King si ais sa * (yh a ss Anta Ei CORNELL UNIVERSITY MEDICAL LIBRARY ITHACA DIVISION. FROM SIMON HENRY GAGH, OLASS OF 1877. Cornell Universi lodern microscopy; a handbook for beginne MODERN MICROSCOPY oil Ny. L : iN Fie. 1.—A Typical Modern Microscope.—Made by W. Watson and Sons to the specification of Dr. Henri Van Heurck, Antwerp, for Photo- Micrographic and High Power work. MODERN MICROSCOPY A thandbook for Beginners and Students COMBINING I. THE MICROSCOPE, AND INSTRUCTIONS FOR ITS USE BY M. I. CROSS — IT. MICROSCOPIC OBJECTS: How PREPARED AND MOUNTED BY MARTIN J. COLE LECTURER IN HISTOLOGY AT COOKE’S SCHOOL OF ANATOMY THIRD EDITION ENTIRELY REVISED AND ENLARGED TO WHICH IS ADDED IIL MICROTOMES: THEIR CHOICE AND USE CHICAGO W. T. KEENER & CO. 90 WABASH AVENUE 1903 PREFACE TO THIRD EDITION. An entire revision of the text has been made in this new edition. Information regarding the microscope, its acces- sories and methods of manipulation, have been brought into line with present-day knowledge and practice. The instructions for preparing and mounting microscopic objects have been extended and are more varied. A third part has been added on the subject of the choice and use of microtomes, for which we desire to express our special thanks to Mr. G. West, of the Botanical Laboratory, Edinburgh. The directions given are compressed within narrow limits, and are designed especially to aid the beginner. Acknowledgments are also due to the pub- lishers of Knowledge, of 326, High Holborn, for permission to use several notes and articles which have appeared in their columns, and to numerous friends who have con- tributed their kind advice and suggestions for the improve- ment of the book. M. I. C. M. J. C. Lonpon, November, 1902. PREFACE TO FIRST EDITION. Tuts handbook is not intended to be an exhaustive treatise on the microscope, nor to give particulars of the various patterns of instruments that are made, of which details can be seen in the makers’ catalogues, but to afford such information and advice as will assist the novice in choosing his microscope and accessories, and direct him in his initial acquaintance with the way to use it. The directions for preparing microscopic objects by Mr. Martin J. Cole are the outcome of a very long experience as a preparer of Microscopic Objects of the highest class, and cannot fail to be of the greatest service to the working microscopist. M. I. CROSS. CONTENTS. PART I. INTRODUCTION. - 1-5 CHAPTER I. THE MICROSCOPE-STAND. On choosing a microscope-stand—Designation of the parts of a © microscope—Different forms of feet compared—The stage— Sub-stage and under-fitting—Coarse and fine adjustments —Two-speed fine adjustments—The limb—The body-tube —The mechanical draw -tube—Tailpiece and mirrors— Binocular microscopes—Microscopes for dissecting—Micro- scopes for metallurgy—Microscopes for travellers—Micro- scopes for petrology and for preparing and mounting—The selection of a microscope—Testing a microscope - 6-47 CHAPTER II. OPTICAL CONSTRUCTION. Optical definitions—Magnifying power, how produced—Objec- tives, apochromatic and achromatic—Immersion objectives —Angular and numerical apertures—The apertometer— Correction for thickness of cover-glass—The use of the correc- tion collar—Testing objectives—The Abbe test-plate—Choice of Objectives — Eyepieces: Huyghenian, compensating, holoscopic, Kellner, projection, and binocular—Standard gauges for eyepieces - 48-83 CHAPTER III. ILLUMINATION AND ILLUMINATING APPARATUS. Monochromatic light and light filters—Sub-stage condensers— Aplanatic apertures of condensers—How to use the con- x Contents PAGES denser — Choice of a condenser — Spot lens and para- boloid—The polariscope — Selenites—Selenite stages—The bull’s-eye condenser — Parabolic reflector — The vertical illuminator 84-106 CHAPTER IV. ACCESSORY APPARATUS. Oil lamps, electric lamps, gas lamps— Revolving nosepiece — Davis’s shutter—Drawing apparatus—The measurement of objects — Micrometers — Troughs — Compressors — Stage forceps—Eye-shade—Hints regarding the care of the micro- scope—Text-books recommended 107-128 APPENDIX. A short note concerning the influence of diffraction on the resolving power of microscopical objectives, and upon the apparent colour of microscopical objects, by Dr. G. John- stone Stoney 124-126 PART II. INTRODUCTION - - 127 List of tissues and organs, and the most suitable hardening, staining, and mounting reagents 128-131 LESSON I. HARDENING AND PRESERVING ANIMAL TISSUES FOR MICRO- SCOPICAL EXAMINATION. Absolute aleohol—Chromie acid and spirit—Potassium bichro- mate—Ammonium bichromate—Miiller’s fluid— Methylated spirit— Decalcifying solution—corrosive sublimate—Picric acid—Formaldehyde—General directions for hardening tissues - 132-136 LESSON II. EMBEDDING TISSUES AND SECTION-CUTTING. Cutting sections with a razor by hand—Embedding in paraffin wax and lard—Infiltrating a tissue with paraffin—Embedding bath—Cole’s microtome and embedding in carrot—Cathcart’s microtome for freezing—Embedding in celloidin—Embedding in gelatine—Cambridge rocking microtome - 186-145 Contents xl LESSON TII. STAINING ANIMAL SECTIONS AND MOUNTING IN CANADA BALSAM, PAGES Grenacher’s alcoholic borax carmine—Staining process—Staining with Ehrlich’s hematoxylin—Double staining with hema- toxylin and eosin—Aniline blue-black—Aniline blue—Golgi’s nitrate of silver methods—Mounting in Canada balsam— Staining in bulk—Fleming’s method for staining nuclei— Weigert-Pal method of staining—Ehrlich’s triple stain for blood corpuscles—Toison’s stain for white blood corpuscles— Fixing and staining sections on the slide—Mayer's albumin method—Shellac method - 145-153 LESSON Iv. STAINING BLOOD AND EPITHELIUM, TEASING-OUT TISSUES AND MOUNTING IN AQUEOUS MEDIA—STAINING WITH PICROCAR- MINE, GOLD CHLORIDE, SILVER NITRATE, AND OSMIC ACID. Double-staining nucleated blood corpuscles—Blood of mammals, non-nucleated corpuscles — Epithelium — Endothelium — Teasing-out tissues—Staining with picrocarmine—Farrant’s medium—Glycerine jelly - 1538-158 LESSON V. STAINING AND MOUNTING MICRO-ORGANISMS. Staining bacteria on cover-glasses—Ehrlich’s method for double- staining—Bacillus twberculosis—Ziehl Neelsen’s method— Gibbe’s double stain—Gram’s method—Leprosy bacillus— Anthrax bacillus—Léffler’s alkaline blue method—Diph- theria bacillus—Glanders bacillus— Kuhne’s method — Schutz’s method—Enteric fever method—Gaffky’s method — Spirillum — Actinomycosis— Weigert’s method—Hezema- tozoa of Laveran — Filaria -- Vermes — Anchylostoma — Trichina spiralis 158-165 LESSON VI. INJECTION OF BLOODVESSELS. Carmine and gelatine injecting mass—Prussian or Berlin blue and gelatine mass— Watery solution of Berlin blue—Injecting apparatus—Directions for injecting—Injecting lymphatics— Injecting lymph-sinuses of glands 165-167 xii Contents LESSON VII. CUTTING, STAINING, AND MOUNTING VEGETABLE SECTIONS. PAGES Bleaching—Staining in borax carmine—Hematoxylin—Dalton Smith’s double stain—M. J. Cole’s double stain—Staining with eosin—Staining hairs on leaves—Treatment of fucus and other alge—Ovaries—Flower buds—Pollens—Pharma- cological specimens—Powdered drugs—Mounting in Canada balsam - - - 168-175 LESSON VIII. THE PREPARATION OF VEGETABLE TISSUES FOR MOUNTING IN GLYCERINE JELLY, ACETATE OF COPPER SOLUTION, ETC. Annular vessels—Scalariform vessels—Spiral vessels—Raphides —Starches—Yeast—Mycetozoa and fungi—Preserving fluid for green alge—Protococcus—Antheridia and archegonia of mosses—Fertile branch of chara—Ferns—Digestive glands in pitcher plant—Aleurone—Marine algee—Corallines 176-185 LESSON IX. CUTTING, GRINDING, AND MOUNTING SECTIONS OF HARD TISSUES—-PREPARING METAL SPECIMENS. Bone—Rock sections—Meial specimens— Etching — Ferrite — Cementite—Pearlite - - - 185-189 LESSON X. PREPARING AND MOUNTING ENTOMOLOGICAL SPECIMENS FOR THE MICROSCOPE — MR. ROUSSELET’S METHOD OF PRESERVING ROTIFERA, ETC. Preparing whole insects for mounting with pressure in Canada balsam — Mounting insects in Canada balsam without pressure—To mount insects in glycerine without pressure— Method of preserving rotifera—Killing, fixing, and mounting Entomostraca - - 189-200 LESSON XI. CRYSTALS AND POLARISCOPE OBJECTS. Orystals—Crystals of silver—Starches— Sections of starch-bearing tissues—Cuticles containing raphides—Cuticles of leaves— Cotton, hemp, wool, silk, flax, ete.—Scales of leaves—Fish Contents xili PAGES scales—Palates—Sections of hairs and quills—Small fine hairs — Horns, hoofs, whalebone, and claws — Decalcified bones—Muscular fibres 201-205 LESSON XII. CLEANING AND MOUNTING DIATOMS, POLYCYSTINA, AND FORAMINIFERA. Cleaning diatoms growing upon alge or shells—To clean fossil diatomaceous deposits—To clean living diatoms—To clean polycystina—To clean foraminifera—Mounting diatoms in Canada balsam—Unselected polycystina, transparent and opaque—Selected diatoms and polyecystina—Foraminifera, unselected transparent and opaque mounts—Spicules of Gorgonia— Spicules of Alcionium—Sponges to show cell structures—The skeleton—Sections of sponges—Collecting and preparing foraminifera, Mr. A. Earland’s method - 206-220 LESSON XIII. DRY MOUNTS. Opaque cells—Transparent cellsa—Opaque mounts of Pollens - 221-222 LESSON XIV. FINISHING OFF SLIDES. Canada balsam—Quick method—Exposure method—Glycerine jelly —Farrant’s medium—Dry mounts—Cleaning off failures - 222-224 PART III. MICROTOMES : THEIR CHOICE AND USE—INTRODUCTION 225 CHAPTER I. RAZORS FOR HAND SECTION CUTTING AND MICROTOMES. Razors—The hone—The strop—Microtomes—-Schanze’s cheap microtome—British microtomes—The Cambridge rocking microtome—The Delépine microtome—Thoma’s microtome —Becker’s Spengel and other microtomes— Reichert’s microtomes—Leitz’s microtomes—Sliding microtomes with automatic feed arrangement—Minot microtome, its action and capabilities—Reinhold-Giltay microtome, its feed adjust- ment and advantages—Schanze’s Model ‘ B’ microtome, its xiv Contents knife-carrier, object-holder, orientating and focusing arrange- ments, and adjustments generally—Schanze’s Model ‘C’ microtome for cutting very thin sections—Addresses of PAGES microtome makers 226-284 CHAPTER II. ACCESSORY APPARATUS. Selection of knives: Henking’s, Jung’s, Weikert’s, and Thoma’s pattern knives and their special uses—Microtome knives in sectional view—Knives necessary for beginners—Sharpening knives, and the manner of holding—The hone, its surface and selection—Process of sharpening—The strop and its prepara- tion—Strop paste—Process of stropping—Testing the knife edge—Infiltrating with paraffin—Baths and ovens—Infiltrat- ing oven used by the writer—Gas regulators, Page’s and Reichert’s, their methods of use 249-261 CHAPTER III. PRODUCTION OF SECTIONS. Gripping subjects for cutting—The use of elder pith and cement —tThe freezing method—Celloidin embedding—Collecting and preserving—Fixation—Testing methylated spirit—De- hydrating and clearing—Final paraffin infiltrating bath— Enclosing the infiltrated object within a cube of paraffiin— Cooling parafiin—Treatment of blocks of paraffin containing embedded objects—Dehydrating—Melting points of paraffin —Cutting sections with Schanze microtome—Treatment of ribbon sections, stretching, mounting, and finishing—Stain- ing in bulk—Books recommended—General treatment of nuclear and cytological subjects—Processes recapitulated 262-285 INDEX - 287-292 LIST OF ILLUSTRATIONS. [The figures contained in this book are intended to illustrate special features described in the text, and should not be regarded as recom- mendations of any particular instruments or makers; at the same time, only such firms are mentioned as are actual manufacturers, and whose work is reliable and of the first grade. | FIG. PAGE A Typical Modern Microscope - - Frontispiece 1. A Typical Student’s Microscope - 8 2. Typical Continental Microscope - 12 3. View of Diagonal Rackwork - 19 4. Sectional View of Adjustable Fittings of Rack and Pinion 19 5. Section of Limb of Microscope to show System of Fine Adjustment - 23 6. ‘Imperial’ Microscope by R. and J. Beck = - 25 7. Spindle Milled Head for giving Two Speeds to Fine Adjust- ment: . 26 8. A Popular Binocular Microscope + 82 9. Simple Dissecting Microscope 35 10. The Aplanatic Magnifier 35 11: Metallurgical Microscope - 37 12. Holder for Metallurgical Work 38 13. Travellers’ Microscope 39 14. Petrological Microscope 41 15. Stand VI., by Leitz - 48 16. Mechanical Stage showing the ‘Sprung’ Fittings with Adjusting Screws - - 46 17. Abbe’s Apertometer 63 18. Correction Collar (Zeiss) 66 19. The Abbe Test Plate - 72 20. Huyghenian Eyepiece 77 21. Holoscopic Eyepiece 80 22. Projection Eyepiece - 81 23. Zeiss’s Binocular Eyepiece - 83 24, Abbe Illuminator (1:20 Optical Part) 89 25. C. Baker’s Achromatic Condenser 91 26. Iris Diaphragm as fitted to the Condenser Carrier 92 27. a Apertures of Condensers - 94 30. 31. Image of Lamp-flame - 96 XVi Inst of Illustrations FIG, PAGE 82. Back Lens of Objective 97 33. Stops for Condensers - 99 34. Polarizer 102 35. Analyzer - 102 36. Darker’s Selenites 103 37. Mica-selenite Stage - 103 388 Stand Condenser - 104 39. Vertical or Disc Illuminator 106 40. Microscope Lamp with Metal Chimney 108 41. Electric Microscope Lamp — - - 109 42. The Dustproof Nosepiece for Three Objectives 110 43. Davis's Shutter 111 44, Abbe Camera Lucida - 113 45. Jackson’s Micrometer fitted to Eyepiece 117 46. Ramsden’s Screw Micrometer - 117 47. Botterill’s Trough 118 48. Rousselet’s Live-cage - 119 49. Rousselet’s Compressorium 119 50. Eye-shade for Monocular Microscope - 120 51. A cheap Embedding Bath 138 52. Cole’s Pattern Microtome - 189 53. Cathcart’s Microtome - 140 54. Rocking Microtome 144 55. Sectional View of Razors 226 56. Sectional View of Strop 228 57. Cheap Microtome by Schanze - 230 58. R. and J. Beck’s Delépine Microtome 233 59. Thoma’s Microtome by Jung - - - 234 60. Diagrammatical View in Section of Thoma’s Microtome 235 61. Kleine’s Automatic Microtome. An Hzamrls of Becker's Workmanship - 238 62. Reichert’s Microtome 239 63. Minot-Zimmermann Microtome, without Knife or Object- holder 240 64. Reinhold- Giltay Microtome : Mechanical Part 242 65. Model ‘B’ Microtome by Schanze 243 66. Model ‘C’ Microtome by Schanze 244 67. Henking’ s Microtome Knife 249 68. Jung’s Microtome Knife, with its Screw Handle 250 69. Weikert’s Microtome Knife, with Straight Handle - 250 70. Thoma’s Microtome Knife, with Curved Handle - 251 71. Three Forms of Microtome Knives in Sectional View 251 72. Infiltrating Oven, Open, showing Gas Regulator, Abereci- meter, etc. - - 259 78. Paraffin Cooled Rapidly (x 250 diameters) - 269 74, Paraffin Cooled Slowly ( x 250 diameters) 269 75. Paraffin Cooled Slowly (x75 diameters) 270 76. Embedded Object fixed on Wooden Cube 270 MODERN MICROSCOPY INTRODUCTION. To attempt to give a historical account of the development of the microscope, tracing it through its pre-achromatic days, and noting the successive improvements that have contributed to the high position it occupies to-day amongst instruments of precision, would alone fill a small volume. Suffice it to say that so recently as the year 1824 Tully constructed his first Achromatic Microscope, since which time every decade has added its tale to the march of progress. In 1883 the President of the American Society of Microscopists, in his annual address, remarked ‘ that lenses, which were believed to have so nearly reached the limit of perfection fifteen years ago, are antiquated now, and the limit of perfection has moved forward like the horizon, and is ag far off as ever.’ Those who are acquainted with the development of the microscope and its appurtenances since that time are aware that greater progress has been attained in recent years than had ever been made before. With the introduction of the system of lenses, termed the ‘ apochromatic objectives,’ microscopical optics were raised to an entirely new plane, for these lenses were of a nature altogether superior to those of their pre- decessors. Subsequently a new and increased variety of optical glasses from which to manufacture their lenses was placed at the disposal of opticians, and from that time 1 2 Modern Microscopy the leading manufacturers have vied with each other in their attempts to provide inexpensive objectives that have some of the most important qualities associated with apochromatism, particularly freedom from spherical aber- ration; in so high a degree have they been successful that the so-called students’ series of objectives by many makers, at what may be considered nominal prices, are in many instances superior to the finest lenses of twenty years ago. This improvement in the construction and performance of the objective has brought in its train com- plementary developments in the sub-stage condenser, and necessarily increased accuracy and improved design in the mechanical construction of the microscope-stand; and any worker who is interested in the scientific and artistic sides of microscopy can view with nothing but intense satisfaction the distinctly progressive nature of everything connected with the instrument itself and the absorbing secrets it reveals. That this onward march will continue is certain, for the issues which depend on the microscope for their solution are ever increasing in number. The demands that are made on it are more exacting and varied than ever before, and there will always be forthcoming the willing brain to devise facilities to meet them. The principal factors which have contributed to the progress that has taken place in microscope construction, optical and mechanical, are: Firstly, the unceasing criticism, impartial examination, and, where merited, recommenda- tion, together with the suggestion fraught of mature know- ledge, skill, and thoughtful consideration on the part of amateur microscopists, and especially a select number whose names are familiar to every worker with the micro- scope; secondly, to the exacting requirements of modern bacteriological research, medical work generally, its adop- tion in numerous branches of education, and in manu- facturing industries of every description. Not many years ago a hospital rarely possessed more than one or two microscopes, and these were usually kept Introduction 3 under glass shades, and intended more for ornament than use. At the present time every hospital student has to provide himself with a microscope, and become practically acquainted with the ultimate structures of organs and tissues, both in health and disease. The microscope has been introduced into many ordinary schools, and teachers are slowly realizing that ‘ we think in pictures,’ and that as sight aids the memory, so an ounce of ocular demonstra- tion is worth a pound of oral description. The result is, that erstwhile dry-as-dust lectures in zoology and botany are now anticipated by students with keen interest on account of the pleasure derived by examination with the microscope of the actual subject of study. By means of the microscope the pharmacist is able to judge of the quality of his drugs; the Medical Officer of Health uses it to detect adulterations, entozoa, and bacteria; the brewer and the baker watch their ferments with it, and it is employed by iron and steel workers, seedsmen, dairymen, clothworkers, handwriting experts, and, in fact, in nearly every vocation it is becoming more and more an indispensable referee. It therefore is something more than a mere tool which magnifies, for it contributes in a vital degree to our well- being and comfort. To the microscopical societies also the evolution of the microscope is due in no small degree, and especially does this apply to the Royal Microscopical Society and the Quekett Microscopical Club, both of which meet in London at 20, Hanover Square. Every improvement in the instru- ment and its accessories that takes place is presented to these societies for criticism, and in connection with both of them, as officers and members, are men who have attained the highest eminence in microscopical science and manipu- lation, whose judgments have influenced and moulded the character of microscopy, and who are ever willing to assist by advice and suggestion any who will avail themselves of their experience. It is most desirable that microscopists should become members of a good microscopical society, 1—2 4 Modern Microscopy and those just mentioned enjoy the highest position in England. Undoubtedly the professional worker has in a large degree to thank the amateur microscopist for the efficiency of the microscope that is to-day available, for he it is that has devoted time and money to the encouragement of the manufacturer. It is surprising that, notwithstanding the pleasures and advantages that are associated with microscopical work, microscopy has not the hold on people of refined tastes that its merits should fairly claim for it. It does not seem to be realized that the microscope will unfold its wonders and beauties without that long and careful study which is the necessary preliminary to the majority of scientific pursuits. Those who may be induced to use a microscope in the first place for pleasure and recreation will quickly find their inclinations leading them to a desire for fuller knowledge concerning the subjects which may come under their notice, and by degrees this instrument will become the means of the acquirement of a very liberal education, for its influence is not merely confined to one kingdom ; it embraces every tangible and intangible subject, whether it be the air we breathe, with its myriads of invisible friends and foes to human well-being, or the floors of oceans, with their minute flinty shells bearing markings which exceed in accuracy the power of any draughtsman to depict, and which in themselves are invisible to the naked eye, many of them measuring but the , 55 of an inch. Astronomy, with all the wonders that are associated with the study, demands many a night vigil, an expensive instrument, and a suitable observatory, and even then there is always a sense of dissatisfaction when accounts are read of observations of fellow-workers who are more highly favoured in that they possess glasses of very large aper- ture, which the average astronomical student could not aspire to. How dependent the observer is on the weather, too ! Introduction 5 Photography, with its manifold uses and the pleasant memories associated with numerous pictures that are secured, cannot be compared with the microscope, more especially for the long winter evenings. For any kind of recreation to produce the mental rest which is required by the man of business, a fresh set of faculties must be brought into play, and no better method can be imagined for the purpose than the introduction to a world whose variety is illimitable, whose form is lovely and unique, and whose subjects can never be met with excepting in the quiet observations through the microscope- tube in the study. Perhaps some people may hesitate to attempt working with the microscope, not caring to use it merely as a means of amusement, and mistrusting their ability to employ it scientifically. They reflect that every department has its untiring, experienced workers, and available ground appears to have been gone over so repeatedly that it would seem hopeless for an amateur to attempt to add to existing know- ledge on any subject. This idea is a mistaken one, and any microscopist who uses his instrument thoughtfully will be surprised at the manner in which the love for the work will grow upon him, and how gradually he will become master of some special department which he has adopted as his own. On this point we would echo the words of a well- known microscopist : ‘It needs no marvellous intellect, no special brilliancy, to succeed in a scientific study; work at it ardently and perseveringly, and success will follow.’ In order that the best results may be obtained, however, there must be a correct understanding of the methods of working the instrument. Facility in this respect can only be acquired as the result of experience and practice, and it is the object of this work to indicate in the plainest manner the rules of manipulation that should be adopted in order to insure success. PART I. CHAPTER I. THE MICROSCOPE-STAND. As one looks through the catalogues of the various dealers, and notices the microscope-stands varying in price from £2 to £40, a feeling of bewilderment arises as to what is essential and what can be dispensed with. We will, then, examine the parts, describe their uses and advantages, and state what is necessary for a beginner. Here let us advise intending purchasers not to buy a microscope unless it bear the name of a manufacturer: a good workman is never ashamed of his handiwork. There are many very inferior instruments that look tempting, but a practical acquaintance with them soon discovers their weak points and inefficiency. Happening to attend the conversazione of a well-known microscopical society, at which there were exhibited over one hundred instruments, it was surprising to note the many makeshifts of micro- scopes belonging to some of the exhibitors—and many of them had probably cost a fair price, too. A manufacturer once remarked to the writer that he was some time ago in a provincial town, when an auctioneer asked him whether he could make him up a job lot of microscopes for sale by auction, as he was very successful in disposing of a certain class of pictures in that way. The disgust of the scientific workman can be better imagined than described. The microscopes often seen in the novice’s possession seem to be The Microscope-stand 7 of this genus, and but little satisfaction is derivable from working with them. If the user is at all progressive, an instrument of this kind is either speedily discarded in favour of a well-made one, or it may, on the other hand, cause him to become disheartened, and attribute want of success to his own incapacity instead of the poor quality of the instrument. Although a good second-hand instrument may be occa- sionally met with, great discretion is required in pur- chasing, because improvements may have been introduced since its manufacture, or some damage may have occurred to the optical parts. If it be obtained from a respectable dealer who understands his business, and will give a guarantee of condition, there is some inducement; but a friend who is up to date in microscopy is generally the best to advise. In all cases before purchasing, a catalogue should be obtained from the maker whose name the instru- ment bears, so that it may be ascertained whether the: pattern is still made, or is antiquated and out of date. It is much better to buy a good stand, capable and worthy of ceceiving additional apparatus from time to time, rather than an inferior instrument that is completely furnished with objectives and accessories. These latter rarely engender pride of ownership, and are often relegated to some obscure corner after a short acquaintance ; whereas, if a good instrument be purchased, with but one objective to start with, there is always a pleasure in working with it, and a peculiar fascination from its quality—a satisfaction in feeling that one has something superior. On page 8 a typical student’s microscope is figured, by reference to which the different parts of the instrument will be made clear. Fic. 1.—A is the stand or foot. . B is the tailpiece carrying the mirror (C) with which light is thrown upon the object. D is the under-fitting, into which are fitted the sub-stage condenser, polarizer, etc. Fic. 1.—A TypicaL SrupEn1’s Microscorr. The ‘Fram’ Microscope. By W. Watson and Sons. The Foot 9 E is the stage on which the object is placed. F is the limb carrying the body (G). At the lower end of the body is a nosepiece (H), having a screw into which the objective is fitted. At the upper end of the body is a sliding fitting called the draw-tube (J), by means of which additional magnifica- tion may be obtained, and into this draw-tube the eyepiece or ocular (K) fits. L is a rackwork, by means of which the body (G) is raised and lowered in order to focus the objective upon the object which is placed on the stage (E). M is the milled head controlling the fine adjustment, which imparts a delicate motion to the body, in order that the objective may be more exactly adjusted than would be possible with the rackwork (L) when using high magnifying power. N is a fitting for forceps, or side silver reflector; and O are the springs with which the object is held in position. We have selected the instrument (Fig. 1) because, from practical acquaintance with it, we are able to strongly recommend it for a beginner’s microscope, worthy of receiving additions from time to time as means may permit. Still, it should only be considered as a typical one. THE FOOT. Since the first edition of this book was issued, a decided reaction has taken place in regard to the form of foot on which the microscope is mounted. There has scarcely been a modern writer of repute who has not urged the necessity and importance of having such a mounting for the microscope as shall secure for it absolute rigidity, whether it be used vertically, inclined, or horizontally for photography. No foot so fully answers these requirements as the tripod pattern. Rarely does it happen that the * * bench or table on which work is done is absolutely level, and the tripod is the only pattern that naturally adjusts 10 Modern Microscopy itself to such inequalities of surface. It cannot, therefore, be too emphatically insisted that microscopists should select this pattern in preference to any other. At first sight this feature may appear to be a somewhat trivial one, and especially so to a novice, yet minor details have a marked significance in the satisfactory execution of his work. It will be found advantageous to have a foot shod with cork, as thereby the microscope is in a degree insulated from vibration, and the risk of scratching the surface of the table on which it is being used is avoided. It must be clearly understood, however, that even this form of foot must be made in correct proportion, or its advantages will be annulled. Next to this foot in point of rigidity we would place the Jackson model, as shown fitted to the instrument on pages 87 and 41. This, when properly made, though somewhat heavier, is bui little inferior to the tripod. Of variants from these two designs there are many, but their rigidity can be tested by placing the instrument in a horizontal position, racking the body out, and then observing if there be a tendency to topple over; if there be, have nothing to do with it. We have purposely left the type of foot known as the ‘horseshoe’ to the last, because we consider it the least advantageous of all. It has enjoyed and still commands a distinct preference on the part of a very large section of workers all over the world, and that section the one that uses the microscope to a greater extent than all others put together—we refer to medical students and medical laboratories generally. It is to be remembered that a great deal of work in laboratories is done with the instrument in an upright position, in which circumstances the condenser and mirrors are perhaps somewhat more easily manipu- lated with a horseshoe foot and pillar than with the tripod. It is claimed also for the horseshoe foot that it is more compact, and. that if it be unconsciously brought beyond The Stage 11 the front edge of the bench it will remain firm, where a tripod instrument would fall completely over. These are just demands. While an instrument mounted on the horseshoe is in a vertical position it stands firmly; but if the body be inclined at an angle, as an amateur usually employs it, even with a large and heavy foot there is a tendency to side-falling. All considered, the tripod has so many advantageous points that it is to be recommended in preference to any other. Our selection of a foot for a microscope would therefore be in the following order : 1. The tripod foot, as shown fitted to instruments on pages 8, 25, 82, and frontispiece. 2. The Jackson form of foot, as fitted to the instrument illustrated on pages 87 and 41. 3. The Continental or horseshoe form, as shown on page 12. THE STAGE. The stage of the microscope on which the object is placed for examination may be divided into two classes : (1) mechanical, and (2) plain. Tue Mecuanicat Stace.— The instruments figured on page 25 and frontispiece are provided with this type of stage, in which, by the turning of two milled heads which are attached to screws, plates are moved in dove-tailed grooves one over the other, in rectangular directions, carrying the object with them. A first-class microscope should be pro- vided with this form of stage; in fact, there is no means so suitable for systematically examining an object as is afforded by it. In addition to these mechanical movements, if a bar be fitted to slide in a vertical direction on the top plate the efficiency of the stage will be greatly increased. The mechanical stage lends itself to the adaptation of further important movements. A means of rotating the object is an essential in most classes of work. For this purpose the lowest plate of the stage is usually fitted to rotate on the NER cml ll Stand I, by E. Leitz, Wetzlar Fie. 2.—TypicaL CoNTINENTAL MICROSCOPE (Showing attachable mechanical stage.) y The Stage 13 fixed centre of the base-plate, the mechanical movements acting above it. It is then termed a concentric rotating stage, the object remaining in the field during the whole rotation of the stage. In mechanical stages of economical construction the rotating plate is occasionally fitted above the mechanical movements, and is carried by them, in which case it does not rotate concentrically. The object can, not- withstanding, be kept in the centre of the field by constantly re-setting it with the mechanical screws during the rotation of the plate. Some stages of the concentric form are arranged to rotate by rackwork and pinion ; although this is not really an essential, it is often convenient; it also prevents the stage from rotating accidentally, especially in photography. When it is provided, it should have the pinion-wheel so arranged that it may be disengaged from the rack and replaced instantly. Centring screws to the concentric rotating stage, by means of which the axis of the stage may be made true with any objective, will be found a useful addition, especially if petrological work is to be done. Divisions to the ‘periphery of the stage for reading the angle through which the stage is rotated are not advantageous for ordinary purposes, but for chemical and petrological work they are a necessity. Finpers to Mecuanican Sraces.—Divided scales, read- ing to parts of an inch or millimetre, fitted to the plates of the mechanical stage, will be found of great utility. By means of such an arrangement, important parts of an object can be noted and subsequently refound. For instance, supposing a specimen were being examined, and an im- portant feature were observed to which future reference would be desirable, it would only be needful to take the reading of the divisions on the stage, and record them on the slide—say, horizontal, 24; vertical, 20. On future occasions, on setting the stage readings at the same points and placing the object in the same position on the stage (for which purpose nearly all mechanical stages have a 14 Modern Microscopy stop-pin, against which the slide can be set), the special feature would be at once in the field of view. These divisions can also be used for roughly measuring objects, the modus operandi of which is given in the instructions for the measurement of objects, page 114. If a mechanical stage be selected, it should be a good ’ one, for if badly made it is far less convenient than a plain stage; also the frictional parts should be sprung, and fitted with adjusting screws, so that compensation may be made for wear and tear. ArracnaBLe MecuanicaL Staces.—In recent years a variety of mechanical stages, which can be attached to or removed from an ordinary plain stage microscope, have been introduced. Some of these possess merit, but, taken as a whole, they are inaccurate in working, and at their best are not for one moment to be compared with the mechanical stage, which has been built as an integral part of the microscope, and no microscopist who wishes to do himself and his work full justice should entertain such a fitting. : For some reason, Continental manufacturers have never fitted their instruments with the mechanical stage as it is understood in England, but have always recommended and arranged for the adaptation of an attachable stage. Where mechanical movements are found to be an essential for certain work, and from reasons of inconvenience or impracticability the plain stage cannot be exchanged for a proper mechanical one, then, and then only, should the attachable form be resorted to. Puan Sraces.—The stage of the microscope shown on page 8 has two flat springs only, to hold the object in position on the surface, and the movement of the object is effected by the fingers. For cursory examinations this answers every purpose; but where systematic work is to be done something more is needed, and this, when a mechani- cal stage is not provided, should take the form of a bar reaching completely across the stage and sliding in a The Stage 15 vertical direction. If properly fitted and sprung, it will travel freely when gently pressed with one hand only. The object is carried by it, and can be moved in a horizontal direction upon this bar. With a little practice the fingers become educated to the work, enabling examinations to be conducted with the highest powers almost as rapidly and systematically as with the mechanical screws. This sliding- bar should further be provided with two flat springs, so fitted that they may be turned inwards to rest on the bar when not required. It is often necessary to set an object at an angle across the stage during observation, in order that some special feature may appear vertically in the centre of the field. If the springs be not provided this cannot be done. Finpers ror Puan Sraces.—The form of finder sug- gested by Mr. Lewis Wright for plain stages is the most efficient for practical purposes. Many proposals have been made, but none equal this one for simplicity. On the right- hand side of the central aperture, one inch of the stage is divided into 50 parts in vertical and horizontal directions. A special feature of interest in an object having been dis- covered, the slide being maintained in a horizontal position across the stage by means of the sliding-bar, it is only necessary to read from the top right-hand corner of the slide the lines against which it lies. A note of same is made on the labels of the object, and the specimen can sub- sequently be placed in exactly the same position, and the subject re-examined. Without the sliding-bar it is some- what difficult to keep the object exactly straight across the stage, but with care, on observing an important feature, the slide can be gently turned until it is in a correct position for taking readings. The great saving of time that is afforded by such a device as this should establish its claim to be placed on every student’s microscope. Several makers have already adopted the arrangement, and it would be a great gain to micro- scopists in general if a uniform position for the divisions 16 Modern Microscopy were agreed upon between them, so that a person noting a special point with his microscope could send the specimen, with the readings marked upon it, to a brother worker, and he, having the same kind of finder on his stage, would at once be able to find the desired spot. The following method would be suitable for the average size of stage: A piece of metal the same size as an ordinary glass slip (8x1 inches) should be adopted as a tool, and ¢# inch from one end and ; inch from the edge a minute spot should be made with a small drill. The metal slide should be placed on the stage with the spot towards the front, and the ?-inch space to the right of the centre of the stage. The drilled spot should then be placed central in the field of a 1-inch objective, and the outer margin of the square of divisions marked off from the right-hand end of the metal slide. The Wright's finder is obviously unsuitable for any other than a stage whose upper surface does not travel. In selecting a stage for a microscope our choice would therefore be as follows: For a first-class microscope: Mechanical movements; concentric rotation; screws to make the rotation quite true with any objective; sliding-bar to top plate and stop-pin for object to go against; divisions to plates of stage reading to parts of millimetre or inch; rackwork rotation to stage; and (optional) divisions to periphery of . Stage. For a second-class microscope: Mechanical movements ; sliding-bar to top plate; non-concentric rotation. For a third-class microscope: Plain stage, with springs to hold object in position ; if provided with sliding-bar or plate as object-carrier, so much the better. The Sub-Stage 17 THE SUB-STAGE OR FITTING UNDER THE STAGE TO CARRY CONDENSER, ETC. Tue Sus-stace.—This consists of a tube which should be 1°527 inches = 88°786 millimetres, or, roughly 13 inches full, internal diameter—termed the ‘ Society’s size.’ It carries illuminating apparatus for condensing the light on the object, the polarizing prism, and other apparatus, referred to on a later page. It is adjusted in a vertical direction to and from the under surface of the stage by means of a rack and pinion, and the ring carrying the apparatus is mounted in an outer collar provided with screws, by means of which the condenser, or other apparatus, can be made exactly central with the objective with which it is working. This central fitting is made to rotate by rack and pinion in some instances for using the polarizer, etc., but this is so rarely needed that it is hardly necessary except for special work. It is essential that the sub-stage should be substantially made, as it is a most important fitting, often too little appreciated. For the sake of economy some makers fit the sub-stage without a rack-work, it merely being designed to slide up and down in the dovetailed fitting; this is neither desirable nor convenient. A fine adjustment, to permit of the con- denser being focussed in the most exact manner, is often provided with the best stands, and it is exceedingly con- venient and of very great importance where high-power work is intended to be done. Often it is wished just to alter the focus of the sub-stage condenser very slightly. In attempting so to do the tension on the milled head of the rack-work is apt to cause vibration, so that the best point of adjustment cannot be at once observed. By communicating this small amount of motion with the fine adjustment the focus is obtained to a nicety. It is also especially convenient where a number of specimens have to be examined. The varied thicknesses of the slips neces- sitate a slight readjustment of the condenser in each 2 18 Modern Microscopy instance, and this can be very quickly done if a fine adjust- ment be fitted to the sub-stage. Further, the modern sub- stage condensers possess such large apertures that their exact adjustment becomes equal in importance to the precise focusing of the objective. Where a microscope is provided with a sub-stage it is necessary to ascertain if it will centre with the objective by means of its screws; this should be done in the same manner as described below for the ‘ under-fitting,’ and the centring screws turned. Also, it is very important that when the sub-stage is racked up or down it should main- tain its centre with the optical axis. But few instruments will stand this test; in consequence of untrue mounting or build the sub-stage goes out of centre—slightly in some cases, considerably in others. There ought to be absolute - truth if everything is square, and any great deviation in this respect should call for rectification. If a fine adjust- ment be fitted to the sub-stage, it may be tested by using the upper surface of the fitting as a stage and placing the object on it; this may be made to adhere with a little tallow or grease. An objective of medium power will prob- ably not focus, the sub-stage being too far away. The nosepiece end of the microscope must therefore be length- ened ; for this purpose remove the prism from an analyzer fitting, and use this fitting as a lengthening adapter. The object is then viewed in the usual way. In the construction of the sub-stage once again the Continental microscopes are not for a moment to be com- pared with their English contemporaries. There is not a single maker of microscopes on the Continent who provides centring screws to his sub-stage. It is simply impossible to do good work without this convenience. There is hardly a worker nowadays who does not have objectives by more than one maker, and it will be found that these have different centres. Continental opticians maintain that their method ‘simplifies work, for the condenser is centred once for all ‘to the objectives that are supplied with the instruments. The Sub-Stage 19 Simple and obvious tests will quickly demonstrate that this is quite fallacious. Tue Unper-Frirtinc.—In the cheaper instruments, instead of the sub-stage as above described, an ordinary plain tube, termed the under-fitting, is screwed into the under side of the stage, and in this tube the condenser or other apparatus is moved up and down to focus. It is shown fitted to the microscope figured on page 8. This must be oy) = Lull Lo A ‘ell Fig. 3.—ViEw oF DIAGONAL Fic. 4.—SECTIONAL VIEW OF RAcKWORK ATTACHED TO ONE ADJUSTABLE Fittings oF Rack OF WATSON AND Sons’ Micro- AND PINIon. SCOPES, SHOWING DOVETAILED FIrrines TO RECEIVE FINE ADJUSTMENT SLIDE AND AD- JusTiInG ScREWs (MARKED A). truly centred with the optical tube, and it is well to test it by placing a small diaphragm in the under-fitting, and with an objective in the body to focus the diaphragm. If it be not central its practical importance is annulled. The additional convenience and necessity of the centring sub- stage cannot be too fully impressed upon the beginner who contemplates doing thorough work. Swift and Son, of Tottenham Court Road, in their new series of ‘ Students’ Microscopes,’ and Watson and Sons, of High Holborn, in 2—2 20 Modern Microscopy their ‘Edinburgh Students’ and ‘Fram’ Instruments, make many of the fittings interchangeable, so that a person who starts with an under - fitting, and subsequently ex- periences the necessity of having a sub-stage, can himself _ remove one fitting and replace it by the other. W. Watson - and Sons have lately adopted a form of centring under- fitting; it is inexpensive and very considerably increases the efficiency of a student’s microscope. It can be fitted to almost any microscope. The great convenience will be found in many instru- ments of being able to swing the sub-stage aside out of the optical axis of the instrument on a hinge-joint fitting. It saves much time to students, especially where two or three powers are constantly being interchanged, and the con- denser may not be required for all of them. Where this arrangement exists it should be adapted in a workman-like and substantial manner, and a proper support given to the fitting when in the optical axis to make it perfectly rigid. The choice with regard to a sub-stage would therefore be— In a first-class microscope: Sub-stage, having rack-work and fine adjustment for focusing, and provided with facilities for centring ; rackwork rotation, if for examina- tion of crystals or for petrology. Second-class instrument: Sub-stage, having rack-work and centring adjustments, and means of lifting aside out of the optical axis. Student’s instrument: The same as the second-class, or with the plain under-fitting ; the latter, preferably with centring arrangement. In any case it is imperative that it shall be of the ‘ Universal’ size. The Sub-Stage 21 ARRANGEMENTS FOR FOCUSING. The next point for consideration is the means of focusing the object-glass. This is done by two adjustments, called the coarse and the fine movements. The former consists of a rack and pinion actuating the body in a very true- fitting dovetailed bearing, as per Fig. 8 shown on page 19. In the illustration it will be seen that the rack is cut diagonally, and this is undoubtedly the best kind, a softer motion being obtainable than with the horizontal form. In order that it may work at its best, each tooth of the rack has to be carefully ‘ground in’—that is, fitted to a leaf in the pinion—and so that the fitted tooth of the rack may always engage the correct leaf of the pinion, it is necessary so to fix the body that, when racked up as high as possible, it may not be withdrawn from its bearings and rackwork, it being, in fact, provided with a ‘stop’ screw. The pinion should have suitable provision by means of adjusting screws for exactly controlling the stiffness of the rackwork action and for taking up slight backlash which may arise in consequence of wear and tear. An illustration of the method adopted to secure this result is shown in Fig. 4, page 19, in which the pinion P is held in position by a block of metal, N, against which pressure is exerted by two screws, one of which, M, is shown in the figure. With this arrangement the most exact relation of pinion to rack can be established and maintained. It has been recommended that microscopists should take their instruments to pieces in order that they may judge of their workmanship ; but in reality a well-made microscope requires to be as carefully put together as a watch, and for a novice to attempt to undo the parts means very probable detriment to the instrument. The name of a first-class maker on an instrument may generally be considered a guarantee of good workmanship, otherwise he could not possibly maintain his reputation. Some cheap students’ microscopes, instead of being pro- 22 Modern Microscopy vided with a rack and pinion for the coarse adjustment of the object-glass, are made with the body to slide in a fixed tube. This is a very rough-and-ready arrangement, and accuracy of centring cannot be maintained as with a rack and pinion. These instruments are, however, largely used in hospitals and medical schools, and possessing one element of advantage—namely, cheapness—other considera- tions are often made subsidiary. Instruments of this kind are generally cast aside or disposed of in favour of an in- strument having rack and pinion after a very short time, and anyone purchasing a microscope with a view of adding apparatus to it would be well advised in having one with a little less apparatus, but with a rack and pinion instead of a sliding body. FINE ADJUSTMENTS. Well-defined attempts have been made by nearly all makers to improve this most important of all movements. The demand for accuracy in this particular has been greatly increased by the growing use of objectives of large aperture which cannot be profitably employed excepting with a fine adjustment of the utmost precision. Its essentials are that it impart a very slow motion and be absolutely free from lateral movement. The fine adjustment that for many years has proved thoroughly satisfactory in the writer’s hands is that made by Messrs. Watson and Sons and applied to all their instruments. It is shown in position on the instrument (Fig. 1) and the working details will be gleaned from Fig. 5. The body is raised or lowered in a dovetailed fitting by means of a lever contained within the limb of the instru- ment, and a pin passing through it transversely acts as a fulcrum. By turning a milled head attached to a micro- meter screw, force is applied to the lever at one end against a pointed rod, attached to the body and entwined by a coil spring, at the other extremity. As the body moves upwards, the spring is compressed against a brass plate, and on the Fine Adjustments 23 micrometer screw being released this spring produces the reactionary power. One arm of the lever is four and a half times longer than the other, consequently the weight of the body at the milled-head end of the lever and the motion imparted are reduced in this ratio. Thus the makers give the weight of a body of one of their instruments as 17 ounces, and this divided by 4} reduces the resistance to 3% ounces. This system has the advantage that the = Fic. 5.—SecTion oF Limp oF Microscore ro sHow Sysrem oF FINE ADJUSTMENT. position of the milled head on the limb is convenient for manipulation, and is not altered when the body is racked up—that is, it is not carried by the rack-work, as in many forms, so that its attachment to a focusing rod of a camera for photo-micrography is easy and convenient. There is also a very simple means of adjustment provided for taking up any slackness through wear. The slide in which the fine adjustment is fitted has sprung slots, to which are fitted screws (shown in Fig. 8, page 19, marked A). By turning these screws slightly, the spring-fitting grips the 24 Modern Microscopy bearing more tightly, and so takes up any wear caused by friction. Any microscopist can thereby adjust his own instrument. This pattern of fine adjustment in modified forms is now made by at least two other houses. It would not be applicable to microscopes other than those in which the so-called Jackson limb is employed, and makers of stands in which a direct acting screw is fitted, as in the Continental form, have exercised considerable ingenuity in providing an efficient movement. Carl Reichert, of Vienna, who has in recent years shown highly progressive tendencies, designed a lever-form of fine adjustment for this latter description of microscope, which has proved even more satisfactory than was originally anticipated. Messrs. Swift and Son, too, have adopted a somewhat similar plan with corresponding success, while Messrs. Zeiss have devised an entirely distinct fine adjustment for their photographic stand, actuated by means of gear-wheels, which produce an extremely slow and precise movement. Powell and Lealand’s instruments are also provided with a fine adjustment having special merit, consisting of a lever actuating a long tube sliding up and down inside the body. This, however, has the disadvantage of being carried by the rack-work when focusing for different powers. Two-speed Fine Adjustments.—Complaint has arisen that the tendency to fit very slow-acting fine adjustments has become a source of inconvenience to students and others who work interchangeably with objectives of different powers fitted on revolving nose-pieces. To meet this Mr. A. Ashe, of the Quekett Club, designed a two-speed fine adjustment, and the plan was carried to a practical issue by Messrs. R. and J. Beck, who now apply it to certain of their instru- ments. It is shown fitted to the microscope (illustrated page 25). It will be noticed that two milled heads are provided for fine focusing instead of the usual one. The upper milled head turns a screw having a coarse thread, moving the body #5 inch for each revolution ; the lower ANE ‘ATES i TN i AG. ‘ ae rs AN ee Fig. 6.—‘IMPERIAL’ Microscope py R, anp J. Beck. (Showing two-speed fine adjustment. ) 26 Modern Microscopy actuates a fine screw which causes a movement of the body 335 inch for each complete turn. At any moment either milled head may be used and time thereby saved when low powers are employed. Where a simpler method may be desired, practically the same result can be secured by having attached to the centre of the ordinary milled-head a smaller spindle, say, one-sixth of the diameter of the former. When a quick movement is required this spindle can be turned between the fingers, and a rate equal to six to one of that obtainable by slowly rotating the ordinary milled-head will be secured. Messrs. Watson and Sons have applied this to their instruments, and the writer has found the working exceedingly satisfactory. When testing the performance of the fine adjustment, a central cone of light must be used; if the light be thrown obliquely there will be of necessity an apparent movement in the direction from which the light comes. With central illumination there should be no shake or displacement whatever in the object when it is focused. Nearly every maker has his own system or systems of fine adjustment, possessing features more or less desirable, but they are mostly modifications of those mentioned here. Some firms adopt a most excellent form of fine adjustment for a superior class of microscope, while in the students’ patterns the method employed is dissimilar and oftentimes useless for high-class work. It Fic. 7.—Sprxprn WOW be far better that efficiency were not 2 be NDLE RB . Minrzep Heap sacrificed in such a manner for the small ae ae saving in cost involved. ADJUSTMENT. Above all things avoid the form of fine adjustment which carries the whole weight of the body of the instrument, or depresses it against a ‘spring, as in the Continental instruments and cheap students’ forms: these are almost worse than no fine adjustment at all, as they invariably soon work loose in the fittings and cause great annoyance. The Limb QT There is also another form, now almost non-existent, which is fitted to the microscope at the nosepiece end, and consists of a milled head attached to the body, by means of which a tube which carries the objective is raised and lowered inside the body-tube against a spring. This can never work thoroughly well, for if two round tubes are perfectly fitted one inside the other, they clutch, and to overcome this, one of the tubes has to be rendered a little eccentric, and con- sequently a lateral shake arises. In the choice of a fine adjustment, therefore, reject the direct acting and the nosepiece forms. THE LIMB. The design of the limb of the microscope is of special importance, because it carries the body and is intimately associated with the fine adjustment. It should be of a substantial shape and strongly made. In our opinion the pattern which is shown in the build of microscopes, pages 8, 25, etc., is to be preferred before that shown on page 12, because in the former, additional solidity is imparted to the body fittings on account of there being no separate adjust- ment which has to act and re-act at the back part—in other words, a limb which carries the body at one extremity, and at the other is acted upon by the fine adjustment through a pillar, cannot in the nature of things be so satisfactory as a limb which carries the fine adjustment instead of being supported by it. Still, it cannot be denied that the method of attaching the limb to the pillar adopted by the majority of the Continental makers, and, for the matter of that, those English manufacturers who include this style of instrument amongst their models, is usually a very sub- stantial one. THE BODY-TUBE. It has always been the custom, in the construction of the full-sized English microscopes, to make the body of fairly large diameter ; while on the Continent the reverse is the 28 Modern Microscopy case, and it is made as small as possible. Owing to the extended use of Continental oculars and objectives, English makers have in recent years adopted the Continental diameter of body to a considerable extent. For photographic purposes itis held by some workers that a fairly large body should be used, but for ordinary visual work we do not consider there is any real advantage in the large over the small tube. The growing system of providing a large outer body with an inner draw-tube to carry the small students’ eyepieces, is a commendable step, and meets all needs. It may here be said that instruments of Continental type have their bodies constructed very much shorter than the so-called English models, the rule being to employ objectives adjusted for a tube 160 millimetres long with the former, while English opticians usually adjust theirs to one of 250 milli- metres. The microscopist who enters enthusiastically into his work invariably has objectives of both Continental and English make, and he therefore requires the convenience of being able to use both perfectly. One or two English opticians make microscopes with a body of 160 millimetres, and a draw-tube sliding inside it, by means of which a length of 250 millimetres can be obtained; but a greater range is often found convenient, and Watson and. Sons in their Van Heurck microscope (shown in the frontispiece), and Mr. Baker in his Nelson model microscope, supply two draw-tubes giving a range of body from 140 millimetres (54 inches) to 310 millimetres (12 inches). One of these draw-tubes works by a rack and pinion: the object of this being to afford facility for adjusting the objective for thick- ness of cover-glass, as described on page 68. This form of body is coming more and more into use, and will be found a very great convenience to the all-round worker. No precise advice can be given without knowing the work intended to be done, but, generally speaking, the short body with the two draw-tubes is much to be preferred to any other. The draw-tube usually has a scale of divisions engraved The Body-Tube 29 upon it to parts of a centimetre or inch. The object of these divisions is to enable a record to be kept of magni- fications at different points of extension, or a note to be made of the lengths of tube that give the most perfect corrections for certain objects and objectives. In all microscopes of medium or high-class, the universal thread should be fitted to the lower end of the draw-tube ; where there are two draw-tubes it should be supplied to the outer one. The advantages of this adapter are numerous. A low-power objective can be used in it which it is often impossible to focus on many stands, owing to the compact- ness of the build and shortness of the movement of the coarse adjustment. With the two draw-tubes, if the outer one have this adapter fitted, nearly 10 inches of separation can be obtained between the eyepiece and the objective. It is further useful for carrying the apertometer objective and the analyzer, described respectively on pages 63 and 102; also the Bertrand’s lens for examining the ‘ brushes’ of crystals, and for many other purposes. It has occurred within the experience of the writer that results obtained on a microscope having a large tube could not be reproduced with the same objective on an instru- ment having a small tube. This was traced to be due to the diaphragm at the bottom of the draw-tube, and it has since been found that in many students’ stands the opening of this diaphragm is as small as 3 inch. This is altogether insufficient, and causes restriction to the passage of rays from the objective. It would be well to see that this diaphragm had an opening of at least 3 inch. TAILPIECE AND MIRRORS. The Tailpiece.—It will usually be found convenient if the arm, or tailpiece, which carries the mirrors, be so mounted as to be turned aside with the mirrors when desired.. This arrangementis of great utility, for it permits of light being readily directed from the lamp through the 30 Modern Microscopy sub-stage condenser for critical work. Opticians favour a rectangular rather than cylindrical tailpiece to carry the mirror gymbal; the reason for this is a little doubtful, but there is probably no distinct advantage in one over the other. Where, however, a cylindrical tailpiece is provided, it will be obvious that the mirror could quickly be swung round out of the axis of the microscope and so obviate the necessity for the swinging of the tailpiece itself, but this is quite a minor consideration. The Mirrors should be plane and concave, hung in a gymbal, giving universal movements, and have a means of adjustment to focus in a vertical direction. The plane mirror is always used with the condenser, spot lens, etc., and with very low-power objectives, but the concave, when the condenser is not employed and the maximum amount of light is desired. A constant source of trouble and annoyance is an imper- fectly worked plane mirror, which will give several reflec- tions of the image of the lamp flame. The plane mirrors usually fitted invariably do this. It should be insisted, in a@ microscope with which high-class work is to be done, that a parallel-worked mirror should be supplied. It is a trifle more costly than the ordinary kind, but the additional efficiency, on account of the reduced number of reflections, is very appreciable. The parallelism of a mirror may be tested by holding it just below the level of the eye in the direction of a row of objects, such, for instance, as chimney- pots ; and on observing the reflections, each subject should stand out singly and clearly. If the mirror is not parallel- worked several reflections of the same object will appear superimposed in the mirror. Care is needful in the use of the concave mirror, if the best result is to be obtained with it. It should be so arranged that the apex of the cone of rays that it transmits may be exactly in focus on the object. Many microscopes are provided with mirrors that are unsuited to the instru- ment, being either too long or too short in focus, and con- Binocular Microscopes dl sequently do not produce good effects. To test the mirror, a piece of white paper should be placed upon the stage of the instrument, which must be set horizontally, and light from a lamp reflected by the concave mirror on this; then, by sliding the mirror up and down on the tailpiece, it can quickly be seen if the focal point can be obtained upon the paper. BINOCULAR MICROSCOPES. We have hitherto been treating principally of the monocular microscope, and this, it must be understood, is the only form that can be used for critical high-power work —in fact, the Continental firms as a rule do not make binocular microscopes at all, regarding them as unneces- sary. Two or three of them, however, make a binocular eyepiece, which will be found described under the head of eyepieces. The advantage of a binocular microscope is, that both eyes can be employed simultaneously, saving the strain on the vision which is apt to ensue through the con- stant employment of the monocular microscope, and the endeavour to see in the best manner the detail in the speci- mens examined. We should recommend every user of the monocular microscope to train himself to work with either eye, keeping the one not in use open; this will be found of the very greatest service. The universally understood bino- cular microscope is provided with a prism, designed by Wenham, which admits of the light going up a direct tube, and reflects light also into a second tube. By this means objects can be seen more naturally than with the mono- cular microscope, for the reason that stereoscopic vision is obtained, and objects having a certain amount of depth may be seen completely with the binocular microscope, whereas with the monocular it would be necessary to focus in successive stages through the entire depth. Especially is this true regarding opaque objects, with low powers. The stereoscopic binocular conveys an impression of the 32 Modern Microscopy objects viewed that is almost startling in its beautiful effect. Subjects stand out in relief, exhibiting their natural con- tour, and at once the worker is able to decide the shape and form of an object in a way that it is impossible to do by focusing through the several planes with a monocular Fic. 8.—A PoruLaR BinocuLaR MIcRoscopE. The Nelson model by C. Baker. instrument. The binocular microscope is par excellence the instrument for the amateur. To him the beautiful appeals in a manner that it perforce cannot do to the scientific man, who, being intent on the pursuit of know- ledge of some obscure point, has no time to notice, or if Binocular Microscopes 33 to notice, cannot linger to reflect upon the esthetic aspect. In the examination of rotifers and other inhabitants of ‘ ponds and rock pools,’ perhaps the most charming subjects that the microscope has ever revealed, the microscopist with a monocular instrument cannot possibly appreciate and interpret structure and movement in the same accurate manner that the binocular enables him to do. These facts should receive careful consideration when a microscope is to be chosen, but it must be borne in mind that the Wenham stereoscopic form of binocular cannot be used with an objective having a higher air angle than 40°. Provision is, however, always made whereby the prism may be with- drawn, and the light then only goes up the monocular or straight tube, and the instrument is to all intents and purposes as useful and convenient as the monocular micro- scope, while the unemployed eye of the observer is rested by looking into the blank binocular tube; the fact of its not being illuminated will scarcely be noticeable. For use with the binocular microscope, the closer the posterior lens of the objective is brought to the prism the better. In fact, some makers have constructed objectives in very short mounts, working quite on to the prism, up to a power of 3 inch. These, however, are not recom- mended. Dr. Carpenter some time since pointed out that when an objective having an air angle exceeding 40° was employed with the Wenham binocular, spherical objects became distorted, and instead of appearing round in shape they became conical. The special apparent advantage gained is therefore useless. Special high-power prisms are also made by two or three opticians, whereby higher powers may be employed with a binocular microscope; but we doubt whether the results obtainable with them are really worth the additional outlay. In those we have seen there has been such a very unequal illumination in the two tubes, that, personally, we should prefer using the instrument monocularly. It must be understood that all vision through the microscope in the 3 B4 Modern Microscopy ordinary way is inverted, that is, the object is seen upside down. A very good form of binocular microscope, devised by Stephenson and made by Swift and Son, erects the image, or, in other words, enables it to be seen the right way up. It is excellent for dissecting purposes, and high powers can be employed with it; still, it cannot be described as an all-round microscope, and would have to be classed with in- struments for special work. Our advice on the question of a monocular or binocular microscope is: If the instrument be required for strictly educational, scientific, or photo- graphic work, the monocular must be chosen. The bulk of the general amateur’s work is done with comparatively low powers, and in such cases the binocular is unquestion- ably of advantage, and to be preferred. If it is proposed to combine scientific with general work, a good plan is to have two separate bodies—monocular and binocular—inter- changeable in the same bearings. The maximum facility is then at the disposal of the user. It should be noted that when the two bodies are chosen, it is well to have centring screws to the rotating stage, as described page 13, because the bodies rarely have identically the same centres, and the stage could not otherwise be made to rotate concentrically with both bodies. MICROSCOPES FOR SPECIAL PURPOSES. Dissecting Microscopes.—The dissecting microscope is usually, so far as the stand itself is concerned, of plain, but substantial construction. It should have a good clear stage, preferably consisting of a glass plate, which can be interchanged with opal, glass, ebonite, etc., substantial supports for the hands; rack-work and pinion to focus the magnifying-lens ; and a gymbal, carrying a mirror on one side and a mat opal disc on the other. In this department Continental manufactures may be considered superior to the English on account of their variety of design, sub- stantial and excellent construction, and economy in price. Microscopes for Special Purposes 35 We figure one by Leitz, which for all ordinary work can be recommended ; in addition to this the same maker supplies Fic. 9.—Srmpite DissEcTING MIcRoscoPE. By E. Leitz. The supports for the hands are attached to the two small buttons shown on the edge of the stage. Fic. 10.—TuHe APLANATIC MAGNIFIER. an erected image dissecting microscope and a series of excellently corrected lowpes, or aplanatic magnifiers after . 3—2 36 Modern Microscopy Steinheil. Zeiss also makes most excellent dissecting microscopes and well corrected magnifiers. Dissecting microscopes of various patterns may also be found in the opticians’ catalogues. For many purposes a magnifier mounted on a plain stand with an extending arm, known as lens-holders or magnifying-stands, will be found very useful. It is eminently desirable that the magnifiers employed be of the aplanatized form, consisting usually of three lenses cemented together, similar to Fig. 10, p. 35. Many of them can be had so arranged in the mounting that they can be used either with a dissecting microscope or be carried in a suitable fitting in the pocket. Microscopes for Metallurgical Work.—Examination of metal by means of the microscope is a comparatively modern study, but there is probably no iron or steel works of standing that is not equipped with suitable instruments both for observing and photographing. By means of the microscope much information regarding both the chemical constitution and mechanical properties are disclosed, but it is especially valuable for the latter. For instance, the structure of steel varies with the degrees of hardness and the amount of heat to which it has been subjected, and it is possible readily to gain definite information concerning the suitability of the metal for the purposes to which it is to be put by means of the microscope. In the manufacture of guns any defect which may have taken place in the heating or quenching of the steel, which would render the gun unsafe or unsatisfactory, can be discovered before the manufacture is proceeded with. Engineers can detect flaws, blow-holes, defective welds, etc., at an early stage, and avoid the trouble incident to the use of imperfect metal in the finished article. Microscopes for this exclusive purpose require no sub- stage or mirror, and although it is customary to employ an ordinary microscope, so that the use may not be restricted to the one especial purpose, two or three well-designed Microscopes for Special Purposes 37 instruments are manufactured for this work alone. We illustrate one by C. Baker on page 87, which has the following features : The main support of the stage is carried in a dovetailed Fic. 11.—MErALLuRGIcAL Microscopz. By C. Baker. fitting, parallel with the body of the instrument, and can be raised or lowered by means of a rack-work and pinion. The stage itself has on the upper surface a levelling- plate on which the specimen for examination is laid. 38 Modern Microscopy Three screws permit of any want of parallelism between the faces of the specimen being compensated for. To illuminate a specimen a vertical illuminator, the con- struction and use of which is referred to on page 105 and which may be fitted at either the eye or objective end of the body-tube, is employed. The lamp and bull’s-eye have to be placed in fixed relation to this vertical illuminator, and it is important that once the illuminant is adjusted no more than a slight movement, such as would be imparted by the fine adjustment, should take place ; this renders obvious the utility of the rack-work for raising and lowering the whole of the mechanical stage. Other microscopes for the same Fic. 12.—Ho.preR FoR METALLURGICAL WORK. specific purposes are made by C. Reichert, of Vienna, and Queen and Co., of Philadelphia. The necessity for a special microscope for metallurgical work has, toa certain extent, been obviated by the introduc- tion of a carrier, which can be constructed to fit any ordinary microscope stage. It is shown in Fig. 12. The subject for examination is held between two jaws, which can be made to approach or recede from each other by means of screws, to which they are attached, the block of metal under examination is gripped between the jaws, and can be set at any desired angle. This is particularly useful where Jarge surfaces are required to be gone over or where the face of a cylindrical piece of metal is to be inspected, the cylindrical portion itself being suspended through the aperture in the stage. The addition of the vertical Portable or Travellers’ Microscopes 39 illuminator, referred to previously, with this fitting renders an ordinary microscope thoroughly serviceable for metal - lurgical purposes, but it would not, of course, have the vertical adjustment to the stage itself. Fig, 18.—TRAVELLERS’ Mrcroscorr. By J. Swift and Son, PORTABLE OR TRAVELLERS’ MICROSCOPES. A large variety of these are made, every maker having some pattern possessing especial merit. Among these may 40 Modern Microscopy be mentioned very excellent models by Swift and Son, R. and J. Beck, Leitz, and Watson and Sons, the last-named offering to construct a body-tube of a size to carry eye- pieces of any desired gauge, so as to save duplication of apparatus. ' The instrument illustrated in Fig. 18, by Swift, is of ingenious and exceedingly compact and efficient design. PETROLOGICAL MICROSCOPES. A variety of microscopes are made for this purpose only, but those who may wish to examine rocks and crystals and to use their instrument for ordinary work as well will find it advisable to have a concentric rotating stage with the periphery divided and to read by verniers, a polarizer having a divided rotating circle, and immediately above it and fitting in the sub-stage with it a condenser of large aperture. The analyzer may be arranged to fit together with divided circles over the eyepiece of the instrument. Cross-webs can be fixed to the diaphragm of the eyepiece, and a calespar plate can be fitted immediately beneath the analyzer prism in its carrier over the eyepiece. In the regular: petrological microscopes these arrangements are already made, and in such, an extra analyzer is usually mounted in a box in the body of the microscope in such a manner that it can be pushed out of the field of view when desired. For petrological study exclusively, the instrument designed by Mr. Allan B. Dick and manufactured by Swift and Son, is usually conceded to be the most efficient pattern thatis made. The special feature of this is that the stage is fixed, and instead of rotating the object, the polarizing and analyzer prisms with the eyepiece, are made to revolve together. Much of the time which ordinarily is occupied in effecting the exact centring of the stage to insure con- centricity of revolution is thereby obviated. Fic. 14.—PzTroLocicaL Microscore. By Swift and Son ; designed by Mr. Allan P. Dick. 42 Modern Microscopy MICROSCOPES FOR PREPARING AND MOUNTING. A plain, substantial microscope-stand, which the user would have no compunction in soiling, is of inestimable value in the preparing and mounting of micro-slides, and a suitable one can be obtained for such a small sum that it is a pity that instruments designed for work of a higher grade should be employed for the purpose. We figure on page 48 an illustration of the type of microscope that will be found very serviceable for this purpose. When we mention that the cost of this is only 20s., that it is pro- vided with rack-work and pinion, and that it has the standard screw for objectives, and receives eyepieces of Continental diameter, it will probably need no further recommendation. THE SELECTION OF A MICROSCOPE. It will be well to summarize the conclusions arrived at in the consideration that has been given to the individual parts of the microscope in the foregoing pages. In the interests of the advancement of microscopy as a science, the best and most suitable means should be commended, and nothing can be said to encourage the perpetuation of such instruments as do not embody the accuracy of adjustment or convenience of design which the modern worker with his beautifully perfect optical acces- sories actually needs in order to derive all the benefit that his lenses are capable of yielding. In perusing the catalogues of the Continental makers it is impossible to find amongst them a single instrument which combines all those refinements which the English manufacturer has with such excellent judgment devised and fitted in his. We have had occasion to refer previously to the fact that the Continental sub-stages are provided with no centring screws, the stages, except in two cases,* have * Photographic microscopes by C. Zeiss and C. Reichert. The Selection of a Microscope 43 Fic. 15.—Stranp VI., py Lerrz. A very inexpensive microscope suitable for preparing and mounting specimens, no mechanical screws, and the fine adjustments, with rare exceptions, which have been duly acknowledged, are far too rapid in their action. 44 Modern Microscopy It is asserted by users of Continental microscopes, whose name is legion, that the British microscope exceeds the needs of the laboratory worker ; the only response to such a criticism from the expert is that there is a want of appreciation and education in matters microscopical in the laboratory. It is impossible to disregard the modern spirit which demands excessive rapidity of work at the cost of excellence and accuracy, and it is not too much to say that the man who examines structures with a good 7,-inch oil immersion objective and an Abbe illuminator has limited his knowledge to an extent which would cause him great surprise if he had but the opportunity of seeing the same subject properly illuminated with a sub-stage condenser having a suitable ratio of aperture to that of the objective and the microscope properly manipulated. The defects of the Continental microscope are in a large measure dimin- ished because of this very restricted cone of illumination which is yielded by the Abbe illuminator. The modern ;;-inch objective having a numerical aper- ture of 1°25 to 1:3 will bear a solid cone of illumination of 0°9, and when it is stated that the Abbe illuminator regularly provided, not only by Continental opticians, but also by English houses, only yields an aplanatic cone, under the most favourable circumstances, of 0°5, it will become only too apparent how utterly restricted the laboratory worker becomes in his work when outfitted with the apparatus which the Continental optician provides. When once the Abbe illuminator is abandoned and a suitable well-corrected condenser is substituted for it (for be it remembered the Abbe illuminator is not even achromatized) it will become necessary to improve the construction of the Continental stand. Centring screws must be perforce provided to the sub-stage ring; fine adjustments to the sub-stage, though not indispensable, will soon be found desirable, and the inven- tive faculty will quickly be brought into play for the provision of a slower acting fine adjustment. As matters stand to-day it is impossible to do other than advise that an English The Selection of a Microscope 45 microscope by one of the well-known makers should be selected, and the following would be the order of preference for the various mechanical fittings : Coarse adjustment by rack-work. Fine adjustment. Compound sub-stage with screws to centre. Mechanical movements for the stage. Mechanical draw-tube. Fine adjustment to sub-stage. Concentric rotation to the stage. Divided scales, as may be found necessary. 9. Other mechanical fittings, such as centring screws and rack-work to the rotation of the stage, rack-work rotation to sub-stage, ete. In amplification of the above we would remark that where questions of economy prevail the sub-stage may be replaced with an under-fitting having centring screws and the mechanical stage with a sliding bar. Many microscopes are made in plain form as a founda- tion on which as a super-structure many of the mechanical fittings can be subsequently mounted ; consideration might with propriety be given by a beginner to such instruments. Sra Le a TESTING A MICROSCOPE. The following are some points to be specially examined when purchasing a microscope: The motions should be perfectly smooth, with no lumpy feeling, and there should be no backlash. This latter can be detected best by holding gently the part that is actuated by the pinion, and then attempting to rotate the pinion. If the pinion rotates at all, or a movement of it can be detected with- out a corresponding motion on the movable part, there is backlash. Then, there should be no shake in any of the fittings. In a badly-constructed microscope, even when the fittings are in their most advantageous position, by holding them and shaking them slightly, a movement in the slides 46 Modern Microscopy can be detected. The body should be racked up a consider- able distance to see whether any rock or shake beyond that of the tension on the bearings can be detected. An instru- ment sound in construction should exhibit none whatever. The stage should be treated in the same way. The next point is to ascertain that the body and limb are perpendicular to the stage; this should be tested with a metal square, such as is generally used by brass workers, engineers, and others, and can be purchased at a tool-shop. If there be any inaccuracy in this respect it is important that it be discovered before the microscope is used, other- wise no objective can possibly work at its best. Reference Fic. 16.—MECHANICAL STAGE SHOWING THE ‘SpruNG’ Firtines WITH ADJUSTING SCREWS. has already been made to the necessity for seeing that there is no lateral movement in the fine adjustment and that the sub-stage centres accurately. Beyond this it is well to put a square between the upper surface of the sub-stage and the under-side of the stage to ascertain that they are parallel and that the sub-stage has been mounted with accuracy. It is also of importance that each fitting in which move- ments are effected should be provided with slots and screws, '' by which the effects of wear and tear can be taken up. This is known as ‘springing’ the fittings, and the system is shown very clearly in Fig. 16. Screws are placed at F and G, which, on being tightened, produce a compression of the fitting in which the slots are made, and wear can quickly be adjusted for. Testing a Microscope 47 A good idea of the comparative quality and finish can often be obtained by examining some hidden or unnoticed part, and observing whether the same care in finishing has been exercised there as in parts that are seen. For instance, if some microscopes be examined underneath the foot, they will be found left in the rough as cast, and merely blackened over; while another instrument will be found carefully finished in that part. It would not necessarily follow that the former was a bad instrument, but it would often be found, if taken to pieces, that there was not a careful fitting in working parts that did not catch the eye, and there would be a probability of its not proving so durable as the better-finished instrument. CHAPTER II. OPTICAL CONSTRUCTION. Preliminary Note. In the former part of this book we have dealt exclusively with the stand, or mechanical means of employing the optical system and accessories; and important as it is that those details shall be very efficient, it is, if anything, still more so that the eyepieces, objectives, and illumi- nating apparatus shall be of the most perfect description, properly adapted and intelligently employed, for on the optical combinations depend the results that are to be obtained with the stand; and although care and trouble may enable a person to use a bad stand, no good stand can ever compensate in any way for bad objectives. It requires constant practice and a long apprenticeship to learn to use the microscope to the utmost advantage. Livery special subject of examination calls for special manipulative treat- ment if it is to be correctly understood and appreciated. Experience alone can guide in obtaining the best result under varied circumstances, and that experience must be based on a knowledge and understanding of correct methods in working. Definitions. Some of the following terms will be made use of in this book, and are constantly met with in literature on micro- scopical subjects ; a brief explanation of them may therefore prove of service. Optical Construction 49 Achromatic Correction.—Owing to the relatively greater dispersive power of flint-glass (containing lead or other heavy metals) as compared with crown-glass, it is possible to produce a combination of a convex lens of crown-glass with a concave lens of flint, which collects rays like a simple convex lens, but which unites two different colours in the same focal point, thus in a great measure correcting the chromatic aberration. Aplanatism.—A freedom from spherical aberration (see below). Apochromatic Correction.—The highest attainable correc- tion of microscope objectives, comprising the correction of spherical aberration for all colours, and the union of three different colours in one focus, thereby eliminating the secondary spectrum. Chromatic Aberration. — White light is the composite effect of a continuous range of colours, passing from red, through yellow, green, and blue to violet (see Spectrum). All transparent media have different refractive indices for these different colours, and as a consequence after their passage through a simple lens the rays do not unite at one focal point. The red rays, being the least refrangible and bending to the smallest extent, unite at the farthest distance from the lens; the orange and green rays unite at points closer to the lens; while the violet rays come to a focus at a point nearest to the lens. The confusion of different coloured images resulting from this dispersion is termed ‘ chromatic aberration.’ Chromatic Over-correction. — A term used when a lens brings yellow or even orange rays to the shortest focus and best correction. Chromatic Under-correction.—A term applied to a lens when rays towards the blue end of the spectrum are best corrected. Thus a photographic lens is visually under- corrected. Diaphragm.—This is generally understood in optical instruments to be a circular opening in a plate that is 4 50 Modern Microscopy used to cut off the marginal portions of a beam of light, and in this sense is referred to in this book. The diaphragm. is often improperly called a stop. Diffraction Spectra.—lIf we look through a finely-ruled grating at a gas or candle flame, we shall see a large number of images of that flame having the colours of the spectrum. This effect is due to diffraction. In the microscope, objects having fine and regularly spaced markings diffract the light in a similar manner, the resulting diffraction spectra being plainly visible at the back of the objective. According to the Abbe theory of microscopic vision these diffraction spectra determine the character of the image seen, the latter becoming less like the real structure when the number of diffraction spectra admitted by the objective is reduced, a faithful representation of the object being obtainable only when all diffracted light of sensible brightness is admitted. A further note on this interesting subject by Dr. G. John- stone Stoney, F.R.S., will be found in an appendix on page 124. Female and Male Screws.—The former is a threaded fitting which receives a screw, and the latter a screw which goes into the female fitting. In the case of a bolt and nut the former would have the male, and the latter the female screws. N.A.— Abbreviation for numerical aperture. See page 60. Negative Eyepiece.—This is an eyepiece for examining an image formed at the diaphragm set between the two com- ponent plano-convex lenses. The Huyghenian is the best- known form of negative eyepiece. O.1.—Abbreviation for optical index. See page 64. Positive Hyepiece.—This is an eyepiece for examining an image situated beyond the field lens. It can consequently be used as a magnifying-glass, etc. Refractive Index.—When a ray of light passes obliquely from one medium into another of different density, the path of that ray is bent or altered in its course. According to Optical Construction 51 the law of refraction, there is a constant ratio for any given two media between the sine of the angle of incidence (being the angle included between the incident ray in the first medium and the perpendicular) and the sine of the angle of refraction, or of the angle included between the ray after refraction and the same perpendicular. The numerical value of this ratio for a ray passing from air into a medium is called the refractive index of the medium. Secondary Spectrum.—In an achromatic lens the chro- matic aberration is corrected for the brightest (yellow or green) rays of the spectrum, and the pronounced colour shown by uncorrected lenses is in consequence removed. A stricter examination, however, shows that rays of a different colour are not brought to the same focus, for owing to the fact that flint-glass, as compared with crown-glass, disperses the more refrangible rays relatively too much, and the least refrangible relatively too little, a peculiar secondary spectrum results from the achromatic combination, the rays corre- sponding to the brightest apple-green part of the ordinary spectrum being very closely united and focused nearest the combination, whilst the other colours focus at increasing distances in pairs, yellow being united with dark green, orange with blue, red with indigo. The composite effect of these colours is best seen with oblique light, causing dark objects to have apple-green borders on one side and purple ones on the other. Semi-apochromatic Correction.—In achromatic microscope objectives of the older type, chromatic defects that are worse than the secondary spectrum are caused by spherical aber- ration of the coloured rays, the spherical aberration being corrected for the brightest part of the spectrum only. Objectives made entirely of glass, and therefore showing the secondary spectrum, are called semi-apochromatic when the spherical aberration is corrected practically for all colours. Spectrum.—A band of colours produced by the splitting up of white light by means of a prism. The order of the colours is: red, orange, yellow, green, blue, indigo, violet. 4—2, 52 Modern Microscopy Spherical Aberration.—Rays of light passing through the marginal portion of a lens come to a focus nearer to the lens itself than those rays which pass through the centre of the lens, and the interval bétween the focal points of rays which pass through the marginal and the central parts of that lens is the spherical aberration. In compound lenses this spherical aberration can be corrected for one or more special rays, and a lens so corrected is called aplanatic. It is only truly aplanatic for the particular rays for which it has been accurately corrected. Spherical Over-correction is present when a lens limits the marginal rays at a greater distance than the central rays. Spherical over-correction is indicated when the marginal rays focus closer to the lens than the central ones. Spherical Zones.—In objectives of considerable aperture the intermediate rays may show decided spherical aberra- tion, although tke central and marginal rays are united. This defect is meant when spherical zones are spoken of. The degree to which spherical zones are corrected deter- mines chiefly how large a cone of illumination, and how deep an eyepiece an objective will bear before ‘ breaking down.’