enter- ing, the intermediate portion being bent away from both these planes as shown in Fig. 86. If water be used above the glass, the emergent ray will be bent up more towards the perpendicular, while when cedar-wood oil, or any of the homogeneous-immersion fluids are employed, the path of the ray will be one continuous line from the under side of the glass slip. A diagram of the passage of a light-ray through glass is shown in Fig. 86. Ordinary glass slides, and also the thin covers, are made from FIG. 86. crown glass having a refractive index varying from 1-5 to 1*525 referred to air as unity: the following table of mean refractive indices of many sub- stances used by the microscopist may not be uninter- esting : H PRACTICAL MICROSCOPY. REFRACTIVE INDICES. Air ................ I'ooo Water ................ -336 Sea water .............. -343 Alcohol .............. -373 Glacial acetic acid .......... '380 Equal parts, glycerine and water . . . . ' 400 Glycerine .............. '475 Oil of turpentine ............ 478 Crown glass ............ I 5 to 525 Homogeneous-immersion fluid ...... 500 Chloride of cadmium in glycerine . . . . ' 500 Cedar- wood oil ............ '512 Canada balsam ............ -532 Flint glass .............. -575 Monobromide of naphthaline ...... '658 Bisulphide of carbon .......... '678 Oil of anise .............. i'8n Sulphur .............. 2*115 Phosphorus .............. 2-224 Plane or flat mirrors reflect an image of the same size as the object, the flame of a gas or oil lamp for instance, and therefore the rays are parallel, and the image is not inverted. When light is reflected from glass, the under side of which is silvered, much of it is lost from several causes ; but when polished metal is employed for the reflecting surface, the rays do not enter the substance of the reflector, and there is less loss of light than in the former instance. We must now turn our attention to the concave mirror, with which all respectable microscope stands are furnished. In this kind, the focus is situated at a point at which the reflected rays meet, and when rays parallel to the axis are brought together after reflection, the meeting point or focus is at an equal distance between the centre of curvature C and the mirror itself, and, consequently, if a luminous object be placed in this principal focus F, the rays emitted by the whole surface of the mirror will be parallel, as seen in Fig. 87. If, however, the luminous point be at a greater distance CONCA VE MIRRORS. 99 from the mirror than the principal focus, or vice versd if the luminous rays fall divergent upon the concave mirror, a focus is obtained at another point called the conjugate focus. In the first case the rays, instead of being parallel, will converge towards L (Fig. 88), while in the second the focal FIG. 87. point of light will be removed further away from the mirror, and the rays proceeding from a lamp may be brought to a focus until the distance between the source of illumination and the mirror has been lessened to the centre of curvature, the rays being then reflected on to themselves. FIG. 88. FIG. 89. If the source of light be placed between the principal focus and the mirror the reflected rays will be divergent, as shown in Fig. 89. Let us now consider the action of lenses upon illuminating rays. In a double convex lens the refracted rays from a parallel pencil of light form a focus very near to the centre of curvature of the lens, and conversely when a lamp is placed in its principal focus a double convex lens may be made to appear the source of light, as shown in Fig. 90. H 2 100 PRACTICAL MICROSCOPY. It will be seen that a double convex lens has a principal focus on either side of it, and therefore the light may be parallelised on either side, but if the source of illumination be placed further away than the principal focus, the rays FIG. 90, will be no longer parallel, but centred in a point at some distance from the opposite side of the lens, as shown in Fig. 91. FIG, 91. These points (/ and L) are called conjugate foci, and do not lie in any fixed plane, but are dependent the one upon the other; it is this movement of the conjugate foci which yields a long working distance from the objective when the body of the microscope is shortened, and requires the object-glass to be approached nearer to the object when the draw-tube is used. Diverging rays can be produced by placing the illumi- nating point between the principal focus and the lens, and THE PLANO-CONVEX LENS. 101 when converging rays fall upon a double-convex lens they are brought to a focus at a point between the principal focus and the lens itself, as shown in Fig. 92. The action of a plano-convex lens, of which our bull's-eye condenser, Fig. 36, is a type, may be studied in the same diagrammatic man- ner. This may be considered as a double-convex lens split down the centre, and so forming two plano-convex lenses ; it is generally used for purposes of microscopy on account of the great work- ing distance of its focus. As already described, parallel rays falling upon a double-convex lens come to a focus very near the centre (radius) of its curvature, but when the same rays fall upon the curved surface of a bull's-eye condenser they are brought to a focus at a distance equal to the diameter of the curvature, or twice the distance of a double-convex lens, as may be seen in Fig. 93 ; and con- versely if we wish to produce rays of parallel light from FIG. 92. FIG. 93. a lamp, the ordinary bull's-eye condenser must be placed twice as far from the luminous point as would be neces- sary in the case of a double-convex lens, a condition of 102 PRACTICAL MICROSCOPY. extreme importance when we consider the heat given out during combustion in most oil lamps. These remarks upon the behaviour of certain lenses and mirrors towards the rays of light may be considered super- fluous, nevertheless, as the student proceeds, he will find that not one word too much has been written. False appearances are often produced by the bad employment of light, and the student is advised to practise many kinds of illumination upon objects with which he may be familiar, so as to acquaint himself with the various appearances which diverse applications of it will afford. When the action of the various mirrors and lenses has been fairly grasped, the student should proceed with some work capable of giving him experience of the manner in which various objects are delineated or depicted under various objectives. In the days of Dr. Goring (1832) when objectives even of high amplification had not surpassed an air angle of 55, and even when achromatics were despised by nearly all working microscopists, it would not have been a difficult task to test an objective for its spherical and chromatic aberrations, by means of the tests we now possess ; but time has changed all things microscopical ; really bad lenses are rarities, and taking objectives to, and including the -inch, it is remarkable how few imperfections they possess. In order to correct perfectly the aberrations of objectives, the practical optician employs the globule of mercury or "artificial star" as a test object, while the accuracy of their setting is examined by studying the reflected image of a flame or the window bars, while the mount with lenses in situ is revolving in the lathe. Beyond mention of it, the " artificial star " test need not be described here; it has been fully treated upon by Dr. Goring in the 'Microscopic Cabinet/ to which the WORKING DISTANCE. 103 reader is referred, while the introduction of a good series of test objects renders the general employment of the former scarcely necessary. It must not be forgotten that great differences exist between test objects. Amongst the slides of Pleurosigma angulatum, sold for this purpose, some are so extremely coarse as not to be a test in any sense of the word, while others are so finely marked that they can only be resolved with the greatest difficulty under a J-inch objective of high air angle. This is also the case with the diatoms Navicula rhomboides and A mphipleura pellucida, used as tests for the highest powers ; so that no reliance should be placed upon statements that such and such a diatom was resolved under a certain objective. Object-glasses for use with the microscope are usually spoken of as possessing the following qualities : 1. Working distance of the front lens from the object ; 2. Defining power ; 3. Flatness of field and freedom from distortion ; 4. Penetrating power ; 5. Resolving power ; and it is to ascertain their excellence, or otherwise, in these directions that test objects are brought into use. WORKING DISTANCE. It has already been shown that the nomenclature of objectives does not presuppose any working distance from the front lens ; in fact, such a thing would be impossible, seeing that the enlargement of the aperture reduces the distance of the front lens from the object, while the amplification remains the same. Their designation, such as a " i-inch objective," indicates only that such an object-glass should possess the same magnifying power as a single lens of i-inch focus, the distance the 104 PRACTICAL MICROSCOPY. front of the system focusses from the object not being considered at all. Gundlach, in a rather abstruse article in the 'American Monthly Microscopical Journal,' ii., 1881, tells us that "working distance" depends upon (i) the focal distance (nominal, it is presumed), (2) the aperture, (3) the number of lenses of which the objective is composed, (4) the pro- portionate curves of the lenses, and (5) the thickness of the lenses. Great working distance is valuable in an objective only when circumstances demand it. Thus, for dissecting, or for the examination of opaque objects, a certain amount of distance is requisite for manipulation and illumination ; but when an object has been prepared and mounted, no more working distance is absolutely required than will admit of the use of the thickest covering-glass, and of the examina- tion of a moderate depth of object. High-angle objectives of low power and consequent shorter working distance will define much better than the smaller apertures, and there is sufficient working distance, even for dissecting, with the i-inch of 25 air angle. There is some difficulty in selecting a i-inch objective. A glass of 40 air angle possesses considerable working distance, being a power well suited for dissections, used in conjunction with the erector ; while a J-inch objective of 80 air-angle scarcely gives o 03 inch of working distance, focussing closer to the object than an ordinary J-inch objec- tive of 85. With the i-inch and all higher powers the working distance is very small, so that often the microscopist is precluded from using covering-glasses of the ordinary thickness. This is the case with all extremely high aper- tures used as dry objectives. Immersion objectives, for the same degrees of amplification, afford much longer DEFINING POWER. 105 working distances than dry lenses, so that it is often pos- sible to use an immersion ^-inch where the covering-glass is too thick for a dry Jth. The author would like to see objectives catalogued by the makers in somewhat the following manner ; the oculars too might be included : Designation. Aperture. At 10 inches with A ocular. Price, Remarks. Numerical. Air angle. Working distance. M a II I -inch ^-inch 1-inch ^2 -inch 0*64 0*94 I*OO 16 25 40 60 8 o 140 180 0-750 O*42O 0*300 0*I50 0*032 0*080 0*038 0*024 O*OO7 50 56 100 106 no 200 2IO 600 6l 5 * Triplet. Dry objective. > > Water-immersion. ) Homogeneous- ) immersion. Oculars A or I Bor2 C or 3 i D or 4 Eor 5 For 6 Amplifying power) in diameters / 5*0 rs I0'0 20-0 30*0 40*0 DEFINING POWER. This property is of the first import- ance in objectives, and has been described by Dr. Goring to mean " nothing more than a destitution of both kinds of FIG. 94, aberration." A well-corrected objective focusses the indi- vidual rays of a pencil of light, both from the centre and periphery, to the same plane as shown at F in Fig. 94 ; but, nevertheless, a more or less distinct image is produced for io6 PRACTICAL MICROSCOPY. some distance from each side of this focal plane. If, how- ever, the rays are not thus corrected, the outlines of edges of the image will be thick and confused, and the glass is said "to be wanting in definition." This fault may be shown diagrammatically in Fig. 95, which is an under-corrected glass, the peripheral rays being FIG. 95. brought to a focus at ///, between the central focus F and the lens itself. Lenses in their primitive state are very much " under-corrected," and can only be employed when it is possible to cut off the peripheral rays by a diaphragm. An over-corrected lens is shown in Fig. 96, from which it may be seen that the marginal rays are thrown further away from the glass, being brought to a focus at /// respectively, while F represents the focus of the central portion. FIG. 96. An objective free from both these defects is said to be free from spherical aberration or aplanatic. These defects in cheap low-angle objectives are corrected (if the term will apply) by the interposition of a diaphragm behind the back lens cutting off the marginal rays. These rays cannot then enter into the formation of the picture, the result being a dark, not very well defining glass of DEFINING POWER. 107 low angle, but long working distance, though not suffi- ciently corrected to work with deep eye-pieces. In the best objectives of high angle the marginal rays are not cut off, but corrected to the very edges by the application of a wider back lens than usual ; the aperture is therefore larger, but the objective possesses less penetration than one of lower aperture, and the working distance has been materially reduced. A small aperture objective may be constructed from one of these more perfectly corrected lenses by the addition of a diaphragm ; such an objective would bear deep eye- pieces and possess a fair amount of penetration, though the working distance may not be sensibly increased, owing to the thickness of the lenses and their various curves not being specially planned with this end in view. The defining power of an objective may be examined by the employment of certain test objects obtainable from Messrs. Norman or Wheeler, until the student has learned how to prepare them for himself. The pollen of the Hollyhock (Althea rasa), shown in Fig. 97, is a useful test ; it must be illuminated as an opaque object, and with a deep eye-piece (the Huyghenian D), the minute spines should be readily and clearly defined. A well cut wood-section, such as is shown in Fig. 98, is also an excellent test of definition, the borders of each FIG. 97. io8 PRACTICAL MICROSCOPY. vessel and cell should be clearly and sharply delineated, there must be no mistiness or blackness of edge. A dark FIG. 98. image shows at once that too much of the peripheral pencil has been cut off, the definition of a small aperture objective being never equal to one" of large angle. The figure shows a sec- tion of the Horse-chestnut stem (^Esculus hippocasta- num). Triplets, such as are shown in Fig. 28, gene- rally break down under deep eye-pieces. The proboscis of the Blow-fly, shown in Fig. 99 a favourite preparation of the late Mr. Topping is an excellent object for testing the defining power of an objective of low amplification ; the outlines may be fairly sharp, yet the details of the pseudo-tracheae will not be clear, well defined, and free from colour under TESTS OF DEFINITION. 109 a deep eye-piece, unless the corrections have been well cared for. The student should, if possible, compare this object under two objectives, one of foreign make yielding an amplifica- tion of 50 diameters with the A eye-piece, the other a i-inch of English construction, possessing an air angle of 30. Another exceedingly good test of definition is a well mounted specimen of the tracheal system of Dytiscus marginalis, delineated in Fig. 100. In this instance the spiral threads should be visible with- out any halo of colour, and clearly separated so as to appear a continuous fibre rolled between two membranous walls. The engraving shows the illusive ap- pearance generally perceived in this ob- ject that of watered silk produced by the contact of the back and front of the spiral, at different in- clinations, when the object is flattened and mounted in the usual way in balsam. In order to discover how the correction for colour has been performed, several objects may be employed ; Dr. Carpenter's test is the section of pine-wood shown in Fig. 101 ; it should be mounted dry, and the small circles (glandulcz) must be well defined and free from colour even with the D eye-piece. Perhaps absolute freedom from colour under deep eye-pieces does not yield the FIG. 100. 1 10 PRACTICAL MICROSCOPY. utmost perfection in resolution; but authorities differ on this point. For higher powers a white petal of the Pelargonium may serve as a test object, it is a moderately severe test for a FIG. 101. FIG. 102. J-inch ; while for higher powers still, the diatom Meridion circulare will show directly whether the corrections have been well executed. Insect scales are generally used for judging the defining power of higher objectives, such as the scale of the Morpho Menelaus shown in Fig. 102, and that of the Podura, Fig. 103. The former exhibits lines in a longitudinal direction with transverse markings, attributed to the corrugation of the internal surfaces of the lining membrane, and which are only to be noticed by the use of a good objective. The Podura scale forms an excellent test of definition ; it was known for this purpose before the appearance of TESTS OF DEFINITION. Ill Pritchard's 'Microscopic Cabinet,' published in 1832; but the true character of the markings was not then known. This scale was then included amongst the " line tests," whilst now, with objectives of wide aperture, it presents FIG. 103. FIG. 104. the appearance shown in the figure. Central light from an achromatic condenser is the best for exhibiting this scale, or it may be illuminated as an opaque object by means of the lamp and bull's-eye condenser. Other insect scales are to be found in use as test-objects, such as those of the Lepisma saccharina shown in Fig. 104, and the battledoor scales of the Polyommatus argus de- lineated by Fig. 105. These are considered much easier tests than those of the M. Menelaus and Podura (Lepidocyrtus curvicollis\ but not so variable in quality. Fig. 106 shows one of the ordinary scales of the Morpho Menelaus, being, however, magnified to but one-half the extent of Fig. 105. 112 PRACTICAL MICROSCOPY. In the use of these test objects, great attention must be paid to the illumination and more particularly to the ad- justment of the lenses for the thickness of the covering glass. But little correction is needed with the screw collar FIG. 105. FIG. 106. adjustment in the water-immersion objective, and still less, if any, is required with the homogeneous system ; never- theless, it should always be added, as then correction is possible, should it ever be required to meet exceptional cases. In the lenses made by Messrs. Ross and Co., all the powers from the ^--inch upwards can be used either as ''dry " or "water-immersion," by merely moving the lenses by means of the adjusting collar near to the mark "wet," thus avoiding the cost of extra fronts and the inconvenience in changing them. Great care is required in adjusting them exactly, so as to get the best performance. 3. FLATNESS OF FIELD AND FREEDOM FROM DIS- TORTION. These properties in objectives may be tested in several ways ; for low powers, a section of a large spine of Echinus, such as shown in Fig. 107, may be used ; a well cut and perfectly flat wood-section ; or perhaps better still, one of Mr. Dancer's exquisite micro-photo- graphs. The whole field should be well defined under SECTION OF ECHINUS SPINE. 113 one focussing, the margin as well as the centre. For higher powers similar objects may be used, but of course of smaller dimensions, while freedom from distortion can FIG. 107. be tested for, by observation of the micrometer placed upon the stage, or a series of lines ruled by Wheeler, varying from one thousand to ten thousand to the inch. 4. PENETRATING POWER. " Penetration," as it is understood now, signifies that property which an objec- tive possesses whereby several planes of an object are I 114 PRACTICAL MICROSCOPY. brought into focus simultaneously. Until very recently the property of penetration was shrouded in mystery and its usefulness often exaggerated, but thanks to Professor Abbe, who has made it a special study, most of the diffi- culties surrounding the subject have now been cleared away. The greatest use of "penetration" is perhaps in the employment of the binocular microscope, for it is only when an object can be seen in its entirety that a true stereoscopic image can be obtained. There is no doubt also that this penetrating power is very useful for general work, such as dissecting; still it should be remembered that a low - angle lens with much penetration will not usually stand deep eye-pieces. Dr. Blackham writes of "depth of focus" as "only a residual error at best" and "that it can be left in a wide-angle lens as well as in a narrow-angled one." Whether this is practically correct, perhaps opticians will be able to tell us, but it seems to the author much more difficult to leave spherical aberrations uncorrected and practically unnoticed in a wide-angle lens than in one of narrow angle. Professor Abbe has demonstrated the fact that penetra- tion or " depth of vision " depends upon two circumstances : first, the accommodation depth of the eye, and secondly the depth of focus, varying with the quality of the images pro- duced from certain distances on each side of the exact focal plane. The eye, being insensible to small defects in the various images, can practically discern the object in its several depths, provided the images compare with sufficient exactness. The accommodation depth is very great with low ampli- fications. Under a magnifying power of 10 diameters it amounts to nearly 2 I millimetres, while with higher powers the image passes quickly into a mere transverse PENETRATION. 115 section. The depth of focus does not diminish at such a rapid rate, but the thickness of an object which can be seen under one focussing decreases accordingly as the amplification increases, and therefore it is most import- ant, where binocular vision is essayed, to use the lowest power sufficient for distinctly recognising the object ; and with transmitted light to employ as narrow a pencil as will sufficiently illuminate it. The following table by Professor Abbe will sufficiently illustrate these remarks : Amplification. 0-50 N.A. a. Diameter of Field. b. Accommoda- tion Depth. C. Focal Depth. d. Depth of Vision. & + C. Ratio a to d. a ~d mm. mm. mm. mm. 10 25-0 2'08 0-073 2'153 ii "6 to i 30 8'3 0'23 0*024 0-254 32'7 , 100 2-5 0-02 0-0073 0-0273 91-6 , 300 0-83 0-0023 O'OO24 0*0047 176*6 , 1000 0-25 0*00021 0-00073 0-00094 266 , 3000 0-083 O*OOOO2 O'OOO24 O-OOO26 3*9 , The higher the numerical aperture of an objective the less will the penetration be, though the defining and re- solving power, with quantity of light admitted, increase with the absolute aperture of the objective, provided the aberrations are well corrected. The following table (see next page) has been abstracted from the 'Journal of the Royal Microscopical Society' for August 1 88 1, wherein may also be found Professor Abbe's paper from which we have largely quoted. From the foregoing considerations, "tests" for pene- tration would seem to be superfluous, seeing that it results from two almost fixed conditions ; nevertheless, it is well to know how it may be observed. A section of frog's lung or of human liver (Fig. 108) is useful for this purpose, as when mounted, the various parts will be found to have con- I 2 PRACTICAL MICROSCOPY. NUMERICAL APERTURE. (n sin = a.) Penetrating Power. (i) Illuminating Power. (') NUMERICAL APERTURE. (n sin u = a.} Penetrating Power. (v) Illuminating Power. (M 52 'SO 658 667 2-310 2*250 I'OO 0-98 000 020 I'OOO 0-960 48 '676 2-190 0-96 '042 0-922 46 685 2' 132 0-94 064 0-884 '44 694 2-074 0-92 OS? 0-846 42 704 2'0l6 0*90 III 0-810 40 714 960 o'88 136 0-774 38 725 904 0-86 I6 3 0-740 36 '34 746 850 796 0-84 0-82 I 9 '220 0-706 0-672 "33 752 770 0-80 250 0-640 3 2 758 742 0-78 282 0-608 30 769 6 9 0*76 316 0-578 28 7 8l 638 0-74 351 0-548 26 794 5 88 0-72 389 0-518 24 806 '538 o'7o 429 0-490 22 820 "488 0-68 471 0-462 20 833 440 o'66 'SIS 0-436 18 847 392 0-64 562 0-410 16 862 346 0-62 '613 0-384 14 877 3 00 0*60 667 0-360 '12 893 254 0-58 724 0-336 io 909 210 0*56 7 86 0-314 08 926 166 0-54 8 5 2 0-292 06 943 124 0-52 923 0-270 04 962 082 0-50 2-000 0-250 '02 980 04O traded somewhat, producing corrugations or folds, and consequently the rays proceed from different planes, so that the test should be to see how much or how little of the total depth can be seen under one focussing, without indistinctness. As an instance of the work specially suited for low angles and consequent penetration, the cyclosis in Vallis- neria spiralis is often cited. The author can assure his readers that this may be easily seen with advantage under a J-inch objective of 80 air angle (0*64 numerical aper- ture), the largest yet made for this power in this country. 5. RESOLVING POWER. Without entering into any of the theories of this property in objectives, it may be briefly RESOLVING POWER. 117 stated that it depends entirely upon large aperture, com- bined, of course, with accuracy of the corrections for sphericity and chromatism. The several pieces of appa- ratus mentioned in the previous chapter are often essential with objectives of wide aperture, for it is ob- vious that the advan- tages would be lost if sufficiently oblique rays did not enter into the formation of the image. The Ross - Zentmayer stand, used with a i-inch or 2-inch objective as a condenser, the radial substage condenser of Messrs. Swift and Son, or the oil - immersion condenser of Messrs. Powell and Lealand, are all capable of producing light-rays of sufficient obliquity. Diatom frustules, as a rule, furnish tests for the resolving property of medium and high-power objectives, to which may be added the insect scales already shown ; but these natural tests are all of very variable quality. Gyrosigma formosum and G. angulatum, shown by Figs. 109 and no respectively, are tests of the resolving property of the \ and |-inch objectives, both of which have been engraved from photographs taken by the late Dr. Redmayne, of Bolton. Navicula rhomboides, shown in Fig. in, is used as a test for the -|th objective and higher powers. It is rather a difficult diatom to resolve properly without accessories, but with the Powell and Lealand oil-immersion condenser it may be managed without much trouble. FIG. 1 08. n8 PRACTICAL MICROSCOPY. Amphipleura pellucida, shown in Fig. 112, is a most difficult diatom to resolve ; indeed, it cannot be accom- plished by any dry objective save of the widest aperture, and even then it requires most careful attention to the details of illu- mination. Dr. Woodward considers this frustule to be the most useful test for immersion objectives of \ power and higher. The resolution into lines is not so FIG. 109. FIG. no. FIG. in. FIG. 112. NOBERTS TEST PLATE. 119 difficult, when compared with that into dots. Mr. Wenham, when describing his new illuminator for diatoms, states that he was never successful in the patient manipulation required to resolve this diatom by the old methods of illumination. Allusion has already been made to the variations likely to occur in the markings upon the frustules of diatoms and the scales of insects. In order to avoid these irregu- larities, the late Herr Nobert of Pomerania issued a series of test lines ruled upon glass, each band containing lines of a definite number to the inch. The most popular is that known as the 19-band plate, containing lines to the inch as under : Band. Number of Spaces per Inch, about. Band. Number of Spaces per Inch, about. I. 11,300 XI. 68,000 II. 17,000 XII. 73,000 III. 22,500 XIII. 79,000 IV. 28,000 XIV. 84,000 V. 34,000 XV. 90,000 VI. 39,800 XVI. 96,000 VII. 455o XVII. 101,000 VIII. 51,200 XVIII. 106,500 IX. X. 56,800 62,500 XIX. 112,000 Herr Nobert often expressed his opinion that the last four bands of this plate would never be resolved by any objective ; but after inspecting Dr. Woodward's photo- graphs of the whole series, he produced another "plate" ruled to the twentieth band, the tenth on which cor- responds to the nineteenth on the old, the twentieth band being ruled at the rate of 200,000 lines to the inch. Herr Moller produces what is called a " test-platte," containing 20 diatoms, mounted dry or in balsam; they are arranged upon the slide in a row, at the beginning and end of which is a specimen of Eupodiscus argus. The 120 PRACTICAL MICROSCOPY. following is a list of the diatoms on this " test-platte," with the number of striations to the inch, as given by Morley : Diatom. Number of Striae to the Inch. T Triceratium favus 2 060 to 080 IO ,800 T' 7 a 16 -7QO rS COO A 2? ,OOO 07 ,000 6. 8.' 9- TO Pinnularia interrupts Stauroneis Phoenicenteron Grammatophora marina . . Pleurosigma Balticum ,, acuminatum . . 26 H 31 42 42 ,500 ,100 ,300 ,500 ,700 ,QOO 26 33 34 ,800 ,000 ,300 ,-200 II. 12. T7. Pleurosigma angulatum .. Grammatophora subtilissima Surirella gemma 43 61 ,800 ,200 ,4.OO > 61 ,700 ,800 M 63 ,OOO 6T , 300 T 5 -^ , CQO 56 T6 Surirella/ gemma 67 ooo ^O ADO 17- 1 8. TO Cymatopleura elliptica Navicula crassinervis . . . 63 79 ,300 ,400 )> 82 S 1 ,200 ,7OO 20. Amphipleura pellucida 92 ,700 92 ,900 It has already been mentioned that the resolving power of objectives depends entirely upon their aperture, with the excellence of their corrections for colour and sphericity. Objectives of low angle are generally made with posterior lenses of such a size as to exclude the extreme uncorrected marginal rays, or if this is not done the margins are cut off by a diaphragm. The utilised portion is very fairly corrected for colour, but nevertheless the spherical aberra- tions, though small, must exist ; on the other hand, wide apertures require large back lenses, the rays from which cannot be cut off by a diaphragm without reducing the angle, and therefore the corrections have to be applied to the more oblique pencils ; and chromatic aberration can scarcely be corrected without at the same time affecting the spherical in a very great degree. IMMERSION OBJECTIVES. 121 The value of wide pencils was first put into practical form by Hartnack, who made in the ordinary course of busi- ness water-immersion objectives utilising pencils of light approaching to 170 in air; a drop of water was placed be- tween the objective and the object, and thus by passing through a denser medium, rays of light entered the object- glass which could not possibly enter from air, giving at the same time a considerable working distance for the higher powers. Some years afterwards Mr. J. W. Stephenson, whose name has been already associated with the erecting binocular, conceived the idea of substituting for water, in the immersion objective, a fluid having the same refractive and dispersive power as crown glass ; fourfold systems upon this plan were calculated by Professor Abbe and made by Carl Zeiss, the optician of Jena, of ^ and -|- equivalent foci. These objectives had a balsam angle of 113: greater than the maximum of 180 in air, in the ratio of 5 to 4. Messrs. Powell and Lealand have recently constructed a homogeneous - immersion objective of T Vmch equivalent focus, with a numerical aperture of I "43 or 140 balsam angle, with two extra fronts, one of which gives an aperture of I * 28 or 1 1 5 balsam angle, while the other provides an aperture of 1*0, or 82 in balsam. With an aperture of ,1*43 the working distance is 0*007 inch; the aperture of I -28 gives a focal distance of 0*016 inch ; while with the numerical aperture of 1*0 the working distance is 0*024 inch. It was the existence of these immersion objectives and their various angular values which led Professor Abbe to investigate the general principles of microscopic vision. He tells us that "the very first step of every understanding of the microscope is to abandon the gratuitous assumption of our ancestors, that microscopical vision is an imitation of 122 PRACTICAL MICROSCOPY. macroscopical, and to become familiar with the idea that it is a thing sui generis, in regard to which nothing can be legitimately inferred from the optical phenomena connected with bodies of large size." Professor Abbe discovered the fact that the microscopical image is the result of diffraction, or the consequence of those changes which are produced in rays of light by their interception by minute particles ; the rays are collected at the back of the objective, where they depict the direct and spectral images of the source of light, reaching in their further course the plane which is conjugate to the object, and give rise there to an interference phenomenon, which gives the ultimate image observed by the eye-piece, and, therefore, the image depends essentially on the number and distribution of the refracted beams which enter the objec- tive.* From this it appears that the larger the number of diffracted rays admitted into the objective the greater like- ness to the object will the image possess, a true image being only produced when all the diffracted rays from the object are admitted. Dr. G. Blackham, in a paper read before the Microscopical Congress at Indianapolis, August 15, 1878, said, "Now, if it is the function of the objective to collect and bring to a focus rays of light too divergent to be received by the unaided eye . . . the more of these lost rays that a given glass can so collect and bring to a focus the better the glass," and " one would naturally expect to find that the improvement or evolution of the microscope was accompanied by an increase of the angular aperture of the objectives, and this, indeed, we find to be the case." Un- * It is clearly beyond the scope of a work such as this to enter fully into the details of this most important question. Those who wish for a more complete dissertation are referred to the April number of the ' Journal of the Royal Microscopical Society' for 1881, where also Professor Abbe's paper "On the Estimation of Aperture " may be found.^ THE VALUE OF WIDE APERTURES. 12$ aided, the human eye will only admit pencils of about 10, and as light is dispersed from every point of an object to an angle of 180, those glasses which approach infinitely near this latter angle must give a more correct image of the object that those of small apertures. When we appreciate the above facts we shall be able to estimate the value of immersion objectives, which enable us to collect and gather to a focus rays which cannot possibly enter the microscope when a film of air exists between the objective and object. The editors of the 'Journal of the Royal Microscopical Society ' have given diagrams of the relative diameters of the utilised back lenses of dry and immersion objectives of the same power, commencing with an air angle of 60 and ending with the homogeneous hemisphere of 1 80 balsam angle, which has nearly the same refractive index as crown glass. They also add, " Thus, if we commence with an air angle of 10 and proceed by successive additions of 10 up to 180 air angle, passing then to 82 balsam angle, and again progressing to the nearest practicable approximation to 1 80 balsam angle, the emergent pencils will show a continuous increase ; there is no break at 180 air angle, nor does anything abnormal appear at that point, but we have a regularly progressive series from the lowest air angle to the highest balsam angle." It has been urged by some, that all those rays equivalent to more than 180 air angle are not image-forming rays, but this assertion has also been dealt with in the same journal. "The simplest experiment of all is to take a homogeneous-immersion objective of large aperture, say i "25 (i 10 balsam angle), and place a stop of tin-foil on the back lens, leaving only a small clear annulus of the extreme marginal rays. With sufficient obliquity of the illumination the image of the object will be seen perfectly delineated either on a bright or dark field." 124 PRACTICAL MICROSCOPY. An immense amount of discussion took place in reference to these lenses in England, nearly all the opponents to the system averring that by the use of an immersion fluid the aperture was cut down ; that, in fact, rays which were equivalent to more than 180 in air were of no practical utility, even if they could be made to enter an object-glass and form an image. The editors of the 'Journal of the Royal Microscopical Society,' on page 305 of the ' Journal/ * write : " The * aperture question ' will ever hold a most prominent place in the history of the microscope, represent- ing as extraordinary a series of mistakes as were ever com- mitted in any branch of science, and in which (down to comparatively recent times) both the leaders and the rank and file were equally involved. * Aperture ' may be said to have been the haschische of the microscopist ; when that has formed the subject of consideration, the simplest and oldest established optical principles have not been disregarded merely, but their very converse tacitly assumed, as if the great optical physicists of this and the previous century had never lived or had written nothing that was worthy of con- sideration ! " Since penning the foregoing remarks, the author has been informed by Mr. J. B. Dancer, of Manchester, that for some years he has practically effected the reduction of aperture in somewhat a similar manner to that indicated on page 107. Instead, however, of screwing in a diaphragm of definite aperture behind the back lens of the objective, he uses an ingeniously constructed graduating diaphragm, screwing into the lower end of the microscope body as an adapter, into the lower end of which the objective is made to fit. Mr. Dancer constructed this, some years ago, to give penetration to wide angle lenses, and has sent the author * Series ii. vol. i. part 2. PROFESSOR ABBE'S NOMENCLA TURE. 1 2 5 the apparatus in question. There has scarcely been time to thoroughly test the apparatus while this sheet is passing through the press the author has, however, tried it upon a ^-inch objective of wide aperture, and this was cut down in such a manner as to enable it to be used with the Wenham binocular without producing any distortion. The working distance of the front lens was not in the least altered by the contraction of the diaphragm, so that the objection to most wide-aperture lenses still remains, when distance from the object, either for illumination or for use of dissecting instruments, is required. Having become somewhat confused in our nomenclature of apertures, on account of the unequal value of the same as expressed in degrees, Professor Abbe introduced a system based on his own experiments and used in harmony with existing but older optical laws. Under his hand the air angle of 180 was identical with the water angle of 97 and the balsam angle of 82, but instead of giving them three separate values he introduced the term " Numerical Aperture," equivalent to I o in each of the three above instances. The numerical aperture is easily obtained by multiplying the sine of the semi-angle by the refractive index of the fluid in which that angle has been measured : this is the meaning of the formula : n sm u = a, where n = the refractive index of the medium ; sin u = the sine of the semi-angle, while a = the numerical aper- ture. A table of natural sines for each degree from i to 90 may be found in Chapter IX. The following table has been printed for many months upon the cover of the ' Journal of the Royal Microscopical Society ' (see next page). 126 PRACTICAL MICROSCOPY. Diameters of the Back Lenses of various Dry and Immersion Objectives of the same Power (i in.) from 0*50 to 1*52 N. A. NUMERICAL APERTURE. (n sin = a.) Angles of Aperture (= 2 it). Illumi- nating Power. O 2 -) Theoretical Resolving Power, in Lines to an Inch. (A. = 0-5269 |x = line E.) Dry Objectives (=!.) Water- Immersion Objectives (= i*33) Homo- geneous Immersion Objectives. ( = i -52.) o / o / o / ;52 .. .. 180 o 2-3I 146,528 48 161 23 153 39 2-25 2-16 144,600 142,672 4 6 .. . . 147 42 2-13 140,744 44 .. .. 142 40 2-07 138,816 42 .. . . 138 12 2'02 136,888 40 .. .. 134 10 1-96 134,960 38 .. .. 130 26 1-90 133,032 36 .. .. 126 57 I-8 5 131,104 '34 .. 123 40 I -80 129, 176 33 . . 180* o 122 6 I'77 128,212 32 .. 165 56 120 33 I'74 127,248 30 .. 155 38 H7 34 I-6 9 125,320 28 .. 148 28 114 44 1-64 123,392 26 .. 142 39 in 59 i'59 121,464 24 147 36 109 20 i'54 119,536 22 . , 133 4 1 06 45 1-49 II7,6o8 '20 .. 128 55 104 15 1-44 115,680 18 .. 125 3 101 50 i*39 113,752 16 . . 121 26 99 29 i'35 111,824 * 14 , , 118 oo 97 ii 1-30 109,896 12 10 114 44 in 36 94 56 92 43 1^25 107,968 106,040 08 ,, 108 36 90 33 ri7 104,112 06 . . 105 42 88 26 I'I2 102,184 04 .. 102 53 86 21 I -08 100,256 O2 .. 100 10 84 18 I'O4 98,328 o 180 o 97 3i 82 17 I'OO 96,400 0-98 157 2 94 56 80 17 9 6 94,472 0*96 147 29 92 24 78 20 92 92,544 0'94 140 6 89 56 76 24 88 90,6l6 0'92 133 5i 87 32 74 30 85 88,688 0*90 128 19 85 10 72 36 81 86,760 O'88 123 17 82 51 70 44 '77 84,832 0-86 118 38 80 34 6854 ;74 82,904 0-84 114 17 78 20 67 6 80,976 0:82 no 10 76 8 65 18 67 79,048 0-80 106 16 73 58 63 3i 64 77,120 0-78 102 31 7i 49 61 45 61 75,192 0-76 9856 69 42 60 o 58 73,264 0-74 95 28 6736 58 16 '55 0-72 92 6 65 32 56 32 52 69,408 0*70 88 51 63 3i 54 50 '49 67,480 0-68 85 41 61 30 53 9 46 65,552 0-66 82 36 59 3 51 28 '44 63,624 ft 0*64 0'62 0-60 79 35 7638 73 44 57 3i 55 34 53 38 49 48 48 9 46 30 : i 61,696 59,768 57,840 v_y 0-58 0*56 IVl 51 42 49 48 44 51 43 14 34 31 55,912 53,984 '54 65 22 47 54 4i 37 29 52,056 [\~\ 0-52 62 40 46 2 40 o 27 50,128 \L/ 0-50 60 o 44 1 38 24 25 48,200 CHAPTER VI. THE COLLECTION OF OBJECTS. IT has been the author's endeavour to persuade the student to take up some special branch of study with the aid of the microscope, and with this end in view the following chapter has been written, showing where certain objects are to be found, what apparatus is requisite for their collection, what to collect, how to collect, and when gathered, how to preserve them for future examination under the micro- scope. Collectors of experience will not require to be informed on many of these points, and therefore to make the chapter interesting to more than the mere student a list of works treating on each subject has been appended, and also the names of several species under each heading in order that the possessor of a microscope may know what slides to purchase should he desire to fill a cabinet in that manner. The author would strongly advise the young student to refrain from flitting hither and thither over the whole range of microscopical objects. It is not enough to be able to name a few rotifers or rare diatoms, such knowledge is of the shallowest kind ; but if he sets himself to work to study the life-history of some hitherto obscure organism, or the anatomy of an insect, the outward form of which he is alone familiar with, he may rely upon it, he will be useful in his generation. Most collectors have their own method of gathering specimens, and are very conservative on this point, but the 128 PRACTICAL MICROSCOPY. telescopic walking-stick with all its fittings as shown in Fig. 113, is an article generally used by all. There are generally supplied with it, a ring to carry a fine muslin net, a ring to hold a bottle, a weed knife, a spoon, and a drag hook for weeds. FIG. 113. Mr. Baker, of Holborn, supplies the article in rather a different form : the bottom ring is not clamped by a screw as shown in the figure, but is furnished inside with a thread, into which is made to screw the neck of one of the York Glass Co.'s bottles. For many purposes a pond scoop is required, such as for scraping the surface of the mud at the bottom of pools when searching for Oscillatoria, &c., and if it is made to screw into the end of the collecting stick it will be .very convenient. It is simply a ring of tinned iron about 5-inches in dia- meter and i -inch deep. Both edges are "wired," as the tinsmiths call it, so that a piece of thin muslin or stout gauze may be stretched tightly over it. It is shown in Fig. 114. Some collectors prefer to secure the muslin over the scoop with a firm elastic band, so that after collecting it may be removed and folded up for transport home and perhaps this is the better plan. FIG. 114. THE TOW-NET. 129 The tow-net, Fig. 1 1 5, is of great use in collecting marine, and even river or lake objects. It is made of fine but strong muslin, tied at the large end round a wooden hoop, while the nethermost extremity is secured round a small wide- mouthed bottle, so that the more deli- cate organisms may find their way into it, and so be out of the way of the currents caused by the passage of the net through the water. The tow-net, as illustrated, is fur- nished with an interior net, which, acting as a valve, prevents the escape of organisms which have once been enclosed. Aquatic organisms, whether animal or vegetable, are met with in all kinds of water ; even the tap water supplied by some of our corporations is ex- tremely rich in specimens, while in clean ponds the collector will not fail to find a host of treasures ; in impure streams and pools, containing sewage and other decomposing matters, only such common animalcula as Para- mecium aurelia are to be found. Other objects are fixed upon stones and weeds under water, and little pieces of dead stick are often found covered with interesting objects. When the water is not rich in specimens, it may be necessary to concentrate them by straining off the super- fluous water, which may be effected by using the filter shown in Fig. 1 16. It consists of two small funnels passing through a cork as shown in the figure, the one which is K FIG. 115. 130 PRACTICAL MICROSCOPY. inverted in the bottle being covered at the mouth with very fine muslin. The water containing the organisms is poured in at the top funnel, while water only issues from the stem of that inverted in the water. FIG. 116. FIG. 117. This operation may be continued until the bottle is well stocked, when the contents may be carried homewards for examination. When an organism is required to be removed from a DIPPING TUBES COLLECTING CASE. 131 bottle of water, a tube or tubes of the form shown in Fig. 117 will be found necessary. They may be cut from ordinary glass tubing by making a cut with the edge of a three-square file, and breaking it in two with the fingers. The sharp edges should then be fused by holding in the flame, finally allowing to cool gradually. The bent tubes may be made by taking a length sufficient for two tubes and softening the middle portion in the flame of an ordinary gas burner or spirit lamp, and when sufficiently softened the two extremities are to be pulled asunder so as to form a couple of tubes of the form of B, Fig. 117, they can then be cut asunder with the file and the edges fused. The form A is produced in a similar manner, the softened portion being drawn out obliquely. It is a great mistake to load oneself with a host of paraphernalia. The labour of carrying a heavy pack often destroys what might otherwise have been an enjoyable excursion. FIG. 1 1 8. A set of half-a-dozen small corked bottles or tubes, and as many small tinned boxes, will complete the collector's outfit. A small but handy pocket collecting case was introduced several years ago by Stanley, of London. (See Fig. 1 1 8.) The objects having been collected, the next thing is to K 2 132 PRACTICAL MICROSCOPY. find out what they are, genus and species, often no easy task for the beginner. One way of getting over the difficulty is by consulting the books mentioned under each heading nearly always to be had at the free libraries often a long, laborious, and unsatisfactory task ; while the other is by sending the specimens in a tube to a friend or naturalist of repute. In the event of one not being known, the author has much pleasure in suggesting the name of Mr. Thomas Bolton, who for several years has established a naturalist's studio in Birmingham.* A word to such inquirers never forget to enclose a stamp for reply : many forget this, and thus the willingness of the naturalist to furnish information gratis becomes a serious tax upon his pocket. We may now pass on to the enumeration of many objects of interest ; but let it not be for a moment supposed that it is possible to give a complete list of objects suitable for microscopic study. The main wish of the author is, to put before the reader something he may collect easily, in the hope that he will become interested in the study of the details of some one of them. ALG^E. The members of this class of Thallophytes may be found almost anywhere, in ditches, streams, ponds, and even in the small pools of water lying in the hoof-prints of animals upon clayey or boggy soils. One of the most interesting of the Algae is the Volvox globator (Fig. 119), which, however, is very uncertain in its habitat. Wherever found it is usually plentiful. All the fresh-water Algse may be collected by the use of the appliances already mentioned. Many Oscillatorise grow upon the surface of the mud at the bottom of pools, and so require the scoop shown in Fig. 1 14. The whole collection, including mud, should be wrapped up in the muslin, and carried home in that state for examina- * 57, Newhall Street, Birmingham. ALG^EANIMALCULA. 133 tion. The remainder of the Algae may be carried home in tubes or bottles, and upon arrival should be emptied into small aquaria formed of wide-mouthed bottles or small propagating glasses turned upwards, the knob resting in a hollow support. Whether minute or not, the gatherings should be examined on the spot with a platyscopic lens, to prevent the loading of one's satchel with useless speci- mens. Marine Algae furnish many beautiful objects for the microscope, and can be easily collected upon many shores. ' FIG. 119. Perhaps of all places, Tenby, Torquay, and the Mumbles near Swansea are the best hunting grounds. The various species of Cladophora, Ptilota, Dasya, Bangia, Ceramium, and Griffithsia all form good objects. Books which may be consulted: Rabenhorst's ' Flora Europaea Algarum ' ; Griffiths and Henfrey's ' Micro- graphic Dictionary ' ; Johnstone and Croall's ' British Sea- Weeds ' ; Hassall's ' Fresh-Water Algae/ ANIMALCULES. Taking this term to apply to the Infusoria and Rotatoria the student will find a good field for study. It is scarcely possible to find a drop of water which has been for any length of time exposed to the air, not containing either Infusoria or Rotatoria. In some waters they are found in but few numbers, while other localities literally swarm with them. In the former case the pond-filter shown at Fig. 116 will be found valuable. The organisms may be taken from the pond or stream by means of the stick and bottle, and after straining, the residual water carried home in the cork tubes or bottle 134 PRACTICAL MICROSCOPY. which the collector may have with him. A word of advice to the student : Do not overcrowd the organisms, and do not leave any portion of the bottle or tube filled with air if they are to be exposed to shaking or concussion. On arriving home the contents of the tubes may be emptied into small aquaria improvised from broken wine glasses, or better perhaps the tube can be stuck through the centre of a large cork and floated in a vessel of water to maintain an equal temperature, when the organ- isms can be easily abstracted as required by means of a dipping tube. Amongst the Infusoria the Eiiglena viridis, Paramecium aurelia, and Coleps hirtus are good objects for study ; of the Rotatoria, A nurcsa longispina has been found in the tap water furnished by the Birmingham corporation, and others are to be met with in the same habitat, notably Triarthra longiseta and Salpina redunca. Melicerta ringens, the tube- building rotifer (Fig. 120) is a beautiful object, and should be carefully searched for ; it is frequently found attached to water plants, such as the Potamogeton crispus, or large-leaved pond-weed; the Ana- charis alsinastrum and Myriophyllum spicatum. FIG. 1 20. ARACHNIDA ANIMAL PREPARATIONS. 135 Books which may be consulted: W. Saville Kent's 'A Manual of the Infusoria ' ; Griffiths and Henfrey's ' The Micrographic Dictionary'; Pritchard's 'History of the Infusoria.' ARACHNIDA. This class of animal life containing the spiders may become very interesting and instructive objects. It seems hardly necessary to say where they may be found, or how to collect them ; but it may be necessary to enjoin keeping them moist when they are required for permanent objects or for dissection ; diluted glycerine or diluted acetic acid will effect this. The respiratory system, the circulating system, the spin- ning organs, and even the eggs, are very interesting, but a, knowledge of dissection must be gained before the student can make a successful study of this branch. The Arachnida are very plentifully distributed the mites or Acarini, such as the Acarus domesticus (cheese mite) and the Acarus farince (flour mite) are good objects for the J-inch objectives used binocularly, either alive or when mounted without pressure. Books which may be consulted: ' Micrographic Dictionary ' ; Blackwall's ' British Spiders ' (Ray Society) ; Walker's ' British Spiders ' (Ray Society). ANIMAL PREPARATIONS. The number of these objects is legion ; and little else can be done here than to say that animal preparations, as a rule, require special preparation and treatment. Still there is the raw material to collect, and this should be carefully preserved, in order that when examined, its characters shall be faithfully delineated. The hairs of animals and scales of fish present no unusual difficulties; but such subjects as skin, tongue, liver, lung, &c., should be reserved until the student has become a moderately expert experimentalist. Frogs, mice, rats, rab- bits, and guinea-pigs are generally pressed into this service. 136 PRACTICAL MICROSCOPY. Books which may be consulted : 'How to Work with the Microscope/ Beale ; Brunton, Foster, Klein, and Sander- son's ' Handbook for the Physiological Laboratory ' ; Syl- vester Marsh's ' Section Cutting ' ; Rutherford's ' Practical Histology.' CRUSTACEA. In this branch are specimens innumerable, the Entomostraca being included under this head. They may all be taken with the appliances already mentioned, and what will do for Infusoria will also be sufficient for Crustacea. Daphnia pulex, the water flea ; Cyclops quadri- cornis ; Cypris tristriata ; A rgulus foliaceus, the fish louse ; Asellus vulgaris, the water wood-louse ; Gammarus pulex, the fresh- water shrimp; Bosmina longirostris (Fig. 121); FIG. 121. Chirocephalus diaphanus, the fairy shrimp, are all to be found in easily accessible ponds during the spring, summer, and autumn. Books which may be consulted: Baird's 'British Entomo- straca' ; 'The Micrographic Dictionary.' DIATOMACE&. 137 DlATOMACE^E. This probably has been the most attrac- tive class to nearly all microscopists. Diatoms are a family of Confervoid Algae, in which the protoplasm is enclosed in silicious valves, generally covered with very fine markings, the nature of which has not yet been satisfactorily made out. They are found in fresh, brackish, and salt water, adhering to plants and stones, or scattered amongst peat, water mosses, or Oscillatoriae, and even upon damp ground. Nothing is easier than their collection, but of course it is not always possible to meet with the specimens desired. Diatoms are often found in the stomachs of fish, especially crustaceans and molluscs, and several species have been found in the internal arrangements of Noctiluca miliaris, a small exceedingly transparent organism of the size of a grain of mustard seed, causing a phosphorescence in the sea. A little experience will enable any one to find and to gather all he may desire. Those living in the city can easily procure many beautiful varieties by simply fastening a muslin bag like an umbrella cover to the hydrant. After securing a quantity of the sediment, empty it into a large fruit jar or other receptacle nearly filled with water, and let it settle. The green, brown, or fawn-coloured scum on the surface of pools, bogs, and marshes, is mostly diatoms, and it may be taken up by means of a spoon or bottle and preserved, always in alcohol and water, or dried upon paper. The living weeds should be taken carefully from their location without much compressing or washing. The finer water plants yield the richest harvest. Fresh-water forms are sometimes found hanging in green-coloured masses from drains, sluices, and water-pipes. To gather from the lake, a net of fine muslin, having an opening in the bottom in which a wide-mouthed phial is tied, may be towed at the 138 PRACTICAL MICROSCOPY. stern of a steamer. The sediment left in the bottom of pails, barrels, and other vessels contains a good supply. To obtain varieties not found at home, open a correspon- dence with gatherers in other localities, who will gladly exchange. Dr. Redmayne, in a short communication to 'Science- Gossip' in 1875, described the simple arrangement for diatom collection shown in Fig. 122. FIG. 122. A cork must be provided which fits the collecting bottle tightly ; this is to be bored with two holes : in each is fitted a glass tube as shown in the figure, one having a slight curve a, the other b bent at right angles an inch from the end. To the tube b is attached a piece of indiarubber tubing about the length of the collecting-stick, and the free end c may be held in the hand or fastened to the stick with a small elastic band. To use the apparatus, the thumb of the right-hand must DR. REDMAYN&S APPARATUS. 139 press the tube firmly against the stick at c and the bottle be lowered until the mouth of the tube is within a quarter of an inch of the diatoms ; the thumb is then to be raised, and if the water is deep the bottle will fill by atmospheric pres- sure, carrying in the diatoms at the same time. In shallow water suction will be necessary to exhaust the air in the bottle, in which case the bulb pipette shown at B in the same figure will be useful as a mouthpiece. In the collection and recognition of diatoms, the student will find Professor Brown's pocket microscope a useful ad- junct, as it is furnished with a deep eye-piece and objectives of an inch, and a fifth of an inch focus. It is shown in Fig. 123. Great care should be taken in the collection of diatoms, so as to have them in as pure a state as possible, as it is not easy to separate them from foreign matter when it is mixed up with them. The late Dr. Redmayne placed the gathering in a long bottle in the sun for a few hours, the lower half of the bottle being covered with black paper. The free diatoms separate themselves from the mud and come to the surface, and can thus be removed. Fossil Diatomaceae also exist in im- mense quantities in various places. The large deposits of guano, the Bermuda deposits, the Berg-mehl in Norway, the Mourne Mountain deposit in Ireland, the recent dis- covery of diatoms in the London clay by Mr. Shrubsole, and the still more recent discovery in Llyn Arenig Bach, about midway between Bala and Festiniog, in North Wales, FIG. 123. PRACTICAL MICROSCOPY. show that the Diatomaceae are, and have been, very widely distributed. Some collectors may consider the pocket microscope shown in Fig. 123 not steady enough for general use, and therefore may prefer the form shown in Fig. 124 as made FIG. 124. by Mr. Browning and others ; it is exceedingly portable and very steady, as the author can testify from its practical use when travelling in North Wales. Amongst the diatoms which may be singled out for examination are Pleurosigma angulatum (Fig. no), P. formosum (Fig. 109), Navicula firma (Fig. 125), N. lyra (Fig. 126), N. rhomboides (Fig. in), Isthmia enervis, Arachnoidiscus Ekrenbergii, Meridian circular e y Diatoma vulgare, and a host of others. Whilst writing this chapter the author has received a tube of diatoms from Mr. Bolton of Birmingham, consist- ing of a number of species found attached to algae in the canal of that neighbourhood ; the most easily recognised DIA TOMSECHINODERMA TA. 141 were: Bacillaria paradoxa, Nitzschia sigmoidea, N. lanceo- lata, Grammatophora marina, A mphiprora alata, Pinnularia radios a, and P. viridis. FIG. 125. FIG. 126. Books which may be consulted: Rabenhorst's 'Flora Europaea Algarum ' ; J. Smith's ' British Diatomaceae ' ; Donkin's ' British Diatomaceae ' ; and Schmidt's ' Diato- macean Atlas.' ECHINODERMATA. The marine objects, star- fishes, sea- eggs, or sea hedgehogs, may always be taken at- low water after a spring tide. In their earlier stages they are ex- tremely interesting, being infusoria-like organisms, and often appearing without any internal structure whatever. They furnish the microscopist with abundance of material. The prickles or spines, hooks, and the pedicellaria make interesting and instructive objects. A section of Echinus spine is used as a test for the flatness of field in low-power objectives, while a section of a small spine may be used for the same purpose with higher 142 PRACTICAL MICROSCOPY. powers. A section of one of these may be seen delineated in Fig. 107. Books which may be consulted: Forbes's ' British Star- Fishes ' ; Agassiz's ' Echinodermata Viv. et Foss.' FERNS require no special apparatus for collecting save a sharp knife and a tin collecting box ; they form a very interesting study. The stem may be double stained, as described in the chapter on the staining and injecting of objects. How many microscopists are there who possess fern preparations in their cabinet, spores and sori, stems in cross and vertical section, double and single stained, and yet are totally unacquainted with the life-cycle of a single species ; who have looked at sori and spores innumerable, and yet never made the effort of allowing these to ger- minate, and to observe them in their various and strange mutations. The Lastrea filix-mas, or male fern, is one of the best species to study for the beginner, and there are many others easily found, such as A thyrium filix-fcemina (lady fern), Scolopendrium vulgare (hart's tongue), Pteris aqui- lina (common bracken),- A diantum capilhis- Veneris (maiden hair), and many others. Books which may be consulted : Moore's ' Index Filicum ' ; 'Handbook of British Ferns' ; Newman's ' British Ferns ' ; J. Smith's ' Ferns, British and Foreign ' ; Hooker and Baker's ' Synopsis Filicum.' FORAMINIFERA. These gelatinous, structureless ani- mals are mostly sought after for the sake of the shells, serving them as a covering. The shells are pierced with holes, through which the animal protrudes its pseudopodia, using them as a means of locomotion. They are found in largest numbers in the sand and mud from the sea- bottom, but may also be found on sea-weeds, and in FORA MINI PER A . the fossil state in chalk, limestone, and other mineral deposits. From sea-soundings they may be procured by dissolving out the tallow in which they are collected, by means of benzine any sort of benzine or benzoline serving this purpose. To obtain fossil Foraminifera from chalk, the pieces must be broken up small by means of a hammer, and then gently crushed in an iron mortar. The powder is then to be placed in a piece of coarse calico, tied up like a pudding, and put into a large basin of water, and well kneaded, until reduced to one-third its original bulk. The milky fluid is then to be poured off, until only one-fourth remains, and the operation of washing, by stirring up with FIG. 127. fresh water, and allowing to settle, repeated many times, until a portion of the residue, on examination under the microscope, shows that most, or all, of the extraneous matter has been eliminated. 144 PRACTICAL MICROSCOPY. The illustration Fig. 127 shows a section of chalk from Gravesend with Foraminifera in situ. Amongst others, Lagena squamosa, Orbitolites compla- natus, Polystomella crispa, and Nodosaria raphanus, are very good objects. Books which may be consulted: Williamson's ' Recent Foraminifera ' (Ray Society) ; Carpenter's ' Introduction to the Study of Foraminifera.' FUNGI. Micro-fungi may be found everywhere, and make a splendid study. Many of them, however, can only be examined successfully when in the fresh state, such as Penicillium crustaceum and Aspergillus glaucus, the common moulds and mildews of our houses. Nearly every plant and tree is attacked at one time or another by some parti- cular species of micro-fungus, so that the student will find plenty of work in this class alone. We have the Puccinia graminis upon the leaves and stems of standing corn, as well as Tilletia caries and Ustilago segetum or smut, which fill up and destroy the whole contents of the ear ; the JEcidium on the berberry bush ; Triphragmium on the leaves of the meadow-sweet ; the blackberry brand, Aregma bulbosum; Coleosporium synantherarum on the colt's-foot ; Cystopus candidus on cabbages ; Peronospora infestans on our potatoes ; Peronospora gangliformis on lettuces ; and Peronospora vici