\' f - THE GETTY CENTER LIBRARY balcowan. The Scovill Photographic Series. Price, Per Copy. No. i —THE PHOTOGRAPHIC AMATEUR.— By J. Traill Taylor. A Guide to the Young Photographer, either Professional or Amateur. (Second Edition.) Paper covers.. $o so Library Edition. ' x OQ No. 2.-THE ART AND PRACTICE OF SILVER PRINTING.—By H. P. Robinson and Capt. W. de W. Abney, R.E., F.R.S. (Third Edition.) Paper covers. Library Ed ition.. No. 3.—Out of Print. No. 4.—HOW TO MAKE PICTURES.—By Henry Clay Price. (Fourth Edition.) The A B C of Dry-plate Photography. Paper covers. Library Edition. No. 5.-PHOTOGRAPHY WITH EMULSIONS.-By Capt. W. de W. Abney, R.E.. F.R.S. A treatise on the theory and practical working of Gelatine and Collodion Emulsion Processes. (Second Edition.) Paper covers, Cloth bound. No. 6.— Out of Print. No. 7.-THE MODERN PRACTICE OF RETOUCHING NEGATIVES-As practiced by M. Piguep6, and other celebrated experts. 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Robinson. Finely illustrated from the Author’s own photographs and containing a photogravure frontispiece of the Author. Cloth bound.■. 1 50 No. 28.-THE AMERICAN ANNUAL OF PHOTOGRAPHY AND PHOTO GRAPHIC TIMES ALMANAC FOR 1889. Finely illustrated. Paper (by mail, 12 cents additional).... 50 Library Edition. 1 00 No. 29.-THE PROCESSES OF PURE PHOTOGRAPHY—By W. K. Burton and Andrew Pringle. A standard work, very complete and freely illus¬ trated. Paper covers...'. 2 o° Library Edition. 2 50 No. 30.-PICTORIAL EFFECT IN PHOTOGRAPHY.—By H. P. Robinson. A new edition. Illustrated. Mr. Robinson’s first and best work. Cloth bound. 1 50 No. 31.-A DICTIONARY OF PHOTOGRAPHY.—For the Professional and Amateur Photographer. By E. J. Wall. Illustrated. Cloth bound. 1 50 No. 32.—PRACTICAL PHOTO-MICROGRAPHY, by the Latest Methods. By Andrew Pringle. Illustrated. No. 33.—THE AMERICAN ANNUAL OF PHOTOGRAPHY AND PHOTO¬ GRAPHIC TIMES ALMANAC FOR 1890. Paper cover, (by mail, 14 ceuts additional). 50 Library Edition, (by mail, 18 cents additional). . . x 00 THE SCOVILL & ADAMS COMPANY, Pu ishers, 423 Broome Street. NEW YORK CITY, U.S.A. Frontispiece. Foot of Spider, X 150 . PRACTICAL Photo-Micrography: BY THE LATEST METHODS, BY ANDREW PRINGLE, F.R.M.S., President of the Photographic Convention of the United Kingdom, 1889 . Joint-Author of “Processes of Pure Photography ' etc., etc. NEW YORK: THE SCOVILL & ADAMS COMPANY. 1890 . C.° . GOT 2sr TO THE MEDICAL PROFESSION, THIS BOOK, PARTLY INTENDED TO PROMOTE PHYSIOLOGICAL RESEARCH BY PHOTOGRAPHIC METHODS, IS DEDICATED. Copyright, 1890 , By The Scovill & Adams Company. THE GETTY CENTER LIBRARY PREFACE. No apology seems to be necessary for this book. So far as I know, no attempt has been made by any person apparently up in the subject to give instructions for Photo-micrography, since time has elapsed for proper trials of the two great and novel features of the Science, Apochromatic Objectives and Color-correct Plates. My experience, though limited in dura¬ tion, has been gained by assiduity and very varied practice, and the whole, practically, of my study has been directed to the investigation and utilization of our two new tools above named. I have written deliberately and confessedly for the Natural and Medical Sciences rather than for those who use the micro¬ scope as a pastime. But I have not overlooked the intelligent investigators of such subjects as diatom-structure and the like; for in objects whose real structure is so problematic lies not only a valuable field for optical research, but much of the future of practical optics, and perhaps the very best manege for the student photo-micrographer. I boast, however, to have written something that will materially help those who study the Natural Sciences in general rather than Optical Science in particular. I have to thank many friends for help of various kinds. Mr. Edward M. Nelson, my instructor in practical microscopy, has placed me under the most marked obligation, for he kindly read my M.S. so far as it touched optical theories, and he directed me where necessary. I owe so much generally to 4 PREFACE. Dr. R. L. Maddox that I need not particularize any late favors. I thank certain opticians for blocks which appear in these pages, notably Mr. Powell, Mr. Swift, Mr. Beck, Mr. Baker, and Mr. Newton. Apology is perhaps due for over frequent reference to a book in the writing of which I took part. It seemed better to refer even to my own writings than to waste space in the present book. Andrew Pringle. CONTENTS Preface, - Description of Plates, CHAPTER I. Introductory and Historical, - Preparations for Work, Optical Apparatus, CHAPTER II. CHAPTER III. CHAPTER IV. Optical Apparatus Continued, CHAPTER V. Illumination. CHAPTER VI. Photo-Micrographic Apparatus, .... CHAPTER VII. Requisites for Photography, - CHAPTER VIII. Solutions for Photographic Operations, CHAPTER IX. On the Selection of Plates, - CHAPTER X. The Condenser and Bull’s-Eye—Their Use and Abuse, CHAPTER XI. The Use of the Eye-Piece or Ocular—Stops—Reflections, CHAPTER XII. Progressive Examples, ..... PAGE. 3 7-10 11 17 22 33 39 47 55 61 65 69 74 79 6 CONTENTS. CHAPTER XIII. page. Exposure, 88- CHAPTER XIV. Development of Gelatine-Bromide Plates, - - - 91 CHAPTER XV. Operations Following Development, - - - 101 CHAPTER XVI. Color-Correct Photography, - 105 PART II. Introductory, ------- 113 . CHAPTER XVII. Printing on Albumenized Paper, .... - 115 CHAPTER XVIII. Gelatine Chloride Paper, - 121 CHAPTER XIX. Printing on Bromide Paper, - 123 CHAPTER XX. The Platinotype Process, ..... 130 CHAPTER XXI. Enlarging, - -- -- -- - 134 CHAPTER XXII. Lantern Slides, ------- 143 CHAPTER XXIII. Use of the Optical Lantern, - 155 CHAPTER XXIV. Immersion, Apochromatics, and Aperture. Oculars, - 162 CHAPTER XXV. Classification of Objects—How to Treat Them, - - 167 CHAPTER XXVI. Black Backgrounds. Opaque Mounts. Polarized Objects, 174 PLATES. Frontispiece. Foot of Spider, X 150. Color of object—Brown-yellow. Objective—16 mm. apochromatie. Screen used—Pale yellow. Ocular—Projection No. 3. Condenser—Lower part of achro. by Swift. Light—Lime. Plate—Landscape gel. brom. Exposure—5 minutes. Development—Pyro ammonia. Plate I. Critical image, X 375. Objective — 6 mm. apochromatie, N. A. • 95, Zeiss. Ocular—Projection No. 3. Condenser—Achromatic 140 deg. Light—Lime. Plate—Lantern-slide gel. brom. Exposure —2 minutes. Development—Pyro ammonia. Injected Villi, Intestine of Rabbit, X 30. Preparation by E. C. Bous- field, L.R.C.P. Color—Very dense orange-yellow. Objective—Swift 1 inch. Screen—None. Ocular—None. Condenser—Lower part of achro. with author’s ground glass (see p. 71). Plate—Eosin.—Light-Lime. Exposure—14 minutes. Development—Pyro ammonia. Hairs on Proboscis of Blowfly. Color of image—Brown. Screen—None. 8 PLATES. Plate II. 1 . A. Margaritaceus, X 150; General appearance. Objective—16 mm. apo. at N. A. ‘30. Ocular—Projection No. 3. Condenser—Lower part of achro. Light—Lime. Plate—Thickly coated landscape. Exposure—40 seconds. Development—Pyro ammonia. 2 . Triceratium. Critical image of secondary structure, X 750. Objective—3 mm. apo. at N.A. 1. Ocular —Projection No. 3. Condenser—Powell and Lealand used dry. Light—Lime. Plate—Thickly coated landscape. Exposure—45 seconds. Development—Pyro ammonia. Plate III. l. “Taste-buds ” in Tongue. X 100. Preparation by E. Klein, M.D., F.R.S. Stain—Logwood. Objective—24 mm. apo. Zeiss. Screen—Pale yellow. Ocular—Projection No. 3. Condenser—Achro. lower part, with author's ground glass. Light—Lime. Plate—Eosin. Exposure—15 seconds. Development—Pyro ammonia. 2 . •Typhoid bacilli in Intestine. X 750. Preparation from the Research Laboratory, Royal College of Physicians, Edinburgh. Stain—Gentian violet. Objective—3 mm. apo. at about Screen—Medium yellow. N.A. 1. Ocular—Projection No. 3. Condenser—Zeiss. Apo. N.A. 1. Light— Lime. Plate—Eosin. Exposure—90 seconds. Development—Quinol-caustic. PLATES. 9 Plate IV. B. Anthracis in blood. X 750. Stain—Deep gentian violet. Screen—Medium yellow. Spermatozoa of Triton, X 1000 . Stain—F uchsine. Screens—1 pale, 1 dark, yellow. Preparation by W. T. Wilson, V.S Objective—3 mm. apo. Ocular—Projection No. 3. Condenser—Achro. at about 140 deg. Light—Lime. Plate—azaline stained. Exposure—35 seconds. Development—Pyro ammonia. 2 . Preparation by G. F. Dowdeswell. Objective—2 mm. apo. Ocular—Projection 3. Condenser—Achro. 140 deg. Light—Lime. Plate—Eosin. Exposure—About 2 minutes. Development—Pyro ammonia. Plate V. "Pneumococci,” encapsuled. x 500 . Stain—Fuchsii e-deep. Screen—None. Objective—oil. Swift. Ocular—none. Condenser—Achromatic 140 deg. Light—Oil lamp. Plate—Ordinary rapid, Exposure—about 9 minutes. Development—Pyro ammonia. 2 . Flagellated Spirilla (? serpens) x 800 . Maddox, M.D. Unstained. Screen—Medium yellow. Preparation by R. Objective—3 mm. apo. Ocular—Projection No. 3. Condenser—Acromatic. Light—Lime. Plate—Landscape. Exposure—2*4 minutes. Development—Pyro ammonia. 10 PLATES. Plate VI. 1. Bacilli Tuberculosis in Lung of Horse, x 750. Preparation by W. Watson Cheyne, F.R.C.S. Stains—Bacilli-red. Tissues—Pale-blue and yellow. Screen—Deep yellow. Objective—3 mm. apo. used at N.A. 1. Ocular—Projection No. 3. Condenser—Powell and Lealand, used dry. Light—Lime. Plate—“ Isochromatic.” Exposure— 2 % minutes. Development—Q u i n o 1 (hydro- quinone) and caustic soda. 2 . B. Tuberculosis by Inoculation. Lung of Rabbit, x 600 . Prepara¬ tion by E. Klein, M.D., F.R.S. Stain—Red and blue. Objective—3 mm. apo. Screen—Medium yellow. Ocular—Projection No. 3. Condenser—N. A. I. Light—Lime. Plate—Eosin. Exposure—minute. Development—Quinol caustic. The reader is requested to note that these illustrations are given not as perfect specimens of work, nor as the best that have been produced by the author ; they are given to illustrate various types of subjects and varied treatment. Fig. 2 of Plate VI. is not a good representation of the original negative. Practical Photo-Micrography. CHAPTER I. INTRODUCTORY AND HISTORICAL. It would be out of place and inconvenient for the writer here to enter into any elaborate apology for the Science of Photo-micrography, or to occupy useful space with a lengthy discourse on the reasons that led him to make a study of the Science. Any one of a scientific turn of mind, and especially one who has more or less mastered the science of Photography, could not, on entering the microscopic world, fail to realize how great a boon Photography might become to microscopy, if the photographer were a good microscopist and the micro- scopist a skilled photographer. Should the world ever possess in one man a skilled and careful microscopist and an experi¬ enced and versatile photographer, a very great step may be expected not only towards solutions of many present enigmas and towards future discoveries, but also towards a very satis¬ factory and very convincing medium for publishing and cer¬ tifying the solutions and discoveries. The potential value of Photography in this line has always been admitted and often dwelt upon; but difficulties, some real, some exaggerated, some imaginary, have always been cited as fatal to the employment of Photography for the delineation 12 PRACTICAL PHOTOMICROGRAPHY. by a graphic method of microscopic images. That difficulties, and often great ones, present themselves is matter of fact, but to smooth over some of these difficulties, to evade some and to conquer others, it is the writer’s ambition to assist the reader. Two classes of photo-micrographer are met with in the ordinary course; the first a microscopist of more or less ex¬ perience and skill, who suddenly bethinks himself that photog¬ raphy seems an easy and rapid way of graphically represent¬ ing his objects and his observations; he never doubts that he may in a very short time master the elementary troubles of photography, and accordingly he embarks in a cockle-shell on the most stormy waves of a great and growing science. The other class of photo-micrographer is the photographer, usually the amateur, who thinks he has conquered the realms of land¬ scape and portraiture, and sighs only for fresh conquests; and so he plunges blindly into the science which of all others re¬ quires practice, perseverence and acute powers of observation. The results are shame and calamity to photo-micrography as a utility, as an educator, as a science. Photo-micrography has not yet taken the place it deserves, demands, and shall finally take. It is much to be regretted, but it is true, that we have so many books on photo-micrography written by men who are only smatterers in microscopy and totally ignorant of anything but the very rudiments of photography. Much has been well written on the subject, but the science is advancing so rapidly that what was in the front one week is “ exploded ” and improved out of knowledge the next week. Even were it desirable, our space makes it impossible, to enter at any length on the history of photo-micrography. Some assign the credit of the first photo-micrograph to one person, some to another; but we confine ourselves to saying that among the earliest workers in this line were Wedge wood, Rev. J. B. Reade (about 1837), Mr. Dancer (1840), Mons. Donne (1840), Mr. Archer (1851), etc. Among the early workers was Dr. R. L. Maddox, admittedly the suggestor of gelatine emulsion for photography, and still alive to view the results of his own work and talents, and to lend a helping hand and PRACTICAL PHOTO-MICROGRAPHY. 13 give valuable instruction to beginners as he most generously did to the writer. No man has worked more perseveringly or more successfully in this branch of science than Dr. Maddox, and no man ever got less reward—beyond that of conscious merit—than our good friend. His photographs of Pleuro- sigma angulatvm X 3,000, of many micro-organisms, and of various other subjects still rank among the best works that have been produced. The work of Dr. Woodward of the U. S. Army was so re¬ markable and so excellent as to mark, or even to make , an era of itself. This scientist was a microscopist of the very fore¬ most rank, a generous government placed at his disposal the very best instruments, and the results amply justified the country’s confidence, for Dr. Woodward’s photo-micrographs of Arnjjhipleura pellucida , of Nobert’s test plates, of the well-known test “ Podura scales,” as well as of many physio¬ logical and pathological subjects have seldom if ever been equalled. If these works of Dr. Woodward’s are ever beaten the superiority will be due to late improvements in optical ap¬ pliances, and to the use of more sensitive “ color-correct ” plates for photography. Dr. Koch, the eminent authority on micro-organisms, has produced fine photographs of bacteria, and Dr. E. M. Crook- shank, Professor of Bacteriology at King’s College, London, has published not only a number of photographs of Bacteria but also, in a succinct and ably written book, his methods of producing his photographs with capital diagrams of his ap¬ paratus.* The Diatomacese, as might be expected from their beauty, have always been favorites with the photo-micrographer, but from their formation they have always proved severe tests for the powers of those who have attempted them. Besides the micrographs of Dr. Woodward we must notice some very fine work by Dr. Mercer, also an American citizen. Drs. Aber¬ crombie and Wilson in England were very successful in photo¬ graphing the diatomaceae, and lately Messrs. A. Truan y Luard * Photography of Bacteria, by Edgar M. Crookshank, M. B., F. R. M. S. London: H. K. Lewis. 1887. 14 PRACTICAL PHOTO-MICROGRAPHY. and Otto 1ST. Witt, the former of Spain, the latter of Prussia, have succeeded in producing a magnificent set of photographs of the Diatomaceae of Hayti, West Indies. These collabora- teurs found that they obtained the best results, or rather the only good results, by using the wet collodion process. The writer, while inclined to traverse this assertion of the superior¬ ity of wet collodion over suitable gelatine emulsion, will have occasion to advert to the modus operandi of these undoubtedly skillful and successful workers in a later part of this book. Dr. R. Zeiss, of Jena, has lately exhibited certain photographs of A. Pellucida and P. Angulatum , which in Britain excited considerable comment of a highly favorable kind. The late Isaac H. Jennings produced some very creditable photographs of diatoms, notably one of N. Lyra / and his treatise on Photo¬ micrography is one of the best in the English language, though late optical and photographic advances have made the book a little out of date.* Another work worthy of perusal, on account of the careful treatment in brief space of the opti¬ cal part of the subject, is that by Dr. E. C. Bousfield, who is not only an adept with the microscope but has been highly successful in the department of photo-micrography, f Still confining himself to work that he has seen, the writer would now draw attention to the magnificent “ critical image- photographs” of Mr. E. M. Nelson,of London. A microscop- ist of long and varied practice, of consummate skill, and pos¬ sessed of an intimate knowledge of microscopic optics, Mr. Nelson has laid himself out for the most difficult branches of photo-micrography, the photography of the highest possible resolutions of such subjects as muscle-fibrils, “ secondary struc¬ ture ” of the diatomaceae, and ordinary diatom structure of the most delicate kind. To Mr. Nelson the writer owes practi¬ cally all the knowledge he has of microscope-manipulation, and to Mr. Nelson’s unstinted instruction and careful explana¬ tions the writer is indebted for any measure of success lie has * Photo-micrography; or, How to Photograph Microscopic Objects. By I. H. Jennings. London: Piper & Carter. 1886. f Guide to the Science of Photo-micrography. By Edward C. Bousfield, L. R. C. P. London : W. Kent & Co. 1887. PRACTICAL PHOTO-MICROGRAPHY. 15 had in this branch of science; not forgetting the unwearying kindness of Dr. Maddox when the writer was an entire novice in everything microscopic. A great many other names would have to be mentioned did this chapter profess to be a history of the science with which we are dealing. Drs. Draper and Sternberg of America have done more than “Yeoman Service ” in this line; Mr. Wen- ham, Mr. T. Charters White, Mr. Shadbolt, Dr. Lionel Beale in England; in France Dr. Miquel and others; in Italy Count Abbe Castracane; and ISTeyt in Belgium—all have made their marks in photo-micrography. Dr. Heneage Gibbes, now Pro¬ fessor of Physiology in Michigan, and Mr. F. EL Evans, of London, have produced some useful photographs of physio¬ logical preparations. So of late years have many other per¬ sons, for the value of photography in this line is daily attract¬ ing more and more attention. It appears to the writer that for the instruction of a class of students in such branches as histology, physiology, pathology, bacteriology, etc., no method can equal the use of an Optical-lantern slide projected upon a screen, the room being temporarily darkened; there are no such difficulties or inconveniences as would at first sight ap¬ pear, and a chapter of this book shall be devoted to the sub¬ ject, in the hope that attention may be called to this very scientific and convenient means of imparting instruction. A few words may aptly be written on what the writer claims as the advantages of photo-micrography over other ex¬ isting methods of delineation. In the first place, “ Personal Equation,” or perhaps “Personal Prejudice,” is almost entirely eliminated. A dishonest man may possibly arrange his pho¬ tography so as to bear out his own previous assertions, but a candid person doing his best to secure truth will be confident * that what his photograph shows represents what his lens “ saw,” and those who see his photograph will know that it re¬ presents the object in one aspect at least. The aspect may be a delusive one through optical mismanagement, but it must be one aspect of the object. Photography certainly cannot lie, but the photographer may be a liar or a fool. For this reason the photo-micrographer must not only be unprejudiced and honest, he must also be a microscopist and know his object. 16 PRACTICAL PHOTOMICROGRAPHY. Many microscopic objects are so fine in substance, others so intricate in structure, that the human hand is unable by any device to draw them anything like accurately. No line visible to the naked eye is too fine for photography to limn, no struc¬ ture too intricate for the pencil of light to follow. By photo-micrography a vast amount of time is saved. A few minutes of work may furnish us with a matrix for a thousand prints, one of which, if possible at all, could not be produced by hand in many hours. By photography are done with moderate ease and complete accuracy many subjects which by hand could not be done at all, witness moving objects, and appearances rapidly changing. Many other claims might be made for photo-micrography, but we shall cite only one more. Of all the intellectual and scientific pursuits that can be named, no one possesses so great, so varied fascinations as photo-micrography. There is mental food and mental exercises for every one; microscopy in all its varied branches and with all its interests—optics, mechanics, chemistry; and best of all, a practical, visible, permanent, use¬ ful result—an education and an educator! Note. —Since the above was written, Drs. Fraenkel and Pfeiffer, of Berlin, have begun to publish a set of photo-micrographs of bacteria which may be called splendid. They use daylight, apochromatic lenses and orthochromatic plates. CHAPTEK II. PREPARATIONS FOR WORK. A great deal of time, trouble and expense will be saved by a careful consideration of certain matters before any attempt is made at actual work. It is probable that after actual work has been carried on for some time alterations may be found necessary and improvements may suggest themselves; but the earlier. lessons of practice and experience will be well nigh wasted if there is not in the first arrangements a certain amount of forethought and fore-knowledge. We shall here, therefore, endeavor to smoothe the way for the beginner by giving hints, as close as possible to directions, on such matters as apartments, and apparatus of a general nature, leaving a more detailed de¬ scription of special apparatus for future chapters. A great advantage will arise from having two apartments communicating with each other for the two branches of the work : the microscopic or exposure work, and the photographic nr development work. If two communicating apartments are not available, two adjacent ones may do almost as well; or with certain evident precautions one chamber may be used for both lines of operation. Wherever the apartment for making the exposure may be, it is of the utmost importance that it be steady; with a good sound floor, and as far as possible removed from house or street traffic. No place is usually so suitable as a basement or half-basement cellar, and if this ground apart¬ ment have floor and walls of cement, and a strong ceiling, it will probably be perfectly adapted for the work. But even such an apartment should be as far as possible distant from street traffic, if the finest photo-micrographic work is intended. Mr. E. M. Nelson, alluded to in last chapter, works in a cellar such as just described, using for his apparatus a base not only itself very heavy but further weighted with several hundred- 18 PRACTICAL PHOTOMICROGRAPHY. weight of lead, yet he never attempts his most critical work till after 9 p. m., though he does not live in a very busy thor¬ oughfare but in a quiet suburb of London. The writer, too, works in a cement-floored and walled lialf-basement in a very quiet village, but has had negatives spoiled by the tremor of a passing ’bus or of a heavy step overhead. There should be no chance, therefore, of vibration or tremor in any part of the apparatus or apartment. And further, the whole of the appar¬ atus actually used to produce a negative—the light, the con¬ denser-system, the microscope, the camera and the plateholder —should all, during exposure at least, be practically one solid mass; in other words, the whole system must be firmly clamped to one base, and that base should be the floor if pos¬ sible. The entire system being clamped to one base, even if that base be not itself above suspicion, moving, all other parts move in unison, so that blurring may be avoided; we have heard of a plan by no means bad, wherein the whole system on a strong base was suspended by cords from a high support. If strong India-rubber formed part of the length of these cords the apparatus was probably very satisfactory, and the idea was surely ingenious. India-rubber cubes have often been found very beneficial as supports for the base-board of a system where tremor is inevitable. If by any chance the illuminant to be used is daylight, whether diffused or direct sunlight, this consideration must naturally play an important part in the selection of an apart¬ ment. If daylight is to be used for the development of the photographic plates, a course which we do not recommend, it is almost essential that the window of the room chosen face the North. The writer has but little experience of daylight as the illuminant for making the exposure in the camera, but where daylight, and especially sunlight, can be depended on, it is perhaps the best of all illuminating agents. In Britain we believe it impossible to obtain anything like uniform re¬ sults, though no doubt at odd times great successes may be, and have been, scored by sunlight. Now that photographic emulsions are made so sensitive to actinic light, the great ad¬ vantage of sunlight is of les3 consideration, while the superior PRACTICAL PHOTO-MICROGRAPHY. 19 equability of artificial light makes it a far more certain factor in our work than ever daylight could be, even at its best and steadiest. The fact that some magnificent work—as of Drs. Woodward, Maddox and others—has been produced by daylight only leads us to regret that these eminent workers had not the advantages we have of equable radiants and very sensitive plates. There are, however, over and above the actinic value of daylight, certain qualities in daylight that make it, in spite of its inferiority in some respects, invaluable if not essential for certain kinds of work, and we do not wish to be taken as despising or rejecting daylight entirely as our radiant. No treatise on this subject would be complete without more than a passing allusion to the use of natural light as the radiant, and accordingly the subject shall be treated in such manner as is within our power. Another advantage worthy of note, though always over¬ looked, pertaining to basement apartments, is the equability of temperature. The writer’s half-basement operating rooms do not vary 10 deg. Falir. in the course of the entire year; never unbearably cold in winter they are a refreshing change from the outside heat of summer. This is of more importance to the advanced photo-micrograplier than might appear; the microscopic apparatus keeps better and works better in such conditions, and in such a room, in cold weather especially. The heat arising from the radiant of whatever kind it be, pro¬ duces much less serious effects on the apparatus, and much less time is required for the parts to “ settle ” into their places. (Seep. 173.) The above remarks are intended for those who propose to enter seriously into the work of photo-micrography. Those who propose only to work at odd times or on the easier sub¬ jects, will not require such perfect preparations, or such per¬ fectly adapted apartments; it will be well, however, for every intending worker to keep our hints in mind and to choose apartments as nearly as possible fulfilling our desiderata. The purely photographic exigencies of the work may be met either completely or partially according to the means and intentions of the worker. Development of a micrographic 20 PRACTICAL PHOTO-MICROGRAPHY. negative is a matter requiring most accurate visual observation, and that by a non-actinic light; the light in question must therefore be equable, ample, and “ safe.” Running water is so great a convenience that many other desirables should, if necessary, be sacrificed for a supply of water from a tap ; for this reason, and also by reason of the waste-pipe, we might pre¬ fer a room otherwise inconvenient if furnished with a tap, sink and waste-pipe, to an apartment in other respects more com¬ fortable and convenient. Critical photo-micrography requires at every stage com¬ plete concentration of thought. The worker must not be dis¬ tracted in the very slightest degree while “ setting up ” his object or while developing his negatives. All steps, therefore, should be taken to avoid confusion in the rooms of work. The fewer the bottles of chemicals in the dark room, and the more neatly they are arranged the better; presses and shelves should be provided for everthing required, which luckily is not much unless great digressions are to be made from the matter treated of in this book. Yery good work may be done with apparatus of ordinary quality, but the very best work will only be done, and can only be expected to be done, by the very best apparatus used with the utmost skill. We therefore counsel our Reader to begin with the best of everything, so far as he can afford to pro¬ cure it. It is false economy to buy mediocre instruments for a start, because if we succeed and persevere we are sure to require better implements as we advance in skill; while if we fail, become disheartened, and “ chuck the thing up,” our shoddy outfit is valueless. It is difficult to draw aline between “ easy ” and “ difficult ” photo-micrography, because each branch has its own difficulties. In low-power work which is often called “ easy” we have to meet difficulties of uneven illumination, uneven surfaces and puzzling colors ; in high-power or wide angle work, we have to contend with difficulties in optics, in vibration, and in photo¬ graphic technique. But always the best instruments produce the best work, though often the best instruments are the most difficult to work to their best advantage. PRACTICAL PHOTO-MICROGRAPH?. 21 If it is true, and we assert that it is so, that the first neces¬ sity is a knowledge of the use of the microscope on our special object, it is no less true that he who begins photo-micrography without considerable experience in photography will have a hard task and many failures. The writer ventures to assert that an enormous deal of trouble and perplexity will be spared to the tyro plioto-micrographer if he practise carefully before¬ hand ordinary photography, specially of varied subjects, as Landscape, Interior, Portrait, and most of all, Reproduction of colored objects, as Paintings. The greatest stress will in this book be put on color-correct, or “ Ortho-Chromatic ” photo¬ graphy, because the writer is well assured that not only has color-correct photography brought about vast improvements of late in photo-micrography, but that color-correct photography is destined to be the means of placing our science in the posi¬ tion which it claims, and will sooner or later hold as the means for the delineation of microscopic images. In a book of Pure Photography, wherein the present author had the advantage of collaboration with Professor W. K. Burton, C. E., the writers have pointed ou t with considerable clearness the effect of variations in Exposure and Development in ordinary photo¬ graphy, and the Reader is strongly recommended to study this book, and to gain as much proficiency as possible in pure photo¬ graphy before starting on the special, but varied, operations required to produce good micrographic negatives and prints.* The present book will, however, treat photography on the supposition of total ignorance on the Reader’s part. The same course cannot be followed regarding the microscopic branch of the subject, for this matter is one of experience and long and close observation rather than one which can, by however much writing, be imparted from writer to reader. * The Processes of Pure Photography, by W. K. Burton, C.E,. and Andrew Pringle, F.R.M.S. New York, The Scovill and Adams Co., 1888. CHAPTER III. OPTICAL APPARATUS. Most of our readers are pretty sure to possess a microscope, some objectives, eye-pieces, a substage condenser and a bull’s- eye. (Note : The term condenser shall hereafter be used as applying solely to the “ substage condenserwhile “ bull’s-eye ” shall be used to cover all condensing or parallelizing instru¬ ments used between condenser and light). Some kind of so- called microscopic lamp is also likely to form part of the out¬ fit. As a rule a microscopic lamp not made specially for microscopy is for various reasons unsuited to our purpose. Such adjuncts to a microscope as a Polariser, a Paraboloid, a Spot lens or other arrangement for “ black ground illumina¬ tion ” shall be treated separately ; till further notice we shall treat of axial transmitted light illumination only. The Microscope. The qualities essential to a microscope- stand for our purpose are perfect rigidity, accurate working of all parts, specially racks, fine adjustments, stage mechan¬ ism, and draw tubes, if any are to be used. The substage ar¬ rangements as to focusing and centering are just as important as the other parts of the instrument. The writer proposes to lay great stress on the accurate use of the condenser, which is often treated in a careless and ignorant way, and sometimes even by certain authorities omitted altogether. When the func¬ tions of a condenser properly used are explained, it will be seen how serious an omission this is. All the best photo¬ micrographic apparatus we have ever seen, and all of which we have seen illustrations that gave any promise of excellence, have been used on the horizontal; on the other hand there are cases where the vertical position of the apparatus, i. e ., the horizontal position of the stage, is necessary. No microscope- stand will meet both conditions unless it swing on a pivot, Fig. 1 24 PRACTICAL PHOTO-MICROGRAPHY. which pivot should be so arranged with the working part of the swing that the microscope shall be steady at whatever angle from vertical to horizontal it is placed. At true horizontality the tube should be stopped by a “ stop ” for the purpose. The body-tube of a stand for our work can hardly be too wide in diameter so long as we do not reach the point of inconvenience in adapting eye-pieces, etc., to the tube. The finest stands made, so far as the writer knows, are those of Powell & Lealand, of London; they are perfect in rigidity, in mechanism, in workmanship and finish, and, though expen¬ sive, their owners say they are cheap in the long run. A very good stand is the “ Wales” pattern of Messrs. Swift and Son, London. (Fig. 2.) The advantage of this form of swing is that the centre of gravity is in the same position, and that a good one, to whatever angle the tube is swung. The writer used this stand for high power work with complete satisfaction, and that he gave it up was due not to the stand itself, but to certain exigencies which arose and could not be met with the then arrangement whereof this stand formed a part. Consequently this stand is highly recommended to those who desire a good stand for our purpose at a very moderate price considering the work, and the efficiency. For simple examina¬ tion of objects, this stand fitted with a Differential Screw Fine Adjustment, is a most satisfactory instrument for any kind of work. Beck’s Stands, which are easily procurable in America, are probably in no way inferior to others, but we cannot speak from personal experience. A most excellent stand is the one now exclusively used by the writer for photo-micrography. In general the stand is the so-called “ Nelson ” pattern, and the one in question was made by Mr. C. Baker, of London, specially for the writer. This stand was made extra heavy and strong in all parts, with a differential-screw fine adjustment most accurately fashioned, and there is a similar fine adjustment to the sub-stage. A very important feature not often attended to in stands of modest pretensions, is the rack work draw-tube for the cor¬ rection of objectives not furnished with “ correction collars,” 25 PRACTICAL PHOTO-MICROGRAPHY, or with insufficient range of collar correction. In critical work with certain objectives this draw-tube is an absolute p IG> 2 .—Swift’s Wales Stand. necessity, and the purchaser of a new outfit is recom mended to insist on a racking draw-tuhe, which will be found conven- lent in all cases, essential in some. 26 PRACTICAL PHOTO-MICROGRAPHY. These are the stands of which we have some experience or knowledge, but there are doubtless other stands well adapted to our purpose. Many of the so-called Students’ Stands are well and strongly made, their low price being due not so much to want of good material and work as to absence of all delicate and perhaps unnecessary mechanism. In particular, the pat¬ tern of Hartnack appears to us commendable among cheap but good microscope stands. One thing, however, in the cheaper class of stands ought always to be carefully tested before a purchase is made, the jine adjustment must work true and without jerk, deviation or “springand it must be firm and not “ give ” in the slightest degree when the tube is at any angle. The long lever adjustment of Powell and Lealand’s best stands is considered the acme of perfection, and the differential screw known as “ Campbell’s ” is also very trustworthy when well made. Mr. Swift has lately devised an additional inde¬ pendent safety spring to be used in conjunction with the Campbell screw fine adjustment. PRACTICAL PHOTO-MICROGRAPHY. 2 7 Doubtless, any well made stand may be used with complete success. Much very fine work has been produced with cheap microscopes not designed for photo-micrograpliy ; so that any one possessing any good microscope and a moderate amount of ingenuity and neatness of hand need not despair of success, should his pocket not admit of the purchase of a new and specially adapted stand. One of our diagrams will show how easily any stand may be used for photo-micrography at an odd time or in a casual way. The Objective, or Object Glass is, on the whole, the most important part of the apparatus. The first essential is that it be “ corrected for photography.”* Objectives made for ordinary purposes of observation are usually “ over-corrected,” a term that requires explanation. The rays which give the best visual effect being less refrangible than those exercising the greatest influence on chemical compounds such as we employ in photography, the latter rays come to focus behind the focal point of the visual rays, and naturally lenses for ocular observation only are corrected for visual and not for actinic rays. Moreover, in order to overcome certain im¬ perfections of image that would arise from the interposition between the object and the lens of the glass disc usually covering the object, and also to meet the fact that ordinary eye-pieces are not made achromatic, the objectives are still further corrected in such a way as still further to separate the visual from the actinic focus. The greater the principal focus {i. e. the lower the power) of the lens, the greater the distance between the foci, so that while with a high power objective the foci may practically correspond, with a low power they are so sensibly separated as to produce a blurred image in photography while visually the image was quite sharp. Of course, this would be fatal to photo-micro¬ graphy. We must, therefore, either procure objectives cor¬ rected for photography, which is the best plan by far, or we must by the use of a supplementary lens so alter the whole combination that the visual and chemical ra} r s shall *The “correction,” if not in the objective itself, may be obtained by use of an ocular made for the purpose, Corrections for chromatic and spherical aberations are, of course, essential to all objectives. 28 PRACTICAL PHOTO-MICROGRAPHY. focus exactly in one plane. This supplementary lens is usually a double convex fixed at the back of the objective¬ cell ; it introduces extra reflecting surfaces which should be avoided, and it sensibly alters the focal length of the objective. By experiment the distance between the foci of any lens may be determined and allowance made before each exposure by withdrawing the sensitive plate from the objective to the amount determined by the experiments; but as the distance differs not only for each objective but also for each distance from object at which the objective is used, it is plain that such a necessity would be an intolerable infliction to most men. Many, if not all, opticians now produce objectives of all powers most accurately corrected for the actinic rays, and there is no extra cost involved. The new apochromatic object glasses used with projection oculars are perfect in this respect, and have so many other valuable qualities that we propose to devote a chapter, or at least a paragraph, to them solely. A much vexed question is that of the angular apertures of objectives not only for photo-micrographic purposes but for general purposes of observation. “ Resolution,” or the power of separating visibly line from line, dot from dot, mark from mark, increases with angular aperture. Thus an objective having an angular aperture of 20 deg. will, with blue light, visibly separate lines about 18,000 to the inch, while a lens of 120 deg. will, under similar conditions, separate lines about 90,000 to the inch. (For further remarks on angular aperture, immersion, and numerical aperture, see Chapter XXIY, pages 162 et ssq.) But as aperture increases certain useful qualities fall off. A quality called “ penetration ” is known to fall off in proportion as aperture increases, and this matter re¬ quires investigation. “ Penetration,” as it is called in micro¬ scopy, “ Depth of Focus,” as it is called in photography, is a supposititious power of focusing, sufficiently sharply to prevent visible blur, simultaneously on several planes perpendicular to the optical axis of the system. Plainly the essence of the debate lies in the amount of blur visible or permissible. In a small photograph a very small area of confusion is permissible, while in a larger photograph a comparatively large area of confusion PRACTICAL PHOTOMICROGRAPHY. 29 is not only permissible, but preferable to over-sharpness from an artistic point of view. In photo-micrography as a science we have nothing to do with fine art, and scientifically speaking no blur or confusion-area is permissible at all. And further a lens cannot by any possibility focus equally sharply on any two planes perpendicular to its axis, and any appear¬ ance of equal sharpness can only be attained by a general sac¬ rifice of sharpness, or by a compromise between absolute sharpness on one plane and absolute sharpness on another plane. When such a compromise is made the image may ap¬ pear sharper generally, but that is simply because there being no absolute sharpness anywhere there is a lack of sharp to compare with unsharp, and so the eye is deceived into an im¬ agination of sharpness. And yet again it is not the case that a compromising or “ diffusing ” lens gives the sharpest image as a whole, for a lens capable of giving the utmost definition on any one plane will certainly show adjacent planes propor¬ tionately sharper than the lens which is incapable of giving thorough definition on any plane. It is our belief that this misleading theory of penetration, promulgated and upheld by great authorities mistaken on this point, has done much mis¬ chief to microscopic optics, and led many an optician and many a worker astray. For purposes of rapid observation of moving objects, where general appearances are desired rather than critical examination, undoubtedly low angle “ compromis¬ ing ” lenses are of the greatest service, but the writer ventures to assert, both as a theory and from careful and repeated ex¬ periment, that better photo-micrographs are produced, and pre¬ sumably better images observed, by well-made, wide-angle lenses than by lenses made for “ penetration,” or stopped down so as to cut off available angle. Certainly many lenses are made so imperfectly that when they are used at their full available angle of aperture aberrations come in that spoil their performance altogether, but this is a mechanical not a theo¬ retical fault. The writer regrets to say that all this is in flat contradiction to what he wrote with reprehensible precipit¬ ancy when he was but a beginner and a “smatterer” in this science. But granting the desirability of this quality of pene- 30 PRACTICAL PHOTO-MICROGRAPHY. tration there is another matter to be considered. While it is true that penetration decreases in direct proportion as angular aperture increases, it is also true that penetration decreases much more rapidly as magnification increases. Penetration varies inversely as aperture but also inversely as the square of magnification. So that a low power with a wide angular aperture may be expected to yield a better result in the matter of penetration than a higher power with an equal aperture. As, therefore, aperture is the means whereby we gain resolu¬ tion and definition, and as magnification can be obtained in other ways than by the use of a high power objective, the ad¬ vantage clearly lies with the use of a low power of wide angle, magnification being obtained by stretch of camera, eye- piecing, or “ camera enlargement ” of the original negative. The limit to the angular aperture of a glass in proportion to its focal length is a difficulty of optical mechanism. Beyond a certain point angular aperture in high proportion to focal length cannot be achieved by practical opticians. There is, however, one defect inseparable from the use of very wide angled objectives, and as it was noticed by Dr. Car¬ penter in his great book, “ The Microscope and its Revelations ” (London: Churchill. Sixth edition, 1881), it may well be put in his own words. After dwelling upon the difficulty of per¬ fectly correcting a wide-angled lens for spherical and chromatic aberrations, and after pointing out the advantages in this respect gained by the system of homogeneous immersion, he proceeds thus: “ But here comes in another source of impair¬ ment— the difference in the perspective views of every object not a mere mathematical point or line which are received through the different parts of the area of the objective .” Dr. Carpenter then quotes in support of his position such high authorities as Dr. Royston Piggott, and Messrs. Dallinger and Drysdale. We might admit this defect more readily if we were certain that “perspective” can exist in a diffraction image, but we still believe that even if Dr. Carpenter’s view be correct, better results on the whole will be obtained by the use of as wide angles as can be used without serious amounts of aberration. PRACTICAL PHOTO-MICROGRAPHY. 31 “ Working distance ” is also apt to be curtailed by largeness of angle. “ Working distance ” is simply the distance between the object and the front combination of the objective. With low powers this is of little moment, but where we come to use high power objectives their performance is not only apt to be impaired by very close working, but there is a danger of dam¬ age to object or objective itself. Homogeneous immersion helps us out of the trouble to some extent, but with cheap immersion lenses of numerical aperture 1.25 and over, we have repeatedly failed to observe objects that happened to have cover glasses thicker than usual. The “power” of an objective is frequently very loosely quoted. “Power” depends on focal length, and the focal length of a compound lens is usually quoted by a supposititious comparison with a single lens of given construction. The real focal length of an objective, and consequently its amplifying power, are very seldom accurately stated, even by the best makers. If it is necessary to know the exact magnification of any object with any objective at any distance, recourse must be had to measurement by a stage or other micrometer. The lower the power of an objective the more difficult it is to give it wide angular aperture, consequently objectives which pre¬ tend to wide angles are usually quoted under their real power, i. e., over their real focal length. And tolerance of eye-piecing is a very important factor in calculations as to power, for a quarter-inch o. g. may stand an ocular of twice the power that a one-sixth can bear, and so after all the one-fourth may come to be the higher power. Well-made objectives will give good images with oculars that will break down inferior lenses, and a lens before purchase should always be tested with a high ocular; there is no better trial that can be rapidly made. There are certain proverbial tests for microscopic objectives, and a glass is quoted as resolving this, that or the other test structure. This is all very well if the tester be the owner of the test object, and know it well. But opticians always have test objects of their own, which, we need not say, are intended for testing lenses to the satisfaction of would-be purchasers. An optician’s podura scale, or “blowfly’s tongue,” is generally 32 PRACTICAL PHOTO-MICROGRAPHY. easier of resolution than any to be found in the cabinet at home, and an optician is likely to choose a pretty even section of an echinus spine as a test for flatness of field. Flatness of field is a highly important quality in an objective for general photo-micrography, and depends chiefly on skilful work on the optician’s part. Want of this quality may be hidden by dia¬ phragms in the ocular, and often is so in badly made apparatus. Finally, it may be said that the tyro microscopist is not capable of making a wise selection of objectives for photo¬ micrography or any purpose, but an experienced worker will be able to select the good lenses at once in all points except actinic correction, and on that point the optician’s word must be taken if trial of the lens is not permitted. The beginner ought therefore to get a skilled friend to choose the lenses for him. We should consider the following to be a complete battery of objectives, provided they were of first-class con¬ struction : 3 inch ; inch or 2 inch; 1 inch ; \; |; -fa. CHAPTEK TV. OPTICAL APPARATUS CONTINUED. The Condenser is almost of equal importance with the objective, and certainly the best object glass cannot be expected to work at its best without a good condenser properly used. A non-achromatized object glass is so evidently useless that no one is likely to be taken in by one, but—perhaps unfortunately —a non-achromatic condenser is often found a very tolerable makeshift for an achromatic one. At all events, those who cannot afford to buy an achromatic condenser need not despair of producing very fine work, though [perhaps the very finest is beyond their reach. For the lowest powers usually employed in our work, how¬ ever desirable a substage condenser may be, certain diffi¬ culties of illumination preclude the use of a condenser, and a bull’s-eye must suffice. But for all objectives of one inch and higher power we strongly recommend the use of a substage condenser ; the bull’s-eye also may be used if necessary. The cheaper and commoner kinds of condenser are non-achromatic, consisting usually of three elements and varying in angular aperture from low figures up to the numerical aperture 1.4 as made by Zeiss and others, the latter being of course oil immer¬ sion condensers. For lower power work—up to (say) the ordi¬ nary four-tenths o. g. of about 90 deg.—the front element should be removed from the condenser; this front is usually fitted with a metal cap pierced with a very small hole for centering purposes. Even the third or lowest element may be used alone as a condenser, but as a rule it is better either to use the two lower elements or to omit the condenser entirely. Achromatic Substage Condensers are now made by all opticians and used, so far as we know, by all good micro- scopists aiming at the best results of either observation or X 34 PRACTICAL PHOTOMICROGRAPHY. delineation. These condensers are usually of three combina¬ tions, and range to the highest attainable immersion apertures. The best that has ever come under the notice of the writer is the Apochromatic Immersion Condenser of Powell and Lea- land, giving an immersion angle of A. A. 1.4 or even higher. The focal length of this condenser is about one-fifth of an inch, so that in the absence of a bull’s-eye—which the writer never uses with this condenser—the illuminated field is but small unless the power of objective used be high ; for critical work this shortness of focus is an advantage. The price of achro¬ matic condensers of high angle is considerable, but their superiority over non-achromatic condensers is great. Messrs. Beck, Swift, and probably all opticians make achromatic condensers to nearly the full extent of the air angle, but one of 140 deg. will be found a very useful condenser where the pocket does not permit of more than a few pounds of expense. Fig. 4*—Iris Diaphragm. The condenser, as will be more fully shown hereafter, is not so much a device for throwing a blaze of light upon the object, as for (1st) focusing the light at a certain point, and (2nd) modifying the angle and the direction of the light. Accord¬ ingly the condenser is furnished with various accessories which must be named here, though their precise use must wait till a later time. A set of diaphragms usually accompanies the con¬ denser, and these have apertures graduated in size from the full aperture of the optical part of the condenser down to very small holes, the smallest of all usually serves for centering. An exceeding great convenience is the Iris Diaphragm, Fig. 4, PRACTICAL PHOTO-MICROGRAPHY. 35 the nature of which explains itself to anyone looking at onr cut. The manufacture, however, is by no means so simple, and the purchaser ought to see that the interior of the aperture is as nearly circular as possible, and that, in closing, the seg¬ ments do not “ jam,” or work very tightly, a very common fault. The segments should not “ lock,” that is should not interlace with each other, for that construction generally means a “ jam.” Another accessory usually accompanying a condenser is a set of “ black-ground discs,” the nature of which will be under¬ stood from figure 6 and the use of which will be treated later. Fig. 5.—Powell and Lealand’s Apochromatic Condenser N . A . 1.4 Frequently also other “ stops ” accompanying the condenser, we figure three of them here, so that their description may be recognised when we come to mention their uses. #®® © Fig. 6.—Stops. i There are various other accessories frequently fitted to the substage of a microscope, sometimes to be used along witli the condenser, sometimes independent of the condenser. Thus a polariser, with or without selenite discs, is often made to fit the substage; colored glasses, technically called “light modifiers” are also common accompaniments of an achro- 36 PRACTICAL PHOTO-MICROGRAPHY. matic substage condenser. Indeed, condensers are frequently mounted so as to carry all or several of the accessories described, and to carry them all at once; while the writer admits the ingenuity, and even the occasional convenience of such arrangements, he hardly ventures to recommend their use. Arrangements of this* kind are apt to be heavy, clumsy, bulky and puzzling, and it is on the whole better to have on the substage at the time of work only as many of these accessories as are actually in use. Still it may suit some tastes and some purses to adopt one of these multiplex arrangements, and one is figured here as a sample of the usual article. Fig. 7.—A Fitted Substage Condenser—Achromatic.— Swift. Another common contrivance for facilitation of work and • for convenience of manipulation is a nose-piece, double, triple, or even quadruple (Fig. 8). While admitting the convenience of this contrivance the writer does not recommend its use, for delicate work at all events. The less load we have on the microscope tube, especially near the objective, the better; cases have occurred of injury to the working of a delicate fine adjustment screw by a heavy nose-piece. Greatly to be preferred for efficiency and for economy, and not a whit behind PRACTICAL PHOTO-MICROGRAPHY. 37 in facility and elegance, is a simple adapter with a bayonet joint made by filing away part of the thread of the adapter, and part of that of the microscope tube. A lens can be screwed into the latter in the usual way just as if the tube- thread were complete. Fig. 8.—Triple Nose-piece. The objective screws into one end of the adapter in the usual way, then the other end of the adapter is pushed straight into the threaded end of the microscope tube and gets a short turn in one direction, when it is at once clamped. These adapters are easily made, small, cheap, and several can be had for a few shillings. No mention has been yet made of oculars, because the writer has no confidence in any ocular except those made specially for projection, and projection oculars shall be treated in a separate chapter along with apochromatic objectives. It is not by any means asserted that no ocular of the ordinary type may be successfully used in photo-micrography, but the writer has never yet been satisfied with the performance in this work of any ocular, achromatic or otherwise, except the projection oculars. It is not denied that in some cases good results have been obtained with ordinary eye-pieces, but the . writer has never yet ascertained on what fortuitous circum¬ stances the success has depended. “ Fortuitous ” is written advisedly, for by its construction an ordinary ocular is evi¬ dently not intended to project an image, except on the human retina. It is to be noted that an ordinary objective is “ over¬ corrected,” a Huyghenian ocular under-corrected, so the two often balance each other. 38 PRACTICAL PHOTO-MICROGRAPHY. Nor does the writer commit himself to any opinion as to the merits of a contrivance known as an “Amplifier,” and con¬ sisting of an achromatic concavo-convex or double-concave lens inserted behind the object glass; he has not himself tried such an arrangement, hut has seen specimens of work produced with its aid which do not seem to speak highly in its favor, though jper contra he has seen very creditable work produced in its presence. Fig. 9.—Bulls’s-Eye on Stand. The bull’s-eye does not require special mention in this chapter; it is usually a plano-convex glass used with its plane surface next to the radiant, and the larger it is, without being clumsy, the better, though by some a small bull’s-eye is pre¬ ferred. A small bull’s-eye, having a shorter focus, gives more brilliant illumination than a large one. CHAPTER Y. ILLUMINATION. Practically we have to consider only five radiants: Sun¬ light direct, daylight diffused, electric light, oxy-hydrogen limelight, and lamplight from some form of oil lamp. Mag¬ nesium light we must at once put aside, because, while it has been successfully used for exposures of very brief duration, it is out of the question for prolonged exposures on the scores of inconvenience and expense, and some of our objects will require prolonged exposures to any light how¬ ever powerful. The light produced by carburetted hydro¬ gen burned at the orifice of an ordinary “ gas-burner ” has several qualities which render it useless for our purpose, and no system known to us of burning this gas alone is at all suited to our purpose. Admittedly, however, the incandescence of certain materials impregnated with such refractory substances as zirconium salts, the incandescence being produced by ordin¬ ary “gas ” suitably used, gives some promise of future utility, though as yet we have not been able to utilize any contri¬ vances such as the “ Welsbach ” burner, and efforts on our part have not been wanting. In past years, before our photographic preparations had at¬ tained the degree of sensitiveness to light that they have now, duration of exposure was often a very important considera¬ tion, and it was little wonder that direct sunlight was invari¬ ably used for certain work, where with even the most power¬ ful of artificial lights the exposure must have extended to many hours. The duration of exposure is per se of little con¬ sequence, but the danger of tremor and change of tempera¬ ture are much more serious matters. The use of direct sunlight involves, in most cases, the use of a^heliostat, and in all cases a vast amount of uncertainty. 40 PRACTICAL PHOTO-MICROGRAPHY. In Great Britain, at all events, the use of direct sunlight may be set aside as not available, certainly as not presenting suffici¬ ent advantages to counteract its enormous disadvantages, and in America there is ho longer the necessity for it that there was in the days of Woodward’s achievements. No work on photo-micrography, however, could pretend to be complete without something more than an allusion to sunlight illumina¬ tion, so we shall present a diagram of the arrangement used by Woodward, an arrangement which in many respects formed the basis of future developments. Messrs. Truan and Witt, in the production by wet collodion of a fine series of photo-micrographs representing certain Dia- tomaecae of Hayti, used an apparatus wherein direct sunlight was projected by means of the mirror of a Chevalier megascope. Dr. Woodward, after using the arrangement figured No. 10, made alterations which he considered improvements and which in some respects undoubtedly were steps in the right direction. He used a room as his camera, supporting his sensitive plate on an easel which was made to run on rails to and from the microscope which, with objectives, was fixed to the window shutter, the light, as before, being reflected through the optical system by a heliostat, The obvious disad¬ vantage of this arrangement was the fact that in case of any tremor the sensitive plate and the optical system might not move together. Dr. Maddox used also a darkened room, but he had in the room a camera, reflecting the sunlight by means of a mirror and prism through the optical system which was fitted to a hole made in the shutter. Non-actinic light was ad¬ mitted into the room by means of suitable “ light-filtering ” media. Many other arrangements might be mentioned with¬ out any notable difference from or superiority over these already touched. An important feature in sunlight illumination is the use of “monochromatic” light. The reader is probably aware that the rays composing a beam of white light are not all equally energetic in producing the chemical action necessary to the production of a photographic image. The waves of light producing the sensation of sight vaiy in length from PRACTICAL PHOTO-MICROGRAPHY. 41 Fig. 10. (From Dr. Beale’s ** How to Work with the Microscope. 42 PRACTICAL PHOTOMICROGRAPHY. crest to crest, from—roughly —-jtoto th to s g „ 0 6 tli of an inch; and of these varying wave-lengths those which exercise the greatest chemical action measure about e - 4 l 0 Q th of an inch, and are what we call “violet” in color. If, then, we can cut off all rays except those which exercise strong chemical action, we shall reduce the general actinic force of the white ray,—for all visible rays have some actinism,—but we shall have less confusion among the rays producing the photographic image, and so our optical apparatus will probably be used at its best, especially if our lenses are corrected for the rays which pro¬ duce the best visual effect, as lenses naturally are when in¬ tended for ocular observation only. Two methods were in vogue for this passing of actinic rays alone: one was the use of a cell containing cupric ammonio-sulphate dissolved in water, which makes a blue solution, and one spectroscopically suitable for the purpose aimed at; * the other plan consisted in the use of a prism which broke up the white ray into its component parts, the blue part alone being allowed to pass through the microscope. A “ diffraction grating ” would yield a still purer spectrum, but so far as we know has not been used for this purpose. In each case the solar ray was passed through the monochromatising medium before it reached the object. Diffused daylight may be utilized by reflection from white cloud or uniform blue sky; but after repeated and careful experiments the writer can not recommend this system of illumination as likely to prove satisfactory to the serious photo- micrographer. If white cloud or homogeneous sky can be depended on, the ordinary plane mirror of the microscope, if of ample size, will answer; all rays in this case being practi¬ cally parallel at their impact on the mirror. Another plan, tested by the writer with greater success, is to omit the mirror and to replace it by a white surface, as very fine filter or blot¬ ting paper, but not a shiny surface as sized paper or opal glass The white surface is to be inclined in a suitable direction at an angle of about 45 deg. to the axis of the optical system. *This solution is, in the writer’s experience, usually far from monochro¬ matic and decidedly inferior to certain qualities of cobalt blue glass for this purpose. PRACTICAL PHOTO-MICROGRAPHY. 43 The electric light has been used with great success in this connection, an arc lamp having been the usual form. There is no doubt that an arc light, provided it is steady, may be expected to work grandly for our purpose. It is in most cases, however, expensive, difficult to work to perfection, and when imperfectly worked, a very serious botheration. As the writer has no experience of the arc light he refrains from making any statements as to its suitability or unsuitability for our pur¬ pose. Incandescent filaments in electric lamps might be made answerable to our purpose, but so far as we know no incan¬ descent electric lamp has yet been found equal even to a good oil lamp, the area of incandescence being too slender in the former. Fig. 11. —Oxy-Hydrogen Tet by Newton. Author’s “ Cut-off.” The writer uses the lime light as his radiant in nearly all his work, and, taken as a whole, this light is as nearly perfect for the purpose as any illuminant at present known. The general form of a lime-jet is well known, and the ordinary form of “blow-through” or “mixing” jet will doubtless suffice for all purposes. The blow-through jet gives a larger area of incan¬ descent lime, but the incandescence is not so perfect, nor the color nearly so good as that produced by the mixed gases. There is no necessity for any great pressure of the gases, pro¬ vided the proportion of one gas to the other is suitable, and the nipple of the jet has a bore suitable to the other circum¬ stances. All these matters maj be settled by experiment, and, in fact, must be so settled. The writer uses a mixing jet, but takes the hydrogen direct from the house main, while he puts the oxygen in a bag and puts thereon only a moderate pressure; 44 PRACTICAL PHOTOMICROGRAPHY. in fact, on a pressure board four feet by three feet, he places a weight of forty pounds.* Figure 11, representing a jet arrange¬ ment designed by the writer, requires some explanation. The jet is an ordinary mixing jet, but has an extra attach¬ ment consisting of three cogged wheels worked from the back of the jet by the cross piece and operating two taps, one on the O and the other on the H tube. These tubes are operated proportionally by the equal cogged wheels, so that the light being once arranged at its best, both of these extra taps being full open, the brilliance of the light can be lessened without injury to the quality by simply turning the cross-piece. But the H tap has a “bye pass,” so that the hydrogen can not be entirely cut off by this tap, while the oxygen is a complete cut off. The result is that the lime never gets quite cold when the gases are not turned up, and there is no need to light the jet each time after it has been temporarily out of use. More- over, by lowering the gases together the worker saves his eyes, and by turning the extra arrangement entirely down leaving only a glimmer of hydrogen burning, the worker saves his time, his lime and his money. The harder the lime the bet¬ ter for our purpose, for a large area of incandescence, unequal in brilliance and color, is most puzzling and pernicious. The jet figured is made by Messrs. Newton & Co., of London, but is open to the public, being in no way “protected.” For a very brilliant light with a small incandescent area—as for work of the greatest delicacy—both gases should be put under heavy pressure in bags, or preferably, cylinders. Mr. E. M. Nelson has his gases in iron tanks, and gets a very fine light indeed. The writer has on several occasions attempted to work out some medium to replace “ limes ” which crack at awkward times, and are at all times liable to disintegration through ac¬ cess of damp. Magnesia pounded very thoroughly for two hours in a mortar with sufficient water to form a paste prom¬ ised well. “ Buttons ” were made with the paste, each button * Since the introduction of Beard’s excellent “ Regulator,” the writer has given up bags, and uses oxygen from the cylinder, pressure being controlled by the regulator. PRACTICAL PHOTO-MICROGRAPHY. 45 impaled with a short length of platinum wire, and then gradu¬ ally dried, first in a heated iron oven, then in coal gas flame, lastly in the oxy-hydrogen flame, as suggested by Dr. Roux of Paris; but the result seemed decidedly inferior to the ordin¬ ary lime. Zirconium oxide was also tried with no better success. Fig. 12. —Swift’ Microscope Lamp. Probably most of our readers will find an oil lamp answer all desired purposes, and if time be no object, a good oil lamp will probably prove entirely satisfactory. If the wick be single and flat, and if it can be turned either broadside or edge to the microscope, the kind of lamp is practically immaterial. But lamps are made specially for this work and several of them may safely be recommended. Mr. Swift’s lamp (fig. 12), for instance, has served the writer thoroughly well, and the lamp figured No. 13 is also well adapted for this work. 46 PRACTICAL PHOTO-MICROGRAPHY. The lamp should be on a heavy stand, and for convenience should slide up and down a stem as shown in our cuts. About an ounce of camphor may be added to each pint of the oil used, which should be paraffin of the very finest quality that can be obtained. Bad oil causes no end of trouble. The wick must Fig. 13.—Baker’s “Nelson” Lamp. be kept well trimmed, and all parts of the lamp scrupulously clean. The slips of glass used in front of the flame in the metal chimney are very apt to break when the flame is turned edge toward the microscope; to prevent this the slips should be rolled in a piece of cloth and boiled for two hours in water, or, still better, in sweet oil. * \ No. 1.—Hairs on Proboscis of Blowfly, X 375. No. Z .—Injected Villi, Intestine of Rabbit, X 30. Plate I. CHAPTER YI. PHOTO-MICROGRAPHIC APPARATUS. After what has been written about the component parts, the entire system of apparatus ought to be easily understood. Ho matter what microscope-stand or what light is to be used, some ingenuity and care will be required to fit the several parts together so that the action of the whole may be efficient and sure. If a complete photo-micrographic apparatus be bought ready for use, of course the purchaser, having once satisfied himself of the accuracy and convenience of the apparatus, need no further trouble himself on this score. Exceedingly good work has been done and may be done again without any special apparatus beyond a microscope and a camera. The microscope has only to be turned to the horizontal, a camera run up to the eye piece end of the micro¬ scope, all light not passing through the optical system excluded by means of a velvet tube or cone passed from a photographic lens tube, (the glasses being removed) to and over the end of the microscope tube, the whole presenting an appearance somewhat as shown in fig. 14. Fig. 14. —Simple Arrangement of Microscope and Camera. It need hardly be said that with such an arrangement great care is necessary to prevent shaking, and to preserve the due relation of parts. In all apparatus for this purpose two salient 48 PRACTICAL PHOTOMICROGRAPHY. necessities must be attended to ; 1st. The light, the condensing system, the object glass and the centre of the sensitive plate must all be axially centred to each other; and 2nd. The object and the sensitive plate must be parallel to each other and per¬ fectly perpendicular to the optical axis of the system. The slightest divergence from either of these relations will entail failure. / Where the intending photo-micrographer possesses the com¬ ponent parts of the system, as microscope, lamp, and camera, and requires only to fix these in suitable position on some base to be used permanently, the matter lies chiefly with himself, and the ease of his operations will depend chiefly on the instru¬ ments he happens to have. We shall figure and describe two arrangements at least; one a sample of an apparatus sold ready made, the other an apparatus built up of miscellaneous materials by the writer, and these are given merely as sug¬ gestions and as examples of what has been found to work well in the writer’s hands. In Britain of late years, several opticians have stocked apparatus of which figure 15 is a type. It presents all the useful features of its class, though other instruments are to be found differing in detail, some details being superior, others in¬ ferior, to those seen in the cut. This apparatus was, the writer believes, designed in the form shown by Professor E. M. Crookshank, and used by him in his photo-micrography of Bacteria. The writer has used an apparatus in all essential points similar to that figured, and had great satisfaction in its use. The stand is so made that when it is necessary to have the stage of the microscope horizontal, as shown in Fig. 15, as when for photography of liquid matter, the base board can be let down to the vertical position; as every part of the optical and photographic and illuminating systems is clamped to the base board, this position is easily attained ; and, except where a lamp burning same liquid is used, one position is as manageable as the other. If an oil lamp furnishes the light a mirror must come into requis- tion. A point on which the writer lays great stress is: the microscope, condensing system and radiant are all fixed to one 49 PRACTICAL PHOTO-MICROGRAPHY. £ platform which turns or a central pivot, so that the optical system can he turned out from the axial line of the entire sys- Fig. 15. —Photo-Micrographic Apparatus by Swift, shown at the Vertical Position. tern, the object can be examined, corrections of the objectives studied, and everything focused and centered with the eye- 50 PRACTICAL PHOTOMICROGRAPHY. * piece, the operator being in a comfortable and convenient position, sitting or standing, as desired. These things being done, the platform is turned back to a “stop” which is so arranged by the maker of the apparatus that the optical sys¬ tem is then axially centered with the photographic system. The focusing on the plane of the sensitive plate alone remains to be performed, and this is done by means of the rod seen in the cut. The rod has a pulley, tiie fine adjustment has a screw with a grooved milled head, and a pulley passing over the rod- pulley and round the grooved milled head operates the fine ad¬ justment at the will of the operator examining the image on the focusing screen of the camera. A “ Hooke’s Joint ” may be used in place of this focusing arrangement, the writer used that contrivance for some time, but on the whole an arrange¬ ment with rigid rod and pulleys is to be preferred. Figure 16 shows the writer’s latest arrangement, which combines some ideas gathered from Mr. Kelson’s apparatus with others of the apparatus last figured, and still others which occurred one by one to the writer as he advanced in experi¬ ence. The platform carrying light and optical system and turning on its pivot is retained, so also is the rigid rod and pulley contrivance. The platform is still “ stopped ” at a certain point, but this time the camera is entirely free on a very heavy teak base to which al*o the swinging platform is attached. In Fig. 15, after the optical part is stopped at the axis, the front only of the camera is run forward to meet the ocular end of the microscope tube; but in Fig. 16 the whole camera is pushed forward to the cap on the tube, and in both cases a smaller cap fixed to the camera fits very loosely inside a larger cap on the microscope tube. There is no difficulty in centering the latter arrangement, Fig. 16, for the centre of the camera focusing-screen is marked, and if the centre of the object coincides with the centre of the ground glass everything must be centered and perpendicular to the general • axis. Any photographic camera will answer for this work pro¬ vided it be light-tight, and reasonably well made. No “ mo¬ tions,” such as “ swing-backs,” are required. The camera PRACTICAL PHOTO-MICROGRAPHY 51 shown in Fig. 16 is one for a plate 7^x5^ inches that hap¬ pened to be in the writer’s possession. The bellows arrange¬ ment is very useful though not essential, and the same applies to the rack and pinion. The available stretch of the camera, 52 PlijCTICAL PHOTOMICROGRAPHY. including the supplementary tapered bellows seen in front, is 30 inches without using the rack and pinion ; an extra length of 10 inches can be added at will. The writer has an attach¬ ment to the apparatus, Fig. 16, by which he can increase, his stretch of camera to 7 feet 6 inches, and his plate to 10x8 inches, but this is hardly ever used and never has been used with real success. A stretch of 40 inches from micro-tube to plate is perhaps the limit of utility, for very few object glasses will stand more than that stretch in the writer’s experience, especially with oculars. The largest useful size of plate is about 7x5 inches, or what is called in England “half-plate,” 6-|x4£ inches. The camera should take a plate of one of these sizes, but should have “ car¬ riers ” or “ kits ” fitting the “ dark slide ” to take 5x4 or 4|x3£ plates, the latter being technically called “quarter- plates.” Square plates are probably better than oblong ones as a rule, so that the carriers may be made for 4^ inch square plates, but plates of unusual sizes are not so readily obtained as common sizes. * One point regarding the dark slide is very important; it should on no account slide into its position in the camera by a long groove, but should be so made as to slide only about an inch, or better still it should fall into a groove at the lower side and be held into position by a catch at the top. There is very little danger of light fog in this region of our apparatus, especially as the room should be darkened (see later); and there is very great danger of moving some part of the apparatus after all is focused if the operator has to exert any force to shove the slide into position. The shutter of the dark slide must also work very easily and sweetly for the same reason. The camera has of course a “ ground glass,” which is used for preliminary examination of the image upon the screen, and in certain cases the ground glass if finely ground is all-sufficient. In any case the ground surface may with advantage be oiled ; but even then the surface is too coarse for focusing images with very fine details. Many devices have been used and recommended for producing a surface sufficiently fine yet with PRACTICAL PHOTOMICROGRAPHY. 53 sufficient grain to show an image. A sensitive gelatine plate exposed for a second to light, developed, fixed, washed and slightly treated with mercuric bichloride gives a good surface; this, we believe, orginated with Mr. "Walmsley. No better focusing surface will be found than a piece of glass with some diamond marks on the front—that is on the side next the light. In the writer’s case these marks consist of a cross, the arms of which are inches divided into tenths with a diamond; when viewing the image with the focusing eye-piece, if the magnification be known, it is easy to measure objects and dis¬ tances at a glance. No image can be seen on the plain glass unless an eye-piece be used to focus the aerial image; the glass in fact is used only as a rest for the focusing eye-piece. The eye-piece used is known as a Ramsden, or perhaps better a Zeiss “ aplanatic magnifier ” may be used. In either case the scratches on the front of the focusing glass must be most care¬ fully focused with the eye-piece ; if is found difficult to focus the scratches, a fly’s wing or some such object may be fixed to the front of the glass* plate and the magnifier set to focus on that. It goes without saying that the focusing screen—or ruled glass or whatever it is—should be in the same plane as that occupied later by the sensitive plate. The rays, however, in ordinary work are at the sensitive plate so nearly parallel that slight “ want of register ” between plate and focus-screen is not so very fatal as many think. Dr. Bousfield, whom no one need fear to follow, uses no glass plate at all, nor fixes his focusing eye-piece at any point as many do, but focuses the aerial image in air alone; none the less the glass plate affords a convenient rest for the Ramsden or Aplanatic, but there must be no heavy pressure of focuser on screen. There are many other ways of producing a focusing screen but probably the best have been here noticed. It is important to have an arrangement inside the camera for starting and stopping the exposure. At “ 7 ” on Fig. 16 is seen the exterior of a simple flap shutter, the flap inside be¬ ing operated by the button outside. Sometimes when a very rapid exposure is required and when consequently there is fear of moving the whole apparatus, we close this flap, open the dark 54 PRACTICAL PHOTO-MIUB^APHY. slide shutter in the usual way, then taking a square of blackened cardboard in (say) the right hand and holding it close behind, but not touching the substage condenser so as to shut all light from the object, we open the flap with the left hand and swiftly raise and lower the card in the right hand, thereafter immediately closing the flap. Where the hand is unable to make a sufficiently quick exposure, we rig up a photographic instan¬ taneous shutter between light and condenser, and proceed with this shutter on the same principles as we did with the card¬ board. There is a fine field open to the mechanic in the designing of apparatus for photo-micrography; but the writer can only say that after long and varied work with the apparatus, figured No. 16, which was put together chiefly by Mr. Baker of London, he is at a loss to suggest any improvement on it. A cell or trough of wood with plate-glass sides contains a saturated solution of common alum or in certain cases a solu¬ tion of cupric ammonio-sulphate. These solutions, or the water of them, should be well boiled to drive off air, which, if the boiling is omitted, rises in bubbles from the bottom when the solution begins to get heated as it does with the oxyhydrogen limelight. This alum cell is necessary when the limelight or any other illuminating arrangement generating considerable heat is used. In high-power work changes of temperature have a very marked effect on the sharpness of image in the negative. (See Fig. 16, 2.) CHAPTER VII. REQUISITES FOR PHOTOGRAPHY. Haying already remarked upon the advantages of ample dark¬ room accommodation, running water and a waste-sink, we may now enter somewhat into detail regarding dark-room arrange¬ ments and articles used in operations purely photographic. In arranging for non-actinic illumination the reader is ad¬ vised to prepare himself at the outset for u color correct ” or “ orthochroinatic ” photography, for the worker at general photo-micrography, if he pay any attention whatever to the words of the author of this book, will very early find himself using color-sensitive plates. This means that whether the the reader proposes to use as a general rule yellow diffused, or clear ruby light, he must provide himself at all events with ruby illumination. If development is to be conducted by day¬ light we recommend that the outer sash of the window be glazed with yellow glass, or at least three thicknesses of “ can¬ ary medium” known to all photographic dealers. For ordin¬ ary plates a sash of ruby glass should be added to the yellow glass, and the best—in fact practically the only good ruby glass—is “ flashed ruby ” on one side and “ stained yellow ” on the other. A splendid glass, if it can be got, is flashed deep ruby on one side and ground on the other side. The three thicknesses of canary medium may suffice if the light shining on it be not very strong, and if a thickness of good ruby glass be added, or two layers of good “ ruby fabric,” the window is probably trustworthy for even ortho-chromatic plates. If arti¬ ficial light be used the flashed and stained ruby glass may suf¬ fice in a single thickness for even color-correct plates, though this must be tested ; it will almost certainly suffice for any or¬ dinary plates; two thicknesses of the canary medium for ordinary plates, with the addition of ruby glass as before for 56 PRACTICAL PHOTO-MICROGRAPHY. “ ortho ” plates, may be taken as “ safe.” To test a light for safety, place a plate of the kind to be used half in the leaves of a book, expose for (say) four minutes at the spot where the operation of development is to be conducted, thereafter de¬ velop the plate as much as possible in total darkness, and it will be easy to discover if the light is unsafe, for, if it is un¬ safe, the half that projected from the book will “ take a tint,” in other words, will develop darker’ than the part which the book protected. It is vastly important in photo-micrography to see exactly what the plate does under development, but in order to examine the plate critically, no great space of time is necessary; the rationale of the light question may therefore be summed up: Use the greatest possible amount of safe light, but do not waste any light; that is, do not expose the plate to light, however safe, when no object is gained by such exposure. Uon-actinic lamps are held in stock by all photo dealers; we figure one: Fig. 17.—Dark Room Lamp.—C arbutt. A wooden sink lined with sheet lead seems preferable to iron or earthenware, and in the sink should be a wooden “ hatch ” or grating on which measures, bottles, etc., may stand and drip. On one side at least of the sink should be a ledge or table sloping down to the sink and lined with lead or covered with rubber or American cloth, so that dripping dishes, etc., may be laid on the slope and their drippings run into the sink. PRACTICAL PHOTOMICROGRAPHY. 57 The end of the water-tap should have a thread by which may be coupled on to it such conveniences as a rose, a rubber tube, etc. If none of these conveniences can be had, the worker must content himself with a jug of water, or a vessel with a rubber tube provided with a tap or a spring clip, the vessel being placed at some height, as on a chair standing on the table. Convenient collapsible rubber sinks with waste pipe can be had in Britain and probably in America. In photo¬ micrography as in all things we must “ Cut our coat according to our cloth.” A set of flat “ developing dishes ” are necessary and not ex¬ pensive. Two of these, the proper size for the plate to be used may be made of papier-mache or ebonite; they should be black. Two others, large enough to hold three or four of the plates may be of porcelain and should be white. Fig. 18. —Developing Tray. Other dishes may be required for printing and other pur¬ poses ; their uses will be seen as we proceed. Some filter funnels; glass measures, say 10-oz., 2-oz. and 2 drams; scales and weights; a drying rack, Fig. 19 ; and, as a luxury, a washing trough, Fig. 20, may complete this branch of the outfit, which a few dollars will cover. For those who object to slightly stained fingers, a hook, Fig. 21, and a “ pneu¬ matic holder,” Fig. 22, may be added. 58 PRACTICAL PHOTO-MICROGRAPHY. The following apparatus will be required beyond the things already mentioned: Fig. 19.—Dry-Rack. Fig. 20.— Wash-Trough. For all kinds of contact printing: Printing frames (see later). For enlarging and reducing—which may be postponed till some practice has been gained in other works: A suitable cam¬ era, or arrangement of other cameras (see later); also a suitable photographic lens. Fig. 22.—Pneumatic Holder. A squeegee. Some glass plates or ebonite sheets a good deal larger than the largest negatives to be made direct. PEAOTIOAL PHOTOMICROGRAPHY. 59 The following articles are useful: A “ Warnerke sensito- meter;” a so-called “Matchless” gas burner, by which the gas can be lowered out of sight without entire extinction; this will be found vastly convenient in the dark room, Fig. 23. The following is a list of chemicals which are certain to be required for development and other operations with gelatine- bromide dry plates: Pyrogallol.1 ounce * Citric acid.1 ounce Sodic sulphite, $ pound ; or potassic meta bisulphite. 1 ounce Sulphurous acid.1 ounce Ammonic or potassic bromide.1 ounce * Liquor ammonia, sp. gr. .880 (see later) ..4 ounces Potassic carbonate pure.4 ounces * Sodic carbonate (crystals).4 ounces f Sodic hyposulphite.1 pound * Potash alum. 1 pound * Hydrochloric acid (comml.; useful for cleaning things) 1 pound Ammonic carbonate.4 ounces Mercuric bichloride {poison) .4 ounces For albumen paper printing on “ ready sensitized ” paper: Auric terchloride (“ chloride of gold,” in sealed tube). 1 tube Sodic acetate, or biborate (“borax”). ... 1 ounce For albumen paper not ready sensitized, in addition to the two last items: Argentic nitrate. . Albumenized paper .... 1 ounce a few quires 60 PRACTICAL PHOTOMICROGRAPHY. For printing on gelatino-cliloride emulsion paper (“ Aristo- type ”): Ammonic sulpho-cvanide. .1 ounce Sodic sulphate.1 ounce For printing on bromide paper, transferotype (Eastman), and for certain lantern-slide plates: Ferrous sulphate (protosulphate of iron).£ pound Potassic oxalate.1 pound Acetic acid (glacial at 52 deg. F.).1 ounce * Sulphuric acid, good comm’l.1 ounce * Sodic chloride (common salt).1 pound Pure talc or “ French chalk ” (for transferotvye).1 ounce For the wet plate collodion process either for negatives or for making lantern slides, a quantity of each of the following: Collodion iodized, or with separate iodizer; solution for clean¬ ing glass plates; argentic nitrate; f distilled or pure rain water; ferrous sulphate in addition to the above, also acetic acid or nitric acid ; sodic hyposulphite extra, or potassic cyanide. For all negative processes : hard varnish, 5 ounces. For all lantern slides : cold “ crystal” varnish, 5 ounces. These quantities are small, but will suffice for a start, and they are arranged as above so that the reader need not amass a large collection of chemicals for processes he does not pro¬ pose to work. * These may be useful for all processes, and may be stocked in larger quantity. f Necessary for almost all processes, and may be stocked in bulk. CHAPTER VIII. SOLUTIONS FOR PHOTOGRAPHIC OPERATIONS. 1st. To develop Gelatine Bromide Plates. The Pyrogallol or “ Pyro ” Solution : Take Sodic Sulphite. Water to about 4 ounces > r, Y dissolve / ounces ) Make slightly acid with sulphurous acid. Then pour into a a Commercial one ounce bottle of Pyro. Make up to nine ounces and filter. Label the bottle “ Pyro—10 per cent. 10 minims==l grain pyro.” Or better: Take Potassic Meta-Bisulphite*.34 ounce, avoir. Water to about.7 ounces This will dissolve easily, especially if the salt is pounded ; then pour into a bottle of Pyro as above and label as above, having made up to nine ounces. Or, lastly : just before using make a sufficient quantity of water acid with citric acid, and with that water make a solu- lution of four grains of pyro to each ounce of acid water. This will constitute one-half of the measure of what will be called a “ Normal Developer.” The writer uses and recommends the Potassic salt formula. But solutions known as “ Sulpho-Pyrogallol ” are sold, pre¬ pared more or less according to the formula of Mr. H. B. Berkeley, the originator of the compound, and these solutions are usually good. As before, 10 minims of the solution=l grain of pyro. * A patent salt to be obtained from The Scovill & Adams Co. 62 PRACTICAL PHOTO-MICROGRAPHY. Alkaline Solutions. Take 1 ounce (chern.) Liquor ammonia fortiss. and dilute to 10 ounces with water. (As soon as any bottle of this solution of ammoniacal gas is opened, an equal bulk of water should be added, or better, water should be added till the hydrometer stands at .920.) In all cases the bottle must be kept closed when not in use, and the stopper of the bottle should be smeared with vaseline. Of course, after this dilution is effected, a double quantity of the solution will be required to represent the quantity given in terms of “ liq. amm. fortiss.” The bottle containing ten ounces (1 oz. ammonia and nine of water) is to be labelled : “ liq. amm., 10 per cent. 1 minim== 1 minim ammonia.” Or, Sodic Carbonate Water to.. Or, Potassic Carbonate Water to. Or, Sodic Carbonate.-J ounce, avoir Potassic “ .. Jounce “ Water to.9 ounces Label: “ Carb. 10 per cent.—10 minims=l grain carb.” Bromide Solution. Potassic or ammonic bromide. 1 ounce, avoir Water to. 9 ounces Label: “Bromide 10 per cent.—10 minims=l grain bromide.” Citrate Solution. Sodic or potassic citrate (or half of each). 1 ounce, £^voir Water to.9 ounces Label: “ Citrate 10 per cent.—10 minims=l grain citrate.” (Note: If chemical weights be used, the solutions are to be made up to 10 ounces, and labelled as above.) 1 ounce, avoir 9 ounces 1 ounce, avoir 9 ounces 63 I PRACTICAL PHOTO-MICROGRAPHY. Fixing Solution. Sodic hyposulphite... 1 part Water. 4 parts to 5 parts Made decidedly alkaline with a carbonate or with liq. amm. Clearing Solution. Concentrated solution of potash alum. 1 pint Citric acid. 3 ounces Hydrochloric acid... 2 drams If used before fixing the acids should be omitted. Reducing Solution. No. 1. (Farmer.) A. Potassic ferricyanide (red prussiate of potash).10 grains Water. 1 ounce B. The ordinary “ hypo” solution. Reducing Solution. No. 2. A. Perchloride of iron. (Druggist’s tincture or satu¬ rated aqueous sol.).2 drams Hydrochloric acid. 4 drams Water to.20 ounces B. Fresh hypo, solution (This is probably superior to the Ferricyanide Reducer for lantern slides.) Intensifying Solutions. a. Mercuric bichloride. 1 part Water. 20 parts Hydrochloric acid.5 part b. Sodic sulphite. 1 part Water... .8 to 10 parts The Ferrous Oxalate Developer. A. Saturated solution at 60 deg. Fahr. of potassic oxalate, B. 1 to 3 aqueous solution of Ferrous sulphate, to each pint of which latter is added sulphuric acid, 1 dram. (Water 3 parts ; Iron 1 part, by weight). Notes: A “saturated” solution of potassic oxalate will hold about 1 part by weight of the salt to 4 parts by weight of water. The water may in each case be boiled to facilitate solu¬ tion. The sulphuric acid is to be put into the water before the iron salt is added. 04 PRACTICAL PHOTO-MICROGRAPHY. A and B will keep a long time if separate, but mixed they will not keep. For precautions in mixing see fater. Solutions For Printing Processes. Sensitizing Bath For Albumenized Paper. Argentic nitrate.40 to 65 grains Water, distilled....,.1 ounce. For the ammonio-nitrate process, and for full instructions on this entire subject, the reader is requested to consult “ The Processes of Pure Photography,” by Professor W. K. Burton, C. E., and the present writer. (New York : The Scovill & Adams Co.) Toning Solutions For Albumenized Paper. Sodic acetate.25 grains Auric chloride (Terchloride of gold). 1 grain Water. 8 ounces Used alkaline. Toning Solution For Chloride Emulsion Paper. See publication as above. Fixing Solution For All Silver Printing Processes. Hypo. Water 1 i made alkaline 5 parts ) Solutions For Platinotype Printing. a. A saturated solution of potassic oxalate. See above. b. Hydrochloric acid, 1 part; water, 60 parts. c. The same as b. Solutions for developing lantern slide plates of various kinds will be found under the heading appropriate to them in Chap. XXII on lantern slides. CHAPTEK IX. ON THE SELECTION OF PLATES. There are not a few able photo-micrographers who assert that the wet collodion process is superior to the gelatine bromide process for photo-micrography. The writer is in¬ clined to dispute this point, on the following grounds: The strongest argument of the advocates of wet collodion is that the deposited metal forming the image is in the wet process in a finer state of division than it can be in any gelatine process. The writer traverses this statement at the outset. The wet collodion process gives a more finely grained image than the rapid gelatine emulsion gives, certainly; but a gela¬ tine emulsion made with certain precautions, and suitably de¬ veloped, yields a metallic image quite as fine in grain as a wet collodion image, if not finer. But the gelatine emulsion for this must be very “ slow,” such as that used for the pro¬ duction of lantern slides in which the visible image approaches a stain more than a deposit in appearance. Moreover, the de¬ posit in the most sensitive plate is so fine as to be incapable of producing the slightest granular effect by direct contact print¬ ing, or even after enlargement of the negative by photographic processes up to at least 4 diameters. And the writer is very strongly of opinion, having worked and thought out the mat¬ ter very carefully, that both in theory and in practice no ad¬ vantage whatever is gained, or can be expected to be gained, by “ camera enlargement ” of a negative, over a negative of the desired amplification produced directly with the micro¬ objective. If, for example, an ultimate magnification of 300 diameters is required, it will be better produced by direct am¬ plification by an objective than by making a negative at “ X 100,” and enlarging 3 diameters in the camera; and an enlargement of 3 diameters, if properly managed will not, 66 PRACTICAL PHOTO-MICROGRAPHY. with the coarsest grained image the writer ever saw produced by gelatine bromide emulsion, show any grain due to the coarseness of the image deposit. We have, in fact, seen nega¬ tives in most sensitive gelatine emulsion enlarged 5 diameters without the slightest appearance of grain. The argument for the wet-plate-and-camera-enlargement is that the ultimate re¬ sult shows greater “ penetrative ” effect, that is to say that there is less apparent difference of sharpness in different planes of the object; the writer made a large series of experiments on diatoms, enlarging always to X 300, by direct micrographic means, and by camera enlargement from original negatives at 100 and 150 diameters; in every case the direct amplification was superior to the “ enlarged ” negative; and the plates used for the 100 and 150 diameter negatives were some wet, some very slow gelatine emulsion, while the negatives direct at 300 were in every case an exceedingly rapid gelatine emulsion. Thus much for practice, the theory is too intricate to follow here. At one time the writer thought that wet collodion would prove superior to gelatine emulsion, and he had got the length of producing a considerable number of wet plate nega¬ tives ; but on using slow gelatine plates on the very same sub¬ ject, which were chiefly flies’ tongues, and minute hairs, he found the gelatine results were in every way equal to the col¬ lodion, while the ease and certainty of the former process were incomparably greater than with the latter, though the writer is well accustomed to the wet plate process. But it is most important to make a wise selection of the gela¬ tine plates to be used. Our objects may be divided into two great classes : the coarse and the delicate. When our objects are by nature coarse in detail, or when they are made actinic¬ ally coarse by staining, we shall have no difficulty in getting contrast in our photographs; but when our objects are very minute, or composed of very minute details, or when they are so stained as to present very little contrast to the background and between each other, the affair is quite different, and a quite differ¬ ent class of plate is required. Where the danger is over-contrast rather than want of contrast, a “thin ” plate, such as is com¬ monly used in portraiture, is the best to use; but in cases of PRACTICAL PHOTO-MICROGRAPHY. 67 great delicacy of detail or color—the real difficulties of photo¬ micrography—we require a plate thickly coated with an emul¬ sion containing a handsome proportion of silver haloid. More¬ over, with the first class of subject, over-exposure is less to be feared than the reverse, so that rapidity of emulsion is rather to be desired than avoided; while with delicate subjects the exposure, even with high powers, is never prolonged to incon¬ venience, except in very exceptional cases where oblique light is used. A good “ portrait ” plate, then, is recommended for the ordinary run of low power work, while for the higher flights of “ critical images,” bacteria and the like, a plate should be thickly coated, not too rapid, and capable of giving a plucky, or even a “ hard,” negative at will. The two great factors in the late advances in photo-microg¬ raphy have been : 1st. The introduction of rapid emulsion; 2d. “ Color correct,” or “ orthochromatic” photography. (We omit for the present a third factor, which is optical in its nature). It may be asserted that the man who wishes to produce photo-micrographs of general utility, and still more, he who aspires to march anywhere near the van of the photo-micrographic army must master orthochromatic pho¬ tography. There is no getting round this fact. The majority of the most useful objects are only to be rendered to the best advantage by color-correct plates, and a large number of ob¬ jects can not be photographically rendered at all without such plates; and there are objects which will not be photographed until orthochromatic photography is perfected. The beginner should, therefore, provide himself with some orthochromatic plates for a start; as he becomes accustomed to their manipu¬ lation he is advised to orthochromatise his plates for himself, if he has facilities for drying plates. The subject of ortho¬ chromatics, though far too wide, as a whole, for full treatment in this book, will be treated as carefully and as fully as the author is able to treat it in a single chapter. Under the heading of lantern-slides we shall give a description of the wet collodion process, which must suffice for the reader’s present needs ; the process given in that chapter will serve to produce negatives as well as positives. More 68 PRACTICAL PHOTO-MICROGRAPHY. complete instructions will be found in “ Processes of Pure Photography.” A dozen or two of ordinary portrait gelatine bromide plates, a dozen or two of very slow thickly-coated plates, and a dozen or two of “ color-sensitive ” plates, will form a sufficient stock for a beginner. If the reader is au fait in the wet collodion process, he will do well to try it in order to satisfy himself—as the writer did—as to the relative advantages of wet coflodion and dry gelatine. The greatest mistake that can be made is to change from one make of plate to another without very weighty reasons. The beginner, especially, should stick to one make of plate and work with it till he can work it well. There are few plates in the market that will not yield a perfect negative when properly used. t CHAPTER X. THE CONDENSER AND BULL’S-EYE—THEIR USE AND ABUSE. In some almost classical books on the Microscope, the Con¬ denser, Achromatic or otherwise, is passed over with little more than mere mention ; its construction is described en passant, but it is easy to see that the writers placed little importance on, even if they understood, its use. It is by no means a long time since even the best microscopists were not wholly aware of the full advantage to be gained by a proper use of the con¬ denser, and the author is informed that Mr. E. M. Nelson played a prominent part among the demonstrators of the scientific application of what is now admitted on all hands to be a matter almost as important as the objective itself. The condenser, as before stated, is not intended merely to throw a blaze of light upon the object, and as it is necessary that the photo-micrographer should thoroughly understand the use of the condenser a diagram and some remarks are here given, which, it is hoped, will elucidate the matter, and still further remarks and diagrams will be found in a later chapter. (See p. 83) Fig. 25. —General Outline of Elements of a Condenser. The substage condenser is made to collect the pencils of light from the radiant H, and to focus these pencils on the f 70 PRACTICAL PHOTO-MICROGRAPHY. object 0. The object-glass is also made to focus on O ; and object-glass and condenser are working at their best when the object, or the critical plane of the object, lies in focus of both object-glass and condenser, and then only. The best resolution of any plane of an object can only be achieved when the object lies in the conjugate foci of objective, and condenser. But the focus of the condenser is not usually long enough to throw upon the object an image of the light large enough to cover evenly the field of the objective and so with low powers we have an image of the light only partially cover¬ ing our field, and while scientifically speaking this is the true critical image of our object, still, as a rule, a photo¬ graph of an image so illuminated would be unsightly. There¬ fore we make a compromise in one of several ways ; we sacrifice to some extent the accuracy of our critical image in order to make a more sightly photograph. And often we may have so much resolving power “ in hand,” so to speak, that we may sacrifice some of it without losing any of the necessary resolution. Be it clearly understood that a critical image is really the image of the radiant with the object intercepting certain pencils of light; this is fact for all cases of axial trans¬ mitted light, but there are cases of oblique lighting and reflected lighting where the object itself becomes the radiant. At pres¬ ent we deal only with axial transmitted light. We effect the compromise mentioned above in various ways,, some better than others. The commonest way is to interpolate between light and condenser a bull’s-eye which collects pencils of light from the radiant and transmits them parallel to each other into the condenser. The result of this is that the rays previously focused on a small area of the object are now spread evenly over the whole field, and if the bull’s-eye is properly used in such a case there ought to be no falling off in the quality of the image. It is important to keep the bull’s-eye at a good distance from the condenser, and the radiant must be at the focal point of the bull’s-eye. This is the usual method of procedure with simple objects and low powers. The bull’s- eye is sometimes used alone as a condenser, being turned with its convex side toward the radiant, and where an angle of not PRACTICAL PHOTO-MICROGRAPHY. 71 more than 125 deg. is required, it answers fairly well in this capacity. Another method of compromising has been much used by the writer for very low power work, and even under certain circumstances, for high power work ; it answers as a makeshift for the low power work, bnt is not recommended for high power where, in fact, it is not necessary. The arrangement is shown in figure 26. G B Fig. 26. Here R is the radiant, B a bull’s eye parallelizing R ’s rays upon a disc of very finely ground glass G, the bull’s eye and disc being so fitted that they can be fixed as a whole piece of apparatus, in front of the radiant. Reference to figure 16 will explain the fitting of this to the front of the lantern; S, in figure 26, being a pinch screw by which the apparatus is fast¬ ened to the front. Here G becomes practically the radiant, and the writer hoped great things for this arrangement, until he found that he could not accurately focus the ground glass, even when oiled, on his object without getting an image of the grain of the ground glass, which was fatal, of course, to accurate focussing of this radiant. But for the lowest power work, where the angular aperture of the objective (as 3-inch, 4-inch, etc.) is very low, and where the use of a condenser is forbidden, this apparatus is strongly recommended and often used by the writer. It might appear that by using, in place of the piano convex B, a double convex, a small disc of very brilliant light would be obtained on G, the size regulated by sliding G to and from B 3 and that this small disc would be very valuable 72 PEACTICAL PHOTO-MICEOGEAPHY. for medium and high power work; but it is not so, for the diffi¬ culty of focusing the image of the ground glass again comes in. It may be worth while to try, in place of ground glass at O, a cell full of milk and water, as suggested by a friend of the writer. There is still another method of obtaining, with a condenser alone, an evenly lighted field. This method consists in mis- focusing the condenser, but we need hardly point out at any great length the danger of this system. If, in Fig. 27, A be the object and C the condenser focused on A, as shown by continuous lines, the image of the light seen with a low-power objective may be too small to cover the field ; if we focus the condenser down (dotted lines), or up (interrupted lines), we shall get an even field of light, but our object will no longer fulfill our condition of being in the foci of condenser and objective at once, and so our image will be inferior. If we focus our condenser and o. g. both on our object, look down the tube without ocular, arrange our condenser aperture so as to file our o. g. with light, and then rack down our condenser, we shall see that our o. g. is no longer fully utilized; but on replacing the ocular we may find our field now evenly lighted. So that focusing down our condenser has entailed loss of aperture. If, by opening our condenser aperture by an iris or by a larger stop we can once more fill our objective with light, probably not much harm will be done to the quality of image by our racking down of the condenser; still the writer decidedly objects in practice to this system of mis-focusing, and recommends any other system in preference to this one. To recapitulate: W ith low powers, where the image of the radient focused on the object does not sufficiently fill the field, PRACTICAL PHOTO-MICROGRAPHY. 73 a bull’s-eye may be placed between light and condenser, at a possible sacrifice. With high power objectives the image of the flame, focused as before, is usually large enough to illu¬ minate the whole field without any bull’s eye. With the low¬ est powers the condenser is usually and preferably omitted. .Note.— In very many cases it will be found convenient and advantageous to use an objective as substage condenser. The objective so used may have an aperture equal to or less than that of the objective used for the' image projection. Those who have good objectives, but only poor condensers, may do well to adopt this system for all work. The writer, for pur¬ pose of experiment, had a mount made fitting his substage and taking his objectives; the addition of a small iris diaphragm made the apparatus complete. The results were so satisfactory that we would not hesitate to adopt this system entirely, were we not already in possession of Powell and Lealand’s fine apo- chromatic, and Zeiss’ achromatic condensers; the former of N. A. 1-4, the latter of N. A. 1, and capable of being used for quite low angles. For example, using in our substage as con¬ denser an apochromatic o. g. of 16 mm. N. A. .30, we resolved with a 2 mm. o. g. P. angulatum, into white areas or black dots, with ease and at will. For ordinary photography of bacteria and the like we find this arrangement, or a similar one, not at all inferior to the use of expensive achromatic condensers. CHAPTER XI. THE USE OF THE EYE-PIECE OR OCULAR.—STOPS.— REFLECTIONS. The ocular in ordinary microscopy is an optical system whereby the aerial image produced by the objective is “ taken up ” and projected, magnified more or less, on the retina of the human eye. The ordinary Huyghenian ocular is made for this sole purpose; it is frequently made non-achromatic It is therefore not to be expected that an ordinary Huyghen¬ ian ocular, particularly one not achromatized, should project a perfect, or even a good image on a flat plate perhaps 30 inches distant from the spot for which the ocular was intended to work. The writer is, however, bound to accept as a fact in the experience of others what his own experiments have in¬ variably failed to verify, viz.: that in some cases an ordinary microscope ocular does project on a screen, distant from the ocular from 20 to 40 inches, a true image, and that a photograph tolerably faithful to nature can be made of that projected image in the usual way. Certainly it is conceivable that by some accidental suitability of ocular to objective such a result may be obtained. The writer, therefore, does not gainsay the assertion that photo-micrograplis of the highest quality and of difficult objects may be produced by the use of the ordinary achromatic eye-piece, but he does say that he has never produced nor ever seen any photo-micrographs that he could call first-rate obtained by the use of the common eye¬ piece sold with ordinary microscopes and used for ordinary observation. On the other hand microscopic objectives are not intended for projecting images upon screens several feet distant, but are constructed so that their best image falls somewhere between 6 and 12 inches up to the microscope tube; and, moreover, that image is intended to be “ picked PRACTICAL PHOTO-MICROGRAPHY. 75 up” carried on, modified, and in many cases and ways cor¬ rected by an ocular. Still the author has seen and produced micrographs without an ocular not easily to be surpassed. One of the latest outcomes of optical science has been the construction by Herr Zeiss, of Jena, on formulae by Dr. Abbe, of a series of oculars arranged for the purpose of projecting the image formed by the objective. The writer may as well state at once that he believes, and has good reason to believe, that in the use of the projection oculars, with other matters of different nature, will lie the future of scientific photo-micro¬ graphy. In the author’s experience these oculars have acted well with objectives made by makers other than Zeiss; for in¬ stance, they have been found to work satisfactorily with a 1-inch, a and a y 1 ^ immersion, all by Swift of London ; also fairly well with a by Reichert. It is matter for congratu¬ lation that the Zeiss (or Abbe) projection oculars are cheap, costing only about £2, or say $10 each. A chapter will be devoted to the apochromatic lenses and “ compensating ” and “ projection ” oculars made by Zeiss. Undoubtedly an eye-piece of any kind, when it can be used without detriment to result, is a great convenience. The cam¬ era does not require to be so long; there is less danger of internal reflections, which must be sedulously avoided, as will presently be seen. But failing a projection ocular, or failiug the necessary coincidence between projection ocular and ob¬ jective, the balance of opinion and the balance of high-class results are probably in favor of the image projected directly upon the screen by the objective. The writer at all events is willing to commit himself to this opinion, and to recommend either a projection ocular or none. Amount of Magnification. To discuss this question w'e may adopt an arbitrary term: “Initial power.” We propose to call the initial power of an objective the amount of magnifica¬ tion (in terms of diameters) given by the objective at 10 inches behind its posterior conjugate focus. (The distance, 10 inches, is about the distance for distinct normal vision, and is chosen for that reason.) A good objective will, as a rule, when properly used, stand a strain of magnification to ten times its initial power. 76 PRACTICAL PHOTOMICROGRAPHY. Thus : A lens of one inch focus gives at about 10 inches up the tube a magnification of 10 diameters; by whatever means we try to get from that lens a magnification of 100 diameters projected as a real image, it must be a good lens if it stands that strain with¬ out breaking down in definition or corrections. In fact none but the very best objectives will tolerate any such strain. By dint of exceedingly skillful manipulation such as only a few men can claim to have acquired, we have known specially fine objectives stand a stretch of their powers even greater than this, but the writer’s own attempts in this direction have always been utter failures, and in the majority of cases six or seven times the initial power is ample to produce a faltering of the lens’ capabilities. And this holds good whatever be the means adopted for increasing the magnifying power of the objective, whether long stretch of camera or high eye-piecing, or the two combined. Thus Zeiss makes two projection oculars for one series of lenses; the first ocular increases the magnification by “ three times ” and may be used without difficulty up to a stretch of about 30 inches from ocular to sensitive plate (giving a power about nine times the “ initial power ”) ; while the higher power projection ocular magnifying “ six times ” will at the same stretch break down the finest objectives, unless the skill of the operator be very great indeed, greater than the writer can claim, certainly. “Stopping down” Objectives. It is common to find at the back of objectives a cell or diaphragm constricting more or less the light-way from the objective to the eye or to the plate. So long as the “ stops ” do not cut off any pencils of light that otherwise would reach the plate or the retina no harm is done; but if the stops are used as a supposed means of reducing aberrations or incorrectnesses of the objective, a great deal of harm is done and a very foolish mistake made. By the time that the pencils of light have passed through the objective the mischief is done, if it is done at all, and a stop behind the objective may hide , but cannot possibly correct any errors. Whatever “ stopping ” is to be done should be done in front of the objective, that is to say in the condenser, or, failing a condenser, in the substage. If, therefore, the reader PRACTICAL PHOTO-MICROGRAPHY. 77 finds a stop at the back of any objective, the aperture being small enough to constrict to any notable extent the cone of rays proceeding from the objective, he is advised to remove that cell or stop, blackening with some dead-black pigment the interior of the objective where it will probably be found bright. The writer owns a ^-inch o.g. which, when purchased, had a ridiculously small stop at the back. The lens was condemned as very poor by several experts who examined it ; but, on re¬ moval of the stop by way of experiment, it was found to be a very good lens indeed, and has been much used by the writer for a certain class of work. Reflections inside the apparatus. The reader is earnestly advised to made sure that there are no “ shiny ” points or sur¬ faces inside any part of his apparatus for photo-micrography. The inside of a microscope tube is almost invariably worn bright at some places, and even the black varnish inside an ordinary camera is often quite capable of causing most trouble¬ some “flares.” The micro-tube should be lined with black velvet, and not only should the interior of the camera be dead black all over, but blackened cards should be put inside, having apertures graduated from front to rear of the appa¬ ratus, so that no light can reach the plate except that which passes through the objective and forms the image on the plate. This is a most important caution and is especially to be ob¬ served when the image is projected by the objective alone without ocular, for as already noted the ocular has the merit of reducing danger from untoward reflections. Whether the ocular is in use or not there should always be, as close as con¬ venient to the front of the sensitive plate, an opaque plaque having a disc of aperture a shade larger than the area of plate being used. Th v plaques may be of cardboard, wood, metal, or any other suitable material, but they must present a dead- black surface. Sometimes the edges of the apertures in the condenser-stop become bright from friction, and this is specially the case with badly designed “ Iris ” diaphragms. The writer has had con¬ siderable trouble with this defect in one of his apparatus, and warns the reader against it. Internal reflections cause uneven * 78 PRACTICAL PHOTO-MICROGRAPHY. illumination, “flares,” and poor thin negatives with back¬ grounds dirty grey in place of dense black. The apartment where microscopic work, visual or photo¬ graphic, is going on, should be as nearly dark as possible. ' I No. 2.—Triceratium, X 750. Plate II CHAPTER XII. PROGRESSIVE EXAMPLES. It is proposed in this chapter to give a few examples of operations for subjects presenting various degrees of difficulty ; taking to start with the easiest class of object likely to be met with, and attacking it with the simplest apparatus likely to be required for any class of work. Example 1. A subject presenting only light-obstruction,with¬ out very delicate marks or structure, and with just enough of color to give actinic contrast, thin and flat; to be photographed with a low power to a magnification not exceeding 20 diame¬ ters; no ocular nor condenser, direct illumination. Subject; a good section of wood. Objective: two inches focus or lower power. If an oil lamp provide the illumination the wick to be turned “ broadside on.” Procedure: Having focused the object with objective and ordinary ocular, arrange the object and light carefully so that the whole field is evenly illuminated; attach the camera to the microscope, the ocular being entirely removed. Proceed to adjust the focus on the ground-glass of the camera; for this the microscope tube will require more or less racking in. In such work as this there is danger of the tube cutting off part of the field; a good wide tube is therefore an advantage. Notes: It is supposed that the reader has studied the foregoing instructions and diagrams with regard to means for preventing access of stray and reflected light to the sensitive plate. The leather cap seen at No. 11, on Fig. 16, is in the writer’s prac¬ tice placed on the end of the tube, and the camera is then pushed forward till the brass cap on the front passes into the leather cap 11, thus forming a light-tight junction. In order to make sure that the lighting of the field is even, the best plan is to remove the ground-glass of the camera, and 80 PRACTICAL PHOTO-MICROGRAPHY. to hold a few inches behind its late position a piece of white paper or cardboard. On this white surface the image is visibly projected, and uneven lighting easily detected. In most cases the ground glass of the camera and the coarse adjustment of the microscope permit of sufficiently accurate focusing with very low powers, but a “ Ramsden ” eye-piece placed on the plain glass focus-screen may be found preferable in conjunction with the fine adjustment of the microscope. Often, however, where a good general appearance is wanted the unaided eye and ground glass are better in practice. Our ground-glass disc illuminated by parallelized rays, as figured ISTo. 26 on page 71 will be found most convenient for this class of work. While the object is being examined in the microscope with the ocular, the part of the object occupying the centre of the field should be carefully noted; this point must occupy pre¬ cisely the centre of the ground-glass, where the camera is attached. If the apparatus is of the construction suggested on page 51, where the microscope and the light are fixed in their relationship to each other, and where the table bearing them rotates to a “ stop ” when the camera is about to be attached, it is important to note at the very first whether the centre of the object coincides precisely with the centre of the ground- glass when the image is seen on the ground-glass. The centre of the latter should be marked in pencil on the ground side of the glass ; this may be done by drawing diagonals and describ¬ ing a little circle round the intersection of the diagonals. The cardboard or other discs recommended on page 77, should have apertures corresponding in size to the sizes of plates to be used. These remarks if carefully noted and acted upon will save trouble in future, and the image being focused on the ground- glass, the dark slide carrying the sensitive plate is inserted in its place, the light shut off, preferably by a shutter working very easily inside the camera, and all is ready for exposure, remarks on which are left for a later page. Example No. 2. A subject similar to No. 1, but more finely marked and smaller, requiring more angular aperture, and still PRACTICAL PHOTO-MICROGRAPHY. 81 practically colorless. Low power, narrow angle condenser magnification about 30 diams. without ocular, 120 with pro¬ jection ocular. Subject a flat diatom as Arachnoidiscus Ehrenbergii , or an Echinus Spine. Objective two-thirds inch or one inch ; Achromatic condenser, front hemisphere removed. Procedure: A. To centre the condenser—( this step must be taken in every case when a substage condenser is to be used.) Placing the pinhole cap on, or a pinhole diaphragm in, the condenser, examine with a low power objective and eye piece, working the substage centering screws, till the- disc of light, (which may proceed from any radiant), occupies precisely the centre of the field. Remove pinhole stop or cap. B. To centre the light. ( This also must be performed in every case.) The condenser being centred as under “ A,” rack out and in the substage and the microscope tube with objective and low power eye-piece, until an image of the light is seen sharp in some part of the field. (The light may require to be moved in order to bring it upon the field.) 'With an oil lamp the wick should present its edge to the condenser. Move the light from side to side and up and down till its image falls directly in the centre of the field. G. To focus objective and condenser on the object. Place the object on the stage so that it occupies the central position, focus the objective on the object and then rack the substage till the image of the light is sharply focused across the object. This is a suitable arrangement so far for obtaining the best possible image microscopically of a certain area and plane of the object. The nearer the light is to the substage, and the longer the focus of the condenser, the larger will be the sharp image of the radiant across the object. D. To spread the light evenly over the field either a bull’s- eye or a diffusing medium—as the ground-glass referred to above—must be placed between the light and the condenser. If the “ ground-glass and bull’s-eye ” arrangement be used, the result is simply to transfer the radiant surface from the lamp to the ground-glass ; if the bull’s-eye alone be used the result is to fill the back of the condenser with parallel rays; and in order to get the most evenly lighted field the condenser will probably 82 PRACTICAL PHOTO-MICROGRAPHY. require to be racked down more or less. The use of the bull’s- eye is so important that diagrams and quotations from papers by Mr. E. M. Nelson, in the English Mechanic , 1884, shall be given in explanation. In the first place the light must be in the focus of the bull’s-eye, and the latter is to be fixed so that the edge of the flame if a wick is used, or the surface of the lime if the limelight is used, is in the focus of the bull’s-eye. In order to ascertain whether the bull’s-eye and light are in proper relation to each other, Mr. Nelson recommends either that the eye be placed in the rays proceeding from the bull’s- eye, or that a condensing lens be placed in the rays and the image thrown upon a white card there examined. In fig. 28, E represents the edge of the flame, P the bull’s- eye and A the image as it ought to be seen on the card. P C D © O «> Fig. 28. B represents the appearance when the bull’s eye is too near the edge of the flame. C represents the appearance when the bull’s eye is too far away from the flame. D shows the appearance when the bull’s eye is focused, but out of centre. In order to get the proper use of the bull’s-eye, it should not be nearer to the substage than a distance of, say, 12 inches. The same papers by Mr. N elson contain information regard¬ ing the substage condenser, so useful that the author ventures to copy some further figures. PRACTICAL PHOTO-MICROGRAPHY. 83 A (Fig. 29) shows a substage condenser and an objective focused on the same point, and the aperture of both equal and fully utilized. On looking down the microscope tube the lens will be seen filled with light as at C. B shows a condenser and an objective still focused on one point, but having their apertures cut down by a stop in the condenser ; the back lens, examined as before, will present the appearance of D. Fig. 29. These remarks and quotations, though, perhaps, not in their proper order here, are to be specially noted in every case where a bull’s-eye is used. The bull’s-eye, as already stated, should be on a stand with a heavy base, and in order to focus the con¬ denser when the bull’s-eye is to be used, the following steps may be taken. The objective being focused, the condenser and light centered, the edge of the bull’s-eye is advanced in front of the light and the image of the metal rim of the bull’s- eye is focused on the object by means of the condenser. At the same time the worker must take care that the segment of 84 PRACTICAL PHOTO-MICROGRAPHY. the circle of the bull’s-eye seen on his field is vertically central, i. e., the imaginary centres of bull’s-eye and field should be in one line. This is difficult to explain, but will be understood on experiment. Lastly, when the field is illuminated, the bull’s-eye being focused and in its place centrally, the worker is to look down his tube and compare what he sees with our figures on pages 82 and 83. If the entire area of the back combination is filled with light, we are utilizing all the aper¬ ture of our objective, and it may be said that, in many cases where it is desirable to utilize every fraction of aperture that our objective possesses, the use of the bull’s-eye is to be recom¬ mended. In many cases the use of our entire aperture intro¬ duces photographic difficulties, but these must be overcome. If a photograph is wanted of the “ general appearance ” of an object, it is well to cut down the aperture much more than would be permissible where a scientific photographic representation is required.^ We are aware that this is heresy to some old workers. Example Ho. 3.—A “ critical image,” with a low power, a condenser, no bull’s eye. Subject: “ Test hairs ” on a blow¬ fly’s proboscis. Objective: A two-thirds, one-half, or one- quarter inch of highest attainable aperture ; this test for a one- quarter inch being, as a rule, too easy. Projection ocular used; magnification from 150 to 400 diameters. See Plate I, fig. 1. A and B, centre condenser and light as before. C, for the two-thirds, or one-half, or four-tenths objective, the substage condenser may be usod without its front hemisphere, unless with its front it has a very low angle, or unless it is not achro¬ matic. Two things must always be observed with regard to the condenser; first, it must have sufficient angle to fill the objective; second, whatever its angle be, if its angle is greater than that of the objective, it must be stopped down till the angles are nearly equal. This time the object is focused care¬ fully with the objective, and then the light is most carefully focused on the part of the “ tongue ” to be photographed. The hairs will now be seen more or less elongated, according as the correction of the objective happens to be more or less accurate. PRACTICAL PHOTO-MICROGRAPHY. 85 Now come in the skill and experience of the worker, for the objective has now to be corrected for the thickness of the cover glass and the position of the test hairs with regard to the cover glass. The correction is to be accomplished by means of “ screw collar,” if the objective has one, or by length of tube, if the objective is without collar. We cannot instruct in this matter. The screw collar or the length of tube is to be altered gradually till the hair under observation is shown as long as it can be shown, as black also and as finely pointed. In critical image work it is better to choose one hair and con¬ fine the attention to that hair; if the object be not flat, of course other hairs will not be equally sharp, but we have noth¬ ing to do with that; our business is to get a perfect image of our one object. If we require the best general photo-micro¬ graph of our “ tongue ” we must set about it by method No. 2. The objective being corrected, and the image perfect as seen with the ordinary eye-piece, the latter is removed and the pro¬ jection ocular substituted; the image is then projected first upon the ground glass of the camera. Assuming that a pro¬ jection ocular of the type made by Zeiss is to be used, we have next to focus the diaphragm of this ocular upon our ground glass, where a round disc due to the diaphragm in the ocular will be seen.* This is easily done by observation, twisting round the moveable part of the ocular, where we shall find a scale and an index put there for the purpose. Next the image, having been carefully centred on the ground glass, is accurately- focused on the plain glass prepared as suggested on page 53. Here again we can not instruct; it is a matter of experience to focus a difficult image. With objectives, other than apo- chromatic, fringes of color are usually found round the critical part of the image ; very often, with ordinary objectives, the focus will be found correct when the colors seen are claret and green; but this depends on the correction for color given to the lens by the optician. The apocliromatic lenses, in our * If the image of the diaphragm as projected is larger, we may have to slew the camera a little in order to see the edge of the projected disc image. 86 PRACTICAL PHOTO-MICROGRAPHY. experience, give little or no color which is not in the object, and we never, with these lenses, find fringes of color. But we > find the focusing as easy with one glass as the other; it is only- after development that the superiority of the apochromatic glasses shows itself unmistakably. When we use u Bamsden ” or “ Aplanatic ” focusing ocular color is often seen in the objects, but we must not attribute that to the objective. If this color is objected to, a very low-power ocular made by Zeiss, and called a “ Searcher Eye-piece,” may be sunk in a plaque of wood, that is to say, the eye-piece may be thrust through a hole in the wood to such a distance that when the wood occupies the place of the ground glass or sensi¬ tive plate, the diaphragm of the eye-piece occupies the critical plane where the image is to fall on the sensitive plate. Several holes may be bored in the plaque of wood, and the eye-piece may then be moved from hole to hole. Under the next set of examples we may put a very large series # of objects—always overlooking, for the present, the pho¬ tographic difficulties of color to which an entire chapter is allotted—such objects, for instance, as the easier diatoms; phy¬ siological, histological and pathological subjects; insect structure and the larger bacteria where no minute structure is to be shown, as flagella. For all such subjects where the magnifica¬ tion required is from forty diameters upwards, the substage condenser with bull’s-eye may be used, and the focus in such cases should be general rather than critical. Below forty diameters the writer avoids the use of a magnifying glass for focusing, believing that a better general focus is obtained with¬ out the Bamsden; he admits, however, that his eyesight is possibly abnormally sharp. As soon as the power used is sufficiently^high to magnify the focused flame-image so as to make it cover the whole field to be photographed; i. e., in all magnifications over say 400 diameters, the writer always dis¬ penses with the bull’s-eye. As already pointed out, the size of the flame-image on the field depends in the first place on the focus of the condenser, in the next place upon the combined power of objective and ocular. As the writer progresses in experience he uses the bull’s-eye less and less in his work. In PRACTICAL PHOTO-MICROGRAPHY. 87 fact he has of late discarded it almost entirely. Perhaps the best plan is to omit the bull’s-eye and use a condenser of such focal length as to project on the object a sufficiently large image of the radiant. Photo-micrography of deep objects, as many diatoms, is a vexed question which the writer prefers to leave undiscussed. The reader must judge for himself whether he is to get the best general appearance of his object, whether he prefers to resolve one plane without attention to any other plane, or whether he will be best suited by a compromise, that is, by a little resolution with fair general sharpness. One thing he need not attempt, viz.: to get perfect resolution on several planes simultaneously. The achievements of the most difficult photo-micrography are vouchsafed only to the most careful and skillful operator. There is no secret in, nor any royal road to the photography of the flagellum of a microbe, or the “ dots ” on P. angulahim. Good optical appliances, absolute freedom from tremor, efficiency in centering, correcting and focusing, are the only secrets. Skill in these matters can only be acquired by long, earnest, unflagging study and practice. An occasional rush will not do for this work; it is necessary to give up to it the entire attention for the time being, and there is no use for any person to attempt this work at any odd moment, for failure is sure to result. Above all we would counsel our reader to study the science of correcting his objectives by collar or tube ; centering and focusing are mechanical, and follow definite and patent rules; “ correction ” is never alike for two objects, and is a matter of sheer accuracy of observation the highest quality a micro- scopist can possess. In a later chapter we shall specify certain difficult and com¬ mon test objects giving hints—and only hints—how the work may be attempted. CHAPTEK XIII. EXPOSURE. There is not a subject more important, nor any more diffi¬ cult to deal with, than this one. We shall make it even more important in photo-micrography than it is in general photog¬ raphy, because we propose to advise the reader not to alter to any serious extent the constituents or proportions of his devel¬ oping solution. It is a more difficult subject here than even in general photography, for whatever be the colors of the ob¬ jects actually photographed in the latter branch, there is al¬ ways a very large amount of reflected white light which to a vast extent lessens the difficulties arising from the colors of the objects themselves. Had we to photograph a landscape entirely by transmitted light, were such a thing possible, the result would be curious; in photo-micrography practically 99 per cent, of our light is transmitted, opaque objects being rarely photographed. We may state at once that we do not approve of the sys¬ tem of trying to give rules for exposure ; rules and tables doubtless assist the beginner at first but leave him helpless in the end. Moreover, it is futile to attempt to give rules, for it is impossible to take into account the most puzzling of all photo-micrographic conditions, that of color. If we worked only on colorless objects, we could easily give a most useful code of exposures, but no such state of thing obtains, a color¬ less object is very rare in photo-micrography. We propose rather to begin at the other end, and to inform our reader how to know after exposure where he has erred; and by this means he will not only very soon arrive at the proper exposure for the particular object in hand, but he will gain experience by every exposure he makes. By far the nearest approach to a scientific judgment of exposures that we know is the table PRACTICAL PHOTOMICROGRAPHY. 89 of Dr. E. C. Bousfield already alluded to (p. 14), as his table is copyright we do not, even with his permission, pro¬ pose to use it here. , The factors on which exposure mainly depends, excluding from our consideration color, are 1. Illumination. 2. Magnification. 1. Illumination. We can no more lay down rules with re¬ gard to this than with regard to color. An oil lamp is the weakest light generally used, the lime light next, magnesium next, the electric light next, sunlight the most powerful. But all depends on how they are used; lime light properly burning and properly used may be much more active on our plate than diffused daylight. As we propose to con¬ sider here artificial light only, and specially the lights we know best, oil and lime, we need only say that the lime light may be from 10 to 50 times as powerful and as actinic as the best oil lamp. The effect of the condenser properly used is astounding to the beginner, the bull’s-eye sometimes increases and sometimes diminishes the force of the light. The beginner after producing a negative will find it very difficult, even with the instructions to follow on later pages, to form a correct opinion as to whether his negative is over, under, or properly exposed. His best plan is probably to show his negative to some experienced photographer who will be able to give a certain amount of help as to the steps to be taken in future. But even the experienced photographer, and in some cases even the experienced photo-micrographer, will be at a loss to decide whether a negative is over or under exposed. In this case the only thing to be done is—if the prints are not satisfactory—to try a longer exposure and a shorter one. Exposures should be varied by geometrical rather than arithmetical progression; that is, if, for example, an exposure of 40 seconds is found to be wrong, it is well to try 20 or 80, or even 10 or 100 seconds rather than 35 or 50. A weak background ( i . e., a ground grey in the negative and dirty white in the print) is a sure sign of one of three faults: 90 PRACTICAL PHOTO-MICROGRAPHY. 1. Under-exposure; 2. Reflected light inside the apparatus ; 3. Too much light, or rather, too much angle not necessarily over-exposure (over-exposure, under certain conditions and to a certain degree, causes also grey backgrounds; but in this case, as we shall see later, the whole image is grey). If we are dealing with a colored object we are practically compelled to disregard all circumstances except that of color. Color upsets every calculation of exposures that human ingen¬ uity can devise. Reds and yellows sometimes cause us to in¬ crease our exposure a hundred fold, but there are reds which, being bluish (as eosin, a favorite stain with many for certain ob¬ jects), upset all our previous calculations. Violets are, of course, as a rule, highly actinic, aud require very brief exposures, but logwood, as an example, stains certain tissues to a violet so full of red that again we may be completely at sea in our exposure. The writer has daily experience of such puzzling conditions. Yellows, in like manner, are in certain objects practically almost chemical opacity, while in other cases their contrast with the white ground is so small as to render great the diffi¬ culty of differentiating between the yellow and pure white. A red, a violet, and a yellow may each be either very easy or al¬ most impossible to render by ordinary photography; a mixture, such as a double stain of violet and red, is very often, with¬ out “ color correct ” or “ orthochromatic ” photography, a com¬ plete impossibility. In view of conditions so common, yet so puzzling, we again submit that any table of, or rule for, exposure would be out of the question : what we may do, and propose to try to do, is to give guides by which the reader, on developing his negative, whatever the subject may have been, may be able to correct at next trial any error he may have made in his first exposure. Appended to each of the illustrations of this book we have noted the exposure given by us in producing the negative, but even this attention on our part is only of minor value on account of our inability to gauge the quantity and quality of of the light actually reaching our sensitive plate. CHAPTER XIV. DEVELOPMENT OF GELATINE-BROMIDE PLATES. Normal Developers. A .—Pyro-Ammonia. Pyrogallol. 2 grains Liq. Amm. .880. 3 minims Potassic or ammonic bromide. 1 grain Water to. 1 ounce B . —Pyro-Carbonate. 1. —Pyrogallol. 3 grains Sodic carbonate.12 grains Water to. 1 ounce Or, 2. —Pyrogallol. 3 grains Potassic carbonate.12 grains Water to. 1 ounce Or, 3. —Pyrogallol. 3 grains Sodic carbonate. 6 grains Potassic carbonate. 6 grains Water to. 1 ounce Notes. —The reader is referred to Chapter VIII for hints as to method of “ stocking ” the above reagents. The stock solu¬ tions there formulated are all so-called “ 10 per cent, solutions,” and in each case a grain or a minim may be obtained by tak¬ ing ten minims of the stock solution. Thus to make the Normal Pyro-Ammonia Developer we take of Stock pyro solution.20 minims Stock Ammonia.30 minims Stock Bromide.10 minims and make up with water to one ounce. 92 PRACTICAL PHOTO-MICROGRAPHY. To make the Normal Pyro-Carbonate Developer No. 3 : Stock Pyro Solution.30 minims Sodic and Potassic Carbonate Solution.120 minims (=2 drams). Bromide...None. Make up with water to one ounce. (No account is taken of the chemicals used merely to preserve the pyro.) Normal Developer G. Ferrous Oxalate. Take of the saturated solution of Potasssic oxalate. 4 parts Ferrous sulphate. 1 part Be careful to pour the Ferrous Sulphate Solution into the Pot¬ assic Oxalate and not vice versa. (Note.—The writer does not recommend the Ferrous Oxalate Developer for ordinary photo-micrographic work. This is not denying that it is excellent in some hands and for some kinds of subject. The reader is advised to try the effect for himself; as a general developer it has some recommendations.) The plate after exposure is, in non-actinic light, (see pp. 55 and 56,) placed film upwards in a black developing tray, and the developing solution is deftly swept over it; the developer must not be poured upon one spot but “ swished ” with a side motion all over the plate, so that as far as possible the plate may be all wetted at once. Some workers prefer to soak the plate, till the gelatine is all wet, in plain water before applying the developer; this procedure does no harm if air bubbles that may form are removed with a clean brush or finger. In all cases the inexpert must be prepared for air bubbles and remove them if they occur. In a certain number of seconds the image ought to begin to appear. The first thing that regulates the time required for this first appearance is the quality or treatment of the gelatine used in the emulsion; this factor need not be taken into much account. But the important matter is to observe the quality of the image at the time of its first appearance, and still more is it important to note most carefully the pace at which the details follow each other. As a rule an overexposed plate will show some of the details before an underexposed PRACTICAL PHOTO-MICROGRAPHY. 93 plate of the same batch would do so. If with any of our “Normal Developers” no image at all is visible after thirty seconds, the plate may be put down as underex¬ posed. The image appears in the following order of rapidity with our three Normal Developers : 1st Pyro-Ammonia; 2nd Pyro-Carbonate; 3d Ferrous Oxalate. This is to be taken as a general rule in comparing the Carbonate with the Oxalate developer. Bromide always slows the appearance, and also the acquisition of density and detail, of the image and free bromide in the developer has a much greater apparent effect on the carbonate and ferrous developers than on the ammonia developer. Half a grain of bromide in the carbonate or fer¬ rous developer has at least as much retarding effect as a grain in the ammonia developer. Bromide restrains both detail and density, but luckily it restrains detail more than density. The chief use of free bromide in the developer is to give us time to watch progress and to stop progress at the proper moment, but bromide is absolutely necessary to prevent fog with certain plates used with the ammonia developer. In the case of the carbonates the carbonic acid evolved acts as a restrainer and retarder; in the case of the ferrous oxalate, ferric bromide is formed in considerable quantity, and restrains and retards de¬ velopment. Hence a ferrous oxalate developer used over and over again works each time more slowly and more feebly than the time before. We deprecate the repeated use of the same dose of pyro-developer; but repeated use of one dose of ferrous oxalate is quite permissible up to a point which will easily be known by muddiness of the solution, and slowness of its action. If the image starts about 10 or 15 seconds after the devel¬ oper is applied to the plate, we must be prepared for over¬ exposure of the high lights at least. With subjects presenting violent contrasts of density or color-actinism, we are almost bound to over-expose the high lights; but with ordinary sub¬ jects the image, if it appears at all in 15 seconds, should show only the highest lights at that stage ; and the half tones of a properly exposed plate will follow the high lights without any lagging or apparent reluctance. In fact, the develop¬ ment should proceed steadily, without stoppage and without precipitancy from start to finish, so far as detail is concerned. 94 PRACTICAL PHOTO-MICROGRAPHY. The details that first appear under any of our normal devel¬ opers must be most carefully scrutinized, because upon their appearance we shall to a great extent base our judgment of exposure. If the first details are pale gray and are hurriedly followed by others almost equally gray the plate is over-ex¬ posed ; if the first details rapidly become black, while other details are either pale gray or invisible, the plate is certainly under-exposed. If the whole plate become very quickly gray over-exposure is certain; if the whole plate become dense black, or dense black and dark gray, a less degree of over-ex¬ posure is the probable cause. Experience, and a certain amount of allowance for the nature of the subject, are both necessary in judging of ex¬ posure by observation of the appearances during development; but when experience has been gained, and a variety of sub¬ jects photographed, the worker will be able very accurately to judge where and how much he has erred—if he has erred —in exposure. But two things are settled: An image homogeneous in color all over is an over-exposed image; an image which is white in any part when development is com¬ plete is an under-exposed image, unless we have a subject requiring absolute blackness in our print, such, for instance, as a “dark-ground” subject. If some part of our object be nearly opaque or highly non-actinic in color, as many patho¬ logical and physiological red stains, we have but two courses open to us. 1st. To deliberately over-expose the high lights or actinic colors to such an extent as will allow development of the opaque or non-actinic parts. Yery often this method is unexpectedly successful, the high lights not being so much overexposed as might have been feared; or 2nd. We must use a color correct plate, cutting off, if necessary, the over-actinic rays by suitable screens. (See chapter XYI.) Another matter resting chiefly on experience, and almost impossible to treat usefully in a book, is the amount of devel¬ opment required. In most cases when details are all “ up,” not necessarily distinguishable but at least developed, the density of the negative after fixation would not be sufficient to yield good prints by any of our usual processes. Now in PRACTICAL PHOTOMICROGRAPHY, 95 the course of development as a rule sufficient bromide is evolved (beyond the soluble free bromide we put into the de¬ veloper), to greatly retard if not to arrest the growth of detail and density. Where the subject itself presents violent contrasts it is well to expose to such an extent that the first dose of developer shall reveal all detail without undue density in any part. Where the high lights or actinic colors are, as above advised, deliberately over-exposed in such subjects, the action known to photographers as the “reversing action of light” comes in, and these over-exposed high lights in place of being densely black in the negatives, undergo the reversing action and refuse to develop density. This, of course, suits us ad¬ mirably in the cases under consideration—of violent contrasts. But if the subject be an ordinary one without violent contrasts, or if we require as nearly as possible a black and white render¬ ing of such objects as diatoms, then we find that when detail is all “ up ” density is insufficient, and we reinforce our de¬ veloper with alkali. In the case of the carbonates, time will produce the desired effect, because though retarders are being evolved there is no volatilization of the alkali; but with am¬ monia as the alkali, not only is fresh bromide being evolved- but the original alkali is evaporating, so we generally add about 1^ minim of ammonia (15 minims of our 10 per cent.) to each ounce of developer as soon as there appears to be a halting in the acquisition of detail or density. To know when to stop development is a very serious matter. Many so-called rules have been laid down, and many hints given on the point, but we have never found any such rule or hint to cover many cases. The image is not fully developed as a rule until some part of it is visible from the back of the plate, any further examination of the back may inform us of the nature of the gelatine, of the nature of our subject, of the degree to which the emulsion has been “cooked,” but that is all. If we have a standard non-actinic light, through the col¬ ored medium guarding which we can see the flame through the plate, we shall by experience come to judge very fairly whether a plate of a batch we know is sufficiently developed. The flame should be barely discernable as to shape through 96 PRACTICAL PHOTO-MICROGRAPHY. the dark parts of the negative; the whole plate should look as if it would be a very great deal too dark but for the fixing operation, But it must by no means be opaque nor equally dark all over, nor must there be any part where the shape of the flame is clearly visible, as through clear glass. Looking at the face (film) of the plate, some parts should seem quite black, others dark-gray, others (the shadows) paler gray but not white. Practice alone, and as before consideration of the various qualities of various subjects, can ever teach us what is perhaps our most difficult lesson next to proper exposure, viz.: when to stop development. In cases of error in exposure, if the error is but slight we can almost always make our negative as good as if we had ex¬ posed correctly. And further, there are certain subjects which cannot by exposure alone without a little “ dodging ” in after processes be rendered to the best advantage. We have sug¬ gested a method of treatment for subjects presenting violent contrasts. We presently shall touch on other abnormal sub¬ jects. But we must now mention another system of development •which has marked advantages for the beginner; its chief dis¬ advantage being that in using it we have not the same useful guides to judgment of exposure. This system is usually called slow development, and as a rule the carbonates are the alkalis used. The difference consists simply in starting development with a very weak developing solution, and adding the reagents if necessary little by little till the full normal dose is reached or the full effect produced. Thus we may start with pyro, 2 grains; bromide, 1 grain; ammonia, 1 minim; and, after (say) 3 minutes add a minim more of ammonia, and so on for a space of from 20 minutes upwards, or we may start with only 5 or 6 grains of the combined carbonates and pursue a similar course. The writer has never in ordinary photography found the slightest benefit to arise from such a prolongation of development, unless it be that in certain cases more contrast of light and shade is obtained by the slow method. The tyro has perhaps more time to make up his mind when to stop de¬ velopment, but in the system recommended by us he has cer- PRACTICAL PHOTO-MICROGRAPHY. 97 tainly 3 minutes, probably 4 or 5, to examine progress, and he can hardly want more; further, by the slow process 15 or 20 minutes will be needed simply perhaps to find out that another exposure is required. Those, however, who lean towards slow development may refer to any of the Scovill series of photog¬ raphic books, where they will find full details. Abnormal Development. It has been said that if the error in exposure has been but trifling a perfect negative may still be made. It is better in most cases to make a fresh exposure, but this proceeding may not at all times suit the worker, and further it is well to gain a little mastery over, and tact in, manipulation of the developer. If appearances cited above show that the plate has certainly been underexposed, the developer should at once be thrown away and the plate washed. Thereafter a fresh developer is applied containing less pyro and bromide. Thus : Pyro 1 to grains. Bromide a grain. Ammonia as before. Or water may be added to the first developer which is to be im¬ mediately reinforced with alkali. If the plate is known before development to be underexposed to a considerable extent, it may be soaked in the alkali and water of the developer alone for a couple of minutes, the pyro and a modified quantity of bromide to be added thereafter. But an underexposed plate as a rule is useless, no matter what we do with it. With overexposure the matter is different, for a plate grossly overexposed can be saved if it is taken in time. The difficulty is to catch it in time and to apply the cure in time. Bromide certainly restrains detail but unfortunately it also keeps back density, and in a severely overexposed plate density of the high lights is the very thing most lacking. If the overexposure be only enough to make the plate very black all over, or as is the usual case, very black and dark grey, the simplest cure is to add water to the developer and to stop development earlier than if the plate were properly exposed. But if the overex¬ posure has been sufficient to make the image grey all over, the best plan is to watch carefully till every detail seems out and then instantly to flood the plate with water containing, for 98 PRACTICAL PHOTOMICROGRAPHY. every three grains or one minim of alkali in the developer, from two to four grains of sodic or potassic, or sodic and potassic, citrate. (See page 62.) This immediately stops all further development of detail, but does not seem to affect density which is gained as follows: Add to the original developer, or to a freshly made one, two to four grains of citrate for each minim of ammonia or for every three grains of carbonate, pour away the water and citrate on the plate and apply the modified de¬ veloper for a minute or two. Then add to the developer a brisk dose of alkali, say two minims of ammonia, and continue development till sufficient density is attained. The carbonates may be restrained so easily and so forcibly with bromide, that perhaps the simplest way when developing with them is to add (say) one grain of bromide for each ounce of developer. Water alone added to the carbonate developer sometimes suffices to overcome moderate overexposure. When the subject dealt with is by nature wanting in con¬ trast it will be found useful to develop with an ordinary de¬ veloper until all detail is up, and then to add citrate to the de¬ veloper finishing as above with the brisk dose of ammonia. The citrate was suggested by Mr. Watmouth Webster, now of Chester, Eng., and from its action in stopping detail but not density in development will be found very useful. We have in this chapter formulated for one ounce of devel¬ oper only; this quantity will develop a quarter-plate; two ounces a half-plate or 7x5, four ounces a 10x8 plate ; but the the beginner is advised to double these quantities till he has become adept in evenly and quickly flooding his plate. During development the plate should be constantly shielded from light even of the non-actinic lamp, being uncovered only at the time and for the purpose of scrutiny. It may be well to recapitulate certain points of this chapter referring to certain appearances shown by a plate soon after the image has started to appear in the developer, normal as on pages 91 and 92. Refusal to appear for one minute, or appearance in highest fights only, bespeaks utter underexposure. High fights grey, shadows wanting—severe underexposure. PRACTICAL PHOTO-MICROGRAPHY. 99 High lights dense black, shadows dirty grey—under¬ exposure. High lights appear after fifteen to thirty seconds, grey at first, gradually darkening, shadows creeping up steadily all the time; finally high lights black and shadows ranging from pale to dark grey—correct. High lights rapidly followed by shadows, the whole rapidly gaining great blackness and density—overexposure. High fights and shadows appearing almost immediately and simultaneously, the whole turning rapidly gray and remaining so—great overexposure. Instant flashing out of the whole image, all remaining a very pale smokey grey—enormous overexposure. The Hydroquinone Developer.* This system of development has of late been greatly elaborated and improved, especially since it was found feasible to use hydroquinone in conjunction with the hydrate or caustic alkalis without damage to the film. Whether or not this is destined to be the standard developer of the future, we are confident in stating that it is a very fine developer at present. Indeed, on account of the ease with which almost any amount of density may be obtained without injury to the shadows, it is probably the best developer for photo-micro¬ graphic negatives. A further advantage is that this developer can be used without the presence of soluble bromides. We give a formula due to Messrs. Thomas & Co., of London; though containing a heavy dose of hydroquinone, it is not really expensive, as one dose of the developing solution may be used for three, four or even more plates. The formula stands thus: a. Sodic sulphite. 1 ounce Citric acid.30 grains Aram. Bromide.10 grains Water to.10 ounces Dissolve and add hydroquinone.80 grains * It is to be hoped that hydroquinone will soon be universally called by its shorter and systematic name : “ Quinol.” 100 PRACTICAL PHOTOMICROGRAPHY. b. Sodic hydrate “ caustic soda ”.80 grains Water (that has been boiled).. .10 ounces The developer consists of equal parts of a and d. The action of this developer is on the whole more gradual than that of a pyro-ammonia developer; the density varies chiefly as the exposure, and the shadows have a tendency to develop more slowly and with greater clearness than when pyro is used. The carbonates may also be used with hydroquinone, but such a developer is much less energetic than the above. The writer frequently uses potassic meta-bisulphite in place of the citric acid and in less quantity. Potassic hydrate is as good as sodic, or better. It is well for the beginner to use the above solution somewhat diluted, say with an equal measure of water. m CHAPTER XV. OPERATIONS FOLLOWING DEVELOPMENT. Fixing, Clearing, Intensification, Reduction. After development is judged to be complete tbe plate is to be carefully washed by non-actinic light under a tap for a couple of minutes or in a few changes of water. It may then be placed in a saturated solution of common alum ; but except in cases where plates have a tendency to frill, this is not neces¬ sary. The plate is then “ fixed ” in a solution of sodic hypo¬ sulphite made alkaline. (See page 63.) The plate is to remain in the “ hypo ” solution not only till the white (un¬ altered argentic bromide) has disappeared from the back, but a considerable time longer. If the plate is removed as soon as the white has gone, it will deteriorate to a certainty after a time. After fixation, the plate must be very thoroughly washed, preferably under a rose tap, for 10 minutes, or in one of the washing-machines sold for the purpose. Several hours are necessary in the latter case. After the hypo is undoubt¬ edly removed the plate maybe “cleared” in the acid alum solution formulated on page 63. In this it may remain for about 5 minutes or longer. It is thereafter to be thoroughly washed, and racked to dry. The beginner at all events is recommended to make a print from his negative before he ventures upon any other step he may think advisable. The negative should be shown to an expert photographer, who will instantly say if there has been any serious error of treatment. If the negative is “thin ” or pale all over and wanting in contrast, yielding on albumenized paper a “ washed-out” look¬ ing print, all gray and no black, intensification may be tried. 102 PRACTICAL PHOTO-MICROGRAPHY. % To Intensify. The negative being above suspicion of hypo is soaked in a flat dish in the solution of mercuric bichloride and acid given on page 63. When the film is as pale gray as it will be¬ come, it is well washed and placed in the solution 5, given on the same page. Here it will very quickly turn black or brown; but it must be left in the sodic sulphite till it is dark when seen from the back as well as from the front. It is then washed, and will be found to have gained greatly in density and printing quality. There are many other methods of intensifying for which we refer the reader to the photographic literature of the day. We have given the one we prefer to all others, especially for photo¬ micrography. As a rule, however, intensification is to be avoided, for it is not only ticklish to perform but doubtful— under conditions not precisely established—as to its perman¬ ence. If the finished negative is too dense, especially if the shad¬ ows are clogged, we may have recourse to “ reduction,” which does not present the same difficulties as intensification. Reduction. To Mr. Howard Farmer we are indebted for a very simple and effective system of reducing. (See page 63.) Soak the plate, carefully freed from acid after the acid alum bath, in solution B, for 3 minutes. Put into a cup a few drops of the solution A, and pour the hypo-solution into the same cup, then returning the mixture to the dish holding the plate. Reduction will at once begin and its progress must be watched. If the action cease before sufficient effect is produced add more of A. When finished wash well and drv. The Reducer No. V 2, given on page 63, may be used. The plate is soaked in A till a gray film more or less marked seems to cover the im¬ age ; the plate is then washed and placed in solution B. Varnishing. If a large number of prints are likely to be wanted from a negative it is well to varnish with a spirituous solution of shel- PRACTICAL PHOTO-MICROGEAPHY. 103 lac, or better with one of the good negative varnishes sold for the purpose. The instructions as to heating the plate are to be carefully followed. Plain collodion makes a very good and usually sufficient protection for the film. What we have to avoid is damp, and of course our negatives must not be scratched. The only objection to varnish is that if appreciably thick it may prevent absolute contact for contact printing, perfect “ sharpness ” being important in photo-micrography. A good negative presents gradations from nearly opaque highest lights, to clear but well marked shadow detail. The shape of a candle flame should be discernible with difficulty through the very densest parts of the negative. There should be no clear glass , but some detail everywhere. The higher lights as well as the deeper shadows should show detail. These remarks hold good for nearly all kinds of subject, but there are exceptions. If in a negative we find dense lights in close juxtaposition with clear shadows the negative is underexposed, unless the subject is for u black and white ” rendering. If any part is absolutely opaque and (1) some parts are quite clear, underexposure, or (2) all other parts too dense, overex¬ posure, is the probable cause. If the negative is grey all over, nothing in it approaching opacity—overexposure, or under¬ development. If the negative is clear nearly all over, underexposure is certain to be the fault. Frilling and blistering are not often met with now-a-days. If a plate frills before it goes into the fixing solution it is a bad look out for that batch; frequently the carbonates if long applied in the developer cause frilling or isolation of the whole film; ammonia has not this tendency. If the blistering or frilling appear on first washing after the hypo, a cure will usually be found in soaking the plates straight from the hypo in a tray of common salt and water, say one part to twenty. Too strong hypo solution may cause blisters. Fog, which is easily recognized when seen, must be traced to its source. If the parts of the plate protected during ex¬ posure, as by the paper mask, or by the rebates or corners of 104 PRACTICAL PHOTO-MICROGRAPHY. the dark slide, are degraded and fogged, either the plates are foggy by nature or the light of the operating room is unsafe. If the protected parts of the plate are quite clear, we must look to overexposure, or failing that to reflections inside the camera. Black streaks across a negative mean stray light reaching the plate, and the sharper are the outlines of the streak the nearer to our plate must we look for the leakage. A hole in the dark slide makes a much sharper streak than one of the same size in the camera front. Uneven lighting is a very frequent cause of failure, but its results on the negative are not difficult to recognize, except as in next paragraph. There are many defects to which plates are liable, but we cannot enter into details of such things here. Uneven coating has often the semblance of uneven lighting ; careless packing often produces details on our negatives not projected by our objectives. Transparent spots of minute size, called “Pinholes” are caused by dust adhering to the film, these spots have as a rule no nucleus; but dust in the emulsion usually produces spots that have nuclei. Each plate must be dusted with a clean camel hair brush before being put into the slide. No. 2.—Typhoid Bacilli in Intestine, X 750. Plate III. CHAPTER XYI. COLOR-CORRECT PHOTOGRAPHY. “ Ortho-Chromatic,” or as we prefer to call it, “ Color- Correct ” Photography, is one of the latest important improve¬ ments in our science. It may be well to explain the rationale of this modification of common processes, and the reader is begged to study carefully our introductory remarks to this chapter. It has been probably remarked by all observant people that while the colors yellow, bright red, and certain greens appear to the eye the most luminous colors, they are in photography rendered as dark, or at least as below the average grade of light, in the landscape, portrait, or copy of a colored drawing. On the other hand the indigo and violet colors of an object seen in the usual way appear to the eye dark or at least sub¬ dued tints; while these colors, when portrayed in the mono¬ chrome of photography, are represented as high lights. This has always been a serious objection to photographic ren¬ dering of many objects, notably of painted pictures, and to a considerable extent of landscape with foliage and sky effects. It is evidently a very serious objection to photo-micrography if we render an object, stained (say) with the dark violet of logwood and the bright red of eosine or magenta, the very opposite of what it really is visually; viz.: bright in the print where the object is dark violet, and dark in the print where in the section there is a fine glittering red. Further, if we have an object entirely red or entirely yellow but in gradations of red or yellow, it is most annoying to get a print of a homo¬ geneous blackness, simply because our plate is practically in¬ sensitive to even the palest of the red or yellow. The diffi¬ culty also is great with an ordinary plate where we have either a 106 PRACTICAL PHOTOMICROGRAPHY. dense yellow object on a pure white ground,or a very pale yel¬ low or pink of such a nature that the contrast between object and ground is very small. Color-correct photography helps us out of all these troubles and many besides; and it may be stated once more, as it has been stated by the writer on former occasions, that no branch of photography has been so much benefitted by “ ortho-chromatics ” as photo-micrography, and that in color-correct photography lies the future of photo¬ micrography. The writer has lately produced such renderings of double stained pathological subjects, prepared without regard to photographic requirements, as a few years ago neither he nor any other person would have been mad enough to attempt. The advances that up to this date have been made in ortho¬ chromatics may be summed up briefly in the statement that our plates have been rendered more sensitive to yellow-greens, yellows, oranges and reds than they could be made formerly, and less proportionately sensitive to blue-greens, indigos, blues and violets. Plates have even been prepared as sensitive to yellow as to blue; but our usual procedure is to make a plate more sensitive to yellows, etc., than an ordinary plate, and a little less sensitive to violets and blues, and to assist the action when necessary by calling off more or less of the blue and violet by means of yellow “ screens.” By this method we have more command over our results than if our plates were as sensitive to yellow as to blue; indeed, if we carried the sensitiveness too far into the red end of the spectrum, we should often have to cut off some of our yellow rays, and we should have great difficulty in finding a suitable light whereby to develop our plates. Still, for a few subjects, a greater sensitiveness to red would be a great boon, though developing difficulties would multiply. In Britain, at least, color-correct gelatine bromide plates may be bought; as a rule, they keep quite well for a few weeks, but we prefer to use them within a few days. Mr. B. J. Edwards, working under the “ Tailfer-Clayton ” patent, produces plates orthochromatized by an eosin process; Mr. J. R. Gotz trades in plates sent from Germany by Vogel and PRACTICAL PHOTOMICROGRAPHY. 107 Obernetter. The best color-correct plates for photo-microg¬ raphy we have ever used were produced by Mr. Edwards for copying purposes. Where sensitiveness to certain regions of the red is required, the “ azaline ” plates of Obernetter are, perhaps, the best in the market; but “ cyanin”-stained plates are the best of all for reds. The photo-micrographer who has time and a good drying cupboard may quite easily orthochromatize his own plates. An emulsion should be chosen capable of giving ample density, not very rapid, and not containing more than 3 parts of iodide to 100 of bromide. The writer has tried, per¬ haps, every published process of any promise for “ staining ” his plates, but finds none so simple or so effective as a process lately promulgated by the talented Mr. Ives, of Philadelphia. The dyes suggested by Mr. Ives are erythrosin and cyanin. The latter seems to sensitize for reds more than any aniline we have tried, but the operations must be carried out prac¬ tically in total darkness. The erythrosin or cyanin may be used as follows: Take of the dye.1 grain Absolute alcohol...4 ounces Soak the plate in this for one minute, allow to dry in total darkness. This will not take long. Then place the plate face upwards in a black tray and cover the tray with another tray or flat cover having a hole in the top into which is let a piece of tubing. Connect the tubing with the water tap and wash the plate in the covered tray for five to ten minutes. In very subdued red light, or in darkness, convey the plate to the drying press; when dry it is ready for use. The drying- box or press must be properly constructed for passing a con¬ stant current of dry air; details of such presses will be found in “ The Processes of Pure Photography ” (Scovill & Adams Company). A book as long as the whole of this one would be required to do full justice to the processes of color-correct photography. We have to allow for such wide variations of color, and there are so many different methods available for meeting our various 108 PRACTICAL PHOTO-MICROGRAPHY. color difficulties, that we can only give the outlines of the principles upon which we work. In the majority of cases a color-correct plate is almost useless unless we use in conjunction with it a “ screen,” but it must be remembered that if we are using an oil light, and particularly a paraffin lamp without camphor in the paraffin, our light is yellow, and to a considerable extent acts precisely as a yellow screen would do. It is necessary to have several yellow screens of different tints, and it is well to have also one or two screens of blue, also of varying tints. A glass of a color known as “ signal green ” will be found useful for pale reds of which eosin stains are types. The colored glasses called “ screens ” ought to have their sides perfectly parallel, (ought to be what opticians call “worked,”) but unless the screens are to be used in some critical position in the light rays, as for example close behind the condenser, we do not insist upon this perfection. We do not find that it matters in the least where the screen is placed so long as it is between the light and the object, and so long as no light reaches the object except such as passes through the screen. The colored glass may be put in a holder about midway between the light and the condenser. If, as in some cases, the screen is screwed into a part of the mounting of the condenser (in which position it is often called a “light-modifier”), then it ought to be “ worked ” glass. A set of most useful screens can be made by mixing aurantia with collodion, and three screens may advantageously be made in this way by mi xing the aurantia in three different proportions, about one grain of dye being first dissolved in one ounce of alcohol. The col¬ lodion may then be poured upon a glass plate, which has been previously thoroughly cleaned, and then rubbed all over with French chalk (powdered talc), the chalk being apparently all removed with a clean dry cloth. If desired, the collodion, which must be in an even film, may be, after drying stripped from the plate and used as a film, or the edges may be varnished, and the film left on the glass. One screen should be of the very palest color, the other two progressively darker. At need any two, or all three, may be used together. We are indebted to Mr. C. H. Bothamley for the suggestion of aurantia. PRACTICAL PHOTO-MICROGRAPHY. 109 Mr. Wellington recommends an alcoholic solution of turmeric, and a solution of picric acid will also be found to be of great service. In any case the screens must, when tested by aid of a spectroscope, cut off some portion more or less of the violet and blue of the spectrum. In cases where we have blue or violet lacking actinic contrast with the white of the background, or likely to be over-exposed before other colors (as yellow or red) can be sufficiently exposed, we cut off some blue or violet by use of our screen, and the paler is our violet or blue the darker must be our screen. But if our violet be very dark, and more particularly if it be a reddish violet, such as logwood shows at times, we shall get a better result in presence of a red or yellow-brown contrast stain, if the latter be pale, by omitting the screen entirely, for the depth of the violet amounts in practice to so much opacity, not to mention the red impurity of the violet stain. But if the contrast red or yellow be also dark the screen may be required unless our plate be very much corrected for color. It is not an uncommon occurence to find preparations very faintly stained with red, and it is still more common to find red stains and also yellow fade after a time. In such a case there is want of actinic contrast between object and ground, and the latter is exposed practically as fully as the former, and so we can not get a white ground in our positive. In such a case a rather dark yellow screen used either with an ordinary or an orthocliromatic plate will prove of great service. Bacteriological preparations seem to be specially subject to this fading, and we have many times got good results with a yellow screen after repeated failures without one. Insect preparations and others similar, where we have yellows approaching opacity at times are very difficult. Sometimes the best result is got by using an ordinary plate without any screen, sometimes a yellow-sensitive plate without screen, some¬ times the latter plate with a screen added, sometimes a cobalt blue screen, and sometimes a signal green. The best procedure depends upon the depth and quality of the yellow in the object. The common flea as usually mounted seems to come out best with an ordinary plate, but we possess one which curiously enough requires not only a yellow-sensitive plate—which pro- 110 PRACTICAL, PHOTO-MICROGRAPHY. duces differentiation in the body-color of the insect—but a yellow screen which prevents the background from being over¬ exposed. It must always be remembered in dealing with orthochromatics that we cannot always make practice agree with theory, and the only way to succeed with some objects is to “ ring the changes ” till we do succeed. Without color-correct photography it would be found well- nigh impossible to photograph any thin section or very minute object stained lightly with .that very useful dye, gentian violet. The same remark applies to another favorite stain, methyl blue. Gentian violet used with an orthochromatic plate in presence of a yellow screen, or illuminated by an oil lamp, is, in our opinion, the most satisfactory stain for nearly all bacteria, for, in fact, all that are amenable to the violet stain. And methyl blue is a very useful contrast stain for such micro-organisms as we choose to stain red. If a preparation be stained with a clear violet so as to show body-details in the object, or if a preparation is double stained, red and blue, then a color-correct plate, generally helped by yellow screen or yellow light, will give renderings quite beyond the reach of an ordinary plate. This we have verified a hundred times. “ Test ” diatoms are sometimes mounted in media having very high refraction indices but of very yellow color. We refer to certain arsenic compounds used for the mounting of A mphipleura jpellucida and such like. Over and above a dif¬ ficulty entailed by the optical principles involved when we are working at very wide numerical apertures, this yellowness of the mounting medium makes it very difficult to get a white background in our print. A yellow-sensitive plate used with¬ out a screen frequently lessens the difficulty alluded to in con¬ nection with the background. In concluding this chapter we can only say that while we apologize for the meagre amount of definite instruction given by us under this head, it must be remembered that the vari¬ eties of color and shade are almost infinite; that unfortunately language is not sufficiently accurate to permit of accurate color nomenclature; and lastly, that experience alone can teach us how best to choose a suitable plate and a suitable screen for each subject as it falls to our lot to photograph it. PRACTICAL PHOTO-MICROGRAPHY. Ill Our latest color sensitizing bath is due to Mr. Bothamley, and gives considerable sensitiveness to yellow, but not to red. The general sensitiveness of the plate is raised about three times, and the aptitude for giving density greatly increased. No washing is required, and the plates keep well. A rather slow plate should be chosen, and the caution about proportion of iodide must be kept in mind. The proper erythrosin is that of the Badische-anilin-and-soda-fdbrih , called “ Erythro¬ sin B.” The use of ammonia in this way is covered by patent in England. The ammonia may be omitted, but five or six times more exposure will be required. (Bothamley.) Erythrosin solution (1 to 1000). 1 part. Ammonia (1 to 10). 1 part. Distilled water .. 8 parts. Bathe the plate for two minutes. Dry. No washing needed. PART II.—PRINTING PROCESSES. INTRODUCTORY. * It is hardly within onr scope, and it is certainly not our in¬ tention, to enter here at any great length into printing processes. After a negative has been obtained the process of producing a print or prints is purely photographic, and full instructions for printing by the many processes available are to be found in many purely photographic books; for example, the reader will find ample instruction on the subject in the book already repeatedly alluded to: “ The Processes of Pure Photography,” by Professor Burton and the present writer, forming one of the series of publications of which the present book forms another item. Still by attention to certain details in the ordinary photog¬ raphic processes points preeminently important to the photo- micrographer may be emphasized, and our present instructions shall tend to work chiefly upon points affecting specially the printing of a photo-micrographic negative. The processes we propose to treat thus briefly are: The albumen paper silver process. The gelatine chloride emulsion process.... The gelatine bromide emulsion process.... The platinotype process. Enlarging on bromide paper. Lantern slides by contact and by reduction. Paper prints by contact. CHAPTER XVII. PRINTING ON ALBUMENIZED PAPER. This process is the one most commonly used by photogra¬ phers for producing their prints in the ordinary course of work ; for our purpose it is well adapted, as it lends itself to the rendering of fine details in a fairly satisfactory manner; the image is not lost and made granular by being deposited in the texture of the paper as in some other processes; but the very finest details either in shadows or in high lights are apt to be lost in the operations of “ toning ” and “ fixing,” how¬ ever carefully these operations may be performed. Shadow details are greatly emphasized and generally improved by the process of enamelling, to which we shall call attention at the end of this chapter. Paper of special quality is coated by special machinery with albumen containing a certain amount of chloride, usually am- monic or sodic. This is floated upon a solution of argentic nitrate and by double decomposition argentic chloride is formed and remains in the albumen layer with a certain neces¬ sary excess of argentic nitrate. Argentic chloride in such conditions darkens under the action of white light; the black parts of a negative prevent access of light to the parts of the sensitive paper in contact with the negative, the light parts of the negative allow the light to pass and affect the paper, and so a “ print ” is obtained. The print thus obtained if not “fixed ” would soon turn black all over, so it must be fixed in sodic hypo-sulphite, but this would leave the print with a very unpleasant color, so before fixing we “tone” the print by causing a layer of gold to be deposited over the image; this gold, being deposited over the peculiar gradations of color proper to the reduced chloride and albuminate of silver, gives various color or “ tones ” according to the color of the sub- 116 PRACTICAL PHOTO-MICROGRAPHY. jacent image and the fineness of division of the gold itself. The fixing operation does not remove the tone produced by the gold; and, moreover, the gold imparts to the image an ability to withstand atmospheric and other actions which but for the gold would soon destroy the beauty of the image. The paper is almost always bought ready albumenized and “ salted,” and very frequently ready sensitized also; in the latter case special processes are resorted to in order to make the paper keep good for a period of weeks or even months. We prefer the ready sensitized paper for general work, and in Britain it is easily obtained at very reasonable prices. If, however, sensitized paper cannot be obtained, the albu¬ menized salted paper is to be floated albumen downwards in a bath of argentic nitrate. (See page 64.) Time of floating 3 to 5 minutes according to temperature and nature of the albumenized paper. The higher the temperature the shorter should be the flotation. The “ bath ” formulated on page 64 should be neutral or slightly alkaline, and must be kept up to strength, for of course each sheet of paper floated on it removes a certain quantity of silver. When the negatives are “ thin,” full of detail but lacking contrast, the 60-grain bath—or a strong bath—should be used and full time of flotation given. “ Hard ” negatives may be printed on lightly sensitized paper, that is to say on paper floated on a weak bath, and only long enough to ensure com¬ plete conversion of the salt in the paper into silver salt. The ready sensitized papers usually give more brilliant prints, or more contrast, than paper sensitized at home and used imme¬ diately without “ preservatives.”. A hard negative is best printed in direct sunlight; a very thin negative should be printed in weak light, or in brighter light behind a sheet of ground-glass. When flotation is finished, the sheet of paper is to be hung up in a cool dry place to dry; nothing must touch the face while it is damp, and the corners should not be allowed to curl inwards and so spoil the centre of the sheet. It is well to blot off each sheet as it leaves the sensitizing bath, having allowed PRACTICAL PHOTO-MICROGRAPHY. 117 the sheet to drip as much as it will. Pure bibulous paper must be used for this purpose. If the sheet after drying is placed between sheets of blotting or pure filter paper previously im¬ pregnated with sodic carbonate and dried, it will remain white and good for weeks. Sensitizing and drying must be per¬ formed in yellow or artificial light. All these matters are most carefully treated in “ Practical Guide to Photographic and Photo-mechanical Printing Process ” by W. K. Burton, C. E. (London, Marion and Co.) and also in the book already alluded to in the Scovill Photographic Series. The negative is laid face upwards in a “ printing frame ” (fig. 30.) and on it is laid the albumenized paper face down- Fig. 30. —Printing Frame. wards, so that the albumen surface is in contact with the gelatine film. The frame is then taken to the daylight and printing commences. As a rule photo-micrographic negatives, if thoroughly good, take longer to print than ordinary photo¬ graphic negatives. In any case the printing must be carried considerably beyond the stage when the print seems to look at its best, as seen by opening one-half of the hinged back of the printing frame. The operations of toning and fixing greatly lessen the depth attained by the merely exposed print. When a batch of prints are printed, having after printing been stored away in some suitable light-tight receptacle, we proceed to the operations of toning, fixing, and washing. The prints are first immersed in water, being well covered therein and kept moving and separate from each other. After 118 PRACTICAL PHOTOMICROGRAPHY. about ten minutes they are moved into a fresh supply of water. If the paper is “ ready sensitized ” this second water should contain a small dose of sodie carbonate sufficient to neutralize the acidity of such paper. If the paper is home-made, and if it is not brick red on entering the second water, a pinch of common salt (sodic chloride) must be added to the second water. The prints must go into the toning-bath red, not violet or purple. As a rule the print should pass through four changes of water before being toned. The toning-bath preferred by us, the only one we think it necessary here to give will be found on page 64. It is to be poured into a white perfectly clean flat dish large enough to hold two prints side by side, and deep enough to allow one print to slip easily under another. The temperature of the toning-bath should be about 65 deg. Fahr., not under 60 deg. nor over 70 deg. The prints are put in face downwards and kept moving. The Toning Solution must be alkaline ; an acid toning solution is the commonest of all amateurs’ printing troubles. The prints ought in from ten to twenty minutes to change from brick red to a handsome color, brown, purple, or warm blue-black according to taste and time. (Prints always turn a little more blue after drying.) Home sensitized paper usually tones more quickly than ready sensitized. It is an ad¬ vantage to get as much gold as possible deposited without turn¬ ing the prints blue ; that amounts to our saying that it is better to tone slowly than quickly. If the bath is too strong in gold the prints turn blue very quickly, but the blue is only superficial and comes away in the fixing bath leaving the print practically untoned. A fairly good rule is to stop the toning when the print has, seen by reflected light, a good color inclined to be warmer than we desire our finished print, seen by transmitted light just a suspicion of blueness in the half tones of the higher lights. After toning, the prints are put into clean cold water and there kept moving if possible. A trace, however slight, of hypo in the toning solution ruins it, the greatest care is there¬ fore to be exercised to keep these solutions apart. The toning bath should seldom be filtered, but gold chloride must be added No. 1.—B. Anthracis in Blood. X 750. No. 2 .—Spermatozoa of Triton, X 1000. Platk IV. PRACTICAL PHOTO-MICROGRAPHY. 119 to replace the quantity taken out by the prints, and each time gold is added the bath should be tested for acidity and alkalized slightly if necessary. The gold salt is usually sold in sealed tubes containing fifteen grains; one of these tubes may be scratched with a file, put into a two-ounce bottle, the tube broken and water added to fill the bottle. One dram of this contains about one grain of gold chloride, and a sheet of paper (17x22 inches) may be expected to take up about one grain of gold salt. From the fresh water after toning the prints are to be put into a fixing bath of sodic hyposulphite, made decidedly alka¬ line, as on page 64. In this they are to be kept separate, in motion, and for a period ranging from 12 to 18 minutes. A very light print may be removed in case of need after 8 or 10 minutes. The prints, if on “ double albumenized ” paper—a quality having a high gloss—are to be put direct from the hypo bath into water containing, to each pint, about an ounce of common salt; this is to prevent blisters, which are apt to occur. Then the prints must be very thoroughly washed, an easy matter if the proper means be taken, a very difficult one if commercial washing machines are the only apparatus used. Our plan is to lay each print down on a slab of glass or vul¬ canite under a rose tap, and to pass a squeegee many times briskly over the back of the print, occasionally turning the print face upwards to get some water, but not any squeegeeing, of course. After a few changes of water and applications such as this, one of the washing machines may be trusted to finish the elimina¬ tion of the hypo, in four to six hours ; the water running con¬ stantly and being syphoned oft from the bottom of the washing trough. After thorough washing, the prints may be dried or blotted off, but before they are quite dry they should be rolled one by one, face outwards , round a ruler or other roller of hard wood, and there left till they Sre dry, or till they retain the outward curl. The effect of glazing or enamelling the face of a print is to produce an appearance not, perhaps, artistic, but in some cases, such as ours, desirable. The shadow details are greatly assisted by this gloss. The simplest way we know to impart a gloss to the albumen print is to place the print, washed but still wet. 120 PRACTICAL PHOTO-MICROGRAPHY. with its face in contact with a sheet of “ ferrotype plate ” hav¬ ing a highly glazed impervious surface. If the ferrotype plate is clean the print, as it dries, will leave the plate either spontaneously or with very slight assistance. A plate of glass rubbed with pure talc powdered (French chalk) may be used instead of the ferrotype plate. A still higher glaze may be obtained thus: Make a solution of good gelatine £ to 1 ounce, water 10 ounces. Swell the gelatine in the water, then dissolve by heat. Take a sheet of glass, free from scratches, clean well, sprinkle over with powdered talc from a pepper box or muslin bag. Rub the talc all over ; then all (apparently) off. Coat the plate with plain collodion. When the collodion is set, but not dry, wash it under a tap or in changes of water till the greasy appearance is gone. Place the collodionised plate face upwards in a flat, clean dish, containing the melted gelatine, submerge the print to be enamelled face upwards in the gela¬ tine, bring plate and print up together face to face; put a sheet of waterproof cloth over the print and carefully squeegee the print to the collodion surface of the glass plate, avoiding air bells. Allow to dry, run a knife edge round the edges of the print, when it will probably jump from the glass with a very highly glazed surface. The drier the print at time of strip¬ ing the higher will be the gloss. It is not absolutely necessary to immerse the plate in the gelatine solution; if the print is thoroughly saturated with the solution, it may be laid down on the collodionised glass and then squeegeed. If these enamelled prints are to be mounted on cards, it is well to pass a card of no great thickness through hot water and to place it behind the print on the glass, while the print is still saturated with gelatine, and then to squeegee all together, placing a flat board on back of the card after squeegeeing and leaving a weight on the board for a considerable time, to pre¬ vent the card from bending away from the print. If the reader propose to mount his own prints of every de¬ scription, he will probably find fresh starch paste as convenient and as efficient as any mountant. Pour a little cold water on the starch, make a thick cream with it, then add hot water. Flour paste must never be used. Gelatine dissolved in water and spirits is with some a favorite mountant. CHAPTER XVIII. GELATINE CHLORIDE PAPER. By some manufacturers paper is coated with emulsion of gelatine chloride (argentic) with a certain excess of argentic organic salt, and paper so prepared is used for “ printing-out ” in a manner very similar to albumen paper. (The term “ print¬ ing-out” is used to signify a process wherein the image is revealed by the action of light alone, in contradistinction to the term “ printing by development,” where after light-action the image is wholly invisible or only faint, and development is required to bring the image to its full vigor.) The practice of printing the gelatine-chloride paper is pre¬ cisely similar to the practice with albumen paper. A few details of difference may be noted as to the properties of the papers. The gelatine-chloride paper gives much more contrast than ordinary albumenized paper, so where we have thin weak negatives the former offers a great advantage. The gelatine paper seems, as a rule, to lose as much depth in operations following printing, but seems to require a considerably larger quantity of gold chloride to produce the toning effect. The printing of the gelatine paper is usually more rapid than that of the albumen paper. After printing with Dr. Liesegang’s paper no washing is required, but the prints are at once toned in a bath made thus: Water.24 ounces Sodic hyposulphite.6 ounces Ammonic sulpho-cyanide.1 ounce Saturated aqueous solution of potash alum.2 ounces Dissolve, and place in the solution some scraps of gelatine- chloride paper for about 24 hours. Filter, then add : Water. 6 ounces Auric terchloride .15 grains Ammonic chloride.80 grains 122 PRACTICAL PHOTOMICROGRAPHY. This bath tones and fixes at one operation. To judge of the color we must look through the print, reflected light will mis¬ lead us. It is not easy to overtone by this process. Obernetter’s Aristotype paper has in our hands worked best with separate toning and fixing baths. Toning Solution. Ammonic sulpho-cyanide.140 grains Sodic phosphate.140 grains Sodic tungstate.100 grains Water.24 ounces Dissolve. Put in scraps of paper as above, filter, then add : Auric terchloride.15 grains Water. 4 ounces Tone to a rich brown color—or into blue if desired—then, after washing, fix in weak hypo ; viz.: Hypo. 1 part Water.10 parts These papers may be dried with their natural surface or may be highly glazed by squeegeeing to ferrotype, vulcanite, talced glass, or oollodionized talced glass, as described in last chapter. i CHAPTER XIX. PRINTING ON BROMIDE PAPER. This printing process is on the whole, perhaps, the most satisfactory and convenient of all for the photo-micrographer. The author always produces by this process prints intended for special purposes of exhibition, which is a fair guarantee of his own opinion, at least. The photo-micrographer whose time is, perhaps, pretty fully occupied with other business will find the bromide paper process convenient from its celerity, and satisfactory on account of the beauty of the results that by a little practice may be obtained. There are certainly difficulties in the process, but as they will all be overcome by care and practice, they need not appal us. The process is one of exposure—to artificial light as a rule— and development, by ferrous oxalate generally. There is no protracted period of printing, no watching of the progress of printing, no toning nor serious washing before fixing. To attain speedily to success, and to ensure repetition of success with the same or similar negatives, the worker should in the first place obtain a standard fight. A u regulator gas-burner ” or an oil lamp always turned to the same height, or a standard candle, or a fixed length of magnesium ribbon, or wire, or a “ unit lamp,” any of these is suitable. W e may also fix either a standard distance from the radiant, varying our exposure, or we may fix upon a standard exposure, and vary our distance from the radiant. Our own preference is to have a normal distance from fight to sensitive surface of, say, 18 inches, and to vary the exposure according to our negative, and this practice we recommend to beginners, though the more experienced will find a marked advantage, without introducing insuperable difficulties, in varying both distance and exposure 124 PRACTICAL PHOTOMICROGRAPHY. to meet certain peculiarities of negative. In varying tlie distances, however, we must not overlook the law regarding the intensity of light, viz : that the intensity varies inversely as the squares of the distances between radiant and recipient; in other words that halving the distance is equivalent to quadrupling the exposure. If the correct exposure at 18 inches be twenty seconds, at 30 about 55 seconds will be required, as 182 : 3 0 2 20:55$. If magnesium ribbon be used it will be found convenient to burn each time a definite length, say one inch, and to vary the distance of the printing frame from the burning wire. A number of prints may be exposed at one operation with magnesium wire by arranging the frames in a circle, or at various parts of several circles, either imaginary or actually traced on the table. Fig. 31. By such an arrangement as shown at figure 31 we may evidently expose with one length of wire or ribbon, burned ai “ X,” eight negatives of three grades of density at one time. “ Bromide paper,” as it is usually called by photographers, is a good quality of paper coated with an emulsion of gelatine- bromide of silver, a considerable proportion of chloride PRACTICAL PHOTO-MICROGRAPHY. 125 being sometimes added. The sensitiveness of the emulsion varies in different makers’ products, but it is usually consider¬ ably higher than any collodion emulsion, and always lower by a good deal than that of such gelatine bromide plates as are used for the production of negatives. In order that the paper may not curl and become unmanageable in aqueous solutions, the gelatine film ought to be very thin; and to counterbalance its thinness it ought to contain a high proportion of the argentic salts, otherwise vigor of image will be difficult to attain. The paper is placed face to face with a negative in a frame in the usual way for contact printing, but this opera¬ tion must be performed in non-actinic light, a simple yellow glass not being sufficient protection against daylight, nor even against ordinary artificial light. If there is any doubt as to which is the face—or gelatine side—of the paper, it may be laid down for a few moments on a flat surface, and it will soon show signs of curling face inwards. Makers usually send out three qualities of bromide paper: A—Thin paper with a surface glossy; B—Thicker paper with surface glossy; C—Thick paper with a mat surface, which is not recommended for our purpose. The sensitiveness does not appear to be affected in any way by the surface, all grades being practically alike sensitive. The development is usually effected by ferrous oxalate, me constituents being used in the proportions of from 4 to 6 parts of the potassic oxalate to 1 part of the ferrous sulphate (see page 63). We recommend the addition of at least one-half a grain of soluble bromide to each ounce of developer. We have assumed a standard light and a standard distance from light to printing frame. We now remark regarding exposure: A dense negative requires long exposure, and vice versa. Long exposure tends to softness, detail, want of contrast. Short exposure tends to brilliance, contrast, pluck. Over-prolonged exposure leads, as a rule, to an unpleasant color, a greenish tint, which is undesirable. Too short exposure leads to “ chalk and soot,” dense black shadows, and glaring whites. 126 PRACTICAL PHOTOMICROGRAPHY. A strong developer (say 4 oxalate to 1 iron) gives good brisk blacks, a weak developer (say 1 to 1) is apt to yield “washed out” tones of the image, while a very weak developer sometimes produces a peculiar appearance of measliness or grain. If we have to print a weak, “ ghostly ” negative, we keep down the exposure and develop with a brisk developer restrained with a full dQse of free bromide, say 1 grain bromide per ounce of developer. If our negative is hard or shows violent contrast, we give a prolonged exposure and use a developer consisting of, perhaps, 1 part of iron to 6 of potassic oxalate, and even to this some water may be added, while the free bromide may be reduced, or even omitted. One strong point, perhaps the strongest point in favor of this process is that it lends itself above all other printing pro¬ cesses to the production of good prints from inferior negatives, and enables us to vary to a very marked extent the prints we may obtain from any one negative. A negative showing little more than a ghost of an image may be made to yield a print actually “ hardwhile a negative of the “ chalk and soot ” order may be made to yield a print of the utmost softness, and these effects may be produced by a mere variation of the ex¬ posure, assisted or unassisted by intelligent variation of devel¬ opment. The manipulation of bromide paper is simple. After the exposure the sheet of paper is plunged into clean water in such a manner as + to prevent formation of air bells; if bubbles do occur they are easily seen and must instantly be removed by hand or with a clean camel hair-brush. As soon as the paper lies flat in the dish it may be transferred face upwards to the developing dish, or if the same dish is to be used for both operations of soaking and washing, the water only requires to be poured out of the first dish and the paper left in it face up¬ wards. The ferrous oxalate developer is now poured over the paper. (See page 92.) The developer should be allowed to act until the shadows of the picture show a good pronounced black as seen by the subdued colored light of the operating room. By this time the highest lights should not show any PRACTICAL PHOTO-MICROGRAPHY. 127 grayness, while the half-tones should be from pale to dark gray according to their nature. In our work there ought always to be some part of the picture perfectly white when development is complete, but often it is impossible to attain this; our best guide then, remains in the vigorous tone of the dark parts. With the developer we have given on page 92 the image ought not to begin to appear for at least 20 seconds, and devel¬ opment with fresh solution may be expected to finish in about two to three minutes. Yery rapid development points to over¬ exposure, which is probably the most serious error that in our special work can be committed. As soon as development is complete, the print is to be flooded with acidulated water, not plain water. A dram of acetic acid (glacial) to one pint of water will do, or a dram of citric acid. If the water smells of acetic acid it will suffice. Three applications of acid water for a minute or so each will suffice to wash out most of the iron, the print being moved in the acid water all the time ; washing in plain water till the acid is eliminated follows, and then the print is fixed in the ordinary bath of hypo, 1 part, water 5 parts made alkaline. In this the prints should stay at least twenty minutes, then they are washed in the manner detailed for albumen prints. (Page 119.) The Eastman Company of Rochester, U. S., prepare not only bromide papers of the very highest quality, but also a modification called Transferotype Paper. For the photo- micrographer “Transferotype” is a most valuable process. The printing and other processes, up to and including the final washing, are exactly the same as for ordinary bromide paper, but as in transferotype the sensitive emulsion (insoluble) is laid over a soluble stratum of gelatine on the paper, it is evident that by dissolving the soluble gelatine we can remove the paper. If we can stick our image-bearing film to a transparent, or semi-transparent, or translucent support, as glass or opal, and then remove the paper, it is evident that we can produce a transparent or translucent positive—reversed as regards right and left certainly, but for us this reversal is of no import. By the following means, then, we can produce by “ Trans- 128 PRACTICAL PHOTOMICROGRAPHY. fer” a lantern-slide, transparent positive, or “opal print.” After the final washing, which in the present case need not be so very laborious as if no further ablution were to take place, we place our transferotype print face to face with a perfectly clean sheet of glass or opal, squeegee the two together, place a double layer of blotting paper over the print, then a flat piece of wood, then on the wood a slight weight, as ‘one pound. A fter about half an hour we place the support bearing the print in water at about 100 deg. Fahr. or higher, and soon the paper will float, or may be carefully lifted off. The film is then washed under the tap, getting a gentle rubbing with the soft pads of the fingers if necessary to remove any gelatine adhering to the picture; acidified alum may follow this wash¬ ing. (See formula on page 63.) After the cleaning with alum the plate is finally washed, dried and varnished with a clear varnish usually called “ crystal varnish.” Bromide papers “ A ” and “ B ” may have a greater or smaller amount of glaze conferred on them by one of the fol¬ lowing methods. A fairly glazed surface may be obtained by squeegeeing the finished and washed prints to a sheet of vulcan¬ ite. When dry they will come, or may be taken from the vulcanite, and will have a very good glossy surface. Talced glass may be used in place of the vulcanite. Or glass talced and collodionized as on page 120 may be used, and this will not only give a very high gloss but will tend to protect the surface from scratches and from damp. Transferotype prints may be dried naturally and the paper removed at any future time. Previous, in such a case, to squeegeeing to the rigid support, the prints must be very thor¬ oughly wetted in water, and they must not have undergone alum treatment. Bromide prints and transferotype prints may be developed with pyro, and by some workers the pyro- developer is used with a view to warmer tones. The hydro- quinone developer as formulated on a later page (page 152) answers most admirably for bromide prints and also, of course, for transferotypes ; for bromide prints on “A,” “ B,” or “ C” paper we consider the hydroquinone developer superior in certain respects to all other developers; the regularity of PRACTICAL PHOTO-MICROGRAPHY. 129 development, the scope allowed for variations in exposure, provided the exposure has been sufficient , and the color of the deposit or image, lead us to recommend this developer with much confidence. But the pyro and hydroquinone develop¬ ers so act upon the soluble substance of transferotype paper as to make the stripping—not by any means impossible nor even difficult—but less easy than after ferrous oxalate development. The image on bromide or transferotype paper can by various proceedings after fixation be toned to various colors; for details of these matters we must refer our reader to general photographic literature. The method of developing bromide papers for enlargements and transferotype paper for lantern slides is practically identical with the method we have given in this chapter. Bromide paper offers a vastly greater prospect of permanence of result than other silver printing processes ; in fact a bromide paper print produced with proper precautions is in point of permanence, as it is in point of beauty, inferior to no purely photographic print that at present we know how to produce. A very pretty effect may be produced by attaching a number of transferotype prints, arranged in an artistic manner, to a sheet of opal. The prints being trimmed with scissors to the desired shapes are, in the bath of plain water after fixing and washing, caused to adhere to the opal plate in their desired positions. The plate bearing the prints is in the usual way removed from the water and the prints are carefully squeegeed into perfect contact. The prints are then allowed to dry , and the stripping in hot water performed thereafter. CHAPTER XX. THE PLATINOTYPE PROCESS. ^ This is a process due mainly to the ingenuity and chemical skill of Mr. W. Willis, of London, England, and is so far pro¬ tected by patent laws that in this country the original platino- type process can only be worked under license from the com¬ pany,* and with, for the most part, materials provided by the company. The process has a strong claim on our attention on account of its almost indubitable permanence, the image con¬ sisting of metallic platinum, and on account of the great beauty of its results under favorable conditions. On these two accounts we think it right to give at least a brief description of the pro¬ cess ; but except for special micrographic purposes—and these practically of one class—we do not put forward this process as • eminently suited to the photo-micrographer. Where extreme fineness and definition of detail is a necessity no printing pro¬ cess by which the image is deposited in the substance of a textile such as paper, can be expected to compete with a pro¬ cess whereby the image is kept on the surface and prevented from “ losing itself ” among the fibres of the textile by a surface-medium such as albumen or gelatine. But where fineness of detail and sharpness of outline are secondary, and indubitable permanence paramount considerations, this platinotype process is to be strongly recommended when the negatives are of high technical quality. But unless the negatives are tolerably good in a technical sense, i. e. un¬ less they show a considerable range of gradation from high light to shadow, and unless they possess a reasonable amount of “ pluck ” or contrast, their rendering as printed in platino¬ type will not be satisfactory nor even tolerable. *We understand that this license is now unnecessary. PRACTICAL PHOTOMICROGRAPHY. 131 The image in platinotype printing becomes under the action of light visible; with the Platinotype Company’s original process of image becomes rather faintly visible, with a new process lately introduced from Germany, the image “ prints right out.” With the latter process, known as Pizzighelli’s, all that is required is to print the image to the required depth and then to “ clear” it with a weak dilution of hydrochloric acid. The Company’s ordinary paper is partly printed and partly developed. For the Hot-bath Development process: The paper is ob¬ tained from the company ready sensitized, it must be kept away from light and perfectly dry. In order that it may be kept quite dry it should be placed in a calcium tube, a light- and-air-tight tube, having at one end a receptacle containing calcic chloride in some shape. Sometimes the paper is bought separately and the materials for sensitizing separately ; in such cases full instructions accompany the goods. By means of varying the proportions of certain ingredients of the sensitizer, we may vary our results within limits or prepare a paper more suitable than the ordinary commercial article to our negatives. Every precaution must be taken with this paper to prevent it getting damp in the least degree. A sheet of India-rubber is put behind the paper and preferably overlapping it in the printing frame. Printing is carried on till the shadows take a very peculiar dirty green brown .color, or until all the details are faintly visible over the print, except, in a few cases, in the very highest lights. After printing, the paper must again be stored with every precaution against light and damp. Any veil produced by light in this process is not, as in the albumen process, removed by the after operations of toning and fixing, for in platinotype there is no analogous “ clearing ” action. Therefore the progress of printing must be examined in the frame as rapidly as possible, and it may be noted that the process of printing is much more rapid with platinotype than with ordinary albumen paper. To develop platinotype prints. Make a saturated aqueous solution of potassic oxalate at 60 degrees Fahr., and heat to 132 PRACTICAL PHOTO-MICROGRAPHY. about 160 degrees in an enamelled flat iron dish, a Bnnsen or other burner serving to keep the temperature up while a suc¬ cession of prints are developed. On this hot solution made alkaline the print is laid for a few seconds; the previously pale image will almost instantly flash into a black picture, the high lights remaining for the time yellow. If the printing or exposure has been too short the developing solution should be hotter, say 180 degs., if the exposure was too long the bath should be more cool, say 110 degs. It will generally be found best to expose to such an extent that the develop¬ ment will be correct at 160 degs. We usually have slightly acidified the bath with oxalic acid and got fine results, but the Company now recommend that the bath be kept faintly alkaline. After development, which, conducted as above, will not oc¬ cupy more than 15 or 20 seconds even with a “cool” solution such as suggested, the print is to be put straight into : Water,... 60 parts. Hydrochloric Acid,. 1 “ Two more baths of the same ingredients are to follow, and the third bath must never show any yellow tinge ; if it does show such a tinge it must be followed by a fourth bath. When No. 1 bath becomes very yellow it should be rejected and the others be advanced each a step, No. 2 becoming No. 1, etc. After the baths the prints are to be washed for about 15 minutes in running water to remove the acid ; they are then finished. Pizzighelli’s paper requires no development, but at time of printing it must be slightly damp, not wet. To damp it we may breathe on it or put it into a box in company with pieces of wet blotting paper. Printing may be partly performed by light,.and finished with cold solution : Saturated Solution Sodic Carbonate. 5 parts. Distilled Water,.100 “ Clearing is performed as above. The latest process put forward by the Platinotype Company is convenient and good. The company send out with the paper PRACTICAL PHOTO-MICROGRAPHY. 133 not only a salt, the nature of which they do not state, but the platinum salt which is not in this case present in the paper at first. Full instructions are given with the paper, and these directions are so complete and accurate that failure is unlikely if the negatives be suitable for the process. The printing is performed in a frame and by daylight, as before, but after the printing, the paper is caused to take up a certain amount of moisture, which it readily does on exposure in a damp apart¬ ment or box, the damping and the printing being kept in certain relations to each other. Then follows development on the “ cold bath,” which gives the name to this process. Devel¬ opment is less rapid than with the hot bath; in fact, the print having been floated for a moment or two on the cold develop¬ ing solution is usually thereafter held in the hands till development is seen to be complete. The clearing process in acid and the washing are as in the hot bath process. CHAPTER XXI. ENLARGING. This is a somewhat important subject to the photo-micro- rapher, as it is frequently inconvenient to take at the first a negative as large as may ultimately he required. “ Enlarging,” as the word is technically used by photographers, will not, as some persons seem to think, help us to get any superior quali¬ ties to those which we can get by direct amplification properly managed, excepting only the one quality of size. If we have to produce a photograph of an uneven diatom exempli gratia , at a magnification of 300 diameters, we shall get it just as well, or better, by direct operation in the camera as by magni¬ fying to 150 diameters in the camera, and then “enlarging” to 300 diameters, always provided that in our original direct operation we do not overtax our instruments. By “ enlarge ment ” we enlarge and accentuate the difference of focal planes in the original object just as much as we accentuate it by direct projection ; and, what is more, we often introduce new aberrations in our system of “ enlargement,” unless we are tolerably au fait in our optics and careful in our operations. In considering enlargement we have two optical systems to attend to. 1st. A system for collecting light and transmitting it through our original negative or positive, or for concentrat¬ ing the light passing through our original at or near a certain point with reference to our projecting system. 2d. Our pro¬ jecting system, which regulates the sizes of our enlarged image and projects it upon our sensitive surface. Our condensing or transmitting system depends chiefly upon the sensitiveness of our photographic receiving surface ; if the latter is very sensitive and our light reasonably actinic, we require no condenser at all. But if either our receiving surface PRACTICAL PHOTO-MICROGRAPHY. 135 is little sensitive as albumen paper, or our radiant little actinic as an oil lamp, then in practice we require an optical system known as a condenser. As we propose to treat of two methods only of enlarging we shall touch but lightly on the subject of condensers. Our purpose is to confine ourselves to the use of such sensitive materials as gelatine-bromide emulsion, in which case we use daylight reflected or diffused, and transmitted in parallel • pencils through the original; or failing daylight, artificial light concentrated and transmitted in converging pencils through the original to a certain point at or near the optical centre of the projecting system. Enlarging by Daylight. Fig. 32. The cut (Fig. 32) shows almost at a glance an arrangement which if carried out in a reasonably workmanlike manner, will fulfil every desideratum for enlargement by ordinary diffused daylight. C is a camera capable of holding in its dark slide the negative from which we propose to make an enlarged print on paper or glass. C is fixed in any convenient manner to the sides of an aperture, A, in a wall or shutter preferably facing the north. F is a reflector of white blotting paper or any mat white surface, not a mirror nor any shiny surface; E is an easel sliding easily along the base-board D ; E must keep at all times parallel to the negative in the camera, and to start with 136 PRACTICAL PHOTO-MICROGRAPHY. the centres of negative, projecting lens and easel should all be in one line. If we are to project our image upon paper—as bromide paper—the paper is simply fixed by drawing pins to E; if our enlargement is to be on glass, E must have a rebate or other contrivance to carry the glass so that it (the glass) shall occupy precisely the position of the surface upon which we focus our projected image. The writer has a square aperture «cut in E; in this aperture he places a sheet of ground glass, so that (no light entering the apartment except through the negative and lens) he can arrange and focus his image with perfect ease and absolute precision. A pane of non-actinic glass, or a sheet of non-actinic paper, may be used to illuminate the apartment which otherwise should be dark. This pane should be covered while focusing is performed. In place of the angled reflector F a sheet of ground glass may be used parallel to the negative, and a few inches behind it; tissue paper will answer the same purpose. The only remarks necessary about the projection lens are that it should be capable of covering at full aperture the whole of the area of negative requiring enlargement. But we strongly recommend the use of a lens of focal length as long as possible, so that only the central rays of its “ cone ” are used. The only real limit to this is the length of the base-board D ; and the easel E may be on wheels, or on a car on the floor of the apartment. This lens ought also to be a doublet lens, one of the so-called “Rectilinear” or “Symmetrical” lenses used in photography. If it have a sufficiently great focal length it need not be stopped down at all, as we have to deal with parallel surfaces, and “ depth of focus ” does not come into requisition. If it is inconvenient to have an entire apartment darkened for this purpose, figure 33 will suggest an alternative. Here we have two cameras joined in a very simple manner, the small one carrying the negative from which we desire the enlargement, the dark slide of the larger camera carrying the sensitive material. In this arrangement we are likely to be troubled by want of stretch in the large camera which will entail a short focus projection lens, this again will entail No. 2. —Flagellated Spirilla (? Serpens). X 800. Plate V. PRACTICAL PHOTO-MICROGRAPHY. 137 stopping down of the projection lens which will entail greatly prolonged exposure. The usual “ motions ” of the large camera “rise and fall” and “traverse” of the front, will enable us to arrange our image on the ground-glass; while the small camera carrying the negative will by its rack and pinion enable us to arrange the magnification. The focusing of course is done on the ground-glass, and with the rack and pinion of the large camera. The small camera’s back is pointed towards the light, and an angled reflector, or ground-glass “diffuser” is used as before. With this arrangement there may be a difficulty in focusing due to want of light; in this case plain glass may be substituted for ground-glass, and the focusing done with an eye-piece of the Ramsden, or “ Zeiss Aplanat ” type. 138 PRACTICAL PHOTO-MICROGRAPHY. In enlarging, as in most photographic processes, the crux of the whole affair is the exposure, and it is just as hopeless here for us to attempt to give rules for all conditions as it is else¬ where. All we can say is that cceteris paribus exposure varies directly as diameters of enlargement. That is to say : with a given negative, given light, given lens, given diaphragm, and given sensitive material, it will take twice as much exposure to enlarge a quarter plate to S-| x 6f as it will take to enlarge it to 6^ x The best way is to make a trial exposure—on a small piece of the paper and at the distance to be used seriously— and to develop it, noting carefully our remarks as to the appearance on development of under and over-exposed prints under the appropriate headings. AVith an arrangement, as shown in Tig. 32, and an average negative, using a 13-inch rectilinear at full aperture {£) and Eastman’s bromide paper, the daylight being of average autumn quality, to enlarge three diameters the writer exposes from five to six minutes with ground glass diffuser, rather less with white reflector alone. But his average photo-micrographic negative is a dense one compared with a landscape negative, not to mention a portrait one. If the lens has to be stopped down to prevent spherical aberration, the exposure will be greatly increased, generally double for each next smaller size of stop as sent out by opticians. A “ table of enlargements ” will be found in this book, and with it the worker may easily reckon approximately the posi¬ tion of lens and easel for any given operation. The camera bearing the lens may at any rate be racked to the desired ex¬ tent, and then the focusing may be accurately performed by sliding the easel. The front motions of the camera may be used to arrange the image suitably on the screen. Focusing, if not done from behind with ground glass as suggested, may be done by viewing the projected image on a white sheet of paper afterwards to be replaced by the sensitive paper or glass, but in our experience the former method is vastly superior. Enlarging by optical lantern: This is perhaps the favorite system among amateurs who are likely to possess an optical lantern for its ordinary use. Some of the laws touching the optics of this system must be noticed. PRACTICAL PHOTO-MICROGRAPHY. 139 In the first place, an artificial light is used, and that light not remarkably powerful or actinic in comparison with day¬ light, and, moreover, the light is used in a lantern and not many inches distant from the original which is to be enlarged. So in the absence of a condenser we should have not only a weak but an uneven light, for the margins of our original would be much less strongly lighted than the centre. We here give a cut which will explain the functions of a con¬ denser. A is the radiant; B , a condenser of two elements ; C, the original negative or positive held in frame E ; the front focus of the condenser falls at a point inside a doublet lens 1), the rays having passed through the original, except some marginal rays which might be used but are stopped by portions of the frame E. Theoretically the radiant should be a point, and that point accurately in the focus of the condenser. Practically we can¬ not get such a point, the electric arc approaches most nearly to a point, the oxy-hydrogen mixing-jet lime-light next, a “blow-through” lime-jet perhaps next, and so on down to the worst of all—a multiple-wicked oil lamp. Still even the three- wicked lamp may in practice be successfully used, especially if we adopt an ingenious little contrivance due to Mr. Traill Taylor (Editor of the British Journal of Photography). Mr. Taylor’s suggestion was a simple converging lens placed between the light and the condenser, the supplementary lens collecting rays that would otherwise not reach the condenser. The area of the condenser must evidently be not less than, and ought to be greater than, the area of the portion of the original we propose to enlarge. The diameter of the condenser 140 PRACTICAL PHOTO-MICROGRAPHY. must at least equal tlie diagonal of the plate or portion of plate we mean to enlarge. Thus the ordinary four-inch condenser of the optical lantern is not of sufficient size to enlarge an entire quarter-plate. Moreover, the larger the condenser the more light it will collect, so that the exposure required, cceteris paribus , varies inversely as the area of the condenser. In practice the light is placed as nearly as possible in the focus of the condenser, the negative and the condenser remain fixed in relation to each other, the lens for projection is racked backwards and forwards till the image is seen sharp on a trans¬ lucent or opaque surface placed to receive the image, this sur¬ face being parallel to the original undergoing enlargement. The easel may run on a track, or may be on castors, we figure a most convenient form of arrangement made by the Eastman Company. Fig. 35. This easel may very aptly be used for daylight enlarging by our first-described process. PRACTICAL PHOTO-MICROGRAPH Y. 141 The front of the lantern must be so arranged that the lens used for projection can be racked to at least twice its own focal length from the original. In this process of enlarging (by the optical lantern) the rela¬ tion between focal length of the projection lens and area of the original plays no part; a diaphragm is of little optical advantage, and to proper selection and use of our condenser we must look for success. We are as much incompetent here as in daylight enlargement to give rules for exposure, but the one rule we did give holds equally good here. If by either daylight or lantern process we wish to produce an enlarged negative , two courses are open to us. 1st: We may make from our original (presumably a negative) an en¬ larged positive, and from that we may print “ by contact ” a negative. For this process we recommend a slow gelatine- bromide plate, such an emulsion as is made for lantern-slides answering admirably; or we may produce our large contact¬ negative by the carbon process, or on gelatine chloride emul¬ sion. (For the carbon process consult “ Processes of Pure Photography,” or u Burton’s Guide to Practical Printing,” etc.) This is probably the better course if we enlarge by daylight. 2d: We may in the first place make a small positive by contact or reduction (according to the size of our original negative), and from this small positive we may make an enlarged nega¬ tive on any suitable sensitive plate or film. A small positive such as would be a good lantern-slide is not so well adapted for this purpose as one almost fogged ; that is to say, the posi¬ tive for this purpose should be very fully exposed, more heavily developed than a lantern-slide, full of detail without absolutely clear glass; and presenting no violent contrasts. Whatever be the sensitive material used to receive the en¬ largement, the after operations are the same as those given by us under the heads of development, etc., of the various sensi¬ tive materials treated in other chapters. Bromide paper and Transferotype, for instance, are treated just as if they had been exposed by contact, errors in exposure will manifest them¬ selves in the same way in both cases. If very sensitive emulsion is used, as for example an ordin- 142 PRACTICAL PHOTO-MICROGRAPHY. ary negative gelatine plate, for making an enlarged negative, the greatest care must be taken to prevent extraneous light reaching the sensitive plate. If a lantern of the ordinary type is used it should be enclosed in a box, the lens only protruding through a fitting aperture. In enlarging, beautiful effects may be produced very simply by judicious “ vignetting” ; an opaque mask having a suitably sized and shaped aperture cut in it may be moved towards and from the sensitive material during the exposure. Such matters are treated in most books devoted to photography. CHAPTER XXII. LANTERN SLIDES. It is almost beyond question that the most useful, imposing and satisfactory method by which we can exhibit to others the result of our micrographic work, lies in the projection of a posi¬ tive image upon a white, and proportionately large, surface called the “sheet” or “screen,” by means of an Optical Lantern. So strongly does the writer feel the importance of this subject that he proposes to devote a chapter to a plea for the use of the Lantern for many purposes. The present chapter will be devoted to a description of certain processes which seem to the writer most suitable for slide-making in the hands of those who are not thoroughly conversant with this branch of photo¬ graphy. It is necessary in the first place to know precisely what qualities we require to obtain in a slide so that it may be a thoroughly good slide, or as nearly perfect as our subject and our negative will allow. The first attribute to a slide, specially of a photo-microgra¬ phic slide, is perfect clearness of the ground. Wherever there is no subject the glass must be quite clear. A good slide laid down on a sheet of white paper, will show the paper, seen through the ground of the slide, perfectly white. There should in such condition be no graying or degradation of the purity of the white in the paper. The image may be of various tones according to the process used, but it must always be “ plucky,” and never pale or “ washed out,” the details m ust stand out clearly from each other, as if etched with the finest needle. A perfect slide is indescribable but once seen will never be forgotten. The image, though plucky, must not be opaque, except where the detail in the microscopic image was opaque. Such 144 PRACTICAL PHOTO-MICROGRAPHY. an image as that of amphipleura pellucida should consist of pure white and absolute opacity, and the margins of black and white must be sharply cut. The microscopic image is really essentially an image of lines and points, but in many cases we have to show our lines so blended that the appearance is not one of lines but of masses resembling the masses of a portrait or landscape photograph. If we have such masses in a slide, they must be full of differentiation, or “ half tone,” as it may be called; anything approaching opacity here will be fatal. As instances of masses we may cite low power images of insects, physiological preparations, homogeneous tissues of any kind, “ resolvable,” perhaps, but not for our purpose “ resolved.” All such masses, then, must show half tone. Essential, we have now seen, to a perfect slide are : Clear¬ ness of highest lights ; opacity of resolved lines and points; half tone in masses ; matter of taste is the color or tone ; con¬ ventionally fixed is the size; and “sharpness” goes without saying. It is necessary that the argentic deposit forming the image be very fine, partaking more of the nature of a stain than of a granular deposit. Makers of plates for slides are so well aware of this, and the processes for producing suitable emul¬ sion for slides are so little prone to yield coarse deposits, that we need do no more than point out the desideratum of a fine grained image. But on this account, if on no other, gelatine bromide emulsion such as is used for negative production, is as a rule, totally unsuited for slide making purposes. “ Wet collodion” stands pre-eminent among processes for the production of photo-micrograpliic slides; on that point we have no doubt whatever. Perfect clearness of lights, complete opacity of lines when desired, sharpness, fineness of deposit, half tone in masses—all are obtained by the wet collodion process with a very reasonable exposure in the camera; for contact printing this process is not so convenient. But in any case camera copying is preferable to contact printing for our special purpose, even if our negative is the same size as our slide is to be. In landscape and portrait slides a warm tone is a very important feature, in the opinion of advanced photog- PRACTICAL PHOTO-MICROGRAPHY. 145 rapliers; in pkoto-micrograpliy tone of slide is a very subor¬ dinate consideration, if, indeed, a black tone is not preferable. Dry Collodion, which in virtue of its tone is perhaps the most suitable of all processes for landscape and portrait slides, gives purity of high lights little if at all inferior to wet collo¬ dion, but for camera copying the slowness of Dry Collodion is against it. Gelatine-chloride plates are most valuable for contact print- ing, giving great clearness and varying tones at will, but for camera copying their excessive slowness is very unfavorable. Gelatine bromide plates made for the purpose work rapidly and may be made to yield clear lights and good tones; but while assuredly the gelatine bromide process is the most con¬ venient, and while its results under suitable conditions are in¬ ferior to none, still the greatest care and much practice are required in order that the working of the process may be mastered. Foggy slides by this process are unfortunately very common. We have now put the merits of these processes fairly be¬ fore our Reader. If he makes his negatives of a suitable size for contact printing we recommend on the whole the gelatine- chloride process, or dry collodion. (The latter process, how¬ ever, we must not detail here for want of space.) But gelatine bromide may answer to perfection. For camera copying we strongly recommend the wet collo¬ dion process, but as we fear few of our Readers will care to face its difficulties, or rather its inconveniences, we give as an alternative the gelatine bromide process. The usual size of a Lantern Slide plate is inches square, as a rule a mask is placed over this leaving an area of image of about 2f inches diameter. To print by contact , the negative is placed face upwards in a printing frame, a Lantern Slide plate is placed face to face with the negative, the frame closed and the exposure made to the light in the usual position, i. e., negative next the light. It is advisable that the extreme edges of the lantern plate be protected from the light by a mask or the rebates of the printing frame. 146 PRACTICAL PHOTO-MICROGRAPHY. Reduction or copying in the camera may be performed most conveniently by the same arrangement as is figured No. 33 in our last chapter. The negative is placed in the “ dark slide ” of the large camera; distance is arranged by the rack and pinion of the large camera, centering by its front motions, while the lantern plate is held in the dark slide of the smaller camera with the rack of which, by aid of an eye¬ piece, focusing is performed in the usual way. The back of the large camera bearing the negative may be pointed at the sky, may be lighted by an angled mat white reflector, or may have a sheet of ground glass or tissue paper placed parallel with the negative and a few inches behind it. No light should get past the edges of the negative into the large camera, but this though a good precaution is not absolutely essential. A still simpler mechanism consists in placing the negative against the pane of a window, lighting the negative either by a reflector or by the sky, and photographing it to the slide size in an ordinary camera. The day light enlarging apparatus figured 32 in last chapter may very easily be utilized for our present purpose, by simply fixing the negative in the window, turning the camera round, and suppressing the easel. The lens used must be rectilinear and may have a focus of 3£ inches or upwards. The author uses a “ Rectilinear Stereo ” lens of about 3| inches focus. The Wet Collodion Process for Slides. In most books of photographic instructions that have any pretension to completeness, an account of the wet collodion process will be found. In “ Processes of Pure Photography ” the subject is treated with sufficient care to enable any one referring to that book to work out the process for himself, and to succeed with the process for our present purpose. We pro¬ pose here merely to accentuate, as it were, certain directions given in the book alluded to. Though a lantern-slide is only three and one-quarter inches square, it will be found almost necessary to use a larger plate with the wet process, and to cut it down after the slide is fin¬ ished. Five inches by four will be a suitable size. The glass PRACTICAL PHOTO-MICROGRAPHY. 147 must be very good, flat, polished, free from scratches, bubbles and other visible flaws. The glass must be cleaned, not only in the usual acceptation of the word, but made chemically clean by soaking for some hours in dilute acid (say nitric acid 1 part, water 10 parts). Thereafter it must be washed, prefer¬ ably under a tap first, and then with a solution of sodic carbon¬ ate. Next it should be well rubbed with a clean cloth dipped in a mixture of alcohol and ammonia ; lastly, it should be pol¬ ished with a perfectly clean and soft chamois leather. For lantern slides we do not recommend any substratum of al¬ bumen. The collodion may be bromo-iodised, as generally sold and used, but the iodide should be in strong proportion, the brom¬ ide being here of less consequence. The collodion should be rather limpid than thick, and should be moderately ripe, neither quite fresh nor very high-colored with age. The “ silver bath ” may be the usual 35-grain one, and must be acid, preferably nitric acid being used. Of course this bath must have the usual trace of iodine, produced by immersion of a coated plate for some hours, or by the direct addition of an iodide. Either the flat or 1 the dipping bath may be used, per¬ haps the latter is here preferable, as dust must be most zeal¬ ously avoided. Operations of coating and sensitising are exactly as given in “ Processes of Pure Photography,” Chap¬ ter YI. The developer may be the usual acid ferrous sulphate, or the ammonio-sulphate, or the pyrogallol. The latter devel¬ oper requires the longest exposure, but gives magnificent results, especially if used with a collodion containing iodides only. Pyrogallol Developer. Pyro. 2 grains Glacial acetic acid.30 minims Water. 1 ounce Alcohol. q. s. This solution should either be used fresh, or the pyro dis¬ solved in alcohol in strong stock solution. The quantity of aicohol, which should be of good quality, depends on the age 148 PRACTICAL PHOTO-MICROGRAPHY. f of the silver bath, as the latter is more and more nearly satur¬ ated with the collodion solvents of the immersed plates, the more alcohol will be required. A fresh silver bath requires little or no alcohol in the developer. Ferrous Sulphate Developer. Proto-sulphate of iron, fresh green crystals.15 grains Nitric acid. 1 minim Water. 1 ounce Alcohol.;. q. s. Ammonio-Sulphate Developer (from “ Processes”). Ammonio-sulphate of iron.77 grains Acetic acid (glacial) .70 minims Water.3 ounces Alcohol.q. s. One or other of these developing solutions is deftly swept over the plate from a cup, and no harm will be done with the iron developers if a little solution should run over the edge of the plate. The iron acts more rapidly than the pyro developer, and the latter should be carefully watched and instantly rejected if it shows signs of turbidity. In each case the solution is to be kept moving over the plate until the image has gained consid¬ erable strength, but none of the solutions is to be poured on and off the plate. As we do not approve of re-development for slide making, we advise that the developer be allowed to act fully, provided *no muddiness appears. The plate is now washed, and fixed with 0 Potassic cyanide. 20 grains Water.-.1 ounce Or, Sodic hyposulphite.100 grains Water. 1 ounce We prefer the cyanide, but it is a dangerous poison, the handiest antidote in case of accidental imbibition being a good draught of the iron developer. After fixing, the plate is to be well washed, particularly after hypo. Frequently the slide at this stage wants “ pluck.” The remedy is intensification, but the remedy must be applied with caution. PRACTICAL PHOTO-MICROGRAPHY. 149 Put into a clean cup about 10 drops of a solution of argentic nitrate, bath strength, acidified with nitric acid, add thereto about 4 drams of the pyro, or iron developer, pour this on the plate, but keep it moving, and beware of turbidity, Wash the plate. The color may not be pleasing at this stage, in which case we resort to toning. Immerse the plate in Platinic chloride. 1 grain Nitric acid. 1 minim Water. 3 ounces Watch the progress and remove the plate when the color is satisfactory. A fine color (engraving black) may be obtained by toning in this solution till the image is bleached and almost disappears, and then lightly intensifying with iron and silver as above. Mr. Armstrong’s Palladium Toning for Slides. Palladic Chloride. 15 grains Water. 15 ounces Nitric acid...a trace For each ounce of toner take 1 dram of this and 7 drams of water. Tone till the color penetrates right through the film, and is visible from back of the plate. After the plate is dried and varnished with a clear varnish, (see page 60) it is cut down with a diamond to proper size. rr Gelatine Chloride Plates for Slides. Print in contact. As even with gas or oil light the exposure may be inconveniently long, a good plan is to burn a measured length (say 1 inch) of magnesium ribbon at certain distances, determined by trial, from the frame. For a beginning the inch of ribbon may be burned at 10 inches distance for a moderately dense negative. The exposure to diffused daylight may be from 5 seconds upwards. If magnesium be used a spirit-lamp may be utilized to ignite the ribbon, a spirit-lamp would require a very long time to impress the plate. Development is performed in a flat dish, the plate being placed therein face upwards, and the developer poured over the plate. 150 PRACTICAL PHOTO-MICROGRAPHY. Mr. A. Cowan’s Formula for Development of G-elatine Chloride Plates. a. Ferrous protosulphate.140 grains Sulphuric acid. 1 minim Water. 1 ounce Add one part of a to three parts of one of the following. For Cold Tones after Short Exposure. b. Potassic citrate.136 grains Potassic oxalate. 44 grains Hot water. 1 ounce For Warm Tones after Long Exposure. c. Citric acid.180 grains Ammonic carbonate. 60 grains Water. 1 ounce Mr. B. J. Edwards’ Formula. a. Neutral potassic oxalate. 2 ounces Ammonic chloride. 40 grains Distilled water. 20 ounces b. Ferrous protosulphate. 4 drams Citric acid. 2 drams Potash alum. 2 drams Water distilled. 20 ounces For developing mix a and b in equal parts. The developer is allowed to act until the image is seen plucky in every part, the plate is then well washed and fixed in “hypo,” of usual strength. If any scum, due to salts in the water, is found, it may be removed by the acid-alum solu¬ tion given on page 63, the solution being poured on to the plate. Beautifully clear slides ought to result from this treatment of gelatine chloride plates. These plates may be made much more sensitive than those usually sold, but, as a rule, the more sensitive plates are inferior in quality. In developing these plates, it is well to develop to full den¬ sity at least, rather beyond than short of it. It is easy and often advantageous to reduce the slides' after fixation thus: (Mr. Cowan.) “ Strong solution of iron perchloride. 34 ounce Hydrochloric acid. 24 ounce Water.20 ounces PRACTICAL PHOTO-MICROGRAPHY. 151 “Followed after washing by fresh solution of hypo.”—(See page 63.) Gelatine Bromide Plates for Slides. In this process we have certainly the quickest method of making slides for the lantern, but it can only be called the best method when worked with care and skill. If the Reader proposes to make all his lantern-slides “ by contact,” he had probably better use chloride plates; but if he objects to the wet-plate process, he will find the process we now describe the best for his purpose if he has to reduce or copy in the camera; while the gelatine bromide process may be made to answer admirably for contact printing. Ninety per cent, of the author’s slides, reduced and contact-printed, are made with commercial gelatine-bromide plates. The exposure required in the camera is from three to ten times less than with wet collodion, the difference of ratios depending upon the sensitiveness of the gelatine plates and the quality spectroscopically of the light. In a very “ actinic” light the difference is minimized, in a “ poor ” light as regards blue and violet rays the gelatine has a greater “ pull.” The bromide plates are many times more rapid than the chloride plates, not less, perhaps, than 20 times. Exposure is made in the usual way, but the light of the “ dark-room ” must be much more subdued or non-actinic than for collodion or chloride plates. This fact is too often disre¬ garded. The developers we recommend are ferrous oxalate and hydroquinone, but alkaline pyro is also available. The ferrous oxalate developer may consist of one part of the .saturated ferrous sulphate solution (page 63), to four, five, or six parts of the potassic oxalate solution on the same page. We always add half a grain of soluble bromide to each ounce of the mixed developer. When this developer is applied the image should begin to appear after about thirty seconds, not before twenty seconds; density and detail should increase steadily up to about four minutes ; when the development is complete the image seen by transmitted light ought to be 152 PRACTICAL PHOTCBMICROGRAPHY. denser than appears necessary, for the fixing makes a great difference. "When the plate is watched by reflected light there should be no sign of degradation of the white where there is no image; as soon as any such “graying” appears the plate must be swilled with water containing enough acetic acid to make it smell distinctly of that acid, the developer being, of course, poured off, either down the sink or into a cup for future use. The developer may be used repeatedly until it either turns muddy or acts too feebly; each time it is used it acts with less vigor than previous times, on account of fresh bromide being formed and on account of oxidation. Unfortunately it is sometimes impossible to avoid a slight degradation of the white of the slide, this being due to the want of density of the negative ground in comparison with the density of the subject-image. If there is good contrast of density between subject and ground there ought to be no sign of deposit on the slide-ground, and any such deposit is probably due to overexposure of the slide plate or unsafe light. A very slight deposit may be removed by the acid alum and iron solution, used after fixation. After the plate has been well washed in the acidulated water, three changes at least, it may be washed under the tap and fixed in the usual hypo-bath. Thereafter it is to be well washed and then the following poured on and off from a height, in order to clear any deposit that may be on the plate, due to lime salts. Saturated Solution of Ferrous Sulphate.4 ounces. Saturated Solution of Alum.16 Hydrochloric Acid... 1 to 2 drams. The IIydroquinonb Developer. (Mr. Edwards.) Mix in the following order:— Sodic Sulphite,. 2 ounces. Water.20 “ Hydroquinone.30 grains. Sodic Carbonate..... 3 ounces. Potassic Carbonate. 3 “ Potassic Bromide..40 grains. This solution will not keep good for very long, but if the water be divided into two parts and the carbonates separated No. 1.—Bacilli Tuberculosis in Lung of Horse, X 750. No. 2.—B. Tuberculosis by Inoculation, Lung of Rabbit, X 600. Plate VI. PRACTICAL PHOTO-MICROGRAPHY. 153 from the hydroquinone, the two solutions will keep for a long time, and may be mixed at time of need. This developer acts very steadily; if exposure has been sufficient failure can hardly occur with suitable negatives and good plates, for the developing action only requires to be watched and stopped at the proper time. Fixing, washing, and clearing may be conducted as usual. This developer is strongly recommended for Bromide prints. Alkaline Pyro Developer. This is not recommended for our purpose unless there be some special desire for warm tones. Sodic Sulphite as a pyro preservative is to be avoided and preference given to potassic meta-bisulphite see page 61. An ounce of developer may be made to contain: Pyro.1 to grain. Potassic Meta-bisulphite.2 to 3 grains. Liquor Ammonia, 880. .. .2 minims. Potassic or Ammonic Bromide.2 grains. Water.to 1 ounce. The rationale of developers, of which the above may be called illustrative, is simply a well restrained development, the potassic salt tending to produce a warm tone, coupled with full exposure and “ willing ” development. Here again fixing and washing are as usual, except that after fixation if a warm tone be desired and not previously obtained, the washing should be little more than a rinse, and the acid- iron-alum solution applied as before will greatly redden the color of the image while clearing the high lights. All lantern slides should be varnished with a clear varnish, usually certain colorless gums dissolved in benzole. To mount a lantern slide. Clean a thin, flawless glass plate 3^ inches square; select and lay in position upon the face of the slide a suitable mask ; on the mask lay the cleaned cover- glass. Wet a gummed “ strip,” which ought to be about fths of an inch wide and 14 inches long ; lay one side of the now- protected slide in the middle and at one end of the wetted strip; turn the slide over side by side, always lifting with it 154 PRACTICAL PHOTO-MICROGRAPHY. the sticky strip and always turning in the corners of the strip and running the fingers along back and front of the slide to make the strip adhere all along. On reaching the last corner, cut or tear off the extra bit of strip, and see that the strip adheres all along its length to the glasses. Lay the slide down face up as it is wished to appear on the screen, and then attach two white marks one at each top corner; these marks for a guide to the lanternist. Transferotype prints when stripped on plain glass make fine slides ; a strong point being that they may first be dried and then trimmed with scissors. They are then well soaked in water and stripped, see page 128. CHAPTER XXIII. USE OF THE OPTICAL LANTERN. Many lecturers and teachers are quite awake to the advan¬ tages that would arise from the use in lecture and class-rooms of the optical lantern, but almost all are deterred from the use of this valuable instrument by a mistaken idea as to the diffi¬ culties attending its use. The writer has conversed on this subject with several Professors of note, and in every case the difficulties were either imaginary or exaggerated. It is admitted that under certain conditions the student will learn more of the nature of his subject by making a careful, even if faulty, drawing of it as seen in the microscope. But in very many cases the value of drawing is confined to the time actually employed upon the sketch ; that is to say, that whatever is noticed at the time of sketching is learned, but nothing more; and the sketch thus executed can never teach anything more. A photo-micrograph not only reproduces details independent of the momentary observation of the operator, but it is a litera scripta which is itself open to fur¬ ther and more leisurely and careful examination, and, more¬ over, it will probably contain details beyond the power of the hand to copy, even if the eye noticed them; and if there is one method more apt than another to lead to fresh discoveries, it is the method of enlarging in the optical lantern. So much for the student. The advantages of the lantern will be even more sensible to the teacher than the taught. The professor presumably knows what points he wishes to demonstrate, though by a sketch he may be unable to dem¬ onstrate them; and every one knows the uncertainty and in- » convenience of demonstration to a class by one or more micro¬ scopes and oral explanations. But let a suitable photograph be produced, a lantern-slide made therefrom, and the image 156 PRACTICAL PHOTO-MICROGRAPHY. projected upon a screen in front of the class, and the professor can point out to all at once, without a possibility of mistake, the appearances and the parts he wishes to discuss. Further, when the plioto-micrographic negatives are to be used for this purpose, the necessary magnification is so greatly reduced in all cases that the original operation is very greatly simplified, and in fact the results are probably better than when a considerable original amplification is necessary in order that the points may be seen. And on account of this smaller magnification the surroundings of the critical object are better rendered. By the process we now discuss the teacher shows things as, and where, they are ; by a sketch he can only show them as he thinks they are or ought to be. B»T£»t NEWTON U Co. I w DB a "•*■▼** LAMO ON MSI ■ K mm j| II 1 P ill1 l lli I msm till Him IB [ ■■■ii ii Fig. 36. —Lantern Microscope. When practicable it is, perhaps, better to show the object by a lantern microscope, or by a special arrangement of lantern and the essential parts of a microscope. Here is figured an arrangement of the kind alluded to (Fig. 36). The remarks of this chapter will, in the main, apply to the use of this in- • strument as well as to the use of an ordinary optical lantern furnished with a “photographic” projection lens, and used with a photographic lantern slide. But it is easy to realize that the use of the lantern microscope PRACTICAL PHOTO-MICROGRAPHY. 157 is limited, because the number of subjects suited to the instru¬ ment is limited. The loss of light in projecting a large image of even a moderately thick object is so considerable, that only in exceptional cases can success be expected. Moreover, as our optical science now stands, the combined difficulties of great magnification, illumination, resolution, and definition are too great to be satisfactorily overcome, except in specially favorable circumstances.* Fig. 36 ( a ). The instrument known as an optical (or vulgo “magic”) lantern is quite common and its appearance familiar to every * Since this was written the author has seen a greatly improved instru¬ ment by the same makers and for the same purpose. It is, however, costly. It is shown in Fig. 36 in its latest and best form. 158 PRACTICAL PHOTO-MICROGRAPHY. one. It is simply a box to hold the condenser, object or slide, and projecting lens in certain relations to each other and to the radiant, and to prevent the radiant from illuminating the screen directly. The screen is usually a sheet of white fabric, cloth or paper, but it ought to be, especially for purposes now under consideration, an opaque white surface, as plaster on a smooth wall. Paper with a smooth but not shiny surface, pasted to a wall or board of sufficient size, will answer every purpose. The diameter of the screen should be about one- third of the length of the lecture room; there is no necessity for much more, and in any case 15 to 18 feet will be the extreme likely in ordinary cases to be convenient. An oil lamp made on scientific principles, due attention to draught being the main feature, will illuminate a ten-feet screen well, an eight-feet one perfectly. Number of wicks*is of less consequence than the position of the wicks with relation to each other, and still more emphatically with relation to the condenser. A lantern full of burning wicks will give less light and worse confusion than one wick or three properly placed. A lantern known in America as the Scovill “ Sciopticon ” is as good as any oil lamp we know. We figure this article Fig. 36 (a). Perhaps the condenser is the most important part of a lan¬ tern. It should be a double one certainly, and a triple is by some considered superior. A condenser which works very well with a wick lamp need not be the best for the lime-light, and the converse holds equally good. Still a condenser which gives an even, brilliant image, free from spherical aberration, with the lime light will suit well for an oil lamp with one or three wicks as usually arranged. Now that oxygen is obtained in our countries so easily, of so good quality and at such moderate prices, we think it prob¬ able that a lecturer who has once used the lime-light would not long tolerate the nuisance of an oil lamp. There is perhaps no necessity for a serious nuisance in connection with oil; and whatever trouble there is will fall not on the lecturer so much as on paid assistants; none the less the inconveniences of lime- PRACTICAL PHOTO-MICROGRAPHY. 159 light are so small compared with those of oil that we recom¬ mend the former in all cases where common house gas is attainable. For any ordinary class rooms the light produced by a “ blow-through ” blow-pipe on a “ soft ” lime will be found ample, either for projecting a photographic slide image on a 12 to 15 feet screen, or an image of the actual object—if suit¬ able at all—or a screen of about 5 or 6 feet diameter, using in the latter case a microscopic objective, in the former a photo¬ graphic lens of the portrait type. For our “blow-through” jet we require one tube of the jet to be connected with the gas supply of the building; the oxygen gas we keep in a steel cylinder under pressure, or in a bag between weighted pressure boards; these with a supply of lime cylinders are all the appli¬ ances required. There is no smell, no cleaning needed, no danger, nor any notable heat. There are several forms of blow-through, or as it is sometimes called, “ safety ” jet. The hydrogen issues and burns at the larger orifice, the oxygen is forced at some pressure through the hydrogen flame, a jet of the two gases burning impinges on the lime, which like other refractory substances gives a brilliant light in com¬ bustion. The area of incandescence with this jet is larger considerably than the area produced by the “mixing-jet,” which gives a light both smaller and brighter. The “ safety ” jet is on the whole better adapted for the lecture-room than the mixing jet, not so much on the score of safety as on that of convenience. A mixing jet is shown in Fig. 11. The objection usually raised in our hearing to the use of the lantern for classes is the difficulty of darkening the lecture- room* There need be no trouble on this point, though there may be some expense if the lectures are given in the daytime. Ordinary folding shutters, if decently fitted, will exclude light sufficiently, but in Britain we have a very good shutter for any purpose, probably the best for all purposes, called Clarke’s Patent Shutter. It is made on the “ Louvre ” principle of strips of metal or wood, and on being raised it coils itself up into a receptacle above the window. These shutters can be pulled in a few seconds, and in the writer’s house they shut out every beam of light that could mar the lantern image. 160 PRACTICAL PHOTOMICROGRAPHY. The axis of the lantern optical system may be horizontal, and the lantern may be raised so that this axis “ produced ” falls upon the centre of the s creen ; or the lantern may be placed on an ordinary table or stand tilted upwards; and the screen tilted the opposite way until it is perpendicular to the optical axis of the lantern. The lantern slides are held in position in the lantern by a device known as a “ carrier.” The simpler this carrier is the better, provided that the slides can be easily and certainly centred. No carrier for our purpose is likely to excel the old “Chadwick,” well known in England, at least; but one of the mechanically centering ones may be used. The projection lens should have a focal length suitable to the size of disc required and the distance from lantern to screen. S— Size of opening in slide in inches. Z>=Diameter of disc in feet. Z=:Distance of lantern from screen in feet. Zh=Focal length of projection lens in inches. r DXF LXS Lx S z = ~ • z = — • f =~d- (From “The Magic Lantern Manual,” by W. I. Chadwick. London : Warne & Co.) A good light is obtained more by careful adjustment of the proportions of the gases than by heavy pressure. It is, of course, necessary to accurately centre the various parts of the optical system, and to get the radiant as nearly as may be in the exact focus of the condenser. The limes should be always kept in a dry or air tight receptacle, and it is well to heat or even bake them for a short time before use. If bag and pressure boards are used the latter must have a free fall, and no person should be allowed to touch the bag when the gas is alight. The weights must be of such shape, or so fixed, as to have no chance of falling off. If any hitch should occur the oxygen is to be turned off first; on lighting up at first the hydrogen is always to be PRACTICAL PHOTOMICROGRAPHY. 161 lighted before the oxygen, and the full brilliance is to be got by turning on the gases alternately little by little. The “ cut off ” jet designed by the writer is specially recommended for the lecture room, gas and trouble $re greatly economized by the simple contrivance (see page 44). Oxygen and hydrogen, if desired, may very conveniently be stored in gas tanks or holders made of sheet iron. The gases may be used directly from such tanks. If bags are used for the two gases, each bag should be conspicuously marked “H ” or “ O,” or the bags should be different in appearance. There is no necessity for an accident even if the gases should get mixed in one bag provided that the pressure be kept up ; but risk, however slight, should be avoided. With the gas in cylinders there is no risk. Postscript .—A learned professor has just suggested another difficulty to the writer. “ l r ou do not,” says this experienced teacher, “ know medical students in the dark! ” The writer admits his inexperience, but would gladly run the risks of gases and students mixed. CHAPTER XXIV. IMMERSION, APOCHROMATICS, AND APERTURE. OCULARS. The two greatest improvements made in our optical appliauces within a period of many years have been, first, the introduction of the “homogeneous immersion” system for objectives; and second, the use of a new glass, having different dispersive powers from glasses previously made. The result of the discovery of this new glass is that in the new apochromatic objectives we have a more complete correction for color than ever we had before; that is to say, the new lenses are corrected not only for two spectrum regions as formerly, but also for a third region. The system of immersion is now so much a matter of ancient history that we need only point out the advantages now universally admitted to arise from the latter development of immersion, viz : Homogeneous or oil-immersion. Before the days of oil-immersion trouble arose from the fact that between the object and the objective two media of different refractive powers intervened, viz : Crown-glass (the cover-glass) and air— or water. When a substance was found having approximately the same refractive index as crown-glass, and when the objective was practically joined to the cover-glass by this substance, viz : an oil, it is easy to see how great a step was gained, provided always that the object was either in contact with the cover-glass, or in a medium nearly equal in refractive power, and in contact with the cover. More oblique rays passed from object to objective, the illumination was better, working distance greater, and in fact there was improvement in almost every respect. By the immersion system lenses can be produced of larger aperture PBACTICAL PHOTOMICKOGKAPHY. 163 than the limit available with dry lenses, and the “ fan ” of diffraction rays is by immersion “ closed up.” To narrate the causes and considerations that led to the adoption of numerical aperture as the basis of calculation of the apertures of objectives would be both tedious and out of place. ¥e content ourselves, therefore, with saying that to Professor Abbe is due the system of calculation now almost universal, and by permission of the Royal Microscopical Society, we give in the latter part of this book a table of apertures, showing the relation between “ N. A.” and angular aperture, together with other matter of great value to the microscopist. It is important to remember that resolution is proportional to numerical aperture but not to angular aper¬ ture. The maximum air angle being 1, we have oil immersion lenses with numerical aperture as high as 1.5, but nothing above 1.43 has, so far as we know, been made practically useful. The immersion system is also used for condensers, and theoretically we ought to be able to use the whole aperture of the highest apertured glasses, but few, if any, glasses are sufficiently well corrected to stand utilization of their entire aperture. (See paper by Mr. E. M. Nelson in “English Mechanic,” No. 1,234, Nov. 1888, for information on this subject,) Messrs. Powell and Leland make a fine apochromatic condenser N. A. 1.4. Zeiss constructs a similar article N. A. 1, while many opticians make non-achromatised oil-immersion condensers up to N. A. 1.4, or nearly so. Apochromatic homogeneous immersion objectives of high N. A. are at present the acme of microscopical practical optics. But while resolving power increases with numerical aperture the quality called penetration decreases as resolving power increases. We have, however, tried to show in an earlier chapter that this “ penetration ” is a bogus quality, and, in fact, a .defect though a deceptive one. Nevertheless there are occasions not a few when moderate sharpness on various planes is preferable to absolute sharpness on any one plane, and in such cases the utilized aperture of the lens may be easily cut down by stopping down the condenser to any desired extent 164 PRACTICAL PHOTO-MICROGRAPHY. short of loss of definition and resolution. Definition and resolution as technical terms must not be confounded. Reso¬ lution consists in visibly separating close markings, definition consists in imaging distinctly small compact objects. In order to obtain the full benefit of the series of apocliro- matic objectives made by Zeiss it is necessary to use in combination with these objectives the “compensating eye¬ pieces” made to go with them. In order to obviate the necessity for an ocular being specially made to suit each objective, Abbe and Zeiss have been bold enough to deliberately introduce in certain objectives certain aberrations which are corrected by the oculars. For the projection of a real image, such as in photo¬ micrography we require, free from aberrations, and visually and chemically correct, Abbe has designed and Zeiss makes a series of “projection oculars.” These are the oculars to which we have referred as being the best, if not the only good, oculars for photo-micrography. In both the compensating and projection eye-pieces Zeiss follows the commendable system of marking the eye-pieces, not by arbitrary and meaningless letters, as “ A,” “ B,” or “ C,” but with a number indicating the amount to which the object¬ ive image is magnified by the ocular ; but it is to be noted in calculating the magnification of an image produced by these oculars, that the figure on the ocular is accurate only for the precise tube length for which the ocular is designed. The No. 4 ocular is intended for the continental tube of 160 milli¬ meters (about 7 inches); and at 160 mm. behind the posterior conjugate focus of the objective, the objective-image is magnified" just four times by the No. 4 ocular ; but if we are caused to alter the tube-length in order to obtain better “ correction for the cover glass,” the calculation no longer can be taken as. accurate, as far as regards our total magnification. The magnification given by a projection ocular and objective combined on a screen at a known distance from the ocular may, with convenience, but only in one case with absolute accuracy, be calculated by dividing the distance in mm. from ocular to screen, multiplied by the number on the ocular, by PRACTICAL PHOTOMICROGRAPHY. 165 the focal length in mm. of the objective. Example: With an objective of 3 mm. focus, a No. 3 projection eye-piece and a screen thirty inches from the shoulder of the ocular, we get a magnification of approximately 750 diameters, 30 inches = eay, 750 millimeters. 750 X 3 (ocular) _ ^ 3 (focus of lens) But this applies with only moderate accuracy when we have, for instance, racked our tube to 10£ inches in place of the 250 mm. (10 inches) for which the ocular and objective are in¬ tended. If, however, our tube length be really 250 mm. our magnification may be taken as almost exactly 750 diars. in the above given example, at least the writer has not been able on experiment to verify any inaccuracy in the figures given. The future advance in photo-micrography—if there is to be any advance in the optical line—will depend upon apochro- matic objectives and condensers, and the use of wide angles. It is vain to say that all the greatest discoveries have been made with low-angled glasses, though the statement may be perfectly true. Had higher angled glasses been used the dis¬ coveries would have been made all the sooner, and our high¬ angled glasses of to-day demonstrate with perfect ease even in unskilled hands what required years of study and the most skilled microscopists to certify in by-gone days. And further, the science of practical optics was, say twenty years ago, far behind where it is now, and the opticians of these days, in achieving high angles, probably introduced such errors of cor¬ rection as made the glasses practically worthless. So the ob¬ servers did well to use well corrected low-angled glasses rather than faulty high-angled ones, and their discoveries were made with low-angled glasses, faute de mieux. The writer is in the constant habit of examining numbers of objects of the most diverse kinds, he has at command high-angled and low-angled glasses, yet even for cursory examination of such subjects as pathological, physiological, bacteriological and diatomaceous objects, he invariably takes as if by instinct the widest angled glass he can find, of suitable 166 PRACTICAL PHOTO-MICROGRAPHY. power. For photo-micrography, as a matter of course, he uses apochromatics, and always at the highest angle possible con¬ sistent with contrast between “ tissue ” and ground. No doubt the old experienced microscopist, accustomed to low-angled glasses, will be sceptical, and may even at first believe in the inferiority of the wide angles, especially if he use an imperfect system of illumination, but we have firm confidence that after a fair trial and a little perseverance and fight with prejudice, the newer glasses will prove victorious, even in the eyes of our fathers of microscopy. If, in any branch, “ want of penetra¬ tion ” can be disastrous, surely that branch is the photo-micro¬ graphic ; yet of all the conditions under which we insist on a wide-angle apochromatic objective, photo-micrography is the chief. CHAPTER XXY. CLASSIFICATION OF OBJECTS—HOW TO TREAT THEM. While we can not attempt to give definite rules for the treat¬ ment, microscopic or photographic, of every object or class of objects that may come under the notice of the Photo-micro- grapher, still in onr somewhat varied experience of various classes of objects we hav.e noticed facts and formed opinions which may be of service to our Readers. In hope, at least, of such service being rendered we shall set down a few points noted in our actual work. Insect preparations are almost always exceedingly difficult to photograph well. In the first place if the insects are of any size and are mounted in cells as they ought to be, and not flattened by pressure as they sometimes are, we have to deal with the optical “ difficulty ” of focusing at once various planes. To meet this trouble the lowest available “power” should be used, and stretch of camera substituted for eye-piecing as a means of getting magnification, and even this does not help us much. Making small negative images and enlarging them will probably not help matters in the least, in our experience “ enlarging ” has never been of any service in overcoming the difficulty to which we allude. In our own work the most marked successes in this line have been obtained by the use of an apochromatic lens of 70 millimetres, or about 3 inches facal length, and a camera stretch of from 5 to 7^ feet. The illumination in a case such as this is apt to be a puzzle, but the best plan is perhaps one suggested to the writer by Mr. E. M. Nelson ; namely : to use as condenser the field-glass of a good “ A ” eye-piece, placed close behind the object, (i. e. between the object and the light), the convex sur¬ face of the field-glass being turned towards the light. Other 168 PRACTICAL PHOTO-MICROGRAPHY. glasses may doubtless be used, but this is the one most likely to be within reach of every microscopist. The other salient difficulty with insects is a photographic one and depends on color. Insects are frequently very dense yellow in color which may be overcome by use of color-correct plates with or without yellow illumination; but when in addition to a densely non-actinic body they have pellucid wings, antennae or legs, the difficulty becomes very great indeed, and skilful is he who can surmount such a concatenation of difficulties. Again the worker must look to careful and intelligent use of color-correct plates and colored screens or light. Exposure must be full for the densest part of the object; development should be sharp and short, and as soon as details are out in the dense parts, the negative should be fixed and intensification resorted to. We have an idea, the outcome of some experi¬ ments, that for this class of work the hydroquinone developer with the caustic alkalies soda or potash is eminently suited. (See page 96.) Pellucid Objects , such as some diatoms, present difficulties of their own which we must notice. The difficulty is partly in the optical, partly in the photographic department of our work. By lowering the angular aperture of our lens we may produce more visual contrast between the pellucid object and the ground on which we see it, but we at once lose definition and resolution. This holds good in the photographic as well as ocular branch. If we lower the force of our radiant-power we improve matters visually by taxing less the “ accommoda¬ ting” power of our eye, but the same step has no advantage in our photography, for lowering the light simply entails in¬ creased exposure. With objects of this pellucid nature our best plan is to cut down the angle of our condenser as much as we can without any loss of definition or resolution, (which is equivalent to using as much as necessary of our objective’s available aperture and no more), keeping the exposure as short as is consistent with getting a black negative ground, using a thickly coated plate, an emulsion replete with silver haloid, and a slow system of development, preferably perhaps hydro¬ quinone with sodic or potassic hydrate (“caustic” soda or PRACTICAL PHOTOMICROGRAPHY. 169 potash.) And though as a rule we consider intensification of negatives after fixing, by mercury or otherwise, an operation little commendable, we must confess that for this particular class of object it has frequently proved vastly useful to us. A fine sample of this class of subject is a “ plate” split off, usually by accident in cleaning, a diatom. Physio-and Pathological Preparations are easy, difficult, or impossible to reproduce well according to their cutting and mounting. Assuming that the points to be portrayed by pho¬ tography are well seen by the eye, it is our business to over¬ come difficulties of staining ; but difficulties of uneven mount¬ ing, bad sectioning and the like are beyond our control. We would protest here both for our own sake and for the sake of pathological and physiological science, against the mania, apparently spreading, for gossamer sections. Granted that there are cases where no instrument we have can cut a section too thin, it is none the less true that those cases are exceed¬ ingly rare. A section cut to show the more minute bacteria, or for histological purposes in general, can perhaps not be too thinly cut, but for ordinary physiological and patho¬ logical research, sections are often cut too thin to be of an} 7 real use; and they are most obnoxious to the photo-microg- rapher. On the other hand tissues imperfectly or improperly prepared for cutting cannot be cut thin enough for any useful purpose, and the mischief of thick cutting is made all the more apparent when staining that would be good for ordinary sections, is used on these “ slabs ” of tissue.* Given, however, a good section of the class of subject now under notice, we ought to produce by photography a repre¬ sentation far beyond the best that manual dexterity can' accomplish. The question resolves itself into one of color- correct photography with careful and skillful correction of the objective specially if the work be histological. If “ cells ” are to be shown in the characteristic state of health or disease, the optical adjustment of all apparatus must be perfect. Epithelial cells, for instance, may be horribly travestied by improper lens correction or false lighting. The lowest power should be used that will show the formation required to be * There are those who maintain, with some reason, that a section can hardly be too thin, provided it is sufficiently stained. 170 PRACTICAL PHOTO-MICKOGRAPHY. shown. If a one inch o. g. has sufficient aperture to show muscle striation, it is folly to use a one-eighth o. g.; if magnifi¬ cation alone is wanted, in such a case we should decidedly recommend camera enlargement. Isolated histological subjects are sometimes very difficult, the difficulties coming under various heads already.touched in this chapter or about to be touched. We allude to such things as epithelium cells, red and white blood corpuscles, sperma¬ tozoa, etc., spread on a slide. These objects do not, as a rule, take kindly to stains, and they are not only more or less rounded but require considerable magnification to make their morphology visible. Here again we must look to color-correct photography to give us contrast, and to accurate correction objectives, both for the actinic focus, and by careful collar or tube adjustment. We usually meet the pale red or blue of such staining by the yellow screens and yellow-sensitive plates. Slow development carefully restrained is indicated, and sometimes intensification necessitated. As a sample of this class we may cite the task of reproducing ciliated epithe¬ lium cells, to show which well, cilia, nuclei, and protoplasm, is not easy. The ease or difficulty of photographing cover glass prepara¬ tions of bacteria depends chiefly on their staining. The stains most frequently used for such preparations are either violet (gentian) or red (fuchsine). Few things are more difficult to photograph than a microbe lightly stained with gentian violet, or very lightly with fuchsine. If the stain in either case be pale, the only hopeful method, is to use yellow screens and color-correct plates, no more angle than necessary being • brought into play. Some organisms badly stained with Bismarck brown have completely baffled all our attempts to photograph them. In the case of pale pink staining, some¬ times the result of faded or abortive fuchsine or eosine stain¬ ing, the signal green glass screen has more than once helped us, but in most cases the yellow screen, theory to the contrary, has proved more useful. If organisms are well stained, the colors vigorous, the ground clear, the material evenly and thinly spread, the difficulties of this class of works are great PRACTICAL PHOTOMICROGRAPHY. 171 only because of the magnification required. We have suc¬ ceeded well in such work with a cheap “ student’s ” oil immersion one-twelfth by Swift, and also with a similar one- twentieth by Reichert, using no ocular in either case; it is within our knowledge that glasses by Powell and Lealand (specially a twenty-fifth), Beck, and Seibert have been success¬ fully used in this manner and for the same purpose. The demonstration by photography of flagella, with which some micro-organisms are endowed, depends upon most accurate correction of objective coupled with suitable mounting and staining. Though many of these organisms are flagellated, it is a most difficult matter to keep the flagella visible in mount¬ ing, and it is little less difficult to photograph them when visible. Test diatoms present the usual microscopic difficulties with, generally, a photographic one “ thrown in,” viz., the difficulty of getting contrast. In these cases angle must not be dis¬ pensed with, for the great point is not only to get resolution, but to get very strong and distinct resolution. The photo¬ graphic difficulties must be overcome by photographic skill and not by sacrifice of optical excellence. The diatoms should be absolutely in contact with the cover glass, to determine which Beck’s “ Vertical Illuminator ” is invaluable, and the medium in which the objects are mounted should be of suit¬ able refractivity. It seems to be the fate of every photo-micrographer of any ambition, sooner or later, to attempt to photograph “ amphi- pleura pellucida in lines.” The first thing is to get a frustule as coarsely marked as possible, but withal clean and flat, mounted in a medium of high refraction index. The “ striae ”* seem usually to run about 95,000 to the inch. The lines can¬ not with any angular aperture open to us be clearly shown by axial illumination, and oblique illumination is attained by put¬ ting into the condenser a disc with a slot of one or other of the following shapes: B being preferable in most cases. This slot is variable in breadth and length, and must be chosen to give the best result by experiment. A strong light should be used, as lime light, for under the best conditions of illumination 172 PRACTICAL PlHOTOMICROGRAPHY. the exposure will be protracted. During the long exposure there is great risk of motion of the apparatus not only from tremor of the apartment, but from heat of the radiant affecting the apparatus. An alum trough is therefore recommended be¬ tween light and condenser. Fig. 37. If the edge of a lamp-flame or the face of a lime cylinder is used as the illuminating surface, the frustule must be arranged vertically on the stage, and the slot of the condenser stop is to run parallel to the frustule. The directions of valve and light are therefore shown thus: Fig. 38, while in the field of the microscope almost the whole will be dark, except the frustule itself, until the high power o. g. is brought into play. PRACTICAL PHOTO-MICROGRAPHY. 173 The successive operations towards “ setting up ” this object may be thus described. A suitable frustule is chosen, a me¬ dium power objective being used to centre the object and the light, and to focus the condenser accurately upon the object. The slotted stop is then inserted in the condenser, and the effect examined with the medium power; little else beside the frustule should be illuminated. Both high-power objective and condenser must be oil immersions and of the best quality. The high-power objective is now put into place and action, and the most accurate correction by collar or tube length ob¬ tained by experiment. The condenser will almost certainly require re-focusing after the high-power glass is brought into play. With perfection of apparatus the white lines ought to appear about four times as broad as the black ones (Nelson). The focusing of the image on the screen is with ordinary achromatic lenses exceedingly difficult, with apochromatics less so. No pains should be spared to get a correct focus, and after it is got the whole apparatus should be allowed to rest for ten minutes at least, the light at full blaze; after this time the image is to be again examined to test for steadi¬ ness. Clearly if the accuracy is lost in ten minutes it is hope¬ less to attempt an exposure of twenty minutes to an hour with lime, not to speak of four to ten hours with an oil lamp. We have exposed for six hours on this diatom with oil, and our usual time with lime is twenty-five to thirty minutes; with these exposures we expect to get a good photographic negative; if there has been no motion of the image we are much pleased and somewhat surprised. There is no special difficulty in de¬ velopment. In spite of all these details, A. pellucida is child’s play to photograph in comparison with such tests as P. angulatum , 8. gemma , and N. rhomboides by axial light and to show “ black dots.” P. angulatum in white areoles, or N. rhomboides in squares with a special disc in the condenser, is infinitely easier than the same in black dots. CHAPTER XXYI. BLACK BACKGROUNDS. OPAQUE MOUNTS. POLARIZED OBJECTS. Photography of certain lustrous objects upon a dark ground is not merely, as some think, a playful way of producing a sensational picture, but is in many cases a really useful method of depicting suitable objects. We have seen a lecture on Diatom Structure illustrated almost entirely by lantern-slides of diatoms on black ground; many crystals, and certain eyes are better seen on such a ground than on a white one. When objects are “mounted opaque” they require to be photographed by reflected light, and the background is then naturally dark, but when objects are mounted in the common way on clear glass, we require for a black ground one or other of several optical instruments. First we mention a “ spot lens,” which, before it became obsolete, was used below the stage, but is now entirely replaced by a “ paraboloid,” or, better, by the addition of certain disc-stops to an ordinary substage con¬ denser. The paraboloid is a very pretty piece of optical ingenuity, sometimes called “parabolic illuminator,” but in practice the condenser with stops will be found superior. For this class of work the achromatic substage condenser is usually furnished with a set of the discs figured No. 5, and there is in the fittings of the condenser a slot or other recep¬ tacle to receive the stops. With powers higher than a quarter- inch it will be found somewhat difficult to work the system properly, still it is not impossible to obtain good results even with immersion glasses. The practice is somewhat as follows: The condenser is centred, the condenser and objective focused as usual on the object, but in the present work a bull’s eye may without detriment, and even with advantage, be used to PRACTICAL PHOTO-MICROGRAPHY. 175 parallelize the rays on the back of the condenser. Now one or other of the black discs is placed in the condenser, which may be racked up and down so that the best image of the object and the blackest ground possible are attained. The point of chief importance is to get the margins of the object perfectly sharp against the black ground, and the secret of suc¬ cess is to use the smallest disc that will give a brilliantly-lighted object with an absolutely black ground. The “ spot lens ” may aptly be likened to the above arrangement minus the power of altering the size of the spot, and the parabolic illuminator may take a place midway between the two, for it has a certain range of adaptation in virtue of a small movable “spot” worked from its lower end. We need hardly say that in this work a prolonged exposure is required, more prolonged than might be expected by the beginner. Fig. 39.—Lieberkuhn. (Beck.) Fig. 40.—Wenham’s Parabolic Illuminator. On the whole, we should prefer a spot lens to a paraboloid, were we compelled to use one or other. When an opaque mount has to be photographed we should use the Lieberkuhn (Fig. 39) when it is available. With the Lieberkuhn the light passes from the radiant round the object to the Lieberkuhn (which is fixed to the objective) and thence reflected back upon the object, which is thereby illuminated evenly from all sides. This necessitates an object mounted on a black disc having a clear space all round it. As the province of the Lieberkuhn is to reflect and focus the rays upon the object, it will be easily understood that the Lieberkuhn has to be made to correspond with a lens of a par- 176 PRACTICAL PHOTO-MICROGRAPHY. ticular focus, so that in practice a Lieberkuhn is required for each objective used with it. Fig. 41. —Mount for Lieberkuhn. Failing a Lieberkuhn for opaque-mounted objects, we may have recourse to any other system of condensing rays of light upon our object. Thus the light may be thrown upon the Fig. 42. —Side Reflector. (Swift.) object through a bull’s eye, which should be placed at an angle as narrow as possible to the plane of the object, i. e., as near as possible to the microscope tube. PRACTICAL PHOTO-MICROGRAPHY. 177 Or an article variously known as a “ parabolic reflector,” a “side reflector,” or a “cup,” may be used in conjunction with a bull’s eye. The action of the cup may easily be gathered from our cut, Fig. 42. A parallelizing glass will be useful here also, and the radiant is to be placed on a line perpendicu¬ lar to the optic axis of the microscope. Polarising apparatus in connection with photo-micrography is often of great value, in the investigation of crystals and crystalline matters. The worker in this line must be prepared for considerable difficulties in color-rendering, but beautiful and useful results have been achieved by some. It is important to use prisms as large as possible, to consider carefully the selenite to be used, and to study the best effects in color-rendering that can be produced by modern photography. Books, etc. Referred to or Consulted Beale, Dr. Lionel S., F.R.S. “ How to Work with the Microscope.” Harrison : London. Lindsay & Blakiston : Philadelphia. Bousfield, E. C., L.R.C.P. “Guide to Photo-Micrography.” London: Kent & Co. Carpenter, Dr. W. B., F.R.S. “The Microscope and its Revelations.” London : Churchill. Crookshank, Edgar M. M.B. “Photography of Bacteria.” London: H. K. Lewis. Jennings, I. H. “Photo-Micrography.” London: Piper & Carter. Koch, Dr. R. “ On the Investigation of Pathogenic Organisms.” Translation in “ Microparasites in Disease,” by Victor Horsley, B.Sc., from “ Mittheilungen aus dem Kaiserlichen Gesundheitsamte,” Vol. 1. Berlin. London : New Sydenham Society. Maddox, R. L., M.D.; Hon. F.R.M.S. “Papers in British Journal of Photography ,” passim, and elsewhere. Naegeli, Prof. Carl, and Schwendener, Prof. S. “ The Microscope in Theory and Practice.” London : Swan, Sonnenschein & Co. Nelson, Edward M. “ Papers in English Mechanic,” passim. Sternberg, Miquel, Truan, De Witt —and others. Journal of Royal Microscopical Society, passim, London. Fraenkel and Pfeiffer. “ Atlas der Bakterienkunde," Berlin, 1889. Abney, Capt. W. de W., R.E., F.R.S. “Treatise on Photography.” London : Longmans, Green & Co. Bothamley, C. H. Papers before Photo Convention, 1887, 1888, 1889, “ Journal of Chemical Industry" ; also “ Photo News," passim. Burton, Prof. W. K. “ Practical Guide to Photographic Printing,” etc. London : Marion & Co. Bur i on, W. K., and Pringle, Andrew. “ Processes of Pure Photog¬ raphy.” New York : The Scovill & Adams Co. Hardwich, T. F., and Taylor, J. T. “Photographic Chemistry.” New York : The Scovill & Adams Co. 180 PRACTICAL PHOTO-MICROGRAPHY. 25 A o <** COo zo ©* iO ^ CO » 3-' 208 8.3 234 9.4 260 10.4 286 11.4 312 12.5 338 13.5 364 14.6 390 15.6 CQ IO <=* 2*o' 150 6.3 175 7.3 200 8.3 225 9.4 250 10.4 275 11.5 300 12.5 325 13.5 350 14.6 375 15.6 • W > •Kk CO CQ 120 5.2 144 6.3 00 m 192 8.3 216 9.4 240 10.4 264 11.5 288 12.5 312 13.6 336 14.6 360 15.7 u w CQ >sk CQ CQ IO w TH 4Q* 138 6.3 161 7.3 184 8.4 207 9.4 230 10.5 253 11.5 276 12.5 299 13.6 w «= CO T-H 345 15.7 o m B 21 A 110 5.2 132 6.3 CO iO ^ 176 8.4 oo 220 10.5 242 11.5 264 12.6 286 13.6 308 14.7 330 15.7 J H fc O 20 A 105 5.3 126 6.3 2- 168 8.4 O 55 « 210 10.5 231 11.6 252 12.6 273 13.7 294 14.7 315 15,8 P* w H 19 A o CO s- o CO 3* 140 7.4 160 8.4 180 9.5 200 10.5 220 11.6 240 12.6 260 13.7 280 14.7 300 15.8 a w O vT 00 y—i 95 5.3 CO yH CD T-i 133 7.4 152 8.4 171 9.5 190 10.6 209 11.6 228 12.7 247 13.7 266 14.8 285 15.8 w « H •** T— H 90 5.8 GO ^ 2- co ^ CQ ^ 144 8.5 162 9.5 180 10.6 198 11.6 216 12.7 234 13.8 252 14.8 270 15.9 s o p* 16 A IO co 00 iO* 102 6.4 o ^ S-’ 136 8.5 153 9.6 170 10.6 187 11.7 204 12.7 221 13.8 238 14.9 255 15.9 Z w w 1C tH 80 5.3 92 6.4 CO in 128 8.5 144 9.6 160 10.7 176 11.7 192 12.8 208 13.9 224 14.9 240 16 u (f) •V* tH 75 5.4 F9 06 105 7.5 120 8.6 135 9.6 150 10.7 165 11.8 180 12.9 195 13.9 210 15 225 16.1 c« C/3 < iJ o Q Z tsk* CO tH 70 5.4 84 6.4 98 7.5 112 8.6 126 9.7 140 10.8 154 11.8 168 12.9 182 14 196 15.1 210 16.2 •V** CQ tH 65 5.4 78 6.5 91 7.6 s» CO S °* 130 10.8 143 11.9 156 13 169 14.1 182 15.2 195 16.3 P O PS 0 H W ”kT tH tH 60 6.4 72 6.5 84 7.6 96 8.7 108 9.8 120 10.9 132 12 144 13.1 156 14.2 168 15.3 180 16.4 *** O tH 55 5.5 66 6.6 77 7.7 88 8.8 99 9.9 TH *"* 121 12.1 132 13.2 143 14.3 154 15.4 165 16.5 0 z 9 A 93 OS 60 6.7 70 7.8 80 8.8 90 9.9 100 11.1 110 12.2 120 13.3 o y CO -rp tH 1-1 140 15.6 150 16.7 H U w CO 45 5.6 64 6.8 63 7.9 72 9 81 10.1 90 11.3 99 12.4 108 13.5 117 14.6 126 15.8 135 16.9 •— » w o 7 A o t- '■* U3 00 <^> ^F cO 56 8 64 9.1 CQ co © T-i 80 11.4 88 12.6 96 13.7 104 14.9 C3 l-l M 120 17.1 w a H 6 A 35 5.8 42 7 49 8.2 56 9.3 S'Ot 89 70 11.7 77 12.8 84 14 91 15.1 98 16.3 105 17.5 h O CO iO 30 6 36 7.2 42 8.4 GO 54 10.8 60 12 66 13.2 72 14.4 78 15.6 84 16.8 90 18 w u z c \ 4 A 25 6.2 30 7.5 35 8.8 40 10 45 11.3 50 12.5 55 13.8 60 15 65 16.3 70 17.5 75 18.8 H CO t-H Q CO 20 6.7 24 8 28 9.3 32 10.7 36 12 40 13.3 44 14.7 48 16 52 17.3 56 18.7 §8 % CQ iO O 21 10.5 24 12 27 13.5 30 15 33 16.5 36 18 39 19.5 42 21 ia tH 10 10 ^ v-i 16 16 18 18 20 20 «g •suaq aqijosno -oj ;u3lBA{nba 10 to 00 C5 d TH tH tH CQ* TH CO TH IO rH PRACTICAL PHOTO-MICROGRAPHY. 181 416 16.6 442 17.7 468 18.7 494 19.8 520 20.8 546 21.8 572 22.1 598 23.9 624 25 650 26 03 4-» *C/3 1 CD £ -o c (4 400 16.7 425 17.7 450 18.8 475 19.8 500 20.8 525 21.9 550 22.9 575 24 600 25 625 26 Ph &, o • 4X G CD u W G CD 384 16.7 408 17.7 432 18.8 456 19.8 480 20.9 504 21.9 528 23 552 24 576 25 600 26.1 c d G T— C 03 rG 4-* C/3 G G O lx 304 16.9 323 17.9 342 19 361 20.1 380 21.2 399 22.2 00C* £>• CO CO CO lO ^ •4 §5 S ^ cj .2 VH bo £ o 288 16.9 306 18 324 19.1 342 20.1 360 21.2 378 22.2 396 23.2 414 24 3 432 25.4 450 26.5 G X 03 £ rC u c £ •*—< o Sx 03 0) 272 16.9 289 18.1 1306 19.1 323 20.2 340 21.3 357 22.3 374 23.4 391 24.4 408 25.5 425 26.6 1-4 C/3 o G 4-* in •H 256 17.1 272 18.1 288! 19.2 304 20.3 320 21.3 336 22.4 352 23.5 368 24.5 384 25.6 400 26.7 C C/3 P X u C G > rO "O lx o C/3 U C/3 CD rG c/3 1-4 176 17.6 187 18.7 198 19.8 209 20.9 220 22 231 23.1 242 24.2 253 25.3 264 26.4 275 27.5 03 G » pH d CD CD £ *-> G £ o Mx ll t-H 4" l Q 160 17.8 170 18.9 190 21.1 200 22.2 210 23.3 220 24.4 230 25.6 240 26.7 250 27.8 expl G (D IO i> G CD CJ Where t c 03 rG ■*-• 144 18 153 19.1 162 20.3 171 21.4 180 22.5 189 23.7 198 24.8 207 25.9 216 27 225 28.1 03 X +-> in io rH T-< (D X o t-H Mx o in G 03 128 18.8 136 19.4 144 20.6 152 21.7 160 22.9 168 24 176 25.1 184 26.3 192 27.4 200 28.6 03 XI T—* o VI C to p £ C/3 G 7 Cylinders For Gases..... 161 Dallinger & Drysdale, Drs.... 30 Damp in Platinotype, Precau¬ tions Against. 131 Dancer. 12 Dark Room Window. 55 Dark Slide, Mode of Fitting to Camera. 52 Darkening Lantern Lecture Room. 159 Daylight Enlarging. 135 “ Deep” Objects, Photography of. 86 Defects in Plates.104 ‘ Definition ” Defined . 164 “ Depth of Focus” . 28 Details, First Appearance of, in Development. 94 Developer, Ferrous Oxalate.. 63 Developer, Ferrous Oxalate Normal. 92 Developer, Pyro-ammonia.... 91 Developer, Pyro-carbonate.... 91 Developer, Quantity Required of. 98 Developer, Quinol or Hydro- quinone... 99 Developers, Normal.... 91 Developing Dishes. 57 Developing Plates, Light For.. 55 Development, Abnormal....:. 97 Development, Manipulations of 92 Development, When to Stop.. 95 Development of Details, Pro¬ gress of. 93 Development of Platinum Prints 131 Diaphragm, Iris, for Condenser 34 Diaphragms, In Ocular. 32 Diaphragms for Condensers . 34 Diatoms, Photos of. 13 Diatoms, Test, How to Treat.. 171 Differential Screw Adjustment 24 Diffraction-grating. 42 Diffused Daylight as Radiant.. 42 Disc Size For Lantern. 158 Dishes For Developing, etc... 57 Donne. 12 Draper, Dr., his Work. 14 Draw Tubes, Rack Work for... 24 PAGE Drying Box. 107 Drying Rack. 58 Dusting Plates. 104 Easel for Enlargements.140 Eastman Co. Bromide Paper... 127 Eastman Co. Transferotype Paper. 127 Eastman Company’s Easel for Enlargement. 140 “ Easy ” and “ Difficult ” Work 20 Easy Objects to Photograph. .. 79 Echinus Spine as a Test. 32 Edwards, Mr. B. J., Isochro- matic Plates, Commercial... 106 Effect of Condenser On Illumination. 89 Electric Arc Lamp. 43 Electric Incandescent Lamp. .. 43 Electric Light as Radiant. 39 Electric Light as Radiant. 43 Enamelling Albumen Prints... 119 Enlarged Negative, To Pro¬ duce . 141 Enlarged Versus Direct Nega¬ tives. 66 Enlarging, Condenser for.. .. 139 Enlarging, Discussion on. 134 Enlarging, Double Camera for. 136 Enlarging, Exposure Required. 138 Enlarging, Lens for.136 Enlarging, Necessaries for... 134 Enlarging by Daylight. 135 Enlarging by Optical Lantern.. 138 Enlarging by Optical Lantern, the Radiant. 139 Enlarging From Gelatine Nega¬ tives. 65 Eosin, For Sensitizing Plates. . 106 Erythrosin, to Use For Plates. 107 Erythrosin Bath, Bothamley’s. 113 Evans, F. H. 15 Examples of Operations. 79 Exposing Shutter. 53 Exposure. 88 Exposure, Duration of. 39 ' Exposure, Greatly Affected by Color. 90 Exposure, Factors Regulating 89 Exposure, Rapid, How to Make 54 Exposure, Tables Deprecated 88 Exposure, to Judge on Devel¬ opment . 98 Exposure Table, Bousfield’s.. 89 Eye-Piece or Ocular, Use of. . 74 Eye-Piecing, Toleration of.... 31 Factors Regulating Exposure 89 Farmer, Howard, his Reducer 63 IV INDEX. Ferrotype Plates for Glazing Prints. Ferrous Oxalate, Normal Devel¬ oper. Ferrous Oxalate Developer.. Ferrous Sulphate Solution.... Field, Flatness of. Field Glass of Ocular Used as Condenser. Fine Adjustment, Differential Screw. Fine Adjustment, Long Lever.. Fixing Albumen Prints. Fixing the Plates . Fixing Solution for Prints.... Flagella of Microbes, To Treat “Flares”. Flatness of Field. Flea, Peculiar Preparation of.. Focal Length Regulates “ Power”. Foci, Visual and Actinic not Co¬ incident . Focusing Bull’s-eye. Focusing Condenser. Focusing Eye-Piece. Focusing Eye-Piece, To Set Focus of. Focusing Surfaces.. Focusing the Projection Ocular Focusing Various Images. Fog, To Discover Cause of.... Fog on Negatives. Frsenkel and Pfeiffer, Drs., their Micrographs of Bacteria Frilling of Gelatine Plates. Gas Cylinders, Safety Of. Gas Tanks... Gelatine Bromide For Lantern Slides.. Gelatine Bromide Plates,Clear¬ ing. Gelatine Bromide Plates, Fix¬ ing. Gelatine Bromide Slides, De¬ velopers. Gelatine Bromide Slides, Ex¬ posed. Gelatine Chloride, Characters of. Gelatine Chloride, Chemicals for Printing on. Gelatine Chloride Lantern Slides. Gelatine Chloride Printing.... Gelatine Chloride Slides, Ex¬ posure. PAGE Gelatine Chloride Slides, Formulae For Developers... 150 Gelatine Chloride Slides, To Reduce . 150 Gentian Violet, To Photograph 110 Gibbes, Heneage. 15 Glass, Cobalt Blue (foot note). 42 Glass for Collodion, To Clean. 147 Glazing Albumen Prints.119 Gold Chloride, in Tubes.119 Gold Toning, Rationale and Use of. 115 Gold Toning Solution. 64 Gotz, Mr. J. R., Color-Sensitive Plates. 106 Ground Glass of Camera. 52 Hartnack, Student Stands.... 26 Heliostat, Generally Required. 39 High and Low Angles. 165 Histological Subjects.170 History.11-12 Homogeneous Immersion.... 162 “ Hook’s Joint” for Focusing. 50 Horizontal Position for Stand.. 22 Hot Bath Process, Platinotype. 131 Hydrochinon For Slides.152 Hydroquinone Developer. 99 “ Hypo,” Solution. 63 Hypo, To Remove from Prints 119 Illumination...,. 39 Illumination as Affecting Ex¬ posure. 89 Immersion, Advantages of.... 162 Immersion, Homogeneous.... 31 Immersion, System Of.162 Immersion for Condensers.. 163 Incandescent Lamp as Radiant 43 Initial Power. 75 Insect Preparations, How to Treat.167 Intensification of Gelatine Plates. 102 Intensification with Mercury. . 63 Introduction. ll Iron Developer for Wet Collo¬ dion . 148 Iron Perchloride for Reducing 63 Ives, His Orthochromatic Pro¬ cess . 107 Jennings, late Isaac H. 14 jets for Lime Light. 42 Judgment of Exposure in De¬ velopment. 94 Koch, Dr. R., his Work. 13 Lamp, Microscopical. 22 Lamps, “ Non-Actinic ”. 56 Lantern, Size of Disc. 158 PAGE 120 92 63 63 32 167 24 26 119 101 64 171 77 32 109 31 • 27 83 81 53 53 52 85 85 104 103 16 103 161 161 151 101 101 151 151 121 60 149 121 149 INDEX. y Lantern Microscopes, Wright and Newton’s. Lantern Slides. Lantern Slides, Dry Collodion. Lantern Slides, Gelatine Bro¬ mide . Lantern Slides, Gelatine Chlo¬ ride . Lantern Slides, Gelatine Chlo¬ ride... Lantern Slides, Qualities of.. . Lantern Slides, Size of. Lantern Slides, Various Pro¬ cesses for. Lantern Slides, Wet Process .. Lantern Slides by Transfer. Lantern Slides on Gelatine Bromide. Lantern Slides to be Varnished Lens, Supplementary for “ Cor¬ rection ”. . Lens for Reduced Slides. Lieberkuhn. Lieberkuhn, Mount for. Liesegang’s Chloride Paper... Light, Lime. Light, Monochromatic. Light, To Centre the. Light for Operating Room, Rationale of. “ Light Modifiers”. Lighting, Even. Lighting Uneven on Negative. Lime Light. Lime Light, Author’s Jet. Lime Light, Jets Described... Lime-Light as Radiant. Limes For Lantern, To Pre¬ serve. Liquor Ammonia, To Use.. Long Lever A djustment . Low Power Work, “Portrait” Plates for. Machines for Washing Prints.. Maddox, Dr. R. L. Maddox’s Camera Described.. Magnesia, To Replace Limes. Magnesium Ribbon Light. Magnification, Amount of. Magnification, as Touching Exposure. Magnification, To Calculate.. Manipulation of Bromide Paper Manipulations of Development Marking Oculars. Zeiss’ System Mercer, Dr., his Work. Mercury, Intensifier. PAGE Methyl Blue, To Photograph.. 110 Microscope, Baker’s “ Nelson ” 24 Microscope, Powell and Le- land’s. 24 Microscope, Swift’s Wales pat¬ tern . 24 Microscope, The. 23 “Microscope and its Revela¬ tions,” The, by Dr. Carpenter 30 Microscope Attachment to Lan¬ tern. 156 Microscopes, Beck’s. 24 Microscopes, “Student’s”.... 26 Miguel, Dr. 15 Mis-focusing Condenser, Dan¬ ger of. 72 Mixing Jet For Lantern. 159 Mixing Lime-jet. 43 Monochromatic Light. 42 Mount for Lieberkuhn. 176 Mounting Glazed Prints...... . 120 Mounting Lantern Slides. 153 N. Rhomboides. 173 Negative, A Good One De¬ scribed.103 Negative Varnish. 103 Negatives, Qualities of. 103 Negatives, To Print Hard and Soft. 116 Negatives, To Reduce Density Of. 102 Nelson, E. M., his Arrange¬ ments. 17 Nelson, E. M., his Work. 14 Nelson, Pattern of Micro Stand 24 Newton & Co., Jet for Lime Light. 44 Neyt. 15) Non-A chromatic Condenser... 33! Normal Developers. 6L Normal Developers. 91' Nose-Pieces. 36 Numerical Aperture. 163 Obernetter, Color - Sensitive Plates. 107 Objects, Deep ”. 86 Objects, Divided Into Classes 66 Objective, or Object Glass.... 27 Objective Used as Condenser. 73 Objectives, Correction of. ... 27 Objectives, Testing. 31 Ocular, Convenience of. 75 Ocular, Use of. 74 Ocular or Eye-Piece, Stops in. 32 Oculars, In General. 37 Oculars, Projection by Zeiss.. 164 PAGE 156 143 145 145 145 149 143 145 144 144 128 151 153 27 146 175 176 121 43 40 81 56 35 49 104 43 43 43 39 161 62 26 67 119 12 40 44 124 75 89 164 126 92 164 13 63 VI INDEX. PAGE Oculars, Prbjection Recom¬ mended. 37 Oculars, Zeiss Compensating. 164 Oculars, Zeiss’ System of Mark¬ ing.164 Oil Lamp as Radiant. 39 Oil Lamps for Microscope ... 45 Opal Prints by Transfer. 128 Opaque Mounts. 174 Operating Room. 55 Operating Room, Furniture For. 57 Operating Room, Window or Lamp for. 55 Operating Room Lamps. 56 Operations Following Devel¬ opment.. .... 101 Optical Lantern, An Objection To. 161 Optical Lantern, for Enlarging. 138 Optical Lantern, Gas For.159 Optical Lantern, Jets For.159 Optical Lantern, Oil Lamp.... 158 Optical Lantern, Plea For Use Of. 155 Optical Lantern, Table of Dis¬ tances, Foci, Disc,Sizes, etc. 160 Optical Lantern, To Get Good Light. 160 Optical Lantern *• Carriers”... 160 Optical Lantern to Darken Room. 159 Orthochromatic or Color-Cor¬ rect plates, Light For Devel¬ oping. 55 Orthochromatic Photography.. 105 Orthochromatic Photography, Late Advances in.. 67 Orthochromatic Work, Samples of.:. 109 Orthochromatics. 21 Orthochromatics,LateAdvances in. 106 Orthochromatics, Rationale of. 105 Orthochromatics, Value of.... 106 Orthochromatising Plates. 107 Over-exposure, to Remedy_ 97 Oxygen Gas For Lantern.158 Oxy-hydrogen Lime Light. 43 P. Angulatum. 173 P. Angulatum , Photo’d by Maddox.:. 13 Paper Albumenized, Printing Parabolic Illuminator.174 “Paraboloid”. 174 Pellucid Objects. How to Treat 168 FAGS Pellucida, A, “ in lines”.171 Penetration, Aperture, Magnifi¬ cation. 30 Penetration. Defined. 28 Penetration and Aperture. 29 Penetration in Relation to Aperture. 163 Permanence of Bromide Prints 129 Perspective, in a Diffraction Image. 30 Photographic Requisites. 55 Photography, Color-Correct.. 105 Photography, Orthochromatic.. 105 Photography, Value of. 11 Photography in General, to be Studied. 21 Photo-micrographers, Classes of. 12 Photo-micrography, Claims for 15 Pigott, Dr. Royston. 30 “Pinholes” in Plates. 104 Pizzighelli’s Platinum Printing Process. 132 Physio and Patho. Prepara¬ tions, How to Treat. 169 Plane Mirror, Use of, With Daylight. 42 Plate Glass to Focus on. 53 Plates, Common Sizes of. 52 Plates, List of for Stock. 68 Plates, Selection of. 65 Plates, to Orthochromatize.... 107 Platinotype, Cold Bath Process 133 Platinotype, Hot Bath Process. 131 Platinotype, Negatives for.180 Platinotype, Solutions for.... 64 Platinotype, To Print on the Paper.131 Platinotype for Permanence... 130 Platinotype Printing. 130 Platinotype Prints, Clearing .. 132 Platinotype Prints, To Develop 131 Platinotype Process of Printing Described. 131 Pneumatic Holders. 58 Polariser. 35 Polarizing Apparatus . 177 Potassic Carbonate, Solution.. 62 Potassic Ferricyanide, Reducer 63 Potassic Meta-bisulphite. 61 Potassic Oxalate, Sat. Solution 63 Powell and Leland,Their Micro¬ scopes. 24 Powell and Leland’s Apochro- matic Condenser. 34 “Power”.. . 31 INDEX. PAGE Power Depends on Focal Length. 31 Preparations, Bacteriological.. 109 Preparations, Faded. 109 Preparations, Family Red.... 109 Preparations, Insect. 109 Preparations, Opaque Yellow. 109 Preparations, Wanting Con¬ trast . 109 Preparations for Work. 17 Preserving Sensitized Albumen Paper. 117 Presses and Shelves. 20 Principles For Color-Correct Work . 108 Printing, Principles of. 115 Printing Frame, To Use.117 Printing in Platinum. 130 Printing on Albumen Paper... 115 Printing on Albumen Paper.. 117 Printing on Bromide Paper.... 123 Printing on Gelatine Chloride. 121 Printing Processes, Solutions for. 64 Printing Processes in General 111 Prints, To Mount. 120 Prisms for Monochromatic Light. 42 Procedure With Easy Objects.. 79 Progressive Examples. 79 Projection, Ocular, To Focus.. 85 Projection Oculars. 37 Projection Oculars.164 Projection Oculars by Abbe and Zeiss. 75 Pyro, for Bromide Paper.128 Pyro, Solutions. 61 Pyro-ammonia Developer. 91 Pyro-carbonate Developer- 91 Pyro Developer for Wet Collo¬ dion . 147 Quality of Gelatine Plates.... 65 Quinol Developer. 99 Quinol Developer for Bromide Paper. 128 Quinol Developer For Slides.. 152 Ramsden Eye-Piece. 53 Rapid Emulsion, Late Advances in. 67 Rapid Exposure, How to Make 54 Rays, Visual and Actinic. 27 Reade, Rev. J. B. 12 Reds, Plates Sensitive To. 107 Reducing Solution, Farmer’s.. 63 Reduction of Density.. 102 Reduction with Iron Chloride 63 Reflections in Apparatus. 77 • • Yll PAGE “Register”. 53 Removing Hypo from Prints... 119 “ Resolution ” Defined. 164 Resolution Increases with Ap¬ erture. 28 Resolving Power in Relation Aperture. 163 Rigidity, Essential to Micro¬ scope . 22 Ruby Fabric. 55 Ruby Glass for Operating Room. 55 Rose Tap. 57 Roux, Dr., His Magnesia S. Gemma. 173 “Safe” Light For Operating Room... 56 Safety of Light, to Test. 56 Safety Spring for Fine Adjust¬ ment. 26 Sciopticon Lantern, Scovill’s .. 157 Scovill’s Optical Lantern. 157 Screen, Aurantia, To Make.. 108 Screen, Signal-Green.108 Screen For Optical Lantern... 158 Screens, Use of With Colors.. 106 Screens, Worked Glass. 108 Screens for Orthochromatic, Must Cut Off Violet and Blue. 109 Screens for Orthochromatic “Screens” in Color Photog¬ raphy. 106 Sections, Thinness of. 169 Selection of Plates. 65 Selenites for Polariser. 35 Sensitizing Albumen Paper... 116 Sensitising Albumen Paper, Bath for. 64 Sensitometer. 59 Shadbolt, his Work. 15 Shaw Bull’s-Eye. 38 Shutter For Exposing. 53 Shutters For Lantern Room... 159 Side Reflector . 176 Signal Green Screen . 108 Simple Photo-Micro Apparatus 47 Sink For Operating Room. 56 Sinks, India Rubber. 57 Sizes of plates. 52 Slides By Contact, To Print... 145 Slides By Reduction. 146 Slides By Reduction, Apparatus for...!. 146 Slides for Optical Lantern.143 vm INDEX. PAGE Slow Development. 96 Sodic Carbonate, Solution.... 62 Sodic Hyposulphite, Solution for Fixing. 63 Solution for Clearing Negatives 63 Solution for Reducing Nega¬ tives. 63 Solutions, Carbonates for De¬ velopment. .62 Solutions for Photographic Op¬ erations. 61 Solutions for Platinotype. 64 “Spot Lens”..174 Squeegee. 58 Squeegee for Removing Hypo from Prints. 119 Standard Light for Bromide Printing.... 123 Sternberg, his Work. 15 Stock of Dry-Plates, List of... 68 Stop for A. Pellucida. 172 Stopping Down Objective. 76 Stops for Black Grounds. 174 Stops in Objectives. 76 Strain of Objective. 75 Stretch Required for Camera.. 52 Student’s Stands. 26 Subjects, Difficult, Treated by Orthochromatics... 109 Substage, Importance of. 22 Substage Condenser, “ fitted ”. 36 Substage Condenser, Use of... 69 Substage Condensers. 33 Sulpho-pyrogallol, Berkeley.. 61 Sunlight, Advantages and Dis¬ advantages of. 18 Sunlight, Uncertainty of. 39 Sunlight as Radiant. 39 Superiority of Apochromatics 85 Surface to Focus Upon. 52 Swift & Son’s Microscope_ 24 Swift’s Multiplex Condenser... 36 Swift’s Oil Lamp. 45 Swift’s Photo-Micro Apparatus 48 Swift’s Safety Spring. 26 Tables of Enlargement and Reduction.180, 181 Tailfer Patent, The. 106 Tanks for gases. 44 Tanks For Gases. 161 Temperature, Equability of in Work-room. 19 Test Diatoms, How to Treat.. 171 Test Diatoms, Yellow Mount¬ ing. 110 Test For “ Safe” Light. 56 Test Hairs, To Photo. 84 tage Test Objects, Opticians’. 31 Testing Lenses, Advice to Beginner in. 32 Tests for Objectives. 31 Thickly-coated Plates, Use of. 67 To Treat Very Opaque, or Non- Actinic Subjects. 94 Toning, Rationale of. 115 Toning Albumen Paper. 118 Toning Bath for Albumen Paper. 118 Toning Bath for Aristotype Paper. 122 Toning Bath for Gelatine Chloride Paper. 121 Toning Solutions for Albumen Paper. 64 Toning Wet Collodion Slides.. 149 Transferotype Paper. 127 Truan Y. Luard, Diatom Pho¬ tos. 13 Truan & Witt, Their Mirror... 40 Tube Correction.... 85 Turmeric, for Screens. 109 Under-exposure, to Rectify.... 97 Uneven Lighting.*.104 Varnishing Negatives.102 Vertical Illuminator, Beck’s.. 171 Vibration, Fatal to Success_ 18 Vibration, to Obviate . 18 Vignetting Enlargements. 142 Violent Contrasts, How to Treat. 95 Visual and Actinic Rays. 40 Vogel, Color-Sen*jtive Plates.. 106 Walmsley, Focusing Surface... 53 Washing Albumen Prints. 119 Washing Machines for Prints. . 119 Washing Paper Before Toning 118 Washing Plates. 101 Washing Trough. 58 Water Supply..... 20 Water Tap, Adjuncts to. 57 Wave-lengths. 42 Webster, G. W., Citrates. 98 Wedgewood. 12 Wellington,}. B., His Screen. 109 Welsbach Burner as Radiant... 39 Wenham. 15 Wenham’s Parabolic Illumin¬ ator. 175 Wet Collodion, Chemicals Re¬ quired for. 60 Wet Collodion, Fixing. 148 Wet Collodion, Intensification 148 Wet Collodion, Iron Ammo- nio-sulphate Developer. 148 INDEX. IX PAGE Wet Collodion, Iron Developer 148 Wet Collodion, Pyro-developer 147 Wet Collodion for Slides. 144 Wet Collodion Process. 146 Wet Collodion Slides, To Tone 149 White, T. Charters, his Work.. 15 Window for Dark-Room. 55 Witt, Otto N., Diatom Photos. 14 Wood-Section, Easy Object... 79 Woodward, Dr., his Work.... 13 PAGE Woodward’s Camera Described 40 *’ Working Distance ”. ..+... 30 Yellow Ground on Daitom Mounts. 110 Zeiss, Apalantic Magnifier..... 53 Zeiss, Dr. R., Photos of Dia¬ toms. 14 Ziisonium Oxide to Replace Lime. 45 Description of Plates after Contents. F rontispiece— Plate I. Plate II . Plate III. Plate IV. Plate V. Plate VI. PAGE .Facing 46 .Facing 79 .Facing 104 . Facing 118 . Facing 136 .Facing 152 TMSGOVILL & ADAMS COMPANY, Suoeessors to the Photo Department ot the SCOVILL MANUFACTURING COMPANY, (Established in 1802.) 4r23 Broome Street, 1STew ‘ T xTork: THE LARGEST MANUFACTURERS OF Photographic Cameras, Apparatus and Supplies IN THE WORLD, FOR AMATEURS, TOURISTS, SCIENTISTS AND PROFESSIONAL PHOTOGRAPHERS. PHOTOGRAPHIC LENSES OF ALL KINDS. Flash Light Compound for Night "Photography. jdll the Latest Photographic Novelties. PROPRIETORS OF The American Optical Co.’s Factory. PROPRIETORS OF s New Haven Camera Factory. PUBLISHERS OF Scovill’s Photographic Series. W. IRVING ADAMS, H. LITTLEJOHN, President and Treasurer. Secretary. THE No. 1 SCOVILL SCIOPTICON, Complete with Double Slide Carrier, $30. No. 2 SCOVILL SCIOPTICON, Complete with Double Slide Carrier, $50. After experimenting with most of the lanterns in the market, we have come to the conclusion that for parlor or small hall exhibitions, chemical and optical experiments, etc., the Scovill Lantern affords, at a moderate price, the greatest number of advantages, and from its simplicity and non¬ liability to get out of order, gives, even in inexperienced hands, results superior to all others. The No. 1. Scovill Sciopticon when packed for carrying, in its own Russia iron case, measures 15 x 10 x 6 inches, and weighs 12 pounds; the case serving as a convenient stand when the lantern is in use. The Case and Body of the Lantern are of Russia iron, and neat and compact in form. That part of the body which surrounds the lamp is double, the outer covering being ornamentally perforated so as to allow a constant current of air to circulate and keep down the temperature. The lamp is of the triple wick variety, and so constructed that the three flames combine, and by the draught of a ten-inch chimney give a brilliant flame. The Condenser is four inches in diameter, neatly mounted in brass, thoroughly ventilated, and arranged with screw flange so that the lenses may be separated and cleaned when required. The Cone, which carries the objective, and the mount of that lens are nickel-plated. The objective is a double achromatic lens of one and a half inch clear aperture and five-inch focus, so that at a distance of twelve feet from the screen, it gives a brilliant picture on disc six feet in diameter. The focus is roughly obtained by sliding the front, carrying both cone and lens; and fine adjustment by a rack and pinion on the objective. The No. 2 Scovill Sciopticon measures, when packed in case for car¬ rying, 18% x 12 x 8%, and weighs 19 pounds. The objective is a double achromatic lens of 1% inches clear aperture and 5% inches focus so that at a distance of about twelve feet from the screen it shows a brilliant picture on disc eight feet in diameter. The lamp has five wicks and is corres¬ pondingly more powerful than the lamp with the No. 1 Sciopticon. THE SCOVILL & ADAMS COMPANY, 423 Broome Street, New York. ii THE MERCER Bl 16 CAMERA. Size, 2fx3i. Price, $7.50. This Camera is provided with a Brass Cone and Plate- holder with Ground Glass attached, to slide back and fortn in the carriage, as desired. Mercer Camera, shown with Microscope to illustrate working. iii THE SCOVILL OUTFIT For PHOTOCRAPHINC with the MICROSCOPE Photographing with the microscope has hitherto been accomplished by the aid of elaborate and costly apparatus, and been applied chiefly to making illustrations for scientific magazines. The process used, that of wet collodion in connection with sunlight, involved the procurement of an expensive heliostat to produce a steady illumination, for with any less powerful light the exposure would necessarily be so prolonged that the coating of the plate would dry and become useless. Now all this is changed, for with the modern improvements in photography which are the result of the introduction of gelatine dry plates, the photographing of microscopic objects becomes as easy of accomplishment as the photograph¬ ing of the beautiful and visible in nature is with the popular amateur outfits. The scientist and microscopist, instead of spending hours in making imperfect drawings, aided by the camera lucida, may in a few minutes, with the assistance of photography, produce a more perfect representation of a minute object than it is possible for the hand of man to do, working con¬ jointly with the eye. Not only can an enlarged image of a microscopic object be formed for illustration, but professors in colleges will find it a ready means to produce negatives of a suitable size from which may be made transparencies or magic lantern slides for exhibition to classes or the public. If this is done in the dayt^ntj. a room from which all white light is ex¬ cluded should be selected; b keep U sed at night, as in most cases it would be, the operations may all be-ick vnned in the midst of a family group for their interest and amusemenirau^ to impart to them knowledge of the mi¬ nute life or organisms of the . .d 'which the microscope alone can reveal. The Scovill Photomicroscopic Equipment — CONSISTS OF — 1 Scovill Special Half Plate Camera. 1 Multum in Parvo Lantern, with Double Condenser. 1 dozen 4Kx5% size B Keystone Plates to make Negatives; also 1 dozen 3 % x 4M size A Plates for Transparencies. Price, Complete, $18.00. The presumption is that you are provided with a microscope. If not, we recommend the purchase of one from a regular dealer in microscopical goods. Circular containiag directions for use sent with each outfit. iv THE f ALMSLEY PHOTO-IICROGRAPHIC CAMERA. Manufactured by the American Optical Co* Now in use by many Colleges and leading Microscopists, and is an efficient, practical and cheap instrument for the purpose. It is made in two forms: the cheaper, No. 1, (selling for $18.00) is adapted only to the making of negatives on plates 334x434, or 434 x 534» as may be necessary. The complete form No. 2 (costing $30) is also a minia¬ ture enlarging, reducing, and copying camera, admirably adapted to the production of lantern transparancies from any size negative up to 434x534* The camera (of mahogany) is square, carrying a Flammang single plate holder for 434x534 plates ; usable vertically or horizontally, and with kits for 334x434 plates. The bellows are in two sections, with a central division of mahogany, which carries a removable partition, to which a suit¬ able rectilinear photographic lens can be attached, for enlarging, reducing, or copying. A light-tight door on one side of this wooden section gives ready access to the lens for inserting or removing diaphragms, or other necessary manipulations, whilst a milled head, accessible from the same opening, clamps the lens-bearing section firmly to the bed of the camera at ^ £ any desired point. The bellows have an extension of two feet in addition to the length of the box, sliding very smoothly upon V-shaped ways, which for greater con¬ venience are made in two sections, firmly attached to each other by wooden dowels, and a solid brass screw, worked by a milled head. The bellows are firmly held at any desired point of extension by a cam, operated by a lever conveniently placed at the rear of the focusing screen which latter is hinged at the bottom, and when not in use, lies out of the way upon the extension bed. v THE SCOV1LL CAMERAS. When ordering, please specify number and sizes of kits, also style of Holder wanted. No. 61. Size, (>y 2 X % l /2 Price, $30.00 No. 62. 8 X 10 a • 35 -oo No. 63. IO X 12 44 • 48.00 N 0. 64. I I X 14 i 4 • 60.00 No. 65. 14 X 17 a • 72.00 No. 66. 17 X 20 a • 90.00 No. 66*^. “ 18 X 22 44 • 100.00 No. 67. “ 20 X 24 44 9 110.00 Special sizes and styles made to order. VI FEW SETS LEFT The American Annual of Photography 1 Photographic Times Almanac for 1887 (SECOND EDITION.) Contains five full-page illustrations— Am Exquisite Photo-Gravure, by Ernest Edwards. A Bromide r*riiit, by the Eastman Company. A Silver Print, by Gustav Cramer, of St. Louis. Two Mosstypes, by the Moss Engraving Company. 197 pages of Contributed Matter consisting of articles on various subjects, by 80 repre¬ sentative photographic writers of this country and Europe. The American Annual of Photography 1 Photographic Times Almanac for 1888 Contains eight (8) full-page high-grade illustrations ; and over ninety (90) original con¬ tributions, written expressly for its pages, by the most eminent photographic writers of Europe and America. THE ILLUSTRATIONS COMPRISE: A Photo-Eithog:r»ph. showing an improved new process, by the Photo Gravure Company of New York. A Photo-Copper-Plate Engraving of a Pictorial Landscape Subject, by E. Obemetter, of Munich. A Melsenbach of “The Old Stone Bridge,” by Kurtz. A 25inc Etching, from the Engraving, which is itself as fine as an Engraving, by Stevens & Morris. A Charming Child Portrait, by Crosscup & West’s improved process. -< Three Mosstypes of popular subjects. And 330 PAGES OF VALUABLE INFORMATION. The American Annual of Photography 1 Photographic Times Almanac for 1889 Surpasses Even Its Two Predecessors. The full-page pictorial illustrations are more numerous and superior in quality than have ever before been presented in any Photographic Annual, and the reading-matter is in accord with the high standard of the illustrations. The full-page pictures are as follows: A Charming Portrait of the Beautiful Eilllan Russell, from a negative by Falk, printed on the new American Aristotype Paper. 'Watering: His Horse. A Characteristic Group, by Geo. B. Wood, of Phila¬ delphia, printed in Highest Grade Photo-Gravure, by the Photo-Gravure Company New York. Yes’m, I’m Coming:. A Picture of the Small Boy. By Mr. F. Gutekunst, of Philadelphia, in his exquisite manner of Mechanical Printing. The Arab Sheik. A Study of Orthochromatic Photography, by William Kurtz, printed by the Meisenbach Process. A Scene in the Tyrolese Alps. From an Original Photograph by Charles Scolik. By the Crosscup & West Engraving Company. The Divers. An Instantaneous Study on a Carbutt Plate, reproduced in Meisen¬ bach. After the Theatre. A Magnesium-light Photograph by Mons. Flammang. By Photo-Electro Engraving Company. By the Sea. An Eastman Bromide Picture. By E. W. Newcomb. Three Mosstypes. Paper Coverg, (By mail, 12 cents extra.) $0 SO Library Edition, “ ■ ■ • J Edition de Luxe, (postpaid) - 2 50 THE SCOVILL & ADAMS COMPANY, Publishers. vii -► TIITC ♦—- ^merieap pijpual of P^oto^rapfyy Hi P^oto^raptyis Ji/T\es ^Imapao —-*-For 1890.—*♦•—^ (FIFTEENTH THOUSAND.) LIST OF FULL-PAGE PICTORIAL PLATES. “ Thomas Edison;** A Portrait of the Eminent Electrician. George M- Allen & Co., New York. “ Babyhood ;** A Tinted Photo-Gravure. The Photo-Gravure Company of New York. “ Putnam’s Escape ;** A Collection of Historic Views. The Crosscup & West Engraving Company, Philadelphia. “ Southern Fruit ;*’ An Orthochromatic Study. The Electro-Light Engraving Company, New York. “At the Barracks ;’* A copy of the great Meissonier picture. William Kurtz, New York. “ Minstrel Party at * John Brown’s Fort.”* Photo-Engraving Co., New York. “John Brown’s Home and Grave.” Lewis Engraving Company, Boston. “Off Duty;” An Instantaneous Study. William Kurtz, New York. “ Minnehaha Falls in Winter.” Levytype Company, Philadelphia. “ Central Park;” In the Menagerie. I. M. Van Ness, New York. “A Merry Tale;” A Child Group. F. Gutekunst, Philadelphia. u The Van Rensselaer Manor House.” Photo-Electro Engraving Co., New York. “ An Improvised Studio.” Electro-Tint Engraving Company, Phila¬ delphia. “The Bats;” A “Flash” Light Photograph in Howe’s Cave. William Kurtz, N. Y. “A Raider’s Resort;” Morgan’s Favorite Rendezvous. M. Wolfe, Dayton, Ohio. “Group of Esquimaux.” William Kurtz, New York. “Diatoms;” Photo-Micrographs. William Kurtz, New York. “ Tropical Luxuriance ;” A Scene in Florida. Moss Engraving Co., New York. “An Arctic Camp.” Moss Engraving Company, New York. “Home of Edgar Allan Poe.” Moss Engraving Company, New York. From the above list ot full-page pictorial plates, it will be seen that The American Annual of Photography and Photographic Times Almanac for 1890, in point of pictures, will command widespread popularity ! Twenty full-page pictorial plates is an unprecedented number of illustrations for such a publication. They show the progress which has been made in photographic illustration, being excellent examples of the latest improvements in photographic printing, and represent more than twelve distinct processes of photo-mechanical reproduction. Price, fifty cent* in paper (by mail, fourteen cents); library edition, one dollar. With the PHOTOGRAPHIC TIMES (weekly, illustruted edition), $ 5 . 50 , post paid. Edition de Luxe, per copy, $ 5 . 00 . THE SCOVILL & ADAMS COMPANY, Publishers. Vlll “The Processes of Pure Photography.” (Number Twenty-nine of The Scovill Photographic Series.) +-by-+ W. K. BURTON, C.E., and ANDREW PRINGLE, Professor of Sanitary Engineering Imper- President of the Photographic Convention ial University of Japan, Author of of the United Kingdom, 1889 , Fellow “ Modern Photography,” “Photo- of the Royal Microscopical graphic Printing, Etc. Society, Etc. Of the two writers, both have zealously followed photography as something more than a mere amusement, for a considerable number of years. One of the writers has studied the science from a theoretical and experimental point, while the other writer’s attention has been almost entirely directed to the production of practical results by the processes known, and by each process as it has been given to the world. As the joint work of two acknowledged authorities in photography, and as every word in it refers to subjects with which the authors are personally and intimately acquainted—with not a direction or a formula given on trust—” The Processes of Pure Photography ” possesses a practical value that justly entitles it to become at once A STANDARD WORK. It not only contains the best of all the processes and methods which have been tried by the experienced authors, but also includes much that is new and has NEVER BEFORE BEEN PUBLISHED. FOLLOWING IS THE CONTENTS OF THIS COMPLETE BOOK Chapter. I.—Introductory and Historical. II.—The Theory of Photography. III. —Apparatus. IV. —The Dark-room. V.—“Negative” and “Positive.” VI.—The Wet Collodion Process. VII.—A Dry Collodion Process. VIII.—Gelatine Emulsion Processes, Pre¬ liminary. IX.—Gelatine-bromide Emulsion. X.—Gelatine-bromide Emulsion,by the Ammonio-nitrate Process, and Precipitation by Alcohol.—Cen¬ trifugal Separation. XI.—Coating Plates with Gelatine-bro¬ mide Emulsion, Drying, Etc. XII./—The Camera in the Field. XIII. —Exposure and Development Gen¬ erally Treated. XIV. —Development of Gelatine-bromide Plates. XV.—Gelatine-bromide Plates—Fixing, Intensification, Reduction, Etc. XVI.—Defects in Gelatine-bromide Nega¬ tives. Chapter. XVII.—Paper Negatives and Stripping Films. XVIII.—“ Color Correct,” or “ Ortho- chromatic Photography. XIX.—Stereoscopic Photography. XX.—Part II. Printing Processes, Preliminary. XXI.—Printing on Albumenized Paper with Silver Chloride. XXII.—Preparation of Negatives for Printing, Combination Print¬ ing, Vignetting. XXIII.—Printing on Plain Salted Paper. XXIV.—Gelatine-chloride Paper for Printing-out. XXV.—Contact Printing on Gelatine- bromide Paper. XXVI.—Rapid Printing Paper. XXVII.—Platinotype, or Printing in Pla¬ tinum. XXVIII.—The “Carbon Process,” or “ Pigment Printing.” XXIX.—Positives and Negatives by Enlargements. XXX.—Lantern-slides. XXXI.—Residues. Price, in paper covers.$2.00 | Library Edition. $2.50 THE SCOVILL & ADAMS COMPANY, Publishers. ix THE OPTIQIfc MNTERN FOR Instruction and Amusement. BY ANDREW PRINGLE, F.R.M.S., President Photographic Convention of the United Kingdom, i88q. FULLY ILLUSTRATED. LANTERN SLIDE MAKING. By the same Author. In Preparation. THE SCOVILL & ADAMS COMPANY, Publishers. The Photographic Instructor. FOR THE PROFESSIONAL AND AMATEUR. (Number Twenty-six of The Scovill Photographic Series.) ScLited Toy "W". X. Lincoln i^.dam.8, With an Appendix by Prof. Charles Ehrmann. The most thoroughly practical instruction book yet published and the most complete, consisting, as it does, of the Comprehensive Series of Practical Lessons issyed to the students of the Chautauqua School of Photography, revised and enlarged, with an Appendix of over thirty pages, on the Nature and Use of the Various Chemicals and Substances Employed in Photographic Practice, besides valuable Tables of References, etc. The original Lessons were contributed by such competent photographic writers as Charles Wager Hull, Superintendent of the Chautauqua School of Photog¬ raphy ; Prof. Randall Spaulding, Superintendent of the Montclair Public Schools; Prof. Karl Klauser, of Farmington, Cotin. ; Dr. Maurice N. Miller, of the University of the City of New York ; John Carbutt, the well-known Dry-plate Manufac¬ turer of Philadelphia; O. G. Mason, of Bellevue Hospital, New York City; Prof. Chas. Ehrmann, Instructor of the Chautauqua School of Photog- graphy; and W. I. Lincoln Adams, Editor of the Photographic Times. Each being an authority on the subject of which he treats. The Appendix is a complete chemistry of reference in itself, and is invaluable to every photographic worker. A glance at the complete Table of Contents show the scope of the book : Lessons. Preface. Introduction. I. Apparatus. II. Management of Apparatus in the Field. III. The Dark-room. IV. Exposing. V. Developing. VI. Fixing, Washing, Varnishing, In¬ tensifying, and Reducing. VII. Printing on Albumenized Paper. VIII. Printing on Various Other Papers. IX. Printing on Permanent Bromide Paper. X. Artistic Printing. XI. Trimming and Moun ting the Prin ts. XII. Spotting and Burnishing the Prints. XIII. Portraiture. XIV. Retouching the Negative. Lessons. XV. Photographing Interiors and In¬ animate Objects. XVI. Copying, Enlarging, and Reduc¬ ing. XVII. Orthochromatic,or Color-sensitive Photography. XVIII. Transparencies, andjHow to Make Them. XIX. Landscape Photography. XX. Stereoscopic Photography. XXI. Light and Lenses. XXII. Photo-micrography. XXIII. Photographing by Artificial Light. XXIV. Emulsion Making. Appendix on the Nature and Use of the Various Chemicals and Substances Employed in Photo¬ graphic Practice. The book is embellished with Five Full-page Pictorial Illustrations , besides numer¬ ous Cuts, Diagrams, etc., illustrating the letter-press. Two hundred pages of valuable Reading Matter. Price, in illuminated paper covers, ... $0 75 Price, library edition, uniform with other numbers of the series, - - - - - 125 For sale by all dealers in photographic goods, or sent by mail, post-paid, on receipt of price, by the publishers, THE SCOVILL & ADAMS COMPANY. XI “Bi^y Plate Makir© [NO. 20 OF THE SCOVILL PHOTOGRAPHIC SERIES]. By GEO. L. SINCLAIR, M.D. Reprinted, from, “The PhotogrphlcTimeswith additions and corrections. Edited by W. I. Lincoln Adams. A SERIES OF PRACTICAL ARTICLES ON •Artjafeui* Brr)ulsi®i) Simple ^ ctppaiii®!) af ll)® Jfpeiciical Experiejgee ® f ®p. THE EMULSION, WASHING, MELTING, A RAPID EMULSION, IT TREA TS OF - GELATINE, PREPARING THE GLASS FOR COATING, COATING. Etc., Etc. Royal octavo, leatherette binding.50c. By E. J. WALL, AN AUTHORIZED AMERICAN EDITION. As a reference book and photographic cyclopedia it is invaluable to every photographer, be he professional or amateur. Concise definitions of all terms used in photography are given in such a manner that they may be easily and quickly found. The book is printed on fine supersized and calendered paper, and bound substantially and neatly in cloth, uniform with the other volumes of The Scovill Photographic Series. It con¬ sists of 240 pages, royal octavo. Price Only $1.50. Each book enclosed in a neat paper mailing case. THE SCOVILL & ADAMS COMPANY, Pablishers, xii Photographic Publications. (Selected from the Scovill Catalogue of Books.) Price, Per Copy. THE KNACK. $o 40 THE CHEMISTRY OF PHOTOGRAPHY.—By Prof. Raphael Meldola. Cloth bound. 2 00 THE FERROTYPERS’ GUIDE.—Cheap and complete. For the ferrotyper, this is the only standard work. Seventh thousand. 75 THE PHOTOGRAPHERS’ BOOK OF PRACTICAL FORMUL^E.-Compiled by Dr. W. D. Holmes, Ph.B., and E. P. Griswold. Paper covers. 75 Cloth bound. 1 50 THE PHOTOGRAPHIC STUDIOS OF EUROPE.—By ,H. Baden Pritchard, F.C.S. Paper. 50 Cloth. 1 00 ART OF MAKING PORTRAITS IN CRAYON ON SOLAR ENLARGE¬ MENTS. (Second Edition). By E. Long. Price. 50 PHOTOGRAPHY APPLIED TO SURVEYING. Illustrated. By Lieut. Henry A. Reed, U.S.A. Cloth bound. 2 50 HISTORY AND HAND-BOOK OF PHOTOGRAPHY.—Translated from the French of Gaston Tissandier, with seventy illustrations. Cloth bound, reduced to 75 A COMPLETE TREATISE ON SOLAR CRAYON PORTRAITS AND TRANSPARENT LIQUID WATER-COLORS.—By J. A. Barhydt. Practical ideas and directions given. Amateurs will learn ideas of color from this book that will be of value to them. And any one by carefully following the directions on Crayon, will be able to make a good Crayon Portrait. 50 ART RECREATIONS.—A guide to decorative art. Ladies’ popular guide in home decorative work. Edited by Marion Kemble. 1 50 PADDLE AND PORTAGE.—By Thomas Sedgwick Steele. Illustraied. 1 50 AMERICAN CARBON MANUAL.—For those who want to try the carbon print¬ ing process, this work gives the most detailed information. Cloth bound. 1 00 MANUAL DE FOTOGRAFIA.—By Augustus Le Plongeon. (Hand-Book for Spanish Photographers.). 1 00 SECRETS OF THE DARK CHAMBER.— By D. D. T. Davie. 50 AMERICAN HAND-BOOK OF THE DAGUERREOTYPE.—By S. D. Hum¬ phrey. (Fifth Edition.) This book contains the various processes employed in taking Heliographic impressions. Reduced to. 10 THE PRACTICAL PHOTOGRAPHIC ALMANAC. 25 MOSAICS FOR 1870,1871, 1872, 1873, 1875, 1882,1885,1886,1887,1888,1889,1890. 25 BRITISH JOURNAL ALMANAC FOR 1878, 1883, 1887. 25 PHOTO. NEWS YEAR-BOOK OF PHOTOGRAPHY FOR;i8 7 o, 1871, 1887,1888 25 THE PHOTOGRAPHER’S FRIEND ALMANAC. 25 AMERICAN ALMANAC OF PHOTOGRAPHY. 25 PHOTO. NEWS YEAR BOOK OF PHOTOGRAPHY for 1890. S o WALDACK’S PHOTO. ALMANAC. 25 xiii WILSON’S WILSON’S QUARTER CENTURY IN PHOTOGRAPHY. By Edward L. Wilson, Ph.D. “The best of everything boiled out from all sources.” Profusely illustrated, and with notes and elaborate ind§x. $4 00 WILSON’S PHOTOGRAPHICS.—“Chautauqua Edition,” with Appendix. By Edward L. Wilson, Ph.D. A most complete photographic lesson-book. Covers every department. 352 pages. Finely illustrated. 4 f0 BURNET’S ESSAYS ON ART. A facsimile reproduction of the costly original edition, Will help every portrait maker, every view taker, who will study them understandingly. They teach the rudiments and the rules of art entire. You cannot appre¬ ciate or understand the enjoyment there is in pictures, and in making them out or indoors, until you have read “Burnet’s Essays ” and studied the 145 etchings which illustrate them.... 4 00 PHOTO-ENGRAVING, PHOTO-ETCHING, AND PHOTO¬ LITHOGRAPHY. By W. T. Wilkinson. Revised and en¬ larged by Edward L. Wilson, Ph.D. Illustrated. 180 pages, all new. Only American edition, Cloth bound. 3 00 WILSON’S PHOTOGRAPHIC MAGAZINE. Edited by Ed¬ ward L. Wilson, Ph.D. A semi-monthly magazine, illustrated by photographs. Monthly $3.00; Semi-Monthly, per year. 5 00 VOGEL’S PROGRESS IN PHOTOGRAPHY. By Dr. H. W. Vogel. Illustrated. . 3 00 THE BOOK OF THE LANTERN. By T. C. Hepworth, Editor of the (London) Amateur Photographer. A complete manual for the Lanternist and Slide Maker. 278 pages. 2 00 THE PHOTOGRAPHIC COLORISTS’ GUIDE. By John L. Gihon. The newest and best work on painting photographs... 1 50 PHOTOGRAPHIC MOSAICS, 1890. An annual record of photo¬ graphic progress (26th year). Cloth bound, #1.00 ; paper cover 50 W' SEND FOR CATALOGUE! OF PARTICULARS. EDMHRD L-. MIL-SON, photo-Book Publisher . 853 Broadway, New York. xiv Edited by W. I. LINCOLN ADAMS. IS ILLUSTRATED EVERY WEEK WITH A FULL PACE PICTURED thus including in the year FIFTY-TWO FULL PAGE PICTURES, making it the best illustrated Photographic periodical in the world. Special numbers contain more than one high grade illustration ; and there are published, beside superb Photogravures, pictorial illustrations, by other photographic and photo¬ mechanical printing processes. The illustrations are carefully selected, and represent the best work of repre¬ sentative American artists. There are also copies of famous pictures, from time to time, to illustrate lessons in art for photographers, accompanied by instructive reading matter. The Editorials and Editorial Notes are of greatest practical value as they are the result of actual practice and experiment, by the staff. Leading Articles by such acknowledged author¬ ities as Prof. W. K. BURTON, on Scientific and Prac¬ tical Subjects. ANDREW PRINGLE, on Subjects of Greatest Value. P. C. DUCHOCHOIS, on Chemical Subjects. W. J. STILLMAN, on Art and Practical Sub¬ jects. Prof. CHARLES EHRMANN, on Dark Room and Printing Practices. And frequent contributions from G. Watmough Webster, F.C.S., of England; CarlSrna, Charles Scolik, Dr. Eder, Prof. Spitaler, Lieut.-Colonel Volkmer, of Austria; Dr. Lohse, Dr. Schnauss, KarlSchwier, Victor Schuman, and F. Mueller, of Germany. Notes from the Every-day Gallery Experiences of such well-known Practical Photographers as W. H. Sherman, H. McMichael, Prof. Karl Klauser, J. R. Swain, J. M. Mora, and John Carbutt. Various Occasional Articles of a Practical Nature, and otherwise, by our favorite contributors: Rev. C. E. Woodman, Ph.D., C. D. Cheney, D.D.S., Henry M. Parkhurst, Charles Wager Hull, C. W. Canfield, Rev. G. M. Searle, Miss Frances B. James, C. M. Brockway, Miss Adelaide Skeel, Miss Mary Scott Boyd. A highly valuable series of Papers on The Photo-Mechanical Printing Processes, by Ernest Edwards, president of the Photo- Gravure Company. “Occasional Notes,’’ by Prof. W. H. Pickering, of Harvard College Observatory. The Chemistry of Photography, by W. Jerome Harrison, F.G.S. < Correspondence—Scientific and practical discussion of important and interesting questions, by practi¬ cal photographers, and letters from all parts of the globe, by intelligent and observing correspondents. Notes and News, Photographic Societies, The Chautauqua School of Photography, 8 UERIES AND ANSWERS, ur Editorial Table, Record of Photographic Patents and Commercial Intelligence. One Year, $5 00. | Six Months, One Month’s trial, - - 50c. $2 50 THE PHOTOGRAPHIC TIMES PUBLISHING ASSOCIATION, Publishers, 423 Broome Street , New York City. XV Trade Mark. “ ECLIPSE,” Sen. 27. —Films and plates are extremely sensitive, and specially intended for quick studio exposures, concealed and detective cameras, instantaneous views, and magnesium flash-light photography. “ KEYSTONE SPECIAL,’’ Sen. 23 to 25. —For portraits, instantan¬ eous views, out-door groups, etc. Blue Label. This plate and our Ortho. 23 Sen., are the best plates for all round work made. “B” Plates, Sen. 16 to 20. —For landscape views and general photog¬ raphy. Admittedly the finest plate for amateurs. “ ORTHOCHROMATIC ” Plates, Sen. 23 to 27, give correct color values. The best plates for landscapes, interiors, and photo-microg¬ raphy. “A” GELATINO-ALBUMEN Plates. —For lantern-slides and window transparencies. PROCESS PLATE. —For use by photo-lithographers, photo-engravers, and zinc-etchers in making intense and clear-line negatives. Sen. 12 . CARBUTTS LANTERN TRANSPARENCY PLATES Yield beautiful results with HYDRO, or EIKO. Developer. CARBUTT’S Flexible “Celluloid” Films. NEGATIVE AND POSITIVE. THE PERFECTION OF FILM PHOTOGRAPHY. Bulk and Weight Reduced to a Minimum. NO HALATION. NO RISK OF BREAKAGE. • For Portrait, Landscape. Interior, Instantaneous, and Flash- Light Photography. Requiring precisely same Treat¬ ment as glass dry plates. Order through. Your Dealer. Carbutt’s Keystone Dry Plates and Flexible Negative and Positive Films for Sale by all Dealers. PRICE LISTS SENT ON APPLICATION. MANUFACTURED BY JOHN C A1TT3TTTT, Keystone Dry Plate ¥ oris. WAYNE JUNCTION, PHILADELPHIA, PA. Bausch & Lomb Optical Co. MANUFACTURERS OF Microscopes, Objectives and Accessories. Photographic Lenses and Diaphragm Shutters. Telescopes, Eye-Grlasses, Lenses, And a large Variety ol otner Optical Instruments. PHOTO-MICROGRAPHIC 1 CAMERAS. Made in three different Forms , adapted to all modern t requirements . Factory and Main Office : 53 I_ 543 North St. Paul St., Rochester, N. Y. P. O. Drawer 29a. Branch Office : 48 and 50 Maiden Lane, New York City. p. O. Box 433. Illustrated Catalogue sent on application free to any address. XVII 4 l 1 I GETTY CENTER LIBRARY CONS QH 251 P95 1890 BKS c. 1 Pringle. Andre* Practical photo-micrography : by the lat 3 3125 00257 2374 1 V .