i 
 
 The Microscope 
 
 Dr. Henri Van Heurck 
 
 TlilHl 
 
 Mm 
 
 mm 
 
■En 
 
 FRANKLIN INSTITUTE LIBRARY 
 
 PHILADELPHIA 
 
 Class.. \5..7...&. Book 
 
 L/ ,&./.. Accession. 
 
 Article V. — The Library shall be divided into two classes ; the first 
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 Article VI. — The Secretary shall have authority to loan to Members 
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 !)Oo/is out at one time, without a written permission, signed by at least 
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THE MICROSCOPE. 
 
PRINTED BY 
 
 E. BAXTER LIMITED, 
 Lewes and London. 
 
Digitized by the Internet Archive 
 in 2015 
 
 https://archive.org/details/microscopeitsconOOheur_0 
 
Raynaud Antwerp phoio&r 
 
THE 
 
 MICROSCOPE 
 
 ITS CONSTRUCTION AND MANAGEMENT. 
 
 INCLUDING 
 
 TECHNIQUE, PHOTO-MICROGRAPHY, 
 
 AND 
 
 THE PAST AND FUTURE OF THE MICROSCOPE. 
 
 BY 
 
 DR. HENRI VAN HEURCK, 
 
 gfc flotany and Director qt^the Ahivi$r£ B9ttffilf£l\tj&r3em ; 
 Late Pfesi&fjii"'.af>l-he 3ielgi«?i. flfieiosfoficfcSyit/y* I »»« 
 
 Professor 
 
 Late 
 
 ! Hon. •F.A:.l/:s: 'and' Aew York M.S. 
 
 ENGLISH EDITION. 
 
 Re-edited and Augmented by the Author from the 
 Fourth French Fdition, and Translated by 
 
 WYNNE E. BAXTER, F.R.M.S.. F.G.S, 
 
 $Uitl) CTljvrc Vlatta unt> uyu>av&a of '250 1'Utts.tvatious. 
 
 LONDON : 
 
 CROSBY LOCKWOOD AND SON, 
 
 7, Stationers' Hall Court, Ludgate Hill. 
 New York: D. VAN NOSTRAND COMPANY, 
 1893. 
 
THE GETTY CENTEP 
 LIBRARY 
 
AUTHOR'S PREFACE TO THE ENGLISH 
 
 EDITION. 
 
 The Fourth Edition of this book had only just appeared when Mr. 
 Wynne E. Baxter, a skilful English microscopist, honoured us by offering 
 to publish an English Edition. 
 
 We cordially accepted Mr. Baxter's offer ; but in the meantime the 
 Antwerp Microscopical Exhibition has taken place, at which the best 
 work of the most skilful manufacturers of the present day appeared. 
 We were especially anxious that any English microscopist who might 
 wish to read our treatise should have the advantage of the recent study 
 which our position, as President Director of the Exhibition, has enabled 
 us to make. 
 
 Materials accumulated to such an extent that we may now issue 
 the present Volume, not as a translation of the Fourth Edition, but 
 as being a really new Work — a Fifth Edition. 
 
 Thus, the instruments of several manufacturers, Messrs. Bausch and 
 Lomb, Crouch, Koristka, Erbe, Adnet, &c, which were not mentioned 
 in the Fourth Edition, are described in this. All the Illustrations 
 (amounting to a considerable number), which in this Volume are numbered 
 with a figure, followed by a letter (e.g., 196, a, b, c, d, e), will not 
 be found in the French Edition. 
 
 We cannot conclude this Preface without expressing the debt of 
 gratitude we owe to Mr. Wynne E. Baxter, who has so generously 
 not only devoted much valuable time to this translation, but under- 
 taken the entire expense of the publication, with the sole purpose of 
 acquainting his fellow-countrymen with a Work, which, he believes, is 
 not devoid of merit. 
 
 But it is not only from a pecuniary point of view that we owe 
 our acknowledgments to Mr. Baxter. " Traddutore traditore," says 
 the Italian proverb. We may truly assert that this Book is one of 
 those rare exceptions in which the proverb does not apply. Mr. Baxter 
 has shown by his work that the niceties of the French language, as 
 well as all microscopical details, are equally familiar to him, and in 
 several cases he has even suggested improvements to us. If, therefore, 
 there be any fault to find with any passage in the Volume, it is the 
 Author, and not the Translator, whom the reader must blame. 
 
 Antwerp, November, i8g2. 
 
EXTRACT FROM THE FOURTH 
 (FRENCH) EDITION. 
 
 " We were not twenty years of age when we wrote the manuscript 
 of the First Edition of this Volume. However, it was only published 
 some years afterwards, at the request of our friend, Arthur Chevalier, 
 who wished to insert in it certain parts of his Etudiant Micrographe. 
 
 The microscope has, indeed, always been our favourite study from boy- 
 hood, and to it we are indebted for some of the happiest hours of our life. 
 
 When we wrote the first manuscript of this Work, microscopical 
 research was not as popular in Belgium and France as it is now. A 
 microscope was then little more than a toy, and our Book, a humble 
 pamphlet of 104 pages, was, indeed, the first Work by which a French 
 reader could initiate himself into the practice of vegetable microscopy. 
 
 Four years after its publication a Second Edition was demanded ; 
 and the Third Edition, of which a considerable number of copies was 
 printed, appeared nine years afterwards. 
 
 The constant application and length of the work necessitated by the 
 publication of our Synopsis des Diatom'ees, has delayed the appearance 
 of this Fourth Edition, which some of our loyal readers have for a long 
 time past demanded. 
 
 There will not, however, be much ground for complaint at the delay. 
 The microscope has made such rapid progress of late years, that theory 
 forces the belief that we can expect but a few further improvements, 
 the most important of which will be a more complete utilisation of 
 the aperture of objectives. 
 
 As in each of our previous Editions, this Work has been entirely 
 re-written, with the exception of a few of the old sections which have 
 been utilised. We are, therefore, practically speaking, publishing an 
 entirely new Work. 
 
 We have treated at some length the recent progress in micro- 
 scopy. Photo-micrography, staining processes, &c, have been set forth 
 with the fullness their importance deserves. 
 
 The same applies to the very important theory of Professor Abbe, 
 the practical application of which has enabled the microscope in a 
 few years to reach the state of perfection which it enjoys at the 
 present time." 
 
 Antwerp, 2nd August, 1891. 
 
TRANSLATOR'S PREFACE. 
 
 The popularity of this Work on the Continent would alone be 
 sufficient justification for an English translation ; but it has, besides, 
 intrinsic merits which will naturally commend it to readers. 
 
 There does not appear to be a work which, while maintaining its 
 elementary character so completely, at the same time deals with so 
 wide a range of microscopical subjects. 
 
 Again, such a book must necessarily be of interest to all who 
 devote serious attention to microscopic work, as a means of comparing 
 the Continental views and modes of thought with those of their own 
 and other countries. For these and other reasons it is believed that 
 this Volume will be acceptable as an addition to microscopical literature. 
 
 An endeavour has been made to avoid terms of expression essentially 
 French, but at the same time to strictly adhere to the meaning of the 
 text. Notwithstanding the trouble which the Author has taken in 
 revising all the proofs, yet it is far too much to hope that the Work, 
 even with that assistance, is altogether free from mistakes. 
 
 The Translator has been fortunate in securing the co-operation of 
 several friends in carrying out his undertaking, but his acknowledgements 
 are specially due to his son, Mr. Reginald T. Baxter (Exhibitioner and 
 Scholar of Clare College, Cambridge), who has afforded very valuable 
 assistance throughout the Work. 
 
 It is hoped that the addition of a somewhat voluminous Index and 
 Vocabulary, &c, will be found useful for purposes of reference. 
 
 A further Work on the Diatomaceae, founded on the Author's classical 
 "Synopsis des Diatomees de Belgique," is being prepared by the Author 
 for translation into English. It will be published uniformly with the 
 present Volume, to which it is intended to be supplementary. 
 
 W. E. B. 
 
 170, Church Street, Stoke Newington, N. 
 18th November, i8g2. 
 
CONTENTS. 
 
 Frontispiece. — Portrait of the Author. 
 INTRODUCTION. 
 
 CHAPTER I. page 
 Elementary Optics ....... i 
 
 Light — General ....... i 
 
 Reflection ........ 2 
 
 Refraction ........ 5 
 
 Lenses and their properties . . . . .7 
 
 Spherical Aberration . . . . . 14 
 
 Aberration of Refrangibility . . . . .16 
 
 Achromatism . . . . . . .16 
 
 Simple Microscope . . . . . .18 
 
 Compound Microscope . . . . . .19 
 
 CHAPTER II. 
 
 Professor Abbe's Theory of Microscopical Vision . .21 
 
 General Results . . . . . . .27 
 
 Special Results . . . . . . .29 
 
 CHAPTER III. 
 
 Experiments on the Application of Professor Abbe's Theory . 32 
 
 1st Exp. — -Identical images by different structures . . 33 
 
 2nd Exp. — No resolution without one spectra at least . . 35 
 
 3rd Exp. — Relation of aperture to resolving power . -35 
 
 4th Exp. — Spectral images with crossed gratings . . -37 
 
 5th Exp. — Effect of one row of spectra only . . 38 
 
 BOOK I. 
 CHAPTER I. 
 
 The Microscope — Generally . . . . -41 
 
 CHAPTER II. 
 
 Detailed Examination of the Parts of the Microscope . 44 
 
 ;'. Optical Parts of the Tube . . . . • 44 
 
 Objectives — Generally . . . . -44 
 
 ,, in Fixed Mounting . . . -45 
 
 „ with Correction Collars . . . -45 
 
 Dry . . . . .46 
 
 ,, Immersion ... .46 
 
X. 
 
 CONTENTS. 
 
 i. Optical Parts of the Tube — {continued). page 
 
 Objectives — Apochromatic . . . . -47 
 
 „ Wide-angled . . . . .48 
 
 Focal distance . . . . . -5° 
 
 Frontal distance (working distance) . . . 51 
 
 Optical quality of a good Objective . . 51 
 
 Abbe's Apertometer . . . . . -53 
 Theoretical Resolution of Objectives in relation to Numerical 
 
 Aperture . . . . -54 
 
 Number of Objectives . . . . .56 
 
 Tests — r. Ordinary Tests. . . . . • 57 
 
 a. Pygidium of a Flea . . -58 
 
 b. Podura . . . ■ . 58 
 
 c. Pleurosigma angulation . . .60 
 
 d. Snrirella Gemma . . . .62 
 
 e. Van Henrckia rhomboides . . .62 
 
 f. Amphipleura pellucida . . -63 
 
 [Phototype plate of Tests] . to face 64 
 
 2. Nobert's Test . . . . .64 
 
 3. Moller's Test and proof-plate . . .66 
 
 4. The Author's Photographic Test . . -67 
 
 5. Professor Abbe's Test . . . .68 
 Oculars — Huygens' . . . . . .70 
 
 Ramsden's . . . . . 71 
 
 „ Orthoscopic . . . . 71 
 
 „ Compensating . . . . 71 
 
 ,, Projection . . . . . .72 
 
 ii. Stage — -Stage, Clips, Mechanical Stage . . -75 
 Hi. The Movements — Coarse and Fine Adjustments . -77 
 iv. Illuminating Apparatus . . . . -78 
 
 1. By Transmitted Light: 
 
 1. Mirrors . . . . . -78 
 
 2. Diaphragms . . . . . -79 
 
 3. Condensers, Abbe's . . . . .80 
 
 . ,, Powell and Lealand's Chromatic Oil . 84 
 ,, „ ,, Apochromatic Dry 85 
 
 „ „ ,, Achromatic Oil . 85 
 ,, „ „ Apochromatic 
 
 Immersion . 85 
 
 Rules for using Condensers . . . .86 
 
CONTENTS. 
 
 xi 
 
 iv. Illuminating Apparatus — {continued). page 
 2. For Opaque Bodies : 
 
 1. Mirrors . . . . . -87 
 
 2. Condensing Lens . . . . .88 
 
 3. Lieberkuhn . . . . . .88 
 
 4. Vertical Illuminator . . . . .88 
 
 CHAPTER III. v 
 
 Accessory Apparatus . ' . . . .89 
 
 1. Micrometers, Ocular, Stage . . . . .89 
 
 2. Camera Lucida . . . . . .91 
 
 3. Polarising Apparatus . . . . . .94 
 
 4. Spectroscope . . . . . . .96 
 
 5. Finders ...... -97 
 
 6. Rotating Nose-pieces, Adapters . . . .100 
 
 7. Reversing Prism, Erector . . . . .102 
 
 8. Binocular and Stereoscopic Microscope . . . 103 
 Abbe's Stereoscopic Ocular . . . . .104 
 
 9. Ocular Comparer of Dr. H. Van Heurck . . . 105 
 10. Transformer of Dr. H. Van Heurck . . .106 
 
 Cover-glass Gauge . . . . . .106 
 
 Artificial Illumination of the Microscope . . . ic8 
 
 1. Lamps ........ 108 
 
 2. Electric Light . . . . . . .109 
 
 BOOK II. 
 CHAPTER I. 
 
 Simple Microscopes . . . . . . .118 
 
 CHAPTER II. 
 
 Projection Microscopes . . . . . .122 
 
 Solar Microscope . . . . . . .123 
 
 Gas Microscope . . . . . . .126 
 
 Photo-Electric Microscope . . . . .126 
 
 BOOK III. 
 CHAPTER I. 
 
 Use of the Microscope . • . . . .129 
 
 1. Situation and Arrangement of the Work Room . .129 
 
 2. Choice of Light ...... 130 
 
 3. Magnification . . . . . . -132 
 
 4. Hygienic Rules for Observation in Microscopical Researches. 133 
 
xii. 
 
 CONTENTS. 
 
 Use of the Microscope — {continued). page 
 
 5. Example of Microscopical Observation . . 1 35 
 
 6. Choice of a Microscope. . . • • • *3 8 
 Makers of Microscopes : — 
 
 Bausch & Lomb Optical Company, of New York . . 138 
 
 R. and J. Beck, of London 
 L. Beneche, of Berlin 
 Bezu, Hausser & Co., of Paris 
 Arthur Chevalier, of Paris 
 H. Crouch, of London 
 Gebruder Fromme, of Vienna 
 
 E. Hartnack, of Potsdam . 
 
 F. Koristka, of Milan 
 E. Leitz, of Wetzlar 
 Nachet, of Paris . 
 Powell & Lealand, of London 
 Carl Reichert, of Vienna . 
 Ross & Co., of London . 
 W. and H. Seibert, of Wetzlar 
 H. R. Spencer, of Buffalo (New 
 James Swift & Son, of London 
 Watson & Sons, of London 
 Carl Zeiss, of Jena 
 
 CHAPTER II. 
 
 Measurement and Reproduction of Microscopical Objects . 245 
 
 1. Measurement of Microscopical Objects . . • 245 
 
 2. Measurement of Magnifying Power .... 247 
 
 3. Drawing Microscopical Objects .... 248 
 Photo-micrography ....... 249 
 
 1. Installations . . . • • • -250 
 
 Woodward's Installation . . . • -250 
 
 Ordinary Method . . . • • -250 
 
 The Author's Installation . . . • -251 
 
 2. Apparatus ....... 252 
 
 A. Nachet, of Paris . . . . • -252 
 
 C. Reichert, of Vienna . . . • -258 
 
 C. Zeiss, of Jena . . . • • -259 
 
 E. Leitz, of Wetzlar . . . . • .261 
 
 Dr. Henri Van Heurck's Apparatus . . . .262 
 
 York) 
 
 T 5 2 
 i55 
 157 
 *57 
 162 
 162 
 168 
 
 i74 
 182 
 190 
 
 i95 
 206 
 212 
 218 
 219 
 221 
 232 
 
CONTENTS. 
 
 xiii. 
 
 Photo-micrography — {continued). page 
 
 3. The Illuminant ....... 264 
 
 Sunlight ....... 265 
 
 Electric Light . . . . .267 
 
 Oxy-hydrogen Light [Mr. A. Pringle's Apparatus] . . 268 
 
 Oil Lamp Light . . . . . .268 
 
 White Cloud Light . . . . . .270 
 
 4. The Photographic Processes . . . . .270 
 
 1. Production of the Negative .... 270 
 
 The Developer . . . . . .271 
 
 Mercier's Hydroquinone and Eosine Perfect Developer 271 
 Normal and over-exposed Negatives . . .272 
 
 Instantaneous and under-exposed Negatives . .273 
 Intensification of the Negative . . . .273 
 
 Fixing Solution . . . . . . 274 
 
 2. Printing on Paper . . . .276 
 
 3. Production of Lantern Slides . . . . 277 
 
 Process for obtaining Micrographs for Projection . 277 
 
 4. General Instructions ..... 280 
 
 Remarks by Dr. R. L. Maddox . . . 283 
 
 Note by Mr. Andrew Pringle .... 287 
 Note by Mr. Thomas Comber .... 288 
 
 CHAPTER III. 
 
 Cause of Errors in Microscopical Observations . . 290 
 
 1. Iridescence and Diffraction . . . . .290 
 
 2. Flying Flies; Dust on Glasses. .... 291 
 
 3. Defects of the Eye ...... 292 
 
 CHAPTER IV. 
 
 Preservation of the Microscope ..... 294 
 
 BOOK IV. 
 CHAPTER I. 
 
 General Rules for Preparing Microscopical Objects . . 298 
 
 § 1. Medium used for Observation, Preparation and Preservation 
 
 of Microscopical Objects . . . .298 
 
 1. Aqueous and Oily Media: 
 
 r. Calcium Chloride . . . . 299 
 
 2. Glycerine ... . 299 
 
 3. Glycerine Jelly ..... 299 
 
 4. Camphorated Water . . . . . 299 
 
XIV. 
 
 CONTENTS. 
 
 General Rules, &c. — {continued). page 
 
 5. Saccharinated Water ..... 300 
 
 6. Fine Oil ..... 300 
 
 7. Liquid No. 28 .... 300 
 
 2. Resinous Media : 
 
 1. Canada Balsam . . . . .301 
 
 2. Styrax and Liquidambar .... 301 
 
 3. Chemical Media : 
 
 1. Monobromide of Napthalin .... 304 
 
 2. Iodide of Methyl ..... 304 
 
 3. Arsenical Medium (refractive index 2-4) . . 305 
 
 4. Varnishes for Cells ..... 307 
 § 2. Chemical Reagents ...... 307 
 
 (a) Iodized Chloride of Zinc .... 308 
 (/>) Solution of Iodine ..... 308 
 
 (c) Mercury Nitrate ..... 308 
 
 (d) Ether ..... 308 
 
 (e) Alcohol ...... 308 
 
 (/) Ammoniacal Cupric Oxide .... 308 
 
 (g) Nitric Acid ...... 308 
 
 (//) Sulphuric Acid ..... 308 
 
 (/) Potassium Chlorate ..... 309 
 
 (/) Caustic Potash ..... 309 
 
 Staining Reagents .... . 309 
 
 (a) Logwood ...... 310 
 
 (l>) Carmine . . . . . -310 
 
 (c) Ammonium Picrocarminate (Ranvier's double stain) 310 
 
 (d) Sodium Sulphindigotate . . . -310 
 
 (e) Nigrosine . . . . .310 
 (/) Iodine Green . . . • 3 1 1 
 
 (g) Fuchsine . . . . . • S 11 
 
 (h) Methyl Violet . . . . .311 
 (t) Double Staining with Anilines . . . 311 
 (j) Methyl Green . . . . .311 
 
 §3. Instruments used in preparing Objects . . . 311 
 
 (a) Razors . . . . . . .312 
 
 (b) Needles . . . . . . -312 
 
 (c) Scalpels . . . . . . -313 
 
 (d) Forceps . . . . .' . • 3 I 3 
 
 (e) Hand-vice . . . . . -313 
 
CONTENTS. 
 
 XV. 
 
 General Rules, &c. — [continued). page 
 (/) Glass Rods and Pipettes . . . . 313 
 
 (g) Hand Sections and Microtome Sections . . 313 
 
 Microtomes, Topping's . . . . .314 
 
 Nachet's . . . . .315 
 
 „ Reichert's . . . . . 315 
 
 Leitz' . . . . . 317 
 
 „ Watson's Cathcart . . . . 317 
 
 „ Erbe's . . . . .318 
 
 Sections by Freezing . . ' . . .322 
 
 ,, Embedding ..... 323 
 
 § 4. Staining Tissues . . . . . . 323 
 
 Mounting Vegetable Tissues with a Single Colour . 324 
 Double-staining Vegetable Tissues . . 325 
 
 1. With Iodine Green . . . . 325 
 
 2. With Picrocarminate . . . . .326 
 § 5. Mounting Preparations ..... 326 
 
 Square Cells ....... 326 
 
 Round Cells . . . . . . -327 
 
 Turn Table ....... 327 
 
 Marie Bath, Chevalier's ..... 328 
 
 „ Adnet's (Naples) . . . -328 
 
 § 6. Preservation of Preparations . . . . .329 
 
 CHAPTER II. 
 
 The Microscopist's Library . . ... . 331 
 
 1. Periodicals . . . . . . 331 
 
 2. History of the Microscope . . . . 331 
 
 3. General Works . . . . . 332 
 
 4. Photo-micrography ...... 333 
 
 BOOK V. 
 
 1. The Microscope in the Past — Notes on the History of 
 
 the Microscope ...... 334 
 
 2. The Microscope in the Future — Letter from Dr. 
 
 Siegfried Czapski . . . . . -357 
 
 Addenda : — 
 
 1. Messrs. Bausch and Lomb's Fine Adjustment, &c. . . 365 
 
 2. Note on Apochromatic Combinations .... 366 
 
 [Plate illustrating path of rays] . . to face 366 
 
 Index and Vocabulary, &c. . . . . -371 
 
The Microscope. 
 
 INTRODUCTION. 
 
 CHAPTER I. 
 
 ELEMENTARY OPTICS. 
 
 Light. — General. 
 
 Light is the agent which produces in us the sensation of sight. 
 
 Light is emitted by luminous bodies themselves, such as the sun and 
 burning substances, or is reflected by bodies which have not the power of 
 emitting it themselves, which are called non-luminous, such as the moon. 
 
 It is admitted that the molecules of luminous bodies are in a 
 state of exceedingly rapid vibration ; and these vibrations, being com- 
 municated to the ether, which pervades all space, are thus propagated 
 and eventually strike our retina. The number of these vibrations in the 
 unit of time produces in us the sensation of different colours. 
 
 The straight line along which light is propagated is called a ray. 
 An assemblage of rays constitutes a beam. The word pencil is also 
 sometimes used to designate a limited quantity of rays. 
 
 Bodies through which light can be propagated are called in optics 
 media; such for example are air, water, and glass. They are further 
 called transparent, translucent, and diaphanous, according as the rays 
 are transmitted more or less freely in contradistinction to opaque bodies, 
 which do not allow luminous rays to be transmitted. Many bodies 
 which at first sight appear entirely opaque still allow some rays to 
 pass through them when they are reduced to excessively thin plates. 
 Such is the case with gold, silver, &c. 
 
 In every medium which is homogeneous, that is to say, having 
 throughout the same composition and the same density, light is 
 
 B 
 
2 
 
 REFLECTION. 
 
 propagated in straight lines. It no longer does so when it meets an 
 opaque body or passes from one medium into another. In this case the 
 light is reflected, refracted, or decomposed. Reflection is the property by 
 virtue of which polished bodies can send back, in certain directions, the 
 luminous rays falling on their surfaces. Refraction is the change of direction 
 which transparent bodies exercise upon light which traverses them obliquely. 
 
 We need not concern ourselves at present about the decomposition 
 of luminous rays which apparently takes place in their passage through 
 pieces of glass cut in a certain manner, and called prisms. 
 
 Reflection. 
 
 In microscopes of the present day the reflection of light is only 
 made use of to illuminate the object under observation. When the 
 microscope is used without a condenser a plane mirror is employed 
 for low and a concave mirror for high magnification. 
 
 Let us examine 
 the course 
 taken by rays 
 falling on a 
 plane mirror. 
 Let A B (fig. i) 
 be the mirror, 
 C a luminous 
 point : a ray 
 emanating from 
 this point and 
 striking the 
 mirror at D 
 will make, with 
 the perpendi- 
 cular to the 
 surface of the 
 Fig. x. mirror at that 
 
 point (which perpendicular is termed the norma/), an angle E D F, called 
 the angle of reflection, equal to C D E the angle of incidence. 
 
 Other rays C G, C H, emanating from the same point, make again, 
 with the normals G I, H J, the angles of reflection I G K, J H L equal 
 to the angles of incidence CGI, and CHJ; thus, all the reflected 
 
REFLECTION. 
 
 3 
 
 I'ig- 3- 
 
 rays seem to diverge from a point C, which lias no real existence, 
 and is called the virtual image of C. Its position as compared with 
 that of C is symmetrical with regard to the mirror. 
 
 The law is again the same 
 when parallel rays C, D, E (fig. 2) 
 strike a plane mirrOr. In fig. 2 
 the angles of reflection are equal 
 to the angles of incidence 
 
 Concave Mirrors. — There are 
 many kinds of these, but those 
 lg ' 2 ' which are employed with micro- 
 
 scopes are usually spherical, that is, representing a portion or segment 
 of a sphere. Spherical mirrors have as their centre 
 of curvature the centre of the sphere of which 
 they represent a part ; thus the mirror A B (fig. 3) 
 has for radius of urvature the radius O A. 
 
 The point C, the middle of the mirror, bears 
 the name of centre 0/ figure, and the infinite straight 
 line which passes through this point and the centre 
 of curvature is the principal axis. 
 Concave mirrors possess the property of concentrating the. luminous 
 rays which they receive, at a single point which is called a focus. 
 
 There is a distinction between the principal focus and conjugate foci. 
 We shall see what these two expressions signify. 
 
 Let A B be a concave mirror, of 
 which C is the centre of curvature. 
 Let us consider it as composed of an 
 infinite number of plane mirrors. The 
 extreme tenuity of luminous rays, and 
 the infinitely small surface on which 
 they can be reflected, permit this sup . 
 position, which thus reduces the laws of 
 reflection for curved mirrors to those for 
 plane mirrors (fig. 4). 
 Let us suppose the luminous rays E F, G H, / T, K L, parallel to 
 one another and to the principal axis, to fall on the mirror ; they 
 will make, with the normals (which will then be the radii C F, C H, 
 CJ, CL) the angles of reflection C F O, CHO, C J O, C L O, equal 
 to the angles of incidence E F C, G H C, / J C, K L C, and all these 
 rays will meet in a point O in the centre of the principal axis C D. 
 
 113 
 
 Fig- 4- 
 
4 REFLECTION. 
 
 This, however, only holds good for mirrors which are but a small segment 
 of a sphere when the angle A B C is but a few degrees in magnitude. 
 
 Let us now consider the triangle C F O : by the laws of 
 geometry the angle F C O is equal to the angle C F E, being alter- 
 nate angles with respect to the parallels E F, CD, intersected by the 
 straight line C F ; now we have just seen that the angle of reflection 
 C F O is equal to the angle of incidence E F C. The angles F C O 
 and C F O are, therefore, equal to one another since they are both equal 
 to E F C, and, consequently, the straight lines C O and O F are equal. 
 Since the luminous rays can approach so as nearly to coincide with 
 the axis C D, it follows that the point O is to a very minute fraction 
 at the middle of the straight line C D. This is the principal focus. 
 
 Let us now suppose that the 
 rays falling on the mirror, in- 
 stead of being parallel, proceed 
 from a luminous point E (fig 
 5), situated at a certain distance 
 from the centre C ; the angles 
 of incidence, and consequently 
 of reflection, will be smaller 
 than in the preceding case. The 
 point F, where the rays re-unite, 
 will, therefore, be found situated 
 F, s- 5- in front of the point O, and 
 
 between that point and the centre C It is called the conjugate 
 focus of the point E. 
 
 It is easy to see that the farther the point E is withdrawn from 
 the centre C, the nearer the conjugate focus will approach the principal 
 focus O until it becomes coincident with it. If, on the contrary, we 
 draw the point E nearer, the conjugate focus will equally approach 
 the centre. If E becomes coincident with C, the rays impinging 
 perpendicularly will be reflected on themselves. 
 
 If the luminous point be placed between C and the principal 
 focus, the conjugate focus will be carried over to the other side of 
 C, so that the conjugate focus of the point F will be E. 
 
 At the principal focus a conjugate focus is found situated at infinity, 
 the reflected rays being parallel ; between the former point and the centre 
 of figure D the focus will be virtual and placed behind the mirror. 
 
 After what has preceded, it will be seen that the experimental 
 determination of the principal focus of a concave spherical mirror is 
 
REFRACTION. 5 
 
 very simple ; it is placed so as to receive a beam of solar rays in 
 a direction parallel to its principal axis, and, by presenting a white 
 card before it, the distance at which the image of the sun thus 
 obtained appears plainest, is tound. 
 
 This point will be the principal focus, and twice its distance from 
 the centre of the mirror will give the radius of curvature. 
 
 The words parallel rays, having been employed several times, it 
 may, perhaps, be asked by what means such rays can be obtained. 
 It should, therefore, be understood that rays which emanate from a 
 point in the sun or stars are considered to be endowed with parallelism. 
 Although these rays are not mathematically parallel, they can, never 
 theless, be considered as such on account of the enormous distance 
 of the bodies producing them. 
 
 Refraction. 
 
 We have previously seen what refraction is. It has only a sensible 
 effect when the rays fall obliquely with respect to the surface of the 
 
 medium considered ; when they fall per- 
 pendicularly on this surface, they 
 continue in the same course in the 
 second medium as they did in the first. 
 
 Consider, now, a ray from one 
 medium falling obliquely upon the sur- 
 face of another ; for example, from air 
 upon glass, and call it B A (fig 6). At 
 the point of contact A, raise the normal 
 CAD; the ray, instead of following its 
 course in the direction of A B produced, 
 is bent tozvards the normal, making with 
 p~ig 6> it an angle E A D (of refraction) smaller 
 
 than CAB, the angle of incidence. 
 Passage of light through media with parallel surfaces. — This takes place 
 whenever a ray emerges from a less dense medium (and consequently less 
 refractive) and enters a denser medium. If we suppose a plate of glass 
 to have parallel surfaces, the ray BAE produced upon emerging from the 
 glass and re-entering the air, will be deflected from the normal the 
 same amount that it was bent towards it in its first refraction, and it 
 
6 
 
 REFRACTION. 
 
 will make an angle of refraction equal to the angle of incidence in the 
 first refraction. 
 
 If the line of its direction be now produced, the ray upon emerging 
 from the glass will be seen to be following a direction parallel to that 
 which it had before entering the medium. This effect takes place in 
 consequence of the two surfaces of the plate in question being parallel ; 
 if they had not been so, the ray would have taken another direction, but 
 never parallel to A B. This property is very important in demonstrating 
 the properties of lenses. 
 
 It is shewn in physics that the sines of the angles of incidence and 
 of refraction are to one another in a ratio which is constant for the 
 same media and for the same colours. This ratio is called the index 
 of refraction; from air into glass it is 3/2, from glass into air it is 2/3. 
 
 Critical angle, total reflection. — We have seen that a ray, passing from 
 one media to another less dense, makes an angle of refraction greater 
 than that of incidence. It therefore necessarily follows that for some 
 angle of incidence, the angle of refraction will be right, and that the 
 refracted ray will emerge parallel to the surface of the plane. The 
 angle of incidence producing this effect is called the critical angle. 
 And, indeed, for every angle greater than this, the ray is no longer 
 refracted but is completely reflected, which phenomenon is called total 
 reflection, and is made use of in a special kind of prism. 
 
 The critical angle for crown glass into air is 41 0 . 
 
 Passage of light in media, with non-parallel surfaces. — Among media, 
 with non-parallel surfaces, a distinction is made between prisms and 
 lenses. Prisms are pieces of glass with plain surfaces inclined towards 
 each other. Knowing the laws of refraction it will be easy to deduce 
 the course of rays in these media. 
 
 Consider a triangular prism, the 
 transverse section of which is ABC 
 (fig. 7), and a luminous point 1): 
 in the plane ABC a ray, starting 
 from this point strikes the prism at 
 E, and is refracted towards the 
 normal DBA since it passes into a 
 more refractive medium ; it then takes 
 the direction E F, and is refracted 
 anew at the point F, where it again emerges into the air, but is deflected 
 from the normal D B C. 
 
 By placing the eye at the point G the light will appear to follow 
 
OF LENSES AND THEIR PROPERTIES. 
 
 7 
 
 a single straight line, and the point D will be seen, not at its true 
 position, but in the direction F d. Prisms then, like plane mirrors, 
 can be used to make a luminous point appear displaced. 
 
 Application of prisms as reflectors. — 
 Prisms used for this purpose are cut 
 in the shape of a right-angled isosceles 
 triangle as A B C (fig. 8). Consider an 
 incident ray E F parallel to AC: the 
 angle E F B will be equal to AC F, 
 and like it an angle of 45 0 ; now the 
 critical angle of glass being only 41 0 , 
 the ray E F will be reflected completely 
 at F, and will follow the direction F G 
 parallel to A B. 
 
 The eye placed at E will then see 
 objects placed at G as if they were on 
 E F produced. 
 
 Fig. 8. 
 
 Of Lenses and their properties. 
 
 Transparent sub- 
 stances (in general 
 glass) cut in 
 such a manner 
 that the luminous 
 rays which traverse 
 them, can be made 
 to converge or 
 Fig. 9. diverge, are termed 
 
 lenses. There are two series of lenses : the convex series, and the 
 concave series. These series comprehend the following forms (fig. 9): — 
 A The bi-convex lens. 
 B The plano-convex lens. 
 
 C The concavo-convex lens or convergent meniscus. 
 D The bi-concave lens. 
 E The plano-concave lens. 
 
 F The concavo-convex lens or divergent meniscus. 
 
 Let us take for granted, in the first place, the same hypothesis as 
 
8 
 
 OF LENSES AND THEIR PROPERTIES. 
 
 we did for spherical mirrors, viz. : — that lenses are formed of an infinite 
 number of small plane surfaces, having as normals the radii drawn 
 from these points to the centres of curvature (see later " Definition 
 of the centre of curvature.") 
 
 Optical Cetitre. — A point, such that every luminious ray, passing through 
 it, is, upon emerging from a lens, parallel to its direction before entering 
 the lens, is called the Optical centre of the lens 
 
 Thus in figure 10 the point O will 
 be the optical centre of the lens L L', if 
 the incident ray A B is parallel to the 
 emerging ray C D : another incident ray A' B', 
 then passing through O, so as to become the 
 emerging ray C D', again verifies the definition. 
 
 We have now to find the position of this 
 point O, but in the first place let us remark 
 that the centre of a sphere, of which this 
 surface of a lens forms a part is called the centre 
 of curvature of this surface of the lens. 
 
 Thus in figure n, let A B C E be a 
 lens, and ABC the surface to be considered. 
 This surface has reference to a sphere 
 A B C D, of which it forms a part. If this sphere has its centre at 
 O, this point will be the centre of curvature of the surface ABC. 
 It should further be noted that the centre of curvature must not be 
 
 confounded with the centre of figure ; 
 this last varies according as a part of the 
 edge of the lens is cut, so as to render it 
 either square, polygonal, or of different forms, 
 and the centre of figure, although it can 
 be made to change with each new form, 
 though not necessarily in the case quoted, 
 will change with each form, while the centre 
 of curvature will remain the same ; we will 
 suppose the lenses to be round like those 
 ordinarily used in practice, and in this case also the position of maximum 
 thickness of convergent lenses and that of minimum thickness of divergent 
 ones usually coincide with the centre of figure. If this condition is 
 not fulfilled the lenses are eccentric, as in the ordinary stereoscope, 
 where they have to be so. We are here supposing that the lenses are 
 always spherical, the centre of figure corresponding to the maximum or 
 
OF LENSES AND THEIR PROPERTIES. 
 
 minimum thickness, according as they are convergent, or divergent. 
 
 To find the optical centre of any lens, the points where the normals 
 and the tangents are parallel must first be found, or what comes to 
 the same thing, the points where the element of the surface is parallel 
 in each corresponding surface. 
 
 Thus, in lenses where one surface is plane 
 this will be seen at once, for it is evident that 
 the point A (the centre of figure of the curved 
 surface) (fig. 12) will have its tangent parallel 
 to C D, or its normal parallel to that of C D, 
 or the element A will be parallel to all the 
 ekrr.ents of the surface C D, each of which 
 amounts to the same thing. 
 
 Now every ray which enters at the point A 
 will emerge from the surface C D parallel to the direction which it 
 had at its entrance, since it passes through a plate with parallel 
 surfaces : then following the definition which we have given of the 
 optical centre, the point A will be this centre. 
 
 All the rays which emerge from it will have a direction parallel 
 to that which they had before entering the surface C D. 
 
 In figures 13, 14, 15, 16 below, 
 B is the centre of curvature of the 
 surface Bi B2 B3 ; A that of the 
 surface Ai A2 A3. 
 
 If any radius B B' be drawn from 
 the centre of curvature B to the corres- 
 ponding surface, cutting it in B', and 
 then if a radius A A', parallel to B B', 
 be drawn from the centre of curvature 
 A, cutting the corresponding surface 
 Ai A2 A3 at the point A', these surfaces 
 at the points A' and B' will have the 
 normals B B', A A' parallel by construction. 
 
 It follows that their elements will 
 also be parallel at these points. There- 
 fore, if a luminous ray enter the lens at 
 A' so inclined as to emerge at B', or 
 inversely, it will act as if it passed 
 through a medium with parallel surfaces ; 
 that is to say, that its deviations before 
 
 
 ^^J~ 
 
 T 
 
 B >' 
 
 A~ ■ 
 
 
 - B 2 
 
 I 
 
 Fig. 14. 
 
 """"""" — 
 
 
 B 
 
 A ; 
 
 B 3 j 
 
 m 
 
10 
 
 OF LENSES AND THEIR PROPERTIES. 
 
 its incidence, and after its emergence, 
 will be parallel ; therefore it will 
 pass through the optical centre O. 
 
 Further, the luminous ray which 
 passes through A and B will not 
 deviate from its course ; it will 
 Fig. 16. consequently also pass through the 
 
 optical centre ; therefore the point of intersection of A B and A' B' 
 will give this centre. Every straight line passing through the optical 
 centre is called an axis; the principal axis is that axis which also 
 passes through the centres of curvature; the others are secondary 
 axes. 
 
 Every luminous ray which follows the principal axis does not 
 deviate from its course. It is almost the same with regard to 
 secondary axes, at least, if the lenses are not too thick, and the 
 secondary axis does not make too great an angle with the principal 
 axis. 
 
 If these conditions are not fulfilled the luminous ray which follows 
 the secondary axis is slightly displaced, although remaining parallel to 
 itself. 
 
 Determination of the Joci of lenses: — A. Convex Series. — Let us take 
 the bi-convex lens as a type of the convex series ; the student who 
 has understood the properties of this will find no difficulty in 
 applying them to other lenses of the same series. Like the concave 
 mirror the bi-convex lens possesses a principal focus and conjugate 
 foci, which can be either real or virtual. 
 
 Let rays mutually parallel to the principal axis pass through 
 a lens. These rays experience a primary deviation upon entering the 
 
 glass, and are bent towards the 
 axis A B C (Eig. 17); on again 
 entering the air they experience 
 another deviation towards this axis, 
 which they intersect at the point 
 Fig. 17> C. Since all the parallel rays 
 
 intersect at this same point, it will be the principal focus of the 
 lens. 
 
 The principal focal length is the distance of the focus from the 
 optical centre. It is this which determines the "number" given to 
 glasses for spectacles, &c. ; thus, according to the old numbering (which, 
 however, is now replaced by numbers corresponding to the divisions 
 
» 
 
 OF LENSES AND THEIR PROPERTIES. I I 
 
 of the metrical system), lens No. 5 is one whose principal focus 
 is at a distance of five inches from the optical centre. 
 
 If luminous rays proceed from a point A (fig. 18), situated on the 
 principal axis, and meet the lens in the form of a divergent beam, 
 
 the same thing as before takes 
 place, only the deviation being kss 
 the refracted rays are concentrated 
 at a point C farther off than the 
 principal focus which is called 
 
 Fig 18. the conjugate focus of A. If 
 
 A is brought nearer, C moves farther away ; if A is withdrawn, C 
 draws nearer without ever becoming coincident with C, so long as the 
 beam is not parallel. 
 
 Secondary foci are foci which lie on secondary axes where the 
 incident rays are parallel to them. There focal distance is the 
 same as for the foci which lie on the principal axis ; neverthe- 
 less, after what we have seen with regard to secondary axes this is 
 only approximately true, and is only absolutely true for ideal lenses 
 having no thickness, but in practice it is sufficiently accurate. 
 
 If we suppose the luminous point A (fig. 19) always situated on 
 the principal axis, but between the lens and its principal focus O, we 
 shall have the course of the luminous rays constructed as follows : the rays 
 which are divergent before entering the lens remain divergent upon emerging. 
 
 They cannot therefore themselves form a focus, 
 but when produced backwards they meet one 
 another in A' on the other side of the 
 principal focus, giving to the eye placed behind 
 the lens the sensation of a focus. This focus 
 Fig. 19. is "virtual," that is to say, one appears to see 
 
 it, but it does not really exist ; it cannot, like real foci, be received 
 upon a screen or ground glass. If A is brought near to the lens, 
 A' approaches the principal focus ; if A is brought near to this point, 
 A' withdraws from it. 
 
 Formation of the images with convex lenses. — We have always assumed 
 in what has preceded, that luminous points giving rise to the foci, were 
 situated on the principal axis of the lens. Let us notice that the 
 formation of foci will be always the same if these points happen to be 
 on secondary axes, the foci being then formed at a certain point on 
 these axes where the divergent rays intersect after passing through the 
 lens. 
 
I 2 
 
 OF LENSES AND THEIR PROPERTIES. 
 
 This being granted, let us further remark that in order to obtain 
 the images produced by convex lenses, the image of an object is 
 formed by the union of the foci, produced by each of the points of 
 this object, the demonstration of which can be simplified by making 
 the construction only for the top and bottom. These two extreme 
 points being found, the position of the object in the image will be 
 determined, since the foci of all the intermediate points will arrange 
 themselves between them. 
 
 Two cases may occur : either the object may be beyond the 
 principal focus with reference to the lens, or within it. Let us suppose 
 
 first that the object A B is 
 beyond the principal focus 
 C (fig. 20); let O repre- 
 sent the optical centre and 
 draw the secondary axes 
 AO, BO. After what we 
 have just seen, the focus of 
 the point A ought to be 
 some point on the axis 
 A O, and that of B some point on B O. Now draw the incident rays 
 A U and B F parallel to the principal axis. These rays will be 
 refracted so as to pass through the second principal focus C and they 
 will cut the secondary axes at the points A' and B'. At the point A' 
 the image of A will be formed and at B' that of B. In the same 
 way it could be shewn that all the points contained between A and 
 B would be formed by foci between A' and B', where we shall 
 therefore have the complete image of the object A B. This image is 
 inverted and real. If we refer to what has been said of foci we shall 
 observe that the farther the object is from the principal focus the 
 more the rays, emanating from each point of the object, approach to 
 parallelism and the smaller becomes the image of an object (though 
 large) and the nearer it approaches the principal focus ('). On the 
 contrary, if the object is brought very close to the focus its image will 
 be formed at a definite distance and will be greatly magnified ( 2 ). 
 
 Let us suppose in the second place that the object A B is placed 
 between the lens and its principal focus. 
 
 Draw the secondary axes A O and B O, and the incident rays 
 
 ( 1 ) Principle of the camera obscura. 
 
 ( 2 ) The Principle of apparatus for Optical Projection. 
 
OF LENSES AND THEIR PROPERTIES. 1 3 
 
 A D and B E (fig. 2 1 ), these rays after being refracted emerge from 
 the lens, diverging more and more from the secondary axes ; but if 
 these rays are produced backwards, cutting the same axes at the 
 
 points A' and B', they 
 will form there the virtual 
 image of the points A 
 and B ; all the other rays 
 emanating from the object 
 A B forming foci between 
 A' and B', we shall there 
 have an erect and mag- 
 nified virtual image of the 
 object A B, which will be observed by an eye looking in the direction 
 of the rays. In this lies the principle of magnifying glasses or 
 simple microscopes as mostly used. 
 
 Determination of the foci of lenses. — B. Concave series. 
 Formation of foci. — Let us take the bi-concave lens as a type. In 
 all lenses of this series only virtual foci are formed, whatever be 
 the position of the luminous point, this being considered as real. 
 
 In figure 22 at the side the 
 parallel rays A B and D E strike 
 the lens O. 
 
 Draw the normals at the points 
 of incidence B and E ; within the 
 lens the rays approach these while 
 they themselves diverge ; draw the 
 normals at their points of emergence F and G ; they deviate from 
 these and still diverge, and therefore do not form any real focus, but 
 if produced backwards they form the principal focus at C, which is at 
 the same time virtual. 
 
 If the incident rays proceed from a point situated on the principal 
 axis, it is evident that these rays, already divergent upon entering, 
 diverge still more and form virtual foci, situated between the principal 
 focus and the lens. 
 
 Production of Images — Concave lenses having only virtual foci can 
 only produce virtual images of real objects. 
 
 Let A B (fig. 23) be an object placed before the lens O ; draw 
 the secondary axes A O and B O, the incident rays A D and B E, 
 and erect normals ; the rays approach these, while they themselves 
 diverge ; upon emerging they deviate from them, and diverge still more, 
 
) 4 
 
 .SPHERICAL ABERRATION. 
 
 Fig. 23. 
 
 takes place, whatever be the 
 
 but if produced backwards they inter- 
 sect the secondary axis at A' and B' ; 
 in the same way the different points 
 situated between A and B group them- 
 selves between A' and B', where a 
 virtual and erect image will then be 
 formed, smaller than the object. This 
 tance of the object from the lens. 
 
 Spherical Aberration. 
 
 Every lens may be considered as formed of a system of prisms, 
 or of parts of prisms (planes tangent to the lens), and having wider 
 and wider angles. Thus a deviation is produced, which becomes more 
 and more considerable, according as the edges of the lens are 
 approached, and which increases more rapidly than is necessary to 
 collect the rays to one focus. 
 
 Fig. 24. 
 
 This way of regarding lenses is identical with that which we have 
 given (page 3) for spherical mirrors, and this unequal deviation for 
 different parts of the lens is called spherical aberration. 
 
 Spherical aberration is a serious defect, which the optician tries to 
 remove as far as possible by different combinations. 
 
 Besides the theoretical figure (fig. 24) given above, we shall give 
 another to make the effect in question more intelligible, and for 
 greater clearness we shall exaggerate the defect. 
 
SPHERICAL ABERRATION. 
 
 ■ point. 
 From 
 this point 
 
 A B be 
 a lens 
 and C a 
 luminous 
 
 Let 
 
 Fig. 25. 
 
 draw the rays C A and C B so as to just graze the edges, and 
 draw other rays C D and C E very close to the principal axis (fig. 25). 
 
 When these rays traverse the lens we find that C D and C E 
 form their focus in F, while A C and C B form theirs in F', situated in 
 front of F. 
 
 If from the point C, other rays be drawn between the extreme 
 rays in the figure, their foci will be found to be between F' and F. 
 The distance F' F will be the longitudinal spherical aberration, and G H 
 the lateral spherical aberration of the lens. The consequence of this is 
 that instead of having at F a single luminous point, as focus of C, 
 we shall have a luminous centre, which will be F surrounded by a 
 diminishing halo, having G H as its greatest diameter, and G F' H its 
 angle of aperture. Every object placed at the point C will similarly 
 present to the eye a central image, surrounded by a second indefinite 
 outline resulting from spherical aberration. 
 
 This is necessarily annoying to the observer, and efforts have 
 therefore been made to remove this defect, in the first place by 
 providing the lenses with diaphragms, that is to say, with small discs 
 of blackened cardboard, or brass, which cover up their edges and 
 only allow the rays to penetrate in the middle. Diaphragms add much to 
 the clearness of the images produced, and this increases in proportion 
 to the smallness of the aperture, but then another inconvenience arises, 
 namely, that the smaller the aperture the fewer are the luminous rays 
 which it lets through, wherefore the object must be strongly illuminated 
 to obtain a sufficiently intense image. 
 
 In optical instruments, in which diaphragms are employed, there 
 are generally several apertures to be used according to the amount of 
 light at one's disposal, and also according to the character of the 
 objects observed. 
 
 Spherical aberration can also be considerably lessened, and some- 
 times even obliterated by using conbinations of lenses instead of a 
 single lens. An optician has to combine these, so that their 
 
1 6 ABERRATION OF REFRANGIBILITY — ACHROMATISM. 
 
 spherical aberrations react on one another, and are thus totally or 
 partially destroyed. 
 
 Aberration of Refrangibility — Achromatism. 
 
 Light used for the purpose of illumination, whether it comes from 
 the sun, or from a lamp, is white. 
 
 When we observe objects without the intervention of lenses, this 
 light appears to us simple and homogeneous. It is no longer so 
 when a prism or a lens is interposed between these objects and the 
 eye. 
 
 Light then is not only refracted, but also decomposed into its 
 elements, which are violet, indigo, blue, green, yellow, orange, and 
 red. All these colours forming the solar spectrum are unequally 
 refrangible. We have mentioned them in their order of refrangibility. 
 Their indices for a slightly refractive and dispersive glass vary from 
 i - 5466, index of the violet rays, to i - 5258, the index of the red rays, 
 while for very refractive and dispersive glasses, and still more so in 
 certain liquids, this difference of index is tripled, or even quadrupled. 
 
 Since white light is decomposed in lenses (a phenomenon called 
 Dispersion), and is at the same time refracted, it follows that the 
 rays do not meet in a single point on the secondary axes to which 
 they are parallel, and there form a focus. 
 
 For example, 
 let two parallel 
 rays CD and 
 E F strike the 
 bi-convex lens 
 A B (fig. 26). 
 These rays are 
 decomposed in 
 the lens at the 
 same time that 
 Fig. 26. they are re- 
 
 fracted, and upon emerging, each of the colours of which white 
 light is composed, will form its own special focus. These foci will 
 be found between G, the focus of the red rays— the least refractive, 
 and H the focus of the violet rays — the most refractive. The 
 distance H G is the Chromatic Aberration. 
 
ABERRATION OF REFRANGIBILITY ACHROMATISM. I 7 
 
 If now objects are observed through such a lens, it will be noticed 
 that their outlines are not sharply denned, and that they are tinted 
 with all the colours of the spectrum, which is naturally a great 
 inconvenience. 
 
 It was thought for a long time that it would be impossible to 
 remedy this defect, but in 1757, Dollond, an English optician, shewed 
 that aberration of refrangibility was virtually nullified by the use of a 
 system of two lenses of different composition placed in juxta-position. 
 
 To understand the effect thus obtained, it is necessary to know 
 that the glass generally used for lenses is crown-glass (silicate of 
 potassium and lime). 
 
 There is also another kind of glass, called flint glass, which 
 consists of silicate of potassium and of lead, very rich in oxide of 
 lead. 
 
 Flint glass has an index of refraction of nearly 1-63, and a 
 dispersive power '059, greater than crown-glass, where the figures are 
 only 1 "5 2 and '036. 
 
 For a long time there existed only one kind of crown and one kind 
 of flint-glass, and the achromatisation of objectives was therefore 
 very incomplete. Only the red and blue rays blended together. 
 
 Researches made mostly in France and England caused the 
 different kinds of glass to be increased in number, and consequently 
 caused an improvement in the construction of objectives. 
 
 But the number of these kinds of glasses was still very limited, 
 and quite insufficient to answer all the requirements of the optician. 
 
 It was then that Professor Abbe, of Iena, commenced his 
 researches, in conjunction with a skilful chemical specialist, Dr. Schott, 
 and an investigation of all elementary bodies, capable of producing 
 vitreous compounds, was systematically conducted. 
 
 This investigation, which absorbed considerable sums of money, had 
 the most fortunate results, and since 1886, the firm of Schott and 
 Co. (Drs. Schott, Abbe, and Zeiss) have been able to offer for sale 
 seventy different kinds of glass, many of which are vitreous compounds, 
 being glass only in appearance, for they do not contain silica. Silicon 
 is replaced by boron in an entire series of flints, and by phosphorus 
 in a series of crowns. 
 
 The new glass commonly called "Schott glass" or "Iena glass," is 
 now employed by opticians throughout the world. 
 
 Professor Abbe has, moreover, introduced several natural substances 
 in the construction of objectives which defy all artificial imitation. 
 
.s 
 
 SIMPLE MICROSCOPE. 
 
 Such, for example, is the fluoride of calcium or "Fluorite" of the 
 mineralogist. The index of refraction of this substance is only i'434, 
 that is to say, considerably less than that of any kind of glass hitherto 
 manufactured, and its dispersion is only half as great. 
 
 If, therefore, a convex lens be made of crown glass, we assume 
 that it is possible to calculate for a concave lens of flint a focal 
 distance such that, when it is applied to the convex lens, it recomposes 
 the white light dispersed by the latter without altering its converging 
 properties. A lens thus corrected is called "Achromatic." 
 
 In physics, achromatism is demonstrated by the use of two prisms : 
 one of flint and another of crown glass. These prisms, in order to have 
 the same dispersive power, should have their angles unequal, since with 
 their angles equal the flint is more refractive and more dispersive ; by 
 making a ray of white light penetrate a prism of flint glass, and 
 receiving it after dispersion through a prism of crown glass, it will be 
 refracted while reconstructing the white light. 
 
 Achromatism is one of the most delicate branches of optics ; it is 
 an essential property for lenses of a good microscope. Although lenses 
 which are made to achromatize one another could be placed in free 
 juxta-position opticians prefer to cement them together with Canada 
 balsam, which prevents the dust from penetrating them and unites 
 them into one single achromatic system. 
 
 Simple Microscope. 
 
 The simple microscope most employed is the magnifying glass, either 
 bi-convex or plano-convex lens, for the very simple theory of which 
 we refer back to what we have said about the formation of images 
 by these lenses. 
 
 But the defects, spherical aberration and aberration of refrangibility, 
 which are very pronounced in this microscope, were soon recognised 
 by the scientific men who employed them ; they abandoned it for 
 high powers, and sought a more perfect instrument which answered 
 better to their requirements. At that time Wollaston invented the 
 doublet. This instrument is, as its name indicates, 
 composed of two lenses which act like a single one. 
 
 These two glasses are plano-convex, set in a 
 single mount and separated by a diaphragm. 
 
 The convexity of each is turned towards the Fig> 27 * 
 
 eye, and the larger lens is nearer the object. 
 
COMPOUND MICROSCOPE. 
 
 19 
 
 Beside achromatism and the diminution of spherical aberration 
 obtained by using a diaphragm, the principal \ 
 advantage which doublets offer, is that, as two Z^Htt^Z 
 glasses are employed, the focal distance of the B 
 lower one can be increased so that the object can Fig. 28. 
 
 be moved farther away, which is a very important consideration in 
 microscopical dissections, where it is necessary to have between the lens 
 and the object to be dissected sufficient room to use the hands and 
 manage the dissecting instruments. The doublets most in use magnify 
 from 5 to 100 times. 
 
 Compound Microscope. 
 
 The compound microscope, reduced to its simple form, consists of 
 two glasses mounted in a tube ; one which is called the objective is 
 turned towards the object, and the other, the ocular, turned towards the 
 observer. Let a microscope (fig. 29. next page) consist of the 
 objective A A' and the ocular B B', and let us suppose that it is 
 desired to observe the object C D. What will be the course of the 
 luminous rays ? First of all place the object C D a little beyond the 
 principal focus of the lens A A'. 
 
 Draw the secondary axes to the points C and D. According to the 
 rules of refraction, an enlarged image C D' of the object C D will 
 be formed, real and inverted. If now we calculate the distance 
 between the ocular and the objective, so that the image C D' is formed 
 between the lens B B' and jts principal focus, the rays emitted by 
 this image will be refracted again in B B' and will form at C" D" 
 a virtual image amplified and erect with respect to the image, but 
 inverted with respect to the object. 
 
 This then undergoes two magnifications However, it is not 
 sufficient that the real image C D' should be formed anywhere 
 between the ocular and its principal focus ; but it must be formed 
 at a point, such that when magnified and rendered virtual, it is to 
 be found at such a distance as to be distinctly seen by the observer. 
 This distance varies with different individuals. It is somewhere about 
 25 centimetres (g*j 5 inches), a little more for long-sighted people 
 (presbytes)', and less for short-sighted people (myopes). 
 
 Either the ocular alone, or the entire microscope, must therefore be 
 capable of being removed so as to modify the position of the image 
 according to the condition of the observer's eyes. 
 
 C2 
 
JO 
 
 COMPOUND 
 
 MICROSCOPE. 
 
 The microscopes in 
 general use are not so 
 simple as the one we 
 have just described. In 
 the first place the objective 
 is formed most frequently 
 of several lenses, causing 
 an increased effect which 
 allows their distance from 
 the object to be greater, 
 and also gives them other 
 very useful qualities. In 
 the next place between the 
 objective and ocular, a lens, 
 called a field glass,is inter- 
 posed, which constitutes an 
 important improvement. 
 
 In the first place it is 
 sufficient by itself to 
 achromatize the rays dis- 
 persed by the objective, 
 next it collects all the 
 rays and draws them 
 towards the centre of the 
 Fig. 29. ocular, and this diminishes 
 
 the spherical aberration. 
 The ordinary compound microscope is therefore arranged as follows : — ■ 
 The rays on emerging from the objective are collected by the field 
 glass and form in the middle of the ocular tube a first real image, 
 magnified and inverted. This image is again magnified by passing 
 through the ocular lens, which arrives at the observer's eye, and is sent back 
 so as to form a virtual image at a certain distance above the objective. 
 
 The whole system is moveable and can be displaced with reference 
 to the object, so as to be able to see the image distinctly. 
 
 The second magnification adds no detail to the image and obscures 
 it considerably if the ocular has strong amplifying power. In 
 constructing microscopes, the aim of the manufacturer should be to give 
 to their objectives the greatest possible magnifying power they can and 
 only to employ medium oculars, especially as the object can always 
 be strongly illuminated, while in the case of the image it is impossible. 
 
THEORY 
 
 CHAPTER II 
 PROFESSOR ABBE'S 
 OF MICROSCOPIC 
 
 VISION, 
 
 And its Results in connection with Practical Microscopy. (' j 
 
 Up to the present we have spoken of light which emanates 
 from a body, as though the theory of Newton were still admissible, 
 that is to say, as if it were still believed that a luminous beam is 
 made up of actual physical rays, and that the meeting of such rays 
 at a given point always produces a light, whose intensity is 
 proportional to the quantity of the rays. 
 
 But it has been known for some time that this idea is untenable. 
 The facts which we know through the studies of Grimaldi, Hooke, 
 Young, Fresnel, and many other scientists, have given rise to another 
 conception of the nature of light, namely (the hypothesis of 
 Huyghens and Fresnel) that the radiation of light is produced by an 
 undulatory motion of the ether. In homogeneous bodies, such as 
 air, water, and glass, these undulations are propagated with the same 
 velocity in all directions. The luminous rays have no real existence, 
 considered as physical straight lines. Only cones of luminous waves more 
 or less narrow (acute, if the word be preferred) are produced when the 
 development of a uniform system of waves is interrupted by means of 
 a screen pierced with holes. In the case of a sufficiently obtuse cone, 
 moreover, the luminous rays can also be regarded as a mathematical 
 abstraction, that is to say, as the normals to the wave surfaces. 
 
 But each point on a wave's surface should be regarded as being 
 itself, in its turn, the centre of an undulatory movement, such that the 
 
 ( 1 ) Professor Abbe's theory of microscopic vision, which is summarized in this chapter, 
 was first published in 1873. It is the practical application of this theory which has given 
 to the construction of the microscope its present phase, and which has realized in a few 
 years a progress beyond anything that what could have been hoped for. 
 
 We may add that we have gathered the principal elements of this chapter from our 
 conversations and correspondence with the learned Professor, and with his skilful assistant, 
 Dr. S. Czapski. Professor Abbe has been good enough to give us permission to 
 announce that the following pages reflect faithfully his ideas on microscopic vision. 
 
22 
 
 THEORY OF MICROSCOPIC VISION. 
 
 effect of a luminous wave, whether entire, or more or less limited, is 
 always made up of the sum of the effects of all the parts of the 
 wave conformably to the principles of interference. 
 
 Every point on the same wave surface oscillates in the same time, 
 and in the same way ; they are in that way distinguished from 
 independent luminous centres, which are distributed over a surface, and 
 whose phases are entirely independent the one of the other. 
 
 Let us endeavour to make what we have just mentioned more 
 comprehensible by applying the preceding data to a special case. 
 
 If at O (fig. 30) we have a point luminous of itself, and if in 
 its neighbourhood we have a system of lenses L, it is in the first place 
 evident that we have only to consider the part of the luminous 
 waves emanating from O, which is enclosed in the cone O A B, that 
 is to say that part which has as base the nearest surface of the 
 lenses, and as apex the luminous point O itself. 
 
 If the system of lenses in question has the 
 property of concentrating at another point, the 
 luminous motion which proceeds from O, then 
 according to the undulatory theory, the system of 
 lenses is said to transform the spherical waves 
 proceeding from O, and concentric with respect to 
 O, into other spherical waves, whose common 
 centre is a point O' towards which they contract 
 (converge being the usual expression) if it is a 
 real focus ; or whence they appear to emerge 
 if it is a virtual focus. 
 
 Let us now assume the case of convergence of 
 which we ' have just spoken, and let us see 
 whether, under such conditions, the point O is 
 actually reproduced at a point O'. Let us 
 observe moreover upon what the size of this point, 
 greater or less, depends. A little reflection suffices 
 to make it clear, that if any one of the waves 
 Fl S- 3°- which contract towards O' be considered; (for 
 
 example R V S, whose apex touches the posterior surface of the lens), 
 then the elementary waves (which can be regarded, according to 
 Fresnel and Huygens, as emanating from the different points on this), these 
 elementary waves, .we repeat, all arrive at a point O', and in exactly 
 the same phase, that is to say, in the same undulatory condition. 
 
THEORY OF MICROSCOPIC VISION. 
 
 23 
 
 In fact, the courses taken by the elementary waves from each of 
 the points of a spherical wave, towards the centre of .the latter, are 
 all equal, because they are the radii of a sphere. 
 
 It is thus evident that the effects of all these waves ought to be 
 added to the point O', and that it is at this point that the greatest 
 possible luminous effect is produced. 
 
 The effect is not, however, absolutely nil at a point in the 
 neighbourhood of O'. Consequently at a like point (O',) the courses 
 taken by the elementary waves are not equal ; thus these waves arrive 
 there in slightly different phases, and so it is impossible that the 
 luminous effect at 0\ should be as strong as at O'. 
 
 But it will only be at a certain distance from O' that the 
 difference of phase will be sufficiently great, in order that the waves 
 should mutually destroy one another, and consequently produce 
 darkness. As far as this distance the brightness will gradually 
 diminish. 
 
 It follows from the preceding consideration that the image of a 
 point O produced by any system of lenses can * never be a point, 
 but will always be a disc, whose brightness gradually decreases from 
 the centre to its edge. 
 
 In continuing our research in this order of ideas, we shall 
 consequently see that there ought always to be a second distance 
 from O', up to which the luminous action of the elementary waves 
 begins again to increase : and that around the disc in question, at first a 
 dark ring, and then another luminous ring should be produced, and 
 that this phenomenon should be repeated several times. 
 
 But the brightness of the first ring, and still more that of the 
 outer rings, is so very small compared with that of the centre of the 
 disc that it need only be taken into account in very rare circumstances. 
 
 Thus we shall be in a position to give an exact account of the 
 circumstances on which the size of the little disc, which forms the 
 image of a point, depends ; that is to say, the circumstances which 
 determine the resolving power of a system of lenses. 
 
 But it is of very little use to pursue further here these 
 considerations, which are very important in the theory of astronomical 
 instruments, and which have been likewise studied by Helmholtz, in 
 the manner indicated above, from a microscopical point of view. 
 
 In fact, contrary to what takes place with celestial objects, those 
 which form the subject of microscopical study are not (at least for 
 the greater part) luminous themselves. Thus there do not exist any 
 
-4 
 
 THEORY OF MICROSCOPIC VISION. 
 
 independent points from which the spherical waves proceed, as we have 
 assumed above. 
 
 Objects studied with the microscope are most often illuminated indirectly 
 by another source of light, with the aid of mirrors and lenses, and the 
 illumination is generally effected by taking advantage of its transparency. 
 
 This circumstance completely alters the conditions of the formation 
 of their image. 
 
 Let us now see then how the image is produced under these 
 new conditions. 
 
 If opposite the lens L (fig. 31) there be a point O, which is not 
 luminous of itself, but which only transmits rays emitted by the luminous 
 source, M, underneath then it will be impossible to obtain by the geometrical 
 re-union of these rays an image which corresponds with the physical image. 
 
 The reason of this impossibility is that the essential condition, on 
 which the concentration at O depends, is not realised, and this 
 essential condition is the identity of the vibrations propagating themselves 
 equally in all directions. 
 
 How then is it possible to form the image of 
 an object composed of points themselves non- 
 luminous ? This is what we are going to enquire 
 into. 
 
 Let an object O (fig. 32) be composed of 
 transparent and non-transparent elements in juxta 
 position. Each of the points of the luminous 
 source M sends out a spherical wave, which 
 eventually strikes the object, and the undulatory 
 theory of light tells us that secondary undulations 
 are then formed, which proceed from all the 
 transparent points of the object. These undula- 
 tions start from different points in the same phase, 
 or in phases which only differ by the slight difference of distance 
 traversed from M to O, to Oi, and to O2. 
 
 These secondary undulations differ then from the primary undulations, 
 which proceed from different points of a luminous object, in the fact that 
 they are capable of interfering with one another, which is not the case 
 with waves proceeding from different points of an object itself luminous. 
 
 Immediately behind the object these secondary undulations produce 
 interferences, which increase the intensity of the light in certain 
 directions, and diminish it, or even annihilate it in other directions. 
 
THEORY OK MICROSCOPIC VISION. 
 
 25 
 
 interference effects are spoken of as diffraction of the object. 
 
 The character of this diffraction 
 depends entirely upon the constitu- 
 tion of the object, that is to say, 
 its size, its shape, its substance, and 
 the arrangement of the elements 
 composing it. 
 
 But as the objective receives no 
 light, other than the oscillatory 
 motion, which is produced by the 
 diffraction of the object ; it follows 
 that all that appears in the micro- 
 scope ought to depend upon this 
 diffracted light, and consequently the 
 diffraction which an object produces is 
 the sole effect an object has on its 
 image. 
 
 The effects of diffraction are 
 observed in a characteristic way 
 when an object is examined having 
 regular structure, e.g., striae or 
 elements arranged at equal distances, 
 whether in a rectangular series, or 
 in a series increasing by any angle. 
 In all these cases the light is 
 diffracted in a way which produces 
 a series of maxima and minima 
 separated and disposed, either in the 
 shape of a fan or bundle (fig. 33). 
 
 The objective 
 projects these 
 divergent maxima 
 and minima 
 emitted by the 
 object in M'M', the 
 focal plane con- 
 jugate to the 
 plane of the 
 luminous source 
 Fig. 33- M - 
 
 Fig. 32. 
 
2 6 
 
 THEORY OF MICROSCOPIC VISION. 
 
 These maxima and minima can be observed by removing the 
 ocular, and looking down the tube of the microscope ; a transverse 
 section of their different beams is then seen. 
 
 The luminous effect produced by the undulations, of which we 
 have spoken above, and which are transmitted by the objective to the 
 plane, conjugate to the object, is the image of the object. This image 
 can be mathematically constructed in accordance with the theoretical 
 views of undulatory optics as an effect of interference of the diffracted rays. 
 
 Hitherto we have considered the effect which an isolated 
 luminous point, emitting monochromatic light, produces. It still remains 
 for us to examine in the first place the effect of white or composite light, 
 and then the action of an extended luminous source, or one composed of 
 numerous points. Let us, to begin with, take the first of these points. 
 
 It is known that the wave length differs for each of the colours 
 of the spectrum. It follows from this that the effect of diffraction 
 produced by the same object for these different colours differs simply 
 in the space, more or less considerable between the corresponding 
 maxima or minima. This difference in the diffraction spectrum is 
 compensated in the formation of the image, produced by interferences, 
 by the difference of wave length, and this in such a way that the 
 final result is the same for all the colours, and consequently the 
 image itself, may perhaps be altogether achromatic ( 1 ). 
 
 We now come to the second of the points, which yet remains to be 
 examined. 
 
 Every extended luminous source may be considered as formed of 
 a number of isolated points, which act independently of one another, 
 and consequently the final result is a simple superposition of the 
 
 ( 1 ) A few supplementary details will make what we have just stated more comprehensible. 
 The diffraction image (that is to say, the section of diffracted beams behind the objective) 
 is in its dimensions simply proportional to the wave lengths. 
 
 If, for example, the object 
 consists of a simple grating 
 of lines, then behind the objec- 
 tive isolated maxima are seen, 
 and in consequence of the 
 difference in the wave lengths, 
 those of red colour are more 
 distant from one another and 
 from the middle of the field 
 than those of blue colour ; 
 tig. 34 and 35. t hj s j s w hy the course, which 
 
 the blue of the spectrum, 1 has to take to the point of the image O', is shorter than the 
 course of the red spectrum i to the same point. And the one exceeds the other by such a 
 quantity that on each side of these courses a similar number of waves of each species of 
 light is produced, so that both arrive at the point O' in the same time, and in the same 
 phase. The same applies to other spectra. If then the interference of all the red spectra 
 produce at O' a red luminous point, then all the blue spectra equally form a blue luminous 
 point, and so on. If on the contrary all the red waves mutually destroy one another by 
 interference, then the same applies to the blue, green waves, &c. 
 
 t;on verficftte • 
 
THEORY OF MICROSCOPIC VISION. 
 
 27 
 
 elementary images produced by these different luminous points in the 
 way described above. We have just passed in review a statement of the 
 points of Professor Abbe's theory ; it now remains for us to examine the 
 principal consequences, which result from this theory. 
 
 General Results. 
 
 1. The image of an object is not a geometrical projection which 
 produces it point for point, as would be the case if the image was 
 formed according to the laws of geometrical optics by the reunion of 
 luminous rays, which diverge from different points of the object. 
 
 The image has no connection with the object, except by that 
 part of the diffracted rays which penetrates the objective. It 
 follows from this that there exists no absolute likeness between the 
 object and its image, for this likeness depends on special conditions, 
 which we shall speak of further on, and when these conditions are 
 not fulfilled every degree of dissimilarity can be produced. 
 
 The same object always gives different images if different parts of 
 the diffracted beam enter the objective of the microscope. 
 
 Different objects can give identical images when only the parts of 
 their diffraction spectra, which are identical in each of them, penetrate 
 the objective, while the dissimilar parts of the spectra are excluded 
 e.g., by diaphragms placed at the back of the objective. 
 
 The two following assertions, which were made for the first time in 
 Dr. Abbe's work, published in 1873, nave been proved by the help 
 of experiments made with gratings (■). 
 
 2. Mathematical analysis, based on the undulatory theory, determines 
 the conditions for similarity and dissimilarity between the image and 
 the object, and enunciates them as follows : — 
 
 (a) The image is ahvays exactly like the object, as if it were a 
 geometrical projection point for point, when the whole of the diffracted 
 pencil (which the object produces by the rays which traverse it) 
 penetrate the objective of the microscope, and when consequently no 
 part of the pencil of sensible intensity is lost. 
 
 (l>) When, on the other hand, the whole of the diffracted light 
 is not received by the objective, then the image differs from a 
 geometrical projection the more according as the lost part of the 
 diffracted beam is more considerable, that is to say, the greater the 
 
 ( 2 ) We give in the following chapter an article where these experiments are described 
 at length by Mr. J. W. Stephenson, after Professor Abbe. However, we have been 
 obliged to ap] end to the above mentioned article modifications, consequent on the new 
 ideas entertained by Professor Abbe, and which have not hitherto been published. 
 
28 
 
 THEORY OF MICROSCOPIC VISION. 
 
 number of the diffracted rays excluded, and the greater the intensity 
 of these excluded rays, the more will the image differ from a 
 projection of the object. 
 
 In this last case the microscopic images appear like an exact image of 
 another object (which may really exist, or be capable of being artifically con- 
 structed, or may be imaginary), which will be constituted in such a manner 
 that its complete diffraction spectra will be identical with that part of the 
 spectra of the observed object, which alone is admitted into the objective. 
 
 3. From these two preceding statements result other consequences 
 relating to the mode of action of the microscope, when account is 
 taken of the deductions that can be drawn from optics upon the way in 
 which different objects are diffracted. 
 
 The greater the dimensions of the elements of which an object is 
 composed, the smaller is the angular area, over which the diffraction effects 
 of this object extend. The smaller the elements become, the greater 
 the angular extent of the diffraction spectrum becomes. 
 
 If the dimensions of the elements are large multiples of the 
 wave lengths of the light, then the intensity of the diffracted light 
 disappears at a very short angular distance from the incident beam. 
 
 Consequently from each incident ray there only proceeds one erect 
 diffracted beam, whose angular amplitude is only a few degrees, of 
 even only a fraction of a degree. 
 
 If on the contrary the dimensions of the elements or of some of 
 the elements of an object are very small multiples of the wave 
 lengths, or are even smaller than that, then the diffracted light of 
 noticeable or even considerable intensity extends a very great distance 
 from each incident ray. 
 
 As the wave length of a light of the same colour, entering different 
 media, is inversely proportional to the indices of refraction of their 
 media, it thus follows that one and the same object produces in a 
 dense medium, (as for example, glass or oil), a diffraction spectrum 
 proportionally less extended than in a medium of rare density (as 
 for example in air). 
 
 When we are concerned with objects composed of very small 
 elements, as for example, diatoms with fine striation, then the total 
 diffraction spectrum can only be produced in a medium having a 
 very great index of refraction, with a proportionally diminished 
 wave length, while in air, and even in water, and more refractive 
 substances, a more or less considerable central part of the diffraction 
 spectrum then occupies an angular extension of 180 0 
 
THEORY OF MICROSCOPIC VISION. 
 
 29 
 
 The result of all the preceding is as follows : — 
 
 Objects whose parts are large compared with the wave length of the 
 light, or even the parts of objects 'whose elements are considerable, 
 will always produce an image, which will be perfectly like them, even 
 when objectives of small aperture are employed, and that because the 
 diffraction beams of very small angular extension which they produce, can 
 always completely penetrate the objective. With objects of this kind, 
 which are comparatively coarse, the action of the microscope gives a final 
 result, which entirely obeys the rules of geometrical optics, provided 
 that the physical conditions are fulfilled. 
 
 On the other hand, with objects which produce a large angular 
 expansion of diffracted light, a part of the diffracted beam is lost, 
 and this loss is greater according as the angle of aperture is 
 smaller. The lost part, with axial illumination, belongs to the peri- 
 pherical part of the beam ; with oblique light it is an eccentric part 
 which is lost. 
 
 The smaller the elements of an object become, the greater should be 
 the angle of aperture (calculated for a given medium) tiecessary to 
 utilize the whole or partial diffraction spectrum of the structure, and 
 consequently to obtain an image of exact or approximate similarity. 
 
 The same angle of aperture, in a highly refractive medium, embraces 
 a more extensive diffraction spectrum, and one which corresponds to a 
 shorter wave length of the denser medium. 
 
 In fact, the angles of aperture for different media (having regard 
 to the more or less complete absorption of the diffraction beams) 
 vary as the sines of the half-angles, multiplied by the indices 
 of refraction of the media, for which these angles of aperture are 
 of service. These have then to each other the same ratio as 
 numerical apertures. 
 
 Consequently, numerical aperture is the measure of the power which 
 an objective possesses of showing more or less faithfully the elements of an 
 object up to a given degree of minuteness. 
 
 Special Results relating to the Formation of the Images 
 of Objects possessing a periodic structure. 
 
 In objects of irregular structure, the diffraction spectrum extends 
 irregularly in the direction of each incident ray. On the other hand, 
 with objects composed of a considerable number of identical elements 
 
3° 
 
 THEORY OF MICROSCOPIC VISION. 
 
 disposed in regular series the spectrum assumes a regular form 
 corresponding to them. Here each of the incident rays, each colour 
 being considered separately, divides itself into a number of isolated 
 rays (maxima of light, which are separated by dark spaces), which 
 dispose themselves regularly about the axial ray. 
 
 In simple periodic structures (striae at regular intervals), the rays 
 form a plane fan of intensity, gradually decreasing on each side to 
 zero : in double periodic structures (with elements disposed in lattice- 
 work form), they form a beam, with a section in the form of a 
 cross, in which the isolated rays are arranged in series in two 
 directions. As regards the formation of the images of similar objects, 
 the following points are ascertained : — ■ 
 
 1. The structure of the object is not represented, that is to say 
 that the lines, or the points ("beads") are not resolved, when only a 
 single one of the rays of the diffraction beam, to which reference 
 has been made above, enters the objective. To obtain the image of 
 a striation, at least ttvo diffracted rays must enter the objective, there 
 being included in them the direct continuation of the incident ray 
 which represents the central maximum of the diffraction spectrum : for 
 example, there must therefore enter the objective, this direct ray plus 
 a diffracted ray. 
 
 To obtain the image of a double periodic structure, the objective 
 must receive at least three rays belonging to different lines of the 
 spectrum. 
 
 2. As the angular distance of the successive rays of the diffraction 
 spectrum, measured by the sines of the angle, is greater according as 
 the striae (or series of elements) are closer together in the object, it 
 follows from this that each magnitude of the interval between the 
 striae necessitate a corresponding magnitude of the angle of aperture 
 of the objective, in order that this latter may be able to receive two 
 rays at the same time. 
 
 The magnitude of the angle of aperture necessary for the above differs 
 according to the direction of the incident ray. That is to say, 
 according as the ray passes through the centre of aperture, or passes 
 through the edge, as is the case in oblique illumination. In this 
 second case, the nearest diffracted ray can be twice as remote from 
 the direct ray as in axial illumination. Here then is the reason why 
 the same objective shows (resolves) with oblique illumination closer 
 striae than could be done in axial illumination. 
 
THEORY OF MICROSCOPIC VISION. 
 
 31 
 
 3. The mathematical determination of the statements enumerated in 
 the last paragraph (2) leads to a numerical formula for the separating 
 power (resolving power) of a microscope : — ■ 
 
 If the numerical aperture of an objective is represented by a, the 
 wave length of the light employed in the observation (this wave length 
 being calculated for air) by A, then we shall have for 8, which is 
 the shortest distance of an object which can be distinguished by this 
 
 objective for purely axial illumination, 8 = —; for the most oblique 
 
 A 
 
 illumination possible, 8 = J — . 
 
CHAPTER III 
 EXPERIMENTS ON THE APPLICATION OF 
 Dr. ABBE'S 
 
 THEORY OF MICROSCOPIC VISION, 
 
 By J. W. STEPHENSON. ( 1 ) 
 
 In my opinion the very important theory of microscopic vision, 
 which has been enunciated by Professor Abbe, has not received in 
 this country the attention it pre-eminently deserves. This theory 
 asserts that the microscopic images produced by certain objects of 
 minute detail, such as diatoms, scales of insects, and other things, 
 are not simply dioptrical images, such as the mere outline of an 
 object, but are the result in most cases of the combination, or fusion 
 together of the central pencil, with certain secondary images produced 
 by the interference of those pencils of light, into which by diffraction 
 the incident ray of light is, in passing through the object itself, 
 decomposed. In other words, the theory asserts that the principal, 
 or central beam of light alone is not sufficient truly to depict fine 
 lines, small apertures, or other minute structural details, but that as 
 far as resolution is concerned, two or more pencils are always necessary 
 to produce the desired effect. These pencils may, or may not, 
 include the principal or dioptric beam, but where the latter is excluded, 
 the image necessarily appears on a dark field. 
 
 Further, the contention of this theory is that when, from any cause 
 whatever (whether from angles formed by the intersection of lines, or 
 the closeness of the lines themselves, whether from the aperture of 
 the object glass, or when by artificial means the diffraction images 
 as seen within the body of the microscope are made similar), the 
 microscopic images themselves will be identical. 
 
 (') This work was presented to the Royal Microscopical Society of London, in 
 January, 1877, by our esteemed correspondent, Mr. F. W. Stephenson. As we have 
 already stated, we have added modifications, which the recent views of Professor Abbe 
 necessitate. 
 
EXPERIMENTS ON THE APPLICATION OF DR. ABBE'S THEORY. 
 
 33 
 
 The diffraction images of a lined object in focus on the stage of 
 the microscope may readily be seen by removing the eye-piece, and 
 looking down the tube of the instrument. Here, with the light 
 central, and the lines on the object parallel, the coloured spectra are 
 distinctly visible, going off on either side at right angles to the 
 direction of the striae, the most refrangible rays being next to the central 
 beam of light. The latter fact is particularly mentioned, as it has 
 an important bearing on the limits of visibility, and on the photo- 
 graphic reproduction of microscopic objects. 
 
 Professor Abbe has supported his views by some very striking 
 experiments, which appear to me to be a complete practical 
 demonstration of the truth of his mathematical deductions. 
 
 I shall describe four or five of those experiments, which impress 
 me as being the most important, and, therefore, the most interesting. 
 
 First Experiment. — The purport of the first experiment is to 
 illustrate the production of identical microscopic images by different 
 structures, when by artificial means the diffraction pencils arising 
 
 therefrom are made similar in number and 
 position, within the tube of the instrument, 
 as previously mentioned. This experiment 
 is made on a grating (') formed of 
 alternately long and short parallel lines 
 (fig. 36) ruled with a diamond through a 
 F 'g- 3 6 - film of silver of extreme tenuity, deposited 
 
 On the left : Grating used in the on the underside of a thin cover-glass, 
 
 first experiment. — On the right : . 
 
 Appearance presented upon removing and subsequently cemented with balsam 
 
 the eye-piece, showing central and ,. , ... ,. 
 
 spectral images. to an ordinary glass slip, the coarser lines 
 
 being about 1790 to an inch (71 per millimetre), and the fine about 
 3580 (142 per millimetre), that is to say, twice as close together. 
 
 This grating gives rise to two sets of diffraction spectra, when 
 placed beneath the objective in focus in the middle of the field ; 
 the set arising from the wider portion being exactly half the distance 
 apart of that arising from the narrower, such distances between the 
 
 ( 1 ) The gratings necessary for these experiments are provided by the firm, Carl Zeiss 
 of Jena, under the name of diffraction plate, and at a price of 15 frs. The plate is 
 sold with an adapter, having a lateral grove, in which to slide a series of diaphragms, 
 with apertures of different shapes. The adapter is screwed to the microscope by its 
 upper part, and to its lower part is screwed an objective, having a given aperture, e.g., 
 the " a a " of Zeiss, or another of the same focus and the same aperture. 
 
 These precautions are indispensable for the success of the experiments, the size of the 
 diaphragms having been calculated for the above-mentioned objective. 
 
 D 
 
34 EXPERIMENTS ON THE APPLICATION OF DR. ABBE'S THEORY. 
 
 spectra being inversely proportional to the distances between the lines 
 themselves ( 1 ). 
 
 Upon removing the eye-piece, these two rows of spectra (fig. 36) 
 are visible, one above the other, as the eye is brought to see 
 successively the air images at the upper end of the tube. 
 
 It is obvious, from fig. 36, that the wider grating gives spectra 
 exactly one-half the distance apart, and therefore twice as numerous as 
 those arising from the narrower, that the latter may be made to 
 coincide with the former, in number and position (as required) by 
 stopping out every alternate ray from the wider grating. 
 
 This is readily accomplished by 
 placing a stop close to the back com- 
 bination of the objective, so constructed that 
 a central slit will admit the central ray 
 only, whilst another slit on each side 
 will admit only the second spectrum of 
 F ' 8 ' 37 ' the wider, and the first spectrum of the 
 
 On the left : Diaphragm with three 
 
 slits, shutting out certain spectral narrower grating (the spectra are cut off 
 
 images, and making those produced by 
 
 the two parts of the grating, identical.— from the centre) (fig. 31). 
 
 On the right : Appearance obtained . ... 
 
 with the diaphragm. The fine lines Upon replacing the eye-piece, It Will 
 
 are in normal condition, the coarse , , , , ,, . . 
 
 ones are doubled in number. now be seen that the microscopic image 
 
 of the narrower lines remains unaltered, but that the wider lines have 
 doubled in number (fig 38) by an apparent prolongation of the shorter 
 
 lines between them, making the two 
 images identical, the lines in the upper 
 part being distinguishable only from those 
 in the lower by somewhat less brightness, 
 which simply arises from the smaller 
 number of real lines, through which the 
 Fig. 38. light can pass. 
 
 On the left : Diaphragm excluding Again, by stopping out all the 
 
 all the spectra of the field (fig. 36), 1 v 5 
 
 excepting the fourth of the coarse and spectra, except the fourth of the wider, 
 
 the second of the fine grating. On the 
 
 right : Effect produced by this dia- and the second of the narrower part 
 
 diaphragm : the coarse lines are quad- , , c r o\ di- 
 
 mpled, and the fine lines doubled in (by the diaphragm of fig. 38), the dlS- 
 
 number. position of the spectra is again rendered 
 
 ( 1 ) We need not mention that this is the fundamental principle of the phenomenon 
 of gratings. 
 
 When a luminous point is looked at in a dark room, through a plate ruled with fine, 
 parallel striae, which are very close together and alternately opaque and transparent, 
 instead of seeing a single spot, a series of images of this spot is seen ranging along a 
 
EXPERIMENTS ON THE APPLICATION OF DR. ABBE'S THEORY. 
 
 35 
 
 similar (they coincide in number and position), and the microscopic 
 images, though changed, will be still found to be identical (fig. 38) by 
 the doubling of the narrower, and quadrupling of the coarser lines ( 2 ). 
 
 01 [ I I I 1 Second Experiment. The object of this 
 experiment is to shew that unless one 
 spectrum at least is admitted the objective will 
 have no power to resolve the lines. 
 The same grating (No. 1, fig. 36) is 
 
 used as in the preceding experiment with a 
 diaphragm having a single central slit, so 
 
 Fig. 39- 
 
 On the left : Diaphragm excluding 
 all the spectral rays from the fine part 
 
 of the grating i.'and all the spectral rays adjusted (parallel to the lines of the object) 
 
 except the two adjacent to the central 
 
 ray in the course part.— On the right : that one spectrum only will be admitted on 
 
 Effect produced : the coarse lines only are , ., . 
 
 visible, the narrow lines are invisible, each side from the coarser grating and 
 none whatever from the finer (fig. 39). 
 
 Whilst looking into the microscope it is seen that by the reduction 
 of the aperture, the fine lines (all the spectral images of which have been 
 excluded) have disappeared and are replaced by a uniform surface of silver; 
 the coarse lines remain in their normal condition as the theory indicates. 
 
 Third Experiment. — The object of this experiment, like the first, 
 is to illustrate the necessity of an angular aperture sufficient to admit 
 
 1 some spectra Irays. The diaphragm has 
 
 a central slit only i-3oth of an inch in size 
 (fig. 40) which is sufficient to exclude the 
 spectral rays of the finer as well as of the 
 courser lines. 
 
 Fig. 40. 
 
 The examination in the microscope 
 
 On the left: Diaphragm excluding shows that even as far as lines 1780 
 
 the. cnp^trol rn\rc On tVio rirrKt • 
 
 all the spectral rays. — On the right: , 
 
 Effect produced : no line is visible. to the inch all resolving power 
 
 has 
 
 line, perpendicular to the direction of the strios of the grating displaying spectra. These 
 are diffraction spectra. 
 
 The more numerous are the striae of the grating in a given space, and consequently 
 the more compact they are, the more dispersed and visible are the spectral images of the 
 luminous point ; the wider apart the striae are, the more compact are the spectral 
 images, i.e., the distance between the spectral images is inversely proportional to the 
 distance between the striae. J. P. 
 
 ( 2 ) This appearance follows from the principle above ; since the spectra which reach the 
 eye through the diaphragm are put at quadruple the distance in the coarse part of the 
 grating they represent the image of lines situated at a distance four times as small ; those of 
 the fine part being doubled in their distance represents to the eye the image of lines 
 situated at distances twice as small. 
 
 To see this appearance distinctly with the "a a'' objective of Zeiss (i^in.) which Dr. Abbe 
 uses, it is necessary to employ a strong ocular No. 5 of Zeiss (No. 5 of Hartnack and 
 Prazmowski, E of the English opticians). 
 
 Every other objective of 1 to 2 inches focus would similarly succeed with a suitable 
 diaphragm. That which is employed with the "aa" objective of Zeiss has three slits, each 
 i-aoth of an inch in size, and having the same distance of i-2oth of an inch between them. 
 
 D2 
 
36 EXPERIMENTS ON THE APPLICATION OF DR. ABBE'S THEORY. 
 
 departed ; the double grating is replaced by a nearly uniform silver 
 band without any trace of lines. 
 
 Only inequalities exist in the outline of the lines which grow visible, 
 and help to render the lines apparent. 
 
 In all these experiments, where a diaphragm, perforated with a slit, has 
 been employed, it will have been observed that the sides of the slit are 
 parallel to the direction of the lines, but it will be found that if the 
 diaphragm is turned so that the slit is perpendicular to the direction of 
 the lines, all the spectra will be admitted, and perfect definition will 
 result, proving that the position of the diaphragm relative to the strife 
 and not its form alone, produces the phenomena in question. 
 
 The same effects of duplication or obliteration of lines may be produced 
 on an object such as Lepisma saccharina, by using higher powers and 
 suitable diaphragms. 
 
 The limit of visability is a direct con- 
 sequence of the demonstration of the fact 
 that no resolution can be effected unless at 
 least two rays are admitted. And as the 
 admission of a secondary or spectral image is 
 Fig. 41. absolutely dependent on the numerical 
 
 On the left : Effect produced by - , , . . f „ t , , 
 
 light of extreme obliquity on parallel aperture of the objective, it follows that 
 
 "he S»E^i!^2** resolvi "g P° wer is ^function of the 
 the edge of the field and only the more ape iture. This aperture has its superior 
 
 refrangible rays of the spectral image 11 r 
 
 remains in the field at the opposite limit 180 0 . When the limit of resolving 
 
 edge (to the right). On the right : .... 
 
 Appearance produced in the tube by a power with oblique light is reached, the 
 
 valve of Pleurosivma Angulatum . .. . . 
 
 (central light). illuminating ray is seen at the extreme end 
 
 of the back lens with the spectral image on the opposite edge, as in the 
 field (fig. 41). 
 
 The rule given by Prof. Abbe for determining the greatest number of 
 lines per inch which can be resolved by oblique light (taking any given 
 colour as a basis) shows that this number is equal to tzvice the number 
 of light waves in an inch multiplied by the sine of half the angle of 
 aperture. 
 
 As the sine of an angle can never exceed unity, the maximum of 
 the above grating will be equal to twice the number of waves contained 
 in an inch for the ray of greatest refrangibility which will afford 
 sufficient light for this experiment. 
 
 With central light the maximum for a given colour will be equal to 
 the number of waves contained in an inch. What the colour of this 
 
EXPERIMENTS ON THE APPLICATION OF DR. ABBE'S THEORY. 
 
 light is it is generally impossible to determine, as the capacity for 
 
 appreciating light varies with different individuals. If, for example, we 
 
 take o*43 n in the spectrum as being sufficiently luminous for vision, we 
 
 find the maximum, as far as seeing it is concerned, to be 118,000 to 
 
 the inch. — But as the non-luminous chemical rays remain in the field 
 
 after the departure of the visible spectrum, a photographic image of 
 
 lines, much closer together, might be obtained. 
 
 How little is gained in resolving power by an excessive angular 
 
 aperture is seen, when it is considered how slowly the sines of large 
 
 angles increase. In making a reduction from 180° to 128J 0 in the angle 
 
 of aperture, a diminution of only i-ioth of its sine is produced, and 
 
 consequently of only i-ioth of the resolving power, with a considerable 
 
 increase in the general utility of the objective: or, if reduced to io6|°, 
 
 we have still on the same hypothesis by this a resolving power, 
 
 capable of defining 94,400 lines to the inch. 
 
 The following experiments are made 
 
 with crossed gratings, and also give 
 
 important results. 
 
 These gratings are produced by 
 
 ruling two sets of parallel lines on silver 
 
 films, the first on the underside of a 
 
 thin cover glass, the second on an 
 
 ordinary slide ; then the two pieces of 
 
 On the left : Grating crossed at S lass are cement ed together by Canada 
 60 0 .— On the right: Disposition of balsam, so that the two sets of lines are 
 
 the spectral images produced. These 
 
 images form, in the little internal in contact, forming an angle of 6o° with 
 
 circle, an arrangement like that which , , ' . . , . , . 
 
 Piewosigma gives (fig. 41, on the right), each other, which produces rhombic 
 figures on the whole surface of the grating (fig. 42). 
 
 Fourth Experiment. — The object of the first experiment, with crossed 
 gratings, is to show that, with a certain arrangement of the light, the real lines 
 can be made to disappear, and are replaced by a set of perfectly distinct 
 spurious lines, parallel to a diagonal of the rhombic figure (fig. 43). 
 
 The crossed grating, examined without the ocular, with central 
 light, gives an arrangement of the spectral images on the field (fig. 
 42, on the right), in which the inner circle of images is identical 
 with that, which Pleurosigma angulation, under similar circumstances, 
 furnishes (fig. 41, on the right). The real lines are made to disappear, 
 and a system parallel to a diagonal of a rhombus is made to appear 
 by employing a diaphragm, with a single slit in the direction of a 
 
38 
 
 EXPERIMENTS ON THE APPLICATION OF DR. ABBE S THEORY. 
 
 diagonal, and the spurious lines will appear parallel to the other 
 diagonal, that is to say, perpendicular to the slit. 
 
 ©tj^ If a diaphragm, perforated with a 
 
 Rg&W^ cross-shaped slit, as on the left of 
 ^^^^^ figure 43 admitting the central ray and 
 ^^^^ three spectral images, the imaginary 
 ^Q$&$^ lines joining these four images, two 
 and two, being perpendicular to one 
 another, two sets of parallel lines will 
 be obtained, the first longitudinal (on 
 On the left : Diaphragm, with cross- thg rig j lt 0 f figure 43), perpendicular to 
 
 transversal branch of the 
 
 shaped slit, admitting central ray, and 
 three spectral rays ; the lines joining the ^ e long 
 four images, two and two are perpendi- 
 cular to one another.— On the right : cross-shaped slit ; the second transversal 
 
 Effect produced: The lines, which appear . , , • j-„„i 
 
 cross at right angles, to the distances, perpendicular to the snort longuuainai 
 Sctra y (: " ^ * *" branch of the slit. These two sets of 
 
 spurious lines will therefore cut at right-angles, although the real lines 
 of the grating cut at 6o°. This effect, evidently, follows in accord- 
 ance with what we have said above, from the fact that two sets of 
 spectral images, parallel to the diagonals of the grating, which 
 cut at right-angles, have been admitted. 
 
 Similar results will be obtained with 
 a grating crossed at right-angles, as that 
 of figure 44, which, examined without an 
 ocular, will give the field as in figure 44, 
 on the right. 
 
 By employing a diaphragm, which 
 Fi g, 44 . only allows the central and a single 
 
 On the left : Grating crossed at spectral ray to pass (fig. 45). the two 
 right-angles -On the right: Field f , j zontal an( J vertical lines 
 
 observed in the tube. 
 
 disappear, and are 
 
 Fig. 45- 
 
 replaced by a single set, composed of 
 parallel lines, and parallel to one of the 
 diagonals of the little squares of the 
 grating (that is to say), to that diagonal 
 which is perpendicular to the slit. The 
 distance between these spurious lines is to 
 that between the real lines, as i : J 2. 
 Fifth Experiment.— The object of this 
 
 On the right : Diaphragm admit- experiment, which is perhaps the most 
 
 ting the central ray and spectral ray. . . 1 u„ 
 
 -On the left: Effect produced : a important of all, IS to ShOW tnat Dy 
 
 set of diagonal lines, perpendicular to admitt j ng mto t h e objective Only one 
 
EXPERIMENTS ON THE APPLICATION OF DR. ABBE's THEORY. 39 
 
 row of spectra, the structure of such an object as that under 
 consideration is absolutely indeterminate. 
 
 In this experiment, the slit diaphragms are entirely discarded, and 
 the crossed grating is examined with a simple circular stop, which is 
 used merely for the purpose of so reducing the angle of aperture 
 that the first row of spectra only shall be admitted. 
 
 The illumination is central, and, upon examination, without an ocular, 
 seven pencils of light are seen in the field ; the first, in the centre, and 
 which is brilliant, is the dioptric pencil; the six others, equi-distant around the 
 margin, are the spectral rays (see the inner circle drawn in the field of fig. 42.) 
 
 Let it now be clearly borne in mind that we are about to examine a 
 structure, which we know to be entirely composed of distinct rhombic markings. 
 
 On replacing the eye-piece for this purpose, we see hexagonal 
 markings over the entire field, as in Pleurosigma angulation, and this 
 effect has been produced simply by so reducing the aperture relatively 
 to the fineness of the object, that the first spectra only are admitted. 
 
 From this microscopic image, we can infer nothing as to the real 
 structure of the object under examination ; we know it to be rhombic, 
 but it appears to be hexagonal. 
 
 But the central ray, and the six coloured spectra, which have 
 produced this result, are identical in aspect with that presented by a 
 single valve of Pleurosigma angulatum, with central light. (Compare 
 the whole field, fig. 42, with the inner ring, which is there drawn). 
 
 This diatom, with central light under the highest powers, and with 
 the largest apertures, necessarily presents the same spectral appearance, 
 in consequence of the fineness of the striae, or holes (whichever they 
 may be), the dispersion being too great to admit the second row of 
 spectra, unless one of the objectives, lately constructed, and having an 
 aperture from 1*5 to i'6 is employed. 
 
 It has now been proved that, with the means employed, no definite 
 inference could be drawn of the real structure of the artificial object, 
 and it is equally certain that the demonstration will apply with equal 
 force to the valve of Pleurosigma Angulatum, the hexagonal markings 
 of which may, to use the words of Professor Abbe, arise from "two 
 sets of lines, or three sets of lines, or isolated apertures of any shape 
 in the object itself" ('). 
 
 ( 1 ) Professor Abbe admits that if optics do not permit of the determination of 
 the nature of certain delicate structures, on the other hand, the method that we have 
 adopted in our works (determination of the nature of fine structures by the study of diatom 
 valves having analogous but much coarser structures), this method, we say, of forming a 
 conclusion by analogy, is the safest means of arriving at the truth. H. V. H. 
 
40 EXPERIMENTS ON THE APPLICATION OF DR. ABBe's THEORY. 
 
 If it were possible to admit the second row of spectra, a nearer 
 approach to a knowledge of the true structure would be obtained; as 
 the larger the number of diffracted rays admitted, the greater the 
 similarity between the image and the object, the keystone of the 
 theory being that " the interference of ALL the diffracted pencils, which come 
 from the object, produces a copy of the real structure" as in a dioptrical 
 image; but it is impossible to admit all the rays, as has been abundantly 
 shown, by the great dispersive power of many fine structures. 
 
 Further illustrations of the formation of hexagonal markings may be 
 found on the same diatom. 
 
 On bringing into focus, with an objective of about "8 numerical 
 aperture, a good specimen of Pleurosigma Angulatum > flat and having 
 distinct-looking lines, and using a broad beam of central light, the six 
 diffraction spectra, before alluded to, may be distinctly seen (without 
 an ocular) within the margin of the back lens of the objective (fig. 
 41). Any two adjacent spectra, combined with the central cone of 
 light, will form an equilateral triangle {a, b, c, for example), and 
 produce the well-known hexagonal markings, that is to say, three sets 
 of lines crossed at 60°, but, as any other pencils forming an equi- 
 literal triangle will also produce hexagonal markings, a new set on a 
 dark field may be formed by excluding the central, and three alternate 
 rays of the six spectral rays. We will take, for example, the three 
 rays, b, d, f (or a, c, e, g) ; the sides of this triangle being longer than 
 those of the common figure in the proportion of J 3 : 1, the new 
 hexagons will be three times as numerous as the first ones, having their 
 sides at a different angle to the median line. The three pencils producing 
 the interference in this case are, as we have said, b, d,f or c, e, g, 
 and not only is this effect seen to be capable of production, but it 
 follows from the theory that there must be visible three other sets of 
 lines bisecting the angles between the common lines, and corresponding 
 to the combinations of the spectra g, c, or f d — b,f or c, e—b, d, or 
 g, e. All these phenomena can be produced by excluding suitable 
 rays. It is easy to get the lines bisecting the angles of the common 
 rows one after the other, and of these, one set parallel to the axis 
 of the frustule. For this purpose, oblique light must be used, and 
 the central beam, and one of the peripheral rays must be stopped out, 
 leaving, for instance, b and /, or c and e, (i.e.), two spectra parallel to 
 the median lines ('). 
 
 (■) The firm of Carl Zeiss has constructed lately a special apparatus for the demonstration 
 of these experiments even by projection on a screen. 
 
The 
 
 Microscope. 
 
 BOOK I. 
 
 CHAPTER 1. 
 
 GENERALLY 
 
 The invention of the microscope is ascribed to Zaccharias Janssen, 
 of Middleburg (Holland), in about the year 1590, and is the 
 instrument which enables us to study objects too minute to be seen 
 distinctly with the naked eye. It is composed of different parts, 
 each of which has a special and clearly-settled use. The microscope 
 is an apparatus which must be manipulated with care, if it is 
 desired that the best results be obtained from it. To commence with, 
 therefore, we will examine the various parts of which it is composed, 
 both those which are essential, and those which are accessory. 
 
 Microscope. — The foot A of the 
 microscope is that part of the instru- 
 ment by which it rests on the table ; 
 it is attached to a single or double 
 pillar, or to a bar, which supports 
 the microscope proper ; the name of 
 foot is also often given to the entire 
 arrangement, which we have just 
 described. In first-class instruments, 
 the microscope is always jointed at 
 the foot, so that it may be inclined 
 at any angle. 
 
 The microscope proper is composed 
 of a tube, terminated at its two 
 extremities by a combination of lenses. 
 This entire arrangement can be made 
 more or less to approach or recede 
 from a small table, which is called 
 Fig. 46. the stage, and on which the object 
 
42 
 
 GENERALLY. 
 
 is placed lor examination. The stage is perforated with an opening, 
 through which the light, coming from beneath can pass, and 
 illuminate the object. 
 
 Let us examine these different parts more in detail. The tube 
 (fig. 46 T) is made of brass, having its inside blackened. It is 
 open at the upper end, so as to receive another shorter brass tube, 
 which is fitted into it with a sliding motion. . This shorter tube 
 carries a lens at each end of it, and is called the ocular. The 
 lower end of the tube is provided with a spiral groove, by which 
 means a system of lenses, called the objective, can be adapted to it. 
 
 The tube is usually composed of two pieces, the one sliding into 
 the other, so that it can be lengthened or shortened for different 
 magnifications. In good instruments, the inner tube is divided into 
 inches and lines or millimetres ( 1 ), so as to indicate exactly the 
 actual length of the total tube used. 
 
 All microscopes have not the same length of tube, and a distinction 
 is made between the English length, which is adopted by all English 
 and American makers, and which is ten English inches (say 255 
 millimetres), and the Continental length, which is from sixteen to 
 eighteen centimetres (say from 6 3 / IC to 7 1 /,., inches), but by drawing 
 out the tube, the length can usually be increased to 20 — 22 
 centimetres (7 4 / 5 — 8 3 / s inches). 
 
 The length of the tube is a matter to be taken into consideration 
 when the microscope is being used. Unless the objective is 
 furnished with a long correction (we shall see later on what this is), 
 which always is the case with English, but seldom with Continental 
 objectives, no objective will give a good image, except when employed 
 with the length of tube, for which it was constructed. To bring the 
 tube nearer to, or farther from the stage, it is made to slide, by 
 giving it a spiral movement in the tube X, which is carried on the 
 arm of the slow movement Z. We have thus, by a sliding movement, 
 what is called the rough adjustment. In instruments of superior 
 quality, the rough adjustment is effected by turning a milled head, 
 attached to a pinion, which acts on a rack fixed to the tube. 
 
 The rough adjustment serves to bring the object more or less into 
 view, that is to say, it moves the tube, so that the distance of the 
 object from the tube (containing the optical part) is such that 
 the details are seen clearly. In addition to this adjustment, every 
 
 (') One millimetre= "0394 of an inch, or roughly i-25th of an inch; one inch — 25K 
 millimetres. 
 
GENERALLY. 
 
 4.5 
 
 complete microscope possesses a fine adjustment as well, which is effected 
 by turning the milled head Z, controlling a micrometrical screw. 
 By means of the micrometrical screw, the tube can be moved 
 a very little distance indeed (e.g.) one hundredth of a millimetre, and 
 even a much smaller in certain instruments. 
 
 The stage P is a small solid table of metal, perforated with an 
 opening opposite the tube. The glass slip, which bears the object 
 for examination, is placed on this stage, and the former can be 
 fixed with two brass springs, which are called clips. Later on, we shall 
 have to return to the parts of the stage. 
 
 The mirror M, which will be found under the stage, enables the 
 object to be illuminated by transmitted light. The light reaches the 
 object, either slightly concentrated when the plane mirror is employed, 
 or strongly concentrated when the concave mirror is used. 
 
 Such are the essential parts of which the microscope is composed. 
 We now pass on to describe these principal constituent parts in greater 
 detail. 
 
CHAPTER II 
 
 DETAILED EXAMINATION OF THE PARTS OF 
 A MICROSCOPE. 
 
 I. THE OPTICAL PARTS OF THE TUBE. 
 
 I. THE OBJECTIVES. 
 
 General Remarks. 
 
 The objective is the most important part of the microscope 
 for the more perfect the objective, the better will be the results obtained 
 by the instrument. 
 
 Objectives of the present day are all achromatic, that is to say, 
 formed by combination of lenses made of flint and crown glass, and 
 even of various other substances, and so combined, that the red and 
 blue rays, which become separated when white light passes through the 
 lenses, are once more reunited in the same plane. 
 
 When the objective is not achromatised, these rays remain 
 separated; the image of the object under examination is then 
 surrounded by a blue and red border, and does not present any 
 clearness. 
 
 It is only since 1885, thanks to the fact that new combinations (glass 
 and pseudo-glass) can be employed in the construction of lenses, 
 that objectives can be obtained, in which the three principal rays of 
 the spectrum are reunited on the same plane. In this way, objectives 
 of extreme perfection are obtained, to which the name of apochromatic 
 objectives is given. They are, unfortunately, extremely expensive, on 
 account of the difficulty of their construction. 
 
 Formerly, objectives were formed of a single lens only, more or 
 less convergent according to the magnification desired. 
 
 Charles Chevalier, who, in 1823, constructed the first achromatic 
 * lenses with short focus, conceived the idea of superposing several of 
 them. He could, in this way, distribute the curvature of a single 
 lens over several lenses. The effect of this was first to render the 
 construction of objectives easier, since each lens in the Chevalier 
 objective was a great deal larger than in the ordinary one ; and secondly, 
 
THE OBJECTIVES. 
 
 45 
 
 the image for a given magnification became much brighter and more 
 distinct. 
 
 Moreover, Charles Chevalier achromatised each of the lenses of 
 his objectives separately; each of them therefore could, if desired, be 
 employed singly. 
 
 The combination forming an objective, actually gives an achromatised 
 image, but the composition of the crown and flint glass in each lens 
 varies. An image is thus obtained, showing the details of the object 
 much better, and having, what is called, greater aperture, but the lenses 
 should remain combined and be employed united, as the manufacturer 
 supplies them. 
 
 Objectives are divided into those whose lenses are fixed in their mounting, 
 and those with correction collars, which have previously been mentioned. 
 
 1. Objectives fixed in their Mounting. — Objectives fixed in 
 their mounting, are those whose lenses are fixed by the manufacturers 
 at a determined and invariable distance. Such an objective can only 
 be employed for a fixed length of tube. 
 
 2. Objectives with Correction Collars. — In 1829, Amici first 
 
 a 
 
 noticed that high-power objectives, 
 which give a perfectly clear image 
 when objects, not covered with a 
 glass, were examined, did not give 
 so good a one when the object 
 was covered, and that the clear- 
 ness of the image increased and 
 diminished, according to the thick- 
 ness of the cover-glass. To 
 remedy this defect, which resulted 
 from spherical aberration, Amici 
 constructed his objectives in such 
 a manner that they could all be 
 used with cover - glasses of a 
 definite thickness. 
 
 Fig. 47- 
 
 d 
 
 In 1837, the celebrated English 
 optician Ross, though ignorant of 
 the discovery of Amici, made the 
 same observation, and to remedy 
 the defect, he invented correction 
 objectives. 
 
• 
 
 46 THE OBJECTIVES. 
 
 In this kind of objective, the two upper lenses (fig. 47, d) occupy 
 an invariable position with regard to one another. They are fixed 
 in a moveable tube, and can be made to recede from, or approach, the 
 lower part, which is fixed, and carries the single or double frontal 
 (fig. 47, e). By turning the ring a, the upper lenses rise or fall, and 
 a coiled spiral spring c regulates the small inequalities of the screw, 
 and above all prevents the "back lash," which is always produced 
 when the sense of motion is changed. The firm of Carl Zeiss 
 engrave on their correction objectives a very practical series of 
 figures. This series of figures, which extends from ten to twenty-five 
 (or less) indicates at once the position to be taken for any given 
 thickness of cover-glass. If we have, for example, to study a pre- 
 paration covered with a cover-glass \\ tenths of a millimetre thick, 
 the division 15 is placed against the index, and we shall have the 
 best position of the lenses in which to study the preparation in 
 question with axial light. 
 
 Objectives may be divided into two classes, dry and immersion. 
 
 3. Dry Objectives. — Dry objectives are those where the object 
 (or rather the slip which covers it) remains at a certain distance 
 (which may be extremely small) from the objective, or where the 
 air intervenes between them continuously. 
 
 4. Immersion Objectives. — Immersion objectives, on the 
 other hand, are put in conjunction with the cover-glass by means of a 
 drop of liquid. Distilled water is the liquid interposed in ordinary 
 immersion objectives ; in objectives called homogeneous, cedar oil is 
 used, either pure or previously thickened by the oxidizing action of 
 the air and by then adding olive oil. 
 
 By the interposition of this liquid, a great number of luminous rays are 
 recovered, which were previously lost in their successive passages from the 
 cover-glass into air, and from that again into the glass of the objective. 
 
 The cedar oil has the same index of refraction (1 *5 15) as crown- 
 glass, which is used in the construction of the lens nearest the object, 
 and which is called the frontal. There is, therefore, in that medium 
 no loss of light by refraction, although there is still a little in the 
 case of water, whose index is only i"336. 
 
 Immersion objectives give a clearer, brighter, and more detailed 
 image than dry objectives. They have, at the same time, a greater 
 frontal distance for the same amplification ; that is to say, the 
 frontal remains at a greater distance from the cover-glass, and 
 consequently work becomes easier. 
 
THE OBJECTIVES. 
 
 47 
 
 It was at once thought that homogeneous objectives realized the 
 limit that could possibly be reached in the construction of objectives. 
 
 But this was not so, for of late the firm of Zeiss have effected an 
 extremely important advance in microscopical optics, first by the con- 
 struction of apochromatic objectives, and also by the manufacture of 
 a special objective, whose frontal is no longer of crown but of very 
 dense flint-glass. 
 
 5. Apochromatic Objectives. — Apochromatic lenses were first 
 offered for sale in 1886 (') by the firm of Zeiss, and were the 
 result of the researches of Professor Abbe. 
 
 From an optical point of view, apochromatic objectives differ from 
 all systems of lenses hitherto used for the microscope, in that they 
 simultaneously realize two conditions relating to the union of the 
 rays of the spectrum in one focus. The first is the convergence 
 of three different rays of the spectrum to a single point on the 
 axis, i.e., in the suppression of the spectrum called secondary, which 
 existed in all achromatic systems in use up to that time. The 
 second condition is the correction of spherical abberration for two 
 rays of different colours, while, up to that time, correction had only 
 been made for a single ray (viz., for that whose colour was the clearest). 
 
 All optical systems constructed up till then (the microscope, as 
 well as other instruments) give a clear image only for the rays of a 
 single colour (intermediate between yellow and green for ocular 
 instruments, and intermediate between blue and violet for photography). 
 
 For all other colours they give images more and more obscured, 
 surrounding the clearest image with a coloured border, or rather, 
 forming a general mist, which envelopes the whole field. Apochromatic 
 objectives give, on the contrary, for all the rays of the spectrum 
 images of an approximately uniform clearness. Therefore with white 
 (compound) light, or even when only some portions of the spectrum 
 intervene (whether when using monochromatic light, or when photograph- 
 ing) objects can be observed and the image is always uniformly clear. 
 
 Further, in the old systems the correction of chromatic aberration 
 is only good for a single zone of the objective ; it becomes worse 
 and worse towards the edge and towards the centre of the lens; in 
 apochromatic systems, on the contrary, chromatic correction is effected 
 for all the zones of the objective alike. For instance, if the 
 Abbe test be used, the appearance of a coloured outline cannot 
 
 ( 1 ) See H, Van Heurck, Journal de Micrographie 1886, page 91. 
 
48 
 
 THE OBJECTIVES. 
 
 be noticed with a very oblique illumination any more than when a 
 moiety or the centre of the lens is illuminated. 
 
 Thus, in the ordinary achromatic systems, even for the zone of 
 the objective where the chromatic correction is the best, the rays of 
 only two colours cut the axis in a single point ; the images of different 
 colours, therefore, only coincide two and two, and present a very 
 marked difference of focus between them. In the systems we are consider- 
 ing, the rays of different colours cut the axis three and three in the 
 same point, so that the space over which the different foci of all the 
 rays of the spectrum (from the optical rays to the extreme chemical rays) 
 extend is from seven to ten times smaller, and can consequently be 
 considered as practically reduced to zero, and this takes place in an 
 identical manner for each zone of the objective. 
 
 The images of the different colours, already very clear in themselves, 
 are then made to exactly coincide, and to act simultaneously so as to 
 produce one and the same effect. 
 
 The practical advantages of these improvements are therefore manifest. 
 The concentration of a much greater light for ordinary ocular observation, 
 or for any other use, with any kind of illumination (central or oblique, 
 white light, or monochromatic) assures an advantage, to these systems over 
 all others hitherto constructed, as much on account of the importance of 
 their effects, as the number of applications of which they are susceptible. 
 
 The natural colours of objects, even to their faintest tints, are 
 faithfully reproduced in the image. They are almost as distinct at 
 the sides as in the middle of the field. It is only on account of the 
 inevitable curvature of the field that the marginal parts have not quite 
 the same focus as the central parts, which should be focused one 
 after the other by means of the micrometrical screw. 
 
 These objectives admit of the use of very strong oculars with- 
 out any loss of clearness and brightness in the image. High magni- 
 fications are therefore obtained with a relatively long focal distance 
 and each of the objectives consequently furnishes a very wide range 
 of different amplifications. 
 
 6. Objectives with very wide Aperture. — If the crown-glass 
 of the frontal be replaced by a substance of greater refractive index 
 the aperture of the objective is considerably increased. About 25 
 years ago, we made experiments in this direction, and we gave an 
 account of them in 1869 (') 
 
 ( 1 ) See Second Edition of Le Microscope par le Dr. Henri Van Heurck, 1869. 
 
THE OBJECTIVES. 
 
 4') 
 
 But, notwithstanding results beyond our expectation, we were 
 obliged to interrupt our researches from want of leisure and a sufficiently 
 perfect plant to bring these experiments to perfection. 
 
 But what an amateur could only sketch, the firm of Zeiss has 
 brought to perfection, and, in fact, has much surpassed what we could 
 have believed possible. 
 
 The objective, which they have constructed, possesses a numerical 
 aperture of i'6^, the frontal lens is made of flint-glass, having an index 
 of 172, and it is immersed in monobromide of naphthaline, whose 
 index is i'6$. 
 
 In comparing these different figures, the objective will be seen to 
 vary considerably from the homogeneous, type. 
 
 We shall describe this admirable objective when speaking of the 
 apparatus of the Zeiss firm. 
 
 Moreover, this objective is not the last word, so to speak, of the 
 science, for Messrs. Zeiss anticipate the possibility of effecting before 
 long some steps in advance in the same direction. 
 
 7. Objectives are usually adapted to the tube by means ot a screw- 
 thread, but, unfortunately, these differ. — There is the French screiv- 
 thread used by Nachet, the German thread adapted by German makers, 
 and lastly, there is the English thread {Society's Screw) employed by 
 the English and American makers, and also by Zeiss. The English 
 thread, which was designed by the Royal Microscopical Society of 
 London, is very superior to the others, because in the construction of 
 objectives it allows the use of lenses of a greater diameter. Let us add, 
 however, that good Continental makers like Nachet, Reichert, and 
 Hartnack, now furnish their best instruments with the English screw- 
 thread. In these instruments it is only necessary to remove the 
 small piece which forms the end of the tube, and which is called the cone 
 to enable objectives furnished with the English thread to be adapted. 
 
 The English thread is however not all that we have to say on 
 this matter. In America the American thread is also employed, which 
 is considerably greater and admits of the use of lenses with a much 
 larger diameter, and thus offers certain advantages. In the first place, 
 the larger the lens the easier it is to make, and consequently the real 
 curvatures approach closer to the calculated curvatures ; then the larger 
 the lens, the more luminous rays it admits — and this, especially in 
 photography, is not to be despised. Moreover, the first and second 
 points which we have just enumerated allow of the use of more powerful 
 oculars with a given objective. 
 
5° 
 
 THE OBJECTIVES. 
 
 Formerly, objectives were classified by numbers or letters — an 
 altogether arbitrary classification, which differed with each maker. Now, 
 nearly all the good makers classify their objectives according to the 
 focus, or focal distance. 
 
 8. Many factors require to be taken into consideration when examining 
 objectives, viz. : — Focal distance, frontal distance and the angle of aperture. 
 
 Focal distance. — The place, where all the rays, received by 
 a lens, are united into a single point, is called the focus of the 
 lens, and the distance of this point from the lens is called the focal 
 distance. 
 
 In ordinary microscopical objectives, constructed with two, three, 
 or four lenses, these united lenses play the part of a single simple lens. 
 
 In reality, the focal distance of such a combination is shorter 
 than that of a simple objective, which will give the same magni- 
 fication. They can nevertheless be compared, and American and 
 English opticians as well as some German makers number their 
 objectives according to their focal value. 
 
 The simplest procedure for measuring the focal distance of an 
 objective is that indicated by Prof. Harting, as follows : — The micro- 
 meter (divided into hundredths of a millimetre for an objective of 
 high power, or into tenths of a millimetre for one of a low power), 
 is placed on the stage. The objective, whose focal distance is to be 
 determined, is screwed into the tube of the instrument, and the ocular 
 is taken out and replaced by finely ground-glass. 
 
 The microscope is illuminated by solar rays, and adjusted so that 
 the lines of the micrometer appear with their maximum clearness on 
 the ground-glass. 
 
 The breadth of the image of a micrometrical division is measured 
 
 by means of a compass or rule (for greater exactness, a certain 
 
 number of divisions may be taken, and the average breadth calculated), 
 
 and then the exact distance of the surface of the micrometer from the 
 
 surface of the ground glass must be measured. The focal distance 
 
 is then found by means of the formula : — 
 
 bh 
 a = — 
 d 
 
 It follows from this formula that the exact focal distance / will be 
 
 a b 
 
 / = 
 
 a + b 
 
THE OBJECTIVES. 
 
 In these formulae : 
 
 a represents the distance of the micrometer from the objective; 
 
 h the real size of a division of the micrometer ; 
 
 d the size of a division of the micrometer in the image ; 
 
 b the distance of the micrometer from the image ; 
 
 Frontal distance. — The focal distance must not be confounded 
 with the frontal distance, for the latter is the space between the 
 objective and the cover-glass when the preparation is in focus, that is 
 when it can be seen with the greatest precision. 
 
 Frontal distance is an important factor in estimating the value of 
 an objective. If the frontal distance is too small, it is necessary to 
 use cover-glasses which are very thin, and therefore difficult to manage, 
 and, moreover, many ready-made preparations cannot be studied by such 
 objectives. 
 
 We must, therefore, now examine the optical qualities which an objective 
 should possess, and this we will do in the following paragraph. 
 
 Optical qualities of a good objective. — Objectives ought to 
 give images whose outlines are clear, well-defined, and showing the 
 details well, and the image formed at the margins of the objectives 
 should be as good as that in the central part. 
 
 These qualities depend on the angle of aperture, and on good 
 correction for chromatic and spherical aberration. 
 
 mmmm^m^m The angle formed by the two extreme 
 
 the cover-glass, and the part of the object shown at any time 
 approximate to a mathematical plane. It is necessary that the angle 
 of aperture of an objective be made proportional to its power ; it 
 would be absurd to give a very great angle to a low power, for it 
 would thus be made to give details, which the eye could not dis- 
 tinguish, because they would be too closely compacted together. 
 
 The angle of aperture of a dry objective is completely different 
 from that of an immersion objective, and as the angular measure ot 
 
 E2 
 
 The greater the angle (fig. 48) C A C 
 and D A D', and the more detail in the 
 image which the objective shows, but, in 
 proportion as this angle augments, so will 
 the frontal lens of the objective approach 
 
 rays, emanating from the object, and 
 utilized by the objective, is called the 
 angle of aperture (fig. 48). 
 
 Fig. 48. 
 
5 2 
 
 THE OBJECTIVES 
 
 aperture was subject to various interpretations, Professor Abbe thought 
 of the happy idea of replacing this measure by another, which is called 
 Numerical Aperture. 
 
 This quantity is represented by the expression : 
 
 0 = n sin u 
 
 where n represents the index of refraction of the surrounding medium, 
 air, water, or oil, and u the half-angle of incidence of the extreme rays. 
 
 Thus, says Dr. J. Pelletan ('), a dry objective, which has 180° as 
 its extreme angle of aperture, will have i as its Numerical Aperture. 
 For in the formula 0 = n sin 21, the index n of air is 1, and the 
 angle u is the half of 180 0 , that is to say 90 0 , the sine of which 
 is 1. The numerical formula therefore becomes 0=ixi = i. 
 
 It is thus seen that a water immersion objective of only 97 0 of 
 angular aperture corresponds in optical power to this dry objective of 
 180 0 maximum angular aperture, and whose numerical aperture = 1. 
 'For the sine of 48^° (the half of 97 0 ) is roughly 752, which, multiplied by 
 1 '33» the index of water, gives 1 as the value of its numerical aperture. 
 
 And the same optical effects would be produced by a homogenous 
 oil immersion objective which has an angular aperture of only 82 0 , 
 for in the formula 0 = ?i sin u the index n of cedar oil used is i"52, 
 and sin u or sin 41 0 is practically '658. Therefore, 0= 1*52 x '658 = 1. 
 
 It will similarly be seen that a water immersion objective of only 
 85 0 angular aperture corresponds to a dry objective, having an angular 
 aperture of 128 0 (in air), because each has the same numerical 
 aperture, viz., '90. Practically, the formula gives for the numerical aperture 
 of the first: 0=ixsin 64 0 = 1 x '90= '90 ; and for that of the second: 
 0= 1 -33 x sin 42^° = 1-33 x -68 = -90. 
 
 Again in the same way, a homogeneous immersion objective 
 («=i*52), having an aperture of only 92 0 , corresponds to a water 
 immersion objective («=i'33) of angular aperture 112 0 , for each 
 has no as its numerical aperture, as is easily seen from the above 
 mode of calculation. 
 
 It will now be observed that if, as we have seen above, a water 
 immersion objective, whose angular aperture is 97 0 and whose 
 numerical aperture is 1, and a homogeneous objective of 82° angular 
 aperture, whose numerical aperture is also equal to 1, correspond to 
 a dry objective of 180 0 angular aperture, whose numerical aperture is 
 equal to 1, then all water immersion objectives, which have an angular 
 aperture greater than 97°, and all homogenous objectives, whose 
 
 ( ■ ) Les Diatom^es, page 150. 
 
THE OBJECTIVES. 
 
 53 
 
 angular apertures are greater than 82 0 , correspond to dry objectives, 
 whose angular apertures would be greater than 180°. 
 
 Now, this result appears absurd. It is difficult to comprehend an 
 objective whose angular aperture would be greater than 180 0 ; indeed 
 it is a thing which cannot be realized in practice with dry objectives. 
 These are the very same considerations which have excited long and 
 animated discussions among microscopists on what is called the 
 aperture question. And one of the reasons which led Professor 
 Abbe to abandon the idea of angular aperture, and adopt numerical 
 aperture in its stead, which is applicable to all cases, was the fact 
 that it has no meaning after a certain limit. 
 
 Numerical aperture is estimated in practice by means 01 a very 
 small and very simple apparatus, called Abbe's Apertometer (fig. 49). 
 
 Fig. 49. 
 
 The apertometer, an apparatus placed on the stage of the microscope, 
 consists of a thick semi-circular disk of flint-glass, 90 mill. 
 (3-546 inches) in diameter. The diametral edge is bevelled so as 
 to give the effect of a reflecting prism, and so send the light 
 horizontally into the axis of the microscope. 
 
 The objective which is being examined is made to focus the central 
 point of the upper surface of the disk. The limits of aperture are 
 determined by the two indicators, which slide on the circumference of 
 the disk, the image of which being projected by the objective into the 
 tube of the microscope, is focussed by means of an accessory objective, 
 which is screwed to the lower end of the draw-tube; the focus is 
 then obtained by moving the draw-tube up and down. 
 
 The indicators, focussed as just mentioned, are then moved to and 
 fro along the surface of the disk, and when their image is on the 
 point of disappearing from the field, their position is read on the 
 two concentric scales drawn on the disk. One of these scales gives the angle 
 of aperture of the objective, the other indicates its numerical aperture. 
 
54 
 
 THE OBJECTIVES. 
 
 The possession of an apertometer is of great service to those 
 manufacturers and microscopists, who are constantly comparing 
 objectives, but evidently the student, who has only a few objectives 
 at his command (as is usually the case) has no need of this ingenious 
 apparatus, and can estimate accurately enough the aperture of his 
 objectives by examining certain diatoms. 
 
 Indeed, diatoms have markings and striae at regular intervals on 
 their silicious frustules, which vary only to a very small extent in the 
 same form. 
 
 Now the power of resolving a certain number of lines or strise 
 within a given space, depends upon the numerical aperture of the 
 objective. 
 
 The aperture of an objective can therefore be estimated 
 accurately enough for all practical purposes by making an examination 
 of certain diatoms. 
 
 The Royal Microscopical Society of London has published a 
 table, which indicates the number of lines which a well-constructed 
 microscope of given aperture should theoretically be able to resolve. 
 
 We give here an extract from this table for the most usual 
 numerical apertures of good objectives. We have reduced to a 
 millimetre, the number indicated for the English inch ( = 25*5 mm.). 
 
 
 Maximum 
 
 number of lines which 
 
 
 Corresponding number of the 
 
 
 an objcuivr cm resolve in a 
 
 
 
 
 3 
 
 V 
 
 a 
 
 
 millimetre. 
 
 
 Corresponding diatom for white light and 
 
 groups in Nobert's tests. 
 
 < 
 
 
 
 
 
 
 
 
 In blue 
 
 
 mean number of stria;. 
 
 
 Test of 
 
 
 In white 
 
 mono- 
 
 By 
 
 
 Test of 
 
 u 
 
 E 
 
 light. 
 
 chromatic 
 
 photo- 
 
 
 30 groups. 
 
 19 groups. 
 
 3 
 Z 
 
 
 light. 
 
 graphy. 
 
 
 
 I '63 
 
 6000 
 
 
 IOOOO 
 
 Amphipleura in beads (soooto 5100) 
 
 
 
 I '40 
 
 5292 
 
 S737 
 
 6972 
 
 
 
 I 30 
 
 4915 
 
 5327 
 
 6474 
 
 
 
 
 I "25 
 
 4726 
 
 5722 
 
 6225 
 
 
 
 19th (443°) 
 
 I'20 
 
 4536 
 
 4917 
 
 5976 
 
 
 
 
 4347 
 
 4112 
 
 S727 
 
 
 
 1 8th (4209) 
 
 I'OO 
 
 3780 
 
 4098 
 
 4980 
 
 Amphipleura pellucida (3700) 
 
 
 16th 3766) 
 
 '95 
 
 3591 
 
 3893 
 
 473 1 
 
 Vanheurckia crassinervis (3500) 
 
 30th (3544) 
 
 15th (359' 
 
 •80 
 
 3024 
 
 3278 
 
 3984 
 
 Vanheurckia rhomboides (2800) 
 
 25th (3000) 
 
 13th (3100) 
 
 •60 
 
 2268 
 
 2458 
 
 2988 
 
 Pleurosigma angulatum (2000) 
 
 16th (2264) 
 
 9th (2215) 
 
 •40 
 
 1512 
 
 1639 
 
 1991 
 
 Pleurosigma Balticum (1500) 
 
 9th (14901 
 
 6th (1550) 
 
 •30 
 
 1 134 
 
 1229 
 
 1494 
 
 Navicula elliptica (1100) 
 
 7th (1091) 
 
 4th (1108) 
 
 ■20 
 
 75° 
 
 819 
 
 995 
 
 Navicula viridis (700) 
 
 4th (707) 
 
 2nd(665 Ins.) 
 
 We give at the same time the diatom which corresponds to this 
 number, and also the group in the Nobert test. It should always 
 be borne in mind that the number indicated is the theoretical figure, 
 and it would be futile to expect to realize it in practice ; but if the 
 
THE OBJECTIVES. 
 
 objective is well-constructed, the resolution of the diatom or the 
 corresponding group of Nobert, can be effected by employing 
 monochromatic blue light. 
 
 This list can be completed in like manner for objectives, which 
 could not possibly be produced at present. Let us note, however, 
 that it is scarcely credible that a numerical aperture of 2'o can 
 be exceeded, or even that that aperture can be attained. 
 
 To go as far as 2*5, it would be necessary that the frontal lens 
 be made of Diamond, or of Realgar, which, indeed, is quite possible. 
 But it would also be essential that the cover and slide of the 
 preparation have the same refractive index, which, is very hypothetical, 
 unless that can be made artificially of large plates of diamond. 
 Moreover, the immersion liquid must have the same index, which 
 is even still more problematical. 
 
 Be that as it may, we have added the last two apertures simply 
 by way of curiosity, and for the purpose of showing how little 
 difference — compared with what we already have — these fabulous 
 objectives would produce. 
 
 In the following list the resolutions are calculated only for axial 
 light ; for oblique light, the figures ought to be about doubled : — 
 
 
 Maximum number of stria? which the object 
 
 ve can resolve 
 
 Numerical aperture. 
 
 
 
 
 
 per millimetre in axial light. 
 
 
 
 In white light 
 
 In blue monochromatic light 
 
 By photography. 
 
 
 a 
 
 
 a 
 
 
 •53 
 
 •49 
 
 .40 
 
 170 
 
 32IO 
 
 347° 
 
 4250 
 
 1 75 
 
 33°° 
 
 3571 
 
 4375 
 
 1 80 
 
 339° 
 
 3 6 73 
 
 4500 
 
 1-85 
 
 3490 
 
 377S 
 
 4625 
 
 1 90 
 
 358S 
 
 3800 
 
 475° 
 
 2'0O 
 
 3775 
 
 4080 
 
 5000 
 
 2.15 (Diamond and Realgar) 
 
 4717 
 
 5100 
 
 6250 
 
 The Nobert test, which we have cited above and of which we 
 shall speak again later on, consists of a series of groups of lines 
 ruled by a diamond on a glass slip. In each successive group 
 the lines become more delicate and closer together. There are two 
 kinds of Nobert tests in existence, the first consists of thirty groups; 
 the first of which shews 443 lines per millimetre, and its last shews 
 3,544. The second Nobert Test, which was the last to be submitted 
 
56 THE OBJECTIVES. 
 
 to microscopists, shews 443 lines per millimetre in the first group, 
 and 4,430 lines in the nineteenth, or last group. 
 
 These tests bear the name of the skilful optician who used to 
 construct them, and who died about five years ago, having kept 
 secret to the last the process which he employed. 
 
 The instrument used by Nobert belongs, at present, to Mr. 
 Crisp, the eminent treasurer of the Royal Microscopical Society of 
 London. 
 
 Fasolt, in America, it is said, makes tests analagous to those of 
 Nobert, but we have no personal knowledge of them. 
 
 The power of exhibiting detail, as we have just seen, depends 
 on the angle of aperture, and is called the resolving power of the 
 objective. 
 
 An objective, whose aberrations are well-corrected, will define well 
 the outlines of the objects which it shows, and it is then said to 
 have good defining power. 
 
 A wide angled objective, we have previously seen, always has a 
 very small frontal distance, and only grasps a mathematical plane, but 
 objectives having a certain penetrating power, that is the power of 
 grasping several planes at one time, and having greater frontal distance, 
 are preferred for histological studies. These objectives have 
 smaller angular aperture, and are theoretically inferior. 
 
 Professor Abbe has satisfactorily established the fact that a certain 
 relation must exist between magnification and angular aperture, and 
 he has published a long paper on this subject in the journal of the 
 R.M.S. of London (»). 
 
 9. On the number of Objectives. — It is absolutely useless, 
 even for anyone who desires to apply himself to the most serious 
 microscopical studies, to have very numerous objectives unless indeed 
 he wishes to devote himself to studies of comparison. 
 
 The following series amply suffices : — 
 
 An objective of 2 inches (48 mm.) whose initial magnifying power is 5 
 „ „ 1 inch (24 mm.) „ „ „ 10 
 
 » \ » (12 mm.) „ „ ., 21 
 
 » \ » (6 mm.) „ „ „ 41 
 
 „ „ i-ioth (2-5 mm.) „ „ „ 100 
 
 The \ of an inch can always be replaced by a i-6th, and 
 the i-ioth by a i-i2th of an inch. 
 
 ( 1 ) On the Relation of Aperture and Power in the Microscope, J.R.M.S., 1883, page 790. 
 
THE OBJECTIVES. 
 
 5 7 
 
 It must be well understood that the objectives should be of the 
 best quality, and that the i-ioth and the 1-12H1 should be immersion. 
 
 The last objective ought, indeed, to be apochromatic and of 
 homogeneous immersion, if it is to be used for all researches. 
 
 The entire series, on the other hand, would not be required, and 
 the two inches and the i-i2th inch could be suppressed if it is only 
 required for certain researches. The last objective can then be a 
 1 -1 2th inch water immersion. 
 
 Let us recapitulate the preceding data : — 
 
 1. Complete and perfect series. 
 Objective of 2 inches ; 
 
 „ of 1 inch apochromatic ; 
 „ of \ inch ; 
 
 ,, of \ or Jth inch apochromatic ; 
 
 „ of ^th or i-i2th inch, homogeneous apochromatic; 
 
 2. Incomplete series, but sufficient for the greater number of researches. 
 Objective of |rd inch, or | inch ; 
 
 „ of Jth inch ; 
 
 „ of 1- 1 2th inch homogeneous immersion ; 
 
 3. Series very incomplete, but sufficient for many ordinary researches. 
 An objective of 1 inch, and an objective of -J-th of an inch. 
 
 Tests. 
 
 1. — Ordinary Tests. 
 
 Certain objects, whose details are well-known and which serve as 
 standards to estimate the value of objectives, are called Tests. 
 
 It is by examining the principal tests in every possible illumination 
 that the young microscopist will become initiated into all the resources 
 of his microscope, and being perfectly aware by the published designs 
 what details the object should show, he will work on until he is 
 able to bring these details clearly into view. 
 
 The number of tests described by authors is considerable, but they 
 may be limited to the six following : — 
 
 Pygidiuni of a flea ; 
 
 Podura plumbea ; 
 
 Pleurosigma angulatuni ; 
 
 Surirella Gemma ; 
 
 Van Heurckia rhomboides and var crasstnervis ; 
 Amphipleura pellucida ; 
 
58 
 
 THE OBJECTIVES. 
 
 If to the above a Nobert test can be added, these will be sufficient 
 for every experiment. 
 
 We will now examine these tests in detail : — 
 
 Pygidium of a Flea. — The Pygidium of a flea is an excellent 
 test to judge of the defining power of an objective. We have 
 employed it for this purpose for more than 25 years, and we are 
 very satisfied with the results that it has given. This test should 
 always be prepared by the student for himself, for we have never yet 
 met with a good specimen produced by a professional mounter. 
 
 Podura. — Under the name of Podura is included the scales of 
 Podura plumbea (Lefiidocyrtis curvicollis). This object, which is almost 
 unknown to continental microscopists, is the favourite test with English 
 observers. 
 
 The scales show markings, which have been compared, with much 
 justice, to notes of exclamation. When the objective is good, and 
 the correction is suitable to the thickness of the cover-glass used, 
 these markings are clearly defined and traversed by a long luminous band. 
 
 The Podura is a test of definition for objectives of less than 
 i-inch, the illumination ought to be quite axial, and the condenser 
 achromatic, and used with a moderately large opening in the 
 diaphragm. 
 
 The Podura has been very specially studied by the late Mr. 
 Richard Beck ('), and we have to thank Messrs. R. and J. Beck for 
 the reproduction of Plate VII (fig. 51), which represents the 
 different aspects, which these scales assume under different conditions 
 
 The Pygidium of a flea (fig. 50) is composed 
 of two lobes, and displays thirty-two to thirty- 
 eight long and stiff hairs, implanted in the centre 
 of as many areolae, and each is surrounded by a row 
 of small cuneiform elevations. The spaces between 
 the areolae are covered with small spines. 
 
 Fig s°- 
 
 An excellent objective ought to show these 
 areolae clearly defined in all their parts, and the 
 elevations ought to appear cuneiform, and not 
 round as they are drawn by Dujardin. Moreover, 
 the colour of the object ought to be a very clear 
 brownish yellow, without the slightest appearance 
 of milkiness. 
 
 (') A Treatise on Achromatic Microscopes, by Richard Beck, London, 1865. 
 
6o 
 
 THE OBJECTIVES. 
 
 In fig. i is seen the appearance ol the markings where the 
 focussing of the object is perfect, and when the correction adjustment 
 of the objective leaves nothing to be desired. 
 
 In fig. 2 the correction of the objective is equally perfect, but the 
 object is found to be either a little over or under-focused. In fig. 3 
 the correction is perfect, but the focussing is very slightly faulty. In 
 figs. 4 and 5 is seen the appearance produced by over or under 
 focussing the objective, combined with an imperfect adjustment of the 
 correction collar, and lastly, in fig. 6, the focussing is perfect, but the 
 correction of the objective is bad. 
 
 The Podura also enables the chromatic corrections of the objective 
 to be well judged of; all achromatic objectives show these notes of 
 exclamation strongly coloured, only apochromatics display but a slight 
 trace of colour. 
 
 Pleurosigma angulatum.— When examined under objectives 
 without sufficient numerical aperture, the Pleurosigma displays valves of 
 a brownish yellow, without any appearance of marks or designs. 
 When the aperture becomes moderate (e.g., about - 6), no mark can 
 yet be seen in axial light with the use of a diaphragm; but if it is 
 illuminated, either obliquely, or what is still better, by means of a 
 large converging cone of light, produced by a condenser used with a 
 diaphragm with a sufficiently large aperture, then on the valve will 
 be seen three series of lines crossing one another, at an angle 
 of 60° (fig. 52). 
 
 With immersion objectives and objectives of large aperture, the 
 valve can be seen covered with perfectly round points. The nature 
 of these points, which have been called beads, has been for a long 
 time doubtful. It was generally believed, and some microscopists still 
 believe, that these are hemispheres in relief. Others say that they are 
 depressions. Finally, we have detailed, in our Synopsis des Diatotnees, 
 the grounds which made us admit that they are cavities in the middle 
 plate of the valve. 
 
 Under certain conditions of illumination, and especially with the 
 new objective of r6o numerical aperture, instead of round points, 
 hexagonals are seen (figs. 53 and 54). It was thought that this was 
 simply an optical illusion, but we demonstrated in December, 1889, 
 that these hexagonals really represented the form of the openings of 
 the lattice work, or middle plate of the valve. The appearance of 
 round points is produced by focussing the upper part of the openings 
 
Fig- 55- 
 
 Fig. 56. 
 
6 2 
 
 THE OBJECTIVES. 
 
 where species of domes are formed by the upper bed of the valve, 
 which closes the openings of the middle plate. 
 
 Surirella Gemma. — The diatom of this name shows very easily 
 the appearance of the lines transverse to its longer axis (fig. 55). It is 
 a more difficult matter to make the longitudinal lines visible, and lastly, it 
 is exceedingly difficult to resolve very clearly the square beads, which cover 
 the valve in slightly sinuous lines (fig. 56). To be very successful in 
 doing this, it is necessary to employ a good immersion objective, and 
 either an oblique illumination, or better still, a large luminous cone 
 of light produced by a condenser of sufficient aperture. 
 
 The transverse striae, which ought to be resolved in beads, are 
 about 2,000 per millimetre in number. 
 
 The Surirella is an excellent test to ascertain if the chromatic and 
 spherical aberrations have been well corrected in objectives of the 
 highest power. With this object the test is examined with the con- 
 denser open fully, as is done for bacteria. Under these conditions of 
 illumination one can estimate clearly and easily any defects of correction 
 which produce a diffused and coloured image. Let us note, moreover, 
 that the best apochromatics alone can give an uncoloured image. 
 
 Van Heurckia rhomboides. — The genus Van Heurckia, created 
 by the eminent diatomographer, A. de Brdbisson, includes a small 
 number of forms very difficult to resolve. 
 
 Two amongst them are the celebrated tests : Van Heurckia 
 rhomboides (fig. 58), and its variety crassinervis (fig. 57). 
 
 Fig. 58. 
 
 These two forms, which are rhomboidal lanceolate, have longitudinal 
 and transverse lines, which cross one another at right-angles, the 
 longitudinal lines are much more robust, and more easy to resolve 
 than the transverse ones, which in the rhomboides are 2,800 per 
 per millimetre in number, and in crassinervis about 3,500. 
 
THE OBJECTIVES. 
 
 63 
 
 Van Henrckia rhomboides can be easily resolved into delicate beads by 
 means of an immersion objective having a numerical aperture of 1-15 
 to 1-20. V. crassinervis (Frustulia Saxonica Rabh.) is much more difficult 
 to resolve. 
 
 Amphipleura pellucida. — This diatom, which is the most 
 difficult to resolve of all those employed as tests, has lanceolate 
 valves and a raphe (median line), terminated by two very elongated 
 nodules. The middle nodule is absent; with much pains, a few 
 traces of it can be found. 
 
 The Amphipleura (fig. 59) has transverse strise which are excessively 
 difficult to see; they are very fine, and number about 3,700 to the 
 millimetre. 
 
 ^^^jgeSglS mIiiii BhtMUIIUBBsgiii^B^ 
 
 The resolution of these strife into beads is the greatest difficulty 
 of the microscopist. They were made visible by us in the first place by 
 means of a i-i2th homogeneous of Zeiss, on a silvered preparation, 
 made by the late Dr. A. Y. Moore. We were able to make a 
 photograph of it in 1884. 
 
 In 1887, we succeeded in photographing by transparency the 
 Amphipleura resolved into longitudinal lines, and into beads. The 
 negative was made with a Zeiss' ^th apochromatic homogeneous 
 objective (N. A. 1*40), and one of our own preparations in the yellow 
 medium and monochromatic solar illumination. 
 
 This photograph was reproduced in the " Bulletins de la Societe 
 Beige de Microscopie," for 30th April, 1887. 
 
 Lastly, in October, 1889, Zeiss' apochromatic objective of N.A. i*63 
 enabled us to photograph the Amphipleura perfectly resolved into 
 beads over the whole surface of the valve. 
 
 It is not difficult to make the longitudinal lines of the valves ot 
 the Amphipleura appear, but these lines are parallel to the edge of 
 the valves. The real lines are parallel to the raphe, or middle rib, 
 and are undulated throughout their entire length, which arises from 
 the beads not being exactly placed one above another. 
 
 The phototype plate, which we give opposite page 64, shows the 
 appearance which the principal tests present when they are examined 
 under the Zeiss' objective of N.A. 1-63, with monochromatic solar 
 illumination, 
 
 ag— ipMp ra ■ 'iiiijiiiiiiiiiiiaiiiiiiail 
 
 Fig. 59- 
 
6 4 
 
 nobert's test. 
 
 The Amphipleura pellucida Kiitz. is represented in the first three 
 figures. The resolution into lines, or striae, is seen in fig. 3, with a 
 magnification of 2,000 diameters. The beads are seen in fig 2, and 
 fig 1 with magnifications of 2,000 and 3,000 diameters respectively. 
 
 Fig. 4. Fragment of a valve of Amphipleura Lindheimeri Grun. A 
 species only distinguished from Amphipleura pellucida Kiitz. by its size 
 being often much larger, and by its striation being coarser. The 
 "beads," or openings of the lattice work, are seen to be square. 
 Magnification 2,500 diameters. 
 
 Fig. 5. Pleurosigma angulatum W. Sm. Focussed exactly on the 
 hexagonal openings in the lattice work. About 10,000 diameters. 
 
 Fig. 6. Pleurosigma angulatum IV. Sm. Focussed on the upper 
 surface of the valve. The upper part of the small domes can be 
 seen, and between them the points formed by the imperfect focussing 
 of the sides, or walls of the hexagons. 
 
 Fig. 8. Surirella Gemma Ehr. magnified 1,000 diameters. 
 
 Fig. 9. Van Heurckia crassinervis Breb. magnified 2,000 diameters. 
 
 All these photographs were made with Zeiss' i-ioth inch of N. A. 
 1 63. 
 
 Monochromatic solar light. 
 
 Special compensating eye-piece 12.— -Condenser i'6. 
 Preparations in a medium of 2 '4. 
 Covers and slides in flints of 172. 
 
 Fig. 7 shows the 19th group of Nobert's test (4,443 lines to the 
 millimetre) : this photograph was made with a i-i2th inch apochro- 
 matic of N. A. 1 -4 of M. Carl Reichert, of Vienna. 
 
 2. — Nobert's Test. 
 
 A skilful German optician, the late J. A. Nobert, living at Barth, 
 in Pomerania, thought of the idea of tracing on glass, series of lines, 
 nearer and nearer to one another. The tests, or proof tables, as 
 Nobert called them, are truly wonderful, and nothing was known up 
 to his death of the process employed by this maker in tracing 
 lines so fine, and so close. 
 
 Nobert produced two series of tests. The first and oldest series 
 contained thirty groups of lines, and cost ^4 16s. od. We give 
 below, according to Prof. Harting, the number of lines in each 
 group :— 
 
Dr H. Van Heubck, phot. J. Mais, pltototyp 
 
 Tests of the Microscope. 
 
 C. Zeiss 2.5 mm. N.A 1,6 Oli.j. glass. Comp. Eye-Kece 12— Condens. N.A. 1,6. 
 MonocliTomatic Sun-Liglit. 
 
nobert's test. 
 
 65 
 
 Group No. 1 contains 443 lines to the millimetre. 
 
 5 
 
 10 
 
 20 
 25 
 3° 
 
 806 
 161 2 
 2215 
 
 2653 
 3098 
 
 3544 
 
 This test has the appearance of an ordinary microscopical preparation. 
 The lines are traced on the centre of a cover-glass, which is fixed 
 to an object slip like an ordinary preparation. 
 
 Of course, the lines become more and more difficult to resolve as 
 the number of the group increases. 
 
 The construction of objectives having improved, M. Nobert 
 produced a new test, having only nineteen groups but in this the 
 19th group is much more difficult to resolve than the thirtieth of the 
 first series. 
 
 The following gives the distance and number of lines of these 
 nineteen groups : — 
 
 Groups. 
 
 In a Paris line according to 
 Nobert. 
 
 [Paris line = o838 inch]. 
 
 In a millimetre according to 
 Harting, III. 374. 
 
 [Millimetre = -0394 inch]. 
 
 Number of lines in each 
 group. 
 
 I 
 
 1/1000 
 
 443 
 
 7 
 
 2 
 
 1/15OO 
 
 665 
 
 IO 
 
 3 
 
 2000 
 
 886 
 
 13 
 
 4 
 
 2500 
 
 1 108 
 
 
 5 
 
 3OOO 
 
 1329 
 
 17 
 
 6 
 
 350O 
 
 155° 
 
 20 
 
 7 
 
 4OOO 
 
 1772 
 
 23 
 
 8 
 
 4SOO 
 
 1994 
 
 25 
 
 9 
 
 SOOO 
 
 2215 
 
 27 
 
 10 
 
 SS°0 
 
 2437 
 
 3° 
 
 11 
 
 60OO 
 
 2658 
 
 34 
 
 12 
 
 650O 
 
 2880 
 
 37 
 
 13 
 
 70OO 
 
 3100 
 
 40 
 
 
 7SOO 
 
 3323 
 
 43 
 
 IS 
 
 80OO 
 
 3544 
 
 45 
 
 16 
 
 850O 
 
 3766 
 
 48 
 
 17 
 
 9OOO 
 
 3987 
 
 5i 
 
 18 
 
 9SOO 
 
 4209 
 
 54 
 
 19 
 
 I OOOO 
 
 443° 
 
 57 
 
 Certain authors have stated that Nobert's tests are of very little 
 use to microscopists ; they say that the tests differ from one another, 
 on account of the varying pressure of the diamond used in making 
 them, the nature of the glass, &c, and that, moreover, they will 
 never be of use to recognize a good objective, because they cannot 
 be examined in very oblique light. 
 
66 
 
 moller's test. 
 
 We expressly contest all these points, and we are pursuaded that 
 these statements arise from an insufficient study of the test, and 
 that its only fault is its costliness. 
 
 In the first place it can be used equally well in central and in 
 oblique light ; then the test is very far from being as variable as it is 
 stated to be ; diatoms are infinitely more so, and the difficulty of resolving 
 them is affected by a number of circumstances (e.g.), age, locality, &c, 
 as all know who have made a special study of these very interesting beings. 
 
 We possess both the tests, and have used them daily for a 
 long time. Although these two series of groups date from very 
 different periods, they are nevertheless perfectly comparable with one 
 another; and a given objective, which enables us to resolve a certain 
 group with one test, resolves the equivalent group of the other 
 test, neither more nor less. 
 
 The late Colonel Dr. Woodward has produced a series of admirable 
 photographs of the nineteen groups, which were made by him for the 
 Exhibition of Philadelphia. This eminent microscopist had the 
 kindness to send us these photographs, and what we observe is 
 in every respect identical with the images obtained by Dr. Woodward. 
 
 These photographs were accompanied by a leaflet, entitled 
 Memorandum on the Nineteen-band Test-plate of Nobert. This 
 eminent savant, who dedicated all his time to the elucidation of 
 the most difficult points in microscopy, and whose competence can- 
 not be here called into question, expresses himself in the following 
 manner in this memorandum ; we happily agree with him on all 
 these points : — 
 
 "The plate of Nobert, which we are about to describe, offers an 
 admirable means of measuring the defining power of the best 
 objectives. It appears better adapted for this purpose than diatoms, 
 which are so generally employed, because in the latter the individual 
 specimens of the same species vary considerably in the fineness of their 
 lines. Although it cannot be supposed that Nobert has always attained 
 just the same degree of precision, an examination of a good number 
 of plates demonstrates that the deviations from the requisite precision 
 are so slight that they are practically inappreciable." 
 
 3. — Test and Proof-plate of Moller. 
 
 Mbller, of Wedel (Holstein), a skilful preparer, manufactures very 
 valuable tests. The ordinary graduated test consists of twenty 
 diatoms placed in a line, more and more difficult to resolve. 
 
PHOTOGRAPHIC TEST. 
 
 6 7 
 
 This test can replace the test of Nobert up to a certain point. 
 Ed. Thum. the skilful diatom-mounter (35, Bruder Strasse, Leipzig) 
 furnishes similar tests. 
 
 We cannot too strongly recommend microscopists to obtain at 
 least this graduated test, which will render them the greatest service 
 in appreciating the relative value of different objectives. 
 
 We give below the number of striae in the diatoms in Moller's 
 test, as well as those in some other diatoms often employed : — 
 
 Number of striae in the diatoms of Moller's Test, together 
 with the corresponding groups of Nobert. 
 
 
 Number of strict of 
 
 Moller's Test. 
 
 Nobert 's Test, in 19 groups. 
 
 
 different specimens. 
 
 
 
 
 
 No. 
 
 Number of 
 transverse 
 
 
 
 
 
 strte (1). 
 
 
 
 400 per mill. 
 
 
 per mill. 
 
 
 (small) 
 
 700 to 900 
 
 
 
 
 400 to 600 
 
 2 
 
 455 
 
 1st = 443 per mill. 
 
 
 1000 to 1200 
 
 
 
 
 
 
 3 
 
 640 
 988 
 
 2nd 665 ,, 
 
 
 
 4 
 
 5 
 
 i°55 
 
 4th = 1 108 
 
 
 
 6 
 
 1330 
 
 5th = 1329 ,, 
 
 
 
 7 
 
 1429 
 
 
 
 1400 to 1500 
 
 8 
 
 1300 
 
 
 
 
 9 
 
 1734 
 
 
 
 
 10 
 
 i73 6 
 
 7th = 1772 ,, 
 
 
 2200 to 2300 
 
 11 
 
 •738 
 
 
 variable 
 
 12 
 
 2429 
 
 
 
 
 13 
 
 2153 
 
 
 
 3000 to 3100 
 
 14 
 
 2531 
 
 9th = 2215 ,, 
 
 
 
 IS 
 
 2225 
 
 
 3000 to 3200 
 
 16 
 
 2650 
 
 nth = 2658 ,, 
 
 
 
 17 
 
 2531 
 
 
 Vanheurckia viridula (medium) . . 
 
 2700 
 
 
 
 
 
 3000 
 
 
 
 
 
 
 
 
 
 
 3400 to 3500 
 
 18 
 
 3193 
 
 13th = 3101 ,, 
 
 
 
 19 
 
 3334 
 
 14th = 3323 ,, 
 
 
 3700 
 
 20 
 
 3614 
 
 Between the 15th and 
 
 
 
 
 
 16th groups. 
 
 4.— Photographic Test of the Author. 
 
 For some years we have employed, as a test, a detail in the 
 structure of the Pleurosigma, which we have only made known to 
 one or two friends up to the present time. 
 
 If the median nodule of the Pleurosigma be carefully examined, 
 there will be seen at each extremity, a small ball, which is terminated 
 by an excessively delicate thread, and lost in the raphe'. 
 
 (') After Morley in Monthly Mic. Jour., vol. xiii., page 241. 
 
 ¥2 
 
68 
 
 PROFESSOR ABBE'S TEST. 
 
 It is probable that this thread is a caniliculum, and that what 
 seems to be a small ball is its opening. Thus the whole appearance 
 is similar to that which presents itself in large navicular 
 
 Whatever this detail which we are describing, 
 I and which is seen most clearly in Pleurosigma Baltuum 
 
 (fig 60) may be, it is of extreme delicacy, and in many 
 cases is very useful. 
 
 We have often used it as a standard test for realizing 
 the defining power of wide angled objectives, but it is 
 used with great advantage in photography. 
 
 Moreover, since the detail is much more delicate than 
 any diatom stride whatsoever, it is rendered invisible by the 
 slightest difference in focussing. 
 Fig. 60. It can, therefore, be used in photo-micrography, 
 
 1st, to estimate the defining power of objectives, 
 2nd, to prove if an objective has a chemical focus. In this 
 case, after the object has been carefully focussed on the ground-glass, it is 
 photogranhed first in blue monochromatic light, then in white light; 
 if it has no chemical focus, the image ought to be just as clear 
 in the two negatives. This experiment enables us to prove that the 
 objective i - 6 N.A. of Zeiss has not a chemical focus which an English 
 microscopist supposed that he had found in it. 
 
 3rd. Lastly, the correct adjustment of magnifying glasses, used for 
 focussing, can be verified, which is of the greatest importance. For 
 this purpose, an apochromatic objective of high power must be used. 
 If the lens is properly adjusted, the object should appear as clear in 
 the negative as when sharply focussed on the ground glass. 
 
 5. — Professor Abbe's Test. 
 
 Professor Abbe's Test is a very simple piece of apparatus of very 
 moderate price, and is absolutely indispensable to every microscopist 
 who desires to do serious work. Indeed, this small piece of apparatus 
 enables it to be possible to determine : — 
 
 1 st. Whether the chromatic and spherical aberrations of the 
 objective are well corrected. 
 
 2nd. What is the best thickness of cover glass for any objective 
 (without collar correction), employed for any given length of tube ; 
 and inversely, what is the best length of tube for a given thickness 
 of cover-glass, with the same object-glass. 
 
PROFESSOR ABBE'S TEST. 
 
 69 
 
 3rd. What is the best position of the correction collar (for an 
 objective possessing one) in each of the two preceding cases. 
 
 All these problems are very easily solved with the Abbe test. 
 
 Fig. 61. Fig. 62. 
 
 This test consists simply of an ordinary glass slip having fixed 
 to its surface (fig. 61) six cover-glasses, of thicknesses '09, "n, -14, '17, 
 '20, and "23 mm. respectively, or in other words, i-ioth, i-gth, i-6th, 
 1 5th, and i-4th of a millimetre. 
 
 The lower surface of each (which is fastened to a glass slip by means 
 of Canada balsam) is silvered, and in this silver film is traced a series 
 of six bands, composed of fine and very close lines (fig. 62.) 
 
 The mode of employing this apparatus is very simple and is carried 
 out in the manner described below ; it can only be effected with a 
 microscope possessing an Abbe condenser or a similar piece of 
 apparatus. 
 
 We begin by sharply focussing a group of lines on the plate, and 
 when that is done the diaphragm is drawn away from the axis in a 
 direction parallel to the lines, and light is rendered so oblique as to 
 allow the luminous cone to pass by the marginal zone of the ob- 
 jective. 
 
 ^ Thus, for example, if the lines appear to lie in 
 
 ■ the direction A (fig. 63), the diaphragm is so 
 
 placed that on looking into the tube after the 
 
 ocular has been withdrawn, one can see the open- 
 ing of the diaphragm in the position C in the 
 circle B representing the opening of the objective. 
 
 If an immersion objective, whose aperture exceeds 
 ro, be used, the glass slip should first be con- 
 nected with the sub-stage condenser by means of 
 a drop of water, glycerine, or cedar oil, in order 
 that the luminous rays may not reach the marginal 
 zone of the aperture of the objective. 
 These preliminary arrangements having been made, we can now begin 
 to make our test. 
 
 Fig. 63. 
 
7° 
 
 THE OCULARS. 
 
 If the objective has a correction collar it should be turned until 
 the position is found in which the delimitation of the lines when 
 examined in the middle of the field appears as clear as possible, that 
 is to say, when the lines show neither nebulosity nor any want of 
 distinctness. 
 
 The nebulosity (or milkiness as it is often called) shows that the 
 correction has been exceeded ; if on the other hand there is under- 
 correction, the edges do not become milky, but they are indistinct and 
 diffuse. If in the latter case, the objective is adjusted so as to 
 obtain as clear an image as is possible, this clearness will not continue 
 when the diaphragm is brought back into such a position as to substitute 
 axial for oblique light ; indeed, to obtain a clear image the focussing 
 must be changed. 
 
 If the objective has no correction collar the best thickness of cover- 
 glass for a given length of tube can be found in the same way, and 
 inversely the length of tube which is most suitable for a given thickness 
 of cover-glass can be similarly determined. 
 
 The above method gives very sure results ; but naturally it requires 
 a little experience to appreciate such nice differences as the above. 
 
 II. THE OCULARS. 
 
 It is the object of the ocular to magnify the image produced by the 
 objective and to render it plainer and clearer. 
 
 Microscopists make use of various kinds of oculars, the most useful 
 forms of which are : Huygens' ocular, Ramsden's ocular, the Orthoscopic 
 ocular, the compensating ocular, and the projection ocular. 
 
 Huygens' Ocular — This is the ocular which usually accompanies 
 microscopes. It consists of two plano-convex glasses, having their 
 convexities turned towards the objective and fixed at the extremities of a 
 tube. The smallest of the glasses, which really magnifies the image, is 
 nearest the eye, and is called the eye-glass. The lower one is called 
 the field-glass or collecting glass ; it diminishes the magnification of the 
 eye-glass, but renders the image clearer. A diaphragm is placed very 
 near the focus of the eye-glass. 
 
 Microscopes are provided with a series of Huygens' oculars. On the 
 Continent they are generally numbered from i to 5 ; in England they 
 are either marked r, 2, &c, or A, B, &c. up to F; in America they 
 often designate oculars by their magnifying power, and speak of them 
 
THE OCULARS. 
 
 71 
 
 as oculars of an inch, J inch, &c. The magnification of Huygens' 
 oculars, ranges from 2-5 to 15. When it exceeds six and seven times, 
 these oculars loose in clearness enormously and become very inconvenient, 
 because the eye has to be put very close to the upper lens or eye-glass. 
 
 Ramsden's Ocular. — In this ocular there are again two plano- 
 convex lenses, but the convexities are turned towards each other and the 
 lenses are nearer to one another. This ocular acts as a magnifying 
 glass and is used almost exclusively for microscopic measurements, 
 because the extreme divisions of the micrometric scale are not distorted 
 as is the case with Huygens' ocular. 
 
 Orthoscopic Ocular. — This ocular is nearly always used for photo- 
 micrography — formerly introduced to assist the ocular ; but this is a process 
 which will, we believe, be abandoned at no very distant date, and be 
 replaced exclusively by the use of the projection or compensating ocular. 
 
 In this ocular the eye-glass is achromatised and is concavo-convex ; 
 the field-glass is bi-convex. 
 
 Compensating Ocular. — This ocular was invented by Prof. Abbe 
 and has only been a commercial article since 1886. 
 
 It consists of a plano-convex lens placed above a bi-convex 
 lens. The lenses are brought close together so as almost to touch, 
 and no diaphragm is placed between them. The upper lens is simple, 
 the bi-convex lens on the other hand is complex ; a series of diaphragms 
 is placed under the latter. Such are the oculars 8, 12, 18, and 27. 
 
 In the weak ocular it is, on the other hand, the upper lens which 
 is achromatised. 
 
 In the two very low numbers 1 and 2, both lenses are simple. 
 
 In numbers 1, 2, and 4, a diaphragm is placed between the two 
 lenses. The weakest eye-pieces, numbers 1 and 2, which are called, 
 searcher oculars, are only intended for a general examination of the 
 preparation. 
 
 The others are the working oculars. 
 
 These oculars are constructed for the purpose of correcting certain 
 defects inherent in apochromatic objectives, and which cannot be 
 eliminated in the construction of the objective itself. They give a 
 perfect image with apochromatic and other wide-angled objectives, but 
 they are not suitable for ordinary small-angled objectives. 
 
 The number of these oculars indicate also the magnification which 
 they give with any objective for the length of tube indicated. 
 
 Thus the ocular 4 amplifies the image given by the objective four 
 
72 
 
 THE OCULARS. 
 
 times. Supposing we have to work with a i-ioth of an inch whose 
 initial magnifying power ij 100 : we should then have 100x4 = 400 as 
 the total magnifying power. 
 
 These oculars are also mounted, so that they can be interchanged 
 without alteration of focus, and all (except No. 27) permit the use of 
 the camera lucida with ease. 
 
 Projection Oculars. — -These oculars, which are also due to Professor 
 Abbe, are designed to project the image presented by the objective on 
 to a screen ; they are therefore eminently suitable for photo-micrography 
 and will ultimately, it is believed, be exclusively used by all photo- 
 micrographers. 
 
 They are indeed very convenient and give remarkably clear and pure 
 images. 
 
 These oculars are composed of a small achromatized bi-convex 
 projecting glass and of a plano-convex collecting lens or field glass, having 
 its curved surface turned towards the objective. A small diaphragm limits 
 the field of the image, and the projecting glass which is moveable 
 can be adjusted as desired. 
 
 The entire system is carefully corrected chromatically, and with special 
 care for secondary chromatic aberrations and for the difference of focus 
 between the chemical and optical rays. 
 
 The image projected on a screen, or on a sensative plate, preserves 
 exactly the same arrangement of all its parts as in ordinary 
 observation. Moreover, the preparation having been arranged as desired, 
 all the subsequent operations consist in replacing the ordinary ocular 
 by a projection ocular, and in screwing up or down the projection 
 system till the edge of the diaphragm is shewn with maximum clearness 
 on the screen or on the ground glass of the photographic camera. The 
 shorter the distance between the screen (or ground glass) and the 
 microscope, the farther should the projection system be withdrawn from 
 the diaphragm ; the projection system ought then to stand out much 
 farther from the tube. 
 
 As soon as the image is quite clear, the ordinary photographic 
 operations may commence. One point, however, must be observed 
 during the preliminary operation, viz. : — That throughout the operations 
 the tube of the microscope should be kept at exactly the same length, 
 that length naturally being that for which the objective is constructed. 
 
 Projection oculars hitherto have been constructed by Messrs. Zeiss, 
 Reichert, Powell and Lealand, and some other makers ; the first two makers 
 
THE OCULARS. 
 
 make different oculars for the Continental tube and the English 
 tube. 
 
 The oculars for the Continental tube (of 160 millimetres) have an 
 amplification of two and four times. Those of the English tube (of 
 250 millimetres) are marked 3 and 6. 
 
 These figures at once indicate the number of times the initial 
 magnification of the objective is multiplied by the ocular, or in other 
 words, the proportion by which the focal length of the whole 
 microscope is diminished. 
 
 Thus the ocular No. 2 diminishes the focal length of any objective 
 just one-half; consequently an objective of 3 m/m ('/ 8 inch) projects 
 with this ocular, at a given distance, an image whose size is absolutely 
 identical with that which an objective, whose focus is 1-50 m/m 
 (7,6 inch), will give 'without an ocular at the same distance. 
 
 The linear magnification of a projected image is the distance between 
 the image and the posterior focus of the lens system divided by the 
 focal length of the system. The posterior focus of the lens system 
 corresponds in the microscope exactly to the upper side of the ocular. 
 
 It follows from the preceding data that the amount of amplifica- 
 tion of an image is obtained for any distance between ocular and 
 screen by dividing this distance (expressed in millimetres) by the focal 
 length of the objective used and multiplying the quotient obtained by 
 the number of the ocular. 
 
 Thus the objective of three millimetres (7s inch) will give with No. 2 
 projection ocular at a distance of 150 centimeters an amplification of 
 1000 times: 
 
 1500 
 
 x 2 = 1000 
 
 3 
 
 However, this rule only applies to long distances ; with short 
 distances the results which the calculation gives are too high. 
 
 The diameter of the image on the screen or on the photographic 
 plate permits about 
 
 7 S of the distance from the image with oculars 2 and 3. 
 
 I3 " " " " » 4 5! 6. 
 
 The image can be taken to as great a distance from the ocular 
 as may be desired. The minimum distance between ocular and image 
 may be 40 centimeters with Nos. 2 and 4, and 25 centimeters with 
 Nos. 3 and 6. 
 
 Weak oculars are preferable for projections made during lectures or 
 
74 
 
 THE OCULARS. 
 
 demonstrations as well as for photography with small magnification or 
 
 with a long camera. On the other hand the strong oculars 3 and 6 
 
 will be preferable when it is desirable to photograph with a short 
 length of camera 
 
THE STAGE. 
 
 75 
 
 II.— THE STAGE. 
 
 The stage should be large enough to allow the two hands to rest 
 on it, and sufficiently thick to resist the pressure of the fingers. Generally 
 it is made of blackened sheet brass, sometimes it is covered with ebonite 
 or with a plate of black glass to prevent its being injured by reagents. 
 
 There are usually two clips on the stage for the purpose of holding 
 the preparation in position. 
 
 In some instruments the stage — then called a rotating stage — can turn 
 right round on its axis, carrying with it the part of the preparation 
 in view ; the result of this is that every part of the object is illuminated 
 in turn, while the mirror remains stationary. This piece of apparatus 
 is expensive, and is now very little used owing to the universal practice 
 of employing condensers instead. However, it is found to be very useful 
 when the object has to be placed in a particular position in order to 
 facilitate the drawing of it by the camera lucida. 
 
 In first-class English instruments only the moveable part of the stage 
 turns. This arrangement is more convenient than the rotary stage, but 
 it- needs to be constructed with great accuracy, which naturally is 
 expensive. 
 
 Mechanical stage. — Large English instruments, and also those of M. 
 Nachet, have stages with moveable carriers, that is to say they consist 
 of several superposed pieces, rendered moveable by means of milled heads, 
 which enable all the parts of the object to be taken in turn across the 
 field of the microscope, and also allow the object, when once found, 
 to be kept in view and in the most advantageous position, either for 
 making a drawing of it or for micrometrical measurement. 
 
 English stages and sub-stages. 
 Fig. 64. 
 
 M. Reichert manufactures a very convenient form of mechanical 
 stage, which can be removed at pleasure, but this apparatus is not quite 
 the same as the English mechanical stage. 
 
76 THE STAGE. 
 
 A similar apparatus is also made by M. C. Zeiss (fig. 65), the 
 
 Fig. 65. 
 
 engraving of which sufficiently explains its construction. As may be seen, 
 it is a simple stage placed on that of the microscope, and fixed to 
 the slow movement pillar by means of the plate B and the screw 
 K, the end of which is lodged in a small hole drilled into the pillar 
 when the stage is first applied. 
 
THE MOVEMENTS. 
 
 77 
 
 III.— THE MOVEMENTS. 
 
 We have already indicated the two movements which every first-class 
 microscope should possess. For the rapid movement, the rack and pinion 
 is very superior to the sliding movement, and by it alone can the perfect 
 centering of the tube in relation to the illuminating apparatus be maintained. 
 
 The slow movement of microscopes is 
 generally produced by two round or 
 triangular tubes, the one fitting into 
 the other, which tend to part from one 
 another under the pressure of a spiral 
 spring. A screw of very fine pitch 
 bearing a milled head enables the space 
 between them to be varied to any desired 
 extent. By examining figure 66 at the 
 side it will be understood that by means 
 of this movement the outside tube, which 
 carries the microscope tube, is displaced, 
 and the objective approaches or recedes 
 from the object. 
 
 In some large instruments the slow 
 movement is extremely accurate and 
 delicate, nor could it indeed be too 
 much so, because the slow movement 
 serves to accommodate the observer's 
 eye, and upon the delicacy of this 
 micrometric movement depends the 
 power of producing the finest optical 
 planes in an object, and consequently 
 the power which enables us, in a 
 Fig. 66. greater or less degreee, to appreciate 
 
 the structure of the object under examination. 
 
 In the microscope constructed according to our specifications by Messrs. 
 Watson and Sons, which we shall describe further on, the slow movement 
 is communicated to the tube by a lever, and this slow movement, 
 which has absolutely no back lash, is of such precision that each of 
 the divisions of the milled head corresponds to 1/1300 of a millimetre. 
 
78 
 
 ILLUMINATING APPARATUS. 
 
 IV.— ILLUMINATING APPARATUS. 
 
 I.— ILLUMINATION BY TRANSMITTED LIGHT. 
 
 The apparatus employed in microscopes for illuminating objects by 
 transmitted light are mirrors and condensers. The illumination is modified 
 
 by diaphragms. 
 
 i. Mirrors — Every microscope is furnished with a mirror; in good 
 instruments the mirror is double, being plane on one side, and concave on 
 the other ; the mirrors of English instruments are much larger than those 
 of continental instruments, and consequently give more light, which is 
 often advantageous. 
 
 It was formerly believed that by means of a plane mirror parallel 
 rays were obtained, and that the concave mirror produced convergent 
 light. This is not so : the plane mirror also gives convergent light, for 
 since all luminous sources at the service of the microscopist for ordinary 
 work give divergent rays, it follows that the rays converge by reflection 
 from the mirror. It may therefore be asked how it is that the plane 
 mirror gives a less intense illumination than the concave mirror. Mr. 
 Giltay (') explains this in a very simple manner. He says that if PQ be 
 
 an opening of finite dimensions, through which 
 rays from a luminous source can alone be 
 admitted (e.g., a window), then the point O 
 will receive no light reflected from the ends 
 A and E of the plane mirror because the 
 incident rays must have travelled along the 
 lines A'A and E'E, which do not pass through 
 P Q. Finally the figure shews that light can 
 pass through PQ which will strike the concave 
 mirror at points a and e situated outside B 
 and D and be reflected into O, which demon- 
 strates that the useful surface of a concave 
 mirror is greater than that of a flat mirror. 
 The entire difference between the two 
 mirrors consists simply in the fact that the concave mirror can receive 
 and reflect on a given point a greater number of rays proceeding from 
 a circumscribed luminous source than can a flat mirror. 
 
 A well-mounted mirror should be capable of being raised and lowered 
 so that the different sections of the luminous cone which falls on the 
 
 Fig. 67. 
 
 (*) E. Giltay. Inleiding tot het gebniik van den microscoop. Leiden. 1885. 
 
THE DIAPHRAGMS. 
 
 79 
 
 object can be utilized ; it should also have the means of projecting the 
 luminous cone at every possible angle so as to give what is called "oblique 
 illumination;" this latter property is obtained by mounting the mirror 
 on a jointed arm. 
 
 2. Diaphragms. — Diaphragms are pieces of copper, pierced with 
 an opening. These are placed so as to intercept more or less of the 
 luminous beam reflected from the mirror. 
 
 There are two kinds of diaphragms : ist, those which cut off" a part 
 of the peripheral rays of the beam of light reflected by the mirror ; 
 2nd, those which intercept a part of the central rays. 
 
 In the first case, either a plate is employed, pierced with apertures 
 of different diameters and fixed so that each of these apertures can 
 be brought immediately under the stage opening, or a tube is used 
 carrying in its upper extremity some brass discs pierced with holes of 
 different sizes and capable of being adjusted at different distances from 
 the stage. 
 
 This second method, which admits of 
 a finer graduation of light is preferable. 
 
 Some time ago a third kind of 
 diaphragm was introduced, called the 
 Iris or Contraction Diaphragm. 
 
 The invention of this diaphragm 
 seems to have been made by Ch. 
 Chevalier. We recollect seeing some 
 twenty years ago, in the collection of 
 Dr. Arthur Chevalier, a model con- 
 structed by his father, and called by 
 Pi„_ gg. him an artificial pupil. It consisted of 
 
 a large number of small metallic plates 
 partially overlapping one another and so arranged that by means of a knob 
 placed on the frame which held them, the opening could be increased 
 or diminished, while at the same time it remained perfectly circular. 
 
 Charles Chevalier, so far as we know, never applied his "pupil" 
 to one of his instruments. It remained merely a model, and like many 
 of the ideas of this indefatigable inventor, was never practically applied. 
 
 The Iris-diaphragm is superior to all others; by its means light can 
 be graduated quickly and with exactness. 
 
 It is often useful to cut off a part of the central rays of the 
 luminous beam in order to reveal very delicate details; to effect this 
 
So 
 
 THE CONDENSERS. 
 
 a glass plate, whose centre is covered with a small black disc, may 
 be placed over a diaphragm with a large opening, or, still better (as is 
 generally done), a similar disc may be fixed in the centre of a diaphragm 
 by means of three thin brass strips. 
 
 Diaphragms are placed below and very close to the stage, when they are 
 used without a condenser. When, however, this latter apparatus is employed 
 the diaphragm is usually placed beneath the lower lens. For certain oblique 
 illuminations it may be placed above the upper lens, but this often occasions 
 inconvenience — as for instance, when an immersion condenser is employed. 
 
 3. Condensers. — The condenser consists of a system of lenses 
 which is placed between the mirror and the stage, and allows a bright 
 cone, containing many more rays than that which the mirror alone 
 produces, to be thrown upon the object. 
 
 The first true condenser was invented by Dujardin, and soon after- 
 wards was brought to great perfection in England. For a long time 
 condensers remained almost unknown on the Continent, but as early as 
 1858 the late M. Nachet, (the first optician of that name, who was a 
 pupil of Ch. Chevalier and set up for himself) had invented a condenser, 
 the optical formula of which closely resembles that of Prof. Abbe's 
 condenser, but unlike the latter it was achromatic. The late M. Villot, 
 Inspector-General of the Imperial Museums at Paris, acquainted us with 
 this condenser in 1865, by forwarding to us an exact plan of the 
 arrangement of the lenses, and of their curvatures. The letter of this 
 microscopist enables us to fix the date. A little later, the collections 
 of M. Mouchet, which we acquired at his death, put us in possession 
 of one of these early condensers, which are constructed very 
 similarly to those of M. Villot. Although out of date, and of an aperture 
 very inferior to that which the firm of Nachet provides to-day (fig. 69), 
 
 we greatly appreciate this old condenser, 
 and still find it occasionally useful. 
 
 It was not until after Prof. Abbe 
 had in 1873 announced the condenser 
 which bears his name, that Continental 
 microscopists began to appreciate the 
 value of these instruments. Now every 
 good microscope possesses an Abbe 
 condenser, and we are persuaded that 
 in addition to this the achromatic 
 condensers which the English employ 
 will also be ultimately employed on the Continent. 
 
THE CONDENSER. 
 
 81 
 
 The number of condensers used is considerable, and every possible 
 form exists in England. We shall content ourselves by describing here 
 the Abbe condenser and those of Messrs. Powell and Lealand, which 
 are really the best kinds in existence 
 
 The Abbe Condenser. — This apparatus (fig. 70), which is at the 
 present time manufactured by the firm of Zeiss and all good makers, 
 mainly consists of a condenser or system of wide-angled lenses, of a 
 diaphragm carrier capable of holding discs with various apertures, and 
 of a plano-concave mirror. These three parts are placed on a single 
 mount, which can be adapted to the microscope by means of a groove 
 under the stage, in which the ordinary mirror is usually placed. 
 
 The flat surface of the upper lens of the condenser should be 
 brought very nearly to the level of the stage, till it almost touches the 
 under surface of the glass slip. The whole apparatus can be raised or 
 lowered at will by means of a rack and pinion, and thus the rays 
 can be focussed on the plane in which the object under examination 
 lies. 
 
 G 
 
8 2 
 
 THE CONDENSER. 
 
 To insert or change a diaphragm, the carrier is drawn to the right, 
 which makes it turn on an axis; the diaphragm is then inserted, 
 and the carrier is returned to its original position by a movement in 
 the opposite direction. The rack adjusted to the carrier enables the 
 opening to be withdrawn from the optic axis in the first place, and 
 then while thus in an eccentric position to be turned about this axis. 
 In this latter movement, the bar which carries the pinion, acts as a 
 lever only. By the first movement, just described, all degrees of oblique 
 illumination are obtained, and by the second, incident direction of the 
 luminous rays is changed at pleasure. 
 
 While the rack is being rotated the catch of a small spring point 
 shows that the position of the diaphragm is centred so as to give direct 
 or axial illumination. 
 
 For observations by transmitted light stops with central apertures are 
 used, and the size of the opening chosen depends on the focal distance 
 of the objective, the nature of the preparation and the intensity of 
 the source of light employed. 
 
 If desirous of examining preparations whose elements are rendered 
 visible, not by the unequal absorption of the light, but rather by differences 
 of refraction, the observer is generally recommended to use the smallest 
 possible stop, which will give sufficient light. On the other hand, 
 when examining some stained preparations, especially bacteria, bacilli, &c, 
 stops of large apertures, or even the condenser without any diaphragm, 
 can be advantageously employed. 
 
 When wishing to make observations on a dark ground, annular 
 diaphragms (which are wheel-shaped) are used ; they are placed 
 like the others in the diaphragm carrier, but always in a central 
 position. In this case, a reduction in the aperture of the objective 
 also is necessary, or at least advantageous, for all powers except the 
 lowest. 
 
 This reduction is effected by means of special diaphragms, which 
 are placed above the upper lens of the objective, or screwed between 
 the cone and the lenses when these parts can be separated. 
 
 In the case of dark ground illumination the useful rays are those 
 which pass through the object. Consequently this mode of examination 
 is unsuitable for the examination of opaque objects. 
 
 Objectives with correction collars cannot be used with this kind of 
 illumination, because in their case the interception of the marginal zone 
 is not easily effected by the aid of a diaphragm. 
 
THE CONDENSER. 
 
 83 
 
 AVhen wishing to obtain with immersion objectives, an illumination 
 as oblique as possible, or an illumination on a dark ground, with high 
 powers, it is advantageous to put a drop of water on the flat surface of 
 the upper lens of the condenser, so that the space between this lens and 
 the glass slip is occupied by a medium, more refractive than air. By 
 this means rays are rendered useful for observation, which, otherwise, 
 would not be able to reach the preparation by reason of the total 
 reflection, which they would undergo in the space of air intervening. 
 The condenser is then called an immersion condenser. 
 
 When a wide-angled condenser is used with oblique illumination and 
 an objective, whose numerical aperture exceeds i"33, the upper lens of 
 the condenser must be connected with the glass slip with a drop of 
 cedar oil, and not with a drop of water. 
 
 For observations with polarised light, the polarising Nicol Prism, by 
 means of a special mounting, is placed in the diaphragm carrier and an 
 ordinary diaphragm is placed over it. The other operations are carried 
 out as before mentioned. Polarised light, in addition to ordinary light, 
 can be used with oblique as well as with axial illumination. 
 
 Films of selenite or of mica are placed above the Nicol Prism on the 
 diaphragm carrier, and a diaphragm is placed over them as before. 
 For all observations with the condenser it is the rule to use a flat mirror ; 
 however, with very low objectives, the flat mirror does not generally allow 
 the field to be regularly illuminated, and the concave mirror is then used. 
 It is only for this reason that it forms part of the apparatus. 
 
 In all the above cases when once the mirror is conveniently 
 arranged for illumination, it is unnecessary to re-adjust it when changing 
 the diaphragms. 
 
 If observations be made with lamp-light, the use of either a bulls- 
 eye condensing lens, the largest possible, or a glass sphere filled 
 with water, is highly recommended, to obtain a unifoim illumination, 
 for otherwise the flame would have to be brought too close to 
 the microscope. The lens or glass sphere is arranged between the 
 lamp and the microscope in such a position that the image of the 
 flame is projected on to the mirror. 
 
 At the present time the firm of Zeiss divides this condenser into 
 three optical parts. The first has a numerical aperture of V2, the 
 second of 1*4, and the third, which only dates from 1889, of i - 6, 
 and is designed on purpose for their new objective of N. A. 1 - 6. 
 
 These three optical parts can be interchanged at will. 
 
 G2 
 
8 4 
 
 THE CONDENSER. 
 
 Messrs. Zeiss just recently have introduced an achromatic condenser, which 
 which can be placed above the mechanical part instead of the previous ones. 
 
 This condenser has a numerical aperture of ro, and is designed 
 especially for photo-micrography, but it is also of the greatest service 
 in ordinary observations. 
 
 On account of the extreme ingenuity with which the mechanical 
 part of the Abbe condenser is made and of the enormous aperture 
 which it gives to the optical part, this piece of apparatus is of 
 inestimable value, and can scarcely be equalled for serious work. It 
 is always a special advantage in photography that the chromatic optical 
 part should be replaced by an achromatic system. 
 
 Messrs. Powell and Lealand's Condensers. — Except as regards 
 the enormous aperture of i 6 lately obtained by the Abbe con- 
 denser, Messrs. Powell and Lealand's condensers effect all that is 
 possible with any other condensers, and moreover they are made 
 with unsurpassed perfection, the achromatic oil condensers especially. 
 
 This firm constructs five condensers, which we intend duly to 
 examine in order. 
 
 Chromatic Oil Condenser. — This little piece of apparatus, as 
 ingenious as it is cheap, is perfectly suited to the resolution of diatoms 
 with oblique light. 
 
 The optical part consists of two lenses placed one above the 
 other and so mounted as not to admit of being unscrewed. The 
 mechanical part consists of a tube which can be fitted to the substage 
 carrying the optical part above it. By means of a small cogged arm, 
 which is pushed to one side, a central opening, giving axial illumina- 
 tion at first and then a marginal opening, giving feeble oblique light, 
 are successively disclosed. By pushing the arm one cog more very 
 oblique light is obtained, and finally when the last cog is reached 
 two marginal openings, mutually at right angles, are disclosed. 
 
 The condenser is used dry, as a water, or as an oil immersion, 
 and in the last case it has a numerical aperture of i'35. 
 
 By replacing the optical part, already described, by another, the 
 upper lens of which has the middle part truncated and set on end, 
 a N. A. of i "4 is obtained. 
 
 This little piece of apparatus can be used not only for diatoms, 
 but in all cases when the Abbe condenser is employed except when 
 very great axial aperture is required. 
 
 Achromatic Condenser. — The Achromatic Condenser of Messrs. 
 
THE CONDENSER. 
 
 85 
 
 Powell and Lealand, has an aperture of 170 0 ; in England it enjoys 
 an immense reputation, and of late by its means the most important 
 discoveries of Beale, Dalhnger, Sec, were made. 
 
 The optical part of this condenser consists of three lenses placed at 
 fixed heights above the diaphragms. These form a double series, 
 placed one above the other. The upper series has openings of increasing 
 size, like ordinary rotating diaphragms. The second series is fitted with 
 circular diaphragms having openings of various forms. The two 
 series can be turned either together or singly, as desired. 
 
 Apochromatic Dry Condenser. — This condenser has been 
 constructed within the last few months ; the corrections for chromatic 
 and spherical aberrations are as perfect as possible ; its N.A. is C95. 
 This condenser is specially made for use in photo-micrography. The 
 optical part is screwed into the same mounting as the previous one. 
 
 Achromatic Oil Condenser. — More perfect than the preceding, 
 the optical part can be used dry, as a water and as an oil immersion, 
 when it has a N.A. of 1-35. 
 
 The optical part consists of four very large lenses placed one above 
 the other and very close together. 
 
 The mechanical part consists of a tube in which a cell, with eccentric 
 movement, receives the diaphragm plates, which are introduced under 
 the lower lens. 
 
 Whenever the diaphragm is changed, the entire condenser has to 
 be lowered, which is very inconvenient. This inconvenience does not 
 exist in the Apochromatic Condenser. 
 
 Apochromatic Immersion Condenser. — This is the most perfect 
 condenser which exists at present. Messrs. Powell and Lealand do not 
 
 Fig. 71. 
 
 Fig. 72. 
 
 The optical part, 
 which has a N. A. 
 of 1 '40, is formed of 
 four lenses placed one 
 above the other, and 
 
86 
 
 THE CONDENSER. 
 
 0 ®®*>## 
 
 Fig. 73- 
 
 the whole is made apochromatic. The mechanical part admits of every 
 conceivable kind of illumination. It consists of a tube, the lower part 
 of which fits into the substage and the upper part holds the optical 
 part (figs. 71 and 72). 
 
 In this tube slides another tube having, like the last mentioned 
 condenser, a ring with an eccentric movement. But since this second 
 tube can be raised or lowered, at will, it is unnecessary to displace 
 the condenser in order to change the diaphragms, and, moreover, the 
 movement of the inner tube allows the diaphragm to be taken a 
 considerable distance from the lenses. The adjustment of diaphragms 
 in a tube (without a condenser) is thus rendered possible, and the 
 illumination can be graduated with the nicest delicacy, while at the 
 same time, by changing the diaphragms, every conceivable kind of 
 illumination can be obtained. 
 
 Rules for using a Condenser.- — Professor Abbe's condenser is 
 mounted in a holder which fits a spring jacket on the microscope, 
 except in instruments quite recently constructed. This has its 
 advantages for ordinary observations, but then, on the other hand, 
 certain researches with flame images cannot be conducted, researches 
 which aim at the solution of the most difficult problems in microscopy ; 
 for example, the discovery of the ultimate nature of the diatom valve, 
 the flagellum of bacteria, &c. The following rules apply to any kind 
 of condenser whatsoever, which can be axially raised and lowered. 
 
 The object is first placed on the stage and focussed as accurately 
 as possible, then by properly adjusting the flat mirror the image of the 
 luminous source is directed to fall on the middle of the field, and the 
 condenser is racked up or down to allow the image to be distinctly 
 seen with maximum clearness. This is called observation in the image 
 of the flame, because this illumination, in which focussing is most 
 frequently conducted, allows details to be observed which are invisible 
 in every other kind of illumination. 
 
 This illumination, however, is sometimes disagreeable, and in obser- 
 vations which do not require great precision the condenser is lowered 
 until the field of the microscope is uniformly illuminated. If the light 
 is too bright, it is modified by means of a blue or neutral tinted glass. 
 
THE CONDENSER. 
 
 87 
 
 If daylight be employed the object is focussed as before, and then 
 the mirror is directed towards a distant object, which is also focussed 
 so that the image coincides with that of the object. The mirror is then 
 shifted without changing the condenser, and is directed towards a white 
 cloud or a wall, on which light falls, or some other luminous source. 
 
 With daylight, as with artificial illumination, a suitable diaphragm 
 must be used, viz. : one with which the object can be seen clearly, 
 without being overpowered with light. Should oblique illumination be 
 required instead of a diaphragm with a central opening, a diaphragm 
 with either an annular or an eccentric opening should be used. In the 
 last two cases the condenser must be raised considerably to obtain a 
 suitable light. 
 
 The following, is the last method here given of illumination, and is 
 one by which any objective can be turned to good account. 
 
 The mirror is turned aside from the axis, and the microscope is so 
 inclined that the image of the luminous source is shewn direct in the 
 same way as when it is reflected by means of a mirror. All the 
 other operations are carried out as with a reflector. 
 
 II.— ILLUMINATION OF OPAQUE OBJECTS. 
 
 Illumination of opaque objects is effected: 1st, with the mirror; 
 2nd, with condensing lenses ; 3rd, with Leiberkuhn's speculum ; and 4th, 
 with the " vertical illuminator." 
 
 1. Mirror. — Some English and American microscopes are so con- 
 structed that the object when placed with the stage, forms as it were, 
 a centre, around which all the parts of the instrument can rotate. Such, 
 among others, is the beautiful "Wenham Radial," made by Ross, in 
 which these movements are carried out in the most complete and 
 
 precise manner. In this and other 
 similar instruments, such as the 
 Ross-Zentmayer, &c, the mirror can 
 be brought above the stage. No 
 other accessory apparatus is there- 
 fore required for illuminating opaque 
 objects, provided that they are 
 examined with objectives, whose 
 frontal distances are sufficient to 
 allow the light to reach the object. 
 This latter provision applies equally to condensing lenses. 
 
88 
 
 THE CONDENSER. 
 
 2. Condensing Lenses.— Condensing lenses are simply plano-convex 
 magnifying glasses of considerable size, supported on a foot ; they can 
 be made to assume any position, by means of their many joints (fig. 
 74). These magnifying glasses when placed between the microscope 
 and the luminous source can be made to concentrate a pencil of 
 condensed rays upon the object. 
 
 3. The Speculum of Lieberkuhn. — 
 This apparatus is but seldom used. It is 
 simply a concave mirror made either of silver 
 or of silvered glass, and so constructed as to 
 concentrate a beam of condensed rays on to 
 the object (fig. 75). The mirror of the micro- 
 scope acts as a luminous source, so that there 
 must be an open space around the object, to 
 allow the rays reflected from the mirror to reach 
 the Lieberkuhn. A special Lieberkuhn is neces- 
 Fig. 75. sary for each objective. 
 
 4. Vertical Illuminator. — This little piece of apparatus, invented 
 by my friend, Professor H. L. Smith, is ingenious and moreover 
 inexpensive. 
 
 The vertical illuminator consists of a small cylindrical brass tube, 
 which can be screwed on to the microscope above the objective, and 
 
 encloses a bar having a disc fitted 
 into its end, which can be rotated 
 about its axis (fig. 76). This tube 
 has a small opening in its side, 
 through which a pencil of light is 
 thrown on to the disc, when in- 
 clined at an angle of 45 °. This 
 pencil is reflected down through 
 the objective, which condenses it 
 on to the object, and is then 
 Fig. 76. reflected back to the eye of the 
 
 observer. This apparatus can be used with any objectives, and gives 
 particularly excellent results with high powers. It renders the resolution 
 of very difficult details possible, and it was by its aid that we first 
 succeeded in photographing the beads of the Amphipleura. The 
 valves were silvered underneath, thus producing an opaque object. 
 
CHAPTER III. 
 
 ACCESSORY APPARATUS. 
 
 In the preceding chapter we have examined the essential parts of 
 the microscope. We have still to describe various accessory pieces of 
 apparatus, by which certain very important researches can be made, and 
 which in many cases are quite indispensable. 
 
 By means of these pieces of apparatus, the sizes of objects can be 
 measured, drawings of them can be made (with the camera lucida), 
 and their nature be more perfectly understood (with the polariscope 
 and spectroscope). With others, an object, once found, can afterwards 
 be quickly found again ; others facilitate work {e.g., the dissection eye 
 piece and the reversing prism), or allow the objectives to be quickly 
 changed {e.g., revolving nose pieces and adapters). These we propose 
 to examine in due order. 
 
 Micrometers. — Microscopical measurements are made by means of 
 micrometers. There are two kinds of micrometers, viz. : the eye piece 
 and stage micrometer. 
 
 Eye piece Micrometers. — The microscopist uses the eye piece 
 micrometer to measure the actual size of the object he is studying. 
 Several kinds are made, but they all depend upon the same principle, 
 as follows : in the focus of the eye-glass is placed a fixed scale, 
 whose graduations are accurately known, so that the size of the image 
 may be numerically expressed in terms of the divisions of the scale. 
 
 The eye piece micrometer as generally used consists 
 of an ordinary ocular containing a glass disc, on which 
 are traced equidistant lines, generally Y 100 °f a centi- 
 meter apart (fig. 77). This glass disc is fixed by most 
 opticians, but it is sometimes made moveable, always 
 Fig. 77. remaining, however, in the same plane ; this last model 
 adopted by Hartnack and Beneche (fig. 78), is more expensive than 
 the ordinary eye piece micrometer, but then, on the other hand, the 
 
9° 
 
 THE MICROMETER. 
 
 edge of the object can be made to coincide with one of 
 the divisions of the scale. 
 
 The eye piece micrometer, which we have just described, 
 is quite accurate enough, but sometimes a much more 
 precise and, at the same time, more convenient micrometer 
 is employed, viz., Ramsden's micrometer, a micrometer with parallel 
 threads, which is used also for astronomical observations. 
 
 An instrument of this 
 kind is manufactured by 
 the firm of Carl Zeiss, 
 composed of a positive 
 ocular mounted above 
 a brass plate, which is 
 divided into equal parts 
 by notches ; in the 
 middle of the plate is 
 found a spider's thread, 
 dividing the field of the 
 microscope into two 
 equal parts. Under- 
 neath this fixed plate 
 there is a brass frame- 
 work, which can be 
 Fi s- 79- moved in the same 
 
 plane, and has fixed to it a second spider's thread placed parallel to 
 the first one. This frame is constructed so that the thread which it 
 contains can be made to coincide with the first thread, and be with- 
 drawn from it any required distance. This latter motion is produced by 
 turning a milled head at the side of the ocular, carrying a cylinder 
 divided into a hundred divisions, which pass successively in front of a 
 fixed index. 
 
 The distance between the notches seen in the field is such that 
 whenever the cylinder arrives just halfway between two successive notches, 
 the thread registers o (Zero). The object is placed so that the threads 
 touch it on both sides ; the number of notches which the moving thread 
 has passed, multiplied by 100, and added to the number registered by 
 the cylinder, gives the relative size of the object. 
 
 With this micrometer excessively small objects can be measured ; it 
 consequently provides very valuable means of measuring the number of 
 the stria; on a diatom in a given space. 
 
CAMERA LUCIDA. 
 
 91 
 
 For the parallel thread micrometer, and also for the ordinary eye 
 piece micrometer, the actual length represented by a division must be 
 previously ascertained, always keeping the tube of the microscope of 
 constant length. We shall show how this is done in the chapter treating 
 on the measurement of microscopical objects. 
 
 Stage Micrometer. — The stage micrometer consists of a glass slip, 
 on which very fine lines at equal distances are engraved. The first 
 micrometers of any use appear to have been made by Lebaillif. We 
 happen to possess the ingenious machine made by this scientist, fifty 
 years ago, for tracing the lines, and it fulfilled its object admirably. 
 Lebaillif went so far as to divide the millimetre into 500 parts. By 
 means of the improvements which have been added to this instrument 
 the size of a division has been greatly reduced. 
 
 The micrometer usually employed has on it one millimetre divided 
 into 100 parts, and is the scale by which the value of a division in 
 the eye piece micrometer is established. 
 
 2. Camera Lucida. — It is the object of the camera lucida to 
 project the image of an object seen in the microscope, on to a sheet of 
 paper in such a way that a person inexperienced in drawing may yet 
 be able to sketch the outline without difficulty. 
 
 Though it often assumes different 
 forms, the camera lucida usually 
 consists of a prism, placed in front 
 of the ocular. 
 
 That of Oberhauser is such a 
 piece ot apparatus, and is very con- 
 venient. It consists of a prism, smaller 
 than the pupil, and is permanently 
 fixed in front of a special ocular. 
 Above this prism is annexed a ring of 
 blackened brass, close to which the 
 eye is applied. Both the paper and 
 the image reflected by the prism can 
 then be seen at the same time. 
 
 The ocular fits into an elbow- 
 piece enclosing a rectangular prism 
 Fig. 80. a t the joint of the elbow. The 
 
 whole is fixed to the microscope 
 when vertical, which is thus transformed into a horizontal microscope 
 
C)2 
 
 CAMERA LUCIDA. 
 
 The eye piece camera lucida can be removed when desired, and the 
 rest of the instrument used as an ordinary horizontal microscope. 
 In figure 80 we give the form of Oberhauser's camera lucida. 
 The apparatus above is very useful for low or medium magnifications, 
 but it is not easy to manage it with high magnifications, because such 
 a great deal of light is absorbed by the two prisms. 
 
 A camera lucida which can be used as easily with high as with low 
 magnifications is that due to Arthur Chevalier, and projects the image 
 given by a vertical microscope, and on a horizontal surface. 
 
 It consists (fig. 81) of a small steel 
 mirror, at a certain distance from which is 
 placed a prism, having its hypoteneuse 
 silvered. 
 
 Nachet's camera lucida is quite as 
 serviceable (fig. 82), and consists of a 
 rhomboidal prism, having one of its 
 oblique surfaces opposite the eye-piece, to 
 which a second very small prism is annexed. 
 The whole is enclosed in a small brass case, 
 whose distance from the eye-glass can be 
 adjusted as required. 
 
 In the camera lucida, now manufactured 
 by the skilful Parisian maker, the small prism 
 is done away with, and the reflection is 
 effected by a very thin coating of gold. So 
 thin indeed is this coating that the object may 
 be seen perfectly through it, while at the same time its reflecting power 
 is sufficient to show the sketch clearly. 
 
 Mons. Nachet also constructs a camera lucida, which can be adapted 
 
 Fig. 83. 
 
CAMERA LUCIDA. 
 
 93 
 
 to the inclined microscope. It is a convenient form and we almost 
 always use it for making our drawings with. 
 
 Professor Abbe has lately invented a camera lucida which can be 
 highly recommended. In this apparatus, represented in figure 83, the 
 paper is seen by means of a double reflection, produced first by 
 the flat mirror Sf>, and then by the silvered surface of a small prism. 
 The microscopical image can be seen directly through an opening cut in 
 the silver on the prism. In consequence of the concentricity of the 
 luminous beams proceeding on the one hand from the microscope, and 
 on the other from the surface of the paper, which reach the eye 
 simultaneously, the point of the pencil can easily be held so as to 
 coincide with the image, and a sketch thus be made on a horizontal 
 plane without any marked deformity. 
 
 The intensity of the light reflected by the paper is regulated by 
 means of two smoke-tinted glasses, which are placed in a groove adapted 
 to the mounting of the prism. 
 
 Figure 84 below shows how a microscope, which can be inclined, is 
 arranged for adapting the camera lucida. When this apparatus is 
 used, the light of the microscope must be so regulated that it is of 
 an intensity as nearly as possible equal to that from the paper on 
 which the drawing is being made ; when the light from the paper is 
 more intense than that of the microscope it is impossible to distinguish 
 the point of the pencil sufficiently well. In this case the light from 
 the paper must be reduced by means of a screen. 
 
 Fig. 84. 
 
94 
 
 POLARIZING APPARATUS. 
 
 3. — Polarizing Apparatus. — Polarisation is a modification of light, 
 in which the vibrations do not take place at all angles as is the case 
 in natural light. Polarized rays when once reflected or refracted cannot 
 be reflected or refracted again in certain fixed directions. 
 
 When any drawing is examined through a crystal of Iceland Spar a 
 double image is observed, which is produced by the incident ray being 
 twice refracted. One of these rays obeys the ordinary laws of refraction 
 and is called the ordinary ray ; the other which does not obey them is 
 called the extraordinary ray. 
 
 Substances, which possess the property ot refracting a ray twice, are 
 said to be endowed with double refraction. 
 
 An apparatus in which Iceland spar is used, called a Nicol prism, is 
 most frequently used for polarising light in the microscope. 
 
 The Nicol prism is a parallelopiped of Iceland spar, whose length 
 is equal to 37 times its thickness. This parallelopiped is cut in two 
 along the diagonal A B (fig. 85) joining the corners of its obtuse angles. 
 
 The planes of section are carefully polished and then 
 cemented together with Canada balsam. Now the index 
 of refraction of this resin (1*549) lies somewhere between 
 the ordinary index of spar (i"658) and the minimum of 
 its extraordinary index (i - 483). 
 
 The critical angle for the ordinary ray in Canada 
 balsam being 69/5, every ordinary ray which strikes at 
 a greater angle undergoes total reflection. 
 
 Let o be a ray which strikes the surface A C 
 obliquely. It will undergo refraction which will change 
 its direction. Let us suppose that it now makes an angle 
 2o - 5° with the plane of section A B, this ray will be 
 the limit of the field in which ordinary rays do not 
 occur since all such rays striking the film of balsam 
 at a greater angle will undergo total reflection. Thus all 
 the rays between the limits 0 and Ae which are ordinarily 
 refracted in the spar, will be reflected so as to form 
 Fig. 85. a luminous cone hg which will be lost on the blackened 
 surface of the prism C B. On the other hand the extraordinary rays, since 
 their index is less than that of balsam, will be transmitted through the 
 cement and will spread themselves when they emerge in the space ik. 
 
 The light, thus polarised by means of the Nicol Prism, called the 
 polariser, and placed under the stage in the place of the diaphragms, 
 passes through the object and is received by the objective above, 
 
POLARIZING APPARATUS. 
 
 95 
 
 at a variable distance after which it meets a second Nicol Prism, called the 
 analyser, which enables us to observe that the light has been polarised. 
 
 Whenever the analyser and polariser are so placed that their surfaces 
 are parallel, the field is illuminated ; but if they be turned so as 
 to be inclined an angle of 90 0 the field will appear dark. 
 
 Polarisation is the more complete as the field appears either darker 
 or brighter. 
 
 Many substances produce characteristic effects on polarised light by 
 which they can either be recognised or even their structure be determined by 
 them ; some appear to be simply lighted in the dark field of the instrument, 
 others at the same time are clothed with the most vivid colours. 
 
 Besides being of great importance to the scientist, polarisation in 
 certain cases brings before the amateur one of the most beautiful sights 
 under his control ; moreover, there is scarcely an instrument that can 
 be called in any way complete without a polarising apparatus. 
 
 Substances which produce any effect on polarised light are called 
 anisotropes ; those on the other hand which leave the field obscure are 
 called inactive substances or isotropes. 
 
 By placing thin films of selenite or mica above the polariser the 
 sensitiveness of the polarising apparatus can be greatly increased. 
 
 The polarising apparatus, usually fitted to microscopes, consists of two 
 Nicol Prisms, one of which is placed under the stage of the microscope, 
 and the other is adapted to the lower end of the body tube immediately 
 above the objective. By means of a milled head or projecting collar the 
 tube containing the prism can be rotated. 
 
 Some makers place the analyser above the ocular, but this arrangement 
 has the disadvantage of considerably limiting the field of vision. M. 
 Harting recommends a position between the two preceding ones, and 
 accordingly places the analyser at the lower end of the ocular tube. 
 
 Messrs. Hartnack and Prazmowski have invented a new kind of 
 polarising apparatus, which they have patented. This apparatus differs 
 from the Nicol Prism, in that the Iceland spar prism is of a particular 
 shape, the analyser is placed between the eye and the ocular without 
 limiting the field of vision. For a complete description of these prisms, 
 the Annates de la Societe phytologique et micrographique de Belgique for the 
 year 1867 should be consulted. 
 
 Sometimes English condensers are provided with a polariser, which 
 can be removed at will (e.g., those of Messrs. Swift) ; that which Messrs. 
 Ross and Co. adapt to their microscopes is also very good, and is called 
 Darkey's revolving selenite stage. 
 
9 6 
 
 THE SPECTROSCOPE. 
 
 Darkens Revolving Selenite Stage. — This apparatus, which is adapted 
 to the sub- or accessory stage, consists of a tube enclosing three metal 
 frames, each of which contains a layer of selenite, which can severally 
 illuminate the polarised field with a different colour. 
 
 Each frame can be rotated separately, and moreover, by means of 
 a little lever, can be withdrawn from the axis of the instrument. In 
 this way one or more of the films can be used simultaneously. The 
 Nicol Prism is screwed on below this apparatus. 
 
 4. Spectroscope. — Mr. 
 H. C. Sorby has invented a 
 spectroscope, which can be fitted 
 to a microscope like an ordinary 
 ocular. 
 
 This instrument (fig. 86) con- 
 sists of a tube containing a 
 series of superposed prisms 
 placed above an ordinary achro- 
 matic ocular, the lenses of 
 which can be made to approach 
 or recede from one another. 
 
 Between these lenses is 
 placed a diaphragm with a slit, 
 the size of which can be in- 
 creased or diminished as desired 
 by means of a projecting knob 
 at the end of the tube, and 
 to one side of this slit a small 
 prism is placed. This produces 
 the spectrum of light which is 
 used in all subsequent obser- 
 vations, and is further reflected 
 by a lateral mirror. In this way 
 the eye receives simultaneously 
 the normal spectrum and that 
 of the object which is being 
 studied. This is indeed a ser- 
 viceable instrument, but naturally 
 it requires a little preliminary 
 practice on the part of the mani- 
 Fig. 87. pulator. 
 
THE FINDERS. 
 
 97 
 
 The firm of Zeiss manufactures a spectroscope depending on the same 
 principles, only it is much more convenient and complete ; and we are 
 constantly using it and are satisfied with it (figs. 87 and 88). 
 
 As well as in Sorby's apparatus the lens in this case can focus on 
 the slit. This can be made wider or narrower by means of a symmetrical 
 movement of two plates, produced by turning the screw F, and its 
 extension is sufficient for examining the whole visual field. 
 
 The length of the slit can be reduced by turning H, so that by 
 properly using the comparison prism, the opening may be completely 
 
 filled by the image of the object. 
 The apparatus under observation is fitted 
 with a comparison prism, having a lateral 
 support, with a spring for fixing the 
 comparison objects and illuminating mirror. 
 All these parts, as well as the ocular, 
 are put together in a circular frame. An 
 Amici prism of strong dispersive power 
 is placed above the ocular, and may be 
 moved about the pivot K, so that it can 
 be withdrawn from the optical axis 
 altogether and be accordingly focussed. 
 Fig. 88. The axial position of the prism is indicated 
 
 and maintained by the spring catch L. 
 A small lateral tube adapted to the mounting ot the prism together 
 with a mirror projects onto the spectrum the image of a scale, whose 
 divisions give in fractions of a micromillimetre the wave lengths of 
 the light in that part of the spectrum on which they are projected. 
 The scale is made to coincide with the spectrum by means of the 
 screw P. 
 
 5. Finders. — A finder is a piece of apparatus with which an object 
 difficult to find ordinarily under the microscope can subsequently be 
 found immediately, e.g., a particular diatom frustule in a preparation 
 which contains many other frustules. Finders are thus of great service, 
 especially to the diatomist. 
 
 In certain English microscopes, as well as in the large model of M. 
 Nachet the moveable stage itself acts as a finder, which is much more 
 convenient. Before examination, the preparation is placed in a fixed 
 position by bringing it in contact with the two brass knobs which 
 are set square. 
 
 H 
 
98 
 
 THE 1TNDERS. 
 
 The different parts of the preparation are then made to pass 
 successively across the field of view, and whenever an object of interest 
 appears a note can be taken of the two figures denoted by the indices 
 on the verniers placed at the two adjacent sides of the stage, so that 
 afterwards the same object can be found again by making the verniers 
 register the same figures. 
 
 When 'hie stage is not graduated the Maltwood finder can be used 
 instead. 
 
 It is a micro-photograph of a series of 2,500 numbers arranged as 
 shewn in figure 89, which represents but a portion of one. 
 
 When on a slide an object is seen which we may subsequently 
 wish to find again, the object is replaced by the Maltwood finder, and 
 
 the two numbers of the square 
 which is seen in the field are read 
 off and noted down on the slide. 
 If subsequently we wish to again 
 observe the object, the finder is 
 placed on the stage and moved about 
 until the numbers noted on the 
 slide appear in the field. Then it 
 is removed and the preparation is 
 placed on the stage in the same 
 position, and the object sought for 
 is found. 
 
 Finally, if the microscope have 
 no mechanical stage another method can 
 be adopted. Two crosses opposite one 
 another, are first marked on either side 
 of the stage and as indicated in figure 
 90. When an object is met with in a 
 
 10 
 8 
 
 II 
 
 8 
 
 12 
 
 8 
 
 13 
 
 8 
 
 14 
 
 8 
 
 10 
 
 11 
 
 12 
 
 13 
 
 14 
 
 9 
 
 9 
 
 9 
 
 9 
 
 9 
 
 10 
 
 11 
 
 12 
 
 13 
 
 14 
 
 10 
 
 10 
 
 IO 
 
 10 
 
 10 
 
 10 
 
 11 
 
 12 
 
 13 
 
 H 
 
 11 
 
 11 
 
 II 
 
 11 
 
 11 
 
 10 
 
 11 
 
 12 
 
 !3 
 
 14 
 
 12 
 
 12 
 
 12 
 
 12 
 
 12 
 
 IO 
 
 11 
 
 12 
 
 13 
 
 14 
 
 13 
 
 !3 
 
 13 
 
 13 
 
 13 
 
 IO 
 
 II 
 
 12 
 
 13 
 
 14 
 
 14 
 
 14 
 
 14 
 
 14 
 
 14 
 
 Fig. 89. 
 
 X 
 
 4" 
 
 Fig. 90. 
 
 preparation which we wish to find again 
 subsequently, two crosses similar and co- 
 incident with those on the stage are 
 marked on the object-slip with a writing 
 diamond, so as to coincide with the crosses 
 on the stage. The object may then be found anytime afterwards by 
 making the crosses on the preparation coincide with those marked on 
 the stage. 
 
 The learned Hungarian diatomographer, Dr. Jos. Pantocsek, of 
 Tavarnok, has invented a very simple and effective finder. 
 
THE FINDERS. 
 
 99 
 
 On the stage are traced a series of horizontal and vertical lines, one 
 
 millimetre apart, the two last of which form the o of the scale, and 
 
 cross one another exactly in the centre of the stage opening. The 
 lines are numbered from o up to 40 (fig. 90^). 
 
 Fig. 9o«. 
 
 In order to find again an object which has been seen in the field, 
 the number merely of the two lines (one horizontal and the other 
 vertical), which coincide respectively with the left hand and upper edges 
 of the slide have to be noted ; whenever the slide is placed on these 
 co-ordinate lines the object will be seen in the field. 
 
 Under the name of diatom finder, Mr. May, of Philadelphia, has 
 invented a small and very simple apparatus, which we have used 
 frequently during the last twelve years. 
 
 It consists of one tube fitting into the other. The inner tube has 
 at its upper extremity a standard English objective screw, and is 
 screwed to the end of the microscope tube after the object in question 
 has been brought into the centre of the field. 
 
 The outer tube, moveable about the first, carries at its lower 
 extremity a diamond point placed in a slightly eccentric position. 
 
 The microscope tube is lowered until it rests by its own weight on 
 the cover of the preparation ; it is then rotated on its axis, and in this 
 way a small circle is drawn round the object to be found again. 
 
 H2 
 
TOO 
 
 THE ADAPTERS. 
 
 The size of the circle depends on the eccentricity given by the small 
 shank which holds the diamond. 
 
 6. Rotating Nose-piece. — Under 
 
 the name of revolver M. Nachet has 
 invented a piece of apparatus which in 
 England is called Brooke's double nose- 
 piece, and which allows the objective to 
 be changed in a moment. 
 
 Figure 91 represents such a rotating 
 nose-piece. Rotating nose-pieces are also 
 made so as to carry as many as five 
 objectives ; we happen to have one of 
 Fig. 91. these, which was made for us by Messrs. 
 
 Seibert and Krafft, of Wetzlar, the 
 centering of which is extremely accurate. 
 
 Adapters. — The adapter is quite a new piece of apparatus, and 
 has only come into use since 1878; it is intended to facilitate the 
 changing of objectives and save the time which would otherwise be 
 lost in screwing and unscrewing. Most probably the first adapter 
 was invented by Ch. Chevalier, who secured his objective by 
 means of a bayonet, a method of great simplicity, rapidity, and 
 preciseness. 
 
 The earliest adapter as used at the present day was invented by 
 Professor Thury, of Geneva, and has been called " Pince Suisse." In 
 this system the objectives are previously screwed on to a ring, and 
 instead of the usual cone and screw-jacket the tube of the microscope 
 is terminated by branches, a kind of pincers, whose two arms are 
 held tight by a spring, but by slightly pressing these can be separated 
 again. The ring is slipped into the pincers by pressing lightly the 
 lower branch, and the objective is held tight in its place, by the spring. 
 
 A steel rim, soldered to the upper branch of the 
 pincers, and which moves easily in the opening of 
 the ring, ensures the centering of the objective. 
 
 After having used this adapter for several years we 
 can warmly recommend it. It only requires a moment 
 to replace one objective by another, and a better centering 
 is obtained than with a screw. 
 
 M. Nachet has constructed an adapter which is 
 a slight modification of the preceding one, which 
 
THP; ADAPTERS. 
 
 lot 
 
 gives equally excellent results. It is represented in 
 figures 92 and 93. 
 
 We think it is unnecessary to mention the 
 numerous systems of adapters which have been 
 suggested in England and America, all of which are 
 more or less like the swiss-pincers, or the bayonet of 
 Ch. Chevalier, but we ought to fully describe an 
 adapter which has recently been invented by Dr. 
 Roderick Zeiss, and called by him the "sliding 
 objective changer." 
 
 This little piece of apparatus, the most convenient and 
 precise of any hitherto made, possesses a mechanism by which each 
 objective can be easily centred by the observer himself; it admits of 
 the use of any number of objectives (figs. 94 and 95). The constituent 
 parts are : — 
 
 Fig- 93 
 
 Fig- 94- Fig. 95. 
 
 a. Tube Slide. — This part is fixed to the tube in the same way as 
 an ordinary nose - piece ; it is screwed firmly to the tube, the slide 
 opening towards the front. 
 
 The direction of the sliding motion is not perpendicular to the 
 optic axis, but inclined at a small angle to it. 
 
102 
 
 DISSECTING EYE-PIECE AND REVERSING PRISM. 
 
 b. Objective Slide. — This slide has the same angle of inclination to the 
 optic axis as the preceding. It follows that when the objective is taken 
 off it rises a little, and no damage is done to the ring of varnish 
 enclosing the mounted preparation. A screw stop turned by means of 
 a watch key is adapted to the end of this fitting, which stops and 
 fixes it in any required position, to which position it can be brought 
 back again after being removed. This screw acts as a centering ad- 
 justment in the direction of the slide. An endless screw, turned by 
 means of the same key, controls the centering in a direction at right 
 angles to the slide. 
 
 Objectives whose settings are so matched that they are approximately 
 in focus when changed can be focussed accurately by means of a 
 mechanism ad hoc, which contains the piece intended to receive them 
 and which is fixed once for all in their proper position by means of 
 a clamping screw. The slides employed to carry the objectives slip 
 with great accuracy into the slide of the other piece, and can be 
 purchased in such numbers as may be required from time to time. 
 
 After carefully centering the objective the same part of the object 
 returns exactly to the middle of the field with each change of objective ; 
 it, moreover, remains as nearly as possible in focus, so that it usually 
 only requires to be slightly adjusted by the micrometer screw. 
 
 7. Dissecting Eye-piece and Reversing Prism. — Under the 
 name of dissecting ocular, a special ocular is referred to, the object of 
 which is to correct the image which, as is well known, is always seen 
 reversed. 
 
 This ocular consists of two oculars placed one at each end of a 
 brass tube. It is, really, nothing more than the terrestrial ocular adapted 
 to astronomical telescopes. 
 
 By drawing out this tube more or less, a pancratic microscope is 
 obtained ; that is to say, one whose amplifying power is proportional 
 to its distance from the objective. Thus, with the dissection eye-piece 
 adapted to M. Hartnack's microscope, the magnifying power varies from 
 6 to 50 diameters with objective No. 2, and from 30 to no diameters 
 with No. 4. 
 
 The dissecting ocular is an excellent accessory when a compound 
 microscope is used for making dissections, or for picking or sorting 
 diatoms ; without it, it is very difficult to direct the needles conveniently, 
 because every movement has to be made in an opposite direction to 
 that in which they are seen to move. 
 
JilNOClTI.AR MICROSCOPE AND STEREOSCOPIC MICROSCOPE. 
 
 To achieve the same object, the late A. Chevalier manufactured a 
 reversing prism, which was placed on the ocular. We highly value this 
 last piece of apparatus, as its use is very convenient for making a series 
 of dissections which do not require a constant change of amplification. 
 
 Arthur Chevalier's reversing prism possesses the advantage of being 
 immediately applicable to any ocular, but it diminishes the field of vision. 
 
 8. Binocular Microscope and Stereoscopic Microscope.— 
 
 The microscope need not necessarily be used with only one eye ; for a 
 long time past microscopes have been constructed so that both eyes 
 can be employed. These are called binocular microscopes, and some 
 of these pieces of apparatus which convey a sensation of objects in 
 relief are called stereoscopic microscopes. 
 
 Binocular microscopes do not give such perfect images as monocular 
 microscopes ; on the other hand, their use is less fatiguing. 
 Stereoscopic microscopes — it is with reference to those of Wenham that 
 we now speak — can only be used with objectives of low power and 
 small numerical aperture. 
 
 The special binocular microscopes, such as are constructed and 
 much used in England, are almost unknown to continental observers. 
 This instrument cannot certainly be made use of for the study of the 
 fine structures of diatoms ; but for the study of elementary histology, 
 entomology, &c, it is both very convenient and pleasant. It is also 
 excellently suited to the study of the form of diatoms, and we venture, 
 indeed, to assert that he who has never examined a diatom with a 
 stereoscopic binocular will not possess so clear an idea of one as he 
 who is acquainted with this instrument. 
 
 In Wenham's binocular microscope, the image, as it emerges 
 from the objective, meets, over a moiety of the opening of the latter, 
 a small prism, which doubly reflects, and consequently sends into the 
 
 Fig. 96. 
 
 To remedy this inconvenience, M. Nachet constructs 
 a special ocular (fig. 96), which encloses a prism 
 with four surfaces, whose angles are calculated in 
 such a way that the rays forming the image are 
 reflected by it three times before they reach the eye, 
 and reverses on the retina the image produced by 
 the object ; now, as the image given by the object 
 is reversed as regards the ocular, that which is 
 produced by the prism is rectified or erected as 
 regards the object itself. 
 
STEREOSCOPIC OCULAR. 
 
 left-hand tube the image given by the right-hand half of the opening 
 of the objective, while that emanating from the left part of the 
 objective reaches the ocular directly and without deviation. 
 
 Abbe's Stereoscopic Ocular. — If Wenham's microscope, of which 
 we have just spoken, is little known on the continent, on the other hand, 
 Abbe's stereoscopic ocular, which can be adapted to every continental 
 microscope, has begun to be known. 
 
 Prof. Abbe's apparatus is adapted to the microscope tube in the place 
 of the ordinary ocular (fig. 97). The luminous pencil emerging from the 
 objective reaches a thin stratum of air, situated between two prisms, 
 where it is bisected ; divided into two parts, one part crosses the 
 upper prism and proceeds to the ocular, which is directly above it, the 
 other part is reflected against another prism, placed at the end of a 
 second tube, which makes an angle of 14 0 with the principal tube. 
 This latter prism transmits the reflected part to the right eye of the 
 observer through a second ocular. 
 
 1/9 ; The two oculars differ, 
 
 although they finally pro- 
 duce equal magnification; 
 the second is calculated 
 so as to compensate for 
 the longer path which the 
 luminous pencil striking 
 it, has to traverse before 
 reaching the eye. 
 
 The second ocular can 
 also be so adjusted that 
 the distance between 
 the two oculars suits 
 D the observer's eyes. 
 
 Above the oculars 
 are two adjustable semi- 
 circular diaphragms; 
 these diaphragms, by 
 bisecting the pencil, pro- 
 duce a sensation of 
 objects in relief. By 
 taking them off, simple 
 binocular vision is ob- 
 
COMPARING OCULAR. 
 
 tained, which is useful in many cases, and which some people find less 
 fatiguing than monocular vision. 
 
 The apparatus can be used with any objective. The loss of light 
 caused by the prisms is insignificant. 
 
 9. Dr. H. Van Heurck's Comparing Ocular.— To scrupulously 
 compare two diatoms, by examining them in succession under the 
 same microscope, is absolutely impossible. The difficulty can be bridged 
 over by drawing the two forms, and then comparing the drawings ; but 
 although this is the best way of studying diatoms, it is a proceeding 
 which takes up so much time that recourse cannot always be had to it. 
 
 In order to proceed with greater expedition, we conceived the plan 
 some time ago of using two microscopes at the same time which gave 
 the same magnification, which were brought close together, so that each 
 of them corresponded to one of the eyes. After some attempts one 
 can manage, by a proceeding similar to what is called "double vision," 
 to superpose the images ; however, this cannot be effected without some 
 fatigue, and it is impossible to continue the work many hours, together 
 as one is often obliged to do. 
 
 Having seen in the Journal of the Royal Microscopical Society for 
 June, 1886, a description of the comparer which M. Inostranzeff was 
 employing to compare the colour of minerals, it occurred to us to 
 construct and try a similar apparatus. We recognised the fact that the 
 instrument was very practical, but it was insufficient for diatoms ■ in 
 the first place the field was partially cut, thus where the prisms join 
 there remained a black band which precluded a perfect comparison, 
 and, lastly, we prefer (for reasons given later on) that the juxtaposition 
 of the diatoms should be made not longitudinally but along half 
 the breadth of the valve. 
 
 M. Carl Reichert, of Vienna, constructed an apparatus according to 
 our specifications which works perfectly. Instead of the two prisms 
 
 being juxtaposed along one of 
 their edges, as is the case with 
 the instrument of M. Inostranzeff, 
 we use instead two prisms of con- 
 siderable height, but of limited 
 breadth, juxtaposed along one of 
 the triangular surfaces (fig. 98). 
 The two images are juxtaposed in 
 Fig. 98. the direction of the height ; the 
 
 carrier of the two prisms is 
 
io6 
 
 COVER-GLASS GAUGE. 
 
 moveable, and can be turned on its axis and fixed in any position 
 whatever. By turning it slightly the line of separation between 
 the two prisms can be made to disappear completely, and indeed so 
 completely that a perfect diatom valve can be constructed with the 
 two half valves, which belong severally to one of the fields. 
 
 It was in this way that we were able to photograph a valve of 
 Actinoptychus splendens, made by the aid of the Comparer, by bringing 
 two half valves into juxtaposition. The photograph has to be examined 
 very carefully to find out where the valves join. 
 
 Comparisons can, therefore, be made as completely as possible, and 
 the images are so clear that even high powers can be used. 
 
 Diaphragms D D cut off in each part of the tube the light 
 coming from the opposite side, which would otherwise be prejudicial. 
 
 10. Dr. H. Van Heurck's Transformer. — It is well known 
 that objectives are constructed for a given length of tube. During 
 our researches, especially those in photo-micrography, we have frequently 
 been annoyed by this fact, and consequently we have endeavoured 
 to remedy it. The small piece of apparatus which we have called a 
 transformer is the result. It consists simply of a perfectly achrotnatic 
 lens, which is either concave or convex, and has a focal length of 
 exactly twenty inches. This lens is screwed on just above the 
 objective, or what is better, at the lower end of the draw tube, so that it 
 can, within certain limits, be taken nearer to or farther from the objective, 
 and can also be turned on its axis so as to obtain the clearest 
 image. 
 
 The concave lens transforms the Continental objective into an 
 English one, the convex lens on the other hand transforms the English 
 objective. 
 
 It can easily be understood how useful is such a transformer, which 
 only costs a few francs, while at the same time such expensive 
 objectives, as all the apochromatics of 1*4 and i - 6 N. A., lose nothing 
 by the use of this accessory. 
 
 We have made experiments with simple lenses, but the firm of 
 Zeiss have been good enough at once to construct for us achromatic 
 lenses of the focal length and form desired. 
 
 Messrs. Bausch & Lomb's Cover-glass Gauge. — The principal 
 object of this ingenious little instrument, represented by figure 98a, the 
 price of which is only the small sum of three dollars (12s.), is to 
 measure the thickness of cover-glasses. Its second object is to intro- 
 duce a new system of cover-glass correction by means of a draw tube ; 
 
COVER-GLASS GAUGE. 
 
 in devising it, the object has been to provide a mechanical means for 
 obtaining the best optical results without any possibility of doubt, and 
 it accomplishes its purpose perfectly. 
 
 Fig. 98(7. 
 
 This instrument is provided with a stand of japanned iron ; cut 
 horizontally through the top is a thread 7 5<J inch pitch and 3 / l6 inch out- 
 side diameter. A recess is cut in the top below the line of the screw 
 and at right angles to it, for placing the covers. The one-half of the 
 top of the stand which receives the micrometer screw is slotted longi- 
 tudinally to the depth of the screw, and is provided with a set screw 
 to take up wear, the other half has the fixed screw, adjustable however 
 for final adjustment. The end of the micrometer screw is milled, but 
 of a smaller diameter, so that no force can be exerted so as to endanger 
 the cover-glass. Fixed on the screw between two nuts is a brass drum 
 with a half-inch face. A knife-edge index finger is fixed to the top of 
 the stand and projects over the top of the drum. 
 
 To the outside diameter of the drum is fixed a strip of glazed paper 
 provided with a series of divisions. The first gives the thickness of 
 cover-glass in one-thousandth inches, the second one-hundredth milli- 
 
108 ARTIFICIAL ILLUMINATION OF THE MICROSCOPE. 
 
 metre. The third indicates the proper tube length with various thickness 
 of cover glass with a non-adjustable J corrected under a tube length 
 of 8^ inches and cover thickness of o - i6 mm.; the fourth gives the 
 tube length of a 1-5 inch objective under the same conditions ; the 
 fifth for a 1-8, and the sixth for a 1-12 for the same conditions of tube 
 length and cover; the seventh is for a 1-6 with the same cover and tube 
 length of 160.0 mm. 
 
 In objectives provided with cover correction the graduation is so 
 arranged as to read to 1-100 mm. No matter what the power of 
 objective, or whether dry or water immersion, the number gives proper 
 correction for a thickness corresponding to it. 
 
 Artificial Illumination of the Microscope. — z. Lamps. — The 
 microscopist is not always free to employ his time as he pleases ; more 
 often than not he is compelled to defer till the evening observations 
 which he could more easily carry out during the day. Fortunately, 
 however, there are but few observations which he cannot make by 
 artificial light ; the study of diatoms and the resolution of tests are 
 made better by the light of a lamp than by day-light. 
 
 An oil-lamp can be used for these observations, but a petroleum 
 lamp is preferable because the light is whiter. Gas can likewise be used. 
 
 An ordinary petroleum lamp is sufficient for most 
 researches. English makers have, however, designed 
 different kinds of lamps, which certainly possess 
 some advantages. Some are sold, costing as much 
 as ^6, and even more. 
 
 It is essential to have a lamp giving a good 
 white light, which depends upon the way in which 
 the burner is constructed, and it must be one which 
 can be used either with the edge or flat of the 
 flame at will, while as much as possible of the unused 
 radiant is shaded from the eyes. The lamp, in 
 figure 99, of Messrs. Watson and Son, perfectly 
 fulfils these conditions at a moderate cost. 
 
 The same makers sell a more complete lamp, 
 furnished with a silvered reflector, a condensing 
 lens, tinted glass, &c. But these luxuries are often 
 unnecessary. 
 
 But at the present time there is an illumination 
 Fig. 99. which is superior to that afforded by petroleum, 
 
 viz., electrical incandescent illumination. 
 
ELECTRICAL ILLUMINATION. 
 
 2. Electrical Illumination. — In February, 1882, (') we published 
 the result of our researches on the use of electrical illumination. 
 
 Since the month of November, 1881, when we determined to employ 
 the incandescent lamps, which had recently been invented, we have always 
 worked with electrical illumination, and it would be very annoying if we 
 had to forego it. 
 
 Electrical incandescent illumination is superior to any other kind of 
 illumination. It possesses the softness of that given by good petroleum 
 lamps, and resolves delicate details almost as completely as monochro- 
 matic light. The delicate striae of Amphipleura and the 19th group of 
 Nobert's test can be seen with perfect clearness. 
 
 Professor Abbe, whom we made acquainted with the result of our 
 researches, has given us the following theoretical explanation of it. He 
 attributes it to two causes. 
 
 1. The greater whiteness of the light. Therefore the light includes 
 more blue and violet rays. Now, as it has been shewn by measure- 
 ments, made by Professor Abbe in different monochromatic illuminations, 
 that the separating power of any given objective increases as the length 
 of the wave of light employed diminishes, it follows that the electric 
 light ought to show delicate detail more easily than does the yellowish 
 light of gas or of lamps. 
 
 2. The specific intensity of electric light being far more considerable 
 than that of any other artificial light, sufficient illumination is obtained 
 with a much more restricted luminous pencil than that which must be 
 used to obtain the same luminous intensity with gaslight, or with 
 diffused daylight. Rays which are much more oblique can, therefore, 
 be used. 
 
 When we first used the electric light, we placed the lamp, which was of 
 rather large dimensions, in a box, in the lid of which was an opening, 
 which had glass fitted into it. The microscope was placed on the box, 
 the mirror having previously been moved to one side of the axis, or taken 
 off. The light of the lamp was then centered by means of a 
 plano-convex lens, and was directed onto the condenser of the 
 microscope. By manipulating the latter, the illumination was modified. 
 
 The lamps used in this arrangement required a considerable electro- 
 motive force, but shortly afterwards Mr. Swan placed at our disposal 
 some much smaller lamps, which only required about 7 volts. 
 
 Mr. Stearn (Mr. Swan's colleague and director of the Swan Company) 
 made still further improvements, and invented some new lamps specially 
 
 ( 1 ) Consult : "La lumiere electrique appliqu^e aux recherches de la micrographie et de la 
 photo-micrographie par le Dr. Henri Van Heurck. 2e Edition. Anvers 1883." 
 
no 
 
 ELECTRICAL ILLUMINATION. 
 
 designed for microscopic illumination. These lamps (fig. 
 100) are of two kinds. Some (A) give a brighter 
 light than the rest, and are intended for experiments 
 in polarisation and photo-micrography ; others (B) have 
 a very short thread, and are suitable for ordinary 
 illumination. They only require an electro - motive 
 Fig. zoo. f orce 0 f volts and a current of 1^ amperes 
 
 which may be supplied by two accumulators, or two Bunsen cells. 
 Their illuminating power is roughly equivalent to one candle, but if 
 
 Fig. 101. 
 
 Microscope arranged for illumination with three incandescent lamps. 
 
ELECTRICAL ILLUMINATION. 
 
 I I I 
 
 necessary they can be raised to a candle power of 2\ for some 
 minutes. 
 
 Messrs. Mawson and Swan have designed a special microscope for 
 the use of Mr. Steam's lamps, which we will now describe. 
 
 The entire microscope (fig. \o\\ is mounted on a metal stand, 
 the lamps are fixed permanently to the instrument, the different 
 metal parts serving as conductors. There are three lamps attached to 
 the microscope. The first A can be used to illuminate opaque bodies, 
 and is attached above the objectives by a collar a, which allows the 
 lamp to be rotated, while the jointed arm E allows it to be withdrawn 
 from or brought near to the object for the purposes of illumination. 
 
 The second lamp B is placed on the sub-stage ; by sliding it in a 
 groove it can be pushed sideways so as to give oblique illumination. 
 
 Lastly, the third lamp C, whose illuminating power is greater than 
 the previous ones, is attached to a jointed arm in the place of 
 the mirror. This lamp is used in experiments in polarisation and 
 photo-micrography, as well as for the illumination of condensers. 
 
 Fig. 102. — Lamp-carrier on stand. Fig. 103. — Lamp-carrier for 
 
 fixing on stage. 
 
 The resistance produced by a coil of steel-wire allows the power of the 
 electric current to be controlled, and a commutator b allows the current 
 to pass in any of three directions to either of the three lamps. 
 
 Lastly, by means of the stands in figures 102 and 103 the electric light 
 can be adapted with more or less ease to any microscope not specially 
 arranged for this work. 
 
112 
 
 ELECTRICAL ILLUMINATION. 
 
 The photophore of M. Trouve is also a very convenient piece of 
 apparatus for electric lighting. As a matter of fact it is simply 
 the apparatus that we devised in 1881 for illuminating the microscope. 
 The photophore consists of a nickel brass tube (all the parts of the 
 apparatus are similarly nickeled), in the middle of which is placed the 
 lamp, which is of a special form with a straight filament. At the back of 
 the tube there is a reflecting mirror of silvered glass, and in the front of a 
 second tube, sliding in the first, is a condensing lens. With this 
 second tube, the space between the lens and the lamp can be regulated, 
 and consequently a pencil of convergent, divergent, or parallel rays can be 
 obtained as desired. The condensing lens being moveable in its mounting, 
 can be turned so that the convex face is in front or behind. 
 
 The photophore (fig. 104) is screwed on 
 to a very heavy foot, carrying a split 
 tube, 20 centimeters (7 4 / s inches) in height, 
 which acts as a spring, over which slides 
 with very little friction, a second tube, which 
 can be fixed at any height. This second 
 tube carries two fastenings, a, d, one placed 
 at the upper and the other at the lower 
 end; each of these fastenings consists of a 
 steel ball with a rod furnished with a screw 
 thread. The ball is held tight between two 
 concave metal plates, the first of which, 
 having a hole in the middle, allows the 
 threaded rod to pass through. The photo- 
 phore screwed on this rod can then, in 
 consequence of the movements of the ball, 
 be placed in any position that may be 
 desired. 
 
 Lately, our learned correspondent, Dr. 
 Engelmann, professor of biology at the 
 University of Utrecht, and one of the most zealous promoters of 
 electric lighting in microscopy, has had a small and very simple 
 apparatus constructed for him, which acts very well, and at the same 
 time fulfils the purpose of a rheostat or light regulator. 
 
 This apparatus (fig. 105), is composed of a brass foot M, carrying 
 a rheostat R, and a lamp carrier S. 
 
 As will be seen the positive current enters by the pillar a, 
 whence it passes to the upper part of the rheostat, through it, and, by 
 
 Fig. 104. 
 
ELECTRICAL ILLUMINATION. 
 
 "3 
 
 means of the brass foot, is carried direct to the lamp. From the lamp 
 the current returns by a fine copper thread to the pillar b, and thence 
 to the negative pole of the electric current. 
 
 The lamp can be raised 
 to different heights by means 
 of the two brass tubes S, 
 which slide in one another, 
 and can also be placed in 
 every possible position by 
 means of a ball and socket 
 as seen in the plate. 
 
 With regard to the rheostat 
 itself, it is composed of a brass 
 cylinder insulated at its foot 
 by a foundation of ebonite 
 or serpentine. This cylinder 
 contains a pile of thin discs 
 formed of a mixture of graphite 
 and gelatine. The resistance 
 of these plates depend upon 
 the relative proportions of the 
 two constituent parts. 
 
 By means of an adjusting 
 screw which terminates just 
 above the rheostat, and which 
 by means of a brass disc 
 tightens or loosens the resist- 
 current can be regulated with 
 
 of the 
 
 ance plates,] the intensity 
 mathematical precision. 
 
 This apparatus being rather feeble for many kinds ol research, 
 M. Kagenaar, engineer of the University of Utrecht, has made a more 
 perfect one in accordance with our instructions, the construction of 
 which Professor Engelmann has kindly superintended. 
 
 This new apparatus is composed of a small brass table, 20 centimetres 
 (8 inches) in length and 10 centimetres (4 inches) in breadth. This table 
 carries at one end the small rheostat lamp- carrier described above, and 
 on it a corresponding horizontal rheostat placed lengthways. The 
 essential part of this latter apparatus is a long tube of serpentine 
 enclosing 120 graphite discs. When the discs are tightly pressed the 
 
 I 
 
ii4 
 
 ELECTRICAL ILLUMINATION. 
 
 resistance attains nearly £ ohm, and by slowly loosening the regulating 
 screw, this resistance can attain 800 or 1000 ohms. The apparatus 
 can therefore, theoretically, be used for very small lamps as well as for 
 very powerful ones ; indeed, by employing 16 Reynier's large-sized 
 accumulators of 12 plates, we have found no difficulty in using suc- 
 cessively lamps of 2 volts and "5 amperes up to 12 volts and 4 amperes 
 heated to a white heat. 
 
 The two small pieces of apparatus fixed on to the brass tablet can 
 be taken off from it if desired, and the lamp carrier can also be turned 
 about its axis and fixed in all azimuths. 
 
 We believe that it would be impossible to produce an apparatus more 
 complete, more powerful, and at the same time more compact than that 
 we have just described. 
 
 After having described the illuminating apparatus it still remains to 
 examine the electrical sources to which the microscopist can have recourse. 
 
 Two or three Bunsen cells are quite sufficient for the small lamps. 
 The vapours given off by these batteries make them unsuited for use in a 
 laboratory, and we do not advise their introduction into the work-room. 
 
 The bichromate batteries provided with a windlass are more manage- 
 able, although they also emit unwholesome vapours. 
 
 Indeed, we cannot advise any but the small portable battery of 
 Trouve' or the Radiguet battery. 
 
 The portable battery of Trouve (fig. 
 
 106) consists of a small ebonite tank D, 
 
 measuring 15 centimetres (6 inches) in 
 
 length by 10 (4 inches) in breadth and 
 
 18 (7 inches) in height, divided into six 
 
 compartments, each being two-thirds the 
 
 height of the tank, intended to form 
 
 as many elements. 
 
 At the bottom, these compartments 
 
 communicate with one another by a very 
 
 small opening. 
 
 The active elements attached at the 
 
 bottom to the cover E, are arranged 
 
 in six ranks, each of which corresponds 
 Fig. 106. ' . 1 
 
 to one of the cells of the ebonite tank ; 
 each ot the ranks constituting an element is formed of two zinc rods 
 placed between three carbon rods. The six elements are coupled together 
 by connections disposed symmetrically and neatly on the top of its 
 
ELECTRICAL ILLUMINATION. 
 
 "5 
 
 cover, which also carries two terminals for establishing the current. 
 
 The battery does not emit any odour, and will feed the photophore 
 lamp for two hours. 
 
 When the battery is exhausted the two milled heads A, B are 
 unscrewed, the rim which they keep in position is removed ; then the 
 handle F is taken hold of, and by raising it the cover and the elements 
 are removed at the same time ; the exhausted liquid is thrown away, 
 and the tank is washed with water; 800 grammes (1 f lbs. avoirdupois) 
 of new liquid previously prepared is then poured in, and the cover, 
 rim and the two milled heads are returned to their original position, 
 and the battery is again in working order. 
 
 The liquid for this battery is thus prepared : into a large stone vase 
 is placed 1 kilo (2 i-5th lbs.) of bichromate of potash, 8 litres (7 quarts) 
 of water is added, and then 2 litres (3^ pints) of sulphuric acid 
 (commercial), which is poured in very slowly in a narrow stream, while 
 constantly stirring it by means of a glass rod. The liquid gets very warm 
 during the operation, and it must be left to cool before being employed. 
 
 With the quantity which we have just mentioned the battery can be 
 charged 1 2 times. 
 
 The small Trouve battery is an excellent one for the microscopist 
 who only desires to use electric light on special occasions. But if he 
 
 desires to have it permanently at his 
 command he would do better to use either 
 the Trouve' windlass battery or the Radiguet 
 battery. 
 
 The Radiguet battery (') is a modification 
 ot the Poggendorf battery, and is a very great 
 improvement upon it. It has all the advantages 
 and the energy of a Bunsen battery, without 
 any of its inconveniences. 
 
 Each element of the Radiguet battery is 
 composed of an impervious stone jar, a carbon 
 cylinder, a porous cell, and a support which 
 amalgamates with its cistern-reservoir. These 
 different parts are placed one within the other 
 as seen in figure 107. 
 
 It is the combined effect of the amalgam 
 support and its cistern, wherein the novelty 
 of the battery lies. 
 
 (■) Radiguet, maker of Scientific Apparatus, 15, Boulevard des Filles-du-Calvaire, Paris. 
 
 I 2 
 
ELECTRICAL ILLUMINATION, 
 
 M. Radiguet had noticed that when the mercury contained traces of 
 zinc, or when it was in contact with red copper, the electrical current 
 had a tendency to distribute the mercury over the entire surface of 
 the copper. This observation induced the skilful electrician to construct 
 the amalgam support. 
 
 As seen in figure 108, this support is composed of a zinced copper 
 tube, to which is attached a kind of basket to receive zinc, which may 
 be granulated, or, better still, in the form of round shot, which have 
 been cast either at the foundry or by one's self; M. Radiguet also 
 supplies them by the kilo at a low price. 
 
 Underneath this basket is a porcelain cistern containing the amalgam 
 by which the the zinc is kept constantly amalgamated. This cistern 
 is connected by a copper bolt to two metallic feet, attached to the 
 bottom of the basket. 
 
 Such is the apparatus that we are going to put in working order, 
 and for that purpose we pour into the outer jar an acid solution of 
 bichromate of soda, and into the porous jar some pure water; the battery 
 will then begin to work at the end of a few hours. If we wish to 
 to make it work immediately we only have to pour a few grammes of 
 sulphuric acid into the water, as thus we anticipate by dialysis what 
 otherwise takes place gradually. 
 
 The battery works on an average for eight days, at 
 the end of which time the acidulated water in the 
 porous vase must be changed. This is not a difficult 
 operation, and can be easily effected in a few 
 minutes, without taking the battery to pieces or 
 deranging it in any way. This is carried out by 
 means of a very ingenious apparatus, which is patented 
 under the name of the Radiguet siphon (fig. 109). 
 
 This siphon has an advantage over all others 
 because it is set in operation by blowing and not 
 by suction ; it is emptied in the same way. These 
 two operations are effected by means of an india- 
 rubber ball. 
 
 To fill, it is sufficient to press the ball slowly 
 once or twice ; to empty, it is also pressed, but this 
 time sharply. 
 
 The large tube of the siphon encloses a narrow 
 tube, which is a continuation of the straight outer 
 tube, and the diameter of the lower end of the large tube is much 
 
 Fig. 108. 
 
ELECTRICAL ILLUMINATION. 
 
 117 
 
 smaller than that of its average section. It follows then that when 
 air is blown in slowly the pressure increases in the large tube, drives 
 back the liquid into the straight branch, and the siphon is found to 
 be full. A rapid pressure on the other hand drives back the liquid 
 completely, and the siphon no longer works. 
 
 The inner jar should be in this way emptied once a week. The 
 outside liquid need only be changed monthly, or perhaps not so 
 often. This operation can also be carried out by means of the siphon. 
 
 Fig. 109. 
 
 SiPHON RADIGUET 
 
 B T - S.G.D.G 
 
 The battery is therefore 
 always charged, and it is only 
 necessary to take it to pieces 
 now and then to thoroughly 
 clean it. 
 
 The electro-motive force of * 
 the battery is about two volts. It requires three elements 
 to give a white light in the small lamps we have mentioned ; 
 however, they are too expensive, and the battery cannot 
 last for any length of time without a diminution of the 
 light. The best means of remedying this decline of electro- 
 motive force is obtained by arranging them in two series of 
 three elements each. 
 
 By arranging the six elements in this way we have been 
 able for a whole week to obtain without any difficulty five 
 hours' illumination at a sitting without any appreciable decline 
 of the light. 
 
 Let us add in conclusion that every battery requires a 
 certain amount of care, and indeed this is the case with 
 every source of electricity. Dynamos and accumulators both 
 require constant attention and electrical illumination by 
 canalisation should be the microscopist's ideal. Let us hope that, at 
 any rate in all large towns, this desideratum will be realised in a very 
 short time hence. At Antwerp proposals have been made by the 
 eminent electrician Van Rysselberghe, and the Municipal administration 
 have accepted them ; the public will therefore in a few months have 
 electric light at their disposal. 
 
BOOK II. 
 
 SIMPLE MICROSCOPES AND PROJECTION MICROSCOPES. 
 
 CHAPTER I. 
 
 SIMPLE MICROSCOPES. 
 
 The simple microscope, which had to serve for all serious study 
 until the means of rendering the compound microscope achromatic was 
 discovered, is essentially composed of a magnifying glass, mounted on 
 a moveable screw, under which the rays are reflected from a mirror on a 
 plate of glass, or brass, attached to the stage. 
 
 At the present day the simple microscope is 
 never used, except for microscopical dissections, 
 which do not require a greater magnification than 
 15 to 60 diameters. 
 
 Formerly biconvex lenses were used. But now, 
 doublets are generally employed, conceived in the 
 first place by Wollaston. Charles Chevalier 
 followed up Wollaston, and made the doublets 
 * Fig. in. which are still in use. Chevalier's doublet is 
 
 a combination of two plano-convex lenses of the same focal length, 
 having their convexities turned towards the eye (figs, no and in). 
 
 A small diaphragm intended to correct spherical aberration is placed 
 between the two lenses, which are of unequal size, but nevertheless 
 of the same focal length. The advantage of doublets over biconvex 
 lenses is that for the same magnification the former gives an image 
 which is much clearer and more free from spherical aberration. 
 
 Many opticians devote themselves specially to the manufacture of 
 simple microscopes, but the firm of Chevalier enjoy the greatest 
 reputation. 
 
 The firm of A. Chevalier make a number of models of simple 
 
 * Figure 110 represents the doublet mounted, and figure 111 shows the arrangement of 
 the glasses : A, upper glass ; B, lower glass ; O, intermediate diaphragm. 
 
SIMPLE MICROSCOPES. 
 
 II 9 
 
 microscopes. One of these (fig. 112) costs £2, accompanied by 
 a doublet magnifying forty times. Another model, better finished (fig. 
 113), has two doublets, and costs £4. • It consists of a heavy 
 foot, rigidly connected with the stage, and carrying a reflecting 
 mirror. The doublet is carried on an arm, which can be moved 
 from front to back by means of a rack, and which can also be turned 
 about its axis so as to describe a complete circle, by which means it 
 can pass over all parts of the stage. 
 
 Fig. 112. 
 
 Fig. 113. 
 
 Lastly, a more carefully finished model (fig. 114) provided with a 
 
 square pillar, costs at present 
 ^5 4s. Chevalier sells his 
 doublets separately. Their price 
 varies as follows : Doublets 
 magnifying, 12 to 120 times, 
 8s. ; the same 130 to 240 times, 
 15s.; the same, 480 times, 16s. ; 
 the same, 500 times, jQi. 
 
 M. Nachet also manufactures 
 J simple microscopes. One of these 
 IS instruments, having a rack with 
 two milled heads for focussing, 
 and a stage carrying two side 
 rests for the convenience of the 
 hands during important dissections, 
 costs £,2 8s. with two doublets (fig. 115). 
 
120 
 
 SIMPLE MICROSCOPES. 
 
 Lastly, a binocular apparatus for dis- 
 section (fig. 116), which Ave examined a 
 little time ago, and by means of which 
 dissections can be seen very vividly in 
 relief, also costs £2 8s. 
 
 This optician also sells the doublets 
 separately. The price is 5 s. for those 
 of 20 to 5 millimetres, focal length, and 
 8s. for those of 5 to 2 millimetres. 
 
 The firm of Carl Zeiss, of Ie'na, also 
 justly merit their reputation for simple 
 microscopes ; and, some years ago they 
 invented a combination which has been 
 adopted by a great many observers, and cannot be too strongly 
 ecommended. This combination consists of an objective composed of three 
 achromatic lenses screwed to the lower end of a 
 tube, on the upper end of which is a concave 
 achromatic ocular. 
 
 These lenses can be used either singly — in 
 which case they magnify 15, 20, or 30 times 
 respectively — or the ocular may be used with 
 
 either one, two, or three lenses at a time, 
 Fig. 116. . . 
 
 giving respectively magnifications of 40, 60, and 
 
 100 times. The focal distance is very large, being 9 millimetres for 
 
 the highest power, and 30 millimetres for the lowest. 
 
 This excellent combination, which can be applied to any simple 
 microscope, only costs 30 shillings. 
 
 M. Nachet manufactures a similar combination for £1 8s. 
 
 In America, Messrs. Bausch and Lomb, of Rochester, make a very 
 ingenious simple microscope, called the compact dissecting and mounting 
 microscope (fig. n 6a), which appears to be very generally used in the 
 United States. 
 
 The base of the instrument, which is of cast iron, is varnished, 
 and is sufficiently large and heavy to insure the perfect stability of 
 the instrument. On one side of it rises a brass pillar, jointed at 
 the base by means of a hinge, and at the stage by means of 
 another hinge. 
 
 The pillar contains a triangular rod of polished cast iron, carrying 
 at the side a rack which can be raised or lowered by means of a 
 pinion. 
 
SIMPLE MICROSCOPES. 
 
 121 
 
 At the upper end of this rod a ring, for carrying the lenses, can 
 
 be adjusted with a 
 sliding motion. 
 
 Inside this ring is 
 a screw thread by 
 which any objective 
 furnished with the 
 English thread can be 
 screwed. 
 
 Two lenses having 
 respectively 2-inch and 
 i-inch focus are 
 mounted in ebonite 
 rings, and can be 
 placed one above the 
 other so as to give 
 magnifications of from 
 five to twenty-five 
 times. 
 
 By means of the ingenious system of hinges, which allows the different 
 parts of the microscope to fall back on themselves, like the leaves of a 
 portfolio, the instrument only occupies a small space. The box 
 containing it, with the various accessories, is only 15 centimetres (six 
 inches) long by 12 centimetres inches) broad and 6h centimetres 
 (2^ inches) high. 
 
 The price of the instrument is $12 (£2 10s). 
 
CHAPTER II. 
 
 PROJECTION MICROSCOPES. 
 
 SOLAR MICROSCOPE; GAS MICROSCOPE; 
 
 PHOTO-ELECTRIC MICROSCOPE. 
 
 The difficulty of exhibiting a series of microscopical objects to a 
 large number of people and the loss of time which this necessarily 
 involves have for a long time suggested some special apparatus by 
 which these inconveniences might be avoided. Such is the origin of 
 projection microscopes. 
 
 These instruments depend on the same principle as the magic 
 lantern ; the rays, coming from a luminous source, fall upon a convex 
 lens (or a combination of lenses producing the same effect) which 
 concentrates them on the object : thence these rays reach an objective 
 which is placed at a distance rather greater than its principal focal 
 length. By such an arrangement a real and greatly magnified image of 
 the object is formed which may be received on a screen at any 
 distance. 
 
 The names of many projection microscopes which are practically 
 the same instruments, vary according to the illuminating apparatus used. 
 Such are the solar microscope, the photo-electric microscope, and the 
 gas microscope. 
 
 All these instrumenls require a completely darkened room, or hall, so 
 that no light, except that furnished by the instrument, can meet 
 the spectators' eyes and so diminish the intensity of the image received 
 on a tightly-stretched screen, which should be made either of strong 
 white paper or a scrupulously white sheet. When the spectators and 
 the instrument are on opposite sides of the sheet the latter must then 
 either be wetted immediately before the experiments, or, better still, 
 sized with two coatings of gelatine. The object of the sizing is to fill 
 up the interstices found between the threads of the sheeting. 
 
PROJFXTION MICROSCOPES. 
 
 123 
 
 The oldest instrument of this 
 kind is the solar microscope, 
 and its invention is generally 
 attributed to Lieberkuhn. How- 
 ever, the researches of M. Harting 
 prove irrefutably that this appar- 
 atus is due to Pere Kircher, and 
 that Lieberkuhn, who made known 
 the instrument in England, and to 
 whom the invention is attributed, 
 only imitated an instrument con- 
 structed by Fahrenheit, who then 
 lived at Amsterdam. It was in 
 this city that Lieberkuhn saw the 
 instrument in the hands of George 
 Clifford and Henri de Raad. 
 
 The solar microscope has 
 undergone very important modi- 
 fication at the hands of Charles 
 Chevalier ; the model as conceived 
 by this eminent optician (fig. 117) 
 we shall now describe. 
 A A B B, is a wooden slab or panel of a shutter having a circular 
 opening cut in it, which must be situated just opposite the tube of 
 the instrument, T. 
 
 a a b b, is a brass slab, which is fixed to the above with a screw- 
 knob, a plane reflecting mirror M can be rotated by means of the 
 knob C, which turns the disc D by acting on a rack and pinion. 
 A second knob C gives the mirror a vertical movement. 
 To give the vertical movement of the apparatus all possible exactness 
 and rigidity, Charles Chevalier placed on the side of the instrument a 
 rack and pinion. 
 
 T is the usual conical tube which carries the large condensing lens at 
 the widened end ; the top of the cone ends in a tube E which 
 receives another tube /, the end of which is furnished, near the object 
 slip, with a second condensing lens which is usually called the focus. 
 
 Charles Chevalier rendered this lens moveable by means of a rack and 
 pinion. The focus of the lens can thus be altered (i.e., the object can 
 be placed more or less near its focus) which is an important feature, 
 
124 
 
 PROJIiCTION MICROSCOPES. 
 
 because some objects require but little light, while others would be 
 consumed or altered in character immediately if they were placed exactly 
 in the focus of the condenser. At E the tube is moveable, which 
 allows the illumination to be regulated. 
 
 The stage N is made of two plates whose distance apart can be 
 modified at pleasure by means of spiral springs. Formerly preparations 
 of a particular shape could alone be used ; by means of the arrange- 
 ment of Charles Chevalier every kind of object can be examined 
 under the instrument, especially transparent troughs having their opposite 
 sides parallel. 
 
 Let us now observe the construction of the magnifying system. 
 H is a square rod which can slide in the tube G by turning the milled 
 head F ; at one end of H and fixed at right angles to it is the piece 
 I, which holds three achromatic lenses K, and in some cases the 
 concave lens L, which we shall mention again later. Close to L is an 
 adjustment screw for exact focussing. 
 
 Charles Chevalier has also applied the adjustment screw to the solar 
 microscope in the case of all slow movements. Moreover, when in 1823 
 he succeeded in making achromatic lenses, he also adapted these to the 
 solar microscope. 
 
 the large condenser, C the focus, L the three achromatic lenses, A the 
 concave achromatic lens. P is a rectangular prism, which may be 
 introduced if required. R R' represents the solar rays reflected at r r> 
 by the mirror M, refracted by the condenser c, and again by the 
 lens C, which concentrates them on the object 0. The rays which pass 
 through the object are received and refracted again by the lenses L, 
 and after crossing each other, form on the screen in front of the 
 instrument an inverted image of the object, the size of which depends 
 on the distance of the screen from the objective. 
 
 We have mentioned that a plano-concave achromatic lens A is made 
 use of (fig. 118); and the advantages obtained from it are as follows : — 
 
 B 
 
 Before showing 
 how the solar mi- 
 croscope should be 
 used, let us first 
 explain briefly the 
 .theory of the instru- 
 ment. 
 
 Fig. 118. 
 
 M (fig. 118) repre- 
 sents the mirror, c 
 
PROJFXTION MICROSCOPES. 
 
 It often happens that the room where the experiments are being made 
 is not long enough to give the magnification desired. 
 
 By placing before the lenses the concave achromatic glass mentioned 
 above, this inconvenience is avoided, because the image produced is 
 much greater than if the lens was not employed. Moreover, the figure 
 will render this effect quite intelligible ; for the rays B B' will project a 
 larger image on a screen at a given distance than will the less divergent 
 rays b b\ The concave lens can be removed if necessary. 
 
 By means of the figure, the effect of the prism P is also clear. 
 It allows the image to be carried forward either sideways, or on the floor, 
 or on the ceiling. 
 
 We will now show how the solar microscope is used. 
 
 The workroom should, if possible, have but one window, and that 
 facing the south. This window should be made perfectly light-tight by 
 means of shutters ; then one of the panes should be removed and 
 replaced by a wooden panel having a circular opening large enough 
 to admit the mirror of the instrument. The plate a a, b b, is fixed to 
 the panel, and held in position by means of the screw knobs, which 
 should be screwed into some nuts placed in the panel for that purpose. 
 
 The mirror M, and the large condenser will then be outside the 
 window. 
 
 The objective carrier L is then removed, and the mirror M is so 
 adjusted by means of the knobs cc, that the sun is reflected by it on 
 to the condenser. Then the fine adjustment of the focus is made use of, 
 until a perfectly clear disc is projected on the screen. The objective 
 carrier has now only to be replaced, and the preparation slipped between 
 the two spring plates of the stage. The object is then focussed by 
 adjusting the lenses by means of the knob F. 
 
 It is now understood that the focus is intended to regulate the light, 
 and that with it the object can be illuminated slightly or strongly 
 by making the focus of the luminous rays approach the latter, or 
 by placing the object actually in the focus. Delicate objects, live objects, 
 &c, should not, therefore, be placed in the focus of the luminous rays, 
 but rather close to it. However, after a little practice one will soon 
 become acquainted with all such necessary precautions. The light must 
 also be carefully watched, because the motion of the earth is always 
 altering the position of the reflector. 
 
 An intimate acquaintance with the objects observed will afford 
 the secret of illumination, and it will be soon discovered when the 
 circulation of the blood of a living animal has to be observed, that if it 
 
126 
 
 PROJECTION MICROSCOPES. 
 
 be placed in the focus of the solar rays, life, and therefore the phenomenon 
 will cease ; at other times this concentration of heat is necessary, e.g., 
 when we wish to crystalise solutions of salts. It is, therefore, impor- 
 tant to know how to regulate the light by the focus. The tube E, 
 which slides into the tube T, may also be used. The image produced 
 by the microscope is received on a screen which should be tightly 
 stretched and parallel to the axis of the microscope. The nearer the 
 screen is, the smaller proportionally will be the image. However, the 
 screen cannot be removed to any distance, because the light will become 
 insufficient. Some white paper carefully stretched over a frame will form 
 one of the best screens. Vegetable paper can also be employed, or the 
 image may be projected on a wall, if very white and smooth. 
 
 By going behind the screen, when it is made of thin paper, the 
 objects can be distinctly seen and drawn. When the wonderful effects 
 of the solar microscope are displayed to a large number of spectators, 
 it is placed behind the screen, so that they may see the object without 
 their attention being distracted. 
 
 To obviate the inconveniences of solar light with special reference to 
 scientific demonstrations, recourse is had to oxy-hydrogen or the electric 
 light. In this case, either the gas or photo-electric microscope is 
 employed, each of which, as previously stated, is simply the solar 
 instrument having a different luminous source. 
 
 The gas microscope, which was first used by Mr. Cooper, of London, 
 was introduced into France by Mr. Warwick, but the apparatus was 
 dangerous ; Charles Chevalier and Galy-Cazalat modified it so as to 
 make it manageable. 
 
 Notwithstanding the utility of the instrument, it can be easily under- 
 stood that its use is restricted. 
 
 The photo-electric microscope was first constructed by Charles Cheva- 
 lier in accordance with the views of Messrs. Donne and Foucault. The 
 instrument has been made more perfect in form and illumination by Mons. 
 Duboscq, the optician, who has made an apparatus which can be worked 
 quite conveniently. Like the gas microscope, the use of the photo-electric 
 microscope is also restricted ; but for regularity of effects and brilliancy 
 of illumination, oxy-hydrogen gas should certainly be preferred. 
 
 When a large audience has to be addressed, very delicate objects 
 cannot perhaps be seen at a certain distance ; the object itself under 
 these circumstances can be shewn either by a photograph or by a 
 good drawing traced on glass. 
 
 Demonstrations with the oxy-hydrogen gas microscope are very 
 
PROJECTION MICROSCOPES. 
 
 127 
 
 instructive. We have made use of them for our classes with great 
 success for more than 20 years, and we have devised a very simple 
 apparatus, easy to manage and one which works with the greatest 
 regularity. For several years our apparatus has been used at the 
 Higher School of Pharmacy at Paris in the classes for materia 
 medica given by Professor Planchon, who praises it very much. More 
 recently the same apparatus has been adopted by the Industrial 
 School of Antwerp, and also by the School of Medicine at Nantes. 
 
 Fig. 119. 
 
128 
 
 PROJECTION MICROSCOPES. 
 
 In this apparatus the luminous source is a cylinder of lime 
 rendered incandescent by a jet of oxygen and hydrogen gas, which 
 may be introduced either separately or mixed as desired. The 
 condenser is three and a half inches in diameter, and the curvatures 
 of the lenses are calculated so as to make use of all the rays emitted 
 from the luminous source. 
 
 The microscope, which is furnished with a coarse and fine adjust- 
 ment is placed in front of the condenser. The objective used is 
 Ross's 2-3rds inch, having an achromatic magnifying glass in front of 
 it. 
 
 The microscope can be instantly replaced by a special objective 
 by which drawings and photographs can be projected on to the screen. 
 
 One of the most valuable properties of this apparatus is that only 
 a distance of from 2 to 3 metres (6ft. 6in. to 9ft. ioin.) is required 
 between the instrument and the screen. The apparatus for photographs 
 notably supplies an image of 2\ metres (8ft. 2in.) in diameter at a 
 distance of 3 metres (9ft. ioin.). 
 
 Oxygen can be produced in about 20 minutes by means of special 
 retorts made of sheet iron, which are not liable to explosion. 
 
 Messrs. Watson and Son also make a very simple microscope, 
 which can be adapted to any projection lantern. This apparatus, 
 which is represented in figure 119, can of course be used for low 
 powers only. 
 
 Let us further notice before concluding, that by means of Professor 
 Abbe's projection oculars any microscope whatever can be tiansformed 
 into a projection microscope. For this purpose it is only necessary to 
 incline it horizontally, to place a luminous source behind the 
 condenser, and to receive the rays emanating from the ocular on an 
 appropriate screen. 
 
BOOK III. 
 
 CHAPTER I. 
 
 USE OF THE MICROSCOPE. 
 
 i. — Situation and Arrangement of the Work Room. 
 
 The aspect of the work room is by no means a matter of in- 
 difference. The best room is one in which the windows face the 
 north or west, or better still north-west. However, if this is impossible, 
 the difficulty can always be met ; a worker in a room facing 
 the south will make better observations by screening the windows with 
 a white blind, which will protect him from the direct solar rays. 
 The same applies to windows facing the west. This last aspect is 
 particularly suitable for experiments with microscopic photography and 
 also for studies with monochromatic light where the sun is necessary. 
 
 A large and very solid table is the most indispensable piece of 
 furniture. It should be placed near the window, and should have at 
 each end as many drawers as possible, so arranged that an observer, 
 when sitting, can open them without moving. The drawers should 
 have a certain number of compartments to receive all the accessory 
 apparatus and tools in ordinary use. 
 
 For some years past we have employed a special table, which 
 appears to have been used in the first place by Mr. Amrhein, of 
 Vienna. This table, the general fitting up of which is similar to 
 that of the table we are about to describe, differs from it in having 
 a glazed frame, which covers it up entirely, and is opened in front 
 and at the top. In our model the hind part can also be partially 
 opened, in order that the light shall not have a second glass plate 
 to traverse during observations by daylight. 
 
 Such a table is of inestimable value for ordinary work. In less 
 than a minute the observer can open it and recommence the work 
 previously interrupted. The instruments and all the implements are 
 found in exactly their previous position under cover lrom dust. 
 
 K 
 
130 
 
 CHOICE OB' LIGHT. 
 
 When electric light is used, as we do, for illumination, the knob of 
 the commutator has only to be pushed to find the object again, just 
 as it was before in the same position, and similarly illuminated, 
 &c. A complicated drawing can thus be easily accomplished though 
 it may have been interrupted and recommenced many times. 
 
 In default of such a table, the microscopes must, after each 
 sitting, be covered over with a glass globe, as the instruments will 
 become damaged if they are taken out of their cases too often 
 and then returned again after each sitting. 
 
 2. — Choice of Light. 
 
 As fir as possible work should be carried on by daylight. If, 
 from unavoidable circumstances, observations can only be made at night, 
 a steady white flame should be used. In the absence of electric light, 
 oil lamps are the best and most economical. For histological work, if 
 the light be too intense, a small ground glass may be placed to 
 advantage on the diaphragm to soften down the light. This can also 
 be accomplished by interposing a blue glass, chosen so that the colour 
 of the glass is complimentary of that of the flame, in which case a 
 white light is obtained. 
 
 During the daytime, sunlight reflected from white clouds, or by a 
 wall of the same colour, should be preferred. The blue colour of the 
 sky does not permit the observation of very delicate details. 
 
 Direct sunlight can never be used in ordinary observations. The 
 softest possible light should be chosen, which should be further moderated 
 by the use of diaphragms, never losing sight of the fact that the smallest 
 diaphragm openings are reserved for the highest powers. First-class 
 instruments allow the light to be modified by raising or lowering the 
 diaphragms ; this is an excellent arrangement, and admits of a perfect 
 gradation of illumination. 
 
 Complete instruments have two mirrors : the plane is used for low 
 powers; the concave, which concentrates a larger quantity of light, is 
 employed for high powers. 
 
 Several systems of illumination, already mentioned above, may also 
 be placed in the luminous path. It is hardly necessary to say that 
 the plane mirror should be employed when an apparatus for concen- 
 trating the rays is used, and that a rectangular prism which has only 
 one reflecting surface is preferable. 
 
 Oblique illumination is a method which possesses great advantages. 
 
CHOICE OF LIGHT. 
 
 It should be employed when we wish to discern clearly the composition 
 of the surfaces of very delicate objects ; as for example, certain diatoms, 
 the striae of which are of extreme tenuity. It is obtained by placing 
 the mirror on one side at different angles to the stage. 
 
 For some time past certain English microscopies have credited 
 oblique illumination with every kind of defect. They declare 
 that results given by this illumination are misleading and unreliable, 
 and that only axial illumination (so-called) given by a condenser with 
 a moderately sized stop is admissable. Several eminent microscopists 
 who have materially advanced the science, e.g., Hon. J. D. Cox, of 
 Cincinnati, have done justice to these absurdities. Professor Abbe in 
 his turn has demonstrated that this professedly axial illumination is in 
 reality a combination of axial and oblique light, and that there is no 
 theoretical reason for attributing to the former any advantages whatever. 
 The time has not, therefore, arrived when microscopists can abandon 
 oblique illumination, as the most powerful means of investigation at 
 their disposal, for any objective whatsoever can shew, with oblique 
 illumination, nearly twice as many lines as can be seen with really 
 axial illumination. When used injudiciously, oblique illumination may 
 sometimes give illusory results, but an experienced observer will never 
 be misled by them. By simply changing the direction of the oblique 
 illumination one can make sure whether a line or strips really exists. 
 
 When opaque objects are examined they should be placed on a 
 slip of a dark colour if they are light, and on a slip of a light colour 
 if they are dark. Then, as ordinary daylight or lamp light is not 
 sufficiently intense, the rays are concentrated in a bundle on to the 
 object by interposing between the light and the instrument a plano- 
 convex lens, the plane side being turned towards the preparation. This 
 lens is always mounted on a single foot in large instruments, and it 
 is important to place it so that its focus falls almost on the object. 
 
 For the same purpose the Lieberkuhn mirror can also be used to 
 advantage. 
 
 Certain objects, however, have to be examined by polarised light. 
 For this purpose, the lower prism must be placed underneath the object, 
 and the cone, at the lower extremity of the tube of the instrument, 
 must be replaced by a piece containing the analyser to which the 
 objectives are then screwed. After having placed a condensing lens 
 above the prism, the polariser can be adjusted until the outline of the 
 objects under observation are clearly focussed. 
 
132 
 
 MAGNIFICATION. 
 
 In certain pieces of polarising apparatus the polariser is mounted 
 above the ocular. These are generally accompanied by a graduated circle 
 and are used for the purpose of measuring angles. They are employed 
 especially in microscopes intended for mineralogy. 
 
 Monochromatic Light. — For some time microscopists, especially those 
 who are engaged in studying diatoms, have used a special kind of 
 illumination, for the examination of stria? which it is difficult to see. 
 They employ monochromatic light, that is to say, they only make use 
 of one of the rays of the spectrum. The blue ray is preferred as 
 by it the best effect can be obtained. Monochromatic light can be 
 produced in several ways : either by decomposing white light by a 
 prism, or sifting the light through a bath containing a solution of 
 ammoniacal sulphate of copper. A stage is fixed in the window on 
 which is mounted a solar microscope, of which the objectives and 
 focus are removed ; a ray of light is introduced into the room, which, 
 by means of the mirror of the solar microscope, is made to strike 
 the mirror of the working microscope. The bath is placed quite close 
 to the solar microscope, and the solution employed should be of a 
 sufficiently deep blue colour. This method of operation was employed 
 for a long time, but since possessing the excellent condenser of Professor 
 Abbe, we have not required so complicated an arrangement. We have 
 been content with making the diaphragm as eccentric as possible, and 
 with receiving the solar rays on the plane mirror, after having passed 
 them through three, four, or five plates of deep blue glass, placed a few 
 centimetres in front of the microscope. The number of blue glasses 
 should be proportional to the intensity of the solar light, and any 
 superfluous light is avoided by a cardboard screen. 
 
 By working in this way, and by using objectives as perfect as those 
 that we now possess, the resolution of the most difficult tests, such 
 as Amphipleura, even in balsam, and that of the last groups of Nobert's 
 tests, can be easily obtained in oblique illumination. 
 
 3. — Magnification. 
 
 It is a general error with persons who are unacquainted with 
 microscopical observations, that much more can be seen when considerable 
 magnification is employed. This is an entirely erroneous idea; the greater 
 number of observations, especially those of vegetable anatomy, being 
 made with magnifications of from 50 to 200 diameters. In studying 
 
HVGIENIC RULES FOR MICROSCOPICAL RESEARCH. 
 
 diatoms we usually employ a magnification of from 400 to 1,000 times, 
 given by a dry apochromatic objective of -95 N.A. for ordinary work, 
 and one with an aperture of 1 '4 for profound study. The compensating 
 oculars 4, 8 and 12 are those we usually employ. The highest powers 
 are rarely employed, and then to investigate only the smallest forms or 
 to elucidate details. Whatever observation be made, the general appearance 
 of the object should always be first studied under a low power. Stronger 
 powers may gradually be employed, but then only when the necessity 
 for it has shown itself. 
 
 4. — Hygienic Rules for Microscopical Research. 
 
 It is often said that microscopical research is injurious to the eye- 
 sight. There is absolutely no foundation for such a statement. Without 
 referring to Leuwenhoeck, who made observations even at the age of 90, 
 and our friend the late M. Adan, who did much work at 85 by means 
 of simple microscopes, which were both rude and fatiguing, we may 
 cite Schacht and Halting, who have both assured us that their sight 
 has not suffered by the continuous observations to which they devoted 
 themselves. AVe have personally used the microscope daily from child- 
 hood ; we have often extended far into the night observations which we 
 commenced in the morning, and that sometimes for many days ; on 
 one occasion we even continued for nearly two months without 
 interruption. We have spent, and still often spend, several hours in 
 examining delicate details in bright solar light, subdued by the ammoniac 
 copper bath or blue glass. With the exception perhaps of very slight 
 shortsightedness our researches have in no way injured our sight. 
 Moreover with proper precaution microscopical researches can never 
 cause injury ; on the other hand, they may cause real danger if recklessly 
 conducted. Mandl cites the case of a microscopist who almost lost his 
 sight through having made all his observations in a dark room in 
 which light was only admitted through a small opening opposite the 
 mirror. One of the most illustrious algologists of our time, the late 
 A. de Brebisson, also wrote us that having spent the winter in making 
 prolonged and fatiguing observations with very brilliant artificial light 
 for the purpose of counting the striae of diatoms, a violent congestion 
 ensued, which forced him to suspend all work for nearly a whole 
 year. But no such trouble need be feared by anyone who will take 
 heed of the following advice : 
 
134 
 
 HYGIENIC RULES FOR MICROSCOPICAL RESEARCH. 
 
 1. Do not make observations directly after a meal. 
 
 2. Let the field of the microscope be comfortably illuminated. 
 Always avoid brilliant illumination, and on no account use solar light 
 for ordinary observations. 
 
 It is only during experiments with polarized light, photo-micrography 
 and with monochromatic light, that solar light can really be employed 
 to advantage. 
 
 3. As soon as your eyes feel at all fatigued suspend your observa- 
 tions at once. This is of the greatest importance. 
 
 4. An excellent hygienic rule,., which has greatly assisted us during 
 the last six years, is to wash the eyes thoroughly every morning with 
 warm water. We use a litre (if pints) of water for this daily ablution. 
 The warm water thus employed produces at first a very slight congestion, 
 followed immediately by an excellent reaction. We cannot too strongly 
 recommend this washing which rests the eyes. Cold water on the 
 other hand gives a momentary calm, followed afterwards by a congestion 
 of the visual organ. 
 
 The eyes, • like other organs, must naturally be used with discretion. 
 However, it is unnecessary to lay further stress on the advice given. 
 The man of science and the simple amateur will appreciate its import- 
 ance at once. 
 
 There is still one question that remains to be answered : How 
 must the microscope be arranged to prevent fatigue ? 
 
 If the reader simply wishes to study preparations in which the 
 object is already covered with a cover-glass, there is no better position 
 for the microscope than an inclined one. It prevents the passive 
 congestion of the head, which is produced by the compression of the 
 blood vessels of the neck, and it prevents flying flies (musccs volitantes) 
 from annoying the observer by continually appearing in the axis of 
 vision. 
 
 But if objects mounted in liquid have to be studied, or if dissections 
 have to be made, or re -agents have to be used, then the inclined 
 position is no longer convenient. The microscope should evidently rest 
 in a vertical position. However, these inconveniencies can be remedied 
 either by fixing a rectangular prism at right angles to the microscope, 
 as we have done for more than 20 years, when using one of the 
 large English instruments, or by placing on the ocular an isosceles 
 prism as our late friend Harting did, who was much annoyed by flying 
 flies. 
 
EXAMPLE OF A MICROSCOPICAL OBSERVATION. 
 
 Professor Abbe declares in one of his works that a well-constructed 
 prism does not affect the image. We agree entirely with this opinion. 
 Our rectangular prism, which has the advantage of ensuring free play 
 to the chest, does not in any way affect the appearance of the most 
 delicate microscopical details, as for example the beads of (he Amphipleura. 
 
 5. — Example of a Microscopical Observation. 
 
 We feel confident that we have now explained the management of 
 the microscope in sufficient detail ; however, being anxious to facilitate 
 the observations of persons who are. not yet accustomed to the use of 
 this instrument, and who are eager to profit by it in every possible 
 way, we believe it will be useful to give an example of the application 
 of the rules mentioned above. 
 
 Let us suppose then a beginner wishes to observe all the striae on 
 a Surirella gemma, which is one of the most difficult tests. If by 
 following our advice he succeeds in accomplishing this, he may be 
 quite sure that the microscope has no secrets which he cannot dis- 
 cover. 
 
 Let us take a good ordinary microscope, without a condenser, but 
 provided with a jointed mirror, and place it a few feet from the 
 window, on a very solid table, the height of which should be such 
 that when sitting the observer can conveniently look down the tube of 
 the microscope in a vertical position. We shall proceed in this case 
 with every precaution, but later on with practice, the student will 
 accomplish this with less labour. 
 
 With this purpose in view let us first try an objective of about 
 half-an-inch ; when it has been screwed to the tube which is furnished 
 with an ocular of medium power, a medium sized diaphragm is 
 inserted. 
 
 Attention must now be paid to the illumination. The reflecting 
 mirror placed in the axis of the tube is inclined, so as to reflect the 
 light through the opening of the diaphragm. We must now experiment 
 until we get a soft white light ; as long as it is not so, you may be 
 sure that the inclination of the reflector is defective. 
 
 You next place the preparation on the stage having a glass over 
 the object, then you rack down the tube just opposite the prepara- 
 tion, and carefully avoid touching it for fear of injuring it ; then 
 holding it in position with the fingers or clips, you gently raise the tube of 
 the microscope until you see the Surirella more or less in focus, you 
 
136 EXAMPLE OF A MICROSCOPICAL OBSERVATION. 
 
 then move the fine adjustment right and left until one of the diatoms 
 in the centre of the field appears quite clear in the form of an eliptical 
 body, divided by a median line, having twelve or sixteen transverse lines 
 on each side. 
 
 But this is but the beginning of what we are to see ; secure the 
 preparation well by means of the clips, so that it cannot get deranged, 
 and take a higher power objective, one-sixth inch for instance, ; you 
 then raise the tube to such a height that the first objective can be taken 
 off and the one-sixth inch substituted for it ; this done, you substitute 
 for the medium diaphragm that which has the smallest opening, and 
 then you lower the tube until it is near the preparation, and raise it 
 again directly until the object appears confused ; then by turning the 
 fine adjustment milled head to the right or left, you ought to 
 see with the greatest clearness what you saw before, very clear indeed, 
 considerably enlarged, but without any further detail. 
 
 Axial light having thus proved insufficient, let us see what can be 
 done with oblique light. 
 
 First remove both the diaphragm and the sub-stage which holds it, 
 so as completely to disengage the real stage of the microscope under- 
 neath ; then incline the shank of the reflector to the right or left, so 
 as to turn it aside from the axis of the microscope, at the same time 
 keeping sufficient light to illuminate the object ; try every possible 
 inclination of the mirror until at last you notice between the transverse 
 lines spoken of above, and parallel to them, other small lines, which 
 will appear very fine, and extremely close together. An experienced 
 eye can, after a very short time, decipher these fine longitudinal lines 
 even with a one-sixth inch ; but to see them clearly and conveniently, 
 an immersion objective must be used. 
 
 Let us take, then, a water immersion objective. Do not displace 
 either the preparation or the mirror; just raise the tube, and take off 
 the one-sixth ; then having substituted the new objective, take a small 
 drop of clean distilled water, and by means of a glass rod, place it 
 on the lower or frontal lens, without touching the latter for fear of 
 scratching it; then screw the objective to the tube; place another drop 
 of water on the preparation, and lower the tube until the two drops 
 intermix : by proceeding in this way all air bubbles are avoided, which 
 may seriously hinder observation. Now substitute a low ocular for the 
 one you have used hitherto. 
 
 The tube must in this case be moved with the greatest care ; the 
 danger is still greater, because the water makes it difficult to distinguish the 
 
EXAMPLE OF A MICROSCOPICAL OBSERVATION. 
 
 137 
 
 space between the objective and the preparation ; the best way to avoid 
 breaking anything is to proceed cautiously, by moving the preparation 
 gently with one hand, while with the other the fine adjustment screw is 
 turned from left to right or from right to left with extreme delicacy until 
 the Surirella is seen clearly ; if you lower the tube too much, the resistance 
 of the preparation, which is immoveable, tells you that you are moving 
 the adjustment screw in the wrong direction, and that you should raise 
 the tube again by turning the fine adjustment screw from right to left. 
 Then you displace the mirror underneath from the axis of the tube until 
 you clearly see the longitudinal undulating lines of the object of your 
 researches. 
 
 However, it may happen that you are not yet quite satisfied. In such 
 a case your only resource now is in the correction collar ; to use this 
 you take hold of the moveable milled collar on the mounting of the 
 objective, which is capable of correcting the effect of the glass slip 
 or cover glass ; you move this from right to left or from left to right 
 until you can see the lines or strias with the greatest clearness. It is 
 generally necessary, when oblique light is used to bring the lenses of 
 the objective closer to one another, which can be effected by turning 
 (in French objectives) the correction collar from right to left ; if, on 
 the other hand, direct light is used, we must turn the collar in the 
 opposite direction, viz., from left to right. 
 
 It is important to remark that to avoid pressing on the preparation 
 and breaking it, and to keep the object in focus, it is necessary to 
 move the fine adjustment either from left to right or from right to left, 
 while the correction collar is being moved in one direction. 
 
 Such are the operations that a beginner should practise when in 
 possession of a microscope, which is sufficiently good to render all 
 ordinary research possible. 
 
 If instead of this apparatus he has a first-class microscope furnished 
 with an Abbe condenser and a homogeneous objective of large numerical 
 aperture, e.g., an apochromatic objective of N.A. i - 4, the operation is 
 much simplified. The ultimate observation should show not undulating 
 lines, but clear isolated points (usually called " beads ") ; this can be 
 easily effected with the condenser if used in accordance with the rules 
 given on page 86 of this work. 
 
 When the work is finished, you have only to raise the tube, to wipe 
 the preparation, and in particular to dry the lens with the greatest 
 care with a well used cambric handkerchief, or with a piece of very 
 clean chamois leather. These details will appear very unnecessary to 
 
BAUSCH AND I.OMB. 
 
 experts ; but it is not for them that they have been inserted ; they know- 
 quite enough about it, and perhaps more than we do. It has been our 
 sole purpose to acquaint those who are inexperienced in managing a 
 microscope, with some of its most intimate secrets. After a few days' 
 experience, if our advice be thoroughly followed, beginners will be 
 considered to be quite expert, and then they will feel indebted to us, 
 we dare to believe, for the information which we have felt a duty and 
 pleasure in giving them. 
 
 Choice of a Microscope. 
 
 In choosing a microscope several points have to be considered, the 
 nature of the researches for which the instrument is intended, and the 
 price one is willing to give for it, being the most important. 
 
 It is of little use for a beginner to obtain a large and com- 
 plete microscope at once, but it is still worse to buy a small instru- 
 ment of inferior quality to commence with. In a very short time it 
 would have to be replaced by another, and then the money spent upon 
 the first would be regretted as an entire loss. 
 
 The most reasonable course is to buy in the first instance a stand 
 which will meet every requirement that can be anticipated. If to such 
 a stand two objectives be added, for instance an \ inch and \ inch, 
 the student can commence practice with these, and he may add to his 
 apparatus at a subsequent date. 
 
 We warn everyone against making this purchase carelessly or at first 
 sight. Only instruments marked with the name of well-known makers 
 should be accepted, and if the manufacturer himself be not directly 
 communicated with, at least no microscope should be purchased of any 
 agents but well-known firms in whom every confidence can be placed. 
 
 Germany, England, Austria, the United States, France, and Italy possess 
 eminent makers; we shall just review the principal firms in these different 
 countries by describing the models which appear to us to be most 
 appropriate for serious research. For a definite choice, the purchaser must 
 consult his own tastes, preferences, and especially the resources of his purse. 
 
 BAUSCH & LOMB OPTICAL COMPANY (North St., Paul 
 Street, Rochester, N.Y., and 50, Maiden Lane, New York). — Messrs. Bausch 
 and Lomb are old-established makers, and their justly celebrated instru- 
 ments hold the first position in the United States. The instruments 
 of these makers are of perfect workmanship and rare elegance. By a 
 
BAUSCH AND LOMB. 
 
 139 
 
 happy mixture of colours, due to the judicious use of coloured varnishes, 
 bronze, brass, silver, nickel, &c, their appearance is very charming. No 
 other microscopes that we know are so attractive to the eye, or produce 
 so pleasing an effect. 
 
 Messrs. Bausch and Lomb make two kinds of microscopes : a 
 special kind, which they call the American, and the Continental. 
 
 Figure uga is a representative of the "Universal Microscope," which 
 is the original model of the American kind. 
 
 The instrument rests 
 on a foot having three 
 legs, terminating in india- 
 rubber pads. The body, 
 properly so called, is car- 
 ried on a pillar, and the 
 joint allows every inclina- 
 tion that may be desired. 
 
 Both the bar of the 
 mirror and that carrying 
 the sub-stage can rotate 
 about the optical axis 
 of the instrument. The 
 upper part of each bar 
 terminates in a circular 
 fitting, which is silver- 
 plated and is divided 
 into degrees. 
 
 The mounting of the 
 mirror slides in a hollow 
 groove in the bar, and can 
 consequently be raised 
 or lowered, while the 
 rotation of the bar gives it 
 every possible inclination. 
 
 The sub-stage, carried 
 on an inner bar, is raised 
 or lowered by a rack and 
 pinion ; it holds, when 
 
 desired, an Abbe condenser, furnished with an Iris diaphragm, and can 
 be turned on its axis so as to assume different oblique positions. 
 
 The stage has a mirror, rotating about its axis, which works very 
 
 Fig. 119a. 
 
T40 
 
 BAUSCH AND LOMfl. 
 
 smoothly, and is furnished with two clips. When desired, the latter 
 can be removed, and a glass stage and slide carrier placed upon the 
 stage proper. 
 
 The tube is moved by a rack and pinion, which works very smoothly 
 and with great precision. It has a double draw tube; its length, when 
 closed, is 17 centimetres (6\ in.), and when fully drawn out, 30 cen- 
 timetres (nf in.). The draw tubes, which are nickeled, slide in a cloth 
 sheath, and consequently their movements are very smooth. The 
 innermost tube is divided into millimetres. At the lower end of the 
 tube is the American screw thread, but a nose-piece, which can be 
 screwed into it, allows the objective with the English screw to be adapted. 
 
 The slow movement adopted by these makers is quite special, and 
 has been patented under the title of " frictionless fine adjustment." 
 
 The essential parts consist of two strong steel springs placed parallel 
 to one another, and fixed at each end. A milled head acts on the end 
 of a lever, which in its turn raises a piece of apparatus, which carries the 
 coarse adjustment. 
 
 The lever is free at its two ends, and turns on a pivot freely round 
 an axis. 
 
 The slow movement thus obtained works very smoothly and with 
 great precision ; it has however the inconvenience of constraining the 
 tube to move in a small circular arc, which is also noticeable in micro- 
 scopes furnished with the parallelogram slow movement as made by 
 Messrs. Seibert, Reichert, Leitz, &c. Nevertheless, this is only apparent 
 when the milled head of the micrometer screw is moved several times 
 round. 
 
 When desired, the makers furnish their instruments with the differential 
 screw, at a fixed price. 
 
 The instrument above described is, as we have said, the model type, 
 but there are various modifications of it. 
 
 In its simplest form it is called the Investigator Microscope. It 
 has then only one moveable bar, which carries both the sub-stage, formed 
 of a simple tube, and the mirror. The price of this stand is $45 (£<))■ 
 The Universal Microscope, previously described, without Abbe condenser 
 and glass stage, costs $55 (;£n). 
 
 When the Universal Microscope is in all respects as described above, the 
 the makers call it the Bacteriological Microscope, and it is sold for $75 (^,15). 
 
 When of a very large size, and mounted between two pillars, with 
 a perfect sub-stage, the instrument is called the Professional Microscope, 
 and costs $135 (£27). 
 
BAUSCH AND LOMB. 
 
 I 4 I 
 
 Lastly, the instrument appears in the form of Wenham's Radial, 
 which will be described hereafter. It then costs $95 (£19), and is 
 sold as the American Concentric Microscope (fig. 119^). 
 
 Fig. 119*. 
 
 It will be noticed that all these forms are in reality the same 
 instrument, and only differ in special adaptations to various branches 
 of study. 
 
 Messrs. Bausch and Lomb's Continental Type are very similar to the 
 continental instruments of Messrs. Zeiss, Reichert, Leitz, &c. 
 
 In the Large Continental Microscope (fig. ii<k) the makers have 
 taken as their model the most perfect instruments of well-known manu- 
 facturers which we shall describe in their turn. The only difference 
 that we notice at first sight is that here the mirror is moveable round 
 the axis. 
 
BAUSCH AND LOME. 
 
 143 
 
 In consequence of the provisions of the McKinley tariff, Conti- 
 nental instruments will find great difficulty in reaching the American 
 
 Fig. 119^. 
 
144 
 
 BAUSCH AND LOMB. 
 
 markets; it is therefore probable that Messrs. Bausch and Lomb have 
 endeavoured to furnish the microscopists of their own country with a 
 type of instrument which appears to them to be popular. As the 
 price of Messrs. Bausch and Lomb's apparatus is only $90 (^18), it 
 is probable that this model is destined to have a great sale in the 
 United States. 
 
 The Medium Continental Microscope (fig. 119^) is a very inferior 
 model and costs $25 (£5). It is similar to those which can be obtained 
 on the Continent at the same price. 
 
 Messrs. Bausch and Lomb's objectives are very numerous. Besides 
 their special ones (the 2-3rds, i-6th, and i-i2th homogeneous) for the 
 Continental tube, they also make three different series for the English 
 length, which they call the student's series, professional series, and first- 
 class series. 
 
 The specimens which we have examined belong to the professional 
 series, and are as follows : — 2 in., f in., and the 1-12 in. 
 
 These three objectives may be counted among the best that we have 
 hitherto examined. The images are very clear, and chromatic correction 
 is so well effected that practically no trace of colour can be found. 
 In this respect the objectives considerably surpass those of the most 
 celebrated Continental firms. 
 
 2 inch (N. A. = -15) and f inch (N. A. = -35), are two objectives 
 intended for preliminary work, and give excellent images. 
 
 1 -1 2th inch (N.A. = ri8) is mounted with correction collar and has 
 a considerable frontal distance. With central illumination, the Pleurosigma 
 and the Bacillus tuberculosis give very fine and pure images ; with 
 oblique light the Amphipleura is resolved with the greatest clearness. 
 The objective suffices for every-day research, even the most delicate. 
 
 The following abridged table gives the principal objectives of these 
 makers : — 
 
 Objectives. 
 
 Students' Series. 
 
 £ 
 
 4 
 
 inches 
 
 angle of aperture 
 
 6° Price 
 
 &6 
 88 
 
 140 
 140 
 140 
 1 12 o 
 
 1 16 o 
 
 2 16 o 
 
 300 
 
 3 12 o 
 
 4 16 0 
 
 2 
 1 
 
 a 
 
 inch 
 
 27 0 
 
 42° 
 
 4 
 
 i 
 
 » 
 
 $9 
 814 
 
 818 
 
 $24 
 
 » 
 
BAUSCH AND LOMB. 
 
 145 
 
 Professional Series. 
 
 4 
 
 inches 
 
 angle of aperture 
 
 10° Price 
 
 $13 
 
 £ 
 2 
 
 12 
 
 d. 
 O 
 
 2 
 
 
 
 iS° » 
 
 $13 
 
 2 
 
 12 
 
 O 
 
 i4 
 
 inch 
 
 
 24° 
 
 36° „ 
 
 40° H 
 
 |iS 
 
 3 
 
 0 
 
 O 
 
 1 
 
 
 815 
 
 3 
 
 0 
 
 O 
 
 i 
 
 » 
 
 
 $iS 
 
 3 
 
 0 
 
 O 
 
 
 
 
 65° ,. 
 
 *i8 
 
 3 
 
 12 
 
 O 
 
 ilS 
 
 »> 
 
 
 13°° .1 
 135° 
 
 I7O" „ 
 
 175° 
 
 $28 
 
 5 
 
 12 
 
 O 
 
 1/8 
 
 j» 
 
 
 $30 
 
 6 
 
 0 
 
 O 
 
 I.'lO 
 
 a 
 
 water immersion 
 
 828 
 
 5 
 
 12 
 
 O 
 
 1/16 
 
 >» 
 
 
 835 
 
 7 
 
 0 
 
 O 
 
 1/10 
 
 „ homogeneous immersion N. A. 
 
 118 „ 
 
 850 
 
 10 
 
 0 
 
 O 
 
 1/12 
 
 >> 
 
 j» > » 
 
 
 855 
 
 11 
 
 0 
 
 O 
 
 1/16 
 
 n 
 
 11 >» 
 
 »> i> 
 
 865 
 
 13 
 
 0 
 
 0 
 
 I 
 4 
 
 1/6 
 
 1/8 
 
 1/10 
 
 1/12 
 
 1/16 
 
 1/25 
 
 First-class Series. 
 
 inches 
 inch 
 
 angle of aperture 
 
 homogeneous 
 
 Price 
 
 45 
 98° 
 140° 
 
 818 
 825 
 830 
 840 
 875 
 
 3 12 
 3 12 
 5 ° 
 
 15 
 16 
 
 8125 25 
 8200 40 
 
 An apochromatic series is in course of construction, but we have 
 not as yet received any information concerning them. 
 
 They also manufacture a very rational series of oculars ; it consists 
 of 2 inch, if inch, \\ inch, 1 inch and -5 inch. Each costs $4 (16s.). 
 
 The makers also manufacture continental, periscopic and holosteric 
 oculars. 
 
 MESSRS. R. & J. BECK (of 68, Cornhill, London, E.C.), is 
 one of the best firms in England, and their instruments are highly 
 valued. 
 
 Their large model, No. 1 (fig. 120), called The International, can 
 be inclined and is binocular. 
 
 The rapid movement is given by a rack and pinion by means of 
 the milled head K, and the slow movement is produced by the inter- 
 position of a lever on which the milled head L acts. 
 
 The body of the microscope is supported on two pillars fixed to 
 a horizontal circular graduated plate B, which can be turned on a 
 fitting, having three projecting feet in the form of a tripod A, to 
 give stability to the instrument. 
 
 The stage is of an entirely special construction. Attached to the 
 end of a disc R, and capable of moving on a pivot round a central 
 point, it can be turned so as to assume different inclinations ; 
 it can even be turned completely round so that an object 
 
 L 
 
Fig. 120. 
 
R. AND J. BECK. 
 
 147 
 
 can be examined with any obliquity of light without fear of its 
 thickness proving a hindrance. The mechanical stage can be given 
 vertical and horizontal movements by means of the milled heads N 
 and O; the side of the stage is divided into 360 degrees to serve 
 as a goniometer. 
 
 An Iris diaphragm, which can be taken off the stage while it is being 
 rotated, has its opening regulated by a rack and pinion and milled head T. 
 
 The sub-stage G, to which can be adapted all the apparatus for 
 illumination usually employed, can be turned about the optic axis 
 when oblique light is employed by means of the piece /, which is 
 rotated by means of the milled head X and works in a large 
 graduated ring Y. 
 
 Upon this ring, whose movements in the optic axis are regulated 
 by the lever Z, the greater part of the angular movements of the 
 microscope will be found to depend. 
 
 A triangular bar V carries concave and flat mirrors, which can be 
 moved in all directions and are independent of the movements of 
 other illuminating apparatus. 
 
 The price of this beautiful binocular stand, together with two pairs 
 of oculars, forceps, &c, is ^50. In the monocular form it only 
 costs £43. 
 
 The second grand model of these makers (fig. 121) is in the same 
 way supported on two pillars, and provided also with a revolving plate. 
 
 Its stage can be rotated about its axis by means of a rack and 
 pinion, and is graduated for the measurement of angles ; it carries a 
 mechanical stage, moving very precisely. The stage can be rotated the 
 entire revolution, the limb being designed to clear the milled heads. 
 
 The sub-stage can be removed when desired, and can be racked up and 
 down by turning the milled head. The condensers and other illuminating 
 apparatus, when axial, are centred by means of two screws acting on 
 a spring. The mirror, which is concave one side and flat on the 
 other, is jointed and mounted on a circular fitting, which slides on a 
 triangular bar. The coarse adjustment is made by a rack and pinion, 
 and the fine adjustment by a lever. 
 
 The whole microscope can move between the two pillars, and 
 can be set at any inclination. This stand, together with two pairs of 
 oculars and several accessories, costs ^35 when it is binocular, and 
 ^28 when only mounted with the monocular tube. 
 
 The instrument No. 3 (fig. 122) only differs from the preceeding 
 one in its smaller dimensions and its support, which consists of a 
 
 L2 
 
Fig. 121. 
 
i5° 
 
 R. AND J. BECK. 
 
 single pillar, on which the microscope can be set at any inclination 
 that may be desired. 
 
 All the other parts of the instruments are almost identical to those 
 of the model just described, except that the mirror is fixed to a 
 tube sliding over a cylinder, and can thus be used for oblique light. 
 
 The binocular instrument is sold for ^30 and the monocular for ,£25. 
 
 The same opticians make a large number of other microscopes, 
 amongst which we shall specify the model called the Binocular National 
 Microscope (fig. 123), and which is remarkable for its moderate price. 
 
R. AND J. BECK. 
 
 The entire instrument is moveable on its axis, and can assume any 
 inclination. 
 
 The stage rotates and has a mechanical stage ; the sub-stage can 
 be centred by means of two lateral milled heads, and its height 
 regulated by a rack and pinion. The mirror is jointed. The binocular 
 tubes have an optional regulator for the distance between the eyes, and 
 the adjustments, coarse and fine, are carried out in the same manner 
 as in the large models of these makers. 
 
 With five objectives, a triple nose-piece, plane and concave mirror, 
 three pairs of oculars, achromatic condenser, Wenham's parabolic re- 
 flector, polarizing apparatus, Wollaston's camera lucida, micrometer, 
 bull's eye on stand for illuminating opaque objects, glass slips, cover 
 glasses, forceps, &c, the instrument packed in a mahogany case, is 
 delivered for ^,38 13s., but monocular it is ^35 2s. 6d. 
 
 We give in the table below the price of the different objectives made 
 by this firm : — 
 
 4 
 
 3 
 3 
 2 
 2 
 
 1 1/2 
 
 a/3 
 a/3 
 
 12 
 
 410 
 
 4 IO 
 
 1/4 
 
 »/S 
 1/8 
 1/10 
 
 1/15 
 1/20 
 1/40 
 
 1/13 
 
 1/16 
 1/20 
 
 inches. 
 
 inch. 
 
 Angle of Aperture. 
 
 9° 
 7° 
 12° 
 Io° 
 17° 
 23° 
 25° 
 
 3< 
 45 
 65° 
 95 
 75 
 120° 
 130 0 
 1 80° 
 
 180 0 
 180° 
 160 0 
 
 Trice. 
 
 with correction collar 
 
 water immersion with correction 
 collars 
 
 with correction collar 
 Homogeneous Immersion Objectives. 
 
 Numerical Aperture. 
 
 1-25 
 
 1.25 
 1.25 
 
 £ 
 
 s. 
 
 d. 
 
 1 
 
 10 
 
 0 
 
 1 
 
 10 
 
 0 
 
 2 
 
 10 
 
 0 
 
 1 
 
 10 
 
 0 
 
 2 
 
 10 
 
 0 
 
 2 
 
 10 
 
 0 
 
 2 
 
 0 
 
 0 
 
 2 
 
 10 
 
 0 
 
 2 
 
 10 
 
 0 
 
 4 
 
 0 
 
 0 
 
 5 
 
 0 
 
 0 
 
 3 
 
 10 
 
 0 
 
 4 
 
 10 
 
 0 
 
 5 
 
 0 
 
 0 
 
 5 
 
 5 
 
 0 
 
 8 
 
 0 
 
 0 
 
 10 
 
 0 
 
 0 
 
 20 
 
 0 
 
 0 
 
 5 
 
 10 
 
 0 
 
 9 
 
 0 
 
 0 
 
 12 
 
 0 
 
 0 
 
 We have had occasion to examine a series of objectives by these 
 makers quite recently (April, 1891). The following is the result of our 
 observations : — 
 
 4-ioth inch of 65°, good objective, resolved the 1 leurosigma feebly 
 in oblique light ; the pygidium was quite passable in axial illumination. 
 4-ioth of gf. This objective enjoys a great reputation, which is 
 
L. BENECHE 
 
 thoroughly justified. It is in fact one of the best, if not the best, of 
 the objectives giving this magnification that we have ever seen. 
 
 The frontal distance is considerable, but nevertheless, the pygidium 
 presents a very beautiful image with axial illumination, and the Pleurosigma 
 and the 6th group of Nobert is resolved by it very fairly. 
 
 With oblique illumination the Pleurosigma and the ioth group of 
 Nobert are perfectly resolved. Considering its great frontal distance, this 
 is, on the whole, one of the best objectives that can be desired both 
 for histological work and the brief study of diatoms. 
 
 1-4H1 inch. A good objective for histological studies, inferior to the 
 previous one for diatoms. The frontal distance is very considerable. 
 
 With axial illumination it gives a very good image of pygidium and of 
 Podura, and it will resolve the 7th group of Nobert. 
 
 With oblique illumination the Pleurosigma is seen well, as also the ioth 
 group of Nobert. 
 
 1 -1 2th inch homogeneous immersion. — It resolves the 9th group of Nobert 
 with axial, and the 13th with oblique illumination. The Amphipleura, 
 in the yellow medium, is passably resolved. The frontal distance is very 
 small. 
 
 L. BENECHE (of 55, Grossbeerenstrasse, Berlin). — L. Beneche 
 is one of the oldest and most zealous makers in Germany, and many 
 distinguished savants have justly complimented him on his work. Messrs. 
 Dippell and Harting speak very favourably of this maker. Schacht 
 values the Beneche objectives highly, and we remember seeing in about 
 1862, during a visit at Bonn to our late learned friend, some micro- 
 scopes made by Beneche which left nothing to be desired at that time. 
 
 This manufacturer makes at the present time several models of 
 microscopes, the principal of which we propose to describe. 
 
 The Grand Model (A). This microscope can be inclined, mounted 
 on a horse-shoe foot, and furnished with a rotating stage; the coarse 
 adjustment is made by means of a rack and pinion, and the fine 
 adjustment by means of a micrometer screw with a graduated head. 
 This microscope is furnished with a jointed mirror and an Abbe illu- 
 minating apparatus having a numerical aperture of 1*4 and an iris 
 diaphragm. 
 
 The alteration of the condenser to enable the use of tube dia- 
 phragms is effected by a sliding groove. It is furnished with a nose- 
 piece for four objectives, a micrometer eye piece, a drawing apparatus 
 after Oberhauser, Nos. 2, 4, 5, 7 ordinary objectives, i-i2th homo- 
 
*54 
 
 L. BENECHE. 
 
 geneous immersion objective, A, B and C compensating oculars, giving 
 magnifications from 30 to 1,500 diameters. The price of the whole is 
 £34 6s. 8d.. 
 
 The Model (A) has a rack and pinion and a micrometer 
 screw for focussing ; the mirror is jointed. It is also furnished with 
 an Abbe condenser, but one of only 1/20 N.A. The instrument is sold 
 with a nose-piece for three objectives, oculars 2, 3, and 4, ordinary 
 objectives 2, 4, and 7, and an homogeneous objective. It magnifies 
 from 30 to 1,100 diameters. It costs £\d 14s. 6d. (fig. 124). 
 
 A third model, lettered B (fig. 125) has a rotating stage, and can 
 be inclined at any angle. It has a horse-shoe foot ; the mirror 
 is jointed; the diaphragm tube can be raised without moving the 
 
 Fig. 125. Fig. 126. 
 
 cover glass. The coarse adjustment is made by sliding this tube, and 
 the fine adjustment by a micrometer screw. Furnished with five oculars, 
 a moveable micrometer eye-piece, Oberhaiiser's camera lucida, as well as 
 2, 4, 5, and 7 ordinary objectives, and a water immersion No. io, the 
 price is £22. 
 
 L. Beneche also makes a microscope for demonstrations, which is 
 represented in figure 126, and is intended to be handed from one 
 to another in a class. It is furnished with a rack and pinion, and 
 gives a magnification of 125 diameters. Its price is £1 15s. 
 
BEZU, HAUSER AND CO. 
 
 155 
 
 We have seen four recent objectives of this maker (September, 1891). 
 The following are the results which they gave : 
 
 No. 4 (J- inch N.A. '40). Very good objective for histological study, 
 bearing well the use of strong oculars. 
 
 No. 7 (i-6th inch N.A. 75). In axial illumination the image of 
 the pygidium is pure and clear ; that of the Podura is good, and the 
 objective also shows Nobert's 6th group. In oblique illumination the 
 Fleurosigma Angulatum is seen in hexagons perfectly and Nobert's 8th 
 group. 
 
 No. 9 (i-i2th inch N.A. 75). In axial illumination, the image of 
 the Podura is excellent ; in oblique illumination Nobert's 10th group is 
 clearly resolved. 
 
 No. 10 ( 1 '15th N.A. i - 2o). Water immersion. Axial illumination: 
 the image of the Podura is very good and very pure ; the Fleurosigma 
 is seen in hexagons very clearly, and broken edges are perfectly clear 
 and with detail ; the Bacillus Tuberculosis is very clear. In oblique 
 illumination the Amphipleura in the yellow medium is resolved, but the 
 image leaves much to be desired. 
 
 M. L. Beneche marks his objectives, when sold separately, at the 
 
 following prices : — 
 
 £ s. d. 
 
 System No. i (angle of aperture 20 0 ) Price 0120 
 
 ii 2 ( ,, „ 25 0 ) .. .. .. .. ,, o 18 o 
 
 11 3 ( » 3°°) 1 1 o 
 
 4 ( ,i „ 34°) 1 140 
 
 5 ( ,, „ So°) „ 140 
 
 „ 6 ( „ „ 60°) „ 176 
 
 7 ( „ „ 95°) 176 
 
 „ 8 ( ,, „ ioo°) ,, 176 
 
 „ 9 ( ,, „ 120 0 ) „ 250 
 
 „ 10 ( „ „ 130 0 ) immersion without correction ,, 300 
 
 „ 10 ( ,, 130 0 ) immersion with correction ,, 4 10 o 
 
 11 ( „ iS°°) 11 -> 6 15 o 
 
 11 12 ( 11 n i7°°) n ,,500 
 
 „ 1/12 ( „ ,, 145 0 ) homogeneous immersion ,, 500 
 
 11 1/18 ( „ ,, 160°) „ ,, 10 o o 
 
 n 1/24 ( 11 11 180°) „ ,, 15 o o 
 
 BEZU, HAUSER & CO. (Rue Bonaparte, Paris.)— These 
 makers, who were formerly foremen in the firm of Hartnack and 
 Prazmowski, took over their business at the death of Prazmowski. 
 
 They have a considerable number of models. 
 
 The original large model, called the Hartnack model, which dates 
 40 back years, is a well-known instrument, and has been figured by 
 all authors. We give a representation of it in figure 128. 
 
 Their new large model, No. VII., differs from the preceding in being 
 constructed more solidly, and furnished with an Abbe condenser. 
 
bezu, HAUSER AND CO. 
 
 The latter is mounted on an eccentric piece, by which it can be 
 placed on one side, out of the optic axis. 
 
 The instrument is made to incline, and furnished with a rack and 
 pinion for rapid movement, and a very precise adjustment screw. The 
 stage is rotary. 
 
 Furnished with Nos. 2, 4, 5 and 7 dry objectives, and No. 9 
 immersion with correction collar, and with five oculars, the price is 
 
 £za 15s- 
 
 A small and excellent model, No. VIII. A. bis. (fig. 127), having 
 
 a rack and pinion for rapid movement, and 
 an adjustment screw for slow movement, costs 
 £16 4s. 
 
 At this price is included Nos. 4, 5, 7 
 and 9, dry objectives, and Nos. 2, 3, 4, 
 and 5 oculars, the first being a micrometer 
 ocular. 
 
 We have obtained excellent results on 
 examining the objectives of these makers. 
 
 Those given by the principal are as fol- 
 lows : — 
 
 No. 6, dry; aperture = '80. Good objective, 
 well achromatised with pure images. 
 
 No. 9, water ; aperture = i'20. This ob- 
 jective, whose actual focus is i-i2th inch, is 
 one of the best water objectives which we 
 
 Fig. 127. have at present examined. It resolves most 
 
 completely for it has shewn us Amphipkura dry perfectly ; it has 
 another and extremely rare advantage in that it gives excessively flat 
 images. The Amphipkura shows its striae from one end of the frustule 
 to the other with a clearness which leaves nothing to be desired. 
 
 Nos. 10 and 13 have each an aperture of no; their resolving 
 power is, therefore, rather less than the preceding. The field is quite 
 flat ; the Amphipkura is perfectly resolved, but the image is not quite 
 so fine as with No. 9. 
 
 Homogeneous. — We have examined two i-i2th inch; one of an 
 aperture of i - io has a very large frontal distance, but has comparatively 
 a small resolving power. The other has a shorter frontal distance, its 
 numerical aperture is i'25 ; the images are of great purity and the 
 resolving power of the objective is considerable. 
 
DR. ARTHUR CHEVALIER — HENRY CROUCH. 1 5 7 
 
 The following are the prices of these objectives : — 
 
 Dry Objectives. 
 
 £ s. d. 
 
 No. 4 (1/2 inch) angular aperture 80 .. .. .. Price 140 
 
 ,, 5 (1/4 inch) „ ,, 120 0 .. .. .. ,, 180 
 
 ,, 6 (1/5 inch) „ „ 120 0 .. .. .. „ 1 12 o 
 
 „ 7 (1/6 inch) „ „ 140 0 1 12 o 
 
 „ 8 (1/9 inch) „ „ 150° .. .. .. ,, 200 
 
 „ 9 (in inch) „ „ 160 0 .. .. .. „ 300 
 
 Water Immersion Objectives. 
 
 No. 9 (1/12 inch) aperture 120° .. Price 600 
 
 IO (l/l6 ,))',! ,| M 8OO 
 
 „ 13 (1/25 „ ) „ „ „ 14 O O 
 
 .» *5 (i/33 » ) •> » .. 18 o o 
 
 „ 18(1/50 „ ) „ „ ... .. „ 4 02 o 
 
 Homogeneous Immersion Objectives. 
 
 No. 9 (1/12 inch) aperture 120° .. .. .. Price 600 
 
 „ 11 (1/18 „ ) „ „ „ 10 o o 
 
 Dr. ARTHUR CHEVALIER (Palais Royal, Paris).— We only 
 mention the firm of Chevalier as being historically noted in optics. 
 
 Our excellent friend, Dr. Arthur Chevalier, died shortly after the 
 war of 1870-71, when, after a desperate struggle, he succeeded in 
 resuscitating, financially speaking, the firm which his father, a man of 
 research, and an inventor, but not a man of business, had left in 
 difficulty. The most hopeful future offered itself to him, but, alas, 
 neither he nor his have obtained any profit from it. Madame A. 
 Chevalier and her two daughters followed him to the grave at an 
 interval of a few years. 
 
 Only strangers inherited the result of the difficult work of poor 
 Arthur, and the house put up to sale was bought for ^6,000 by M. 
 Avisard, who is still its present proprietor. 
 
 We have not examined the instruments of this firm since M. Avisard 
 took possession. The stands are still such as we described them in 
 1878, but we do not know if the objectives have undergone any 
 improvement. 
 
 Arthur Chevalier was a high-minded man and well read, not only 
 in microscopy, but also in all natural and medical science. If 
 fortune had smiled on him at an opportune moment, and had he 
 lived for a few years longer, he would probably have effected a 
 substantial progress in microscopy. 
 
 HENRY CROUCH (66, Barbican, London) is a well-known maker 
 whom we mentioned favourably in the third edition of this work. His 
 large instruments, which we had occasion to examine carefully fifteen 
 years ago, were well-made and finished. 
 
Fig. 127a. 
 
HENRY CROUCH. 
 
 x 59 
 
 The best instrument of this maker is called the Grand Model 
 (fig. 127a), and is sold either as a binocular or monocular. In the 
 first case the price is ,£42, and in the second ^,34. 
 
 The rapid movement is produced by a smoothly-working rack and 
 pinion, with inclined teeth ; the very sensitive slow movement is of a 
 special patented pattern, with the details of which we are not acquainted. 
 
 Fig. 127^. 
 
i6o 
 
 HENRY CROUCH. 
 
 The stage has rectangular adjustments, controlled by separate milled 
 heads, and fitted with verniers. It is round and its edges are graduated. 
 By means of lateral milled heads each objective can be centred. 
 
 The sub-stage is furnished with screws at right angles for centering, 
 and with a rack and pinion for focussing the condensers. 
 
 The mirror, plane on one side and concave on the other, is three 
 inches in diameter, and mounted with a double joint. 
 
 Fig. 127c 
 
HENRY CROUCH 
 
 161 
 
 Some parts of the instrument are made of brass, and others of 
 polished bronze, thereby producing a very pleasing contrast. 
 
 The instrument called the Premier Microscope (fig. 127^) is of a 
 much simpler character, and rather smaller in size; it only costs ^17 17s. 
 when binocular, and at this price are included objectives of 1 inch and 
 \ inch, a pair of oculars, a stand condenser, and a mahogany case. 
 
 The rapid and slow movements are the same as those of the pre- 
 viously described instrument. 
 
 The sub-stage is furnished with a screw for centering, an adjustment 
 for raising it, and it can also be turned aside by an eccentric hinge. 
 We do not approve of this form of hingeing, because it does not allow 
 a fixed centering to be maintained. 
 
 Mr. Crouch also makes a large number of other microscopes, more 
 or less expensive, and more or less to be recommended, which we are 
 unable to describe here ; we must content ourselves with mentioning 
 the student's model (fig. 1 2 7<r). 
 
 This microscope, which was highly commended by the late Dr. 
 Carpenter, is binocular ; it has a stage, with rotary movement, and is 
 furnished with a glass slide carrier. 
 
 The diaphragm has various sized apertures, and the mirror allows 
 oblique illumination. It is accompanied by an objective of 1 inch and 
 another of \ inch, and the price is only £12 15s. 
 
 " After considerable use of this instrument," says the late Dr. 
 Carpenter, "I can strongly recommend it to such as desire to possess 
 a binocular at once cheap, good and portable." 
 
 Mr. H. Crouch makes two series of objectives : one, called the " Best 
 Series," is unknown to us ; the other, called " Second Series of small 
 angular aperture," contains objectives intended for the Continental tube. 
 We have had occasion to examine some recent specimens, which we found 
 very good for ordinary work, and gave the following results : — 
 
 1 inch and 2-31TI inch. — These objectives have large frontal distances, 
 give very good definition, and are especially suited for elementary research 
 in the histological laboratory. 
 
 i-6th inch. — Very good objective, similar to Zeiss' D. The image 
 of the pygidium is very much as it should be ; with axial illumination 
 Nobert's 6th group is well resolved. With oblique illumination Nobert's 
 9th group and the Pleurosigma Angnlatum are seen very well. 
 
 i-i2th homogeneous (N.A. 1*25). — The Bacillus Tuberculosis and the 
 Podura give very clear images, with axial illumination, which also shews 
 
 M 
 
162 
 
 GEBRUDER FROMME — DR. E. HARTNACK. 
 
 Nobert's 16th group quite as well. Oblique illumination shews Nobert's 
 1 8th group very passably, as well as the large Amphipleura (No. 33) 
 of Moller's test. 
 
 The following are the prices of the different objectives in Mr. Crouch's 
 " Usual Series "■ — 
 
 £ l 0 0 
 
 3 inches 
 2 ,, 
 1 ■> 
 2/3 inch 
 1/2 ,. 
 1/4 ., 
 i/S .. 
 1/6 ,, 
 1/8 ,, 
 1 10 ,, 
 
 1/15 - 
 1 12 ,, homogeneous 
 
 12 
 20° 
 
 25° 
 40 0 
 
 ™S° 
 no° 
 
 IIO° 
 
 120° 
 120° 
 
 120° 
 
 1.25° N.A. 
 
 1 5 o 
 1 5 o 
 
 1 10 o 
 
 220 
 220 
 220 
 
 2 IO O 
 2 15 O 
 4 IO O 
 
 o o 
 
 GEBRUDER FROMME (iii., 21, Hainburgerstrasse, Vienna.)— 
 We mention this firm as being the makers of a small and very ingenious 
 microscope, constructed after the specifications of a skilful amateur 
 diatomophile, M. Anton Amrhein, Jr., of Vienna. 
 
 The microscope is cylindrical, and has only a rack and pinion 
 movement, which is, however, sufficient for the use for which it is 
 intended, viz. : the immediate examination of collections made during 
 excursions. 
 
 The microscope has at its side a tongue, which fits into a groove 
 at the upper end of a stick. 
 
 At the bottom of this stick is a strong steel point, by means of 
 which it can be easily driven into the ground. Thus the microscope 
 is at a convenient height for observation, and one's hands free for work. 
 
 The price of this little apparatus is jQi. 
 
 Dr. E. HARTNACK (of Potsdam, near Berlin).— This celebrated 
 maker who succeeded his uncle Oberhaiiser, and was so long established 
 in Paris, removed into Germany in 1870, and established himself at 
 Potsdam, his native town. 
 
 M. Hartnack is always in touch with the most recent progress in 
 microscopy, and is universally regarded as one of the best makers of the 
 day. 
 
 The large model of this maker (fig. 128) is well known. It has 
 been devised in its entirety by M. Hartnack, and forms the type of 
 what is called the Continental Model. This microscope was used 
 twenty years ago by a large number of serious microscopists. It has at 
 the present time undergone modifications of increasing importance, but 
 
DR. E. HARTNACK. 
 
 163 
 
 nevertheless it is a model which is remarkable for its stability and for 
 its general construction, by which every desirable improvement can 
 be added to it. 
 
 The foot, which is of horse-shoe form, is very heavy ; to it is fixed 
 two vertical pillars, between which the instrument is suspended. 
 
 The mirror, jointed for use with oblique light, can be raised or 
 lowered. 
 
 The stage is rotary and covered 
 with black glass to prevent injury 
 when acids are used. 
 
 The diaphragm holder has a 
 very large opening, in which con- 
 densers, polarising apparatus, &c, 
 can be adapted as desired. 
 
 The tube is moved by rack 
 and pinion, and the slow move- 
 ment is very precise. 
 
 A model still better known is 
 the Small Horse-shoe Microscope, 
 called No. 8. The instrument is 
 composed of a very heavy foot of 
 brass, of horse-shoe form, which 
 supports a cylindrical pillar, to 
 which is attached the stage and the 
 rest of the instrument. The mirror 
 is fixed to a stem to allow it to 
 be moved obliquely. 
 This stage is very thick, solid, and stationary. Underneath it are 
 two grooves, in which slides a piece of apparatus, having in its centre 
 a tube, in which diaphragms, as well as polarising or illuminating 
 apparatus can be inserted. 
 
 A milled head, which forms the lower end of the small tube, 
 allows it to be raised or lowered as desired under the stage. The 
 whole can be removed when oblique light is used. 
 
 The rapid movement is given to the instrument with the hand 
 by sliding the body of the microscope in a tube ; the slow move- 
 ment is made by means of a micrometer screw, of which we have 
 already spoken. 
 
 This instrument, accompanied with objectives Nos. 4, 7 and 9, the 
 
 M2 
 
 Fig. 128. 
 
164 
 
 DR. E. HARTNACK. 
 
 last of which is immersion provided with correction collar, and oculars 
 2, 3 and 4, the first of which is a micrometer, is sold for £16 12s. 
 
 Microscope No. 4 (fig. 129), differs from the preceding in having 
 a larger stage covered with vulcanite, and by the addition of an Abbe 
 condenser of N.A. i - 25, which can be raised axially by a micrometer 
 screw. 
 
 This condenser is furnished with an Iris diaphragm. 
 
 The stand of this instrument costs £6 15s. ; when accompanied by 
 objectives 4, 7 and I. (homogeneous immersion), together with three 
 oculars, besides being jointed for placing in an inclined position, the 
 price is increased to ^18 5s.. 
 
 The New Microscope, No. J 
 (fig. 130) is distinguished from all 
 others of this maker by the simple 
 action of the condenser. 
 
 Under the stage is a circular 
 mounting, carrying the Abbe con- 
 denser of N.A. 1 '40, capable of 
 being centred and furnished with 
 an Iris diaphragm. 
 
 The latter slides in a groove 
 and can, in addition, be turned 
 on its axis. The combination of 
 these two movements allows oblique 
 illumination in all directions. 
 
 The condenser can, by means 
 of a rack and pinion, be moved 
 towards or away from the stage, 
 which is very large. 
 
 The mirror is double, very 
 large, jointed and capable of being 
 placed in all positions. 
 
 The slow movement is very 
 precise and very even, the rapid 
 movement is made by rack and 
 } pinion. 
 
 The whole instrument can be 
 inclined, and can, moveover, be 
 increased in height as required by 
 means of a stem, which is inclosed in the pillar of the microscope. 
 
DR. E. HARTNACK. 
 
 I6 5 
 
 The price of this stand, furnished with all these improvements, 
 ;£i2 ; in a simple form it only costs ^9 16s. 
 
 Fig. 130. 
 
 Dr. E. Hartnack's objectives preserve their old reputation, and are 
 counted amongst the best there are. 
 
 No. 2 is intended for feeble magnification; its power varies from 25 
 to 40 diameters. 
 
 No. 4 is an excellent objective. It is very, clear, and its powers of 
 
1 66 
 
 DR. E. HARTNACK. 
 
 definition and penetration are very remarkable. The magnifications vary 
 from 60 to 180 diameters. 
 
 No. 5 is also very good, and its focus is also sufficiently great to 
 allow moderately thick cover glasses to be used. It gives a series of 
 magnifications, varying from 120 to 360 diameters. 
 
 No. 6 (i-6th inch), is one of the best objectives that we know. 
 While having a frontal distance sufficient for all kinds of work, the 
 objective has an aperture of "86, and the chromatic and spherical 
 corrections leave nothing to be desired. It resolves perfectly Vanhairckia 
 rhomboides. 
 
 Nos. 7, 8 and 9 deserve the same praise, and are nearly as good 
 in all respects as the previous ones. No. 9 has a numerical aperture 
 of "95. The water immersion objectives have an aperture of about 
 1 "15 j all are very good, and show sufficiently well the Amphipleura. 
 
 The homogeneous objectives have a similar numerical aperture, the 
 images are very good. We, however, find that No. 3 (i-24th inch), 
 while showing bacteria well, in the case of diatoms leave something to 
 be desired. 
 
 E. Hartnack makes his homogeneous objectives with correction collars 
 when their maximum effect is desired. He finds, as we do, that the 
 correction is absolutely necessary for the study of diatoms. 
 
 A short time ago E. Hartnack completely changed the formula; of 
 his objectives. We have received two objectives of the new construction, 
 No. 1 (i-i2th inch) homogeneous, and No. X, water immersion. Each has 
 a numerical aperture of 1*27, which has never before been attained 
 by any maker on the Continent for a water immersion objective. 
 These two objectives are admirable, they resolve all known tests; the 
 homogeneous objective gives a rather brighter image than the water 
 objective. 
 
 Since these lines were written, Dr. Hartnack has again modified 
 these objectives. We have just received from the maker (May, 
 1 891) a series of objectives quite recently constructed. 
 
 The following is the result of the trials to which we have submitted 
 them : — 
 
 No. 5 (focus 1 -5th inch) N.A. '90. — Good objective, the image of the 
 pygidium is clear, and Pleurosigma is resolved with axial illumination. 
 
 Nos. 6, 7, 8 and 9. — These objectives, which are respectively about 
 i-6th, i-7th, i-ioth and i-i2th inch, have each a numerical aperture 
 of -95. 
 
 They therefore only differ in their magnifying power. 
 
DR. E. HARTNACK. 
 
 All of them give a good image of the pygidium which is quite clear 
 and pure, and they resolve Pleurosigma perfectly with axial light as well 
 as the ioth group of Nobert. With oblique light the ioth group of 
 Nobert's test can be seen when mounted in the yellow medium. 
 
 The Homogeneous No. i is i-i2th inch focus, and of N.A. i^o. 
 The images are pure and clear, and the frontal distance is very consider- 
 able. 
 
 The Podura gives a good image, slightly coloured. The Amphipleura 
 shows its striae finely resolved. Nobert's test dry shows the 12th group 
 in axial, and the 19th in oblique illumination. 
 
 Apochromatic Objectives. — There are two apochromatic objectives, the 
 first has a focus of i-i2th, and the second i-i8th, each has an 
 aperture of 1*35. 
 
 The images are good and absolutely without colour, the Podura is 
 of a fine black. With axial illumination, the two objectives show 
 very well the 12th group of Nobert and the 19th with oblique 
 illumination. 
 
 The frontal distance of the two objectives is considerable, and both have 
 good definition, though we found the i-i2th considerably superior to 
 the i-i8th. With the first, the resolution of the Amphipleura is finer, 
 and the tendency to resolve into beads is greater. 
 
 The following table summarises the principal particulars relating to 
 the objectives of Dr. Hartnack : — 
 
 Number, 
 
 Equivalent Focus. 
 
 Numerical Aperture. 
 
 Price. 
 
 
 1. 
 
 
 -Achromatic 
 
 Objectives. 
 
 
 
 
 
 
 
 
 £ s. 
 
 d. 
 
 t 
 
 
 
 35' nim - 
 
 
 O 16 
 
 0 
 
 2 
 
 
 
 25" 
 
 
 O 16 
 
 0 
 
 3 
 
 
 
 IS" 
 
 
 i 4 
 
 0 
 
 4 
 
 
 
 IO' 
 
 •5 
 
 1 4 
 
 0 
 
 5 
 
 
 
 S^ 
 
 '9 
 
 1 8 
 
 0 
 
 6 
 
 
 
 375 .> 
 
 ■95 
 
 1 12 
 
 0 
 
 7 
 
 
 
 3'4 
 
 ■95 
 
 ' 1 12 
 
 0 
 
 8 
 
 
 
 23 
 
 ■95 
 
 2 0 
 
 0 
 
 9 
 
 
 
 2' 
 
 95 
 
 3 0 
 
 0 
 
 9 
 
 water immersion 
 
 
 2"2 ,, 
 
 ■95 
 
 6 0 
 
 0 
 
 10 
 
 
 
 i-6 
 
 1 25 
 
 8 0 
 
 0 
 
 11 
 
 
 
 1 "4 
 
 1 25 
 
 10 0 
 
 0 
 
 12 
 
 
 
 1 '3 
 
 1-25 
 
 12 0 
 
 0 
 
 I. 
 
 homogeneous immersion 
 
 
 20 , , 
 
 1-3 
 
 7 10 
 
 0 
 
 LI. 
 
 
 
 1 5 
 
 1 "3 
 
 10 0 
 
 0 
 
 
 2. Apochromat 
 
 1C 
 
 Homogeneous Immersion Objectives. 
 
 
 
 
 
 20 mm. 
 
 1 '35 
 
 12 10 
 
 0 
 
 
 
 
 i'33 .. 
 
 1 '35 
 
 '5 0 
 
 0 
 
i68 
 
 F. KORISTKA. 
 
 F. KORISTKA (47, Rue S. Vittore Grande, Milan.) — F. 
 Koristka is a new maker, who has only been established about ten years. 
 His name is still little known out of his own country, but in Italy his 
 reputation is firmly established. At the present time his instruments are 
 in fact used in all the scientific establishments of his country, and justly 
 so, for if the excellent quality of his objectives is unsurpassed, the brass 
 work of his stands is as carefully turned out. All the different parts of his 
 apparatus are elegantly finished and work admirably, of which we have 
 personally assured ourselves. 
 
 This manufacturer makes a very considerable number of models, the 
 greater part of which resemble Zeiss' stands ; consequently, we will merely 
 mention the large instruments. No. I., la, and II., each of which are 
 furnished with an Abbe condenser, and are moved by rack and pinion. 
 In addition, No. I. has its upper stage rotary, No. la a rotary stage only, 
 while No. II. is similar to No. I., without the upper rotary stage. 
 However, we will describe in greater detail three stands which are 
 deserving of being specially recommended to workers. 
 
 We have, first, the Medium Petrological Stand (fig. 130^). This 
 model, which is very commonly to be found in the natural history 
 laboratories of Italy, can be used not only for petrological research, 
 but also for the most delicate research in botany, animal histology, &c. 
 
 The instrument can be inclined, and is furnished with an Abbe 
 condenser, the Iris diaphragm of which is arranged to receive the 
 polariser. This diaphragm carries besides a small projecting notch 
 which, from 90° to 90 0 , strikes against a spring so as to indicate the 
 principal positions. 
 
 The stage is furnished with a circular scale in divisions, and a 
 vernier, enabling readings of 10' to be made. 
 
 The tube of the microscope carries a stereoscopic ocular with the 
 four quartz of Bertrand. There is also an ocular for the observation 
 of axial images,' Mohl's tablets, and Klein's quartz. 
 
 The lower end of the tube is furnished with an apparatus for centering 
 the objectives. 
 
 This stand as we have described it, together with an analyser, having 
 a circle divided into degrees, costs ,£20 16s. 
 
 The complete apparatus, enclosed in a box, costs ^34, when, in 
 addition to the accessories already enumerated, it is furnished with three 
 objectives, one of them being 1-12U1 inch homogeneous immersion, three 
 oculars, one of which has a micrometer, and a triple nose-piece. 
 
Fig. 1306. 
 
1 70 
 
 F. KORISTKA. 
 
 Fig. 130c. 
 
Fig. 1301/. 
 
172 
 
 F. KORISTKA. 
 
 microscopic research, can be inclined, the rapid movement being by 
 rack and pinion, and furnished with an Abbe condenser, which can be 
 raised and lowered by means of the milled head, which is seen between 
 the two pillars which support the stage. This stand costs ^9, with an 
 Abbe condenser and ordinary diaphragms, and ^9 16s. with an Iris 
 diaphragm. 
 
 Model VI. (fig. 130(f), very elegant and carefully furnished, only 
 costs £5 4s. It can be inclined. The rapid movement is by sliding. 
 It is furnished with a very precise micrometer screw and a simplified 
 Abbe condenser, and is sufficient for all ordinary research. 
 
 As we have previously stated, this maker's objectives may be classed 
 among the best. We have had occasion to examine an almost com- 
 plete set of the date of December, 1891, and the following is the 
 result of our examination : — 
 
 Achromatic Objectives. 
 
 No. o (focus 38mm. = about \\ inch) N.A. '12. 
 
 No. 1 ( ,, 30mm.) N.A. -15. 
 
 No. 2 ( ,, 20mm.) N.A. - 2o ; and 
 
 No. 3 ( „ i5*5mm., or § inch) N.A. '28 are four low powers, 
 formed by the combination of two achromatic lenses. All four give 
 clear and pure images, bear oculars of relatively high magnifying power, 
 and are very useful as searchers or for observations in elementary 
 histology. 
 
 No. 4 (focus 12mm., or \ inch) N.A. '44 is the first ternary 
 combination. The images are very good, and the Pleurosigma balticum, 
 as well as Nobert's 4th group, can be resolved in oblique light. 
 
 No. 5* (focus 8 - 5mm., or \ inch) N.A. 77, resolves Nobert's 7th 
 group in oblique light, and gives good images in axial light. 
 
 No. 6 (focus 6mm., or \ inch) N.A. "50 shows feebly the Pleurosigma 
 angulation in axial, and clearly in oblique, illumination. 
 
 No. 6* (focus \ inch) N.A. - 8o gives a good image of the Pleurosigma 
 angulation in axial illumination ; in oblique illumination Nobert's 10th 
 group is seen very well. 
 
 No. 7* (focus 4mm. or i-6th inch) N.A. -85, and No. 8* (focus 
 2 "8mm. or i-8th inch) N.A. - 88, are two excellent objectives, which 
 only differ in their magnifying power. Both with axial illumination resolve 
 
F. KORISTKA. 
 
 173 
 
 perfectly Nobert's 8th group, and give a good image of the Podura 
 and pygidium. With oblique illumination Nobert's nth group and the 
 large form of Vanheurckia rhomboides in Moller's test are resolved with 
 clearness. 
 
 i-i2th homogeneous immersion N.A. 1*30 is a very good objective. 
 With axial illumination the Bacillus tuberculosis is very good, and Nobert's 
 12th group is well resolved. In oblique illumination the Amphipleura 
 of Moller's test, as well as Nobert's 19th group, are seen perfectly. 
 
 i-i5th semi-apochromatic, homogeneous immersion differs only from 
 the previous one in its magnifying power being somewhat larger and 
 the purity of the images observed being considerably greater. 
 
 Apochromatic Objectives. 
 
 1 6mm. (2-3rd inch N.A. ^o). — The images are clear and pure. 
 Nobert's 3rd group can be resolved with axial illumination. 
 
 8mm. (i-3rd inch N.A. ^o). — The pygidium of a flea and the Podura 
 are seen with admirable clearness with ocular No. 18 ; with axial illumination 
 Nobert's 7th group can be resolved. With feeble oblique illumination the 
 image of the Pleurosigma is very good. 
 
 3mm. (i-8th inch N.A. -95). — The images of the pygidium of the 
 flea and of the Podura are admirable in axial illumination, which also 
 enables the resolution of the Pleurosigma angulatum and Nobert's 8th 
 group. In oblique illumination Nobert's 12th group and Moller's Van- 
 heurckia rhomboides are resolved. 
 
 3mm. homogeneous (i-8th inch N.A. 1-30) and 2mm. (i-i2th inch 
 N.A. 1-30) are two excellent objectives, which do not essentially differ 
 except in their magnifying power. The images are pure and colourless, 
 the Surirella and the Podura show that their aberrations are well 
 corrected. With oblique light all the tests are perfectly resolved, but we 
 find that the images given by the 3mm. are finer than those by the 
 2mm. 
 
 2mm. (r-i2th inch N.A. 1.40). The images are clear and brilliant; 
 the definition of Bacteria are very fine, and all the known tests are 
 shown in an irreproachable manner. We consider this objective as 
 beyond comparison. 
 
 The following is a summary of the principal points concerning the 
 Koristka's objective : — 
 
174 
 
 E. LEITZ. 
 
 No. of Objective. 
 
 Numerical Aperture. 
 
 Equivalent Focus. 
 
 Price, with Fixed Mounting. 
 
 
 Achromatic 
 
 yj dj cciivcs. 
 
 
 
 
 £ s- d. 
 
 a 
 
 
 
 40 mm. 
 
 O IO O 
 
 b 
 
 
 
 32 „ 
 
 O IO O 
 
 c 
 
 
 
 25 „ 
 
 0 10 0 
 
 I* 
 
 
 
 28-50 „ 
 
 116 0 
 
 
 '12 
 
 58 
 
 IOO 
 
 I 
 
 ' I c 
 
 30 
 
 10 0 
 
 2 
 
 "20 
 
 20 „ 
 
 140 
 
 
 •28 
 
 I S 5 i> 
 
 140 
 
 A 
 
 
 12 
 
 180 
 
 5* 
 
 70 
 
 8'S 
 
 I 12 O 
 
 6 
 
 'So 
 
 6 „ 
 
 18 0 
 
 6' 
 
 ■80 
 
 6 „ 
 
 I 12 O 
 
 7 
 
 70 
 
 4 >, 
 
 112 0 
 
 7* 
 
 ■85 
 
 4 11 
 
 I l6 O 
 
 8 
 
 75 
 
 28 „ 
 
 I 16 O 
 
 8* 
 
 •88 
 
 28 „ 
 
 24O 
 
 9* 
 
 •88 
 
 21 1, 
 
 280 
 
 i-ioth Water 
 
 i"io to 117 
 
 21 „ 
 
 3OO 
 
 i-i5th ,, 
 
 
 »7 11 
 
 S 4 0* 
 
 i-i2th Homogeneous 
 
 125 to 1 30 
 
 2"r „ 
 
 5 4° 
 
 i-i6th „ 
 
 
 16 „ 
 
 7 '3 0 
 
 i-iSth „ \ 
 semi-apochromatic J 
 
 I 30 
 
 21 „ 
 
 800 
 
 
 Apochromatic Objectives. 
 
 
 Dry 
 
 ■30 
 •60 
 
 16 „ 
 8 ,, 
 
 400 
 7 12 0 
 
 Homogeneous 
 
 '95 
 
 4 >. 
 
 8 0 0" 
 
 1 30 
 
 2 ,, 
 
 16 0 0 
 
 
 1 '40 
 
 2 ,, 
 
 22 0 0 
 
 Dry 
 
 '95 
 
 3 11 
 
 8 0 0* 
 
 Homogeneous 
 
 1-30 
 
 3 .1 
 
 16 0 0 
 
 
 
 'with collar adjustment. 
 
 The semi-apochromatic i-i6th is accompanied with two compensating 
 oculars, which are included in the price of £8. The price of Huygen's 
 oculars is 7s. 6d. Compensating oculars vary from 16s. to jQi 4s. 
 
 E. LEITZ (of Wetzlar), who has taken the workshops of Kellner, 
 manufactures microscopes of good quality and at a reasonable price. 
 
 One of these instruments (fig. 131), called in the catalogue the 
 Large Inclining Microscope (Stativ I.) has two circular revolving 
 stages, which can be easily substituted for one another. 
 
 One is without accessories, while the other, of new construction, 
 is furnished with a carrier, the movement of which in one direction is 
 made by a horizontal screw, and in the other by means of a vertical 
 friction pinion. Its movement can be registered by two scales. The pre- 
 paration when fixed on the stage by two spring clips is only just 
 above the lens of the Abbe condenser, with which the instrument is 
 furnished, and thus enables the total illuminating effect to be preserved. 
 The condenser is provided with an Iris diaphragm, and can be racked 
 
E. LEITZ. 
 
 i7S 
 
 Fig. 131. 
 
176 E. LEITZ. 
 
 up and down. The foot of the instrument is heavy and of horse-shoe 
 form ; the draw tube is graduated, and is provided with the English 
 screw thread. The micrometer screw for slow movement is very precise, 
 and the rapid movement is effected by rack and pinion. The price of 
 this stand without accessories is £18. The complete microscope — that is 
 to say, furnished with a nose-piece for three objectives, Abbe's camera, 
 lucida polarising apparatus, stage micrometer, eye-piece micrometer, nine 
 ordinary objectives, two homogeneous immersion objectives, i-i2th and 
 i-i6th, and four oculars, giving a magnification of 27 to 1,690 diameters — 
 costs £50. 
 
 The slow movement of this microscope, as well as Leitz's other large 
 instruments, depends on a principle called " k pointe renversee," which 
 
 is that at present used by first-class Con- 
 tinental makers. We have already illustrated 
 at page 77 the principle adopted by the 
 firm of Zeiss. We give here that employed 
 by Leitz (fig. 131^). 
 
 As can be seen, the spiral spring presses 
 at its lower extremity, against the end of 
 the prism inside, and at the upper extremity 
 against a piece of steel fixed in the outer 
 slow adjustment tube, which also carries the 
 body of the microscope. The screw of the 
 slow movement is hollowed out and encloses 
 a steel rod, which presses against the piece 
 of steel just mentioned, and, by means of 
 this rod, the movement is communicated to 
 the spring. Fine adjustment, "a pointe 
 renversee" works very easily in both directions. 
 Another large microscope, Stativ ia. 
 (smaller than the preceding) (fig. 132), has a rotary stage with centering 
 arrangement. 
 
 The system of lenses composing the Abbe illuminating apparatus can 
 be easily taken off to make way for the diaphragm tube, taking care 
 first to remove the Iris diaphragm. 
 
 By adopting this arrangement, either the condenser or the diaphragm 
 tube can be used, while the rack and pinion movement, in both cases, 
 regulates the light. 
 
 The instrument, moreover, possesses a graduated draw tube and all the 
 movements described above for Stativ 1, such as the inclining, rack and 
 
 Fig. 131a. 
 
E. LEITZ. 
 
 177 
 
E. LEITZ. 
 
 179 
 
 micrometer, three ordinary objectives, numbered 2, 4, and 7, a i-i2th 
 immersion objective, and four oculars (giving a magnification of 40 to 
 1,310 diameters). Fitted as above this microscope costs ^20. 
 
 Stativ II. (No. 9) (fig. 133), which is also called the Medium 
 Microscope, is entirely of brass, with horse-shoe foot. It inclines, but 
 
 Fig. 134' 
 
 N2 
 
 I 
 
i8o 
 
 E. I.EITZ. 
 
 cannot move about the optical axis. The stage is covered with vulcanite, 
 as a protection against acids. The rapid movement is effected by rack 
 and pinion, and the slow movement by a very precise micrometer screw. 
 The Abbe illuminating apparatus is made simply with Iris diaphragm, 
 and can be moved up and down by means of a special screw. The 
 tube diaphragms are adjusted on a piece of apparatus, which slides 
 between grooves. The mirror, plane on one side and concave on the 
 
 other, is jointed so as 
 to allow of oblique 
 illumination. This stand 
 is sold for £15 5s., 
 and is then accompanied 
 by 3, 5 and 7 ordinary 
 objectives, by a i-i2th 
 homogeneous immersion 
 objective and oculars i., 
 iii., iv., which give a 
 magnification of 70 to 
 1,100 diameters. 
 
 Another medium 
 microscope (No. 14, 
 Stativ 111.) (fig. 135), 
 can be inclined, and has 
 a rotary stage having its 
 rapid movement by 
 sliding, and its slow 
 movement produced by 
 a micrometer screw. 
 
 The illumination is 
 made (as in Stativ II.) 
 with simple Abbe con- 
 denserand Iris diaphragm 
 which can be moved up 
 and down by means of a 
 special screw. The 
 mirror is double (plane- 
 Up and concave), and allows 
 of oblique illumination. 
 
 The price of this 
 stand is only £4. 
 
£. LEIT2. 
 
 Furnished with a nose-piece for two objectives, with Nos. 3 and 7 
 ordinary objectives, with a homogeneous immersion objective, and oculars 
 I. and III., the price is -£1$ 5s. 
 
 Leitz also constructs cheaper microscopes, and amongst others a 
 Medium Upright Microscope (No. 15) (fig. 134), that is to say, incapable 
 of being inclined, and furnished with a rapid movement produced by 
 sliding the tube, and slow movement for focussing by a micrometer 
 screw. The illumination is effected by a mirror, and the diaphragms 
 are placed in a tube. With Nos. 3, 5, 7 and 9 ordinary objectives, 
 and oculars I. and III. it gives a magnification of from 84 to 880 
 diameters, and costs ^9 15s. 
 
 The objectives of this maker are of excellent quality, and more- 
 over are very cheap. 
 
 The following are the results which the specimens of the series in 
 our possession give, and which date from August, 1891 :— 
 
 No. 2 (focus 30 mm. N.A. '14) is an excellent objective for low 
 magnifications and for photographs of entire objects. This objective is 
 used by Mbller for taking a photograph of his famous preparation of 
 4,000 forms as a whole. 
 
 No. 3 (2-3rd in. N.A. '28) is a good objective for ordinary research 
 in Vegetable Anatomy ; the images are pure and clear. 
 
 No. 4 (A- in. circa. N.A. -45) is a good objective for studies in 
 Vegetable Histology ; the Pleurosigma balticum is passably resolved 
 with oblique illumination. 
 
 No. 6 (] in. N.A. '82). P/eurosigma angulation is clearly resolved 
 with axial illumination and condenser open. 
 
 No. 7 (\ in. N.A. '85). This object is one of the maker's best. 
 Bacillus tuberculosis is shewn very clearly ; the Pleurosigma is resolved 
 admirably with axial illumination, which also shews Nobert's 7th 
 group. With oblique illumination the 12th group can be resolved. 
 
 No. 8 (i-ioth in. N.A. -87). This objective differs from the 
 preceeding only in its magnifying power; moreover, we prefer the first, 
 the images of which appear to us to be better. 
 
 i-i2th in. homogeneous (N.A. i'3o). The jury of the Antwerp 
 Exhibition has spoken of this objective in very laudatory terms as one 
 "of great clearness and of exceptional resolving power." Our own 
 opinion entirely coincides with that of the jury. The objective shewn 
 at the exhibition (we have since examined two other specimens, which 
 only differ very slightly from it) is indeed extremely remarkable. The 
 image of the Podura and of Surirella gemma are very slightly coloured 
 
l82 
 
 NACHET. 
 
 and the Bacillus tuberculosis is seen with excessive clearness. With 
 axial illumination Nobert's 12th group is resolved with this objective. 
 With oblique illumination Nobert's 19th group is seen very clearly, 
 and the Amphipleura is shewn distinctly beaded. The price of this 
 remarkable objective is not at all exorbitant, costing only ^5. 
 
 The objectives manufactured by M. E. Leitz are classed as follows : — 
 
 Number of Objectives. 
 
 Equivalent Focus. 
 
 Numerical Aperture. 
 
 Price. 
 
 
 
 Achromatic Objectives. 
 
 
 
 
 
 
 
 
 £ 
 
 s. 
 
 d. 
 
 I . . . . 
 
 
 44" mm. 
 
 .09 
 
 0 
 
 15 
 
 3 
 
 2 
 
 
 3°' » 
 
 •14 
 
 0 
 
 15 
 
 3 
 
 3 
 
 
 18 ' „ 
 
 •28 
 
 0 
 
 15 
 
 3 
 
 4 
 
 
 io- 
 
 '45 
 
 1 
 
 4 
 
 10 
 
 s 
 
 
 5* „ 
 
 77 
 
 1 
 
 4 
 
 10 
 
 6 
 
 
 4'4 
 
 •82 
 
 1 
 
 10 
 
 0 
 
 7 
 
 
 3 '2 
 
 •85 
 
 1 
 
 10 
 
 0 
 
 8 
 
 
 2'S 
 
 ■87 • 
 
 2 
 
 0 
 
 0 
 
 9 ...... 
 
 
 2 '2 ,, 
 
 ■87 
 
 3 
 
 0 
 
 0 
 
 10 Water immersion 
 
 
 2'I „ 
 
 1 '10 
 
 3 
 
 5 
 
 0 
 
 n ,, ,, 
 
 
 17 » 
 
 
 4 
 
 10 
 
 0 
 
 1/12 Homogeneous immersion 
 
 
 2 2 „ 
 
 1 30 
 
 5 
 
 0 
 
 0 
 
 1/16 „ „ 
 
 
 I'7 
 
 1 30 
 
 7 
 
 10 
 
 4 
 
 
 Apochromatic 
 
 Objectives. 
 
 
 
 
 
 
 16' mm. 
 
 '3° 
 
 4 
 
 0 
 
 0 
 
 
 
 8- 
 
 ■65 
 
 5 
 
 0 
 
 0 
 
 
 
 4' 
 
 '95 
 
 7 
 
 10 
 
 4 
 
 Homogeneous immersion . . 
 
 
 2' ,, 
 
 1 35 
 
 15 
 
 0 
 
 0 
 
 NACHET (17, Rue St. Severin, Paris). M. Nachet is, at the 
 present time, the oldest maker in France, and, thanks to continuous 
 progress in his factory, he maintains his position in the front rank. 
 Indeed, with this manufacturer, it is not the maker who predominates 
 but the microscopist, the man of research, the artiste. While holding 
 the first place among Continental makers, he has appreciated the merit 
 of English instruments ; in addition to his simple and low-priced, but, 
 nevertheless, good microscopes, he furnishes instruments, which for 
 perfection, elegance and finish of the brass-work, rivals the instruments 
 of our neighbours over the sea. Like Charles Chevalier, M. Nachet 
 is a savant who is thoroughly conversant with all the resources of optics 
 and mechanics. His inventions are numerous and have been much 
 appreciated by microscopists. His microscopes are of various forms ; he 
 has unique models for special use — chemistry, petrology, college 
 demonstrations, &c. 
 
 We will proceed to consider his principal models. 
 
NACHET. 
 
 183 
 
 His best is what M. Nachet calls his Large Perfected Model 
 Microscope. 
 
 This microscope (fig. 136) may be either monocular or binocular. 
 
 Fig- 136. 
 
NACHET. 
 
 It is suspended on an axis by which means it may be inclined, and 
 then fixed in any position between the horizontal and the vertical ; 
 it is constructed under the best conditions for solidity and precision 
 with reference to movements and centering. The focussing adjustment 
 is made by means of a rapid movement by a rack and pinion, and by 
 means of a slow movement by a micrometer screw, acting on the pillar 
 carrying the body. This micrometrical movement, in consequence of 
 a new arrangement (the reverse of the spiral spring), is of very 
 remarkable precision and smoothness, while at the same time entirely 
 rigid, in consequence of the extent of the friction surface in the 
 prismatic pillar, which has enabled him to do away with the second 
 slow movement contained in his former large model. The milled head 
 is graduated by divisions for measuring thicknesses, which severally 
 correspond to i-5ooth of a millimetre. The body tube can be drawn 
 out, and is divided into millimetres. 
 
 The stage is mounted so as to rotate, and is furnished with a 
 moveable table, with rectangular movements, produced by very delicate 
 adjusting screws, to move the position of the object in any direction. 
 A rectangular rule fixed in the table checks the preparation, and 
 by means of two scales at right angles to one another, the co-ordinates 
 can be determined ; by means of their numerical values inscribed on the 
 plate, the point which it is desired to see later on, can be found again. 
 
 When desired, a very useful arrangement can be adapted to the stage 
 for the study of rare or valuable preparations. It is a piece of apparatus 
 formed of two small mirrors ; one concave, placed on a level with the 
 stage on the left, and moveable in every direction, so as to send a ray 
 of light across the other placed opposite on the right, and inclined at 
 an angle of 45 0 , to reflect the luminous pencil vertically. The image 
 of the end of the objective being brightly illuminated is projected on 
 to the small mirror at the right, and at a single glance one may be 
 assured whether the objective is in contact with the preparation or not. 
 The bed of the immersion liquid allows the ray of light grazing the 
 objective to traverse, even when the lens is all but in contact with 
 the glass ; this method which has been applied for twelve years to some 
 of the large models, is of great service, and will become more and more 
 genera] as an accessory when using very high powers. 
 
 The illumination is produced with a double mirror, plane and concave, 
 mounted on joints, and capable of being turned in all directions, to give 
 the effects of oblique light and variations of distance. Between the 
 stage and the mirror is a groove system, having an easily fitting slide, 
 
NACHET. 
 
 which is moved by a lever and carries the sub-stage for changing 
 the condenser and for producing different illuminating effects ; moreover, 
 the axle, about which the sub-stage rotates with an eccentric movement, 
 carries a small slow adjustment, by which objects can be illuminated at 
 convenient distances from the object with the greatest nicety. 
 
 Fig. 137. 
 
 With these microscopes, M. Nachet provides a complete series of 
 his objectives, a triple nose-piece, four oculars, a goniometer, a condenser 
 of large angular aperture, which we have already mentioned, a complete 
 polarising apparatus, a dark ground illuminator, a stage micrometer, 
 
i86 
 
 NACHET. 
 
 an eye-piece micrometer, a condensing lens on stand, a collection of 
 dissection instruments, &c. The whole, enclosed in a mahogany box, 
 costs £80. 
 
 The Grand Microscope Stand, No. 2 (fig. 137), is very solidly 
 mounted on two pillars, and suspended on an axis ; the rapid adjust 
 ment is effected by a very precise rack and pinion, and the slow 
 movement by an adjusting screw ; a plane and concave mirror mounted 
 on joints is capable of producing all the effects of oblique light. It 
 has also a rotating stage and stage carrier, with a plate of black glass 
 superposed and furnished with an adjusting screw for shifting the 
 object. 
 
 The illuminating apparatus resembles the former eccentric arrange- 
 ment ; that is to say, the condensers, tube diaphragms, polarising 
 apparatus, &c, can be arranged away from the stage and then brought 
 under the object ; only the movement for bringing it nearer, and the 
 focus adjustment is made in the pillar instead of being made in 
 the central tube. This pillar is composed of three cylindrical tubes, 
 the external tube sustaining the arms, which carry the illuminating 
 apparatus ; this slides easily, and can be turned round the second, which 
 terminates in a small adjusting screw, by which the condenser can be 
 focussed, and the object withdrawn very slowly indeed. The rectilinear 
 movement working of the microscope along the axis of the microscope 
 is ensured by a system of springs, connecting the supporting arm with 
 a vertical knob (not shewn in the drawing), which serves to guide it. 
 Under the tube of the condenser is a rotating cell carrier, with an 
 Iris diaphragm, which can be placed in all positions. 
 
 This microscope, which costs ^36, includes six objectives, Nos. 2, 
 3, 5, 6, 7 ordinary, and No. 9 homogeneous immersion, with correction 
 collar. 
 
 There is, in addition, a triple nose-piece, three oculars, wide-angled 
 condenser, a stage micrometer, an eye-piece micrometer, a camera lucida, 
 a bull's eye on stand, &c. 
 
 A much more simple microscope than the preceding, and, moreover, 
 sufficient for every kind of research, is the New Inclining Model 
 Microscope (fig. 138). 
 
 In this instrument the stage is fixed and furnished with a black 
 glass. The quick movement is effected by rack and pinion, which is 
 very precise, as is also the slow movement. 
 
 The condenser is of wide angle, and can be moved up and down 
 
NACHET. 
 
 l8 7 
 
 by a fine adjustment. The instrument is accompanied by three dry 
 objectives, Nos. 3, 5, and 7, and one homogeneous immersion, No 
 9. There are, in addition, three oculars and a bull's eye, mounted 
 on a stand for illuminating opaque bodies. The price of the whole, 
 which includes nearly every requisite for ordinary researches, is only 
 
 &9 4S- 
 
 Fig. 138. 
 
 Lastly, a microscope within reach of any purse, and one which, 
 moreover, is suitable for serious work, is the Small Modified Erect Stand, 
 No. 13 (fig. 139). 
 
i88 
 
 NACHET. 
 
 The foot is made of iron, its 
 slow movement is very precise, and 
 the mirror is jointed for oblique 
 illumination. The diaphragm con- 
 sists of a rotating plate. 
 
 This stand, together with an 
 ocular and objective, No. 7, is sold 
 for ^3 12s. However, we do not 
 advise its purchase when fitted as 
 above. At least a No. 4 objective 
 inch), costing jQi, should be 
 added, and then it will be suit- 
 able for most of the ordinary 
 kinds of research. 
 
 The objectives of this maker 
 are very numerous. We shall 
 examine the following, which are 
 those mostly used, and of which 
 we have examples of recent 
 date : — 
 
 No. 2 (2 inches). Excellent 
 objective for work with a com- 
 pound microscope ; suitable for 
 dissections, sorting diatoms, &c. 
 
 No. 5 (\ inch). The numerical 
 aperture of this objective is only 
 •6, corresponding to a resolving power of 23 lines in every hundredth of a 
 millimetre (•00394 inch). The resolution of difficult tests must not, there- 
 fore, be expected of this objective, for the formula of this combination 
 has not been made up with this object ; its limit of good resolution ceases 
 at Pleurosigma Balticum, but for histological work, as well as for general 
 examination of diatoms mounted with a cover glass, the objective is of 
 great value. It has a very large frontal distance, as well as a considerable 
 magnifying power, and for many years we have constantly employed it 
 for the purposes just mentioned above. 
 
 No. 7 (i-9th inch). A good objective, with a numerical aperture of 
 0-87. The frontal distance is considerable, and the images given by it 
 are clear. The limit of resolution is found with the large forms of 
 Vanheurckia rhomboides Breb, which it resolves fairlv well. 
 
NACHET. 
 
 No. 10 water immersion (r-iSth inch, N.A. i - 2o). The image of the 
 pygidium is clear and pure. The Podura is clear, and of a good black 
 colour, which signifies quasi apochromatic achromatisation. The P/eurosigma 
 
 resolved well in axial illumination, and the edges of fractures clearly 
 shew the "stamp paper" perforations. In the same illumination Nobert's 
 ioth group is resolved and the bacilli of tuberculosis are clearly defined. 
 
 In oblique illumination Nobert's 16th group can be distinguished. 
 
 No. 10 oil immersion. This objective is in reality an apochromatic, 
 although M. Nachet does not call it so. In axial illumination the 
 pygidium and Podura are clearly defined, and the latter is of a good 
 pure black. Nobert's group is seen very well. The Pleurosigma is clearly 
 defined, as is also the bacillus of tuberculosis, which displays very clear 
 images. 
 
 In oblique illumination the Amphipleura is well resolved, but 
 the valve appears slightly deformed. By perfectly adjusting the 
 correction collar Nobert's 19th group becomes visible in the yellow 
 medium. The frontal distance of this objective, as well of previous one, 
 is considerable, and work is easy. 
 
 The following table gives the details relating to M. Nachet's 
 objectives : — 
 
 Number of 
 Objective. 
 
 Equivalent Focus in 
 English inches. 
 
 Angle of Aperture 
 and 
 
 Numerical Aperture. 
 
 Pr 
 
 Fixed Mounting. 
 
 ce. 
 
 With Correction Collar. 
 
 
 
 
 £ 
 
 s. 
 
 d. 
 
 £ 
 
 s. 
 
 d. 
 
 I 
 
 
 
 1 
 
 4 
 
 0 
 
 
 
 
 
 
 
 0 
 
 8 
 
 0 
 
 
 
 
 2 
 
 2 
 
 
 0 
 
 16 
 
 0 
 
 
 
 
 2a 
 
 
 
 0 
 
 9 
 
 6 
 
 
 
 
 3 
 
 I 
 
 20° 
 
 0 
 
 16 
 
 0 
 
 
 
 
 4 
 
 i /a 
 
 4°° 
 
 1 
 
 0 
 
 0 
 
 
 
 
 5 
 
 1/4 
 
 8o° 
 
 1 
 
 4 
 
 0 
 
 
 
 
 6 
 
 1/7 
 
 120° 
 
 1 
 
 8 
 
 0 
 
 2 
 
 16 
 
 0 
 
 7 
 
 i/9 
 
 120° 
 
 1 
 
 12 
 
 0 
 
 3 
 
 4 
 
 0 
 
 8 
 
 1/11 
 
 1 40° 
 
 
 16 
 
 0 
 
 5 
 
 4 
 
 0 
 
 9 
 
 
 160° 
 
 4 
 
 0 
 
 0 
 
 6 
 
 0 
 
 0 
 
 
 
 Water Immersion. 
 
 
 
 
 
 
 8 
 
 1/10 
 
 
 3 
 
 4 
 
 0 
 
 5 
 
 4 
 
 0 
 
 9 
 
 1/14 
 
 ri6 
 
 4 
 
 0 
 
 0 
 
 6 
 
 0 
 
 0 
 
 10 
 
 1/18 
 
 1 24 
 
 
 
 
 8 
 
 0 
 
 0 
 
 
 Homogeneous Immersion. 
 
 
 
 
 
 9 
 
 1/14 
 
 I'2Q 
 
 6 
 
 0 
 
 0 
 
 8 
 
 0 
 
 0 
 
 10 
 
 1/20 
 
 I 25 
 
 8 
 
 0 
 
 0 
 
 10 
 
 0 
 
 0 
 
 11 
 
 1/2S 
 
 I - 2S 
 
 
 
 
 14 
 
 0 
 
 0 
 
 12 
 
 1/40 
 
 1-30 
 
 
 
 
 20 
 
 0 
 
 0 
 
190 POWELL AND LEALAND. 
 
 For some months past M. Nachet has also commenced the manufacture 
 of apochromatic objectives and compensating oculars. M. Nachet has 
 adopted the classification at present employed by makers of these 
 objectives. The following are those which he makes : — 
 
 £ s. d. 
 
 16 mm. (2-31'd inch) N.A. C30 .. .. .. .. .. Price 400 
 
 8 mm. (1-31'd inch) N.A. o'§5 „ 500 
 
 4 mm. (i-6th inch) N.A. C95 (with correction) .. .. ,, 740 
 2 mm. (i-i2th inch) homogeneous N.A. 1-30 .. .. „ 16 o o 
 
 The compensating oculars cost : — 
 
 I 
 
 4 
 
 6 
 
 8 
 
 12 
 
 18 
 
 £1 
 
 16/- 
 
 16 - 
 
 £1 4 s. 
 
 £1 4S. 
 
 £1 
 
 Projection ocular costs £1 16s. 
 
 Of these objectives we have had occasion to study the 2 mm. 
 (i-i2th inch), which was submitted for examination to the Jury of the 
 Antwerp Exhibition. The examination shewed that this objective is 
 excellent. 
 
 The images are perfectly pure and clear, without any trace of 
 secondary spectrum. With axial illumination the Pleurosigma and Bacillus 
 Tuberculosis give irreproachable images ; with oblique illumination the 
 Amphipleura is resolved with perfect clearness. 
 
 POWELL & LEALAND (170, Euston Road, London). Messrs. 
 Powell and Lealand occupy quite a unique position in the microscopic 
 world. Their workshops are small, the number of instruments which they 
 produce are few, but every piece of apparatus, marked with their name, 
 is an artistic production, perfect in all its details. Moreover, both 
 instruments and objectives of these makers are in the greatest request, 
 and are used in England by all serious microscopists. 
 
 This firm only constructs three principal models. The large micro- 
 scope, No. 1 (fig. 140), is an instrument supported on a tripod stand, 
 the legs of which, being wide spread, ensures perfect steadiness in every 
 portion of the instrument. Cork plates inserted underneath the feet add 
 to the stability of the instrument, while at the same time they protect 
 the table from being rubbed by the brass. 
 
 When the instrument is inclined in a horizontal position the optic axis 
 is 25 centimetres (10 inches) from the surface of the table. 
 
 The body of the microscope is of such a length that the optical 
 distance between the posterior principal focus of the objective and the 
 
Fig. 140. 
 
192 
 
 POWELL AND LEA LAND. 
 
 anterior focus of the ocular is about 10 inches ; as we have already said, 
 it is very important that the optical length of the tube should not vary ; 
 its variation, when the image is not altered, does not exceed J inch 
 with objectives of \ inch and upwards. 
 
 The 2 inch ocular has the largest field glass that can be used with a 
 binocular, viz. : \h inch. 
 
 The arm, on which the body is fixed, is 5f inches long, which 
 not only gives a clearance of 3^ inches from the optic axis, but also 
 permits the use of a long fine adjustment lever. The moveable nose- 
 piece is 3 inches long; it is held at both ends, so that any rocking 
 movement is simply impossible. The fine adjustment screw is placed 
 immediately behind the pivot which holds the bar ; this prevents it from 
 communicating vibration to the bar. The coarse movement is effected 
 by rack and pinion. 
 
 The stage is fixed to a strong brass bracket ; a carrier is fitted to it, 
 possessing rectangular movements, made by rack and pinion, and by 
 a screw, giving movements of 1 inch in each direction. The milled 
 heads which produce these movements do not follow the movements of 
 the stage, but always remain at least \ inch from the moveable parts 
 when they arrive at the end of the course. 
 
 The stage can, by means of a rack and pinion, be rotated through a 
 whole circle. The optic axis passes through the centre of rotation. 
 
 A .graduated arc of silver enables angles to be measured. The move- 
 able plates of the stage are furnished with verniers registering movements 
 of 1 -1 ooth of an inch, and can be used as finders. The divisions 
 correspond to the divisions of a Maltwood Finder. 
 
 The opening of the stage is very large, so that the finger may be 
 inserted for the purpose of feeling the working distance between the 
 objective and the preparation, with the view of preventing the breakage 
 of the cover glass. 
 
 The sub-stage has two mechanical movements, viz., rectangular and 
 rotary. The sub-stage is centred by milled heads. 
 
 The mirrors — plane and concave — are large, and fitted on a double- 
 jointed arm. All the moving parts are sprung, and can be tightened up 
 if they show the slightest play. 
 
 The instrument can be instantly changed into a binocular microscope 
 by unscrewing the monocular tube at the arm and replacing it by the 
 binocular tubes, and inserting the prism. 
 
 The price of this instrument as a monocular, with two oculars, is. 
 
POWELL AND LEALAND. 
 
 *93 
 
 £42. Its price is ^45 when the sub-stage is furnished with fine 
 adjustment for focussing. 
 
 The large compound microscope, stand No. 2, although smaller 
 than No. r, is almost similar in construction. It differs, however, in 
 four points: 1st, the moveable plates of the stage have only a maximum 
 motion of f inch instead of 1 inch. 2nd, the stage is not rotating, 
 but the upper part of the carrier, which keeps the preparation in 
 position, can be turned about the optic axis. 3rd, the carrier is without 
 verniers; and 4th, when the instrument is inclined in a horizontal position, 
 the distance between the optic axis and the surface of the table is 
 less than 25 centimetres (10 inches). This monocular instrument with 
 two oculars costs ^28. 
 
 Further, for the sum of £,20, the compound microscope No. 3 
 can be obtained. It is of smaller dimensions than No. 2, and the sub- 
 stage is without rotary and rectangular mechanical movements, which are 
 replaced by mechanical eccentric centering movements. 
 
 The objectives of these makers, as we have previously said, are of 
 the highest finish. We have at different times examined a certain number 
 of them, but we give detailed notes only of the two following, which 
 we have had in constant use. 
 
 i-Sth inch new formula, which is in reality a i-ioth inch, is an 
 objective of great beauty. It has two frontals, one to be used dry and 
 the other as an immersion. The first has rather a short working distance, 
 which becomes troublesome when examining many preparations, but that 
 of the second is much longer, and allows cover glasses to be compara- 
 tively thick. The optical power of the two frontals scarcely differs; but 
 we think we may say that we prefer the objective as an immersion. 
 
 The total angle of aperture is 140 0 . Measured in air by Govi's 
 method, we find the useful angle to be 127°. 
 
 The images given by this objective are of extreme purity and clearness, 
 leaving absolutely nothing to be desired. 
 
 With central illumination, and with a very small diaphragm, we have 
 been able to obtain perfect resolution of the 9th group, and feebly 
 of the 10th group of Nobert's 19 group test. 
 
 With the same illumination the Plairosigma is also very clearly 
 resolved into beads, and the objective shows the pygidium of a flea 
 perfectly. The definition, moreover, is as good as it can be, and the 
 depth of focus is considerable. All the requisite conditions for histological 
 study are, therefore, perfectly fulfilled. 
 
 The objective is equally efficient with oblique illumination. All the 
 
 o 
 
i 9 4 
 
 i OWELL AND LEALAND. 
 
 usual tests, including the difficult Amphipleura pellucida, are resolved by 
 simple lamp illumination. 
 
 The use of monochromatic sunlight, obtained by means of a bath of 
 ammoniacal sulphate of copper, has enabled us to resolve the last 
 number of Nobert's 19th group test. 
 
 i-8th inch aprochromatic of last year may be considered as one of the 
 finest objectives made up to the present time. The N.A. is 1*4, 
 the luminosity is very considerable ; the images are very bright, sharp, 
 and colourless. With axial illumination Nobert's 12th group is visible, and 
 with oblique light the 19th group is perfectly resolved. Amphipleura 
 is resolved into beads, with oblique monochromatic sunlight. 
 
 i-ioth inch apochromatic, N.A. 1*50, is a very remarkable objective, 
 and considerably superior to the previous one. The images which it 
 gives are very beautiful. Unfortunately its price is so large as to make 
 it inaccessible to most workers. 
 
 Ot the many objectives made by Messrs. Powell and Lealand, we 
 will mention the following : — 
 
 Nominal Focus. 
 
 Numerical Aperture. 
 
 Price. 
 
 
 
 
 Dry Objectives. 
 
 
 
 
 
 
 
 
 
 
 £ 
 
 s. 
 
 d. 
 
 
 4 inches 
 
 
 ■08 
 
 
 1 
 
 IO 
 
 0 
 
 
 3 
 
 
 •09 
 
 
 2 
 
 15 
 
 0 
 
 
 
 
 13 
 
 
 2 
 
 IS 
 
 0 
 
 
 1 .j 
 
 
 •26 
 
 
 3 
 
 3 
 
 0 
 
 
 2/3 
 
 
 '39 
 
 
 s 
 
 0 
 
 0 
 
 
 1/2 
 
 
 '57 
 
 
 s 
 
 0 
 
 0 
 
 
 1/4 
 
 
 '94 
 
 
 8 
 
 10 
 
 0 
 
 
 1/10 ,, 
 
 
 '94 
 
 
 10 
 
 0 
 
 0 
 
 
 
 Water Immersion Objective. 
 
 
 
 
 
 1/8 „ 
 
 
 1 "26 
 
 
 12 
 
 0 
 
 0 
 
 
 
 Homogeneous Immersion Objectives. 
 
 
 
 
 
 1/8 „ 
 
 
 1-29 
 
 
 12 
 
 0 
 
 0 
 
 
 1/8 „ 
 
 
 i'S 
 
 
 35 
 
 0 
 
 0 
 
 
 i/ 12 
 
 
 *'S 
 
 
 40 
 
 0 
 
 0 
 
 
 1/20 „ 
 
 
 1 5 
 
 
 60 
 
 0 
 
 0 
 
 
 
 
 Apochromatic Objectives. 
 
 
 
 
 
 
 1/8 „ 
 
 
 1 "40 
 
 
 25 
 
 0 
 
 0 
 
 
 1/10 „ 
 
 
 1-40 
 
 
 25 
 
 0 
 
 0 
 
 
 I/ 12 ,'i 
 
 
 i - 4o 
 
 
 2 5 
 
 0 
 
 0 
 
 
 1/10 „ 
 
 
 i'SP 
 
 
 5° 
 
 0 
 
 0 
 
 
 1/20 ,, 
 
 
 1 '40 
 
 
 40 
 
 0 
 
 0 
 
 Messrs. Powell and Lealand also make anew i-i2th inch apochromatic 
 homogeneous immersion objective with N.A. 1-40, which is sold for ^15. 
 This objective is a rapid photographer. 
 
CARL REICHERT. 
 
 195 
 
 Messrs. Powell & Lealand's Condensers. 
 
 Achromatic Condenser of '99 N.A. Price ^880 
 
 Achromatic homogeneous condenser, N. A. 1/3 .. .. .. ,, 12 o o 
 
 Chromatic condenser .. .. .. .. .. .. ,, 220 
 
 Apochromatic homogeneous immersion condenser, N.A. i'40.. ,, 15 o o 
 
 For details of these, see the chapter on condensers, pages 84 and 
 85. Compensating oculars, £6 10s., for the series of oculars 10, 20 
 and 30, which form a very useful series. 
 
 CARL REICHERT (26, Bennogasse, Vienna).— Although only 
 established a few years, Mr. C Reichert, who was a pupil of Dr. 
 Hartnack, has rapidly taken a prominent position among the makers 
 of the present time. His stands are very elegant, and manufactured 
 with extreme care ; while his objectives are certainly accounted to be 
 amongst the best of our day. 
 
 The largest model (fig. 141) of this maker can be inclined; the 
 rapid movement is produced by rack and pinion, and the slow move- 
 ment is effected by a micrometrical screw of great precision, and very 
 carefully manufactured, moved by a graduated milled head. The draw 
 tube is divided into millimetres. The foot, of horse-shoe form, is very 
 heavy, and of great stability. 
 
 The stage, which is round and rotary, is divided into 360 0 , and 
 can be used with a goniometer for the measurement of angles. It is 
 fitted with a mechanical stage (fig. 144/'), which possesses rectangular 
 movements, and is furnished with verniers. 
 
 The illumination apparatus comprises a plane and concave mirror, 
 an Abbe condenser of a numerical aperture 1*20 or i'4o, an Iris 
 diaphragm, the aperture of which can be varied from 1 to 30 millimetres 
 (i-24th inch to 1 and i-5th inches). The diameter of this aperture is 
 marked on the graduated edge of the diaphragm by the small arm 
 which is used to modify it. The diaphragm has an eccentric movement, 
 enabling oblique illumination, in all directions, to be obtained. All 
 the parts which compose the illuminating apparatus are fixed on one 
 piece, to which a rack and pinion communicates a vertical movement 
 with reference to the stage. The condenser can be removed, and re- 
 placed by a diaphragm tube. The price of the stand by itself is 
 £18 8s. The instrument, enclosed in an elegant mahogany case, 
 with lock and key, is sold complete for ^70 8s. In this case 
 the stand described above is provided with dry objectives, Nos. ia, 
 2, 4, 6, and 8a; water immersion objective 10, and homogeneous 
 objective 19; Huygen's oculars I., III., IV.; compensating oculars 
 
 02 
 
Fig 141. 
 
CARL REICHERT. 
 
 197 
 
 4 and 12, eye-piece micrometer II., stage micrometer, camera lucida 
 (the Abbe model), polariscope, saccharimeter with support and reserve 
 
Fig. 142^. 
 
CARL REICHKRT. 
 
 I 99 
 
 tube, spectroscopic eye-piece, bull's eye on stand for the illumination 
 of opaque bodies, dissection microscope magnifying 10, 20, and 30 
 diameters, hand magnifier after Steinheil, triple nose-piece, microtome, 
 100 glass slips, 100 cover-glasses, 12 glass slips with pillar, collection 
 of dissecting instruments, including 2 scalpers, 1 bistoury, 1 lancet, 2 
 preparing needles, 1 pair of straight and 1 pair of curved scissors, 2 
 pairs of pincettes, and 1 section knife. The instrument, with these 
 accessories, magnifies from 16 to 3,000 diameters. 
 
 Another large stand (fig. 142), somewhat smaller than the last, has 
 a fixed square stage, fitted with a mechanical stage, which can be 
 removed at will. This stage is not thick ; two guides hold the 
 preparation at the sides, and maintain it in position firmly. These 
 guides are moved by means of milled heads, which act on a rack 
 and pinion for longitudinal, and on a screw for transverse motion. 
 Abbe's illuminating apparatus, furnished with an Iris diaphragm, is adapted 
 to it which can be lifted and lowered by means of a rack and pinion. 
 The mirror is plane and concave. This instrument, which can be 
 inclined, has a graduated draw tube : its rapid movement is effected by 
 rack and pinion, and its slow movement by a very carefully constructed 
 micrometer screw. The foot is of horse-shoe pattern. Without 
 accessories this microscope is sold for j£g 5s. 
 
 It is usually fitted with Nos. 2, 4, 6 and 8a ordinary objectives, a 
 homogeneous objective 18, oculars IV. and V., a nose-piece for three 
 objectives, eye-piece micrometer II., and costs then ,£25 5s. 
 
 The arrangement of the Abbe condenser in this microscope is quite 
 special. Although Mr. Reichert has for many years carried this 
 arrangement out it has never yet been described. Figures 142a and 
 142$ illustrate its details. 
 
 It will be seen that the condenser H C can be raised or lowered 
 the length of the side bar P by means of the milled head T. 
 
 A steel bar />, working in the aperture a, maintains the centering 
 of the condenser during its movement. This bar is shorter than the 
 hole or recess, so that when the condenser goes beyond the bar it 
 can be turned aside as is shewn in fig. 142/;. 
 
 The other movements are similar to those previously described, with 
 the exception that in Mr. Reichert's either the entire apparatus 
 (condenser and diaphragm carrier) can be turned aside, or only the 
 diaphragm carrier. 
 
 Stand No. III. (fig. 143) is an elegant and very well-constructed 
 
200 
 
 CARL REICHERT. 
 
 instrument. Although the price is only the moderate sum of ^5 16s., 
 it is furnished with a rapid movement by rack and pinion, slow 
 adjustment by micrometrical screw, and is jointed, so that it can be 
 
 Fig- 143- 
 
 placed in an inclined position. There is a draw tube. The diaphragm 
 tube is adapted so that it can be raised or lowered, by means of a rack 
 
CARL REICHERT. 
 
 201 
 
 and pinion, for the regulation of the light. This diaphragm can be 
 easily removed and replaced by a simplified Abbe condenser, furnished 
 
 with an Iris diaphragm. 
 This instrument is 
 generally sold with a 
 triple nose-piece, two 
 oculars Nos. II. and 
 IV., and three objectives 
 Nos. 3, 7a, and 18b, 
 giving magnifications 
 varying from 50 to 1,500 
 diameters. The price is 
 then ^15 6s. 
 
 Among small stands 
 we will mention No. 
 IV. (fig. 144, 144a), which 
 is an erect microscope, 
 furnished with a coarse 
 adjustment made by slid- 
 ing the body tube, and a 
 fine adjustment made 
 with a micrometer screw 
 acting on a lever. The 
 diaphragm is formed of 
 a rotating disc, which 
 is slightly convex, and 
 has circular openings 
 of different diameters. 
 The plano-concave mir- 
 ror is jointed, and 
 allows the light to be 
 used obliquely. The 
 price of this instrument, 
 furnished with objec- 
 tives 3, 6 and 8, oculars 
 II. and IV., magnifying 
 50 to 880 times, is 
 £(> 5s. 
 
 Fig. 144. 
 
202 
 
 CARL REICHERT. 
 
 Fig. I44<7. 
 
 We will now examine the objectives of this Viennese manufacturer. 
 The following are the results obtained : — 
 
 I. — Achromatic Objectives. 
 
 No. 5 inch) N.A. 073. — A very good objective, giving very clear 
 and pure images. It resolves the Pkurosigma Angulation dry, and in 
 Moller's new test it shews Vanheurckia Lewisiana and Vanhearckia 
 Rhomboides fairly well. The resolving power is also great, and the frontal 
 distance moreover is very great, enabling the superficial examination 
 of objects with cover-glass. With electric light Nobert's 6th group can 
 be resolved in axial and his 10th group in oblique light. 
 
 No. 6 (i-5th inch) N.A. -83. — With central light Nobert's 7th group is 
 resolved; with oblique light the 10th group is; its frontal distance is large. 
 
 No. 7a (i-6th inch) N.A. '83. — Does not differ from the last, except 
 that the magnifying power is rather larger. 
 
CARL REICHERT. 
 
 203 
 
 No. 8a (i-9th inch) N.A. '83. — Central light, Nobert's 7th group; 
 oblique illumination, the nth group, very well. 
 
 No. 9 (i-i2th inch). — We have examined two objectives of this num- 
 ber, one with an aperture of - 86 and the other of -98. The first, 
 with the exception of the magnifying power, has the same qualities as 
 the previous one; the second resolves in oblique light Nobert's 12th 
 group. The images are very sharp, and the frontal distance sufficiently 
 large to admit of the use of thick cover-glasses. 
 
 Nos. X. and XI. water immersion.- — These two objectives are mounted 
 with correction collars. The numerical aperture of the first is 1*22, and 
 that of the second is riS. The two objectives are very good, the images 
 pure, clear and strong. One of our Nobert's tests, mounted in the 
 yellow medium, shewed the 1 8th group very clearly, with No. X. objective. 
 The Amphipleura pellucida in Moller's new test plate is shewn fairly well. 
 In the yellow medium it is shewn perfectly ; however, in both 
 cases, when its maximum aperture is used, the frustule is slightly 
 deformed. 
 
 Homogeneous. — We have examined two numbers : the i-i5th and the 
 i-2oth inch. Both are without correction collars, but, if required, 
 they are made with them. The first ( 1-1 5th) has a numerical aperture 
 of 1 '26, the second of i'29. 
 
 These two objectives are of great excellence, and are in no way 
 inferior to the best that we have .hitherto examined. The images are 
 excessively pure. The i-2oth shows Nobert's 19th group (yellow medium) 
 so well resolved that there is no difficulty in counting the lines. The 
 length of the frontal distance is sufficient to admit of the use of thick 
 cover-glasses, which is seldom the case with other objectives of the 
 same focus in our possession. 
 
 The Amphipleura is perfectly resolved throughout its length. The 
 aperture of this objective therefore is quite perfect. We can say 
 that few objectives give so flat an image. 
 
 II. — Apochromatic Objectives. 
 
 Mr. Carl Reichert was the first maker to follow the firm of Zeiss 
 in the manufacture of apochromatic objectives. We gave a description 
 of his apochromatic objectives in the Bulletin de la Societe Beige de 
 Microscopie, dated 30th June, 1888. 
 
 His apochromatic objectives are six in number at the present time, as 
 follows : — 
 
204 
 
 CARL REICHERT. 
 
 An objective of 2-31-d inch. 
 
 i-3 r d „ 
 „ 1 -6th „ 
 
 i-i2th „ N.A. 1-25. 
 h N.A. 1-30. 
 „ N.A. 1-40. 
 
 We have had occasion to make an exhaustive examination of the 
 i-6th, and of two of the i-i2ths. 
 
 The i-6th, a dry objective, which Mr. Reichert calls a "4 millimetre 
 focus," has undoubtedly the greatest resolving power of any that we 
 have hitherto examined. 
 
 It has a correction collar, and is made for the Continental tube 
 of 16 centimetres (6^ inches); the correction is so extensive that a 
 longer tube can be used without the image materially loosing brightness. 
 
 The numerical aperture is given by Mr. Reichert as "95. We 
 however find that that figure is surpassed. 
 
 The images are very bright and pure. With axial illumination the 
 Pleurosigma angulation mounted dry, as well as in styrax, is completely 
 resolved ; the marks of exclamation in the Podura (the only real test of 
 apochromatic perfection) are clearly defined, are of a good black colour, 
 and possess long light strokes, which are perfectly colourless. 
 
 With oblique illumination, the Vanheurckia rhomboides of Moller's test 
 is resolved. With monochromatic sunlight the stria? of the Amphipleura 
 can be seen, but its definition leaves much to be desired. 
 
 In the yellow medium and with electric light, the 10th group of 
 Nobert's test can be seen with axial illumination, and the 15th group 
 can be perfectly resolved with oblique illumination. 
 
 Although this resolving power is excessively large for a dry objective, 
 nevertheless, the frontal distance, though naturally very small, is still 
 quite sufficient for all ordinary observations. One of the 1-12U1 inch 
 objectives which we have examined is made for the Continental, and 
 the other for the English tube. 
 
 The first has an aperture of 1*3, the second of 1*4. 
 
 The i-i2th (2 mill.), made for the Continental tube, leaves nothing 
 to be desired. With axial illumination the marks of exclamation on the 
 Podura are clear, and of a good black colour, also the light stroke is very 
 long. 
 
 All the tests, including Nobert's 19th group, are resolved by this 
 objective with the greatest clearness. 
 
CARL REICHERT. 
 
 205 
 
 We have not succeeded in resolving the Amphipkura with perfect 
 clearness into beads by transmitted light. We think that the aperture 
 of 1 '3 is not sufficient for that purpose. 
 
 The second objective intended for use with the English tube is quite 
 similar to the previous one, but the image surpasses in purity that given 
 by the preceding. 
 
 III. — New Semi-Apochromatic Objectives. 
 
 Mr. Carl Reichert calls by this name objectives which only differ 
 from apochromatics in the absence of fluorite in their composition. 
 
 We have in our possession one of these objectives, a 1-15 th of 
 N.A. r 4 . 
 
 The price of this objective is only ,£10. The images of the Podura, 
 Szc, have a little red colouring, as well as a slight secondary spectrum. But 
 with the exception of this trifling defect, which can only be appreciated 
 in photo-microgiaphy, the objective is quite irreproachable, and the most 
 difficult tests, such as Nobert's 19th group, are resolved with the 
 greatest clearness and without much difficulty. 
 
 We cannot too strongly recommend this objective to every conscientious 
 worker who does not wish to make photo-micrography his favourite study, 
 since the objective is quite suitable for the most difficult researches, 
 which an extensive use of this objective has sufficiently demonstrated. 
 
 In conclusion, we will give a summary of the different objectives made 
 by Reichert : — 
 
 Number of Objective. 
 
 Focus. 
 
 Numerical Aperture. 
 
 Price, 
 
 Mill. 
 
 Inches. 
 
 £ s. d. 
 
 
 
 Dry Objectives. 
 
 
 O 
 
 60-5 
 
 2 1/2 
 
 
 0 11 0 
 
 I 
 
 40 O 
 
 1 1/3 
 
 
 0 11 0 
 
 xa 
 
 40 O 
 
 1 i/3 
 
 
 0 17 0 
 
 2 
 
 30 0 
 
 1 
 
 
 0 17 0 
 
 3 
 
 iS'S 
 
 1/2 
 
 '34 
 
 0 17 0 
 
 4 
 
 92 
 
 i/3 
 
 '5° 
 
 140 
 
 \b 
 
 I2"I 
 
 2/S 
 
 '35 
 
 140 
 
 5 
 
 5 '4 
 
 i/4 
 
 •65 
 
 180 
 
 6 
 
 4 3 
 
 i/S 
 
 77 
 
 1 10 0 
 
 7 
 
 28 
 
 1/8 
 
 ■82 
 
 1 10 0 
 
 7<z 
 
 3 6 
 
 1/6 
 
 ■82 
 
 1 16 0 
 
 8 
 
 2*2 
 
 1/10 
 
 •87 
 
 1 15 0 
 
 8a 
 
 28 
 
 1/9 
 
 •87 
 
 200 
 
 9 . 
 
 2 0 
 
 1/12 
 
 '95 
 
 2 12 0 
 
 *9 
 
 2.0 
 
 1/12 
 
 '95 
 
 3 10 0 
 
206 
 
 ROSS AND CO. 
 
 Water Immersion Objectives. 
 
 IO 
 
 18 
 
 
 1/12 
 
 From 1 10 to 1 '20 
 
 O 
 
 10 
 
 0 
 
 *IO 
 
 18 
 
 
 1/12 
 
 
 4 
 
 10 
 
 0 
 
 *II 
 
 1 3 
 
 
 1/18 
 
 
 s 
 
 10 
 
 0 
 
 
 New 
 
 Semi-Apochromatic Objectives. 
 
 
 
 
 12 
 
 
 
 1/6 
 
 From 1 '30 
 
 8 
 
 0 
 
 0 
 
 18 
 
 i '8 
 
 
 1/12 
 
 « ,, 1-30 to 1-35 
 
 8 
 
 0 
 
 0 
 
 19 
 
 1 '3 
 
 
 1/18 
 
 ,, 1 30 to 1-35 
 
 12 
 
 0 
 
 0 
 
 18a 
 
 18 
 
 
 1/12 
 
 , , 1 40 to 1 -43 
 
 10 
 
 0 
 
 0 
 
 i8i 
 
 18 
 
 
 1/12 
 
 ,, i '20 to 1 25 
 
 S 
 
 0 
 
 0 
 
 lqd 
 
 1 '3 
 
 
 1/18 
 
 , , I '20 tO I '25 
 
 8 
 
 10 
 
 0 
 
 
 
 Apochromatic Objectives. 
 
 
 
 
 Dry 
 
 16 
 
 
 2/3 
 
 '3° 
 
 4 
 
 0 
 
 0 
 
 
 8 
 
 
 i/3 
 
 "5° 
 
 1 
 
 12 
 
 0 
 
 
 4 
 
 
 1/6 
 
 '95 
 
 8 
 
 0 
 
 0 
 
 Homogeneous 
 
 2 
 
 
 1/12 
 
 
 12 
 
 0 
 
 0 
 
 
 2 
 
 
 1/12 
 
 1 30 
 
 16 
 
 0 
 
 0 
 
 
 2 
 
 
 1/12 
 
 1 40 
 
 20 
 
 0 
 
 0 
 
 * These objectives have correction collars. 
 
 Mr. Ferdinand Van Heurck, Rue du Moulin, Antwerp, is the 
 authorized agent for Belgium and Holland for Mons. Carl. Reichert. 
 
 ROSS & CO. (112, New Bond Street, W., London). The firm 
 founded by the celebrated Andrew Ross still enjoys a well-merited 
 reputation. They make a large number of stands, of which we 
 will describe two which are of exceptional merit. 
 
 The Microscope called Wenhanfs Radial (fig. 145), was made with 
 the special object of obtaining the largest amount of light in every 
 direction with oblique illumination, and in all its inclining and rotating 
 movements the centre of the object is taken as the common centre 
 of rotation. 
 
 To attain this object Mr. Wenham, the originator of this type, has 
 furnished it with the following contrivances : — 
 
 r. A sector sliding in two grooves allows it to assume every position 
 between the vertical and horizontal. 
 
 2. The entire optical apparatus can be inclined either to the right 
 or to the left without moving the foot. 
 
 3. The entire optical apparatus can be rotated about the illuminating 
 apparatus. 
 
 4. The mirror and sub-stage can be used in any oblique position 
 that may be desired; the stage of the microscope contains the centre of 
 these movements. 
 
Fig- I4S- 
 
208 
 
 ROSS AND CO. 
 
 5. The stage can make a complete revolution about the optic axis, 
 and can be centred for different objectives. 
 
 6. The illuminating apparatus can be used in all positions that 
 may be desired, and can, while the microscope remains fixed, describe 
 nearly a complete circle about the latter. 
 
 7. The slow movement consists of a new system, and is the first of 
 its kind made. It works easily and with the greatest sensibility, and 
 when once an object is focussed it remains in exactly the same focus 
 so long as the milled head of the adjustment screw is not touched. 
 
 In this slow movement the inner tube, which is guided for some con- 
 siderable distance, travels when pushed by an excessively fine graduated 
 screw. At the same time, a spring placed in a barrel outside the tube 
 presses a brass cylinder, which fits exactly into the barrel and constantly 
 tends to draw it back into the tube of the microscope. 
 
 The milled head of the fine adjustment placed at the end of a 
 long steel bar is close to the ocular. At first sight this position 
 appears inconvenient, but one soon gets used to it, and ultimately 
 it proves to be an excellent arrangement. 
 
 The form of this microscope differs from all that we have seen, 
 and is likely to surprise, if not to displease, at first. 
 
 These fears are without foundation ; many years' use of the instrument 
 has proved it to be one of the best pieces of apparatus in our collection, 
 very convenient for actual work, and, moreover, indisputably stable. 
 
 Figures 146, 147, 148, and 149 show the different positions which 
 
 Fig. 146. Fig. 147. 
 
ROSS AND CO. 209 
 
 the instrument can assume without a moment's preparation, which thus 
 proves its utility. 
 
 The price of this stand varies from ,£42 with the monocular, to £50 
 with the binocular tube. 
 
 Fig. 149. 
 
 The second large model is the Ross-Zentmayer No. 1 Microscope 
 (fig. 150). The stage of this instrument is thin, and the sub-stage, 
 can be made to assume every possible inclination round the object 
 which is the centre of all the movements of the instrument. As in 
 the Radial, the mirror can, if desired, illuminate the object from 
 above, and all the angles of inclination can be read off the graduated 
 scale, which is engraved on the upper part of the pillar which carries 
 the sub-stage and mirror. 
 
 There is a mechanical stage ; its coarse adjustment is made by rack 
 and pinion, and the fine adjustment by lever. 
 
 V 
 
2IO 
 
 ROSS AND CO. 
 
ROSS AND CO. 
 
 211 
 
 This instrument is hung between two pillars, and can be fixed at 
 any inclination. 
 
 The price of this stand is ^"29 4s. with the monocular tube, and 
 ^3 1 with binocular. 
 
 Messrs. Ross & Co.'s objectives are constructed by a special method, 
 invented by Mr. Wenham. They are reduced to three simple lenses, one 
 of which is of dense flint, and the other two of crown. 
 
 The following is the result of an examination of these objectives : — 
 
 y 2 inch. — Images excessively clear. With central light, definition is 
 perfect, and the pygidium of a flea is seen in a very remarkable manner 
 for so low a power. 
 
 With oblique light the Pleurosigma angulation can be resolved, but the 
 colouring is, however, too strong. 
 
 1-7 tk inch. — This objective is excellent; as well as Messrs. Ross 
 and Co.'s i-5th, i-ioth, i-i5th, and i-25th; they can be used as dry 
 or immersion objectives as desired, by simply altering the correction. 
 The extreme angle of aperture of this objective is 130°. It shows 
 the 8th group of Nobert's test very well with central light. Its frontal 
 distance, which is very large, makes it a very valuable objective for 
 ordinary use. The purity of the images leaves nothing to be desired. 
 
 1-15M inch. — The angle of aperture of this objective is 150 0 . It 
 shows the 9th group of Nobert's test fairly well in central light. 
 
 The images are very pure, and its frontal distance is pretty con- 
 siderable. 
 
 The pygidium of a flea is admirably defined with central light, and 
 with monochromatic oblique light all known tests can be resolved. 
 
 The following is the series of objectives manufactured by Messrs. 
 Ross & Co. : — 
 
 Series of low powers. 
 
 
 
 
 Magnification. 
 
 
 Price, 
 
 Objective. 
 
 Aperture. 
 
 
 
 
 
 
 
 
 A 
 
 B 
 
 C 
 
 D 
 
 £ s. d. 
 
 *4 inch 
 
 9° 
 
 12 
 
 18 
 
 25 
 
 40 
 
 111 6 
 
 *3 „ 
 
 IO° 
 
 15 
 
 20 
 
 35 
 
 5° 
 
 220 
 
 3 
 
 12° 
 
 15 
 
 20 
 
 35 
 
 5° 
 
 3 3 0 
 
 
 12° 
 
 25 
 
 40 
 
 60 
 
 IOO 
 
 220 
 
 2 „ 
 
 15° 
 
 25 
 
 40 
 
 60 
 
 100 
 
 3 3 0 
 
 
 1 5° 
 
 35 
 
 60 
 
 95 
 
 150 
 
 220 
 
 si „ 
 
 20° 
 
 35 
 
 60 
 
 95 
 
 
 3 3° 
 
 *i „ 
 
 iS° 
 
 5° 
 
 80 
 
 125 
 
 200 
 
 220 
 
 1 „ 
 
 25° 
 
 5° 
 
 80 
 
 125 
 
 200 
 
 3 10 0 
 
 2/3 ,, 
 
 35° 
 
 80 
 
 130 
 
 200 
 
 300 
 
 3 10 0 
 
 P2 
 
212 
 
 W. AND H. SEIBERT. 
 Series of Ordinary Objectives. 
 
 
 
 
 
 Magnifications. 
 
 
 
 
 Price. 
 
 Objectives. 
 
 Aperture. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 A 
 
 B 
 
 C 
 
 I) 
 
 E 
 
 V 
 
 £ 
 
 s. d 
 
 i/a inch 
 
 45° 
 
 IOO 
 
 160 
 
 250 
 
 400 
 
 SOO 
 
 800 
 
 4 
 
 4 0 
 
 1 *l a •! 
 
 8o° 
 
 TOO 
 
 160 
 
 250 
 
 400 
 
 500 
 
 800 
 
 
 S 0 
 
 i 3/ ID » 
 
 6o° 
 
 16S 
 
 26 S 
 
 410 
 
 660 
 
 820 
 
 1300 
 
 { 
 
 10 0 
 
 3/ ro .. 
 
 QO° 
 
 165 
 
 265 
 
 410 
 
 660 
 
 820 
 
 1300 
 
 5 
 
 10 0 
 
 ! i/s .. 
 
 8 5 ° 
 
 120° 
 
 250 
 
 400 
 
 620 
 
 1000 
 
 I2SO 
 
 2000 
 
 5 
 
 S 0 
 
 >/s .. 
 
 250 
 
 4OO 
 
 620 
 
 1000 
 
 I250 
 
 2000 
 
 6 
 
 6 0 
 
 1/7 
 
 I3O 0 
 
 34° 
 
 54° 
 
 8 S o 
 
 1300 
 
 I7OO 
 
 2700 
 
 7 
 
 7 ° 
 
 1/10 „ 
 
 1 4O 0 
 
 50O 
 
 800 
 
 1200 
 
 2000 
 
 25OO 
 
 4000 
 
 9 
 
 9 ° 
 
 1 i/iS 
 
 1 5O 0 
 
 75° 
 
 1200 
 
 1800 
 
 3000 
 
 37OO 
 
 600O 
 
 12 
 
 12 0 
 
 1 25 >. 
 
 160° 
 
 1200 
 
 2000 
 
 3100 
 
 5000 
 
 62OO 
 
 I OOOO 
 
 21 
 
 0 0 
 
 W. & H. SEIBERT (formerly Seibert and Kraft), ot 
 Wetzlar, Prussia. — This firm manufactures excellent microscopes ; the 
 mechanical part, being as carefully constructed as the optical, leaves 
 nothing whatever to be desired. Their large stand (fig. 151) has a 
 very heavy brass foot, from which rises an arm which bears the entire 
 mechanism in such a manner that the microscope can be inclined at 
 all angles. The instrument is rotary. The double stereoscopic tube 
 can be replaced by a monocular tube. 
 
 The instrument has three movements : a rapid movement by rack 
 and pinion, and two slow movements — one for medium magnification 
 and the other for very high powers. These two movements are said 
 to be frictionless, like the one we shall describe later on, and 
 that all shaking is quite prevented. The milled head of the 
 adjustment screw for high powers is graduated. 
 
 The Abbe illuminating apparatus is moveable, and can be centred 
 by means of springs, and is furnished with an Iris diaphragm. 
 
 The entire illuminating system can be easily removed and replaced 
 by the usual sub-stage apparatus, such as the diaphragm tube, 
 polariser, &c. 
 
 The stage, made after English models, is moveable in two direc- 
 tions and furnished with verniers. This stage can be removed at 
 will, and can be replaced by a smooth round stage, graduated and 
 furnished with a centering screw, allowing the glass slip to be moved 
 within narrow limits. 
 
 Enclosed in a box the stand just described costs ^19; without the 
 mechanical stage it costs only ^15. 
 
Fig. 151. 
 
214 
 
 W. AND H. SEIBERT. 
 
 The complete apparatus 
 is accompanied by a nose- 
 piece for five objectives, eye- 
 piece micrometer with 
 optional ruling, polarising 
 apparatus with graduated 
 circle, Abbe's camera lucida 
 system, bulls-eye on stand for 
 the illumination of opaque 
 objects, compresser and 
 stage micrometer. More- 
 over, the instrument has 
 four oculars and all the 
 ordinary objectives from 
 No. O O to No. VII. b ; 
 apochromatic objectives of 
 1 6 mm. (2~3rd in.), 8 mm. 
 (i-3rd in.), and 4 mm. 
 (i-6th in.), homogeneous 
 immersion objectives of 2 
 mm. (i-i2th in.) and 1*5 
 mm. (i-i8th in.) with N.A. 
 1 '30, as well as compensa- 
 ting oculars 2, 4, 6, 8, 12, 
 and 18. 
 
 Test objects, slips with 
 11 hollows, glass slips, and 
 p cover - glasses, usually ac- 
 company the microscope, 
 which is enclosed in a 
 strong polished mahogany, 
 box with brass fittings, costing ^95 10s. 
 
 Stand No. 2 (fig. 152), though of smaller proportions, is in con- 
 struction almost identical with the last. It has only two movements 
 for focussing: the rapid movement by rack and pinion, and the slow 
 movement by micrometrical screw, the head of which is graduated. 
 There is a draw tube, divided by a scale. The stage is rotary, and 
 the illuminating apparatus consists of an Abbe condenser, furnished 
 with an Iris diaphragm. The different pieces of apparatus which 
 
 Fig. 152. 
 
W. AND H. SEIBERT. 
 
 accompany it consist of a nose-piece for four objectives, a moveable 
 eye-piece micrometer, polarising apparatus with graduated circle, camera 
 lucida, bulls-eye on stand, ordinary objectives Nos. O to VII. b, an 
 
 homogeneous immersion 
 objective 1-12 inch with 1*3 
 numerical aperture, and simple 
 oculars Nos. O and I., and 
 periscopic oculars Nos. II. and 
 III. giving magnifications from 
 18 to 1,500 diameters. The 
 instrument, enclosed in a 
 strong box, with test objects 
 and other accessories, such as 
 glass slips and cover-glasses, 
 is sold for ^46 10s. 
 
 Microscope No. 4 (fig. 
 153), with an ordinary stage, 
 can be inclined, and is fur- 
 nished with rapid and slow 
 movements. Illumination is 
 effected by a plano-concave 
 mirror. Under the stage a 
 system of tube diaphragms is 
 fixed, sliding into grooves. 
 
 An Abbe condenser, a 
 triple nose-piece, ordinary 
 objectives I., III., V., and 
 1- 1 2th homogeneous immer- 
 sion of N.A. 1 '3, oculars I. 
 and III., the latter with 
 jj, micrometer test objects, glass 
 slips, &c, are the accessories 
 which usually accompany this 
 instrument. Thus equipped, 
 stand No. 4 gives a magnification of from 30 to 1,500 diameters, and 
 costs ^24 3 s. 
 
 The same firm have a considerable number of other models. One 
 of them, No. 5 (fig. 154), which we have examined at leisure, deserves 
 all praise, and is remarkable for its cheapness. Furnished with objectives, 
 
2l6 
 
 W. AND H. SEIBERT. 
 
 Nos. I., III., V., and VII. b., oculars O, I., and III., with micrometer 
 (affording magnifications from 30 to 1,500), and furnished with the usual 
 
 small objectives, costs £\2 12s. 
 
 The stand has a horse-shoe foot. The 
 stage is very low. The body tube is without 
 a draw tube ; it is moved by sliding it in 
 a thick cylinder, a portion of the inside of 
 which is covered with cloth. The screw of 
 the slow movement is under the stage, 
 and the movement differs completely from 
 those we hive usually seen. 
 
 Lastly, the outside tube of the rapid move- 
 ment terminates at the side with a square 
 piece of apparatus, some centimetres in 
 length, the end of which is fixed in the 
 cylinder. This piece of apparatus is pressed 
 from above downwards by a spring, and is 
 held at the side by four solid arms, which 
 move diagonally. By screwing or unscrew- 
 ing the milled head of the fine adjustment 
 at the bottom of the cylinder the micro- 
 scope tube is raised or lowered. This 
 system is perfectly rigid ; the objects remain 
 immoveable during the focussing, and the 
 makers warrant that it will never shake. 
 
 The following are the principal objectives 
 manufactured by Messrs. W. and H. Seibert : — 
 
 Fig. 154- 
 
 1 . — Ordinary Objectives. 
 
 No. OO. of 2h inches, numerical aperture '09 
 
 I- 1. J i !> ti i. " 22 
 
 i inch ,, ,, '22 
 
 i ,, ■■ "6o 
 
 •90 
 
 •90 (witl: 
 
 II. 
 IV. 
 V. 
 VI*. 
 IX. 
 X. 
 
 ,, I 
 
 7th inch 
 
 1 1 I 
 
 -12th ,, 
 
 „ I 
 
 -8th ,, 
 
 1) 1 
 
 -12th „ 
 
 
 2.- 
 
 16mm. (2-3rd 
 
 8 
 
 . U-3rd 
 
 4 
 
 , (i-6th 
 
 3 
 
 , (i-8th 
 
 3 
 
 ,, (i-8th 
 
 2 
 
 (i-i2th 
 
 2 
 
 (i-i2th 
 
 i-5 
 
 „ (i-i6th 
 
 i-5 
 
 ,, (i-i6th 
 
 correction) .. 
 1 '30 (homogeneous immersion 
 
 i'3° » » 
 
 £ s. d. 
 140 
 o 18 o 
 o 18 o 
 170 
 116 o 
 3 15 o 
 600 
 
 Apochromatic Objectives. 
 
 £ s. d. 
 
 inch) N.A. '30 .. .. .. ..300 
 
 ,, I „ "60 .. .. ., ..400 
 
 ,, ) ,, '95 (correction) .. ..600 
 
 ,, ) ,, 1.30 (homogeneous immersion) 16 o o 
 ,, ) ,, 1.40 .. .. .. . . 21 o o 
 
 ) i"30 13 15 o 
 
 ., ) .. i"4° 14 IS 0 
 
 , , ) , , 1 "30 . . . . . . . . 16 o o 
 
 „ ) ,, I '40 .. .. . . 21 O O 
 
VV. AND H. SEIBERT. 
 
 217 
 
 Messrs. W. and H. Seibert's objectives are of excellent quality. 
 We know their apochromatics of recent date, but we have not seen 
 examples of their present ordinary objectives. We will, therefore, examine 
 the two kinds separately. 
 
 1. — Achromatic Objectives. 
 
 Nos. O. and r.- — Lower power very good ; they have a focus of 
 iijth and 1 inch respectively. 
 
 No. 2. — Excellent objective of £ inch ; it shews the pygidium of 
 a flea fairly well. The details of a transverse section of / inus are 
 seen with great clearness, but there are some slight traces of iridescence. 
 
 No. 3 (1-31-d inch). — This objective, which has 38 0 of useful aperture, 
 gives in the same manner a very slight red border to the section of 
 riiu/s. It shows the pygidium of a flea very well. 
 
 No. 4 (1-4U1 inch) has a useful aperture of 80°, shows the pygidium 
 well, and resolves in central light Hipparchia Janira, as well as the 
 14th group of Nobert's test. 
 
 No. 5 (i-8th inch) has a useful angle of no"; resolves clearly 
 Nobert's 16th group with central light. With oblique light it resolves No. 
 15 of Moller's tests. 
 
 No. 6 (i-i2th inch, with correction). Good objective, having an 
 angle of 124 0 . It shows the Pkurosigma with central light, and resolves 
 Nobert's 7th group very well with the same light. With oblique illumina- 
 tion Moller's 15 th number is perfectly, and his 16th is feebly seen. 
 
 No. 7 immersion and correction (i-i6th inch). This objective resolves 
 Nobert's 8th group very clearly with central light and the Amphipleura 
 with monochromatic light. 
 
 2. — Apochromatic Objectives. 
 
 8 mill. (i-3rd inch). We had occasion a year ago to examine one 
 of these objectives belonging to a friend. We have mislaid the notes 
 which we took at that time, but we recollect that the images were 
 very pure, that the Pkurosigma was seen with oblique illumination, and 
 that the photographs of textile fibre, for which we used it, left nothing 
 to be desired. 
 
 1*5 mill. (i-i6th inch) homogeneous immersion, dating from 25th 
 May, 1 89 1. Excellent objective, having a very considerable frontal dis- 
 tance for its focal length. 
 
 The Pkurosigma angulatum is resolved clearly into hexagons with 
 axial illumination, the pygidium giving very clear and pure images. 
 
2l8 
 
 H. R. SPENCER. 
 
 The Podura gives very pure and colourless images. Lastly in the 
 Nobert test the 12th group is clearly resolved with axial illumination, 
 and the 19th very well with oblique illumination. 
 
 The Antwerp Exhibition afforded us an opportunity of examining the 
 other apochromatic objectives of these makers. All of them gave perfect 
 images and left nothing to be desired. 
 
 H. R. SPENCER, (515, Rhode Island Street, Buffalo N.Y.). — 
 The firm of Spencer has been renowned for some time ; the first 
 information which we have about this house dates from 1848 ; at that 
 time this optician's objectives were noted as surpassing those made in 
 Europe. Shortly after, in 1852, Spencer manufactured a i-i2th inch 
 objective having 174^°, the first having so large an angle of aperture. 
 
 The firm of Spencer has always maintained its old reputation. We 
 are not acquainted with their stands, but their objectives deserve all 
 praise. At different times we have used excellent ones of their make, 
 and we have in our possession one of their latest, made about a year 
 ago. 
 
 This objective, which enjoys an immense reputation in the United 
 States, is a i-i2th inch, N.A. 1-37. 
 
 The objective has a correction collar ; it can be used as a water 
 immersion for axial, and with an homogeneous mixture for oblique, 
 illumination. 
 
 This liquid need not be the oil of cedar, and indeed the maker 
 advocates a special liquid, consisting of a solution of chloride of tin 
 in glycerine. 
 
 The construction of this object is quite special, and differs slightly 
 from the general type now adopted. The optical system consists of a 
 double frontal, surmounted by a single moveable and very thick 
 lens. 
 
 The images given by the objectives are coloured red slightly, depending 
 probably on its construction ; but with the exception of this slight defect 
 we consider the objective to be one of the best of the present date, 
 and we have obtained some of our best photographs with it. 
 
 With axial illumination the pygidium and the Podura give very clear 
 images, and Nobert's 12th group is well resolved. With oblique light 
 Nobert's 19th group shows up well, and with both electric and sunlight 
 the resolution of the Amphipkura into beads is easily obtained. 
 
JAMES SWIFT AND SON. 
 
 219 
 
 JAMES SWIFT & SON (81, Tottenham Court Road, London) 
 are well-known manufacturers of microscopes and photographic lenses. 
 
 Fig- 155- 
 
220 
 
 JAMES SWIFT AND SON. 
 
 Messrs. Swift's stands are numerous, the most interesting being the 
 
 following : — 
 
 The Challenge Microscope, very similar to Messrs. Watson's 
 Scientist (described later) ; this instrument, which was made in the 
 first place by Messrs. Swift, differs from that of Messrs. Watson 
 principally in the fine adjustment, which is of a special form, and in 
 the stage. The price of this instrument with i inch and \ inch 
 objectives, bull's-eye condenser on stand, in cabinet, is £20. 
 
 The Challenge A (fig. 155) is a simpler form, and only costs ^15 8s.; 
 but the illuminating system is too elementary for serious work. To 
 this the sub-stage must be added, entailing an additional charge of £2 6s. 
 
 The Seaside Microscope has still to be noticed. 
 
 This microscope, as its name suggests, is made with the object oi 
 occupying the smallest possible space. It is furnished with a folding 
 stage, rapid movement by sliding the tube, slow movement 
 enclosed in the leg of the tripod, upon which the instrument stands, 
 an ocular, and an inch objective (figs. 156 and 157). It can be packed 
 in a box 15 centimetres (6 in.) long, 7 centimetres (3I in.) wide, and 
 5 centimetres (if in.) deep. The price of this microscope is ^3 10s. 
 
 The small pocket microscope, represented in figures 158 and 159, 
 should also be mentioned. It can be conveniently carried in 
 
WATSON AND SONS. 
 
 221 
 
 Fig. 158. Fig. 159. 
 
 the breast pocket, for when the 
 apparatus is closed it is only 7 
 centimetres (3 inch) in length and 2 
 centimetres (f inch) in diameter. One 
 of these figures represents the micro- 
 scope closed, and the other the instru- 
 ment ready for use. The preparation 
 C is held firm by a small tube B, 
 containing a spiral spring. The rapid 
 movement is made with the tube D, 
 and the slow movement by the tube 
 E, which contains the ocular. 
 The microscope furnished with an 
 ocular and objectives of 1 inch 
 and i-5th inch, costs ^5. 
 
 Not having seen any recent 
 objectives of this maker, we cannot 
 give an estimate of their value. 
 Those which we examined in 1878 
 were good (see Third Edition, p. 172). 
 
 WATSON & SONS, (313, High Holborn, London). — This firm 
 only commenced to construct microscopes seriously a few years ago, but 
 the progress which it has made in this branch has been so rapid and 
 pronounced that upwards of 3,000 have already been produced. 
 
 The stands made by this firm are numerous, but we will only describe 
 the most important : — 
 
 The Swinging Sub-stage Microscope is of the Zentmayer model. The 
 instrument is mounted on double pillars, fixed to a rotating graduated 
 base plate, which rests on a tripod foot. 
 
 The stage is excessively thin, consisting of a base plate, having a 
 rotary motion, on which rests a second very thin plate, to which 
 vertical and horizontal motions can be given by means of two milled 
 heads, placed one above the other and acting upon one axis. 
 
 The rapid movement is given with a rack and pinion having oblique 
 teeth, and the slow movement is produced by a lever, which communicates 
 to the tube a very even and precise movement. The tube is binocular, 
 and carries a Wenham prism, which can be removed when desired, so as 
 to render it monocular. 
 
2 22 WATSON AND SONS. 
 
 The sub-stage can describe almost a complete circle about the stage, 
 which forms the centre of all the movements of the microscope. A 
 graduated scale enables the angle of inclination used to be registered. 
 
 Fig. 160. 
 
WATSON AND SONS. 223 
 
 The price of this instrument, furnished with a binocular tube, is 
 only ^25. 
 
 The Scientist's Microscope (fig. 160) is an instrument supported on 
 a massive foot, with two upright sides, between which it is mounted. 
 
 Fig. 161. 
 
224 
 
 WATSON AND SONS. 
 
 The mirror can be inclined at any angle, and the sub-stage, which can 
 be centred, can be withdrawn from the axis of the instrument. 
 
 The mechanical stage consists of superposed plates. 
 
 The rapid movement is obtained by a rack and pinion having diagonal 
 teeth, the slow movement by a very sensitive lever. 
 
 The tube is binocular. 
 
 The apparatus costs from ^17 to £20, according to its finish. 
 
 The Edinburgh Microscope (fig. 161) is an instrument which Messrs. 
 Watson have already supplied in large numbers, and of which they 
 make several kinds, of different degrees of completeness. 
 
 One of these, " which may be considered as the medium model, is 
 called stand C. 
 
 The foot is of a horse-shoe pattern ; the microscope is mounted 
 on a single pillar, and can be inclined. 
 
 The mirror can be adjusted obliquely, sideways, and forwards. 
 
 The sub-stage can be raised and lowered by rack work, and adjustment 
 screws allow the condenser to be centred. 
 
 The body tube is moved with a rack and pinion, and the fine 
 adjustment, which is effected with a lever, is of great precision. 
 
 The price of this stand is only £6 6s. 
 
 But the most perfect instrument of these makers, one we may add, 
 of the most precise and most convenient which at present exists, is 
 that which the makers- call Dr. Henri Van Heurck's Microscope 
 for the study and photography of Diatoms and all delicate 
 researches. 
 
 In this microscope (fig. 162), which Messrs. Watson and Son have 
 been good enough to make according to our specification, we have 
 endeavoured to combine convenience for ordinary work, with the utmost 
 possible precision, at a comparatively low price. 
 
 Messrs. Watson have admirably carried out all the plans which we 
 have submitted to them, and the instrument they have produced may 
 be justly considered, as we shall see later on, to be a perfect 
 instrument, and one which has realised in various respects a degree 
 of perfection which has not hitherto been attained. 
 
 The base of the instrument is of the horse-shoe form, and is 
 bronzed ; at the three points on which it stands are three holes, 
 in which slightly projecting pieces of cork are inserted. This cork 
 reduces any vibration which may be communicated to the instrument 
 by the table on which it rests, and also prevents the instrument from 
 
226 
 
 WATSON AND SONS. 
 
 slipping and avoids the scratches which the sharp angles of the base 
 would otherwise make on a polished table. 
 
 A substantial brass pillar, jointed in its upper part to allow the 
 instrument to be inclined, supports the whole microscope. The 
 microscope is also made with a brass tripod, to meet the require- 
 ments of certain purchasers (fig. 162^). When the instrument is inclined, 
 this tripod gives a little more stability than the pillar; on the other 
 hand, the pillar allows the various pieces of apparatus placed in the 
 sub-stage to be managed with greater ease when the instrument is used in 
 a vertical position for photo-micrography. The microscope can be 
 fixed at any angle by means of a clamping screw, although the instru- 
 ment is so well balanced as to render this almost unnecessary. 
 
 To reduce vibration to a minimum, all the parts of the instrument 
 are made as if cast in one piece. The stage support is in one 
 piece, and is prolonged into the articulation at the top of the pillar ■ 
 in the same way the limb fits into the stage support and is fixed by 
 six screws, so that the whole has the same rigidity as if it formed a 
 single piece. Finally, the two pieces fitted together are traversed by 
 a bolt on which is the clamping screw for the inclination. 
 
 The stage turns on its own axis ; and instead of small gliding plates 
 to keep the rotating circle to its bearings, there are three plates with 
 double springs. The result is that a very soft rotation is obtained, and 
 at the same time a perfect rigidity in any position, and this also without 
 using a cog-wheel, which generally gives little jerks after it has been used 
 some time. 
 
 The mechanical movements (horizontal and vertical) are effected by 
 two superposed plates, as in the old Ross stands, actuated by lateral 
 screws. The object rests upon a sliding bar, provided with a stop pin 
 and clamping screw. 
 
 For ordinary work, the sliding bar can be replaced by a fixed plate, 
 provided with two clips. 
 
 The horizontal and vertical movements have a range of 25 mm. (1 
 inch) : the divided scales (finders) allow a reading of the movements to 
 i-iomm. by means of verniers. At the request of the purchaser, the 
 stage can be made with centering movements ; such is the case in the 
 instrument in our possession. 
 
 The limb incloses the fine adjustment, and carries the tube in front; 
 both the coarse and fine adjustments move in bearings which can be 
 loosened or tightened as required. A screw attachment at the upper 
 part of the limb fixes the instrument firmly in the horizontal position 
 
228 
 
 WATSON AND SONS. 
 
 when it is required to photograph in that position. We, however, seldom 
 use it, infinitely preferring photographing with this microscope in the 
 vertical position, as we have described in the chapter of this work on 
 " Photo-micrography." 
 
 The fine adjustment is of exquisite delicacy, and of precision 
 surpassing that of all other microscopes in our collection. Each turn of the 
 milled head of the fine adjustment corresponds to i-i3th of a millimetre; 
 so perfect is the adjustment that it is possible in certain cases to estimate 
 to a hundredth of a turn, i.e., to i -1300th of a millimetre. As the 
 mechanism of the fine adjustment acts in an opposite direction to that 
 of Continental microscopes, we have marked on the milled head the 
 letters M (monter) and D {descendre) to indicate the direction in which 
 it is necessary to turn to make the body tube move up and down. 
 
 The mechanism of the fine adjustment is represented by figure 162A 
 As will be seen, the milled head acts on the end of a lever with 
 a fulcrum at C. 
 
 A spiral spring coiled round a steel bar presses at its upper end on 
 a plate attached to the dovetail bar which carries the microscope tube, 
 and counter-balances so well the weight of the tube that the back thrust 
 on the lever becomes very slight, this being in proportion to the tension 
 that the maker gives to the spring. 
 
 This lever system very materially diminishes the resistance at the 
 actuating milled head, and saves wear and tear. Also, a much finer 
 movement is obtained by this means, the rate of movement being 
 reduced by the long arm of the lever on one side of the fulcrum, 
 and the short arm of the other, in the same ratio as the weight. 
 
 The limb is placed between the two guides seen in figure 162^. 
 One of these guides can be tightened by the two screws AA, so as 
 to hold the tube perfectly steady with an entire absence of shake. 
 The pieces are so well fitted that the makers guarantee that the 
 apparatus is absolutely free from any lateral movement. 
 
 The tube is very large, being 42 mm. (i'656 inches) in diameter, so 
 as to be able to be used in all photo-micrographic researches. It 
 has also both a draw tube and an optional extension by rack work. 
 With the draw tube closed it is 142 millimetres (5^ inches) long, less 
 than is required for use with Continental objectives. With the draw tube 
 entirely drawn out, it is 24 centimetres (9^ inches) long. To make it 
 larger still a milled head is turned, which acts on a rack and pinion, 
 carrying a third tube, in reality a second draw tube. This tube enables 
 the length to be increased to 32 centimetres (12^ inches). There is, 
 
WATSON AND SONS. 
 
 229 
 
 therefore, every possible length that can be desired for the correction of 
 apochromatic objectives, made either for the Continental or English length 
 of tube. The innermost tube is the narrowest in the upper part, so that it 
 can be blackened internally over half the space covered by the eye-piece; 
 thus, all internal reflection, which is the cause of so much trouble in photo- 
 micrography, is absolutely prevented. 
 
 It might be preferable to line this tube with black velvet. 
 
230 
 
 WATSON AND SONS. 
 
 The lower end of the draw tube is provided with the Society screw 
 for use with the Abbe apertometer, &c. 
 
 Fig. 162c. 
 
 The mirror is carried by a rod, having lateral movements; it can also 
 be slid up and down within a moderate range. With regard to the sub- 
 stage, which we have purposely reserved to the last, we have here to call 
 attention to improvements which are not found in any other microscope. 
 
 The condenser can of course be centred, and it can be raised or 
 lowered by means of a rack and pinion, but, in addition, a fine 
 adjustment of great delicacy is applied. Up to the present time, in 
 the few microscopes to which a fine adjustment of the condenser has 
 been applied (an adjustment so necessary in certain cases, and not 
 
WATSON AND SONS. 
 
 231 
 
 yet sufficiently appreciated), the focussing has been simply effected by 
 a screw, which does not produce a very slow movement, and there 
 has always been loss of time (back lash) in the changes of direction. 
 
 Here the fine adjustment is produced by a lever, as in the slow 
 movement of the body tube, and the milled head of this adjustment 
 is placed above the stage and quite close to the fine adjustment screw 
 of the body tube. By this means therefore it is possible to obtain a 
 very great precision and to adjust the two slow movements simul- 
 taneously with one hand. 
 
 The arrangement of the condenser, as planned by us (and employed 
 for several months with all our microscopes) affords, we believe, 
 important advantages. This adjustment consists of an Iris diaphragm, 
 surmounted by the lens holder. Between these two pieces slides a 
 plate, removeable at will, provided with a central rotating ring, which 
 serves for the reception of the diaphragms which, by means of the 
 plate, is capable of being placed at any eccentricity that the numerical 
 aperture of the objective used will allow. 
 
 The lens holder is adapted to receive the different Abbe condensers, 
 the Zeiss achromatic condenser and also adapter plates allowing the use 
 of all the excellent condensers of Powell and Lealand. It is thus seen 
 that the mechanical part, allowing every central and oblique illumination 
 desired, remains invariable, but is capable of receiving all possible optical 
 combinations. 
 
 In fact, we have combined in this instrument all the conditions of 
 perfection which long experience in microscopical work has taught us, and 
 Messrs. Watson and Sons have realized all our desiderata with a precision 
 and care which was quite beyond our expectations. If we add that this 
 apparatus, so perfect, only costs ^16, and consequently less than the 
 large Continental models, it will readily be admitted, we believe, that the 
 makers have rendered a real service to serious workers by the construc- 
 tion of this stand. 
 
 Our opinion on this point has been confirmed by the jury of 
 Micrographic Exhibition at Antwerp, who have expressed their views on 
 Messrs. Watson's stands c.s follows : — 
 
 "The perfection of these stands is extraordinary, and the fine adjust- 
 ment of the body tube, which is effected by means of a lever, acted on 
 by micrometer screw, is of incomparable smoothness and precision. 
 
 "Among the instruments exhibited, the stand called 'Dr. Van 
 Heurck's microscope,' because it was made according to the specifications 
 of this microscopist, has appeared the most remarkable to the jury. 
 
232 
 
 CARL ZEISS. 
 
 The elegance of the microscope, the extreme precision of all its 
 movements, the ease with which it can be used for all photo-micrographic 
 and micrographic work, and lastly its comparatively moderate price, have 
 largely influenced the jury to attribute to Messrs. Watson the high 
 recompense which has been awarded to them." 
 
 We cannot at present say much about the objectives of these makers, 
 Messrs. Watson having entirely abandoned their original formula of 
 construction, and their new series, with the exception of the i inch and 
 i -6th inch, not being yet ready. 
 
 The i inch is excellent. The images of histological objects are 
 excessively pure and well defined, and the objective bears very strong 
 oculars, without deforming the images. 
 
 The i-6th inch is a very good objective for ordinary work. It 
 properly resolves the Pkurosigma with oblique illumination, has a consider- 
 able frontal distance, and the images which this objective give are 
 excessively brilliant and well defined. For histological work the objectives 
 leave nothing to be desired. 
 
 CARL ZEISS (Carl Zeiss Strasse, Jena). — The firm established 
 by the late Dr. Carl Zeiss is at the present day the most important in 
 the world. 
 
 Dr. Abbe, professor at the University, and a partner in the firm, is 
 at present the managing director, and this scientist, who is also the 
 highest contemporaneous authority in scientific optics, has under his 
 skilful management made such improvements in the construction of 
 objectives that one may infer that the progress which has already been 
 realised in connection with the ordinary use of the miscroscope, is 
 only the least that will be accomplished. 
 
 The models made by the firm of Zeiss are nine in number. We 
 shall describe the principal ones. 
 
 Stand la (fig. 163) is the most perfect of these models. 
 
 The object is roughly focussed by a precise rack and pinion. The 
 fine adjustment has been described on a previous page. 
 
 The draw tube is divided into millimetres. The stage is furnished 
 with a rotating ebonite disc, which can be centred to the optic axis by means 
 of two lateral screws, which compress a counter spring, inclosed within 
 the stage. This stage can be removed in an instant, and a mechanical 
 stage substituted for it. 
 
 The illuminating apparatus consists of an Abbe condenser of 1-40 
 
2 34 
 
 CARL ZEISS. 
 
 N.A., with an Iris diaphragm, furnished with an eccentric move- 
 ment. 
 
 The instrument is inclinable, and can be fixed in any position by 
 means of a clamping lever. 
 
 The price of this stand with the mechanical stage is ^20, and £15 
 without it. 
 
 The stand for photo-micrography is very similar to the last (fig. 164). 
 
 It differs, however, in the tube, which is shorter, and in the stage, 
 which is a mechanical stage, with excessively slow movements. The 
 upper table of the stage rotates. 
 
 As in the previous instrument, the movements of the mechanical 
 stage are recorded by verniers, reading to i-ioth of a millimetre. 
 
 This microscope, furnished with a special achromatic condenser, which 
 is indispensable for photography (that of diatoms only excepted, as they 
 can be photographed with Abbe's chromatic condenser), costs ,£21 5s. 
 
 In addition to these large special models, the firm of Carl Zeiss 
 manufactures others which are more suitable for the less ambitious 
 worker. 
 
 Thus stands II. a and IV. a are still perfect instruments, and sufficient 
 for all ordinary work. 
 
 Stand II. a (fig. 165) has its coarse and fine adjustments like those 
 of the preceding models. 
 
 The stage is furnished with a revolving vulcanite disc, which can 
 be centred as in stand I. a. 
 
 The illuminating apparatus is as perfect as that of the preceding 
 stands. 
 
 Its price is ^14 10s. 
 
 Stand IV. a (fig. 166) only differs from the preceding in the stage, 
 being fixed and without the revolving disc. The illuminating apparatus 
 can be raised and lowered, but the Abbe condenser is only 1*20 N.A. 
 The price of this stand, together with an Iris diaphragm, is £10 15s. 
 
 Stand VI. (fig. 167), which costs jQt, 5s., is however sufficient for 
 such ordinary observations as do not require the extreme precision which 
 is at the present day so sought after. The stand is a good type of the 
 ordinary microscope used twenty years ago by most microscopists on the 
 Continent. 
 
 The body has a draw tube ; rapid focussing is effected by sliding, and 
 its fine adjustment is precise. 
 
 The stage is fixed, having a tube or diaphragm carrier, which is 
 
Fig. 164. 
 
236 
 
 CARL ZEISS. 
 
 fitted to the under surface by a bayonet point ; the whole of this can be 
 removed when oblique light is required. 
 
 The mirror, plane on one side and concave on the 'Other, is fitted to 
 a jointed bar giving oblique illumination in all positions. 
 
Fig. 166. 
 
2 3 8 
 
 CARL ZEISS. 
 
 The instrument can be inclined at any angle as far as the horizontal 
 position. 
 
 We will now examine the principal objectives of the firm. As we have 
 already stated, their ordinary objectives are excellent, and the apochromatic 
 
CARL ZEISS. 
 
 2 39 
 
 ones include specimens whose perfection has not been equalled by any other 
 maker. 
 
 We will first take the ordinary series and examine the objectives called 
 CC, D, DD, and F, with which we are well acquainted from a prolonged 
 usage. 
 
 All of these give perfectly clear images, and define the pygidium of 
 a flea and a section of Pinus : all have equally a comparatively large 
 working distance, which is very convenient. The following are the 
 results obtained with reference to the resolving power of those we have 
 examined; such examination has been made by means of a lamp and 
 a microscope furnished with an Abbe condenser. 
 
 CC. (objective of |- inch, N.A. 71). With perfectly central light 
 Nobert's 16th group and Moller's No. 8 are feebly resolved. With 
 oblique light Moller's No. n is seen perfectly. 
 
 D. (i-6th inch, N.A. "65). We cannot praise this objective so 
 highly, but as long as we have had it in our possession we have 
 used it daily, and prefer it to all our objectives of the same amplifica- 
 tion for ordinary observations. The clearness of the images is very marked, 
 and the working distance is so great that we can dispense with the 
 use of a cover-glass when making hasty observations. With central light 
 the objective resolves Nobert's 15th group and Moller's No. 8 very well. 
 With oblique light No. 11 of the same test (Pkurosigma Angulatum in 
 balsam) is seen very well. 
 
 F. (i-i4th inch, N.A. '90). This objective is an excellent one, and 
 justly deserves the great reputation it enjoys among microscopists. It 
 resolves Nobert's 17th group and Moller's No. 10 passably well with 
 central light. With oblique light it resolves the first 17 specimens in 
 Moller's test plate, i.e., all the specimens which are resolvable with 
 light which is not monochromatic. 
 
 Apochromatic Series. — We have made considerable mention of 
 apochromatic objectives on page 47, to the manufacture of which we 
 owe our introduction to Professor Abbe. We have, therefore, only 
 to add a few words on the principal types of this series. 
 
 Objective of 24mm. (1 inch) N.A. "30. This objective is intended 
 for the English tube. It is an excellent objective, especially for 
 photographic work with low powers. The images given by it are 
 excessively pure, clear, and bright. It is very convenient for vegetable 
 and histological work, but for diatoms its N.A. is too small. 
 
 Objective of 8mm. (i-3rd inch) N.A. - 6o. This objective is only 
 
240 
 
 CARL ZEISS. 
 
 for use with the Continental tube. It is a very good objective, enabling 
 the Pleurosigma to be resolved, and has a very large working 
 distance. 
 
 Objectives of 6mm. (i-4th inch) and of 4mm. (i-6th inch), both 
 having N.A. '95 ; the first is made exclusively for the English and the 
 second for either the English or the Continental tube as the purchaser 
 may desire. These two objectives represent the apochromatic specimens 
 for ordinary work. The image of the pygidium of a flea is admirably 
 clear and pure. The Pleurosigma is resolved with great ease. These 
 are the two objectives which we use daily for the ordinary study of 
 diatoms and for determining their species. With the appropriate oculars, 
 12 and 8 respectively, a magnification of 500 diameters is obtained, 
 which is amply sufficient for everything but the examination of the 
 valve structure. For ordinary histological work these objectives are quite 
 sufficient. 
 
 Objective of 2'$mm. (i-ioth inch) water immersion N.A. 1-25. 
 The pygidium and the Podura give very good images, and the Bacillus 
 tuberculosis is clearly defined. With axial illumination Nobert's 12th group 
 is seen pretty well, and the Pleurosigma shews clear and very black beads. 
 With oblique illumination the Amphipleura prepared in the medium 24 
 is resolved very clearly, and the same test mounted dry is fairly well 
 resolved. 
 
 Objective of ynm. (i-8th inch) N.A. 1*4, homogeneous immersion. 
 According to our observations, extending over six years, this objective is 
 the best which exists at present ; we know no other which is its complete 
 equal. 
 
 The images of the pygidium and the Podura are of perfect purity ; 
 the bacilli shew excessively clear outlines ; the Amphipleura is clearly 
 resolved into beads, and the image of a diatom resolved into striae is 
 perfectly pure, and equally so throughout the extent of the valve. 
 Even with great obliquity the image remains fiat, and the valve is not 
 deformed in the slightest. 
 
 With axial illumination Nobert's 1 2th group is seen perfectly, and 
 with oblique illumination the 19th is resolved without difficulty. The 
 frontal distance of this objective, notwithstanding its great N.A., is 
 considerable. 
 
 Objective of 2'5mm. and Numerical Aperture i-6. — This 
 objective is the first of a new series commenced by Zeiss. It will 
 probably be followed by one or two other numbers. 
 
CARL ZEISS. 241 
 
 This objective has the largest resolving power hitherto attained. 
 Unfortunately it is difficult to use, and excessively costly, consequently 
 its practical use for ordinary work is not great. Its real use is for the 
 elucidation of details, which it has been impossible to ascertain by 
 objectives of the most perfect construction up to the present time. 
 
 We will examine this admirable piece of apparatus in greater detail. 
 
 The objective is i-ioth inch, and of course apochromatic. 
 
 Its numerical aperture is 1*63, and its construction is such that the 
 whole of this enormous aperture can be utilised. 
 
 However, for this purpose, it requires special conditions, viz. : — 
 
 1. A cover-glass of high refractive index, which must be at least i'6. 
 
 2. The object must be embedded in a medium, which has equally an 
 index of i - 6 at least. 
 
 3. Lastly, if it is desired to utilise the whole aperture with oblique 
 light (that is to say, if the maximum resolving power is required of 
 this objective with the above illumination), the slips must also be of 
 flint, with an index of refraction of at least i'6. 
 
 All these conditions have been realized by Messrs. Zeiss, who have 
 also made a special ocular, which suppresses the slightest trace of 
 colour, and a condenser whose front lens is made of flint, and allowing 
 the most oblique illumination. 
 
 The objective is formed of five lenses, superposed in the following 
 order : — 
 
 1. A simple frontal lens, greater than a hemisphere which is made 
 of flint, and has an index of refraction of 172. 
 
 2. An achromatic lens formed of two simple lenses. 
 
 3. A simple lens of crown glass. 
 
 4. An achromatic lens consisting of three simple lenses. 
 
 5. A correcting achromatic lens formed of three glasses. 
 
 In three of these lenses crown glass is replaced by fluorite. 
 
 The liquid used for immersion is monobromide of naphthaline, the 
 refractive index of which is i - 65. 
 
 To resolve tests with greater ease the slip and cover-glass are 
 made of a medium having the same index as the frontal lens, i.e., 172. 
 
 According to Professor Abbe's formulae an objective of 1*63 can 
 resolve — 
 
 1. With axial daylight illumination ... 3,000 lines per millimetre. 
 
 2. With oblique illumination ... 6,000 „ „ „ 
 
 By using the objective for photography from 5,000 to 10,000 lines 
 can be resolved ; the resolution in this case naturally depends upon 
 
 R 
 
242 
 
 CARL ZEISS. 
 
 which part of the spectrum the plates used are most sensitive, and for 
 which part the spherical correction is the most perfect. 
 
 Let us now notice the advantages of the optical combination described 
 above. 
 
 With oblique illumination the Amphipleura shows the entire valve 
 resolved in beads, as clearly as we can see the Pleurosigma with the best 
 objectives in our possession, although we find that these beads are 
 closer together than our previous incomplete resolutions had led us to 
 believe ; measurements made from several of our photographs show 
 that our Amphipleura has from 3,600 to 3,700 tranverse striae, and 5,000 
 longitudinal striae per millimetre. It is not therefore surprising that 
 previously we had so much difficulty in seeing all those beads. 
 
 But it is only for resolving these beads that oblique illumination has 
 to be used. All the other difficult tests, Van Heurckia crassinervis 
 Breb {Frustulia Saxonica), Surirella Gemma, and even the transverse 
 striae of the Amphipleura are resolved with axial illumination with a 
 large diaphragm. 
 
 Let us add that, latterly, by employing oxy-hydrogen light and 
 focussing the objective in the image of the flame we have succeeded in 
 resolving the Amphipleura into beads with axial illumination, but with 
 a larger diaphragm which gives a mixture of axial and oblique rays. 
 
 We have in our possession a negative made under these conditions. 
 
 The time of exposure adopted was, unluckily, much too short. The 
 beads, though quite visible, are much too faint to be printed on paper. 
 
 Pleurosigma angulatum gives an image differing from those which we 
 have been accustomed to see of late. The objective allows the grating 
 to be perfectly focussed, and the beads no longer appear round, but 
 hexagonal. 
 
 Van Heurckia, Surirella, Amphipleura, &c, show square beads, with 
 a slight tendency here and there towards the hexagonal form. 
 
 The luminosity of the objective is very great. We have obtained 
 vigorous negatives of the beads of the Amphipleura (with a magnification 
 of 2,000 and monochromatic solar light) in six minutes, while with the 
 previous apochromatics at least ten minutes are required for a magnifica- 
 tion of 1,000 diameters with the most oblique light. 
 
 It was with this objective that the photographs of the tests, which 
 appear in this volume, were made. 
 
 This objective is, as we have said, expensive. Its price is ^40. 
 
 In conclusion, we give here the price and magnifying power of 
 objectives made by the firm of Zeiss : — 
 
CARL ZEISS, 
 i. Achromatic Objectives. 
 
 243 
 
 
 _ 
 
 
 
 Magnifying Power. 
 
 
 
 
 
 Objectives. 
 
 If 
 
 £ 5 
 
 Equivalent focal 
 1 e ng"t h . 
 
 
 
 
 
 
 
 Price. 
 
 
 = a 
 
 
 Ocular 
 
 Ocular 
 
 Ocular 
 
 Ocular 
 
 Ocular 
 
 
 
 
 
 
 1 *' 
 
 2. 
 
 3- 
 
 4- 
 
 5- 
 
 
 
 
 
 
 mm. inches. 
 
 
 
 
 
 
 £ 
 
 S. 
 
 d. 
 
 (11 
 
 
 40 1 9/16 
 
 7 
 
 10 
 
 15 
 
 20 
 
 
 0 
 
 12 
 
 0 
 
 az 
 
 
 35 1 3/8 
 
 11 
 
 16 
 
 23 
 
 3° 
 
 
 0 
 
 12 
 
 0 
 
 <*3 
 
 
 30 1 i/4 
 
 20 
 
 3° 
 
 40 
 
 5° 
 
 
 0 
 
 12 
 
 0 
 
 a* 
 
 
 38-26 ij/i 
 
 4-8 
 
 7-14 
 
 I0-2O 
 
 r 5-3° 
 
 
 2 
 
 O 
 
 0 
 
 aa 
 
 •17 
 
 26 1 
 
 25 
 
 35 
 
 47 
 
 60 
 
 77 
 
 1 
 
 7 
 
 0 
 
 A 
 
 ■20 
 
 18 3/4 
 
 37 
 
 5° 
 
 70 
 
 90 
 
 "5 
 
 1 
 
 4 
 
 0 
 
 B 
 
 "35 
 
 12 1/2 
 
 60 
 
 85 
 
 115 
 
 145 
 
 185 
 
 1 
 
 10 
 
 0 
 
 C 
 
 •40 
 
 7 i/4 
 
 105 
 
 145 
 
 200 
 
 265 
 
 325 
 
 1 
 
 16 
 
 0 
 
 D 
 
 •65 
 
 4'3 J / 6 
 
 175 
 
 240 
 
 325 
 
 420 
 
 54° 
 
 2 
 
 2 
 
 0 
 
 E 
 
 '85 
 
 27 1/9 
 
 280 
 
 39° 
 
 535 
 
 680 
 
 865 
 
 3 
 
 6 
 
 0 
 
 F 
 
 ■90 
 
 1-85 1/15 
 
 
 585 
 
 790 
 
 1000 
 
 1275 
 
 4 
 
 4 
 
 0 
 
 H water immer- 
 
 
 
 
 
 
 
 
 
 
 
 sion 
 
 I"20 
 
 2 '4 1/10 
 
 320 
 
 440 
 
 610 
 
 770 
 
 985 
 
 5 
 
 10 
 
 0 
 
 / water immer- 
 
 
 
 
 
 
 
 
 
 
 
 sion 
 
 I - 20 
 
 i-8 1/14 
 
 430 
 
 585 
 
 810 
 
 1030 
 
 '3i5 
 
 7 
 
 4 
 
 0 
 
 1/12 homogeneous 
 
 
 
 
 
 
 
 
 
 
 
 immersion 
 
 I - 20 
 
 20 1/12 
 
 385 
 
 53° 
 
 73° 
 
 925 
 
 1 180 
 
 8 
 
 0 
 
 0 
 
 1/12 homogeneous 
 
 
 
 
 
 
 
 
 
 
 
 immersion 
 
 1 '35 
 
 20 1/12 
 
 385 
 
 53° 
 
 73° 
 
 925 
 
 1 180 
 
 15 
 
 0 
 
 0 
 
 .£, F, H and / with correction are £ 
 
 t extra. ' Each of the oculars costs 
 
 7s. 
 
 
 
 2. Apochromatic Objectives. 
 
 
 
 
 Magnifying Power. 
 
 
 
 Objectives. 
 
 Numerical 
 Aperture. 
 
 
 
 
 Oculars. 
 
 
 
 Price. 
 
 
 
 
 
 2. 
 
 4- 
 
 8. 
 
 12. 
 
 18. 
 
 
 
 
 *24mm. 
 
 1 inch 
 
 
 •30 
 
 
 21 
 
 42 
 
 83 
 
 I2 5 
 
 187 
 
 281 
 
 7 0 
 
 0 
 
 16mm. 
 
 2/3 
 
 
 '30 
 
 
 3 1 
 
 62 
 
 125 
 
 187 
 
 281 
 
 
 5 0 
 
 0 
 
 *i2mm. 
 
 1/2 „ 
 
 
 CO 
 
 
 42 
 
 83 
 
 167 
 
 250 
 
 375 
 
 562 
 
 8 10 
 
 0 
 
 8mm. 
 
 i/3 
 
 
 •60 
 
 31 
 
 62 
 
 125 
 
 250 
 
 375 
 
 562 
 
 
 6 10 
 
 0 
 
 *6mm. 
 
 $ :: 
 
 
 '95 
 
 1 62 
 
 83 
 
 167 
 
 333 
 
 500 
 
 75° 
 
 1125 
 
 11 0 
 
 0 
 
 4mm. 
 
 
 
 '95 
 
 125 
 
 250 
 
 500 
 
 75° 
 
 1125 
 
 
 9 0 
 
 0 
 
 3mm. 
 
 1/8 „ 
 
 
 1-30 
 
 83 
 
 167 
 
 33° 
 
 667 
 
 1000 
 
 1500 
 
 
 20 0 
 
 0 
 
 3mm. 
 
 1/8 ,, 
 
 
 1 '40 
 
 83 
 
 167 
 
 333 
 
 667 
 
 1000 
 
 1500 
 
 
 25 0 
 
 0 
 
 2 - smm. 
 
 11 
 
 
 1 '25 
 
 100 
 
 200 
 
 400 
 
 800 
 
 1200 
 
 1800 
 
 
 15 0 
 
 0 
 
 2mm. 
 
 I / I 2 ,, 
 
 
 1 30 
 
 125 
 
 250 
 
 500 
 
 1000 
 
 1500 
 
 2250 
 
 
 20 0 
 
 0 
 
 2mm. 
 
 I/I2 „ 
 
 
 1 '40 
 
 125 
 
 250 
 
 500 
 
 1000 
 
 1500 
 
 2250 
 
 
 25 0 
 
 0 
 
 
 
 3. New A 
 
 pochr 
 
 amatic Objectives. 
 
 
 
 
 3mm. dry 
 
 '95 
 
 83 
 
 167 
 
 333 
 
 498 
 
 667 
 
 1000 
 
 1500 
 
 10 0 
 
 0 
 
 2"5 monobromide 
 
 1 "60 
 
 IOO 
 
 200 
 
 400 
 
 600 
 
 800 
 
 1200 
 
 1800 
 
 40 0 
 
 0 
 
 1 '5 homogeneous 
 
 1 '3° 
 
 167 
 
 333 
 
 667 
 
 1000 
 
 1334 
 
 2000 
 
 3000 
 
 22 0 
 
 0 
 
 The Objectives marked with an asterisk 
 
 (*) are made for the English tube only, 
 
 
 the others, 
 
 except the 8mm., can 
 
 be' obtained 
 
 for the English tube. 
 
 
 R2 
 
244 CARL ZEISS. 
 
 The price of compensating oculars is as follows : — 
 
 I. For the Continental tube — 
 
 1248 12 18 
 
 £1 £1 £* £* ios - £* IOS - £ l 5 s - 
 
 2. For the English tube — 
 24 8 12 18 27 
 
 £* 5 s - £ l SS. £*■ 15s- £1 ios. £1 ios. £1 5s. 
 The price of projective oculars is as follows : — 
 
 1. For the Continental tube, 2 and 4] 
 
 2. For the English tube, 3 and 6 
 In the above series of oculars each number denotes the magnifyin 
 
 power. 
 
 [£2 each. 
 
CHAPTER II. 
 
 MEASUREMENT AND REPRODUCTION 
 OF MICROSCOPICAL OBJECTS. 
 
 PHOTO-MICROGRAPHY. 
 
 z. — Measurement of Microscopical Objects. 
 
 Several means are employed for measuring the size of objects 
 seen through the microscope. We shall only mention here the easiest 
 and most general method, viz.: that in which the eye-piece micrometer 
 previously described is used. 
 
 With an eye-piece micrometer the maker usually includes a table, 
 giving the relation that exists between the ocuiar and the different 
 systems of lenses employed. If necessary, such a table can easily be 
 calculated by the student himself. To do this, he has only to place 
 on the stage a millimetre, divided into one hundred parts, and observe 
 how many divisions of the eye-piece micrometer (one-tenth of a millimetre 
 in value) go to a division of the stage micrometer. 
 
 If, for example, two divisions of the stage micrometer (one- 
 hundredth of a millimetre in value) corresponds to i2 - 5 divisions of 
 the eye-piece micrometer, we obtain as the value of a division of the 
 
 eye-piece micrometer 02 = - ooi6 of a millimetre. 
 "•5 
 
 Having determined this relation, in order to calculate the actual 
 size of an object, we have only to ascertain the number of divisions 
 of the eye-piece micrometer which it occupies, and to multiply this 
 number by the figure expressing the relation between the eye-piece and 
 the objective used. Thus, suppose an object occupies six divisions 
 and the relation is represented by '002, we have '002 x 6 = -012, 
 i.e., the size of the object examined is twelve thousandths of a 
 millimetre. 
 
246 
 
 MEASUREMENT OF MICROSCOPICAL OBJECTS. 
 
 However simple and easy this calculation may be, it, nevertheless, 
 is tedious, when a large number of measurements have to be made, 
 as is, for example, the case when diatoms are being studied ; con- 
 sequently we have, for some time past, substituted for it a more 
 convenient method. 
 
 The ocular, which we have constructed for the purpose, carries a 
 side piece, in which is fitted a small brass frame, larger than the 
 diaphragm, sliding in two grooves, which can be withdrawn by means 
 of a screw. In this frame is placed a thin glass plate, on which is 
 marked decimal divisions, and graduated so that with a i-ioth inch 
 objective, each small division corresponds to a thousandth of a millimetre. 
 The large divisions of the micrometer are therefore equivalent to a 
 hundredth of a millimetre. By making the divisions in this way, all 
 other objectives give whole number measurements, any small differences 
 being removed by properly adjusting the tube of the microscope. 
 
 Lately, this kind of ocular has been also adopted by the firm of 
 Zeiss, who now make their No. 6 compensating ocular give, with 
 their apochromatic objectives, an exact value for each division. However, 
 we find the base adopted by Messrs. Zeiss for the value of the 
 divisions to be much less convenient than our own. 
 
 With our own ocular we can make in a few minutes, without any 
 chance of error, a series of measurements, which by the ordinary 
 method would take ten times as long. 
 
 The size of microscopical objects can be expressed in several ways. 
 M. Harting has with good reason suggested that the millimetre be 
 discarded as the base of measurement, and that the unit be the 
 thousandth of a millimetre ( - ooi mm.), which he proposed to call 
 a mikron, and represent by fi. An object measuring 15/x would there- 
 fore be i5-i,oooths of a millimetre, which might also be written 
 '015 mm. 
 
 This mode of expression is far more rational than having to quote, 
 
 for example — of a millimetre, &c, since fractions convey but an 
 293 
 
 inadequate idea to the mind, because it is necessary, in order to 
 appreciate their value, to reduce the unit of measurement to decimal 
 fractions. 
 
 Experience has shown us that a thousandth of a millimetre is 
 much too small a unit for measuring diatoms. We are nearly always 
 obliged to use a large number, without any adequate reason, since 
 their size varies so much that their maximum may be double their 
 
MEASUREMENT OF MAGNIFYING POWER. 247 
 
 minimum size. In this case then the hundredth of a millimetre should 
 be used, and we are thus in the same position as when both the 
 metre and centimetre are used in expressing macroscopic measurements. 
 
 2. — Measurement of Magnifying Power. 
 
 It is a very easy matter to measure the magnifying power of a 
 microscope. Two methods are open to us. 
 
 In the first place, a micrometer (or a millimetre divided into a 
 hundred parts) having been placed on the stage, its image can, by 
 means of a camera lucida, be projected on to a sheet of paper, placed 
 at a distance of 250 millimetres (10 inches) — the conventional distance 
 of distinct vision — from the ocular. A certain number of lines, ten 
 for example, projected on the paper are then traced with a pencil. 
 Upon the scale thus drawn on the paper is placed a two-decimetre rule 
 divided into millimetres, and the number of centimetres which a 
 division of the drawn scale occupies is ascertained. If a division of 
 the drawn scale is equal to a centimetre, the micrometer, divided into 
 hundredths of a millimetre, the magnification will be 1,000. By drawing 
 ten divisions, and taking the mean, a more exact measurement will be 
 obtained. It is evident that in the case supposed (the magnification 
 being 1,000) ten divisions will be equal to a decimetre. By drawing 
 several divisions we render the fractions of a centimetre more noticeable. 
 It can thus be easily ascertained that ten divisions, occupying ten 
 centimetres together with two millimetres, the magnification will be 1,020, 
 while if only a single division had been taken the small fraction would 
 not have been observed. 
 
 The second method, called double vision, is more difficult and 
 requires some practice. As before, the paper is placed at a distance 
 of 25 centimetres, and the left eye is directed down the microscope 
 whilst the right eye looks at the paper. The two images then coincide, 
 so that the projected lines of the micrometer can be drawn on the 
 paper; from this point we proceed as before. 
 
 This method requires a great deal of practice, but it has great 
 advantages with high powers, because the field of the microscope is 
 already dark enough, and by using the camera lucida, which further 
 diminishes the light, serious difficulties arise. 
 
2 4 8 
 
 DRAWING MICROSCOPIC OBJECTS. 
 
 3. — Drawing Microscopic Objects. 
 
 Microscopic objects can be drawn either by double vision or with 
 the camera lucida. The paper having been placed on the table, 
 preferably at a distance of 250 millimetres from the ocular, provided 
 that it does not make drawing too inconvenient or fatiguing, the image 
 of the object, projected on the paper, is drawn in with a pencil. 
 
 It is very necessary in this case that a suitable illumination be found 
 by experiment, shewing distinctly at the same time the object, and 
 the tip of the pencil. To ensure success, the paper and object should 
 be illuminated as nearly as possible the same. This is done by 
 either suitably fixing the mirror or shading the paper with a screen. 
 Experience and practice will assist us most here. 
 
 It is most important to always signify on each drawing what magni- 
 fication has been used. This is easily done, because we have only to 
 measure once for all what the magnifying power of the objective used is, at 
 the distance at which one draws. This magnification is expressed by a 
 fraction, whose numerator is the amplication, and whose denominator 
 
 is unity, thus indicates a magnification of 500 diameters or 500 
 
 times the unit which represents the real size of the object. 
 
 As we have said before, it is not essential that there should be a 
 distance of 250 millimetres between the camera lucida and the paper. 
 
 Preference is given to this distance, because the magnification is 
 then immediately known, and, moreover, it is the distance of normal 
 vision with most observers. There is nothing to prevent us from 
 drawing directly on to the table, if that distance is more convenient, 
 as is often the case. The only essential point is to note the exact 
 magnification used, and also it is advisable to state what objective has 
 been employed. It can be easily understood that any given magnification 
 for the drawing of a delicate object will be much more valuable if 
 made with an apochromatic objective of i' 4 N.A., than if it were 
 obtained with a powerful ocular and a low power which had an N.A. 
 of only "4. 
 
PHOTO-MICROGRAPHY. 
 
 All that the eye can see through the microscope can be reproduced 
 by the sensitized plate. Sometimes indeed this discloses details which the 
 observer cannot see, for a large number of rays, e.g., the ultra-violet, 
 which do not affect our eyes, act nevertheless on the sensitized photo- 
 graphic plate. 
 
 From the time when photography was first discovered, the idea of 
 employing this new art for fixing microscopic images was constantly 
 entertained. As early as 1844 Donne and Leon Foucault published 
 their "Atlas cPanatomie microscoftique" which was designed from daguerreo- 
 types. 
 
 We believe that Bertsch, about 1857, produced the first paper proofs. 
 Among the very first workers Belgium can claim M. A. Neyt, of Ghent, 
 who worked with Bertsch at photo-micrography, and who at the present 
 day still occupies one of the first positions among photographic enthusiasts. 
 The work, published in 1887, by Messrs. E. Van Beneden and A. 
 Neyt, on Ascaris megalocephalus, included some particularly successful 
 photographs, although the thick ink, used in printing, gives a very 
 poor rendering of the remarkable clearness of the negative, as is 
 invariably the case. M. A. Neyt devoted himself to astronomical 
 photography at a time when it had scarcely been thought of, and he 
 has published a very remarkable photograph of the moon. 
 
 At the International Exhibition at Antwerp we exhibited some photo- 
 graphs taken by us in 1862. 
 
 Among micrographers who have devoted themselves successfully to 
 photo-micrography, the late Dr. Woodward and Dr. Maddox, of whom, 
 fortunately, the science has not yet been deprived, must be mentioned first. 
 
 Nachet, De Brebisson, Castracane, Hon. J. D. Cox, C. Houghton 
 Gill, Pringle, Dr. Neuhauss, Dr. Moitessier, R. Koch, Th. Comber, 
 Dr. R. Zeiss, and our excellent pupil, M. Gife, of Antwerp, have done 
 credit to photo-micrography, and produced remarkable photographs. 
 
 When, in 1865, in the first edition of our work on the microscope, 
 we set forth the processes then in use, photo-micrography could only 
 boast of a few adepts. At that time, indeed, it was neither convenient 
 nor agreeable to micro-photograph. 
 
 Besides being troubled with photographing with collodion, the 
 staining of the hands, the very long exposures during which the coating 
 
250 
 
 PHOTO-MICROGRAPHY. 
 
 dried unevenly, the frequent spoiling of the negatives from being unable 
 to always anticipate the correct time of exposure, &c, shewed that an 
 altogether special contrivance was required, and that only an intense 
 source of light could be used ; sunlight nearly always had to be resorted 
 to, and the cuproammoniacal bath, together with the heliostat, were 
 indispensable to anyone who wished to do serious work. 
 
 A great change has taken place since then. Photo-micrography, thanks 
 to gelatinobromide, is at everyone's disposal ; indeed, it is a process 
 that should be used by every serious microscopist, for it is by no means 
 difficult, and can render most valuable assistance. It can quickly 
 and conclusively test the truth of any statement made by the 
 observer, and it can also be made to keep an exact record of all 
 his researches. Moreover, it can often accomplish more than direct 
 observation can, for, as we have already stated, certain rays, e.g., the 
 ultra-violet rays, still affect the sensitized plate when the eye is no 
 longer able to perceive them. 
 
 We shall describe in succession : — 
 
 1. The installations. 
 
 2. The apparatus used. 
 
 3. The illuminant. 
 
 4. The photographic processes. 
 
 § 1. The Installations. 
 
 Three typical installations can be mentioned : — ■ 
 
 1. Woodward's Installation. — The microscope is placed in a horizontal 
 position, on a firm support, e.g., some masonry. 
 
 In front of the instrument is placed the source of light; behind 
 the microscope slides, on rails, a carrier, to which is attached the frame 
 containing the sensitized plate. Of course, the whole room is kept as 
 dark as possible, and no other light is admitted but that which passes 
 through the lenses of the microscope. 
 
 This is an excellent method, and our late friend, Dr. Woodward, 
 of Washington, produced negatives which have never yet been surpassed, 
 but this is only suitable to one who is willing to make a speciality of 
 photo-micrography. 
 
 2. The ordinary method. — The microscope is placed in a horizontal 
 position, the ocular is either removed or left in, and the end of the 
 tube of the microscope is connected with an ordinary photographic 
 
PHOTO-MICROGRAPHY. 25 1 
 
 camera. The image is focussed on the ground glass, which is then 
 raised and replaced by the slide containing the sensitized plate, proceeding 
 in just the same way as in ordinary photography. 
 
 As a source of light, daylight, electric light, or even a petroleum 
 lamp, can be used. 
 
 This is the arrangement usually adopted by micro-photographers. 
 
 3. The Author's Installation. — After having employed the two preceding 
 methods for a long time, we definititely abandoned them, for reasons 
 given by us, in March, 1885, in the "American Monthly Microscopical 
 Journal," which we will summarize here. In our opinion photography 
 ought not to be an end, but a means, in the hands of the micrographer, 
 i.e., photography should be employed not for the pleasure of producing 
 photographs, however beautiful or interesting they may be, but as a sub- 
 stitute for the camera lucida, when we desire to represent complicated 
 and extremely delicate details, or still more when an irrefutable proof 
 is necessary in support of some new and contested fact. 
 
 Now for such a purpose the preceding methods are not very con- 
 venient ; it is by no means a matter of certainty, when we are 
 dealing with an object having very delicate details, that the same 
 conditions of illumination can be again obtained when the microscope 
 has been deranged : in any case one would only succeed after great loss 
 of time ; we have therefore thought of a really practical method, which 
 we have carried out in two different ways. 
 
 First, by a method giving, without trouble, small photographs. 
 For this, we have constructed a small camera, which is very light, 
 and capable of holding a sensitized gelatine-bromide plate 5! c. x 4^ 
 c. (2 - 2in. x i - 8in.). The front of the camera terminates in a copper 
 tube, carrying an achromatic concave lens or amplifier. The tube of the 
 camera fits nicely into the tube of the microscope, and the amplifier is 
 placed at such a distance from the sensitized plate, that when the image 
 is sharply in focus with Powell and Lealand's No. 1 ocular, it is 
 also in focus on the sensitized plate. This effect, thus produced 
 automatically, gives with Zeiss's i-i2th homogeneous objective a magni- 
 fication of 300 times. If a larger magnification is required, the photo- 
 graph must be enlarged. 
 
 A ground glass can be substituted for the frame when we wish 
 to use a different objective to that for which the apparatus is 
 constructed. 
 
 It is evident how very convenient this arrangement is. The apparatus 
 is always at hand and in working order. If an interesting object 
 
252 
 
 PHOTO-MICROGRAPHY. 
 
 is seen, in a moment, without deranging the microscope, the photograph 
 desired can be taken by simply changing the ocular. 
 
 This arrangement has been very useful and of great service to 
 us, but we have modified it for some time since, so as to obtain, 
 without enlargement, photographs of every size up to 5 x 7. This 
 is our second method, which we find is preferable whenever details or 
 work requiring great precision have to be photographed. The 
 apparatus, which is extremely simple, will be described in the following 
 section. 
 
 § 2. Apparatus. 
 
 The apparatus, which appear in manufacturers' catalogues, are very 
 numerous. We shall review those which to us appear to be the most 
 ingenious. 
 
 A. NACHET (17, Rue St. Severin, Paris). M. Nachet, who is 
 himself a distinguished photo-micrographer, sells several very ingenious 
 pieces of apparatus. 
 
 Fig. 168. 
 
 The large Micro-photographic Apparatus (fig. 168) is a fine instru- 
 ment, consisting of a solid base with grooves made very exactly, so 
 as to allow the part of the camera marked B, which carries the 
 bellows, to slide for a distance of two metres (6^ft.) It is also provided 
 with a rod at the side in two parts, terminating near the microscope 
 in a pulley A, to which is attached either a small piece of string or 
 
PHOTO-MICROGRAPHY. 
 
 253 
 
 an india-rubber band, communicating with the fine adjustment milled 
 head. If we wish to work with short extension, the base board is 
 folded by means of the hinge G; the apparatus is most frequently 
 used in this manner. If, on the other hand, we wish to work with 
 long extension, the base board is replaced and the end of the move- 
 able rod F is united again to the button D, on which is bored a 
 tapering groove, so as to make the two rods rigid, the hinder part of 
 the rod resting on a guiding post, which can be taken off at will in 
 order to allow the table to be folded. One can focus on to the 
 ground glass or inside on a Bristol board by opening the lower side 
 door V. This camera can hold plates of all sizes up to the full 
 French plate (18 x 24 c. = 7 x gh inch). 
 
 The microscope is connected with the camera by means of a system 
 of tubes, which excludes all light from without. 
 
 Nachet has adapted to his photographic microscope a very useful 
 modification (fig. 169). It is a rectangular box, containing a total 
 reflexion prism, which can, when desired, by means of a rack and 
 pinion, be introduced across the path of the rays coming from the 
 objective. Every facility is thus obtained for putting the object in the 
 position required, for arranging the illumination in the best manner, &c. ; 
 and when all is found to be satisfactory the prism is withdrawn, and 
 nothing further has to be done beyond sharply focussing on the ground 
 glass. 
 
 Nachet also manufactures an apparatus for instantaneous proofs, which 
 works perfectly. 
 
 Fig. 169. 
 
2 S4 
 
 PHOTO-MICROGRAPHY. 
 
 This instrument (fig. 170) is 
 based on the principle of a micro, 
 scope, with two body tubes. 
 Above the objective (fig. 171) a 
 prism reflects the image into the 
 body tube at the side containing 
 
 Fig. 170. 
 
 Fig. 171. 
 
 the ocular. Observation is thus 
 carried on as with an ordinary 
 inclined microscope. The 
 camera, solidly mounted on 
 pillars, contains the sensitive plate, 
 from which all light is shut off, 
 the reflecting prism itself serving 
 as a shutter to the objective. 
 While the observer is examining the object his finger is placed on 
 the catch A, but when he is satisfied with the image, a slight pressure 
 on the catch withdraws the prism and allows the image to pass for a 
 very short time. With a vivid light, such as the sun, electric light, or 
 oxy-hydrogen light, the negatives are produced perfectly. 
 
 Focussing on the glass screen is produced by a special arrangement 
 of lenses, contained in the ocular tube, by means of which each 
 observer must regulate his focus once for all in such a way that when 
 the image is quite clear in the ocular it is equally in focus on the 
 photographic plate. 
 
PHOTO-MICROGRAPHY. 
 
 255 
 
 A small camera (figs. 172 and 173), which can be sufficiently under- 
 stood by the illustrations, appears to be a very good apparatus for 
 elementary photography. The apparatus is neither cumbersome nor 
 
 Fig. 172. 
 
 expensive, and by it photographs can be taken with the microscope in 
 any position. 
 
 Lastly, Nachet's large inverted microscope (fig. 174) is also an excellent 
 apparatus for photography. With this it is only necessary to replace the 
 ocular by a camera. 
 
 Thus arranged, it is an instrument of extreme stability, and probably 
 superior to any other combination. It is not affected by tremors, either 
 from passing vehicles or from any other cause. We have had the curiosity 
 to experiment with it by the side of our gas engine, and have obtained 
 
256 PHOTO-MICROGRAPHY. 
 
 in that position perfectly clear images. Our friend, M. Neyt, writes us : 
 " I have employed it while a hole was being pierced in the kitchen 
 wall." 
 
 Fig. 174. 
 
 The only inconvenience is that projection oculars cannot be used with 
 it. When this apparatus is used with apochromatic objectives, a 
 concave achromatic lens, which will make the focus of the objective 
 coincide with the surface of the sensitive plate must be placed im- 
 mediately above the objective. 
 
PHOTOMICROGRAPHY. 
 
 With this arrangement the instrument gives perfect results, which 
 are obtained with extreme ease. We have obtained excellent negatives 
 of the Amphipleura with it. 
 
 CARL REICHERT (viii., 26, Bennogasse, Vienna). M. Carl 
 Reichert makes two kinds of photographic apparatus. 
 
 The large apparatus (fig. 175) is mounted on a table of polished 
 wood, which has at one end a camera, moved by a rack and pinion, 
 
 Fig. 176. 
 
 and at the other end the illuminating apparatus, which consists prin- 
 cipally of a large mirror moved by clockwork, a condensing lens L, 
 and a bath with parallel sides d, in which is placed either a solution 
 of alum or a coloured solution. The microscope is placed in the 
 centre. The illustration sufficiently explains the details of construction. 
 
PHOTO-MICROGRAPHY. 
 
 2 59 
 
 The other apparatus (fig. 176) is intended to be placed above the 
 microscope ocular. This apparatus is very convenient in all cases 
 where only a small microscope is used, and where only preparations 
 which do not require extreme precision have to be photographed. 
 
 DR. CARL ZEISS (Carl Zeissstrasse, Iena). The firm of Zeiss 
 have made a special study of photography, and one of the heads of 
 the firm, Dr. R. Zeiss, has acquired a great reputation for his photo- 
 micrographical work. A large number of the illustrations which appear 
 in their Photo-micrographical catalogue are really very beautiful speci- 
 mens, and show that he has known how to avail himself fully of 
 the exceptional resources at his command. The catalogue of this firm 
 present various models for workers. 
 
 Fig. 177. 
 
 The Large Apparatus (fig. 177) is an excellent instrument, very 
 complete and perfect, but it is too complicated for ordinary work. Its 
 costliness, moreover, puts it out of the reach of all but the more 
 wealthy photographers. 
 
 Its principal parts consist of a special microscope M, which is placed 
 with the illuminating apparatus on a table made of cast-iron. A second 
 table of the same material, which can be withdrawn from the first, carries 
 the camera K and a jointed hand lever, by which the fine adjustment of 
 the microscope can be regulated. The front part of the camera can be 
 swung back and be used as a vertical apparatus. 
 
PHOTO-MICROGRAPHY. 
 
 Fig. 178. 
 
 The optical parts (fig. 178) on 
 the smaller tube consist of a plane 
 mirror G, two screens F and E, 
 and of a support for the baths H. 
 
 An electric arc lamp and its 
 accessories can also be adapted to it. 
 
 This arrangement, as may be 
 seen, fulfils all the worker's require- 
 ments, but we repeat that the 
 apparatus is too complicated and 
 too expensive. The truth of this 
 has, we know, been felt by Messrs- 
 Zeiss, who have for some time past 
 produced a simpler but sufficiently 
 complete apparatus. 
 Lately Messrs. Zeiss have con- 
 structed a vertical apparatus (fig. 179), 
 composed of a solid camera of ample 
 length, sliding up and down a rigid 
 bar, divided and carried on a large 
 cast-iron tripod. 
 
 The support bar S can be turned 
 round on its axis in the iron socket 
 B, which rises from the centre of the 
 tripod. A clamping screw A enables 
 the bar to be fixed in every position 
 of its rotary movement. 
 
 These arrangements are very happily 
 conceived. They render the apparatus 
 really useful, and enable it to be used 
 in researches requiring the greatest 
 precision. 
 
 Various useful accessories can be 
 obtained, when desired, from the makers 
 separately. Amongst others are : — 
 
 A diaphragm apparatus, mounted on a 
 stand, with a complete series of diaphragms 
 and ground glass focussing plates. 
 
 A bath carrier with two baths. 
 
 A bi-convex lens mounted on a 
 stand with tripod, &c. 
 
PHOTOMICROGRAPHY. 
 
 26l 
 
 M. E. LEITZ, of Wetzlar, also makes a small photographic 
 apparatus, which differs from the preceding ones by being adaptable 
 to the microscope when in an inclined position. 
 
 The apparatus consists of a camera suspended between two 
 pillars which, by means of a clamping screw, can be fixed at any 
 inclination. The inner tubes of the pillars can be drawn out by 
 
 Fig. 180. 
 
262 
 
 PHOTO-MICROGRAPHY. 
 
 means of rack and pinion adjustments (fig. 180), and so the length 
 of the pillars can be increased. Thus photographs can be made at 
 different magnifications. They must* however, be kept within sufficiently 
 restricted limits, because the camera cannot be lengthened to any 
 great extent, especially when the apparatus is in an inclined position. 
 
 DR. HENRI VAN HEURCK'S Photo- Micrographic Apparatus. 
 The apparatus, which we have used for the last three years, and which 
 is the result of every kind of experiment, has the advantage of being 
 perfectly stable, as cheap as possible, and extremely simple. Any intelli- 
 gent carpenter can make it. 
 
 Several workers have come and asked us to photograph various 
 objects for them: one some starch or fibre; another, histological tissue; 
 and another, diatoms, &c. We have never had occasion to make a 
 second experiment. 
 
 Our apparatus (fig. 181) consists of an oblong wooden box mounted 
 solidly on four legs, (') carrying at the top the frame for the ground 
 
 glass screen, &c, and it can be placed 
 on the work table on which the 
 microscope stands. In the bottom of 
 the box is an aperture, which can 
 be closed at will by a small funnel 
 of black cloth attached to it. The 
 height of the legs is sufficient to 
 allow the tube of the microscope to 
 pass through the aperture into the 
 box, so as to be from five to ten 
 centimetres (two to four inches) above 
 the bottom. 
 
 The apparatus is always used in 
 a vertical position. For thus alone 
 
 can work be done conveniently, and 
 Fig. 181. . ■" 
 
 in this position only can such fluid 
 
 liquids, as the essence of cedar, mono- 
 bromide of napthaline, iodine of methyleine, &c, which are now used 
 as immersion liquids, be kept in place on the condenser and prepara- 
 tion. 
 
 (') It would be more convenient to have only three feet, but then we should consta 
 be interfering with the movements and illumination, 
 
PHOTO-MICROGRAPHY. 
 
 263 
 
 The great essential difference between our apparatus and every one 
 similar to it is, that the whole front of the box opens. 
 
 Owing to the breadth of the box, which is 25 centimetres (9! inches), 
 one's head can be very conveniently placed inside the camera, where 
 the eye of the observer is screened from all light but that coming 
 from the tube of the microscope. The image can thus be focussed 
 
 Fig. 181a. 
 
264 
 
 PHOTO-MICROGRAPHY. 
 
 under the most favourable conditions. When this is finished, the 
 ordinary ocular is replaced by a projection ocular (if that is necessary) 
 and the camera door is closed. 
 
 By standing on a chair or stool, the focus is tested for the last 
 time on plate-glass (ground-glass being unable to give sufficient precision) 
 by means of a focussing lens, and the plate is then exposed. 
 
 The outside dimensions of the box are fixed and invariable, but a 
 system of racks in the inside allows the distance between the sensitive 
 plate and the microscope to be modified. However, experience has shewn 
 us that this is needless, for by combining apochromatic objectives with 
 various compensating oculars, any series of magnifications may be obtained. 
 
 It is the invariable length of the arrangement which gives such 
 rigidity to the apparatus, and enables the most delicate objects and 
 their minutest details to be photographed. Our camera is 50 centi- 
 metres (19I inch) in length. If this dimension be increased, it is 
 impossible to manage the fine adjustment conveniently while looking at 
 the image on the plate-glass. 
 
 This length is really more than sufficient. With an objective of 
 i-ioth inch, magnifications of 500 and 1,000 diameters, with projection 
 oculars 3 and 6, are obtained, and of 2,000 and 3,000 diameters, with 
 compensating oculars 12 and 18. 
 
 It was in this way, with an objective of N.A. i*6, that we produced 
 the negatives of the " Tests " which appear in this Work. 
 
 Messrs. Watson and Son make our photographic apparatus, and 
 supply it for ^4 15s. (fig. 181a). 
 
 § The Illuminant. 
 
 To obtain good negatives an illuminant, which is both convenient and 
 powerful, must be employed to produce what in photography is called 
 contrast, i.e., that which sets off the object against the background. 
 If the light is too feeble the image will always be flat, and the details 
 will only be slightly visible or not at all. 
 
 The luminants in photo-micrography may be classed according to 
 their intensity, as follows : — 
 Sunlight. 
 
 Electric arc light. 
 
 Electric incandescent light. 
 
 Oxy-hydrogen light. 
 
 Light from a mineral oil lamp. 
 
 Light reflected from white clouds. 
 
PHOTO-MICROGRAPHY. 
 
 265 
 
 Sunlight. — In order to use sunlight suitably, a heliostat must be 
 employed. The simplest, most practical, and, at the same time, least 
 expensive heliostat that we know of is that of Prazmowski, which is sup- 
 plied in Paris by Messrs. Bezu, Hauser and Co., and in Potsdam by 
 Hartnack. We have used one for several years, and think it deserving 
 of all praise. 
 
 The instrument (fig. 182) consists of a solid piece of clockwork 
 which, once in 48 hours, turns an axis on which can be adjusted a 
 square mirror, which is thus set in rotation. On the circumference of 
 the drum, containing the clockwork, is placed a clock dial, on which 
 the hours are divided into intervals of ten minutes. The drum itself 
 is held by a support which is fixed to a horizontal surface, and allows 
 it to be inclined so that the axis of the clockwork coincides with the 
 direction of the earth's axis (in the place where the instrument is 
 being used). 
 
 This direction, which is given by the latitude of the place, need 
 not be known by the operator, the instrument setting itself automatically, 
 so to speak, both with regard to latitude and the declination of the 
 sun corresponding to the day of the year, at the same time. 
 
 Moreover, when it is set, the instrument will be fixed in the posi- 
 tion demanded by the latitude, by means of a clamping screw acting 
 
 on a limb which carries 
 latitudes marked from 
 o° to 70 0 . 
 
 To set the instru- 
 ment, after the clock- 
 work has been wound 
 up, it is placed on a 
 perfectly horizontal sur- 
 face, and having re- 
 moved the mirror, a 
 metal rule, diametri- 
 cally placed with regard 
 to the dial, is fitted 
 into the axis of the 
 clockwork, which passes 
 through it like a spin- 
 dle. The two extremities of this rule end in perpendicular arms : 
 the shorter one is pierced with a small hole (a pinnule in fact), the 
 longer one is marked with divisions corresponding to the equation of 
 
266 
 
 PHOTO-MICROGRAPHY. 
 
 time and the declinations of the sun for every ten successive days. At 
 the end of the shorter arm the rule is pierced with a slit, through 
 which the numbers of the hours engraved on the dial can be seen. 
 To set the apparatus at any hour of the day, the rule is turned round 
 the axis, like the hand of a watch, until the figure representing the 
 hour and fraction of the hour at which operations are made (the time 
 must be taken from a well-regulated watch), appears in the slit, and the 
 division representing it on the dial coincides with an index placed on 
 the edge of the slit. 
 
 To set the instrument definitely it has only to be turned horizontally 
 on the table, and inclined more or less on its support, until a ray from 
 the sun passing through the hole of the pinnule is depicted on the line 
 of declinations, upon the branch opposite the rule, while, at the same 
 time, a small image of the sun falls exactly on the point corresponding 
 to the day of the year. 
 
 This operation takes but a few moments, and, as can be seen, is 
 very easy. 
 
 This done, the instrument is set ; the screw regulating the inclination 
 on the circle of latitudes is screwed up, the rule is removed, and the 
 shank of the mirror, which can be turned without acting on the 
 clockwork, is pushed into the axis of the clockwork. A steady horizontal 
 ray is thus obtained, which can be reflected on to another plane mirror, 
 placed at a short distance, and fitted with ball and socket adjustment 
 so that the ray can be turned in any desired direction. 
 
 We may add that if the hour is not exactly known, the instrument 
 can be regulated with sufficient accuracy by setting it at about mid-day. 
 One can, however, operate by first setting it at nine o'clock in the 
 morning, and then again at three in the afternoon. In each of these 
 operations a line is traced on the table with a pencil, the foot of the 
 instrument being used as a ruler. These two lines form an angle, which is 
 divided into two equal parts by a bisector, along which the foot of the 
 heliostat is placed. In this way the instrument is set for mid-day. 
 
 The clockwork specially constructed for this heliostat is very exact 
 and solid; it has an anchor escapement, and can move a mirror a great deal 
 larger than the one adapted to it. By means of a small dial placed 
 on the drum, and divided into sixty minutes, on which a minute hand 
 moves, the regularity of its movement can be verified. The hour dial, 
 and the division into days on the square, are enamelled, and consequently 
 prevent accidents and mishaps. The instrument can be used in al! 
 places situated between the equator and a latitude of 70 0 , 
 
PHOTO-MICROGRAPHY. 
 
 267 
 
 The heliostat is accompanied by a second mirror, mounted on a 
 heavy foot, with ball and socket motion, and the whole is enclosed in 
 a box, which is of small dimensions and very portable. To use the 
 heliostat, it is first arranged in position, and then fixed according to 
 the instructions which have just been given. Then the large mirror is 
 placed so that the solar rays strike the second mirror, which is mounted 
 on a foot and placed near the instrument. 
 
 The larger mirror as it turns sends the solar rays on to the mirror 
 of the microscope, which is placed in the most convenient position for 
 illuminating the object. Notwithstanding these successive reflections, which 
 might be expected to absorb a great deal of the light, sufficient, however, 
 remains for obtaining a good negative in an exceedingly short time. 
 
 It is superfluous to say that in the path of the rays should be 
 placed a vessel having parallel sides, containing a coloured liquid, giving 
 a monochromatic light, and, at the same time, serving to protect the 
 eye-sight while focussing, &c. If apochromatic objectives be used, this 
 vessel can, if required, be dispensed with during exposure, although it 
 is much better to leave it ; but if simple achromatic objectives are used 
 it is indispensable to leave it in position. The cuproammoniacal solution, 
 giving blue monochromatic light, is to be preferred. 
 
 This solution is obtained by dissolving one part of powdered sulphate 
 of copper in four parts of liquid ammonia. 
 
 A certain amount of distilled water is then added, so as to give 
 a moderately dark solution, depending on the intensity of the sunlight, and 
 also on the distance between the two sides of the vessel, i.e., upon 
 the width of the coloured liquid. 
 
 Electric Light. — The voltaic arc light has little to recommend it 
 because it is so unsteady and difficult to produce. Incandescent light 
 is, on the other hand, very advantageous, and we invariably use it when 
 we cannot employ sunlight, which very rarely happens in our climate. 
 We have already shewn in the first part of this work how the electric 
 light is produced and used. We will only here add that when using 
 the incandescent light we prefer the photophore, and we generally arrange 
 it so that the light of the luminous source reaches the condenser 
 directly without the intervention of the mirror. 
 
 By using electric incandescent light and Zeiss's achromatic con- 
 denser, which gives a wide image of the luminous source, photographs 
 can be taken in the image of the flame without difficulty with 3mm. 
 (i-8th inch) and 2mm. (i-i2th inch) apochromatic objectives. The 
 whole secret lies in using suitable lamps, having a sufficiently large 
 
268 
 
 PHOTO-MICROGRAPHY. 
 
 and straight filament. Mons. Trouve, of Paris, supplies such lamps, 
 which leave nothing to be desired. 
 
 Oxy-hydrogen Light. — Oxy-hydrogen light, which is produced by 
 a jet of oxygen and hydrogen, and makes a cylinder of lime incandescent, 
 is admirably adapted to photo-micrography, and English amateurs use it 
 very largely. It is, however, in every respect inferior to electric light. 
 
 A burner with separate jets is generally used. Ordinary house gas 
 is used instead of hydrogen, and the oxygen is bought ready made, 
 compressed in steel cylinders. These cylinders, which are proved to a 
 pressure of 250 atmospheres, contain the oxygen compressed to 125 
 atmospheres. The pressure naturally diminishes as the gas is used ; it 
 is therefore necessary, in order to have constant pressure, to make use 
 of a "pressure regulator" which is provided, if desired, with the 
 cylinders. When a cylinder is empty it is returned to the maker, 
 who supplies another in its place. The cylinders can, therefore, be 
 bought or hired. 
 
 There is a manufactory, where oxygen is compressed, in the Rue 
 Gavarni in Paris. 
 
 We have already stated that ordinary gas can be taken from the 
 mains, but this can only be done where separate jets are used, for when 
 the " mixed jet " is used, in which case the two gases are mixed before 
 being lighted, it is absolutely necessary to employ two india-rubber bags, 
 and to submit each to an equal pressure, otherwise an explosion may occur. 
 
 When ordinary gas is not at hand, a special apparatus can be used, 
 in which ether vapour can take the place of gas. Although inferior 
 to the light produced by ordinary gas (which again is not equal to 
 that given by pure hydrogen), the light obtained in this way is very 
 serviceable. The apparatus can be bought at Antwerp, from Messrs. 
 Van Neck and G. Brand, and in London, from Messrs. Watson and Son, 
 and others. 
 
 We give here an illustration of the photographic apparatus, made 
 as specified by our friend Mr. A. Pringle, the skilful English photo- 
 micrographer. This apparatus (fig. 183) is specially intended for use 
 with the oxy-hydrogen light. 
 
 Lamp Light. — By means of an oil lamp very good negatives may 
 be obtained, especially if the magnification adopted is only moderate. 
 
 Mr. J. D. Cox, one of the most skilful American diatomists, has 
 thus obtained an entire series of photographs, which he has used to 
 illustrate his beautiful work on the structure of the valves of diatoms. 
 However, long exposures are necessary when an oil lamp is used, and 
 
270 
 
 PHOTO-MICROGRAPHY. 
 
 thus the focussing is liable to vary, and, moreover, there^ is very little 
 contrast between the object and the background. 
 
 White Cloud Light. — What we have just stated about lamp light 
 applies also to light reflected from white clouds. It can only be 
 used for very low powers, and always with the risk of an alteration 
 in the focussing. 
 
 4. — The Photographic Processes. 
 
 It still remains for us to describe how to take a photograph. We 
 shall do so very briefly, especially as the subject is very simple, and, 
 thanks to the wonderful invention of " gelatino-bromide," by our friend the 
 skilful English microscopist, Dr. Maddox, photographic processes are so 
 well known that at the present day almost every one has some know- 
 ledge of photography. 
 
 The production of a photograph involves, as is very well known, 
 two operations, the production of the negative and the paper print. 
 
 1. The Production of a Negative. — A negative is produced with our 
 apparatus in the following way : — After having placed in the field of 
 the microscope the object to be photographed, the camera is arranged 
 above the instrument, so that the end of the tube is 5 or 6 centi- 
 metres (2 or 2\ inches) above the bottom of the interior. All light 
 outside of the tube is carefully stopped, first by fastening round the 
 tube the small bag made of black material, and then by sliding over 
 the tube of the microscope a small disc of black cardboard. The hole 
 in this disc is such that it just rests on the projecting rim of the tube. 
 
 The object is then carefully focussed and suitably illuminated in 
 accordance with the instructions which will be given further on. 
 
 When the image appears to be satisfactory, a projection ocular is 
 substituted for the ordinary ocular, and the camera is then closed. 
 
 It must then be ascertained whether the image falls well in the 
 centre of the giound glass screen ; if so, a plane glass is substituted 
 for the ground glass screen. With the assistance of a lens correctly 
 focussed for the under surface of this glass, one can see whether the 
 details of the image are as clear as possible ; if not they can be 
 corrected by means of the fine adjustment. 
 
 A card screen is now placed in front of the microscope to prevent 
 any light reaching the instrument. The dark slide containing the 
 sensitive plate is now substituted for the glass screen, and then the 
 plate is exposed by removing the card screen for whatever length of 
 
PHOTO-MICROGRAPHY. 
 
 271 
 
 time is thought to be right. This length of time having elapsed, the < 
 card screen is returned to its place to cut off the light, the dark slide 
 is closed, and taken into the dark room. The dark room should be 
 quite light-tight ; all light, whether natural or artificial, used during the 
 development of the plate, should be filtered through orange or ruby-red 
 glass. It is even advisable to cover up the developing dish with a card 
 until the image has made its appearance. 
 
 Development is either effected by placing the plate in the developer, 
 which is previously poured into a developing dish, or by pouring the 
 developing mixture over the plate, in a developing dish, film upwards. 
 
 The developer is kept constantly moving over the surface of the 
 exposed film. When the image does appear with every detail, and has 
 acquired sufficient density, which can be tested by looking through 
 the plate held up to the light, it is then carefully washed under a 
 jet of water, and afterwards placed in the fixing bath. 
 
 The plate is fixed when no white part is seen on its under-surface. 
 It is then washed afresh, and placed for some minutes in a saturated 
 solution of alum, which has the advantage of preserving the gelatine 
 film, at the same time rendering it more transparent, and making 
 it adhere firmly to the glass carriage. 
 
 The plate, having been taken out of the alum bath, is placed upright 
 against the sides in a large tank containing soft water. It is left there 
 for 11 or 12 hours, but the water of the tank is changed two or three 
 times, so that the last traces of the fixing solution may be dissolved and 
 removed by the running water. The plate, when taken from the water, 
 is allowed to dry of its own accord (without the intervention of heat, 
 which dissolves the gelatine), and then printing can be commenced. 
 
 Developer. — There are a large number of substances which can be 
 used for developing the photographic image. Iron, pyrogallic acid, hydro- 
 quinone, eikonogen are used most frequently, and can be made up 
 according to the formulae, which are found printed on every box of plates. 
 
 All these developers, in the hands of a skilful manipulator, give 
 good results, provided the plate has been given the correct exposure. 
 The whole secret lies in correctly estimating the exposure and in 
 managing the developer skilfully. 
 
 After having tried a great many formulas, we invariably use either 
 "P. Mercier's Perfect Hydroquinone and Eosine developer" (') or a 
 formula which we give below. 
 
 ( 1 ) Faubourg, St. Martin, 158, Paris. 
 
272 
 
 PHOTO-MICROGRAPHY. 
 
 Mercicr's developer, which keeps well for a long time, provided it 
 is preserved in stoppered bottles and kept in the dark, has the advantage 
 of allowing a certain amount of latitude in the time of exposure, while 
 at the same time a correctly-exposed negative can be given any required 
 density. 
 
 We shall now give some supplementary instructions as to development. 
 
 If the exposure has been good, the image begins to show itself in 
 three minutes ; makes its appearance gradually, first the sky and the high 
 lights, then the half-tones with all their details. These are allowed to 
 develop and the image to lose itself in the shadows until the plate is 
 clouded, and begins to assume a nearly uniform tint over its entire 
 surface, except round the edges, which, remaining a pure white, shows that 
 there has been no fog. 
 
 The operation should take at least ten minutes or a quarter of an 
 hour, according to the sensitiveness of the plates. When the image takes 
 longer to appear, whether from under exposure or from want of sensi- 
 tiveness in the plate, or because a bath which has become too weak is 
 employed, the only thing is to await it cautiously ; it always appears 
 as vigorous and detailed as possible, and unless the plate has been 
 exposed to a light it is never fogged ( 1 ). 
 
 As many as ten or twelve plates can be developed in the same 
 bath without its getting dark, i.e., without its losing much of its strength, 
 especially if it is revived from time to time with a little fresh solution — 
 100 to 150 grammes ('/ 6 to \ pint) of liquid being enough for a dozen 
 plates 9 x 12 (3! x 4! in.). 
 
 Care should be taken not to throw away old baths (having been used 
 several times) ; they develop slower, but they are excellent for ordinary 
 work. 
 
 Normal Negatives and over-exposed Negatives. — It is well-known that 
 the great difficulty in photography is to calculate the exact time of 
 exposure depending on the illumination of the subject to be taken, the 
 quality of the plates, and that of the objective, &c. 
 
 With the developers that we used before the above, Ferrous oxalate, 
 pyrogallic acid, and often even with the ordinary hydroquinone baths, 
 
 ( 1 ) It is precisely the slow development peculiar to this developer, which gives the 
 beauty and energy to negatives thus obtained, even with half the ordinary length of 
 exposure ; the liquid has time to re-act on the entire thickness of the exposed film, and the 
 high lights stand out in relief from the darker parts, while objects not so light and details 
 come out equally well. 
 
 Further, owing to the slowness of development, this developer can be manipulated as 
 desired, either by forcing or retarding it, according to the requirements, so that the negative 
 can be withdrawn at exactly the right moment, which therefore always ensures satisfactory 
 results. 
 
PHOTO-MICROGRAPHY. 
 
 273 
 
 an over-exposed plate fogs, assumes a uniform grey tint, and is some- 
 times irremediably lost. 
 
 The Perfect Developer allows a considerable error in the time of 
 exposure to be allowed for, and disappointing miscalculations to be avoided. 
 
 It is sufficient to recollect that when the time of exposure has been 
 almost correct, about three minutes are required with a fresh bath, four 
 minutes with an old bath, for the image to commence to show itself in 
 the high lights. If it appears before this, it is caused by great over- 
 exposure j in this case the plate must be taken out and placed in an 
 older bath, or the bath used must be simply weakened with distilled 
 water. When it is known beforehand that the plate is much over- 
 exposed, a commencement can be made by putting it in an old 
 bath and leaving it there to develop slowly, to try it ; it can be 
 brought on to the desired end by adding, if required, a few drops of 
 new developer from time to time. By following these simple instruc- 
 tions, whenever it is unknown if the exposure has been correct, every- 
 thing is reduced to a matter of care and attention, and there ought 
 never to be a failure. 
 
 With correct exposure the silver is reduced to a beautiful black 
 tint; over-exposure produces a sepia equally good for printing. 
 
 It is evident that an over-exposed negative must not be kept too 
 long in a fresh bath without watching. 
 
 When suitably developed, a plate, although much over-exposed, 
 can be brought out with a light tint as if it had been under-exposed, 
 and it only requires intensifying with bi-chloride of mercury and with 
 ammonia to change the colour and make it capable of producing excellent 
 prints. 
 
 Instantaneous and under-exposed Negatives. — The Perfect Developer, by 
 reason of its extreme energy, is admirably adapted for development of 
 these kinds of images. It is sufficient to use a new bath, and to leave 
 the exposed plate in it for a quarter of an hour, and more if necessary, 
 until, by examining it with transmitted light, it is seen to require no 
 further density. Several successive new baths can be used with advantage, 
 but in these cases it is very necessary to cover the dish with a screen, 
 to avoid even the direct light of the ruby lamp. 
 
 In this way excellent instantaneous negatives are obtained as good 
 and sharp as can be desired, and more vigorous than with any other 
 negative. 
 
 Intensification of the Negative. — A negative which is too thin can be 
 intensified by known methods. One of the simplest and most effective 
 
 T 
 
474 
 
 PHOTO-MICROGRAPHY. 
 
 is mercury intensification. The plate, already fixed and well washed, is 
 placed in a five per cent, solution of bi-chloride of mercury, containing 
 five per cent, of common salt, where it is left to be bleached until it 
 attains the degree of intensification desired. It is then washed in plenty of 
 water, and then sprinkled with a mixture of two or three parts of 
 ammonia to six or eight parts of ordinary water. It darkens while it 
 is acquiring density. 
 
 The ammonia can be replaced by the hydroquinone developer diluted 
 with water, but this method, which is very effective, demands that the 
 preliminary washings be a great deal more thorough. 
 
 One can often obtain good prints with a thin negative by printing in 
 the shade, or in the sunlight with several thicknesses of paper interposed. 
 
 The formula we use instead of the perfect developer is as 
 
 follows : — 
 
 Hydroquinone ... ... ... ... 8 gr. 
 
 Sulphite of soda (chemically pure) ... 50 gr. 
 
 Carbonate of soda ... ... ... ... 50 gr. 
 
 Bromide of potassium ... ... ... 1 gr. 
 
 Eosine ... ... ... ... ... 1 centi.-gr. 
 
 Distilled water ... ... ... ... 1 litre. 
 
 We invariably use a fresh bath, which serves us for several plates ; 
 if the image comes out too quickly we add some water, if it comes 
 out too slowly we strengthen it on the other hand by adding several 
 small quantities of the fresh bath. 
 
 Hydroquinone is inconvenient, in consequence of the rather large 
 graining which it shews on the sensitive film ; therefore, when we require 
 very delicate detail, and especially when the negatives are intended for 
 enlargement, we now prefer Eikonogen, which gives a very delicate and 
 transparent image. The following is the formula we use : 
 
 Eikonogen ... ... ... ... .... 3*5 gr. 
 
 Pure carbonate of soda ... ... ... 2 - 5 ,, 
 
 Pure sulphate of soda ... ... ... 3-5 „ 
 
 Water ... ... ... ... ... ... ioo-o ,, 
 
 We proceed as with the hydroquinone bath, and when necessary we 
 add a few drops of a concentrated solution of bromide of potassium. 
 
 A bath previously used, to which a certain quantity of fresh bath 
 is added, gives very harmonious negatives. 
 
 By operating with care, negatives can be produced as fine as with 
 collodion. 
 
 Fixing Solution. — To fix gelatino-bromide plates we use a solution 
 
PHOTO-MICROGRAPHY. 275 
 
 of 200 grammes (7 ounces) of hyposulphite of soda, dissolved in one 
 litre (if pints) of water. The required quantity of this bath is poured 
 into a dish which holds enough to allow the plate to be well covered. 
 This should always be used fresh. As soon as it gets brown it ought to 
 be thrown away, otherwise it has a prejudicial effect on the plate, by 
 interfering with its perfect transparency. 
 
 Before closing the chapter we ought to say a word on the sensitive 
 plates used. 
 
 All plates are not equally adapted for photo-micrography. It is 
 necessary that the film should be sufficiently sensitive, the grain as fine 
 as possible, and the image capable of penetrating evenly through it. 
 
 It is a rare thing to find all these conditions fulfilled. It is now 
 eight years since we pointed out, in our work on electric illumination, 
 the Ilford plates, which at that time were sold under the name of 
 Marion plates, from the name of the dealer who had for the moment 
 got the monopoly of their sale. 
 
 Since then we have made numerous experiments, and tried very 
 different kinds. Each time that we tested a new sort of plate we 
 always imagined that we had discovered a marvellous plate, and that 
 we were bound to obtain quite phenomenal results. 
 
 Unfortunately, this is never the case, and one only finds that new 
 difficulties have been created. Each time we have to begin quite afresh, 
 and again experiment on exposure, developers, &c, &c. After endless 
 disappoinments we come back to the Ilford plates, and we strongly 
 recommend this brand, which is universally sold throughout London. 
 
 Negatives intended for reproduction by phototype must not be made 
 on glass, since the image being thus reversed often causes inconvenience. 
 We have not up to the present been able to remedy this defect. But 
 now for some months past some very fine and rapid films have been 
 produced, which are stretched on a metallic frame, which can be 
 managed as easily as glass plates. From the experiments we have lately 
 made with them, we are enabled to recommend them. They are made 
 by Messrs. Victor Planchon & Co., of Boulogne-sur-mer (France), and 
 sold under the name of Pellicides auto-tendues. Their price only slightly 
 exceeds that of extra rapid glass plates. 
 
 For Bacteria the ordinary Ilford plates are not suitable. In this 
 case special plates, called Isochromatic plates, are necessary. Those which 
 have given the best results are Carbutt's Orthochromatic plates, and 
 Vogel-Obernetter's Eosine plates, by Otto Perutz, of Munich. 
 
 T2 
 
276 
 
 PHOTO-MICROGRAPHY. 
 
 Ilford plates can also be used if they are previously placed for two 
 minutes in the following bath : — 
 
 Aqueous solution of erythrosin (to 1-1, oooth) ... 25 cent, cubes. 
 Alcoholic solution (to i-5ooth) of cyanine ... 1 cent. cube. 
 Liquid ammonia ... ... ... ... ... 4 cent, cubes. 
 
 Water ... ... ... ... ... ... ... 175 cent, cubes. 
 
 When the plates are taken out of the bath they are placed on 
 blotting paper and rapidly dried while sheltered from the dust. The 
 plates, prepared in this way, can only be preserved for a few days. 
 
 2. Printing on paper. — When photographing the details of a very 
 delicate structure, the author of the negative alone is able to make a 
 suitable print on paper. But in the case of ordinary negatives we 
 advise microscopists to have the printing done by an experienced pro- 
 fessional photographer, after having given him the necessary instructions. 
 The net cost under such circumstances will often be less than if the printing 
 were done by oneself, for if one cannot have an eye constantly on 
 the printing, there is a risk of spoiling a good deal of paper. 
 
 Printing on paper is easy in any case, and it is simply a matter of 
 time and patience. We will describe the operation : — 
 In the first place we will give the formulae : — 
 
 1.— Silver Bath. 
 
 Silver nitrate ... ... ... ... ... ... 15 
 
 Water ... ... ... ... ... ... ... 70 
 
 Alcohol ... ... ... ... ... ... ... 30 
 
 2. — Toning Solution. 
 
 Water ... ... ... ... ... ... 4 litres. 
 
 Auric chloride ... ... ... ... ... 1 gramme. 
 
 Calcium hypochlorite ... ... ... .. \\ grammes. 
 
 Chalk, a small quantity so as to make the bath alkaline. 
 
 3-— Fixing. 
 
 Water ... ... ... ... ... ... ... 100 
 
 Soda hyposulphite ... ... ... ... 15 to 20 
 
 The albumenized paper, which can be bought ready prepared, and 
 should be of the first quality, is stretched over the surface of the silver 
 bath, which has been previously poured into a glass or porcelain dish. 
 After contact for three or four minutes, the paper is taken out by holding 
 one of its corners and fixed, by means of a wooden clip, to a cord 
 stretched across the dark room, and there left to dry. 
 
PHOTO-MICROGRAPHY. 
 
 277 
 
 The sensitized paper is then placed behind and in contact with the 
 surface of the negative on which the image is, then both are enclosed in 
 the printing frame and exposed to the light. 
 
 When the image appears to be sufficiently printed, the paper is 
 immersed in the toning solution and there left until it has acquired a 
 pleasing tint. 
 
 The print is then immersed in the hypo bath, from which it is removed, 
 when, on examining it by transmitted light, the unaffected parts appear 
 quite white. 
 
 It is, finally, left for twenty-four hours in a tank of pure water, which 
 should be renewed two or three times during that period. 
 The print is then dried and mounted. 
 
 M. Mercier some time ago introduced a new paper, called Paper iso- 
 toned with uranium nitrate, which gives very beautiful images, and only 
 requires, after printing, to be fixed with hypo. This paper is very con- 
 venient, and can be obtained from all good vendors of photographic 
 materials ; and we advise all photo-microscopists to give it a trial, because 
 we feel sure that they will be as satisfied with it as we have been. 
 
 3. Production of Lantern Slides. — Lantern slides can be obtained 
 by means of special silver chloride plates. These plates are printed 
 with artificial light — just like printing on paper — with an exposure of 
 from two to three minutes. When the plates have been exposed, they 
 are developed like silver bromide plates, or if special ones are used, 
 the instructions printed on the box must be followed. 
 
 The Ilford positive plates (called special transparency plates) have a 
 great and deserved reputation. 
 
 At the Antwerp Microscopical Exhibition, Messrs. Auguste et Louis 
 Lumiere, of Lyons, shewed some new plates which they had invented, 
 on which objects shewed the colours which were seen on them when 
 observed in the microscope. 
 
 These positives were used in the conferences held at the exhibition, 
 and were held to be a great success. We have to thank Messrs. 
 Lumiere for the following note, in which the mode of obtaining these 
 results is explained. 
 
 Process for obtaining micrographs intended for projection. — The positive 
 photographic images hitherto used for projection have generally the 
 following inconveniences : — 
 
 1. They are deficient in transparency. 
 
 2. They are monochromatic, either black or bistre, except when 
 they are hand-painted, in which case they partly lose their authentic 
 
278 
 
 PHOTO-MICROGRAPHY. 
 
 character, which constitutes the principal advantage of photographic 
 prints. 
 
 In the case of positives representing microscopical preparations this 
 is most inconvenient. 
 
 Photographs in black, indeed, give only a very imperfect idea of 
 such preparations, which are generally of very bright tints. 
 
 It is a matter of some interest to search for a process by which 
 they can be produced mechanically and more faithfully. 
 
 We have been able to attain this end, and we have obtained 
 double colourings by combining photographic processes with the methods 
 of colouring microscopical preparations. 
 
 The best images have been obtained by proceeding in the following 
 manner : — 
 
 We first chose a paper, called carbon paper, the film of which is 
 poor in colouring matter. It is indispensable that the positive be 
 very clear if it is desired that its tint should not sensibly act on 
 the colouring, which should be definitely given to them. This paper 
 is rendered sensitive by a solution containing : 
 
 Water ... ... ... 650 parts. 
 
 Bichromate of potassium 25 ,, 
 
 Alcohol 350 „ 
 
 In summer the solution should be cooled ; its temperature ought not 
 to be more than i5°C (= 59F). 
 
 After the paper has been immersed for five minutes, it is suspended 
 to dry in a position screened from light and dust. 
 
 The paper is then exposed under the negative in the Chassis-press 
 and the carbon process is carried out according to directions. 
 
 In a word the sensitizing and the printing are both carried out 
 with the precaution which that process demands, but which we will not 
 here dwell upon. 
 
 The hardness of the impression is determined by means of a 
 photometer. 
 
 When the register of the photometer shews that the exposure is 
 sufficient, the image is developed according to the known methods, 
 on a thin polished glass, the surface of which has been previously 
 rubbed perfectly clean, the print being applied to the polished side. 
 
 Care should be taken that the print is completely developed. As 
 soon as this is the case, the positive is washed in cold water, immersed 
 in alcohol for ten minutes, and then left to dry. 
 
PHOTO-MICROGRAPHY. 
 
 279 
 
 If the work has been properly done, the print is feeble, and some- 
 times scarcely even visible. 
 
 To colour it, the requisite solutions are prepared, the colours employed 
 in microscopy, or those which are nearest them, such as violet, methylene 
 blue, gentian, violet coton, blue, magenta, macarat, demethylated safranine, 
 malachite green, &c. 
 
 The concentration which appears most suitable varies from 1-100 to 
 1-500, according to the solubility and strength of the substance. 
 
 If any colour is insoluble, or only slightly soluble in water, it is 
 dissolved in alcohol, as dilute as possible, and the solution mixed with 
 water. 
 
 A large number of other substances could, of course, be added to 
 the list of colouring agents mentioned above, but the fact must not 
 be lost sight of that certain aniline colours are rapidly altered by 
 light. Those having this property must be rejected. 
 
 The solution chosen to colour the positive is poured over the image. 
 In a few seconds the liquid will penetrate the gelatine, which will 
 retain the colour, and take a bright tint identical with that of the 
 microscopical preparation, if the right colour has been chosen. 
 
 When the colouring is too intense it should be copiously washed 
 with water. Generally the colour will be slowly and uniformly diminished. 
 
 The effect of washing can be easily watched, and must be stopped 
 at the right moment. 
 
 The decolourizing action of water is generally sufficient when malachite, 
 macarat, or methylene blue are used. 
 
 When the washing is insufficient, alcohol should be used instead of 
 water, but the decolonization is effected much more rapidly than in the 
 previous case, and consequently the operation must be conducted with 
 greater care. 
 
 Treatment with alcohol must always be followed by subsequent washing 
 in common water. 
 
 The effect of alcohol is rapid with methylene violet and magenta. 
 Decolourization is much more difficult with coton blue and safranine. 
 
 For this reason the last-mentioned colours should be used in a more 
 diluted state, in order that their action can be followed more closely, and 
 to prevent the necessity of having recourse to decolourants. 
 
 With these brief instructions it is easy to obtain double stains 
 like those seen in certain microscopical preparations ; when preparing 
 microbes for instance, the microbe is frequently coloured red, and 
 the ground substance blue. 
 
280 
 
 PHOTO-MICROORAPHY. 
 
 To produce the same effect on the photographic positive, it is first 
 treated with an intense red tint, but such that it is not opposed to 
 ultimate partial decolourization of the print. A one per cent, solution of 
 magenta will be found best in this case. 
 
 After this treatment, the print is found to be coloured throughout. 
 To fix our ideas let us pursue this case ; the microbe is seen to be 
 a deep red colour, and the ground substance a clear red. 
 
 We then proceed to partially decolourize it, first with water, then, 
 if necessary, with alcohol. When the ground begins to lose its colour 
 it should be treated afresh with that colour. 
 
 A weak solution must be used, such as diluted solution of coton 
 blue of 1-500. 
 
 The grain of the ground glass, which acts as a support to the 
 negative proof, is detrimental to the transparence of the positive. 
 
 For projection, it is important to varnish, in order to destroy the 
 grained appearance of the surface. The projected images are then much 
 more brilliant. 
 
 The varnish below seems to us to be suitable for this purpose : — 
 Benzine ... ... ... ... ... 300 parts. 
 
 Gum Dammar ... ... ... ... 5 „ 
 
 It is applied cold like collodion. 
 
 The varnishing can be avoided by replacing the ground glass by 
 transparent glass, but then the gelatine is often liable to become 
 unfixed during development. 
 
 Positives produced by this method, when projected on a screen, 
 show very superior effects to those in black obtained by ordinary 
 methods. 
 
 4. General Instructions. — We still wish to give some general informa- 
 tion which we have purposely reserved till now, so as not to interfere 
 with the general course of our remarks. 
 
 1. In order to reproduce the image on the sensitive plate with 
 the greatest amount of clearness and detail, the image of the luminous 
 source must coincide as far as possible with the image of the object. 
 This is absolutely necessary whenever any high power is used, and 
 the object is photographed in axial illumination. 
 
 If the magnification is small, on the other hand a convergent beam 
 of light must be used, so that the image of the luminous source falls 
 above the lenses of the objective. 
 
 2. It naturally follows from what has just been said that a microscope 
 intended to be used for photography must be furnished with a condenser. 
 
PHOTO-MICROGRAPHY. 
 
 281 
 
 The excellent Abbe condenser is not always quite sufficient here, except 
 for oblique illumination. A condenser required for the photographic 
 microscope should be achromatic, and should give as broad an image 
 off the luminous source as possible, so that the whole field is 
 uniformly illuminated. 
 
 The achromatic condenser of Zeiss, which has been specially con- 
 structed for the requirements of photography, perfectly fulfils the end 
 for which it has been designed. 
 
 In certain cases, however, we prefer Messrs. Powell and Lealand's 
 apochromatic condenser, of N.A. 14. In other cases again, as 
 mentioned elsewhere by Dr. R. Zeiss, it is found very useful to employ 
 weak objectives, achromatic or apochromatic, such as the 16 mm. 
 (2-3rd inch) Zeiss' A., AA., &c. For low magnifications an achromatic 
 bi-convex lens is used as a condenser. What condenser one uses must 
 depend a little on circumstances. The essential is to illuminate the 
 image suitably, proportionately to the numerical aperture of the 
 objective, without destroying detail or occasioning a curved and, 
 consequently, deformed image, by using a cone disproportionately 
 illuminated. 
 
 Dr. R. Zeiss in his treatise on photo-micrography lays down the 
 general rule that both for direct observation and for photographing 
 coloured objects (except bacteria), the cone of illumination ought to be 
 of sufficient size to cover a third of the free aperture of the objective. 
 This can be ascertained by removing the ocular, and looking down the 
 tube of the microscope, after having previously placed in front of the 
 eye a piece of smoked glass, to protect the retina from the too brilliant 
 effect of the light. The smoked glass is absolutely necessary when 
 sunlight is used as the illuminant. 
 
 3. In the two preceding paragraphs we have had histological objects 
 specially in view. On the other hand, when it is desired to photograph 
 the delicate striae of diatoms with oblique light, nothing is better than 
 the Abbe condenser, N.A. 1-40. Even with axial illumination we fre- 
 quently use this condenser to great advantage. 
 
 Ordinary gelatino-bromide plates, although excellent for the repro- 
 duction of colourless objects, are not so suitable when certain objects 
 are to be photographed (e.g., bacteria artificially stained), which require 
 long exposure, and are, moreover, very difficult to reproduce. 
 
 If the bacteria were always coloured with Bismarck brown, the 
 photography of their micro-organisms would not present such difficulties, 
 for this colour being only slightly active enables very good negatives 
 
282 
 
 PHOTO-MICROGRAPHY. 
 
 to be secured by ordinary processes. Unfortunately, many of the 
 bacteriological preparations are coloured with red aniline dyes, the chemical 
 nature of which makes it impossible to obtain negatives with sufficient 
 contrast. It is, therefore, necessary to find a medium to diminish the 
 actinic power of the rays of red light which pass through the bacteria, 
 so as to retard the impression of them on the sensitive plate. This 
 problem, which was once thought insoluble, has been solved by M. 
 Henri Schleusner, of Antwerp, by a process as simple as it is ingenious, 
 simply by colour compensation, i.e., by placing between the objective 
 and the photographic plate a green glass of such a tint that it most 
 completely neutralises the red light. An image is thus obtained, com- 
 posed of black bacteria on a green ground, two colours, the contrast 
 of which, relatively to their actinic power, is much more considerable 
 than that which exists between yellowish white and the red of the 
 uncompensated image. 
 
 By this process the time of exposure becomes necessarily very long, 
 but the negatives thus obtained leave nothing to be desired, and they 
 have secured for M. Schleusner Dr. Koch's warm congratulations. 
 
 Since then equally satisfactory results have been produced in photo- 
 graphing preparations of this kind, by using plates called " isochromatic," 
 which are treated with eosine, a substance which has the same effects 
 as M. Schleusner's compensating glass, by retarding the appearance of 
 certain colours. 
 
 Isochromatic plates were first used in photo-micrography by M. 
 Schleusner and ourselves in researches made together in 1884. Since 
 then they have been used by all bacteriologists. We have already, at 
 page 275, described the best makers. 
 
 Such are the principal rules which should be observed by any 
 beginner in photo-micrography. In this, as in so many other subjects, 
 it is only possible to give general instructions which the worker himself 
 must modify a little, according to his personal tastes and inclination. 
 In order, however, to present instructions of greater extent and diversity, 
 we have asked three of our friends to favour us with a note of the 
 way in which they carry out their work. 
 
 These are the most skilful English photo-micrographers, and may 
 justly be considered as masters of the art ; their observations will be 
 all the more valuable to our readers, as the branches of study in 
 which these skilled workers are engaged are dissimilar. Dr. Maddox, 
 the celebrated inventor of gelatino-bromide, which has so fortunately 
 altered the whole aspect of photography, is occupied with histology 
 
PHOTO-MICROGRAPHY. 
 
 283 
 
 and diatoms ; Mr. Comber devotes himself exclusively to diatoms, and 
 his prints are regarded as unequalled ; Mr. Andrew Pringle, the 
 author of an excellent treatise on photo-micrography, gives special 
 attention to bacteria. 
 
 We strongly advise beginners to study carefully the remarks, with 
 which this chapter closes. 
 
 Remarks by Dr. R. L. Maddox. — "As you have expressed a 
 desire that I should tell you how I worked in photo-micrography, I must 
 go back to the days of Collodion. I do not exactly recollect the 
 date of my first efforts, but I believe it was in 1863-64. 
 
 " As soon as possible I arranged a small room, with a window 
 facing the south, furnished with shutters, having two small panes of 
 deep orange glass. The lower shutter had an opening in the 
 middle to admit the rays of the sun projected from the outside 
 by means of a Dubosq solar camera, or a heliostat with two 
 mirrors. All the operations, collodionizing, sensitizing, exposing, and 
 developing were carried out in the room with the greatest ease. The 
 base or table which supported the solar camera when the microscope 
 was used, with the plate holder and its shutters, was made of strong 
 board, blackened, bound with iron, and resting on four triangular 
 feet, arranged so as to give great stability against any vibration. 
 
 "This board was 5 feet in length and 12 inches in width. 
 
 " The heliostat was placed outside the open window ; it was 
 furnished with two mirrors and arranged so as to project the reflected 
 rays along the axis of the microscope, which was placed in a 
 horizontal position. Although with the solar camera it was easy to 
 use convergent solar rays or a parallel beam, the latter was preferred ; 
 oblique light was rarely used. Outside the underneath shutter, 
 and exactly in the path of the reflected light, was often placed 
 a vertical bath with parallel surfaces, containing a solution of 
 sulphate of copper-ammoniac. The light, furnished either by the solar 
 camera or the heliostat, was projected directly on to the condenser of 
 the microscope. The microscope, which was one of Ross' large 
 models, had wider and shorter tubes than usual, and was fixed on a 
 well-finished mahogany stand. The whole was carefully adjusted so 
 that all the axes corresponded and the pieces of apparatus arranged to 
 slide between the guides on the blackened board, while the frame 
 for the sensitive plates was made so as to be inclined in different 
 directions, in order that the surface of the sensitive film could be 
 placed correspondingly parallel in case of any deviation in the mounting 
 
284 
 
 PHOTO-MICROGRAPHY. 
 
 of the object. The image was projected on Bristol cardboard and 
 examined with a lens. 
 
 " In front of the cardboard was placed a screen with an aperture in 
 the middle, depending on the size of the field desired, and a second 
 screen with a smaller opening was placed nearer the microscope tube. 
 Exact focussing was made by means of an adjusting rod tipped 
 with a small grooved wheel suspended under the blackened base 
 board ; a loop of waxed silk cord passed through a slot in the base 
 board, connecting the pulley of the rod with the divided head of 
 the fine focussing screw. 
 
 " To increase the magnification, as the room did not admit of the 
 use of a longer board, an achromatic concave lens was used, placed, 
 according to the necessities of the case, from two to three inches 
 behind the objective, in preference to any eye-piece. 
 
 "When using polarizing apparatus I employed light direct from the 
 heliostat. Sometimes a large achromatic prism was used in place of 
 the condenser of the microscope. For stereoscopic negatives, a half 
 diaphragm, which could be rotated round the objective, was placed before 
 the front lens for low powers ; for those of short focus an arrange- 
 ment was made to project the rays of the condenser at a small angle 
 (9 0 to 1 3 0 ) on to the opposite sides for the two images, but it was 
 very difficult to illuminate a second time the same part exactly and 
 to the same extent as on the first occasion. 
 
 " Some of the objectives were corrected for photography, but others 
 had the ordinary visual focus. From time to time different changes 
 in detail were made as occasion required. Such was briefly the modus 
 operandi at this period. 
 
 "Since the year 1871, when I announced the substitution of 
 gelatino-bromide for collodion, which two years later began to be a 
 commercial article, and which now has extended in an extraordinary 
 manner through all countries, my method of work may be described 
 as follows : — 
 
 " Having changed my residence, and seeing that the sensitive plates 
 of gelatino-bromide required precautions which would have been useless 
 with collodion, I chose for my illuminant the flame of a good 
 paraffin lamp with a large wick. The blackened board with its four 
 feet was preserved, but I added another above it. On this upper 
 base board rested the camera with bellows at such a height that its 
 centre corresponded with the centre of the tube of the microscope 
 when placed horizontally. To assist the explanation, a diagram is 
 
PHOTO-MICROGRAPH V. 
 
 28 5 
 
 given (fig. 184). The front of the camera is fixed, and is pierced 
 in the centre for two brass tubes, of which the inner one can be 
 moved by sliding it sufficiently to enter the shortened draw tube, thus 
 forming a closed connection between the microscope and the camera. 
 The sliding tube and the draw tube are lined with black velvet. 
 
 Fig. 184. 
 
 " The back, part of the camera can be slid between guides on 
 the upper base board and fixed at any distance. The edge of this 
 base board has a scale of half inches and is pierced with holes for 
 fixing the camera ; in the space between the two base boards is a 
 rod passing through holes made in the vertical supports carrying 
 the upper base board. This adjusting rod ends in front in a 
 bit of stout brass wire or fine rod, which enables the tip, furnished 
 with a small pulley with a very narrow groove to be slightly displaced. 
 This pulley, by means of a loop of waxed silk braid, works the 
 milled head of the fine focussing screw. At the other end the rod 
 is furnished with a wooden knob. The rod slides easily, but without 
 too free a movement, in the holes of the supports, and can be 
 manipulated in any part of its length to fix the final focus. The 
 space between the two base boards is very useful for putting down any 
 small objects for a moment on the lower board. In front of the 
 camera is placed a thin board, furnished with a central pinion, 
 which fits into a hole in the stout black base board ; this allows the 
 small board to be turned on its axis while resting on the base 
 board, and is provided with guides to receive and retain exactly in its 
 place the mahogany stand fixed to the foot of the microscope. 
 
 Further, in front is the lamp mounted on a support, so as to be able 
 to be raised or lowered, or moved to either side. These movements 
 
2 86 
 
 PHOTO-MICROGRAPHY. 
 
 are made by means of screws, so that the axis of the flame can be 
 accurately regulated with the axis of the objective. The base of the 
 lamp stand is furnished with a small flap to support a reflector or 
 metal mirror. The glass of the lamp is surrounded with a large 
 blackened tube, pierced both in front and back with a hole facing the 
 axis of the flame. The edges of the aperture in front are furnished 
 with grooves for retaining in place a coloured glass slide.. The 
 lamp stand can be fixed at different distances from the microscope, 
 and in the intervening space a condensing lens can be placed, in 
 order to render the diverging rays from the lamp parallel. 
 
 " By rotating the board which supports the microscope, and lamp, 
 and turning up the mirror, the microscope can be used in the 
 ordinary way, but an adapter is required for the eye-piece. 
 
 "Before making a photo-micrograph precaution is taken to correctly 
 centre the object on the stage, and to make the axis of the sub- 
 stage condenser correspond with the axis of the objective. The lamp 
 having been lighted and centred beforehand, the small board support 
 is turned back, the ocular removed, and the microscope lowered to a 
 horizontal position between the guides on the small board, then the 
 connection is made by the sliding tube and the silk cord is carefully 
 passed round the head of the fine focussing screw and the pulley on 
 the rod. The camera is then closed up, and the image examined on the 
 ground glass screen. If the object is of a delicate nature, the ground 
 glass is replaced by a piece of plain glass of the same thickness, having 
 on its front surface some fine lines drawn upon it with a writing diamond. 
 The image is again examined to ascertain if the image and the fine lines 
 exactly coincide. The plain glass can be replaced by a wooden block 
 or screen, having one or several holes in it, in which an ordinary ocular 
 can be fixed, its exact position having been determined beforehand, so 
 that the image corresponding with the ruled lines on the plain glass 
 shall be in correct focus. In the body proper of the camera is placed 
 a blackened diaphragm, pierced to limit the field and prevent the action 
 of reflected light. After the focussing, some minutes are allowed to 
 pass, to see if the heat from the lamp may not have disturbed the 
 focus. Then after having stopped the transmission of the light, by 
 placing a card between it and the sub-stage condenser, the plate-holder, 
 with the sensitive plate, is inserted in its place and its slide withdrawn, 
 the card removed after a few moments, and the exposure made ; the 
 length of time required depending on many factors, as the distance 
 of the image, the power of the objective, the rapidity of the plate, the 
 
PHOTO-MICROGRAPHY. 
 
 287 
 
 nature and colour of the object, &c. After the exposure, I have always 
 the habit to replace the plain or ground-glass screen and to again examine 
 the image, in case that the negative obtained should not correspond with 
 the image seen, or that the focus has been deranged by vibration. Often 
 the lighting is effected direct, that is, without any lens between the light 
 and the sub-stage condenser. 
 
 " When a projection eye-piece is used, I prefer to finish the adjust- 
 ment, as far as possible, before placing the microscope in position. If 
 preferred, by turning the small board round with the microscope in place 
 upon it, the object can be examined and arranged whilst sitting down ; 
 but generally the examination is made whilst in a standing posture. Every 
 precaution is taken to avoid all reflected light not in the axis of the 
 microscope, by hanging a black curtain upon the wall nearest the 
 instrument, and by excluding the light from the window by curtains. 
 The camera and microscope are also covered by a black velvet cloth. 
 As photo-micrography is accompanied by many details, the explanations 
 needed are somewhat long. Lately, I have experimented with another 
 method of lighting the object, which differs from that generally employed, 
 and which has some promise of utility. Upon the surface of the ordinary 
 plain mirror of the microscope is fixed a tablet in fine porcelain, and of 
 perfect whiteness. The focussing of the object being made by daylight 
 reflected from the surface of the tablet, sunlight is then thrown upon the 
 surface of the porcelain by a large condensing lens, in such a manner that 
 the solar rays illuminate a sufficiently large surface, so that the reflected 
 light does not show in the resulting negative the apparent motion of 
 the sun. The final focussing is easily made during this soft, yet strong, 
 illumination. This method has given me some results that I scarcely 
 expected. The plan requires a different position of the apparatus, unless 
 a heliostat be used. The development of the image and other operations 
 are conducted in the dark room in the ordinary manner." 
 
 Note by Mr. Andrew Pringle. — " For my special work I use 
 an apparatus constructed on the same principles as those of the apparatus 
 manufactured by Messrs. Swift (fig. 183). That is to say, the end 
 of the tube carrying the ocular is supported by a firm and rigid stand. 
 The part of the apparatus which bears the tube is long and strong. 
 The manner of focussing is different, and the fine adjustment is obtained 
 by Campbell's differential screw. But the general conditions always 
 remain the same ; the greatest possible rigidity of all the parts, with 
 the most delicate working. 
 
 "I almost always use oxy-hydrogen light. The point of incandescence 
 
2 88 
 
 PHOTO-MICROGRAPHY. 
 
 should be very small and as brilliant as possible ; for this purpose I 
 use the mixed jet and as hard a lime as possible. I also almost always 
 use a doublet quasi-achromatic lens, for rendering the rays thrown on 
 to the condenser parallel. 
 
 "Sometimes I use Zeiss' achromatic condenser N.A. ro; and some- 
 times Powell & Lealand's oil immersion achromatic condenser N.A. 1*4. 
 
 "According to the colour of my objects, I use ordinary plates, with 
 a very thick film of gelatino-bromide emulsion, or orthochromatic plates. 
 As a rule, Edwards' isochromatic plates give me the best results, but 
 for very deeply-coloured red objects I require plates treated with an 
 ammoniacal solution of cyanin of a strength of 1 to 10,000. I prefer 
 apochromatic objectives to all others for every kind of photo-micrography. 
 
 "I generally use as developer, hydroquinone, alkalized with a caustic 
 alkali, such as potassic hydrate. 
 
 " But for certain objects and for special results, I use other developers, 
 such as pyrogallol or Eikonogen. 
 
 " For diatoms, of which my experience is limited, and for bacteria, 
 of which I have a large experience, I generally use caustic hydroquinone, 
 Thomas' formula, diluted with 50 to 70 per cent, of water, according 
 to circumstances. 
 
 " To screen the light in treating coloured objects, I use a great 
 number of colours, e.g., yellow, blue, rose glasses, Zettnow's liquid, having 
 always a regard to the colours of the object. To succeed completely I 
 believe it is necessary to use the best apparatus, with the greatest skill, 
 in both microscopy and photography. 
 
 Note by Mr. Comber. — "The heliostat, with which I work, con- 
 sists of an ordinary brass time-piece, provided with an extra motion, 
 so that its spindle turns round once, instead of twice, in the twenty- 
 four hours. It is set at an angle corresponding to the latitude of the 
 station in which it is to be used, so that when its highest point is 
 turned due south, the clock is in the plane of the equator, and 
 its spindle parallel to the axis of the earth. 
 
 " Its motion then exactly compensates for the rotation of the 
 earth. Fitting on the spindle is a small plane reflector, of speculum 
 metal, with universal motion ; and when this is set to reflect the sun's 
 rays in the direction to which the spindle points, that is towards the 
 north pole of the heavens, the beam remains without any apparent 
 motion. 
 
 "When the sunbeam crosses the optical axis of the microscope 
 is placed a fixed mirror (plane), inclined to such an angle (half the 
 
PHOTO-MICROGRAPHY. 
 
 289 
 
 latitude of the station), as will reflect the beam horizontally along the 
 axis of the microscope. 
 
 " In the practical use of the apparatus, the points respecting 
 which I am accustomed to use great care, beyond, of course, the 
 cover-correction of the objective, and the adjustment of the projection 
 eye-piece to the distance of the sensitive plate, are the following : — 
 
 " 1. That the optical axis of the whole apparatus is directed due 
 north and south. This is requisite to ensure a motionless illuminating 
 beam. 
 
 " 2. That the achromatic condenser is exactly centred and the cone 
 of illumination absolutely axial. Even a very slight degree of obliquity 
 is apt to produce ' diffraction fringes,' and give rise to images more 
 or less false. 
 
 "3. That no lens, insufficiently achromatised, forms any part of the 
 system. The use of a common ' bull's eye,' which some have recom- 
 mended, I find to introduce aberration, and to yield an image more or 
 less confused. 
 
 " 4. The width of the illuminating cone should vary I believe according 
 to the objective used and the nature of the object. 
 
 "Using my own 2mm. Zeiss' apochromatic objective N.A. i - 43, my 
 experience is that the best delineation of these objects, such as 1 test ' 
 diatoms, is given when the illuminating cone is such as will fill about 
 two-thirds of the back lens of the objective ; but for objects of 
 considerable thickness, a somewhat narrower cone. As a general rule, 
 a very narrow illuminating cone produces ' diffraction ' effects, a very 
 wide one ' haze,' arising, I think, from certain imperfections in the 
 corrections of the objective. I should be only too glad to possess an 
 objective which would stand an illuminating cone of as large an N.A. 
 as itself." 
 
 u 
 
CHAPTER III. 
 
 CAUSE OF ERRORS IN MICROSCOPICAL 
 OBSERVATIONS. 
 
 g i. — Iridescence and Diffraction. 
 
 Iridescence, which not very long ago rendered the use of the 
 compound microscope impossible for serious observations, may now be 
 said to exist no longer. In the objectives made by our best opticians 
 it is practically eliminated, even if there does remain some colour 
 produced by the secondary spectrum, but this is no consequence, 
 except in photography. It is for this reason that in the latter work 
 nothing can equal apochromatic objectives. 
 
 Iridescence cannot therefore any longer occasion inconvenience, except 
 in the case when too strong a light is used, and then it can be made 
 to disappear by suitably arranging the light. 
 
 We cannot, however, say this of the effects produced by diffraction, 
 namely, the illusory existence of double outlines of objects, which is 
 all the greater as the magnification used is increased. Sometimes in 
 the place of a double outline, a triple or even a quadruple one is seen. 
 An experienced observer will never be deceived by diffraction phenomena. 
 There is, moreover, an easy means of ascertaining if apparent lines 
 have a real existence : by successively illuminating the object when 
 possible, first by transmitted light and then as if the object were opaque. 
 It can easily be understood that in the latter case, in consequence of 
 the very nature of the diffraction lines, not the slightest trace of them 
 will remain. 
 
 It can also be ascertained whether the existence of a line is real or 
 not by changing the direction of the illumination. Thus, by illuminating 
 obliquely first from the left side and then from the right, the line, if 
 it is real, will not change ; on the other hand, it will disappear or 
 will come from one side to the other if it is illusory. 
 
 Finally, the effects of diffraction are in a great measure avoided by 
 
FLYING FLIES. 
 
 291 
 
 not using too strong a light, and especially by rejecting sunlight 
 altogether. 
 
 § 2. — Flying flies (Musca volitantes ) ; Dust on the glasses. 
 
 Observers frequently see, especially when a brilliant artificial light is 
 used, lines of dark points, often like rows of threaded beads (fig. 185), 
 which obstinately fix themselves on the object which is being examined. 
 They are called "flying flies" (Musae volitantes), and these images 
 are produced by filaments or corpuscles in the vitreous humour of the 
 eye. The existence of these bodies in the eye has been demonstrated 
 by researches made in common by Messrs. Harting and Schroeder 
 Van der Kolk. Probably no one is exempt from flying flies, but there 
 is no cause for anxiety about them. 
 
 Experience has proved that the use of the microscope does not 
 increase them, for the flies which sometimes annoy us at the present 
 time are exactly the same as those we noticed fifteen or twenty years 
 ago. They have not increased in any way, and the drawing which we made 
 of them . at the time is almost the counterpart of what we see now. 
 
 Flying flies are more visible and tenacious 
 one day than another ; a brilliant light or 
 physical fatigue renders them more apparent. 
 We sometimes perceive these flies with 
 ordinary light when looking up at the 
 sky or at a strongly illuminated surface, 
 such as the ground covered with snow. At 
 other times, notwithstanding the most fatiguing 
 examination of diatoms, we have remained 
 for entire weeks without being aware of 
 their existence. 
 
 About twenty years ago, after prolonged 
 observations of the sun's disc, the retina of 
 our eyes became congested, and flying flies 
 became so abundant that for a long time we 
 were unable to devote ourselves to any 
 serious observation. Professor Harting advised us to adapt an elbow 
 ocular to our microscope, so that the corpuscles might be withdrawn 
 from the axis of the eye, and not inconvenience the sight. 
 
 This means succeeded perfectly, and we recommend it to anyone 
 who may find himself in the same trouble. 
 
 U2 
 
292 
 
 DEFECTS OF THE EYE. 
 
 When much annoyed with flying flies the best course is to suspend 
 observations. A few minutes' rest is sometimes sufficient to restore the 
 eyes to order. 
 
 We therefore repeat that there is no reason to feel anxious about 
 flying flies ; they are not produced by the use of the microscope, and 
 they do not, as is often supposed, indicate any disease of the eye. 
 
 Minute particles of dust often adhere to the surface of the glass 
 slips, and to objectives and oculars. With care these cannot cause mistake. 
 The spot where they may be found can easily be recognised by turning 
 the ocular on its axis, and by moving the glass slip. Particles of dust 
 on the lenses of objectives destroy the clearness of the image. 
 
 When a liquid containing very attenuated corpuscles is examined 
 under the microscope, they appear to change their place as if they 
 were animal objects. 
 
 By observing the movement with much attention it is soon noticed 
 that it resolves itself into a species of oscillation of the corpuscles. 
 These displacements, which are a phenomena of molecular attraction, 
 have been called the molecular movement or Broivnian movement. 
 
 § 3. — Defects of the Eye. 
 
 Astigmatism. — The human eye is rarely perfect; perhaps it is never 
 so. The principal defect and the one, with which it is always more 
 or less tainted, is called Astigmatism, and is in most cases caused 
 by the unequal curvature of different meridians of the cornea. 
 
 If the figure at the side (fig. 186) be looked at 
 with one eye (while the hand is held before the other) 
 and held at the distance of normal sight, the black 
 lines will then appear to be neither equally black nor 
 equally clear throughout ; a certain part will appear 
 grey and more or less extended, and this extended 
 part will probably not be the same for each of the eyes. 
 It is well to examine the eye made use of in observations, and to 
 place in the direction in which the lines appear the clearest the fine 
 details (e.g., the striae of diatoms) which may have to be studied. 
 
 We had occasion, some years ago, to observe the enormous influence 
 that astigmatism can exercise on the appearance of fine details. 
 
 We have on several occasions tried to point out the transverse strise 
 of the Amphipleura to a very competent and skilful diatomist, who 
 always refused to acknowledge the existence of strise that we could 
 see distinctly. 
 
DEFECTS OF THE EYE. 
 
 293 
 
 It then occurred to us that astigmatism might be the cause of 
 this, and an examination of his eyes confirmed this view ; the eye of 
 our friend was found to be greatly deformed in the direction in which 
 the diatom valve was placed. It was only necessary to turn the 
 diatom round a quarter of a circle for him to see the lines with the 
 greatest clearness. 
 
CHAPTER IV. 
 
 PRESERVATION OF THE MICROSCOPE. 
 
 The microscope is a delicate instrument, and great care must be 
 taken of it if we want it to retain its original value. We, therefore, 
 think that a few words on this subject may be useful. 
 
 The instrument should be preserved under a bell-glass. In this 
 way it will be screened from the dust which penetrates even into the 
 best made boxes, and it will always be close at hand and ready for 
 making observations. It is, moreover, impossible to take out and put 
 back the instrument into a box many times without either receiving a 
 few little blows or being placed in a slightly strained position, which 
 after a time inevitably interferes with the centering. 
 
 The lenses being the most important part of the microscope 
 must be taken the greatest care of. If they are covered with dust 
 it should be carefully wiped off with a very clean brush, from 
 which all grease has been previously removed by washing it in ether. 
 
 It is an excellent rule to dust each lens after use before putting 
 it away. 
 
 If any dampness or dirt has attached itself to the lenses they should 
 be delicately wiped with a piece of well used fine linen. Chamois 
 leather, which is also sometimes used for the purpose, is also very 
 good, provided it has been well washed and is very soft. 
 
 Re-agents must never be used without the intervention of a cover- 
 glass, and care must always be taken that the liquid does not wet 
 the objective, which might otherwise become tarnished and corroded. 
 Should such a thing happen, the lens should be immediately washed 
 in distilled water and carefully dried. Similarly the lower lens of an 
 immersion objective is liable to be injured if it is not wiped before 
 being put away. 
 
 Homogeneous objectives should be cleaned with a piece of linen 
 or chamois leather, moistened with benzine. 
 
 The microscope must also be preserved from every kind of vapour. 
 If the body of the microscope slides with difficulty, it should be 
 
PRESERVATION OF THE MICROSCOPE. 
 
 2 95 
 
 moistened either by breathing on it or with a little saliva and then 
 immediately wiped with a piece of linen and briskly rubbed. The 
 same should be done for the tube in which the body of the instrument 
 slides. 
 
 An important point to observe is never to lay aside the instrument 
 with an objective left in, without at the same time leaving an 
 ocular in ; without this precaution small particles of ' dust penetrate 
 into the tube, whatever may be done, and fall onto the upper lens, 
 which then has to be cleaned ; the more often the lens is cleaned the 
 greater chance there is of it deteriorating. 
 
 And now a last recommendation. Keep the temperature of the 
 room neither too high nor too low ; if possible, it should not exceed 
 70 0 to 8o° Fahrenheit, nor be below 2° or 3 0 of freezing point, in 
 order that the variations of temperature may not affect the Canada 
 Balsam with which the lenses of the objectives are cemented. In 
 any case the instrument must be preserved from rapid changes of 
 temperature, which are much more injurious than slow variations. 
 
 Apochromatic objectives require special care. Many of the glasses 
 of which they are composed are very tender, and liable to attack. 
 Messrs. Zeiss condemn their use in tropical regions ; they should be 
 preserved from every kind of vapour, however slightly caustic or acid 
 it may be. It is also necessary, especially with the i"6, to avoid all 
 rubbing, because the flint glass is very easily scratched. 
 
 When cleaning a lens it should always be rubbed in a circle, as a 
 scratch concentric with its circumference is less serious than one across 
 the lens. 
 
 In an excellent little book (') Mr. E. Bausch gives some very good 
 hints for preserving the microscope. We reproduce it here, although 
 some of it repeats the recommendations we have just made. 
 
 To take care of a Stand. — One of the first rules should be to keep 
 the instrument free from dust. This may be done in the manner above 
 described. If dust settles on any part of the instrument, remove it 
 first with a camel's hair brush, and then wipe carefully with a chamois 
 skin, with the grain of the finish and not across it, as in the latter 
 case it is likely to cause scratches. Keep the working and sliding parts 
 absolutely free from dust, as this will make the parts to grind together, 
 and will soon cause play. 
 
 Use no alcohol on any part of the instrument, as it will remove 
 
 (') "Manipulation of the Microscope," by Edward Bausch. 
 
296 
 
 PRESERVATION OF THE MICROSCOPE. 
 
 the lacquer. As the latter is for the purpose of preventing oxidisation 
 of the metals, it is important to observe this rule. In using the draw 
 tube, impart a spiral motion. In instruments which have no cloth lining 
 a straight up and down movement should be employed, as the tube 
 will otherwise become scratched. 
 
 If it becomes necessary to lubricate any of the parts, use a slight 
 quantity of soft tallow, or good clock oil. 
 
 In an instrument which is in constant use it sometimes happens that 
 the pinion works loose, and occasionally to such an extent that the 
 body drops of its own weight. Tightening screws are provided to take 
 up the play : in the Professional, American, Concentric, Universal, 
 Physician, Biological, and Library microscopes, these are situated in the 
 back of the pinion. In the Investigator Model and Family microscopes 
 they are seen in the slide by removing the body. In using a screw- 
 driver, grind its too large surfaces so that they are parallel, and not 
 wedge-shape, and so it will exactly fit in the slot of the screw head. 
 
 In inclining the stand, always grasp it at the arm, and never at 
 the tube, as in the latter case it may loosen the slide, or tear off 
 some of the parts. 
 
 Where repairs or alterations are necessary, always have these made 
 by the manufacturers ; they can, from the system of duplicate parts, not 
 only do it cheapest, but best. 
 
 To take care of Objectives and Eye-pieces. — It is necessary to keep 
 these as free from dust as the stand, indeed even greater cleanliness 
 should be observed. 
 
 When indistinct dark specks are seen in the field lens, and some- 
 times in the eye lens also, the dust may be removed with a camel's 
 hair brush, but when this is not sufficient, use a well-washed piece 
 of linen, such as an old handkerchief. On account of its fine texture 
 chamois skin is desirable, but as it is fatty at first it should never be 
 used until after it has been well washed. The same method applies 
 to cleaning objectives. Clean an immersion objective immediately after 
 it has been used, first by removing the fluid with a moist linen and then 
 by using a dry piece. 
 
 Be particular to keep the objectives in a place where they are not 
 subject to extreme and sudden changes of temperature, as the unequal 
 expansion and contraction of the glass and metal may cause the cement 
 between the lenses to crack. Also keep them from direct sunlight. . 
 
 Screw them into the nose-piece, and unscrew by grasping the 
 milled edge. 
 
PRESERVATION OF THE MICROSCOPE. 
 
 2 97 
 
 Avoid any violent contact of the front lens with the cover-glass. 
 Usually the latter suffers, but it is also liable to damage the former. 
 
 Above all, no one but the owner should handle the microscope and 
 its accessories. One person may be expert in manipulating one kind 
 of instrument but find it difficult to work with another. The fine 
 adjustment always causes the greatest difficulty, as in some instruments 
 its movement is the same as that of the micrometer screw, while in 
 others it is quite different, and thus the objective, as well as the object, 
 are endangered. 
 
BOOK IV. 
 
 CHAPTER I. 
 
 GENERAL RULES FOR PREPARING 
 MICROSCOPICAL OBJECTS. 
 
 § i. — Medium used for Observation ; Preparation and 
 Preservation of Microscopical Objects. 
 
 Objects which are to be examined under the microscope are placed 
 on a glass slip, which is called a slip or slide (French lame), and are 
 covered over with a second and smaller slip of very thin glass, called 
 the cover or cover-glass (French lamelle). 
 
 It would be well, as has been proposed by Professor Leo Errera, 
 to call these only by the two words, " lame " and " lamelle." 
 
 Two principal sizes are adopted for object glass : the size called 
 Giessen's (48 x 28mm.), which is only used in Germany, and the 
 " English size," where the slips are 25 millimetres (1 inch) in breadth 
 by 76mm. (3 inches) in length. These slips are sold all ready made and 
 rounded at their edges ; the bevelling of the edges prevents any injury 
 to the hands, and at the same time preserves the stage from scratches. 
 
 The cover-glasses are of various sizes ; for ordinary work square 
 cover-glasses are used, 21 or 18mm. (f or f inches) square. In the 
 case of valuable preparations, or those mounted in resinous media, &c, 
 round cover-glasses are employed, 15 or i2^mm. (3-5ths or \ inch) in 
 diameter. 
 
 The thickness of cover-glasses for ordinary work is about \\ tenths 
 of a millimetre (3-5ooths of inch). 
 
 The slides usually sold vary very much in thickness, but for 
 very delicate work the maximum thickness should not exceed 1 millimetre 
 (1-2 5th inch), otherwise condensers of large aperture cannot be conveniently 
 used. 
 
AQUEOUS AND OILY MEDIA. 
 
 2 99 
 
 The object ought never to be examined dry, but it should be 
 immersed in an appropriate liquid, i.e., one which does not affect it; for 
 instance, if the object is an organic substance the liquid should be one 
 which does not inflate it by absorption, or which does not deprive it 
 of its water, or further, which does not dissolve it. 
 
 For ordinary examinations of vegetable organisms, water is a very 
 convenient liquid, and is that used in a large number of cases. But 
 generally water is not sufficient for certain tissues which require to be 
 brightened up, i.e., rendered more transparent, also water is not sufficient 
 for permanently preserving tissues. It is, therefore, necessary to have 
 recourse to preservative media, which differ greatly according to 
 circumstances. We will consider those which we most frequently use. 
 
 1. — Aqueous and Oily Media. 
 
 1. Chloride of Calcium. — This is used as a saturated solution. The 
 proportions are one part of chloride to three parts of distilled water. 
 The solution is filtered, and well preserved from dust. It is used for 
 transparent objects. Chloride of calcium, which was very largely used 
 twenty years ago, is now rarely employed. 
 
 2. Glycerine. — Glycerine is used pure, and must on no account be 
 contaminated. It is used for objects which are slightly transparent, such 
 as sections of wood, &c, and also for preparations of starch, which would 
 undergo alteration in the chloride of calcium. 
 
 3. Glycerine Jelly. — Glycerine jelly is an excellent medium for all 
 vegetable tissues which do not require to be strongly illumimated. The 
 English have used it for a long time. In Germany it is called " Kaiser's 
 Gelatine." Its preparation is very simple. One part of the best gelatine 
 is allowed to soak and swell up in six parts of water. After being left 
 for twenty-four hours, the whole is heated with a Marie-bath, until 
 completely dissolved ; to this is then added seven parts of distilled 
 glycerine, and a 100th part of carbolic acid. It is then stirred until 
 completely mixed, and filtered through fine flannel. The jelly is preserved 
 in bottles with wide necks, and well stoppered to prevent the access 
 of any dust. 
 
 4. Camphorated Water. — This is the only liquid which we have 
 found to be suitable for preserving the delicate spiral bands of chlorophyl 
 in some algse, such as the Spirogyra. These spirals are destroyed by every 
 other solution. To prepare camphorated water, take a flask half filled 
 with water, add to it three or four drops of spirits of camphor and 
 
3°° 
 
 AQUEOUS AND OILY MEDIA. 
 
 shake it well. More spirits of camphor is then added, and the 
 whole well shaken again. This process is repeated until a considerable 
 bed of powdered camphor floats on the surface. The liquid is then 
 filtered and preserved in a well-stoppered bottle. 
 
 5. Saccharinated Water. — Unless camphorated water is hermetically 
 sealed the camphor which it contains after a while escapes, and then 
 the liquid is no longer capable of preserving objects. 
 
 The experiments which we have made during the last three years 
 have proved to us that it may be advantageously replaced by a solution 
 of a gramme (15^ grains) of Fahlberg's saccharine in a litre (if pints) of 
 water. This solution may be made with boiling water, and preserves 
 for any length of time. 
 
 6. Fine Oil. — We use the fine oil employed by watchmakers instead 
 of the essential oils recommended by most authors. The advantages 
 which we find are that ordinary black varnish can then be used as a 
 cement and preparations be easily made. Oil is used for pollens, 
 aleuron, and some other objects. 
 
 7. Liquid No. 28. — We invented this liquid twenty years ago for 
 animal histological preparations, but afterwards we found that it was 
 useful for a large number of vegetable substances ; for drug preparations 
 intended for microscopical research in materia medica we find that 
 nothing is better. When it is used pure, without the addition of 
 water, it possesses the important property of not evaporating ; we have 
 preserved for many years a series of preparations of different objects 
 placed in a drop of this liquid between a slip and cover-glass without 
 any cement, and we have found no alteration whatever has taken place. 
 This liquid is used as follows : — To a part of the liquid an equal quantity 
 of distilled water is added, and this second liquid is placed in a bottle 
 and put on one side. When used, the object is placed in a watch glass 
 and covered over with a small quantity of the liquid. It is then left 
 for some minutes or for some hours (according to the more or less 
 transparency of the object) to evaporate spontaneously. When the 
 object has become sufficiently transparent, the preparation is made 
 according to the usual method in the same liquid in which it has 
 been cleared, or if it is not too transparent, it can be prepared in a 
 drop of liquid No. 28. 
 
 For vegetable substances we prepare it thus : — 
 
 Uncristallisable honey sugar, thickened by boiling until it registers 33 degrees 
 of the acidimeter.. .. .. .. .. .. .. .. 300 grammes 
 
 Cristallisable acetic acid . . . . . . . . . . . . . . 25 „ 
 
 Alcohol at 90 degrees 5° „ 
 
 Add distilled water until the liquid marks 28 degrees by the acidimeter. 
 
RESINOUS MEDIA. 
 
 301 
 
 In order that the honey sugar may be good, it is necessary that 
 it should be old. The liquid which floats on the honey is what we 
 mean by honey sugar, and should be obtained from the druggist. 
 This liquid is left in a bottle to itself for some months, and afterwards 
 the upper part, which is the part used, is separated from the rest. 
 
 2. — Resinous Media. 
 
 1. Canada Balsam. — This is rarely used except for diatoms and 
 very opaque objects. Canada balsam is 
 advantageously preserved in a bottle of the 
 special form represented in figure 187. 
 
 2. Sty rax and Liquidambar. — We introduced 
 these two substances into microscopical technique 
 in 1883 (') to replace Canada balsam. They 
 were promptly adopted by diatomists who now 
 no longer use Canada balsam, which after a time 
 becomes brown and resinous, while the new 
 products keep without alteration, and become 
 Fig. 187. by age absolutely colourless. 
 
 Styrax is the balsam given out by Liquid- 
 ambar Orientals Mill, from Asia Minor and liquidambar is furnished by the 
 Liquidambar Styraciflua L., from North America. 
 
 The following is the descriptive note which we published on these 
 media, in July, 1884: — 
 
 " Since the publication of the note, which we had the honour of 
 presenting to the Society in last year's issue, the new products which 
 we then made known have been most favourably received by micro- 
 scopists. The most competent diatomphiles at the present time use 
 styrax exclusively, and numerous papers have been published in Germany, 
 America, England, and France, recommending its use." 
 
 "Many media," writes the celebrated preparer, A. C. Cole ( ; ) "such 
 as phosphorus, monobromide of naphthaline and others, have been tried, 
 but it is unnecessary to enter into details concerning them, since the 
 desired effects have been recently obtained by Dr. Van Heurck, who 
 has succeeded in providing an admirable substitute for balsam in gum 
 styrax, which yields the best possible results. It will probably be found 
 that diatoms mounted in gum styrax are less liable to accidents than 
 
 ( l ) " Billetin de la Soc. Beige de Microscopie," 30th June, 1883. 
 
 (=) A. C. Cole, The Methods of Microscopical Research, London, 1884. 
 
302 
 
 RESINOUS MEDIA. 
 
 Balsam ' mounts,' as the latter becomes resinous in time, and the 
 covers are liable to ' spring,' the result of which is the appearance 
 of prismatic colours in the Balsam, which are not only a great eye- 
 sore, but also sadly deteriorate the slide. Gum Styrax may be 
 considered absolutely permanent and unalterable. Purified Styrax 
 contains a granular substance, which must be removed by dissolving 
 it in chloroform and filtering the solution. The solution thus obtained 
 is used in the same manner as Canada balsam. Styrax solution is 
 even easier to work with than Balsam, and air bubbles are not 
 produced in it by the application of heat, &c." 
 
 Mons. Dippel ( 1 ) has equally recommended styrax and Grunow 
 ( 2 ) insists that difficult forms prepared with styrax show details almost 
 as well as dry preparations, which are now generally condemned on 
 account of the alterations which they inevitably undergo after a short time. 
 
 M. Kitton ( 3 ) has demonstrated that styrax shows clearly in certain 
 diatoms (for example in Polymyxus Coronalis, Bail.), details of structure 
 which are visible neither in dry preparations, nor those made with 
 Canada balsam. The same diatomist recommends styrax for the 
 preparation of sections of wood, insects, &c, and Professor Strasburger 
 ( 4 ) recommends the same product for diatoms, and for rendering visible 
 the nucleus of vegetable cells previously coloured with hcematoxylin. 
 " A solution of styrax in chloroform," writes this professor of botany, 
 "possesses the high index of 1*63; the colour of this hcematoxylin is 
 well preserved by it, the cystoplasm becomes invisible, while the 
 details of the nucleus appear with the greatest clearness ( 5 )." 
 
 Styrax is, therefore, destined to play an important part in the 
 laboratory of the microscopist, and we are now pleased to be able to 
 announce a new property of this substance ; instead of becoming further 
 coloured by time and light, as in the case of Canada balsam, preparations 
 in styrax become absolutely colourless. For the publication of our 
 "Types of Diatoms" (which now includes 275 slides), our preparers 
 have already completed several thousand examples, and we can testify 
 that at the present time those of the first series are quite colourless. 
 This confirms our advice to expose the raw styrax to the light and air 
 for some time. 
 
 ( J ) Dippel, Botanishe Centralblatt, 1883. 
 
 ( 2 ) Grunow, Botanische Centralblatt, 1883. 
 
 ( 3 ) Kitton, Science Gossip, 1883. 
 
 (4) E. Strasburger, Das Botanische Practicum, J6na, 1884. 
 
 (5) E. Strasburger, Das Botanische Practicum, Jdna, 1885. 
 
RESINOUS MEDIA. 
 
 " In our previous note we expressed regret that the liquidambar, 
 which surpassed styrax in its beauty, in the ease with which it could 
 be managed and the magnitude of its refractive index, could not be 
 obtained in Europe. Since then, thanks to one of our best friends, Dr. 
 Jose Clairac, who is head of the histological laboratory of the Military 
 Hospital, of Havana, we have been able to obtain the raw liquidambar 
 in quantity. 
 
 " In this state, the liquidambar is found in the form of a sticky, 
 grayish mass, containing various fragments of wood and bark ; it is very 
 similar to the appearance of raw styrax. 
 
 "The raw liquidambar should be warmed with a Marie-bath, in a 
 mixture of equal parts of genuine coal benzole and absolute alcohol ('). 
 The solution is then filtered and evaporated at such a temperature that 
 the mass becomes slightly brittle at about io° C. ( 2 ). The mass is 
 then again placed in the same solvent as above. The solution should 
 be quite liquid. 
 
 " The following is the way in which we now use the solutions of 
 styrax and liquidambar : — 
 
 " We begin by placing the cover-glasses on a large glass slip, and on 
 each of them, by means of a pipette, we place a large drop of distilled 
 water, on which we allow a drop of liquid, containing diatoms, to 
 fall ( 3 ). These diatoms disperse throughout the drop of distilled water, 
 which consequently is slightly disturbed. 
 
 " The cover-glasses thus charged with diatoms are covered over with a 
 hand-glass, and left to dry naturally. 
 
 " When this is completed, the cover-glasses, taken one by one, are 
 heated to red heat on a plate of platinum, and then put back on 
 the large glass slip ; a drop of very liquid solution of styrax or liquid- 
 ambar is then placed upon them, and they are again left to dry under 
 the hand-glass. 
 
 " After a few seconds the layer whitens ; but this circumstance need 
 cause no apprehension (which does not happen when the solution is 
 with chloroform), and at the end of twenty-four hours the benzole has 
 completely evaporated. The cover-glass is then placed on the glass slip 
 and slightly warmed, preferably with a Marie-bath. A slight pressure 
 
 ( 1 ) Recent research has shown us that this mixture is the best solvent of styrax. 
 Benzole or alcohol alone are not sufficient. 
 
 ( 2 ) Without this preliminary operation, the preparations would not harden. 
 
 (3) We preserve our diatoms in alcohol, which we decant before use, and substitute 
 distilled water. 
 
3°4 
 
 CHEMICAL MEDIA. 
 
 with forceps, drives away all air bubbles, if there are any, as well as 
 any superfluous medium, which can be taken off when it is cold." 
 
 3. — Chemical Media. 
 
 1. Monobromide of Naphthalin. — Monobromide of naphthalin was 
 introduced into microscopical technique, in 1880, by Professor Abbe. 
 
 This substance, which is a colourless, oleaginous liquid, has somewhat 
 the disagreeable odour of naphthalin, but to this one soon gets 
 accustomed. It has a refractive index of i"658. It is used for preparing 
 diatoms, and as an immersion liquid for the objective of N.A. i - 6o. 
 
 Monobromide of naphthalin is not volatile, but it is very irritating 
 to the eyes ; the greatest care, therefore, should be taken not to touch 
 them with the fingers when using the liquid. 
 
 Monobromide of naphthalin dissolves most resinous bodies. The 
 preparations must, therefore, be enclosed by several coats of very liquid 
 glue, and when these are thoroughly dry, by placing over them a thick 
 solution of gumlac. 
 
 Monobromide of naphthalin, as well as the other chemical media 
 which we shall afterwards mention, is used specially for diatoms. 
 
 Any object inserted in a liquid is visible according to the amount 
 of difference between the refractive indices of the medium and the object. 
 
 The silica of diatoms has a refractive index of i'43, the index of 
 water is 1*33; the difference between these two indices, viz., 10, expresses 
 the visibility of diatoms in water. 
 
 Canada balsam has an index of i'S4, the corresponding visibility is, 
 therefore, 11. 
 
 Pure styrax has an index of r6, consequently the visibility is 17. 
 The monobromide having an index of r66, the visibility is 22. 
 
 2. Iodide of Methyl. — Iodide of methyl is a colourless liquid, very 
 dense (2 - 27 at 22 0 ), slightly volatile, with an odour somewhat like chloro- 
 form, smelling slightly of iodine. 
 
 Its index of refraction 1743, and consequently its visibility is 31. 
 
 By saturating the iodide of methyl with sulphur, the index is increased 
 to 1787, and the solution has then not less than 36 as its visibility. 
 
 Iodide of methyl is destined to fill an important place in miscroscopy. 
 It will serve as the immersion liquid for the next objective, with a higher 
 index, which the firm of Zeiss may make, unless indeed a liquid with a 
 still higher index shall be found, but this is not very probable. 
 
CHEMICAL MEDIA. 
 
 30S 
 
 3. Medium having the high refractive index 0/ 2*4 or arsenical 
 medium. — In 1884 our friend, Professor H. L. Smith, made the important 
 discovery of his highly refractive yellow medium. We have good reason 
 for calling this discovery " important ;" however, to fully appreciate the 
 service rendered to diatomists by the American savant, one must be making 
 a study of diatoms and have realized how difficult, I should say almost 
 impossible, it was to examine the striation of the most minute forms. 
 
 The yellow medium of Professor Smith consists of realgar dissolved 
 in bromide of arsenic. It is not quite the same as the article known 
 as realgar in commerce, which is a substance as brittle as glass, brownish 
 yellow in colour and quite opaque, but rather the realgar known to 
 mineralogists, which is of a beautiful ruddy yellow colour and is per- 
 fectly transparent. When Professor H. L. Smith acquainted us with the 
 formula of his medium, he produced this realgar by melting two parts 
 of sulphur with one part of metallic arsenic, and fusing the mass at a 
 red heat for several hours. 
 
 When we had made the realgar in this manner, we found out after 
 a few trials that the product could be obtained more easily, and at the 
 same time in a purer state, by fusing together in a retort one part of 
 sulphur and 17 parts of arsenious acid, and by raising the temperature to the 
 point at which the product distills. Realgar thus obtained by distillation 
 has quite the appearance of the natural realgar of mineralogists. 
 
 The realgar is then dissolved by heat in a test tube in some 
 tri-bromide of arsenic, also obtained by distillation, and a syrupy 
 liquid is thus produced of a yellowish green colour, almost black 
 when seen in bulk. 
 
 The diatoms having been fixed on the cover-glass by dessication 
 are covered with a drop of the liquid medium, and the cover-glass 
 is then turned over on to the slip, and the latter is raised to a high 
 temperature by a spirit lamp. Large air bubbles are thus produced, and 
 the medium assumes a dark red colour, while at the same time the 
 bromide of arsenic evaporates. 
 
 When the air bubbles cease to form and the bromide of arsenic 
 has completely evaporated, the heating is stopped, the cover-glass gently 
 pressed, and the whole left to cool slowly. 
 
 In cooling, the medium loses its red colour, and finally assumes a 
 pale yellow tint. 
 
 Such is the modus operandi; it presents no difficulties, but the 
 dangerous vapours which are thrown off during the operation, should 
 be carefully avoided. 
 
 x 
 
306 
 
 CHEMICAL MEDIA. 
 
 The medium, prepared in the manner indicated above, presents 
 two great inconveniences ; in the first place, the liquid deteriorates very 
 quickly and can only be preserved in hermetically, sealed tubes ; 
 secondly, two-thirds of the preparation gets spoiled often very rapidly 
 without any apparent cause for the alteration. 
 
 We have made numerous experiments with a view to remedy these 
 defects, and have at last discovered a process giving a solid product, 
 which can be preserved without deterioration in a bottle with a ground- 
 glass stopper ; preparations which we have made with it have so far 
 kept excellently and in perfect condition. 
 
 We prepare our medium as follows : — 
 
 In a glass flask or retort 30 parts by weight of flowers of sulphur 
 are dissolved in 10 parts of bromine, and thus is obtained a solution 
 of sulphur in bromide of sulphur (S 2 Br 2 ). 
 
 After perfect combination, 13 parts of finely pulverized metallic 
 arsenic are added, and the whole is heated until the arsenic is 
 completely dissolved. 
 
 It is then turned into a porcelain evaporating basin and heated 
 in the direct flame, being stirred all the while with a glass rod until 
 a drop of it, when cooled, becomes very brittle. 
 
 The medium is then turned out upon a cold plate ; after cooling, 
 the whole is broken up into pieces, which can be preserved in a bottle 
 fitted with a ground-glass stopper. 
 
 The product, which forms a beautiful vitreous mass, of a greenish 
 yellow colour, may be termed the first step. 
 
 Its index of refraction is Nd = 2 "1203, say 2'i2, according to the 
 calculation for which we are indebted to Messrs. Zeiss. 
 
 By heating it for a longer time, until the mass becomes very thick, 
 the index of refraction is raised to 
 
 Nd = 2-2534, say 2-25. 
 
 During its preparation some of the sulphur evaporates, and by suitably 
 raising the temperature the index can be increased to 2*4. 
 
 These two products can be used indifferently ; but they are both — and 
 especially the second — rather difficult to melt. They can, if desired, be 
 dissolved just before use in a little bromide of arsenic, but then the 
 inconveniences of Professor H. L. Smith's above-mentioned medium may 
 again be met with. 
 
 We find that preparations in the arsenical medium are very liable to 
 undergo alteration. To preserve them intact the following precautions 
 must be observed : — 
 
CHEMICAL AND STAINING REAGENTS. 
 
 1. The preparations should be made in very dry weather, in the 
 open air and in the sun. 
 
 2. The arsenical medium should be heated until it cracks more or 
 less, when cooling. 
 
 3. The preparations should be made very quickly ; the covers and 
 slides should be warm just when the medium is applied. 
 
 4. As soon as the preparation is completed, and while it is still 
 lukewarm, a thick layer of gum lac should be applied round the edge 
 of the cover-glass. 
 
 5. The preparations should be kept in a dry place. 
 
 Varnishes for Cells. — Varnishes are used for the purpose of making 
 cells, in which microscopic objects are preserved in appropriate media, 
 as will be described in the following paragraphs : — 
 
 The simplest of these gums consists of a thick solution of gum lac in 
 alcohol, its gummy part being got rid of, either by allowing it to 
 settle, by filtration, or by decanting. However, we cannot guarantee 
 that this gum will keep well, because it is very liable to get detached 
 from the glass. 
 
 For this reason we use with advantage Schwarzer Maskenlack No. 
 3, sold by Besseler (Schiitzenstrasse, No. 66, Berlin), which is probably 
 an alcoholic solution of gum lac, mixed with some resin and lamp 
 black. But as it is difficult to obtain, a thick solution of black 
 bituminous varnish, to which a small quantity of wax dissolved in 
 turpentine has been added to avoid cracking, will do as well. 
 
 All the compounds which we have considered in this chapter, and 
 those to be noticed hereafter, can be obtained at the "Laboratory of 
 Technical Chemistry" of Mr. Ferdinand Van Heurck, Rue de Moulin, 
 Antwerp. Test preparations, and especially those of Amphipleura mounted 
 in the arsenical medium, can also be obtained there. 
 
 2 — Chemical and Staining Reagents. 
 
 The following chemical reagents are those most frequently used for 
 research in vegetable anatomy : — 
 
 Iodized chloride of zinc. 
 Solution of Iodine. 
 Mercury Nitrate. 
 Ether. 
 
 Ammoniacal cupric oxide. 
 
 X2 
 
3 o8 
 
 CHEMICAL REAGENTS. 
 
 Nitric acid. 
 Sulphuric acid. 
 Potassium Chlorate. 
 Caustic potash. 
 
 Iodized Chloride of Zinc. — Its action is the same as a compound of 
 sulphuric acid and iodine, but the blue colour which it imparts to 
 cellulose varies according to the degree of concentration. Its blue 
 colour changes to violet or red at the expiration of twenty-four hours. 
 
 According to Schultz the iodized chloride of zinc should be prepared 
 as follows : — 
 
 A solution of zinc in hydrochloric acid is evaporated to the con- 
 sistency of syrup : it must be kept stirred with a strip of zinc. Iodide 
 of potassium must then be added until it is saturated. The operation 
 is completed by adding iodine and water in sufficient quantity. 
 
 Solution of Jodine. — Is used to colour the membrane of the cell 
 as brilliantly as its contents. 
 
 It is prepared by adding 5 centigrammes of iodine and 15 centi- 
 grammes of iodide of potassium to 30 grammes of water. 
 
 Mercury Nitrate. — This is employed in solution. It imparts an intense 
 red colour to all substances containing nitrogen ; it takes at least a 
 quarter-of-an-hour to act, but a better and more rapid effect is obtained 
 by slowly heating the preparation. 
 
 Ether.- — For the purpose of dissolving fixed and essential oils, as 
 well as resins. 
 
 Alcohol. — Used for the same purpose as ether, but above all it is 
 useful for removing air from vegetable sections. Before preparing the 
 latter they are immersed for some minutes in a watch-glass containing 
 alcohol. 
 
 Ammoniacal Ctipric Oxide. —This is prepared by dissolving in liquid 
 ammonia, cupric oxide, which has been recently precipated and is still 
 moist. Ammoniacal cupric oxide dissolves cellulose. 
 
 Nitric Acid. — This turns intercellular membrane, and matter containing 
 nitrogen yellow. After the nitric acid has been made to react upon 
 them, they should be floated in liquid ammonia. It is also used 
 in Schultz's macerating process, hereafter described in the article on 
 potassium chlorate. 
 
 Sulphuric Acid. — When concentrated it is used in researches on 
 pollens and spores ; in a diluted state (three parts of sulphuric acid to 
 one of water) it is used to colour cellulose blue. To produce this 
 effect the preparation is first soaked in solution of iodine, and then 
 
STAINING REAGENTS. 
 
 309 
 
 having removed the superabundant solution of iodine, by using some 
 blotting paper, a drop of sulphuric acid is added, and the preparation is 
 covered with a thin lamina of glass. The blue colour changes in 
 twenty-four hours to a violet or red colour. 
 
 Potassium Chlorate. — Used in the macerating process discovered by 
 Schultz. The object is taken and cut into thin slices, which are placed 
 on a glass slip ; they are then covered with a quantity of powdered 
 potassium chlorate equal to them in volume, and a few drops of nitric 
 acid are added. The glass slip is then heated for from one to three 
 minutes with a spirit lamp. 
 
 After the reaction, they are washed by painting them over with water 
 several times with a brush. In this way the cells are isolated. 
 
 Caustic Potash. — This is used in solution, and as a rule with the 
 assistance of heat. It is employed to dissolve fat, and intercellular 
 matter, as well as lignin and suberin. Schacht advises that it be 
 preserved in the powdered state because, as he says, the solution 
 attacks cork stoppers, and in bottles fitted with glass stoppers, it forms 
 between the neck and the stopper a silicate which prevents the bottle 
 from being opened. 
 
 Staining Reagents. — The staining reagents frequently used at 
 the present time for throwing into relief certain tissues and details are 
 innumerable, but the botanist may be quite satisfied with those which 
 we are about to mention. As our friend, Mr. Arthur C. Cole, (') 
 well says in his little work, Methods of Microscopical Research, which 
 we cannot too strongly recommend : — 
 
 " We advise the histologist, on the point of commencing work, to 
 ask himself the following preliminary question : What am I about to 
 do? Do I desire to make instructive preparations capable of elucidating 
 details of structure, or only a spread of tint and colour ? " 
 
 The solution should not be uncertain. We recommend the following 
 list of dyes : — 
 
 Logwood or Hcematoxylin. 
 Carmine. 
 
 Ammonium Picrocarminate. 
 Sodium Sulphindigotate. 
 Nigrosine. 
 
 ( J ) Mr. Arthur C. Cole (29, Thurleigh Road, Streatham Common, London, S.W.), 
 is probably the most skilful and best informed preparer of the present day. Some years 
 ago he published, under the name of Studies in Microscopical Science, an admirable 
 series of preparations of animal and vegetable histology, and also of pathological tissues. 
 These preparations were accompanied with photographs and coloured plates, together 
 with descriptive text for each series of preparations, written by the best English specialists. 
 
STAINING REAGENTS. 
 
 Fuchsine. 
 Iodine green. 
 Methyl green. 
 Double staining. 
 These are prepared as follows : — 
 
 Logwood. — Take 60 gr. of dried extract of logwood, and 180 gr. 
 of powdered alum. Mix them thoroughly together for some time in 
 a mortar, adding by degrees 300 gr. of distilled water. When the 
 solution is thoroughly mixed, filter. To the filtrate add 20 c.c. of 
 absolute alcohol and preserve the dye thus made in a well-stoppered 
 bottle. In order to act well, the dye should have been made for 
 some days before being used ; the older it is the more excellent it 
 becomes. The solution we now use will soon be twelve years old, 
 and acts wonderfully. 
 
 Carmine. — Take 4 gr. of the best carmine, rub it thoroughly in a 
 mortar with 8 gr. of concentrated liquid ammonia. Leave the mixture 
 for 24 hours, then add 120 gr. of distilled water saturated with borax, 
 and pound the whole again in the mortar. Then filter and preserve 
 in a flask fitted with a ground-glass stopper. Carmine gives a stain 
 very similar to that of logwood, but many observers prefer logwood 
 as being less fatiguing to the eyes. On the other hand, for photo- 
 graphic reproductions carmine is preferable. 
 
 Ammonium Picrocarminate or Ranvier's double stain. — The following 
 formula, due to Weigert, gives the best results: — 
 
 Mix in a mortar 2 gr. of carmine and 4 gr. of ammonia ; at 
 the end of twenty-four hours add to this mixture 200 gr. of saturated 
 solution of picric acid and a little acetic acid until a precipitate is 
 formed by shaking ; leave for twenty-four hours, filter, and add liquid 
 ammonia until the liquid becomes clear. Then dilute to a one per 
 cent, solution, i.e., to 1 gr. of the solution made as above, add 99 gr. 
 of water. This liquid colours protoplasm a yellowish red and the 
 nucleus a bright red. Its action should be somewhat prolonged. 
 
 Sodium Sulphindigotatc. — This stain is obtained by making a saturated 
 solution of sulphindigotate of soda with distilled water. 
 
 Nigrosine. — This product has been introduced into the laboratory by 
 the skilful Belgian botanist, M. Leo Errera, professor of the University 
 of Brussels. 
 
 It colours the nuclei of cells a very deep blue, while all the rest 
 of the cellular tissue remains practically colourless. After remaining 
 but a short time in an aqueous solution of nigrosine, the microscopical 
 
USE OF MICROTOMES. 
 
 3" 
 
 section is washed with distilled water. It can then be mounted in 
 glycerine jelly if we wish to study the photoplasm and the part of 
 the nucleus formed by the Achromatine of Flemming. If, on the 
 other hand, we wish to study the chromatine (— nuclein), then it 
 should be washed in alcohol cleared, with essence of cloves, and 
 mounted in Canada Balsam or styrax. 
 
 Some technical study on anilines which we pursued some years 
 ago enable us to point out that if all aniline colours are rapidly 
 destroyed by the light this does not apply to nigrosine which remains 
 unaffected even when indefinitely exposed to the sun. 
 
 loditie Greeti. — This is obtained by making a saturated solution of 
 this product in distilled water. It is then made up to a i per cent, 
 solution with distilled water. 
 
 Iodine green is a multiple stain which, when applied to tissues, 
 colours them in different tones of the same colour. 
 
 Fuchsine. — i gr. of fuchsine is dissolved in 30 gr. of alcohol, which 
 is then diluted with water. Fuchsine is very far from being a stable 
 colour, as it is soon destroyed by light. The same remarks apply to 
 the following : — 
 
 Methyl Violet, which is prepared in the same way. 
 
 Double staining with Anilines is effected by first staining with an 
 alcoholic solution of Fuchsine, and then with one of methyl violet. 
 
 Methyl Green, which is prepared in a concentrated aqueous solution 
 and afterwards diluted with water. 
 
 This stain renders grains of chlorophyl very conspicuous, and 
 Professor Slrasburger, after having diluted it with a 1 per cent, solution 
 of acetic acid, uses it for nuclei. Nuclei, which are not in the act 
 of division, are coloured a very light green, while Nuclear plates in 
 every phase of division assume a very intensely green colour. 
 
 This stain has always given us excellent results, but we cannot 
 guarantee its preservation. 
 
 § 3. — Instruments used in preparing Objects. 
 Use of Microtomes. 
 
 The instruments and accessory apparatus which we use are razors, 
 scalpels, forceps, a hand-vice, evaporating basins, a spirit lamp, some 
 glass rods and pipettes, and some watch-glasses. 
 
 When we want to make some good preparations of wood or 
 botanical drugs, a good microtome should be added. 
 
312 
 
 USE OF MICROTOMES. 
 
 Razors. — The razors should be of the finest quality. We prefer 
 those which have been made by the English firm of John Barber. 
 Some should be hollow and some not. The first are used for delicate 
 substances and the latter for hard objects, such as wood, &c. 
 
 Razors are never supplied by the grinders with a sufficiently sharp 
 edge ; moreover, they should be sharpened afresh after making a few 
 sections. One should therefore be able to sharpen the razors oneself. 
 For this purpose we use an excellent, but unfortunately rather expensive, 
 composition (ios. to jQi, according to the size of the tablet), called 
 The Celebrated Magnetic Tablet (Rigge, Brockbank & Rigge, 35, New 
 Bond Street, London, and 5, East Street, Brighton). After passing the 
 razor several times over this composition, taking care to hold the razor 
 very flat, the operation is completed by passing it five or six times 
 over the strop on the other side, on which is spread with the finger 
 some of RimmePs genuine diamond dust, which can be purchased at 
 any of Rimmel's perfumery shops. We have razors which have been 
 treated in this way, possessing an extraordinarily sharp edge, and 
 although we have used them daily for several years, we have never 
 found it necessary to have them ground. 
 
 For some years razor strops have been sold in Paris which, for 
 microscopical purposes, surpass all others, These strops, or so-called 
 strops, are known as cuirs donateurs. The substance of which they are 
 made is none other than the tissue of the stem of the American aloe, 
 on which is rubbed some grease and fine emery powder. We highly 
 recommend these strops, which were brought to our notice by the late 
 Mr. Bourgogne, Sen. They have proved quite marvellous. 
 
 To make sure that the razor is sharp enough, a hair is placed between 
 the thumb and first finger in a convenient position, and then it should 
 be possible to cut the hair right down by pressing gently with the razor 
 at a distance of four or five millimetres above the thumb. 
 
 Needles. — The needles, mounted in handles, which are generally sold 
 with microscopes, are of little use. It is necessary to have a needle- 
 holder, in which needles can be inserted. Such needle-holders can be 
 obtained in Germany, at a small cost. They consist of a pointed rod 
 of wood, ending at the top in a brass shank. This is split into four 
 parts, in the middle of which the needle is inserted, and held there 
 by means of a screw, the outside of the brass shank having a thread 
 (fig. 188). 
 
 The needles used should be as fine as possible. Generally Nos. 11 
 and 12 are employed. 
 
USE OF MICROTOMES. 313 
 
 Needles are used for microscopical dissections ; but when light objects 
 have to be transported, they are taken up with hair pencils, slightly 
 moistened, as needles often damage microscopical objects. 
 
 Fig. 188. 
 
 Scalpels (fig. 189) are needles, the ends of which terminate in an 
 edged blade in the form of an iron lance. They are used for dissections. 
 
 Fig. 189. 
 
 Forceps (fig. 190) are used to take hold of small objects. It is 
 necessary to have some with a smooth inside surface and others grooved. 
 
 Fig. 190. 
 
 The Hand-vice (fig. 191) is used to press objects, such as leaves 
 between elder pitch, in order to make transverse sections. 
 
 Fig. 191. 
 
 Glass rods and pipettes are used to take up drops of reagents, and 
 evaporating dishes and watch-glasses to insert objects in appropriate 
 liquids, such as alcohol and ether, so as to get rid of the air which 
 is found in sections. 
 
 Hand sections and Microtome sections. — All sections which are required 
 in the study of vegetable anatomy can be made with the hand ; but 
 
3H 
 
 USE OF MICROTOMES. 
 
 when we wish to make sections of a definite size, to show the whole of 
 the tissues of some organ, or when very fine and large sections of hard 
 and ligneous bodies are required, recourse must be had to microtomes. 
 
 Hand sections are made in a simple manner. The object to be 
 cut is held tight between the thumb and first finger of the left hand 
 so as to project as little as possible, and the forearm is pressed 
 against the chest in order to render the hand steady. Then with 
 the right hand holding the razor firmly at the base of the blade, a 
 superficial cut is made rapidly to make the surface on the object even. 
 The subsequent cuts are made slowly by pressing the blade forward, 
 and at the same time moving it from base to tip. 
 
 Before and after each cut the razor and body to be cut is moistened 
 with a mixture of equal parts of water and alcohol. 
 
 The sections adhere to the razor. They are taken off with a 
 fine hair pencil, and placed immediately in a watch-glass containing 
 a mixture of water and alcohol. After remaining a short time in this 
 first medium they are removed either to distilled water or to pure 
 alcohol, according to the nature of the medium to be used in the 
 ultimate mounting. 
 
 moistened by means of the breath, and sections are made with the 
 razor in the same way as any tissue. 
 
 We will now proceed to the consideration of microtomes. 
 
 The simplest of these instruments which suffice in most cases is 
 Topping's Microtome made by the firm of Chevalier (fig. 192). 
 
 Bodies too thin or too small 
 to be held between the fingers 
 
 are placed in a cylinder of elder 
 pith or of carrot cut lengthways 
 for the insertion of the object, 
 and then pressed in the hand- 
 vice represented above in figure 
 
 Fig. 192. 
 
 Pulverulent bodies, such as 
 pollen, &c, are inserted in a 
 drop of a solution of gum arabic 
 and sugar, placed on the surface 
 of a transverse section of a rod 
 of elder pith. When it is all 
 dry the surface of the gum is 
 
USE OF MICROTOMES. 
 
 315 
 
 Its price is The object to be cut is enclosed in the tube, and by 
 
 means of an inner knob a small part is made to project, and this is 
 cut off with a very sharp razor. After each section is made a drop 
 of water and alcohol must be placed on the object. 
 
 piece of brass which slides in a bronze groove. This addition enables 
 the cutting to be done with greater regularity, especially with large 
 and hard objects. No object if properly prepared can effectively 
 resist such an instrument. 
 
 There has been a very remarkable progress of late years in the 
 manufacture of microtomes. The microtomes of the present day are 
 instruments of extraordinary precision, in which all the resources of the 
 mechanic have been utilized, and which could not have been produced, 
 except by makers having a special experience in the matter. 
 
 Among these makers we may mention as of the highest excellence 
 Messrs. Carl Reichert and Chr. Erbe. 
 
 Carl Reichert's large microtome (fig. 194), is, as we have already 
 said, an instrument of great precision, and it is made after the design 
 of Rivet. 
 
 The stage carrying the object is moved forward on an inclined plane, 
 the length of the slide bar being 30 cm. (one foot). The carriage, 
 with the object clamp, is moved upwards on the inclined plane by 
 means of a micrometric screw, which enables sections to be cut to a 
 thickness of - ooi mm. This micrometric screw possesses a piece of 
 mechanism, by which each completed section is told by the catch of 
 a wheel. The microtome has also an object clamp, as recommended 
 at the Zoological Station at Naples, and an arrangement for fixing 
 
 A more complicated model is 
 sold by Nachet for £2 (fig. 193). 
 Differing more particularly from 
 the previous one from the micro- 
 metric screw having an index by 
 which the thickness of the object 
 can be ascertained. 
 
 Fig- 193- 
 
 Microtomes still more perfect 
 are made. In the one we 
 generally use, which was made in 
 our amateur workshop, the section 
 cutter is managed by means of 
 a wheel, and fixed on a thick 
 
USE OF MICROTOMES. 
 
 3*7 
 
 crosswise two knives, which are 13 and 16 cm. (5 and 6£ inch) 
 respectively in length. 
 
 A similar, but simpler, microtome is that sold by Leitz (fig. 195) 
 for j£6. In this apparatus the knife is moved by a crank piece of 
 mechanism fixed on the support. Consequently, a section can be made 
 with greater precision, more evenness, and less exertion. 
 
 Messrs. Watson & Sons make an excellent little microtome, called 
 in England the Cathcart Microtome, after the name of its inventor. 
 This microtome, which figure 196 sufficiently explains, is specially intended 
 
3*8 
 
 USE OF MICROTOMES. 
 
 for making sections of frozen tissue, but an accessory tube enables it 
 to be used for ordinary unfrozen tissues. We have used it for several 
 
 Fig. 196. 
 
 years, and can recommend it with confidence. The smallness of its 
 cost {jQi is.) places it within reach of all. 
 
 Erie's Microtomes. — The Antwerp Microscopical Exhibition made 
 known to us the excellent apparatus manufactured by a skilful specialist, 
 Mr. Chr. Erbe, of Tubingen. 
 
 His apparatus, which are all most carefully made, are also com- 
 paratively cheap and very remarkable. Moreover, the Jury of the 
 Exhibition gave him their highest award — the diplome d'honneur — and 
 testified to the praiseworthy character of his productions. 
 
 Mr. Chr. Erbe makes a whole series of microtomes, but we will 
 specially notice Thomas' Microtome, Erbe's Universal Microtome, the 
 large Freezing Microtome, Hughes and Lewis', Cathcart's, and Cathcart 
 and Erbe's. 
 
 Thomas' Microtome. — This instrument (fig. 196a) is well known in 
 laboratories. It is palpably only Rivet's microtome, with the woodwork 
 
USE OF MICROTOMES. 
 
 319 
 
 which that skilful microscopist used, replaced by cast iron — a substitution 
 which enables the instrument to be constructed with greater precision. 
 
 In the Rivet-Thomas' microtome the knife is fixed on a block of 
 iron, which is moved forwards and backwards with the hand, to make 
 the sections. The iron block is guided in a groove, and is only borne 
 
 Fig. 196a. 
 
 on three ivory points, which slide on a well-polished plane, well oiled 
 before being used. 
 
 The object can be held in forceps of different forms, which can 
 be placed in any desired position. The forceps are fixed to an iron 
 block, which slides on an inclined plane. A micrometric screw presses 
 the block forward as far as may be desired ; this space, which may be 
 adjusted to a mikron, determines the thickness of the section. 
 
 Erbe's Universal Microtome (fig. 196^) differs from the previous one 
 in that the object does not slide on an inclined plane, but is pressed 
 from below, upwards, by a very precise micrometric screw. The object 
 is held in a similar manner in forceps, which can be placed in any 
 desired position. 
 
 The knife is held on an iron block, and only rests on three ivory 
 points ; but in place of a cast-iron plane, these points slide on glass 
 slips, which diminish the friction. 
 
 This microtome has very exact movements, and as the object is held 
 more firmly than before, it can never be displaced, which may however 
 happen in the previous instrument, in which the block carrying the 
 object is only held by its weight. 
 
320 
 
 USE OF MICROTOMES. 
 
 Fig. i<)6b. 
 
 The Large Freezing Microtome (fig. 196*-) is only a modification of 
 
 the last ; the appara- 
 tus, which the illus- 
 tration sufficiently 
 explains, is specially 
 intended for sections 
 of frozen tissue. 
 
 Erbe's Cathcart 
 Microtome in no way 
 differs from that of 
 Messrs. Watson's, 
 which is represented 
 in fig. 196. 
 
 The Cat hear t- 
 Erbe Microtome (fig. 
 196^) differs from 
 the previous one in the form of the knife, which is supported on a 
 cast-iron block, sliding in a groove, and also in the micrometric screw, 
 which is more precise. 
 
USE OF MICROTOMES. 
 
 321 
 
 Fig. 196^. 
 
 Lastly, Hughes and Leivis's Microtome (fig. 196^) is an apparatus 
 intermediate between the last two, both in its price, which is only 
 jQi 19s., and in workmanship. 
 
 Fig. 196?. 
 
 But whatever microtome be used, the objects which are cut with 
 these instruments have to undergo a previous preparation, more or less 
 complicated. 
 
3 22 
 
 USE OF MICROTOMES. 
 
 The objects must be softened (wood, shell, &c.) or hardened (soft 
 tissues) which, in the case of fresh vegetable substances, is best accom- 
 plished by freezing ; and lastly, they must, as a rule, be embedded in 
 some other substance. 
 
 In animal histology, the embedding medium used is paraffin, which 
 is poured round the object, when placed in a suitable mould. Vegetable 
 tissues are conveniently enclosed, either in elder pith or pieces of carrot, 
 which can be obtained fresh nearly all the year round. 
 
 Preliminary softening may be effected by placing woody substances 
 either in pure water or in water to which a small quantity of soda 
 has been added. The time during which these substances should remain 
 in the medium will vary from a few hours to several days, according 
 to the hardness of the object. 
 
 Tissues and objects to be frozen must be previously treated in a 
 solution of gum and sugar until they are well soaked. 
 
 This solution is obtained by dissolving on the one hand four parts of 
 Senegal gum in six parts of distilled water, and by preparing on the 
 other hand syrup of sugar, similar to that used by chemists, and which 
 is made by dissolving nine parts of refined sugar in six parts of boiling 
 water and boiling it again for a moment before taking it off the fire. 
 
 These two solutions when made are mixed in the proportion of five 
 parts of the gum solution to three parts of the syrup. To this mixture 
 one per cent, of carbolic acid should be added to prevent fermentation. 
 
 Tissues deposited in this mixture can remain there as long as may 
 be desired. They will be preserved there unaltered. 
 
 These preliminaries being disposed of we may pass on to the mode 
 of operation. 
 
 Sections by Freezing. — From the tissue to be cut, a small piece not 
 more than three millimetres thick is taken, the gum in it is squeezed 
 out with a fine cloth. Then in a little gum previously placed on the 
 stage of the micrometer, the piece to be cut is inserted, and covered 
 over with gum on all sides by means of a brush. 
 
 The jet of pulverized ether which is then directed on the piece 
 of the object by means of the special pulverizer, furnished by the 
 makers with the microtome, immediately freezes the mass, and enables 
 sections to be cut. It should be observed that the freezing must be 
 carried only to a convenient degree, and no further. The freezing 
 is correct when the sections can be easily made, as for example when 
 cutting cheese. When the freezing is carried too far the mass is too 
 brittle, and the tissues are torn, 
 
STAINING TISSUES. 
 
 323 
 
 By embedding large diatoms in this way — Pinnulariaj, Triceratia, 
 Coscinodisci, &c. — in a drop of frozen gum, suitable sections can be 
 made of them. However, in this case, much necessarily depends on 
 chance, as there is no means of placing the diatoms in a satisfactory 
 position to obtain the sections in the desired direction in the gum solution. 
 
 Sections by Embedding. — While delicate tissues are advantagously cut 
 by congelation, those of greater consistency, such as stems, stalks, 
 &c, as well as animal tissues which have been hardened by being 
 placed for some time in alcohol, can be easily cut after having been 
 previously embedded in a suitable substance. 
 
 Paraffin is generally used for this purpose, but this substance, in 
 most cases, is not as suitable as a small piece of fresh carrot, because the 
 natural moisture which the latter contains favours the sliding motion of 
 the knife. Then again, when the section has been made, the remains of 
 the carrot are easily removed, while on the other hand, the paraffin 
 crumbles, and moreover, the use of carrot does not necessitate the 
 employment of heat as is the case with paraffin, which must be 
 previously melted and afterwards allowed to cool round the object. 
 
 First, by means of a knife, a cylindrical piece of the carrot is cut 
 lengthways, so as just to fit in the cylinder or the forceps of the 
 microtome. This cylinder should be from 3 to 5 centimetres (one to 
 two inches) in length. 
 
 Having made the cylinder, it is divided lengthways into two parts, 
 and hollowed out so that the objects to be cut, having been placed in 
 the hollow thus formed, the two parts can be fitted back again to one 
 another, and maintained in position by a thread, so as to press the 
 inclined object very slightly. 
 
 The cylinder is now pressed into the pincers of the microtome, and 
 sections are then cut in the usual manner. 
 
 After each section has been cut a few drops of alcohol, mixed with 
 water, are placed on the surface, and the knife is also dipped in a 
 dish containing the same mixture. By this means the section is detached 
 from the knife and floats off in the liquid. These are afterwards with- 
 drawn, by decanting the greater part of the liquid, and then lifting 
 each section with a fine brush. 
 
 § 4. — Staining Tissues. 
 
 It would be difficult to determine at what period the staining of 
 tissues was first thought of, but it is certain that for a very long time 
 only carmine was used for this purpose. It was only about fifteen years 
 
 Y2 
 
324 
 
 STAINING TISSUES. 
 
 ago that different colours were attempted, and aniline dyes in the first 
 place became the fashion, and were used for all kinds of things. 
 
 If the reader will trust us, he will avoid all colours of this kind ; 
 the effect produced is very pretty and the result interesting, for the 
 moment, but there is no certainty that the results will be durable; and 
 from what is known of aniline dyes he may be assured that these 
 colours will last but for a short time. 
 
 We have in our possession several preparations of bacteria made by 
 skilful mounters, which were very pretty to look at when we first received 
 them, but now no bacteria are visible. 
 
 If therefore it is desired to make preparations of coloured tissue, 
 only colours whose lasting qualities are known, should be used. Their 
 number is unfortunately very limited, and only logwood, carmine, picro- 
 carminate of ammonia, and probably also nigrosine can be recommended 
 with certainty. Methyl green appears also to be sufficiently stable. 
 
 If a person confines himself to these colours, writes Mr. A. C. Cole, 
 he will be certain to find his preparations intact after any lapse of time; 
 especially if he mounts his preparations strained with logwood in Canada 
 Balsam, and his preparations strained with picro-carminate in glycerine 
 jelly of good quality. 
 
 Our own experience corroborates the statements of our esteemed 
 correspondent. Preparations, which we made in this way three years 
 ago and which since then have been constantly exposed to the light, 
 are as bright now as they were on the day they were made. 
 
 These preliminaries being disposed of, we will now mention how 
 preparations are to be made with the colours alluded to. The tissues 
 must undergo a series of operations, viz. : decolonisation, staining, 
 washing, dehydration, clearing, and finally, mounting. 
 
 Mounting Vegetable Tissues with a single colour. — As soon as the 
 sections (which have first been immersed as they were obtained in the 
 mixture of water and alcohol), are ready, they are lifted and transferred 
 to a second mixture of equal parts of water, alcohol, and glycerine. 
 This is what Bourgogne, Senr., called his preservative mixture. The 
 entire pieces or the sections can be left there for any length of time 
 without altering, if the bottle is well closed. 
 
 But we will continue our operations by passing our sections through 
 a preliminary decolourisation. 
 
 If the tissue is very delicate, immersion in strong alcohol for some 
 time is sufficient to decolourize ; but hard and highly-coloured tissues, 
 such as that of a woody stem, requires the action of Eau de Javelk, 
 
STAINING TISSUES. 
 
 325 
 
 This liquid can be made by taking, first, half a litre (7-8ths of a pint) 
 of water, and adding to it about 60 gr. of calcium hypochlorite, leaving 
 it to dissolve for twenty-four hours, but shaking it from time to time. 
 
 Next make a saturated solution of sodium carbonate (common washing 
 soda), in water, and after decanting the solution of calcium hypochlorite 
 to it is added by degrees the solution of soda, until it no longer 
 forms a precipitate. 
 
 The Eau de [avelle thus obtained, is then filtered, and kept well 
 stoppered in a dark place. 
 
 The sections to be bleached are then immersed in a small quantity 
 of this fluid, and are left there until the bleaching is complete, and then 
 washed several times. Lastly, they are again left for twenty-four hours 
 in distilled water, to which has been added 8 to 10 drops of nitric acid 
 to the half litre. 
 
 At the end of this period the sections are inserted in alcohol for an 
 hour or less, and then we proceed to stain, choosing preferably logwood, 
 which admirably sets in relief all structural details and the cell walls. 
 The staining will be systematically carried out by following carefully 
 the following steps. 
 
 1. Raise the sections from the alcohol and immerse them in water 
 for a few minutes. 
 
 2. Place the sections for ten minutes in 3 per cent, solution of alum. 
 
 3. Stain the section with logwood by immersing it in a small quantity 
 of the normal solution, then wash it in water until it is judged that 
 the colour is sufficiently intense. 
 
 4. Place it again in the solution of alum, so as to remove the stains 
 from the surfaces. 
 
 5. Wash thoroughly in water. 
 
 6. Place it in alcohol for at least two hours. 
 
 7. Deposit the section on the surface of some essence of cloves, 
 poured into a watch-glass. When the section sinks to the bottom it is 
 quite prepared, and can then be mounted in Canada Balsam. 
 
 Double Staining Vegetable Tissues. — These can be stained either iodine 
 green and carmine or ammonium picro-carminate. 
 
 1. With Iodine Green. — 1. Place the section in an alcoholic solution 
 of iodine green for one or two hours. This solution should be in 
 the proportion of 15 centigr. of green to 30 gr. of alcohol. 
 
 2. Immerse the section in alcohol for six minutes. 
 
 3. Place it in water for a minute. 
 
 4. Immerse the section in the carminated solution for two hours. 
 
326 
 
 MOUNTING PREPARATIONS. 
 
 This solution is made by dissolving in a gentle heat ; i gr. of carmine, 
 previously pulverized, in a gramme of liquid ammonia, and then adding 
 75 gr. of water, and filtering the whole. 
 
 5. Carefully wash with water. 
 
 6. Insert it for ten minutes in alcohol. 
 
 7. Float it on the essence of cloves, and finish as in the previous 
 case of single staining. 
 
 II. With Picro-carminate. — This process gives results which cannot 
 be surpassed. 
 
 1. Place the section in alcohol for an hour. 
 
 2. Stain in the picro-carminate for a period, varying from half-an- 
 hour to three hours. 
 
 This solution can be made according to the instructions previously 
 given for picro-carminate. It can be bought ready-made, but it is 
 much better when made by oneself as follows : — 
 
 Dissolve a decigr. of carmine in 2 gr. of distilled water in a gentle 
 heat, then add to it 30 gr. of distilled water. 
 
 Next dissolve 4 decigr. of nitric acid in 30 gr. of alcohol, which 
 has been slightly heated. 
 
 Mix the two solutions. The resulting solution obtained should be 
 filtered every time it is used. 
 
 3. Wash in alcohol. 
 
 4. Immerse again in picro-carminate. 
 
 5. Wash again in alcohol. 
 
 6. Float on the essence of cloves, and mount in Canada Balsam 
 (according to A. C. Cole). 
 
 § 5. — Mounting Preparations. 
 
 Many preparations are made in liquids, and so one should first make 
 the cell which is to contain the liquid. 
 
 For this purpose one should take the glass slip intended to carry the 
 object, and paint on it with a brush, two streaks of black varnish as seen 
 in figure 197. 
 
 The varnish should be left to dry, and when it is thoroughly so, 
 one, two, or three fresh coats of varnish are added, and are again left to dry. 
 Of course, the number of coats of varnish applied to the glass must vary 
 according to the thickness of the object intended to be enclosed in the cell. 
 
 When the varnish is again thoroughly dry, a drop of the liquid to 
 be used is deposited in the middle of the glass. This liquid naturally 
 varies according to the nature of the object. The latter is then placed 
 
MOUNTING PREPARATIONS. 
 
 in the liquid, and covered over with a cover-glass. During this operation, 
 if any air bubbles form in the cell, the cover-glass is half raised gently, 
 and the bubbles are got rid of by pricking them with the point of a 
 
 Fig. 197. 
 
 needle and inclining the slide. The cell having been deprived of all 
 air, any liquid that has overrun the side is dabbed up with a piece 
 of blotting-paper, and a coat of black varnish is given to the upper 
 surface of the cover-glass in the same direction in which the first coats 
 were placed, so as to moisten the latter. The preparation is then laid 
 aside for twenty or thirty minutes. At the end of this time, the varnish 
 being somewhat dry, and the cover-glass adhering more or less firmly 
 because the upper coat has softened the lower ones, two coatings are 
 added to the sides of the cover-glass, so that the brush touches the 
 cover-glass and the slip at the same time. When these coats are 
 dry, two or three others are successively added, at some days' interval, 
 so that the cell shall be perfectly closed. There is then nothing 
 more to do besides labelling the preparation. In Germany two strips of 
 glass are often stuck to the two ends of the cover-glass with potassium 
 silicate, so that the preparations may be superposed without fear of 
 damaging the cover-glass. This process is a bad one, as there is no 
 guarantee that the silicate will last. 
 
 Round Cells. — Most mounters instead of making square cells, such 
 as we have just described, prefer to use round cells, which are more elegant, 
 but not so firm. These cells are generally used in mounting diatoms dry. 
 
 To make round cells a small apparatus, called a turn table (fig. 
 198), is used. 
 
 The slips having been fixed 
 by means of two clips on the 
 turn table, a brush is steeped 
 in black mastic, which should 
 be somewhat fluid ; and having 
 F 'g- 198. set the rotating table in motion, 
 
 the brush is lightly applied so as to form a circle of the desired size. 
 When the cell is somewhat tightly stuck, the object is placed within it, 
 together with a drop of appropriate liquid. The cover-glass is then put 
 in its place and held there for a time, pressing on the cell, either by 
 
328 
 
 MOUNTING PREPARATIONS. 
 
 Fig. 199. 
 
 means of small weights or of a spring. When it has stuck sufficiently 
 fast it is wiped, and the sides of the cover-glass washed, and a fresh 
 coat of mastic applied, so that the sides of the thin glass are 
 thoroughly united to the slip. 
 
 With preparations in Canada Balsam, a drop of the balsam is placed 
 on the slip, which is gently warmed. 
 
 When the balsam is liquified the object 
 is deposited in it, avoiding the formation of 
 air bubbles ; but should they occur, they 
 should be got rid of by pricking with a needle. 
 Then a cover-glass, having previously been 
 slightly warmed, is placed over it and gently 
 pressed. Nothing then remains but to remove 
 the excess of balsam which has run out from 
 the sides of the cover-glass. This can be effected 
 by gently rubbing it with a piece of linen, 
 moistened with alcohol. 
 
 The small Marie-bath (fig. 199), manu- 
 factured by the firm of Chevalier, is very 
 useful for liquifying the Canada Balsam on the slip. 
 
 The Naples Marie-bath, by M. Adnet (26, Rue Vanquelin, Paris), 
 which was figured for the first time at the Antwerp Microscopical 
 Exhibition, answers the same purpose and is in other ways an apparatus 
 
 capable of rendering great 
 service in the laboratory of 
 every microscopist. This in- 
 strument, which is made of 
 copper, consists of a small 
 copper boiler, bearing at its 
 side a heating plate, on which 
 preparations can be placed. 
 In the centre of the boiler 
 is a small portable glass 
 chamber, on which cultures of 
 bacteria can be made, &c. ; 
 the latter can also be made 
 in tubes, which can be placed 
 as desired in the round 
 openings, which may be seen 
 in the illustration. In the 
 
 Fig, 199a. 
 
PRESERVATION OF PREPARATIONS. 
 
 329 
 
 small pots figured, paraffin or glycerine jelly can be liquified. A series of 
 paraffin moulds may also be seen in the figure. 
 
 A regulator, with a metallic membrane situated underneath the boiler, 
 enables the temperature to be regulated with the greatest accuracy. The 
 heat is produced by a small Bunsen burner, fixed to the regulator. By 
 means of a bent thermometer, the mercury reservoir of which is inserted 
 in the stove, the temperature of the whole apparatus can be told. 
 This very complete little apparatus costs ^4. 
 
 Liquidambar and Media of high Indices. — We have previously described 
 how preparations are made in these media, and there is therefore no 
 necessity to revert to the subject. 
 
 § 6. — Preservation of Preparations. 
 
 We have still a few words to say upon the preservation of micro- 
 scopical preparations. 
 
 Above all, damp is what has most to be guarded against ; it easily 
 filters through the best cements, destroys their adherence with glass, and 
 consequently renders the preparation useless after some time. Cold is 
 
 also capable of exercising a most 
 vexatious influence. Preparations 
 therefore should be kept in a con- 
 venient apartment, where there are 
 no injurious influences to be feared, 
 and should be examined from time 
 to time, in order to repair, where 
 necessary, the cements which have 
 become detached or cracked. 
 
 Preparations should be arranged 
 so as to lie flat. This system is 
 almost exclusively adopted in England, 
 and enables any preparation to be 
 found directly ; it has only the 
 inconveniences of being rather costly 
 and cumbersome. 
 Fig. 200. Grooved boxes are more economical 
 
 and more manageable. Preparations can then be placed on the shelves of 
 a library ; but care should then be taken in this case not to put the 
 boxes flat, but upright— like books — so that the sections immersed in 
 liquids cannot become deranged, for this inevitably occurs in the other 
 position. 
 
33° 
 
 PRESERVATION OF PREPARATIONS. 
 
 It never does to trust to the memory and to put on one side a 
 preparation which has not been suitably labelled. The label ought to 
 have written on it all the details which it is convenient to know, the 
 nature of the object, colouring, if any be used, the nature of the medium 
 used, &c, &c. Any of these details may have importance at any 
 moment. 
 
CHAPTER 11. 
 
 THE MICROSCOPIST'S LIBRARY. 
 
 Under this title we have collected together a number of books which 
 the microscopist, and especially the botanical microscopist, may with 
 advantage consult. 
 
 i. — Periodicals. 
 
 London (J.R.M.S.) — "Journal of the Royal Microscopical Society." 
 (London : Williams & Norgate). A part appears once every two months. 
 
 Since Mr. Crisp gave it its present form, in 1878, this journal has 
 been the most important in the world, and is absolutely indispensable 
 tc the microscopist who wishes to keep himself in touch with the 
 science. The J.R.M.S. publishes the transactions of the R.M.S. and the 
 original papers presented to the society. 
 
 It publishes, in addition, an analysis of all articles interesting to 
 microscopists, as well as descriptions of all old instruments whenever 
 they are discovered, and also of all newly-invented apparatus. 
 
 Pelletan. — "Journal de Micrographie," by Dr. Pelletan, Rue de Berne, 
 Paris. It has been published since 1877, and is always in touch with 
 all novelties, and equally indispensable to French readers. 
 
 W. H. Berens. — "Zeitschrift fur Wissenchaftige Mikroskopie und fiir 
 Mikroskopische Technik;' von Dr. Wilh. Jul. Behrens. Braunschweig, 
 1884-1892. 
 
 New York. — "Journal of the New York Microscopical Society." 
 New York, 1885-1892. Its articles are often very important. 
 
 Brussels. — " Bulletin de la Societe beige de Microscopic" Brussels, 
 1876-1892. It publishes the transactions of the meetings and original 
 articles by members of the society. 
 
 Antwerp. — "Annales de la Societe Phytologique et Micrographique 
 de Belgique." Antwerp, 1864, and following years. This publication 
 is temporarily interrupted. Original articles by its members. 
 
 2. — History of the Microscope. 
 
 Deby (Julien.) — "Bibliotheca Debyana" London, August, 1889. 
 Not publicly sold. A catalogue of a valuable library of microscopical works. 
 
332 THE MICROSCOPIST'S LIBRARY. 
 
 Harting. — "Das Mikroskop," second edition. Braunschweig, 1865. 
 
 Mayall. — " Cantor Lectures on the Microscope," by John Mayall, 
 Jun. London, 1886. 
 
 Peragallo (H.) — " Histoire sommaire du microscope compose et de 
 ses recents perfectionnements." Toulouse, 1883. 
 
 3. — General Works. 
 
 Adan (H. P.) — "Le Microscope, coup-d'oeil discret sur le monde 
 invisible." Brussels, 1873. 
 
 This is the first edition of the following work : — 
 
 Adan (H. P.) — "Le monde invisible devoile. Revelations du 
 Microscope." Brussels, 1870, with 24 plates. 
 
 This work, intended for the microscopical beginner, as well as 
 those whose interest is merely one of curiosity, is the best written of 
 all popular works. Written in that clear and elegant style which 
 characterises M. Adan, it is able to sustain the attention of the reader 
 without flagging, notwithstanding the dryness of the subject. 
 
 Beale (Lionel S.) — " How to work with the Microscope." Fifth 
 edition. London, 1880 (excellent). 
 
 Behrens. — " Leitfaden der Botanischen mikroskopie." Braunschweig, 
 1890. We have borrowed from this work figures 52, 53, 55, 56 and 
 59, which the editor has been good enough to lend us. 
 
 Carnoy. — "La Biologie cullulaire." Lierre. (Only the first part hitherto 
 published). 
 
 Carpenter. — "The Microscope and its Revelations." 1891. New 
 edition, edited by Dr. Dallinger, who has entirely re-modelled and 
 considerably improved and increased this work. 
 
 Chevalier (Arthur.) — "L'Etudiant micrographe." Paris, 1882. 
 
 Cole (Arthur C.) — "The Methods of Microscopical Research." 
 London, 1884. 
 
 Dippel (Leopold.) — "Das Mikroskop und seine Anwendung." 
 Second edition. Braunschweig, 1882. 
 
 Francotte (P.)— "Manuel de Technique microscopique." Brussels, 
 1887. (The application of the microscope to animal histology, com- 
 parative anatomy, &c.) 
 
 Giltay (E.) — " Inleiding tot het gebruik van den Microscoop." Leiden, 
 1885. 
 
 Giltay (E.) — "Seven objects observed by the Microscope." Leiden, 
 1890. 
 
 An excellent small work for beginners. 
 
THE MICROSCOPIST'S LIBRARY. 
 
 333 
 
 Griffith & Henfrey. — "The Micrographic Dictionary," 4th edition. 
 London, 1S83. In 8vo, of 829 pp., with 812 figures, and 53 plates 
 partly coloured. 
 
 Harting (P.) — " Het Mikroskoop, deszelfs gebruik, geschredenis en 
 tegenwoordige torstand," 4 vols. Utrecht, 1848-54. 
 
 The German edition which follows is much more complete and impor- 
 tant. 
 
 Harting (P.) — "Theorie und Algemeine Beschrebung des Mikro- 
 skopes." Second edition, 3 vols, in 8vo. Braunschweig, 1866. 
 
 Latteux (Dr. P.) — " Manuel de Technique Mikroscopique." 3 vols. 
 Paris, 1887. Excellent, especially in connection with human histology. 
 
 Naegeli und Schwendener. — "Das Mikroskop : Theorie und 
 anwendung desselben." Leipzig, 1867. (An English translation of 
 Parts I. to VII. was published in 1887). 
 
 Pelletan (J.) — "Le Microscope, son emploi et ses applications." In 
 8vo, with 278 figures and 4 plates. Paris, 1866. 
 
 Robin (Ch.) — "Traite" du Microscope." In 8vo, with 317 figures and 
 3 plates. Paris, 1871. 
 
 Schacht (H.) — " Le Microscope et son application speciale a. l'e'tude 
 de l'anatomie vegetale." Paris, 1865. Rather out of date, but a book 
 which can still be very advantageously studied. 
 
 Trutat (E.) — " Traite elementaire du Microscope." Paris, 1883. 
 
 Van Heurck (Dr. Henri). — " Synopsis des Diatomees de Belgique." 
 In Imperial 8vo, 2 vols., text and tables, and an atlas of 139 plates, 
 containing 3,100 figures, drawn from nature. Antwerp, 1880-85. 
 
 4. — Photo-micrography. 
 
 Marktanner-Turneretscher. — "Die Mikrophotographie. Hilfsmittel 
 natur wissenschaftlicher Forschung." Halle, 1890. 
 
 Moitessier (A.) — " La Photographie appliquee aux recherches 
 Micrograph iques." Paris, 1866. 
 
 Neuhauss (Dr. R.) — " Lehrbuch der Mikro-photographie." Braun- 
 schweig, 1890. 
 
 Pringle (Andrew). — " Practical Photo-micrography." New York, 
 1890. 
 
 Zeiss (Dr. R.) — " Special-Catalog fiber apparate fiir mikrophoto- 
 graphie." Iena. 
 
BOOK V. 
 
 THE MICROSCOPE IN THE PAST AND 
 IN THE FUTURE. 
 
 i. — The Microscope in the Past. 
 
 Notes on the History of the Microscope. 
 
 It follows, from a passage in Seneca, that the ancients knew that 
 when writing was observed through a glass globe filled with water, the 
 characters appeared considerably enlarged. But they attributed the magni- 
 fying power to the water, and every fact shows that glass lenses remained 
 unknown to them. All assertions to the contrary made on this subject 
 rest on error. 
 
 In 1285 Salvino d'Armato degli Armati, of Florence, discovered the 
 art of working glass and manufacturing glass spectacles. 
 
 It is known how tins occurred. The glass taken from the crucible, 
 after being slowly cooled, is divided into thin plates by means of a saw. 
 In Zeiss' workshops this division is effected by means of a circular steel 
 saw, the periphery of which is spread with diamond dust. 
 
 When plates or discs have thus been obtained of the desired thick- 
 ness, they are carefully examined with a lens, and all parts rejected 
 
 mercilessly which shew either 
 globules (fig. 201) or veins 
 (stria;) (fig. 202) which are 
 occasioned by the matter when 
 in a state of fusion not having 
 been stirred with the requisite 
 care, or not having been 
 Fig. 201. Fig. 202. fused long enough, or again, 
 
 not having been cooled with the necessary precautions. 
 
 This cooling is, in fact, a matter of great importance. It should 
 be carried out very slowly, otherwise the glass cracks ; and in some 
 cases it should be prolonged for weeks and even months. 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 335 
 
 Glass which has on its surface globules or striae — and especially the 
 latter — will give more or less cloudy images, in consequence of the 
 unequal refractions which occur in the different parts of the lens. 
 
 After the glass has been slit into plates, it is cut up into pieces of 
 approximately equal in size to that required, by grinding them with sifted 
 and moistened sand in moulds of a suitable shape, placed on a lathe 
 or special apparatus, generally turned in large workshops (such as those 
 of Zeiss) by steam. 
 
 When the glass is roughly shaped, and the desired curvature nearly 
 obtained, the serious and final operation is commenced. This work is 
 carried out by means of a special horizontal lathe, called an "optician's 
 lathe." 
 
 This lathe (fig. 203) consists of a solid table, usually of walnut 
 wood. On the left is a vertical shaft, fixed in collars, and terminating 
 
 Fig. 203. 
 
 in a point, which pivots in a metal centre placed on a horizontal 
 support underneath. 
 
 To this shaft is fixed a fly-wheel, and at its upper extremity a 
 piece of iron placed horizontally receives a wooden handle. 
 
336 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 On the right of the lathe there is a shaft similar to the preceding 
 and furnished with a pulley. The pulley and the fly-wheel are connected 
 by a leather band. By means of a screw thread, the tool is fixed at 
 the upper end of the right-hand spindle. This tool, which is of brass, is 
 concave for convex lenses, and is then called a basin (fig. 204) ; it is 
 convex for concave lenses, and is then called a ball (fig. 205). 
 
 Fig, 204. Fig. 205. 
 
 To make these tools it is first necessary to have what is termed a 
 gauge. This is made by cutting out of a plate of brass or steel a 
 curve, depending on radius required. The tool is then shaped by 
 turning it on an ordinary lathe until its surface accurately coincides 
 with the curve of the gauge. 
 
 The small piece of glass, one of the surfaces of which ought first 
 to be made more or less plane by sawing, after being roughly shaped, 
 is then roughly ground, which is done by working it in the iron 
 moulds (ball or basin) by means first of emery Nos. 1 and 2, and then 
 finer with Nos. 3 and 4. 
 
 Finally it is finished in a very accurate basin by means of emery 
 No. 5. 
 
 When the glass has been made smooth, softened, refined, and 
 polished, which is accomplished by successively using putty powder, of 
 different degrees of fineness, moistened with water, it is worked to the 
 correct curve. 
 
 During all these operations the glass is attached to a small cork 
 handle, called a holder. The lens is fixed with a mixture of pitch 
 and wood ash, which is softened by heat for attaching, and cools 
 quite hard. > 
 
 The finished lens is set into its mounting on the lathe, if it is 
 intended to be left in the state of a simple lens ; or if it is to be 
 achromatized, it is first fixed to its complimentary lens with Canada 
 Balsam. 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 337 
 
 The finished lenses are then matched to one another : formerly, and 
 this is still the case in some workshops, the mounting is done empiri- 
 cally, i.e., the lenses are fixed one above another and experiments 
 are made until the required quality of image is obtained. 
 
 By this means excellent combinations can be obtained, but the 
 majority of such objectives are more or less defective, and many 
 lenses have to be rejected. 
 
 In Zeiss' workshops these combinations are regulated by very precise 
 and ingenious apparatus, and the objectives are only tested in the 
 microscope when they are entirely completed. 
 
 Such are briefly the processes used at the present time in the 
 manufacture of lenses. Formerly lenses were made having a slight 
 curvature, intended only to be used as spectacle glasses ; by successive 
 steps they were improved, their curvatures were made greater and 
 greater, and the lenses becoming smaller ; it was thus that they were 
 brought by gradual stages to be employed as magnifying glasses and 
 simple microscopes. 
 
 An engraving is in existence, copied from a picture by Raphael, 
 dating from about 15 13 — 1520, in the Palais Pitti, at Florence, where 
 Pope Leon X. is represented looking at some minatures with the 
 assistance of a magnifying glass. 
 
 It was while arranging some glasses for spectacles, that Zaccharias 
 Tanssen is said to have discovered the compound microscope. 
 
 However, it will be convenient to treat the history of the simple 
 and that of the compound microscope separately. 
 
 Simple Microscope. — We have seen above how the manufacture 
 of spectacle glasses led to the construction of magnifying glasses, and 
 how these, becoming more and more convex, and consequently more 
 and more powerful, were able to justify the title of microscope. 
 
 The first microscope, though questioned by different authors, was 
 called by the name of vitrum pulicariujn, and consisted of a small 
 cylindrical box (fig. 206) ; the magnifying lens was set in the 
 upper cover, and the lower cover or bottom was made up 
 of two small pieces of glass, between which was deposited 
 the object for examination, which generally was a flea ; and 
 in this way the above name was given to the instrument. 
 
 There was also in use a seed microscope, a little toy, 
 such as may still be found in the windows of small 
 
 Fig. 206. 
 
 opticians and other shops. It was composed of a small brass 
 cylinder, the sides of which were open throughout the greater part of 
 
 Z 
 
338 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 its length, and it was inside a glass tube to admit the light. The 
 upper extremity was fitted with a lens, and at the bottom was placed 
 a mixture of different seeds. 
 
 Another early form of the microscope consisted of a magnifying glass 
 set in a mount and carried on a small foot. A needle was fixed 
 at a short distance from the lens and the object for examination 
 was stuck on the point of the needle. 
 
 It was this sort of microscope 
 which gave rise to the microscope 
 of Leeuwenhoeck, and with which 
 this illustrious microscopist made 
 his splendid discoveries. 
 
 Leeuwenhoeck's microscope 
 (figs. 207 and 208) differed from the 
 previous one in the perfection of 
 his lenses, which magnified highly, 
 and in the fact that the point which 
 carried the object was capable of 
 being raised or lowered by means 
 of a screw stem. The stem could 
 spring back easily, and by means 
 of a small screw-nut the object 
 could be brought nearer to the 
 lens, so as to be focussed perfectly. 
 
 Leeuwenhoeck had microscopes 
 of different magnifying power ; one 
 is known to have given 270 diameters. The lenses are bi-convex 
 and very well made, and Harting was able with the last-mentioned 
 one to observe the fourth group of Nobert's Test. 
 
 Simple microscopes, all more or less similar, were made by different 
 microscopists, all of whom used cut lenses ; but some opticians and 
 microscopists replaced these by lenses of cast glass. 
 
 These, it appears, were first invented by Hooke, who introduced 
 them in 1665 in his Micrographia. 
 
 A great number of microscopists used and described them ; among 
 these Pere della Torre, of Naples (1776), appears specially to have 
 succeeded in constructing globules of high magnifying power. 
 
 The last savant, who appears to have been successfully occupied in 
 producing such globules, is Lebailfif. The method employed by 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 339 
 
 this skilful observer has been described by Ch. Chevalier in his treatise 
 on the microscope, as follows :— 
 
 Two conditions are necessary in order to obtain perfect globules : 
 ist, it is necessary that they have a perfectly spherical form given to 
 them ; 2nd, it is important that the glass employed is pure and without 
 bubbles. 
 
 To avoid bubbles, a small piece of glaziers' glass must be taken, 
 which can be easily melted and is fair'/ pure. If the flame of a candle, 
 intensified by a blow-pipe, be directed full on the glass it will melt, and 
 often by directing the point of the flame to the end of a crack, one can 
 even succeed in giving to the fragment an elongated form (it is well that 
 their breadth should not exceed from 5 to 6 millimetres). In this way 
 the fraying is avoided, which the best diamond produces at the edges of 
 the strips cut off by it ; the bubbles almost always result from these 
 frayings which persistently remain in spite of the fusion. 
 
 The ends of one of these fragments is cemented to two pieces of 
 glass, or to the ends of two small tubes : then taking hold of these 
 accessory appendices with the two hands, the central fragment is placed 
 in the hottest part of the flame, and soon it takes almost the form of a 
 cylinder and half a millimetre in diameter. When a sufficient length 
 of glass has been thus made it is best to examine it with a magni- 
 fying glass to select the purest parts ; then they are placed back in 
 the fire to be lengthened into threads, the thickness of which should 
 be proportional to that of the globules desired. If it should happen 
 that sufficiently long fragments of glass become detached, it is useless 
 to attempt to solder them to the appendices. 
 
 It now remains to melt the globules, and at the same time keep them 
 spherical in form. To accomplish this, a piece of glass thread is taken 
 and cut at one end in the flame, for breaking it would give rise to some 
 inequalities. Then the thread is held by one of its ends with a watch- 
 maker's pincers, and the opposite end is inserted in the flame. It con- 
 tracts into a globule, which will not drop until it touches the pincers, 
 because they keep the piece of thread, which they hold, from becoming 
 hot enough to melt and unite with the globule. This globule remains 
 suspended by a very delicate strand, which is placed at the side of 
 the mount, so that the clearness of the vision may not in any way be 
 obstructed. 
 
 The smaller these globules are the rounder they are. 
 To test their purity they are taken up with the pincers by their 
 strand and held up to the light. They will appear perfectly clear if they 
 
 Z2 
 
34° 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 are pure, and studded with black spots if they contain striae or bubbles. 
 A spirit lamp is preferable to a candle, being less liable to dim or 
 stain the work. 
 
 We have in our possession some of the material which Lebailhf 
 used in the manufacture of his glass globules, as well as several globules 
 mounted by him. We can certify that the images they give are very good. 
 
 The simple microscope was considerably improved by Wilson about 
 1740. Wilson furnished his instrument with a mirror, and mounted it 
 on a foot (fig. 209). 
 
 The object was placed between 
 two glass slips, which were held tight 
 between two small brass plates. A tube 
 with a screw-thread enabled the object 
 to be raised or lowered so that it 
 could be examined in distinct vision. A 
 spiral spring pressing from above down- 
 wards holds the plates close, and counter- 
 acts at the same time the back lash of 
 the screw used for focussing. 
 
 Wilson's instrument was in great 
 repute, and was imitated on all sides. 
 
 We have in our possession a very 
 interesting specimen, bearing the in- 
 scription "Jacobus Lommers fecit 
 Fi S- 2 °9- Utrecht, Ao. 1758." The various lenses 
 
 have a screw-thread, which enables them to be adapted to a short tube, 
 furnished with an ocular, in order to transform the simple into a 
 compound microscope. 
 
 Cuff improved the simple in the same way as he did the compound 
 microscope, i.e., he made the stage moveable so that work could be 
 carried out more conveniently. 
 
 The microscope which Cuff made, about 1750, for Ellis, is still the 
 type of those made at the present day. 
 
 The instrument (fig. 210) consists of brass pillar A, which screws 
 on the box and bears an arm ending in a ring, in the groove of 
 which is fitted a glass disc, which serves as the object carrier. Opaque 
 bodies are placed on a black spot near the centre of the disc. On 
 the right side of the ring, pincers intended to hold insects can be slipped 
 into a hole there, which is made for this purpose. A cylindrical rod 
 C, which turns in a hollow cylinder, soldered to the pillar A, enables 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 341 
 
 the arm D, which carries the lenses, to be raised or lowered as desired. 
 
 This last-mentioned arm slides back- 
 ! d ■ wards and forwards in the socket which 
 
 can move easily in every direction, and is used to illuminate, from 
 underneath, the object which is above it. 
 
 If the simple lens A in this instrument is replaced by the doublet 
 C, which we have described at pages 18 and 19, we have an instru- 
 ment which can be usefully employed at the present day. 
 
 Towards the end of the third decade of the present century Raspail 
 found fault with Cuff's microscope, and added to it a number of useful 
 modifications. The rod C was replaced by a rack and pinion, and the 
 backward and forward movement of the rod D was produced by a screw. 
 
 The instrument also had fitted to it diaphragms and a series of 
 accessories, and the arm E, which carried the single lenses, was made so 
 that an achromatic compound microscope could be substituted. Thanks 
 to his Traite de ckimie, in which the instrument was warmly recom- 
 mended, this microscope was much used, and became known only 
 as RaspaiFs Microscope. 
 
 But the popularity of this instrument could not long be maintained. 
 The compound microscope was constantly being improved and soon 
 it definitely supplanted the simple microscope, the use of which diminished 
 to what it is at the present day, i.e., it became a convenient auxiliary 
 for microscopical dissections. 
 
 Compound Microscope. — The invention of the compound micro- 
 scope is attributed, as we have already stated, to Zaccharias Janssen (fig. 
 211), a small optician of Middleburg, whose residence was attached 
 to the church. 
 
 Fig. 210. 
 
 is at the end of the rod C, and these 
 different movements enable the lenses to 
 be brought over the whole surface of 
 the object carrier. The lenses are fixed 
 in the ring at the end of the arm D 
 by means of a screw-thread. They are 
 of two kinds : one E, mounted in the 
 centre of a concave silver mirror, is used 
 for examining opaque objects ; others, 
 without a mirror and similar to that 
 illustrated at F, are used for examining 
 transparent objects. Under the object 
 carrier is a concave glass mirror G, which 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 Fig. 211. 
 
 ZACCHARIAS JANSSEN, 
 
 Inventor of the Microscope, 
 
 Facsimile 
 
 After P. BORELLUS, De Vero Telescopii Inventore. 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 343 
 
 Lately, the Italians have claimed the invention for Galileo — heaping 
 wealth on the rich — but nothing has hitherto occurred to weaken 
 the serious and conscientious researches of Harting, and it is proved 
 that Galileo's instrument, formed of a convex objective and a concave 
 ocular, is not in any way the same as Janssen's instrument. 
 
 Harting's researches, the particulars of which are given in great 
 detail in his interesting pamphlet ('), shows that Janssen's invention can 
 be referred back as far as 1590. We are not acquainted with the exact 
 arrangement of the instrument, but it is known that it was composed 
 of one or two tubes fitting one in the other, and terminated by convex 
 lenses. 
 
 An instrument, called "Janssen's Microscope," is still preserved at 
 Middleburg, which, according to Harting, can be referred back as far 
 as his period, without its being the original instrument. This instrument 
 appeared at the Antwerp Exhibition. 
 
 This microscope, an exact fac-simile of which, made under the super- 
 intendence of Mr. Mayall, is in our possession, consists of four tubes 
 made of iron and soldered together, coated inside with tin. 
 
 The outer tube A (fig. 212) is of the greatest diameter, and con- 
 tains the tubes B and C, which slide into it. 
 
 Fig. 212. 
 
 The tube B contains within itself a fourth tube B', having at its 
 lower end a bi-convex objective lens of 3 J inches focus. 
 
 The ocular lens, of about 3 inches focus, is plano-convex, and is 
 held by a ring made of iron wire in a wooden cell. The tube C 
 which contains it is terminated at the upper end by a concave diaphragm. 
 
 The tube B' which contains the objective, is terminated at the upper 
 extremity by a diaphragm, flush with the end. 
 
 The tube B is terminated at its lower extremity by a diaphragm. 
 
 In using the microscope, B' is pushed right down into B, and 
 the tubes B and C are drawn out as far as possible from the exterior 
 tube A. The instrument is directed towards the object. 
 
 ( 1 ) De twee gewichtigste nederlandsche uitvindingen op naturrkundig gebied — without 
 place and date ; if we are not mistaken, the late Professor Harting sent us our copy about 
 1865. 
 
344 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 Fisr. 
 
 213. 
 
 The oldest microscope, of which we possess a figure, is that 
 
 of Hooke (fig. 213), which 
 is represented in the Micro- 
 graphia of that author, published 
 in 1665. 
 
 The tube was about 18 cent. 
 (7 inches) in length, and was 
 formed of four tubes fitting 
 into one another, and capable 
 of being drawn out to increase 
 the amplifying power. Beyond 
 the objective and ocular, there 
 was an intermediate glass, but 
 this Hooke withdrew in his 
 delicate observations. 
 
 The tube could be inclined, 
 and focussed by means of a screw-thread, in which it ended. Only 
 objects illuminated from above could be observed by it, and for this 
 purpose a glass globe concentrated the rays of a lamp on the object. 
 
 Campani, who lived at the same period, had on his part invented 
 a microscope without a field-glass, but one which allowed transparent 
 objects to be seen by directing the instrument towards the light. 
 
 It is impossible in the space at our disposal, to enumerate all the 
 apparatus invented one after the other, and which had but a slight 
 reputation. For this reason we shall not speak of the microscope of 
 
 Pere Che'rubin, of Orleans (167 1), 
 who also invented a binocular micro- 
 scope, a similar and very curious 
 microscope being in M. Nachet's 
 collection ; nor of that of Grindelins 
 (fig. 214) (1687), very similar to the 
 previous one in its appearance, but 
 very singular in the arrangement of the 
 glasses of the ocular and the objective. 
 
 From this we come at once to 
 the very interesting microscope of 
 Pere Bonanni, figured in his Micro- 
 graphia, published in 1691. 
 
 This microscope is very remark- 
 able for its appliances. 
 
 214. 
 
THE MICROSCOPE IN THE PAST AN I) IN THE FUTURE. 
 
 345 
 
 It k is horizontal (fig. 215), and possesses a rapid movement by rack 
 and pinion, and a slow movement by screw. 
 
 Fig. 215. 
 
 The tube contained three lenses, and Bonanni used three similar 
 tubes, giving different magnifications. 
 
 The object for examination 
 was held tight between two 
 springs and lighted by a lamp, 
 whose rays were concentrated 
 on the object by a true con- 
 denser formed of two lenses, 
 which could be brought nearer 
 to or withdrawn farther from 
 the stage. 
 
 This instrument, as may be 
 seen, is very perfect for that 
 period, and presents the germs 
 of all the appliances adopted 
 at the present time. 
 
 John Marshall, a London 
 optician, made, about 1704, a 
 microscope which enjoyed a 
 great reputation (fig. 216), and 
 only differs from the last in 
 having a series of objectives. 
 This microscope can be in- 
 Figi 2l6- clined by means of a ball and 
 
346 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 socket joint M ; first, it is focussed roughly by sliding the whole tube 
 along the square pillar K. After fixing it at a convenient height, by 
 means of the clamping screw H, " the focussing is effected by the slow 
 screw movement at F. 
 
 The object is placed on the stage C, which can be moved in any 
 direction. The bottom of the stage is formed of a sheet of glass. 
 
 In the first instance, Marshall illuminated the object in a very incon- 
 venient manner by means of a condensing lens R. Afterwards he replaced 
 this by a plane mirror. 
 
 In Marshall's instrument, one of which we have in our possession, there 
 are six objectives, each of which can be successively screwed to the tube. 
 
 They form a well-combined series ; when the tube is closed, the 
 following are the magnifications given by the objective : — 
 No. 1 ... ... ... 4 diameters. 
 
 ,» 2 
 » 3 
 » 4 
 11 5 
 6 
 
 When the tube is drawn out the magnification is increased by a quarter. 
 
 A peculiarity which we have not seen elsewhere noticed consists in 
 the fact that the ocular lens is made of smoked glass, which has 
 evidently been made so as to render the coloured edges of the object 
 less evident. This end is completely attained, and with the low powers 
 the images are remarkably clear. 
 
 It is curious that no subsequent maker of non-achromatic microscopes 
 ever thought of imitating Marshall in this arrangement. 
 
 Marshall's microscope was superseded some years afterwards (about 
 1738), by that of Culpeper and Scarlet (fig. 217). In this instrument 
 the focussing was effected by sliding the outer tube as in many modern 
 instruments, and the stage was supported by three pillars. 
 
 The mirror could be set in all directions, and the aperture of the 
 stage was made to receive either a diaphragm X, or a piece of 
 apparatus with a spring N, intended to hold the preparation. 
 
 Various accessories seen in the figure accompanied the instrument. 
 
 We have in our possession a series of instruments of this form, 
 dating from different periods, for it was manufactured for a long time ; 
 and we still remember a celebrated old Antwerp optician, named 
 "Ongania," who died in our childhood, and who had similar instruments 
 in his shop window. 
 
 7 
 
 12 „ 
 
 25 
 
 5° 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 347 
 
 The most perfect example of this form of instrument that we possess is 
 of English origin, and it belongs to the middle of last century, according 
 to the written instructions which accompany it. With it are five 
 supplementary objectives. 
 
 The popularity of this microscope 
 arose from its being able to be made 
 entirely of wood and cardboard ; also 
 the Nuremberg makers took it up and 
 made it in large numbers. One of 
 these microscopes,called The Nuremberg, 
 was sold at Ongania's for twelve shillings. 
 
 However, the arrangement of the 
 instrument was inconvenient, and above 
 all, it was difficult to manipulate the 
 preparation advantageously. This 
 objection was soon recognised, and the 
 stage in consequence was made free from 
 any obstruction. 
 
 Thus it is that in a Nuremberg 
 instrument in our possession, which 
 belonged to an Antwerp physician 
 named Vervliet, who died in 1764, 
 the tube carrier has at the side two 
 arms one above the other which can 
 be made to slide on a solitary bar, to which also 
 the stage is attached ; to this bar the tube 
 carrier can be fixed at any height (fig. 218). 
 
 The final focus was effected by sliding the 
 tube in its carrier. 
 
 The maker's name is not known, but the 
 instrument bears his mark (fig. 219), burnt in 
 with a red hot iron, which is also found on 
 other Nuremberg microscopes in our possession. 
 
 During the second half of the last century 
 several makers appeared who were true 
 connoisseurs, and who produced 
 offering points of real interest and 
 innovations that are remarkable, such 
 as Cuff, Jones, Adams, Martins, 
 
 Fig. 217. 
 
 Fig. 218. 
 
 and Dellebarre. 
 
 instruments 
 
 Fig. 219. 
 
34* 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 Cuffs instrument was patented in 1744, and displays many ingenious 
 arrangements (fig. 220). The stage is free, and can take several 
 pieces of apparatus above and a diaphragm underneath. 
 
 The mirror can be set 
 in any direction. The rapid 
 movement is produced by 
 sliding the case C along the 
 bar through the square plate 
 B. 
 
 The adjustment screw c 
 was used for exact focussing. 
 Lastly, the whole microscope 
 body could be taken off the 
 ring a in order to change 
 objectives. In the specimen 
 
 Fig. 220. 
 
 Fig. 221. 
 
 in our possession there are six very well made objectives. 
 
 Adams made several microscope models. The first (fig. 221), dating 
 from 1776, is without a stage; the lenses are arranged on a rotary or 
 revolving plate ; there is a fine adjustment, the milled head of which 
 is concealed in the box above the tripod which carries the instrument. 
 
 But the microscope models made by Adams are numerous. His 
 variable microscope (fig. 222) was mounted on a cog-wheel, 
 which 'enabled it to be inclined at any angle. The rapid movement 
 was produced by a rack, and the fine adjustment by an adjustment 
 screw copied from Cuff. 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 349 
 
 We have in our possession a pocket microscope of this maker, 
 which is a little gem. In its main features it approaches Cuff's 
 microscope, but the fine adjustment is effected by a milled head 
 above the limb, as in microscopes of the present day. 
 
 The whole microscope can be slid into a dovetail fixed on the 
 box. 
 
 Fig. 222. 
 
 The instrument is packed away in the same box, which is covered 
 with leather, and is only 18 cent. (7 inches) long by 10 cent. (4 inches) 
 broad, and 4^ cent, (if inches) deep; it can therefore be easily put 
 in the pocket. Accompanying this microscope are six objectives and 
 also the numerous accessories used at that period. 
 
35° 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 Dellebarre's microscope was described, in 
 1778, in a report presented to the Academy of 
 Sciences of Paris. The whole instrument is 
 carried on a tripod, supporting a round plate, from 
 which springs a square pillar (fig. 223). The 
 various pieces of apparatus slide over this pillar, 
 and can be fixed to it with clamping screws. 
 
 These pieces of apparatus, commencing from 
 the base, are as follows : — 
 
 1. Plano-concave mirror of large size. 
 
 2. Condensing lens. 
 
 3. Stage formed of a large ring, on which 
 rests a glass slide 0, at least 7 cent. (2f inches) 
 in diameter. The stage can be brought closer 
 to the objective by means of a rack and 
 pinion. 
 
 The body of the microscope is carried on a 
 ring ; it can be moved backwards and forwards, 
 and also sideways, so as to sweep the entire 
 extent of the stage. 
 
 There are three objectives. 
 But the special characteristic of Dellebarre's 
 microscope is that the ocular consists of no less than six lenses, 
 which can be combined in any way desired. 
 
 The field is excessively large, and observation is very wearisome ; 
 Dellebarre consequently abandoned it, and the instrument which we now 
 have before us has an ocular formed of only two lenses, one of 
 crown glass and the other of flint. The objective is of white crown. 
 
 Dellebarre claimed achromatism for his combination, and false colouration 
 is not so perceptible in it as in other microscopes of this period. 
 
 This microscope has been much praised. The Athen'ee des Arts of 
 Paris awarded to the maker a silver medal, the "22 Floreal an XI." 
 
 This medal was lent to us by his great grandson, who treasures it 
 exceedingly, to exhibit at the Antwerp Exhibition. 
 
 Louis Francois Dellebarre was a Frenchman of noble birth ; his 
 true name was de Streck ; he was born at Abbeville on the 16th 
 August, 1726. In consequence of his religious opinions he was obliged 
 to take refuge in Holland, where he assumed the name of Dellebarre. 
 Afterwards he returned to Paris, where he died on 16th March, 1805. 
 
 Fig. 223. 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 351 
 
 But his children, who had tasted Flemish liberty, returned to Holland 
 at the death of their father, and there established themselves permanently. 
 
 Fig. 224. 
 MARTIN'S MICROSCOPE. 
 Martiris Large Universal Microscope, which was made about 1780, 
 and of which a fine example, formerly belonging to Queckett, is in 
 London, is a very elegant instrument, expensively and stylishly set up 
 (fig. 224). 
 
 The foot is a tripod, the pillar is triangular and jointed at the 
 base so as to admit of it being inclined. This is produced by the 
 action of an endless screw, moved by the key H, acting on a wheel. 
 
 The mirror is jointed for oblique illumination, and moveable in all 
 directions. 
 
35 2 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 Another stage, Q, furnished with micrometrical movements, can be 
 substituted for the stage O. 
 
 Lastly, the body of the microscope FR can be taken off, and a 
 catadioptric microscope M be substituted. 
 
 The entire instrument is made with great precision and extraordinary 
 care for this period. 
 
 We have still to mention Jones's Microscope (A.D. 1798) ; that 
 called The Perfect Microscope evidently served as the model of Ch. 
 Chevalier's universal microscope, to which we shall subsequently refer. 
 
 We have now reached the present century, during which the 
 microscope has made such enormous progress. 
 
 Towards the end of the last century, in 1791, Beeldsnyder, a 
 Holland amateur, had made an achromatic objective, consisting of two 
 convex exterior lenses of crown glass, together with an intermediate 
 bi-concave lens of flint. 
 
 The lenses were made carefully, and the objective, screwed on a 
 good Amici mount, give a very suitable image. 
 
 His countryman, Van Deyl, went much farther. After having made 
 an achromatic lens, at a date which has not been fixed, he recommenced 
 work in 1807, when he had already reached his 70th year. 
 
 ago. 
 
 Unfortunately Van Deyl does not appear to have himself made the 
 experiment which would have effected a complete revolution in the 
 microscope. 
 
 He made a microscope after Jones' 
 model, and adapted to it two achro- 
 matic lenses, one having 18mm. focus, 
 and the other only 13. The lenses 
 are concavo-convex, having the concavity, 
 which is very slight, turned towards 
 the object. 
 
 Fig. 225. 
 
 We well remember the experiments 
 we made with this instrument, and 
 on one occasion with Harting, at 
 Utrecht. The images were clear, and 
 when the lenses were placed one above 
 the other, the images were not inferior 
 to those produced by objectives of 
 equal power twenty or thirty years 
 
THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 353 
 
 We must now proceed as far as 1824 before meeting with another 
 substantial improvement. At this period Selhgue had a microscope 
 constructed by Messrs. Vincent and Charles Chevalier, which was called 
 achromatic. 
 
 In the instrument (fig. 225) which was constructed from the specifi- 
 cations of Selligue, a mechanician, there are three or four plano-convex 
 achromatic lenses superposed (an idea due to Ch. Chevalier) ; the convex 
 surface is turned towards the object. Between the ocular and the objective 
 is introduced a bi-concave lens, which is intended to increase the 
 magnification. 
 
 Fig. 226. 
 
354 
 
 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 The lenses are i2^mm. (\ inch) in diameter, and of about 37mm. 
 (i| inches) focus. 
 
 Immediately above the upper lens there is a very small diaphragm, 
 the aperture of which is scarcely 2^mm. (i-ioth inch) in diameter. 
 
 The images do not shew much colour, but the objective is unable 
 to resolve any delicate detail. Thus the scales of Macroglossa Stellatarum 
 show no detail, while previous microscopes which were not achromatic 
 shewed the longitudinal lines. 
 
 It appears that but few of these microscopes were made, for examples 
 of them are very rare. We believe that at present none is known to 
 exist but that which we have just described. 
 
 Vincent and Ch. Chevalier themselves bore the expense of all 
 experiments and the contruction of the microscope, called " The Selligue." 
 Moreover, they were justly incensed that their name was not mentioned 
 in the report presented to the Academy. They made their discontent 
 known in a pamphlet, which they published in 1825, in which they at 
 the same time described a similar microscope constructed by them, as 
 they say erroneously, according to the specifications of Euler, published 
 in 1774, the single lens of which had a focus of 8 mill. (J-inch) and 
 a diameter of 4 mill. The crown and the flint were in this case for 
 the first time joined together with Canada Balsam. 
 
 Two years afterwards, in 1827, Amici took his achromatic compound 
 microscope, represented in figure 226, to Paris. The microscope which 
 we have in our possession was made about 1835, and is just like 
 that in the figure. The rapid movement is produced by rack and 
 pinion k, and the slow by a micrometer screw /. 
 
 The ordinary tube could be supplemented by an elbow-piece, 
 containing a prism A. 
 
 The stage // is far too small for serious work, and the instrument 
 lacks stability. 
 
 There are four objectives ; some of the lenses can be combined in 
 different ways with one another, and also with some accessory lenses. 
 The object of these combinations is to give a clear image, with cover- 
 glasses of different thicknesses. 
 
 The image given by these objectives is very remarkable for this 
 period. Thus the objective marked O • : \ gives a good image of the 
 pygidium ; it becomes even excellent when the lens marked 6 is mounted 
 on it. 
 
 But the instrument of this period, which rapidly acquired a 
 considerable reputation, was the instrument which Ch. Chevalier 
 
THE MICROSCOPE IN THE I'AST AND IN THE FUTURE. 
 
 355 
 
 produced in 1834, to which he justly gave the name of The Universal 
 Microscope. 
 
 In fact, this instrument could be used, with an elbow-piece, as a 
 horizontal microscope, as well as with the vertical tube ; the tube could 
 also be reversed, the objective being turned towards the sky for obser- 
 vations in chemistry, and lastly, the compound tube could be taken off 
 in an instant, and a simple microscope be substituted for it. 
 
 The coarse adjustment was by rack and pinion, and the fine by a 
 very complicated, but very precise, micrometric screw. 
 
 The microscope had a rotary diaphragm, with a series of apertures ; 
 and these, at pleasure, could be turned aside from the axis of the 
 microscope by inclining it. 
 
 We will mention how, according to Chevalier himself, the various 
 transformations in the universal microscope came about. 
 
 Figure 227 represents the instrument which is screwed to a brass foot. 
 In this microscope the tube T is moved by gently sliding it in the 
 
 tube Z, the stem D turns round 
 in the piece S. The tube 
 carrying the objective turns also 
 at P, so as to be able to 
 change the latter. 
 
 The prism carrier V slides 
 into the tubes A and Z, to 
 make a horizontal * microscope. 
 To make the microscope vertical 
 the piece V, with ne body of 
 the instrument Z, is first > re- 
 moved, then the prism carrier 
 V is separated from the body 
 Z, and lastly, the latter is 
 pressed on to the tube A, and 
 in this manner the instrument 
 becomes a vertical one. 
 
 The piece A P is adjusted 
 on the stem D by means of a moveable pin, having a knob g. The 
 piece V, which enters the tube A by slightly pressing it, enables the body 
 Z and the prism carrier V to be turned round on an axis, which 
 passes through the objective Y. 
 
 The stem C is fixed at the lower end to the piece E by means 
 of the headed pin G. 
 
 AA2 
 
356 THE MICROSCOPE IN THE PAST AND IN THE FUTURE. 
 
 The mirror H moves round in a circle so as to admit of oblique 
 illumination. The stage is moved by the pinion O. The adjusting 
 screw Q produces the slow movement. The variable diaphragm R is 
 fixed on a piece fitted with a hinge and pivot, so that it can at any 
 time be removed. We have already seen how the horizontal can be 
 changed into a vertical microscope ; to transform it into a simple micro- 
 scope, it is only necessary to remove the body of the microscope, which 
 is fixed to the support on the piece PA by means of the knob g; 
 ring for the purpose of holding the doublets is substituted for it, and 
 also fixed in position by the knob g. It is very easy to make a 
 chemical microscope of it : the tube Z and the prism carrier, which 
 move over the tube A, are made to describe a quarter of a circle 
 from right to left. This done, remove the pin G, and lift the pillar C 
 on the hinge E, then make it describe a semi-circle on the pivot S ; 
 let the whole turn on the hinge so that the apparatus becomes horizontal, 
 and you will have an excellent arrangement for chemical experiments. It 
 will only be nesessary to replace the stage and place it above a shelf 
 carrying spirit lamps, if we require to use heat during the observations. 
 
 This microscope may also be used at every desirable inclination by 
 turning it on the hinge E. If we want to use direct light, the apparatus 
 may be set up horizontally on the hinge E, or after that it can again be 
 easily placed in a horizontal position by turning it through a quadrant 
 of a circle on the pivot S. 
 
 Finally, by means of the hinge E and the pivot S, it is easy to 
 conceive that the microscope may be placed in every desired position. 
 If this description has been read attentively, it can be seen that the 
 universal microscope combines every desirable advantage. 
 
 The universal microscope in our possession is accompanied by two 
 objectives. 
 
 The first is a half-inch having a N.A. of 0*2. 
 
 The second is a 1-5U1 inch which has a N.A. of 0-27. 
 
 r Ihe images of histological objects are very good, but with this feeble 
 aperture one should not attempt to look for detail in diatoms. 
 
 A little later, Ch. Chevalier constructed very powerful objectives, 
 following exactly the same methods. A half-inch in our possession is 
 capable of resolving Nobert's 6th group. At this point we may bring to 
 a close what may be considered as the ancient history of the microscope. 
 The present form of microscope, i.e., the true modern microscope, dates 
 from the time when Amici introduced different kinds of glass into the 
 construction of lenses : in particular, various kinds of flint, and from 
 
THE FUTURE OE THE MICROSCOPE. 
 
 357 
 
 the time when each glass was no longer separately achromatized. It 
 is this last modification in their construction which has enabled their 
 numerical apertures to be increased, and more and more delicate details 
 to be resolved. 
 
 The introduction of immersion by Amici, about 1855, was the 
 second important transformation that the modern objective has under- 
 gone, and has been the means whereby the splendid results obtained 
 by contemporary manufacturers have been realized. 
 
 2. — The Future of the Microscope. 
 
 Dear Sir, 
 
 When, during your last visit to Iena, we discussed several 
 questions concerning microscopical optics, we also very naturally came 
 to speak of the future reserved for this powerful instrument of investiga- 
 tion, as far as scientific knowledge and the technical means at our 
 present disposal could allow us to divine. 
 
 I observed on this occasion, that, in my opinion, the greatest 
 progress we could still realize would have to be brought about in a 
 direction which had not been pursued as far as it reasonably could 
 have been. However, even in this case great advance has been made, 
 and we are drawing near to its possible limits. I promised to commit 
 my opinion on the matter in writing, and now I am about to fulfil it. 
 Let us first establish the fundamental formula, giving the real magnifying 
 power of the microscope. Let 8 denote the smallest interval in a 
 regular structure which can be resolved with an optically-perfect objective ; 
 let A denote the wave length, in vacuo, of the light employed, and a 
 the aperture of the system ; then for central light we shall have the 
 formula : 
 
 A 
 
 8 = — 
 a 
 
 both according to Abbe's theory and that of Helmholtz. 
 
 An increase in the power of the microscope is generally measured 
 by the smallness of the elements of a structure that can be clearly 
 distinguished ; we can therefore denote the power by the smallness of 8. 
 Now, as in the formula, 8 is simply the quotient of two magnitudes 
 we can diminish it in two ways, either firstly by making a larger, or 
 secondly by making A smaller. 
 
 In the first method, we know by the works of Abbe and 
 Helmholtz, that increasing the magnitude of a, i.e., the aperture oj the 
 
358 
 
 THE FUTURE OF THE MICROSCOPE. 
 
 objective is the method which has been adopted successfully by every 
 optician for improving the microscope. 
 
 Let us examine to what limits this method can be carried, and 
 how near we are to them at present. 
 
 We shall assume that a = n sin u, a formula in which « 
 represents the index of refraction of the medium in front of the 
 frontal lens of the objective, and n the angle that the extreme ray 
 capable of penetrating it makes with the axis of the lens. This angle 
 cannot, for purposes of ordinary geometry, exceed about 65° in any 
 case ; for there must always be a certain distance, no matter how 
 small, between the object and the objective, both for inserting a 
 cover-glass and for purposes of focussing. It follows then that sin it 
 cannot exceed 0-95. There is therefore as soon as this geometric 
 limit is reached, which usually happens with very powerful systems, no 
 other means open to us except that of increasing the value of the 
 index of refraction of the medium placed in front of the objective. It 
 was this that led to the principle of itnmersioti. It must, however, here 
 be mentioned that it is not sufficient to interpose between the cover- 
 glass and the frontal an immersion liquid of as high a refractive index 
 as possible, but it is also necessary that between the object and the 
 immersion medium there is no layer, however thin it may be, whose 
 refractive index is lower than that of the medium, unless the 
 aperture of the objective, however high the index n of the immersion 
 liquid may be, is reduced to the magnitude d = n where ri is the 
 index of the layer of lowest index lying between the object and the 
 immersion liquid. I have already mentioned this in my note entitled 
 " Ueber ein system von der Aperture r6o, &c." (On a system of an 
 aperture of 1 '6o, &c, J.R.M.S., 1890). 
 
 Now the greater part of preparations require a cover-glass. The 
 index of refraction of cover-glasses usually employed varies from i - 52 
 to 1 "53. This kind of cover-glass is easy to manufacture, and can 
 therefore be supplied at a reasonable cost ; moreover, they are very 
 convenient to use, but they can only be used so long as the aperture 
 of the objective does not exceed the value 1 "45. 
 
 To pass this limit, the cover-glass, as I have shewn in my previously 
 mentioned note, must be made of highly refractive glass, which offers 
 innumerable difficulties. 
 
 It is very true that the glass works of Jena, belonging to Messrs. 
 Schott & Co., have turned out glass of every possible refractive index, 
 even up to 2 00. But cover-glasses made of such glass are very 
 
THE FUTURE OF THE MICROSCOPE. 
 
 359 
 
 expensive. They cannot be produced by blowing, but they have to 
 be made in the same way as are the plane slips supplied by opticians. 
 They have to be reduced by sawing to a thickness of about 015 to 
 o"2mm. (i-i6th to i-i2th inches), then they have to be polished. In 
 the first place a very large quantity of expensive glass is lost during 
 these operations, and then the manual labour absorbs still more money. 
 But this is not all, for, when the cover-glasses are obtained in this 
 way, it appears that while they are being used they offer many 
 inconveniences, which are not met with in the ordinary ones, and 
 with which no one perhaps is better acquainted than yourself, dear 
 Mr. Van Heurck. Then, as has been mentioned, the figure represen- 
 ting the index of refraction of every medium between the object and 
 the frontal should at least be equal to that representing the aperture 
 of the objective. Also, the object should be prepared in a medium 
 of an index at least equal to the above. This is not impossible, for 
 we have in our possession media whose refractive indices exceed 2'o. 
 
 But these media and the method of employing them present in- 
 conveniences. Such are the preparations of arsenic and phosphorus, 
 which have to be heated while the object is being prepared. Now 
 these substances, when heated, give off very poisonous fumes, and may 
 also occasion explosions. It follows that preparations made with such 
 substances endanger the life of the operator. 
 
 Moreover, the inconveniences arising when an objective of N.A. 
 1 '6 is used, eclipse any others that may arise, for these substances 
 attack the flint of which the cover-glass is made, which becomes dull, 
 and hinders vision after a variable length of time, which is sometimes 
 very short. 
 
 It is our intention to see whether this inconvenience can be 
 avoided by using objectives of large aperture, whose construction will 
 yet have to be attempted, but there is no reason for saying definitely 
 that it will be possible. On the contrary, it can be anticipated from 
 now, that cover-glasses of very refractive flint will always be more 
 sensitive to re-agents than ordinary cover-glasses of crown ; and, con- 
 sequently, the number of useful media will be fewer in number. It 
 may also be foreseen that preparations made with these special glasses 
 will be, incomparably, more expensive than those made with the 
 ordinary crown. 
 
 But, besides these objections, it should be remembered that 
 media, like the above, would be bound to affect the substances in which 
 objects are prepared ; if these, as is so in most cases, are organic 
 
3 6o 
 
 THE FUTURE OF THE MICROSCOPE. 
 
 substances, they would, if the present methods of preparation were used, 
 be either attacked or even destroyed, either by the medium or the 
 increase of temperature. Such alteration would ensue, even when the 
 naked eye could not observe it. The majority of organic substances 
 will therefore always, as well as now, require refractive media, whose 
 index is generally 1-35 and, at the most, 1/6. Such are the reasons 
 which will ever prevent the optical power of the microscope from being 
 increased by increasing the aperture beyond the limit which we have 
 already reached, and we shall be obliged to see whether we cannot find 
 the solution of the question in some other way. 
 
 II. Another actually does exist, and as has already been mentioned 
 above, it consists in diminishing the value of A which represents the 
 wave length of the light used during observations. 
 
 By decreasing A a diminution of n is produced in all the immersion 
 liquids which have a normal dispersive power, which diminution 
 corresponds to an increase acting on the magnitude of 8 in the same 
 sense as the diminution of A. However, this is, comparatively speaking, 
 too insignificant a factor for us to dwell on here. 
 
 If we make observations with ordinary sunlight (light reflected from 
 white clouds), then a large number of different wave lengths would 
 act simultaneously, i.e., those of the visible spectrum. But the absolute 
 energy (physical or mechanical), of the solar rays is not the same in 
 all parts of the spectrum, no more in the same way is the eye equally 
 sensitive — to a physically equal excitation of the retina— to all the 
 colours, i.e., the physiological energy of the rays of different colours is 
 itself also different. The result of this is that the intensity of the 
 impression which white light from the clouds produces on the normal 
 eye, varies according to the wave length, by a law which can be 
 represented by an undulating curve whose maximum height is given by 
 A = 0 55/x. The eye then receives the most vivid impression of rays 
 of this wave length, and of those very near to it, and this impression 
 is so powerful that the production of other wave lengths, which are 
 greater or smaller, is either quite destroyed or at least rendered inactive, 
 except in the case in which their properties are equivalent to those 
 which produce the image A = 0-5 5/x. If, on the other hand, one can 
 successfully suppress these rays of a wave length of o - 55/x, both 
 those which act most energetically and those of a greater wave length — 
 whether really or physiologically — and consequently only the shorter rays 
 reach the eye, then in favourable cases, and noticeably when using 
 sources of light which are too intense, then, I say, one can succeed 
 
THE KUTURK OF THE MICROSCOPE. 
 
 361 
 
 within certain limits in making them active in their turn. Indeed, it 
 is too well known how that the resolving power of any objective is 
 increased when it is used in a blue light, obtained either with a 
 complex apparatus for producing monochromatic light, or even by 
 simply using solutions or glass absorbents ('). It can then be seen 
 that striae, &c, which remain invisible in white light, are with the 
 same objective perfectly resolved, all other conditions remaining the 
 same. Indeed, the eye is always sufficiently sensitive to rays which 
 have a wave length of 044/x for obtaining on the retina a very 
 vivid image in these rays by themselves. Now a reduction from 0*55 
 to 0-44 gives the same ultimate optical effect as if the aperture of an 
 objective were raised from 1*40 to 175. As is evident, a very con- 
 siderable progress is realized in this way by excessively simple means. 
 
 This is the reason, as has been established by Helmholtz and 
 others, why photography is a means of increasing the resolving power 
 of tlie microscope. However, practice has not in every case confirmed 
 this theory, because a very important factor, which is indispensable 
 for its realization, has been neglected. Indeed, to obtain the required 
 result it is always necessary that the objective used in photography 
 for rays of feeble wave length be made so as to give with such rays 
 quite as perfect images as with the white light usually employed. 
 Now this is not the case as a general rule. On the contrary, with 
 achromatic objectives of the ordinary kind, i.e., the only ones 
 which have been available for some years, such images could not be 
 obtained. When the objective was corrected so as to give good images 
 with light of a wave length C55/X then images obtained in light of 
 o - 44/x were so bad, i.e., the chromatic and spherical corrections of the 
 objective were so defective in this illumination that the theoretical 
 affirmation of the increased resolving power could not be demonstrated. 
 The difficulty was then got over, as it used to be done, and as it is 
 still done now, with ordinary photographic objectives. The objective 
 was corrected spherically for the most active rays in photography, and 
 the chromatic correction of the objective was made so that the image 
 formed by the rays of o"55/* nearly coincided with the photographic rays. 
 Consequently, with the naked eye, one could focus the photographic 
 image with sufficient accuracy. However, the result thus obtained stil 
 left much to be desired, for, firstly, the active optical image was bad; 
 it was spherically under-corrected (insufficiently corrected) and chromatically 
 over corrected (too much corrected) ; secondly, the active part of the 
 
 ( 1 ) See pp. 124 — 125 of this book. 
 
362 
 
 THE FUTURE OF THE MICROSCOPE. 
 
 spectrum in photography was incompletely concentrated (or re-united) ; 
 the images which correspond to the different active wave lengths of 
 the spectrum coincided neither in position nor in size in consequence 
 of the chromatic under-correction of this part of the spectrum. 
 These various images cannot therefore combine with each other, on the 
 contrary, there is a danger of them mutually destroying each other. 
 An attempt has also been made to remedy this in some measure by 
 employing only a very small part of the spectrum. It is evident 
 that the intensity of the light is much reduced in this way, which 
 has its inconveniences. In every case the objectives specially constructed 
 by opticians for photo-micrography can never be advantageously employed 
 for observations and inversely. 
 
 The immense advance which has been realized in this way, by the 
 construction of apochromatics, consists in the fact that in these objectives 
 the images given by the different wave lengths of the spectrum, up to 
 the violet inclusive, coincides for all practical purposes exactly in position 
 and size. Also, the advance realized by these objectives is much greater 
 in photography than in direct observation. Again, since the introduction 
 of these objectives, photo-micrography has made enormous progress, and 
 the cases in which structures that are invisible, or scarcely visible at all 
 in direct observation, have been photographed and made to appear quite 
 sharp, are numerous. 
 
 However, in my opinion, all that can be desired is not yet supplied 
 by these objectives. We can only hope to reach the maximum resolving 
 power, as I have already mentioned above, by employing rays of feeble 
 wave length, all other rays being excluded ; for when the rays of large 
 wave lengths help to produce the photograph, then the same effects as 
 we have already noticed when using white light, may also be produced ; 
 that is to say, the longest waves predominate and conceal the feeble 
 image given by the shorter rays, for the sensitive plate has not really 
 the property as is often attributed to it, owing to the imperfection of 
 the eye. On the contrary, and this can be done and well understood 
 from our knowledge of chemistry and the practique of photographic 
 processes. We have therefore really nothing to wait for, neither ortho- 
 chromatic plates, nor filters of green, yellow, or even brown light. 
 Besides, it is well known that very often the natural colour of the 
 object necessitates the application of like filters, if the detail of the 
 object is wanted to be very pronounced. There are some photographic 
 reasons which demand these conditions. The hope of being able to 
 advantageously photograph this kind of object is quite excluded. 
 
THE FUTURE OF THE MICROSCOPE, 
 
 3 6 3 
 
 However, by that we do not mean to convey that photographs of 
 such objects are unable to have their advantages, but that these advan- 
 tages are of quite a different nature, which we have no room to mention 
 here. 
 
 'The cases in which photography with the objective can present 
 advantages over direct vision, i.e., shew further than this last and the 
 limit to which these advantages can go, depend on the two following 
 circumstatices : — 
 
 I. That the objective be corrected so that the image to be obtained 
 with the short rays A = x be in itself sharp, and coincide in position 
 with the point where it is so for the eye, unless it cannot be focussed. 
 This correction, which is obtained so far for A = 0^44 or for A = 0 5, 
 can also be obtained for much shorter rays. One can find and deter- 
 mine, by photography itself, the index of refraction of glasses which 
 would have to be used, so as to make the objective use up those rays 
 which have no marked action on the retina of the eye ; both the 
 optician, the manufacturer, and also the mathematician, actually have at 
 their disposal sufficient means to construct objectives which could be 
 handed over to the microscopist, thus giving him a priori the assurance 
 that they are suitably corrected for the invisible rays, without having 
 to use the eye to control it. The same thing should be done for 
 every desired limit of A. 
 
 II. In the second place, the light coming from the short rays in 
 question must be photographically active. We here encounter four 
 considerations of a second kind : firstly, the rays from the luminous 
 source should have in this part of the spectrum the required short 
 wave length and be sufficiently intense ; secondly, the rays of large 
 wave length should be intercepted by appropriate filters, without at the 
 same time on that account diminishing the intensity of the rays of 
 short wave length too much ; thirdly, the photographic plate should be 
 sufficiently sensitive to the light employed, and if the plate has its 
 maximum sensitiveness when light of the required wave length is used, 
 then the same effect will be produced as by using filters ; fourthly, all 
 the media between the luminous source and the photographic plate 
 should allow the short rays in question to penetrate them. 
 
 This last condition, in my opinion, marks the farthest limits we can 
 reach. It is known that ordinary glasses only admit a very small fraction, 
 o"3, of the rays. The difficulties which arise, in consequence of using 
 rays of short wave length, are enormous, and an idea of their number 
 can be formed by reading the works of physicists who, like Cornu and 
 
364 
 
 THE FUTURE OK THE .MICROSCOPE. 
 
 Schumann have photographed the ultra-violet part of the spectrum. I 
 conclude by saying that if luminous rays of a wave length of 0*35/* 
 be used, the farthest limit is thus given, which will be attained in a 
 given time unless the question becomes inconceivably complicated. 
 
 But even to reach this limit considerable work, is necessary, in which 
 opticians, physicists, photo-chemists and micrographers will have to take 
 part. But their united efforts will produce a sufficiently substantial 
 effect to make the subject attractive. Indeed, by employing a light 
 of 0-35 instead of ordinary daylight, composed of rays of a mean wave 
 length of A = 0*5 5/x, the same advantage would be gained as if 
 the apertures of our present objectives were increased from i'4o to 
 
 2'20 ! 
 
 Accept, dear Sir, my best wishes, 
 
 Dr. S. CZAPSKI. 
 
ADDENDA. 
 
 i. Messrs. Bausch and Lomb's Fine Adjustment. — We 
 
 have already described this fine adjustment briefly on page 145, but 
 having lately been able to procure a wood-cut illustration of it, we 
 give it here (fig. 228), and at the same time take advantage of the 
 occasion to describe it in greater detail. 
 
 Fig. 328. 
 
 aa represent two strong blades of finely-tempered steel, set parallel 
 to each other, and securely fastened at one end to the back of the 
 case d, and at the other to the arm e, which carries the rack and 
 pinion ; b represents the micrometer screw, which is fitted to the 
 upper part of the upright arm c; f is the pinion; g the rack; and h the 
 tube. Two screws fasten the adjustment case d to the pillar c. An 
 arm projects from the part e and passes into a recess in the pillar c. 
 The springs support the entire weight of the body, and as their tension 
 
366 
 
 ADDENDA. 
 
 is upward, the projecting arm presses continuously against the micro- 
 meter screw b, and it is evident that whatever distance is traversed 
 by the screw, the arm e, and consequently the whole body, must move 
 through the same distance. The only points of contact are at the 
 ends of the springs aa, where they are both fastened respectively at d 
 and e and on the micrometer screw ; and, as in the former, there is 
 absolutely no friction, there is no wear ; while any that may eventually 
 arise in the latter, is made up for by the force of the springs. 
 
 Since page 140 and those which follow were printed, we have 
 had occasion to examine a i~5th inch objective of these makers, with 
 correction collar in their Professional Series. It has an angular aperture 
 of 130 0 or a N.A. of about "91. The images are very clear, and 
 with axial illumination the PI. angulatum can be resolved. With oblique 
 illumination both the striae of the large forms of Vanheurckia rhom- 
 boides and the 12th group of Nobert's test can be seen well. 
 
 Before concluding this note we desire to modify the assertion we 
 made at page 144, that the medium Continental microscope "is a very 
 inferior model." It is almost unnecessary to explain that we were not 
 denouncing the manufacturer's work in the instrument, but that we 
 found the microscope insufficient for many serious and delicate re- 
 searches, and that, consequently, we advised the purchase of a model 
 of superior make. 
 
 2. Note on Apochromatic Combination. — No precise informa- 
 tion has hitherto been published on the arrangement of lenses in 
 apochromatic objectives and in compensating oculars. It is therefore 
 believed that the following note will be read with interest, supplying as 
 it does, the above-mentioned omission, and also giving the path of rays 
 in these pieces of apparatus. 
 
 As will be seen in the adjoining illustration (fig. 229) the general 
 path of the rays does not differ from that which takes place in 
 instruments which are not apochromatic. The objective projects into the 
 diaphragm of the ocular (the small arrow in the diagram) a real image 
 of the object inverted and enlarged. The object, which is placed 
 near the front focur. of the objective, is not shown in the figure] on 
 account of its small size. 
 
 With compensating oculars Nos. 4 and 6 the image, formed by the 
 objective, is diminished in size; and, moreover, the divergence of the 
 principal rays (axis) of the pencil undergoes a change by passing 
 through the field lens, just as happens in the Campani's (Huyghen's) 
 oculars, usually employed in microscopes. 
 
Fig. 229. 
 
ADDENDA. 
 
 3 6 9 
 
 In oculars Nos. 8, 12 and 18, a type of which appears in the 
 illustration, there is no field lens; the eye lens itself fulfills both 
 functions : firstly, by rendering the principal rays convergent, and then 
 by increasing, i.e., by diminishing the divergence of the pencil emanating 
 from different points of the object. The eye lens therefore (i.e., the 
 front lens of the combination in these compensating oculars), produces 
 entirely the essential refractive effect, while the sole purpose of the 
 triplet underneath is only to correct the image of chromatic and 
 spherical aberration effects; its focus is very long, so that pencils leave 
 the triplet with very nearly the same divergence that existed between 
 the principal rays (axes) and their secondary rays when they entered 
 the triplet. 
 
 The image produced by the oculars is drawn in the figure very 
 near the point of vision, as actually happens with a myopic eye. With 
 a normal eye the image will be situated at almost double the distance, 
 say 25 centimetres (10 inches), the distance of normal vision. 
 
 The distance for short sight has been purposely adopted in the figure; 
 if that corresponding to a normal eye had been taken, the distance 
 would have been double, and the magnification also, so that in that 
 case the arrow 2 would have been too large for the size of the page. 
 
 With regard to the objective its specific action may be described 
 as follows : the simple frontal lens, which is nearly hemispherical in 
 shape (indeed, in the case in the figure, which represents a 3mm. 
 (£ inch) objective, with N.A. i - 4o, it is even more than hemispherical) 
 is employed, after Amici, for all high power objectives. 
 
 The object of a lens of this form is to bring about a sufficient 
 change of divergence in the pencil emanating from the different points 
 of the object (i.e., to enlarge the object) without at the same time 
 introducing great corresponding chromatic and spherical aberrations. 
 
 If the object were placed at the true aplanatic point of the spherical 
 surface, and if the immersion fluid were perfectly homogeneous, i.e., if 
 the coefficient of refraction of the immersion liquid was exactly the 
 same as that of the front lens, then the virtual image of the object, 
 produced by this front lens alone, would be entirely free from aberration, 
 and aplanatic for one of the colours of the spectrum. 
 
 In practice, however, this is not so, because often it is advantageous 
 to intentionally introduce slight aberration so as to be able to destroy 
 it more completely afterwards. 
 
 This front lens reduces the aperture of the pencil from 1*40 to 
 0-65. 
 
 BB 
 
37° 
 
 ADDENDA. 
 
 The double lens which follows is used for the same purpose in all 
 high-power objectives, firstly, to still further diminish the divergence of 
 the pencil (in the system in the figure this diminution reduces the 
 aperture from o - 65 to 0*29), and secondly, that that amount of chromatic 
 and spherical aberration may be intentionally introduced, which can be 
 most perfectly corrected by the succeeding lenses. 
 
 It is characteristic of apochromatic objectives that this front part of 
 the system should consist of a simple plano-convex lens and a triplet. 
 In many systems of objectives there is after the simple front lens an 
 additional simple convex meniscus, which increases the effect of the front 
 lens. 
 
 It has been found most advantageous that the front part of the 
 system should consist of a simple lens, which produces the total 
 dioptric effect desired, and a triple combination, which produces scarcely 
 any convergence (magnifying effect or power), and which only so affects 
 aberration as to eliminate the chromatic difference of the spherical 
 aberration for all the zones simultaneously. 
 
 Lastly, the final upper lens, which is also a triplet, is used to destroy 
 the secondary spectrum. 
 
 The simple lens in the middle (the third lens from the object under 
 examination) makes all the rays converge sufficiently, i.e., this lens alone 
 produces the ultimate magnification which was commenced by the two 
 lower lenses, while both the two triplets that follow have almost infinite 
 focal distances. They therefore only act on the rays as two planes with 
 parallel faces; they do not magnify at all, but simply correct the 
 chromatic and spherical aberrations as already mentioned above. 
 
INDEX. 
 
 A 
 
 Abbe (Dr. E., professor, Jena, Hon. F.R.M.S.), 
 his apertometer 53, apochromatic objectives 
 47, camera lucida 93, compensating oculars 71, 
 condensers 81-83, diffraction p'ate 33, projec- 
 tion oculars 72, stereoscopic eye-piece 104, test 
 plate 68 
 
 Apochromatic objectives due to 47, intro- 
 duces fluorite in lenses 18, numerical aperture 
 adopted by 53, relation of aperture and magni- 
 fication 56, researches on vitreous compounds 
 
 17, rule as to greatest number of lines resol- 
 vable 36, theory of magnifying power 358, 
 theory of microscopic vision 21-31, experiments 
 on the application of the theory 32-40, ex- 
 planation of value of electric light 109 
 
 Aberration [L. aberratio, a wandering, from ab, 
 from and erro, I wander] of refrangibility 16, 
 chromatic 16, spherical 14 
 
 Accessory apparatus 89-117, Abbe's stereoscopic 
 eye-piece 104, adapters 100, binocular micro- 
 scope 103, camera lucida 91, comparing oculars 
 105, cover-glass gauge 106, dissecting eye-piece 
 102, electric lamps 109, finders 97, micrometers 
 89, nose-piece 100, polarizing apparatus 94, 
 reversing prism 102, spectroscope 96, trans- 
 former 106 
 
 Achromatic condensers [Gk. a priv. %pwfia, 
 Xpiofiarog colour], Powell and Lealand 84-85, 
 Zeiss 84 
 
 Achromatic lens corrected for chromatic aberration 
 
 18, explained 44, first constructed by Ch. 
 Chevalier 44, compared with apochromatic 
 objectives 47-48, Beeldsnyder's 352, Van Deyl's 
 35 2 
 
 Achromatic objectives, Bausch and Lomb's 144, 
 Beck's 151, B^neche 155, Bezu, Hauser and 
 Co. 157, Crouch 162, Hartnack 167, Koristka 
 172,174, Leitz 182, Nachet 188, Powell and 
 Lealand, 193, Reichert 202,205, Ross 211,212, 
 Seibert, 217, Watson and Son 232, Zeiss 239, 
 2 43 
 
 Achromatine of Flemming 311 
 Achromatism 16, demonstrated in physics 18 
 Actinoptychus splendens photographed with 
 
 comparing ocular 106 
 Adam's microscopical observations when aged, 
 
 133 
 
 Adams' microscope 348,349 
 Adan (H.P.) Le microscope ; Le monde invisible 
 332 
 
 Adapters 100, Chevalier's 100, Nachet's 100, 
 Thury's Pince Suisse 100, Zeiss' 101, adapter 
 sold with diffraction plate 33 
 
 Adjustments for focussing 42, 43, 77 ; see Rack and 
 pinion ; Fine adjustment 
 
 Adnet (Paris) Naples marie bath 328 
 
 Air angle 52 
 
 Alcohol [C 2 H s OH ; Sp. gr. 0795] 308, 303 
 
 American screw thread 49. 
 
 American Monthly Microscopical Journal 251 
 
 I Amici (Giovanni Battista, b. 1786, d. 1863, 
 professor of Modena), corrects cover-glass 
 aberration 45, compound microscope 354, 
 introduces different kinds of glass in objectives 
 356, introduces immersion 357, Amici prism 
 in Sorby's spectroscope 95, in Zeiss' 97 
 
 Ammoniacal cupric oxide 307,308 
 
 Ammonium picrocarminate 309,310 
 
 Ampere, Andr<5 Marie, French electrician, b 
 1775, d. 1836. . , , 
 
 Ampere [from Ampere, as above] the electro- 
 magnetic unit of current strength (Sym. C), 
 no 
 
 I Amphipleura pellucida as test 63, photographing 
 the beads 63, 88 
 Amplifier 251 
 
 Amplifying power of lens or lens system, see Prof. 
 
 Abbe (J. R. M.S. ,1884, p. 348 ;) Giltay (J. R.M. S. , 
 
 1885, p. 961) 
 Analyser 95 
 
 Angle [L. angulus, a corner], of incidence 2, 5, of 
 reflection 2,5, critical angle 6, of aperture 29, 
 51, of refraction 5, see Visual angle 
 
 Angular aperture 29,51, sufficient to admit 
 spectral rays 35, factor in choosing objective 50, 
 compared with numerical aperture 52, see Wen- 
 ham (J. R.M. S., 1878, p. 321, 1879, p. 270, 271) 
 
 Aniline (C6 H 7 N) stains fade 311, double staining 
 with 311 
 
 Anisotropes [Gk. djnaog, 1 unequal, rpo7ri/, 
 turning], 95 
 
 Antwerp, electric lighting, 117. 
 
 Antwerp International Exhibition 1862, photo- 
 graphs at, 249 
 
 Antwerp Microscopical Exhibition, 1891, micro- 
 tomes 318, Jansson's microscope 343. 
 1 Antwerp Microscopical Society 331 
 .! Apertometer, Abbe's 53, see Carl Zeiss, 
 description with instructions for its use 
 (J.R.M.S., 1878, p. 19); Prof. Abbe (J.R.M.S., 
 1880, p. 20) 
 
 Aperture (L. apertura, from aperio, I open) 29, 
 45, 51, see Angular aperture ; Numerical 
 aperture — wide angled objectives 48, essentials 
 for objectives with high aperture 55, see also 
 Prof. Abbe on the 1 elation of aperture and 
 power (J.R.M.S., 1882, pp. 300-309, 460-473 ; 
 
 1883, pp. 790-812) and on the mode of vision 
 with objectives of wide aperture (J.R.M.S., 
 
 1884, pp. 20-26) ; Prof. Stokes (J. R. M.S., 1878, 
 p. 139) ; Mayall (J.R.M.S., 1879, p. 134) 
 
 I Aplanatic objective [Gk. a, priv., TrXavao), 
 I wander], an objective in which all the rays 
 passing through it are brought to an identical 
 conjugate focus, virtual image when aplanatic, 
 369 
 
 Apochromatic immersion condenser [Gk. uttoj, 
 from, xpw/tia, xpwjiaroe, colour] by Powell 
 and Lealand 85, apochromatic dry condenser 
 by Powell and Lealand 85 
 
 Apochromatic objective, defined 47 ; note 
 on apochromatic combinations, 366, advan- 
 
 BB2 
 
372 
 
 INDEX. 
 
 tages of 362, first sold by Zeiss 47, result 
 of researches by Abbe 47, compared with 
 achromatic 47, require special care 295, use in 
 tropical regions condemned 295, Hartnack's 
 167, Koristka's 173-174, Leitz' 182, Nachet's 
 190, Powell and Lealand's 194, Reichert's 
 203-206, Seibert's 216-217, Zeiss 239-243, Zeiss' 
 first announced (J.R.M.S., 1886, p. 849), 
 Powell and Lealand's first shewn (J.R.M.S., 
 1886, p. 1 1 10), see Abbe (J.R.M,S., 1887, 
 p. 20) 
 
 Amrheim (Anton, Vienna), his work-table 129 
 Arsenic, Bromide of [As.Br 3 ] used in Prof. 
 
 Smith's yellow medium 305 
 Arsenical medium (index 2 4) 305 
 Arsenious acid [H 3 As0 3 ], white oxide of arsenic, 
 
 used by author in production of yellow medium 
 
 3^5 
 
 Artificial illumination 108 
 Aspect of work room 129 
 
 Astigmatism [Gk. «, priv. criy/(a, a spot or 
 point] 292 
 
 Axis I L. axis, an axle], principal axis of mirrors 
 
 3, principal and secondary axes of lenses 10 
 Axial illumination 32, 131 
 
 B. 
 
 Back lash (= lost time) the jarring reaction or 
 striking back of a screw or wheel or set of 
 connected wheels when the motion is not 
 uniform, or when sudden pressure is applied 46 
 
 Balsam, Canada, 301, styrax and liquidambar 301 
 
 Battery of objectives 56-57 
 
 Bausch (Edward) hints for preserving the micro- 
 scope 295 
 
 Bausch and Lomb Optical Company 138, univer- 
 sal microscope 139, American concentric micro- 
 scope 141, large continental microscope 142, 
 medium continental microscope 143, objectives 
 144, cover-glass gauge 106, fine adjustment 365 
 
 Beale (Lionel S. ), how to work with the micro- 
 scope 331 
 
 Beam (A.S. beaut} defined 1 
 
 Beck (R., optician) studies on podura scale 58 
 
 Beck (Messrs. R. and J., opticians), plates illustrat- 
 ing podura test 59, international microscope 
 146, second grand model 148, small first-class 
 microscope 149, binocular national 150, 
 objectives 151 
 
 Beelsnyder's achromatic objective 352 
 
 BiJneche (L. , optician) model A microscope 153, 
 model B 154, dissecting microscope 154, 
 objectives 155, micrometer 89 
 
 Benzol, benzole or benzine [C6 H6 ] a colourless 
 liquid produced from coal tar, of great solvent 
 power. Mixed with styrax 303 
 
 Berens (W. H.) on the microscope 331, the 
 microscope in botany 332 (there is an English 
 translation of this work by Rev. A. B. Hervey, 
 Boston, U.S.A., 1885) 
 
 Bertsch first produces photographic paper proofs 
 249 
 
 Besseler (Berlin), gum sold by, 307 
 
 Bezu, Hauser & Co. (opticians, Paris) 155, small 
 
 microscope 156, objectives 157 
 Bi-concave lens 7 
 Bi-convex lens 7 
 
 Binocular[ L. bin us doubl e , oculus an eye]eye-piece, 
 Abbe's 104, as to Tolle's see American Journal 
 
 of Science, 1864, p. in ; 1865, p. 212; M.M.J. 
 ([871) p. 45 
 
 Binocular microscope 103, see Ahrens (J.R.M.S., 
 1885, p. 959) ; on the conditions of orthoscopic 
 and peudoscopic results in binoculars, see 
 Prof. Abbe (J.R.M.S., 1881, p. 203); Nelson 
 (J.Q.M.C., 1892, p. 45), 
 
 Bonanni (Philip, author of Observationes circa 
 viventia qua; in rebus non viventium reperiun- 
 tur) microscope 345, condenser 345 
 
 Books on the microscope 331-332, on photo- 
 micrography 333 
 
 Boron [sym. B. At. wt. 11. tri-atomic]. Prepared 
 by igniting boron fluoride with potassium. 
 When crystallized, it is nearly as hard as 
 diamond, and less susceptible of oxidation. 
 Used in Schott glass 17 
 
 Boxes for microscopical preparations 329 
 
 Br^bisson (A. de, algologist) creates genus Van 
 Heurckia 62, injures sight 133 
 
 Brookes' double nose-piece 100 
 
 Brown (Dr. Robert) discovers Brownian move- 
 ment or pedesis, 1827 
 
 Brownian movement in microscopical objects 
 292, see Pedesis 
 
 Brussels Microscopical Society 331 
 
 Bulls-eye or condensing lens 87,88, see Nelson 
 (J.R.M.S., 1891, p. 309); Eng. Mec. 1884, pp. 
 68, 157, 263, 282 ; J.R.M.S., 1885, p. 713 
 
 c. 
 
 Calcium chloride [CaCl 2 ] as preservative 
 medium 299 
 
 Camera lucida 9r, Oberhausser's 91, Chevalier's 
 92, Nachet's 92, Prof. Abbe's 93, how used 93, 
 used in measuring magnifying power 247, see 
 H. Schroder, J.R.M.S., 1883, p. 813 
 Camera obscura, principle of 12 
 Campani (Giuseppe, of Bologna) microscope 344 
 Camphorated water as preservative medium, 299 
 Canada balsam (refractive index t'549), bottle 
 for 301, use 311, use in nicol prism 94, lenses 
 joined together by 354 
 Carbolic acid [C6 H s OH] used with glycerine 
 jelly 299 
 
 Care of microscope 294, of lenses 294, of stand 
 295, of objectives and eye-pieces, 296, of 
 apochromatic objectives 295 
 
 Carmine 309-310 
 
 Carnoy, La biologie cellulaire 332 
 
 Carpenter (Dr. W. B.), the microscope and its 
 
 revelations 332 
 Castracane [Conte Abate Francesco, Rome, 
 
 Hon. F.R.MS.], as photo-micrographer 249 
 Catadioptric [Gk. kcitci down Sia through ott — 
 
 root of bpc'uo I see] microscope 352 
 Cathcart microtome 317, 320 
 Cathcart-Erbe microtome 320 
 Caustic potash [KHO] 308,309 
 Cedar oil for homogeneous immersion 46, index 
 
 of refraction 46 
 Cells for mounting preparations 326, round cells 
 
 327 
 
 Centimetre, — a hundredth part of a metre = 
 '3937 ' nc h i 10 millimetres = one centimetre. 
 
 Central beam insufficient to produce images of 
 minute objects 32 
 
 Centre, see Optical centre. 
 
 Centre of curvature of spherical mirrors 3, of 
 surface of lens 8 
 
INDEX. 
 
 373 
 
 Centre of figure 3, not centre of curvature 8 
 Changer, Zeiss' sliding objective, 101 
 Chemical media 304, reagents 307 
 Cherubin (Pere of Orleans) binocular microscope 
 344 
 
 Chevalier (Arthur) 157, his camera lucida 92, 
 reversing prism 103, his father's artificial "pupil" 
 79, author of L'etudiant micrographie 332 
 
 Chevalier (Charles) achromatic lenses 44, 353, 
 aehromatised each lens 45, invents adapters 
 100, invents Iris diaphragm 79, copies Jones' 
 microscope 352, superposes lenses 353, makes 
 globules 339, Topping microtome 314, Selligue 
 microscope 353, universal microscope 355, 
 powerful objectives 356, solar microscope 123, 
 gas microscope 126, photo-electric microscope 
 126 
 
 Chloride of calcium [Ca Cl 2 ] 299 
 
 Chloride of zinc [Zn Cl 2 ] iodized 307, 308 
 
 Chlorophyl, staining 311 
 
 Choice of light 130, of a microscope 138 
 
 Chromatic aberration [Gk. xptj/ia, xp<l> r icirot; 
 colour] 16, corrected 17 
 
 Chromatine, how stained, 311 
 
 Clairac (Dr. Jos6) obtains raw liquidambar 303 
 
 Classification of objectives 50 
 
 Cleaning lenses 294, 295 
 
 Clips for stage 43 
 
 Cloves, essence of, 311 
 
 Cold, effect on preparation, 329 
 
 Cole (ArthurC, F.R.M.S.), on styrax as medium 
 301, staining 309, author of Methods of micro- 
 scopical research 332 
 
 Collar, correction, see Correction collar objectives 
 
 Collodion [Gk. kuKKu, glue, and eidog] resem- 
 blance], pyroxylin or gun cotton dissolved in 
 ether or ether and alcohol. Photographing with 
 24Q 
 
 Colouring reagents 307 
 Colours, how caused 1 
 
 Colours of solar spectrum 16, variation in indices 
 
 of refraction 16 
 Comber (Thomas, J. P. , F.R.M.S. ) as photo-micro- 
 
 grapher 249, on photography with sunlight 288 
 Commutator, instrument for reversing electric 
 
 current 130 
 
 Comparing ocular, Van Heurck's, 105, photo- 
 graphing with 106 
 
 Comparison prism in spectroscope 97 
 
 Compensating oculars 71, invented by Abbe 71, 
 Koristka's 174, Nachet's 190, Powell and 
 Lealand's 195, Zeiss' 244 
 
 Compound microscope, see Microscope, com- 
 pound. 
 
 Concave [L. concavus, con, and cavus, hollow] 
 mirrors described 3, when used 78, lenses 7, 
 virtual images in concave lenses 13 
 
 Concavo-convex lens 7 
 
 Condenser [L. con and denso, I make close] 78, 
 80-87, invented by Dujardin 80, Nachet's 80, 
 Abbe's 80,81-84, Powell and Lealand's 84, 
 chromatic oil 84, achromatic oil 85, dry 84, 
 apochromatic dry 85, immersion 83, rules for 
 using a condenser 86, Bonanni's 345. As to 
 illumination with, see E. M. Nelson, Eng. Mec., 
 1884, pp. 68, 157, 263, 282, J.R.M.S., 1885, 
 p. 713, as to its history, construction, and 
 management see Nelson (J.R.M.S. 1891, p. 90 ; 
 J.Q.M.C., 1890, p. 116) 
 
 Condensing lenses 88, Marshall's 346, see Bull's- 
 eye 
 
 Cone, moveable piece of apparatus at lower 
 
 extremity of tube in some microscopes 131 
 Conjugate [L. conjugo I couple], foci of mirrors 
 
 3,4, of bi-convex lenses 10,11 
 Convergent [L. con together, vergo I incline], 
 
 meniscus lens 7 
 Convex [L. convexus, con together, and veho I 
 
 draw] lenses 7 
 Cooper, gas microscope 126 
 
 Cornu photographs ultra-violet of spectrum 364 
 [ Correction collar objectives, described 45, invented 
 by Ross 45, Zeiss' figures on 46 
 
 Coverglass, size 298, thickness 5r, 298, objectives 
 corrected for 45, Bausch and Lomb's cover- 
 glass gauge 106 
 
 Crisp (Frank, formerly Secretary and Editor 
 and now Treasurer of R.M.S.), owns Nobert's 
 test instrument 56, gives new form to J R.M.S. 
 331, author of Notes on aperture, &c. (J.R.M.S. 
 1881, pp. 300-360), translator of Naegeli and 
 Schwendener on the microscope 333 
 
 Critical angle [L. criticus from Gk, ko'ivui I dis- 
 criminate], defined 6, of crown glass into air 6, 
 in the Nicol prism 94 
 
 Crossed bi-convex lens, a bi-convex lens where 
 the radii of the surfaces are not equal to one 
 another. 
 
 Crouch (Hy., optician) 157, grand model micro- 
 scope 158, premier 159, student's 160, objectives 
 161 
 
 Crown glass. Crown and flint glass first joined 
 with Canada Balsam 354, composition, disper- 
 sive power and index of refraction 17 
 Cuff, simple microscope 340, compound 348 
 Culpepper and Scarlet's compound microscope 
 346 
 
 Cupric oxide [CuO] 308 
 
 Curvature, centre of, 3, radius of 3 
 
 Czapski (Dr. Seigfried, of Jena, sub-manager of 
 Zeiss' workshops and Prof, Abbe's assistant), 
 on diffraction theory 21, on the future of 
 the microscope 358, on objective r6 N.A. 
 359, see also J.R.M.S., 1890, p. 11 ; 1891, 
 p. 814 
 
 D. 
 
 D = the sodium lines in the spectrum. When 
 suffixed to fi denotes that the refractive index 
 is measured with reference to these lines, 
 
 Damp, effect on preparations, 329 
 
 Dallinger (Revd. Dr. W. H., Past President 
 and now secretary R.M.S.) edits Carpenter 
 on the microscope 332 
 
 Darker's revolving selenite stage 95,96 
 
 Dark ground illumination 82 
 
 De Br£bisson (Alphonse) as photo-micrographer 
 249 
 
 Deby ( Julien,F.R. M. S.) Catalogue of microscopical 
 works 331 
 
 Decimetre, tenth of a metre = 3^3704 inches = 
 
 10 centimetres — 100 millimetres 
 Decomposition of light 2, 16 
 Defining power, on what depends 56, tests of 
 
 pygidium and podura 58, see Royston-Pigott 
 
 (J.R.M.S., 1879, p. 784) 
 Dellebaire (Lewis Francis) microscope 347,350 
 De Streck 350 
 
 Diaphanous [Gk. $ia through, (paivu I shine], 
 having power to transmit rays, as glass 1 
 
374 
 
 INDEX. 
 
 Diaphragm [Gk. Sia through, tppaarro) I break off] 
 
 described 79, Iris diaphragm 79 
 Diatoms, measurement of 246,247, as tests of 
 
 resolving power 54, diatom finder 99 
 Diffraction [L. diffrango, diffractum, I break in 
 
 pieces] of object 25, images 26, 32-40, spectrum 
 
 28, 29, 32-40, gratings 32-40, gratings supplied 
 
 by Zeiss 33, Abbe's diffraction plate 33, 
 
 descriptive appearance by 290, phenomena, 
 
 how best avoided 291 
 Dioptrics I Gk. Siotttoikos from, Sid through, and 
 
 root o7ro — to see], the science of diffracted light. 
 Dioptric beam 32, image 32, pencil 39 
 Dippel (Leopold) recommends styrax 302, author 
 
 of Das mikroskop 332 
 Dispersion [L. dispergo, dispersum I scatter] 
 
 defined 16 
 Dissecting ocular 103 
 
 Divergent [L. di for dis asunder, vergo I incline] 
 meniscus lens 7 
 
 Dollond (John, optician, b. 1706, d. 1761) corrects 
 chromatic aberration by composite lens 17 
 
 Donne' and Leon Foucault, photo-electiic micro- 
 scope 126, atlas d'anatomie niicroscopique 249 
 
 Double refraction 94, staining 311, vision 105, 
 247, image 94 
 
 Doublet invented by Wollaston described 18,19, 
 magnification 19 
 
 Drawing microscopic objects 248, see also J. 
 Anthony on drawing prisms, J.R.MS, 1884, 
 
 P- 6 97 , . . . 
 Duboscq (optician) photo-electric microscope 
 126 
 
 Dujardin (Felix, French naturalist and microsco- 
 pist, b. 1801) invents first true condenser 80, 
 on pygidium of flea 58 
 
 Dust on oculars and objectives 292 
 
 E. 
 
 Ebonite (caoutchouc mixed at a high tempera 
 ture with sulphur) used as covering of micro- 
 scopical stages 75 
 
 Electro-magnetic units : — Of resistance (ohm), 
 of electric motive force (volt), of current 
 strength (ampere), of quantity (coulomb), of 
 capacity (farad) 
 
 Electric illumination 109, equal to monochro- 
 matic 109, Abbe's explanation 109, for photo- 
 micrography 260, see also S. H. Stearn on the 
 use of incandescent lamps as accessories to the 
 microscope (J.R.M.S., 1883, p. 29). 
 
 Electric microscope 110 
 
 Embedding tissues 323, see also B. W. Richard- 
 son (J.R.M.S., 1882, p. 474). 
 
 Emission theory of light (advocated by Newton, 
 circa. 1672) supposes that individual particles 
 pass from the luminous body to the eye, and 
 that each ray of light passes from the sun to 
 the earth — opposed to undulatory theory 21 
 
 Engelmann (prof. Hon. F.R.M.S. ), electric lamp 
 112,113 
 
 English length of microscope tube 42, size of 
 
 slip 298, screw thread 49 
 Erbe (Ch., optician, Tubingen) microtomes 318- 
 
 321 
 
 Erector or reversing prism 103 
 Errera (Leo., professor, Brussels Univ.) intro- 
 duces nigrosine 310 
 Ether [L. Aither, Gk. aiOlip from a'iOio I light 
 
 up], a hypothetical medium of extreme elasticity 
 and tenuity, supposed to pervade all space 
 (solid bodies not excepted), every vacuum, and 
 between the particles of all transparent bodies, 
 and that by its movement we get light and 
 heat, 1 
 
 Ether [(C 2 H 5 ) 2 0], a light volatile inflammable 
 fluid, produced by the replacement of the 
 hydrogen of organic acids by alcohol radicles 
 308 
 
 Euler (Leonhard, Swiss mathematician and phy- 
 sicist, b. 1707, d. 1783, credited with the 
 Selligue microscope 354 
 Evaporating basin 311, 313 
 Extraordinary ray in polarized light 94 
 Eye, long sight and short sight 19, defects of 
 (astigmatism) 292, muscas volitantes not evi- 
 dence of defect of 291, how sight protected in 
 microscopical research 133,134 
 Eye-piece 42, 70-74, see Ocular. 
 Eye-piece micrometer 89 
 
 F. 
 
 Fahrenheit (of Amsterdam) his solar microscope 
 123 
 
 False images produced 39 
 Fasolt (of America) tests 56 
 Field-glass, effect of 20 
 
 Finders 97, moveable stage as 97, Maltwood's 98, 
 by marks on stage 98, Pantocsek's 98, May's 
 diatom finder 99, see also W. Webb on an 
 improved finder, (J.R.M.S., 1880, p. 750). 
 
 Fine adjustment, the slow movement of micro- 
 scopes. This is effected in microscopes of the 
 Jackson model by moving the whole body by 
 a lever in the limb as (e.g.) in the Van Heurck 
 model (J. R. M.S., 1891, p. 399), or by Swift's 
 patented side lever, which isalso used for moving 
 the nose-piece (J. R. M.S., 1881, p. 297), or by 
 Campbell's form of differential screw (J.R.M.S., 
 1886, p. 324-325), and in bar movement micro- 
 scopes by simply moving the nose-piece as in 
 that used by Andrew Ross, consisting of a lever 
 of the second order inserted within the bar and 
 worked by a micrometer screw, the nose-piece 
 being between the screw and the fulcrum, and 
 that used by Powell and Lealand consisting of 
 a lever of the first order, the fulcrum being 
 between the screw and the nose-piece. — 43, 77, 
 Bausch and Lomb's frictionless 140, 365, 
 Watson's 229, Leitz' "a pointe renversde" 176 
 
 Flame images, observations in 86 
 
 Flea glass 337 
 
 Flemming, achromatine 311 
 
 Flint [Si0 2 ] glass, composition and index of 
 refraction 17, dispersive power of 17 
 
 Fluorite [CaF] fluoride of calcium or fluor spar, 
 found in veins generally associated with metallic 
 ore in Scotland, the Lake district, Cornwall, 
 Saxony, Bohemia, &c, used by Prof. Abbe 18, 
 index of refraction 18, as to Brun's artificial 
 opal as a substitute for fluorite in the manu- 
 facture of achromatic lenses, see J.R.M.S., 1892, 
 pp. 552-555, as to testing apochromatic objectives 
 for fluorite see Nelson (J.Q.M.C., 1892, p 122), 
 as to fluor spar at Oltscheren see J.R.M.S., 
 1890, p. 661 ; see Prof. Abbe on fluor spar 
 (J.R.M.S., 1890, pp. 274, 392) 
 
 Flving flies, see Muscas volitantes. 
 
INDEX. 
 
 375 
 
 Focal distance 50, 51, measurement of 50, 
 51, see Dr. Czapski (J.R.M.S., 1892, p 678). 
 
 Focal length (principal) defined 10 
 
 Focus I L. focus a fire] of mirror 3, of lens defined 
 50, of convex lens 10, of concave lens 13, of 
 solar microscope 124, 125, principal focus 3, 4, 
 5, conjugate foci 3, 4, 10, 11, virtual focus 4 
 
 Foucault (Leon) 249 
 
 Foot of microscope 41 
 
 Forceps 311, 313 
 
 Formulae, focal distance 50, magnification of 
 projected image 73, numerical aperture 52, 
 real magnifying power of microscope 357, 
 resolving power 31 
 
 Francotte (P.) Manuel de technique micro- 
 scopique 332 
 
 French screw-thread 49 
 
 Fresnel (Augustin Jean, physicist, b. 1788, 
 published memoirs on diffraction of light 1826, 
 double refraction 1827, constructed dioptiic 
 system of lenses 1819, d. 1827), studies in light 
 21, undulatory theory 21 
 
 Fromme (Gebruder) stick collecting microscope 
 162 
 
 Frontal distance (working distance) 46, of 
 objectives 51, greater in immersion objectives 
 46, in apochromatics 48 
 
 Fuchsine (an aniline red) 310, 311 
 
 Future of microscope 358 
 
 G. 
 
 Gauge for thickness of cover-glass io5 
 
 Galileo's claims as inventor of microscope 343, 
 see Prof. Govi (J.R.M.S, 1889, p. 574). 
 
 Galy-Cazalat's gas microscope 126 
 
 Gas microscope 126 
 
 German screw-thread 49 
 
 Giessen's size of slip used in Germany 298 
 
 Gife (of Antwerp) as photo-micrographer 249 
 
 Giltay (Dr. E., of Wageningen), as to plane and 
 concave mirrors 78, author of Works on 
 microscope 332 
 
 Glass, see Cover-glass, Crown-glass, Flint glass, 
 Slip or slide. 
 
 Glass rods 311, 313 
 
 Globules as lenses, making, 339 
 
 Glycerine [Gk. y\uKvq sweet], a transparent 
 colourless liquid, with a sweet taste, obtained 
 from natural fat by saponification with 
 alkalines, or by the action of superheated 
 steam [C 3 H 3 (OH) 3 ], as preservative medium 
 299 
 
 Glycerine jelly as preservative medium 299 
 Grain (apothecaries' weight) — i-20th scruple = 
 
 6*48799 centigrammes. 
 Gramme (centigrade unit of weight) = 10 
 
 decigrammes = i-ioth decigramme = 
 
 iS'4i3o8no36 grains. 
 Gratings, Zeiss' diffraction 33, fundamental 
 
 principle of 35 
 Graphite = plumbago or black lead 113 
 Green, methyl 310, 311, iodine 310, 311 
 Griffith and Henfrey s Micrographic dictionary 333 
 Grimaldi (Francesco Maria, philosopher, b. 1618, 
 
 d. 1663), studies in light 21 
 Grindelius (Pere Francis) microscope 344 
 Grunow (A. , Hon. F.R.M.S.) recommends styrax 
 
 302 
 Gum lac 307 
 
 H. 
 
 Hnematoxylin or logwood 309, 310 
 Hand-vice 311, 313 
 
 Harting (P., professor, Utrecht), author of Das 
 mikroskop 332, Het mikroskoop 333, lines in 
 Nobert's tests calculated by 64, how to avoid 
 effects of musca; volitantes 291, elbow ocular 
 microscope for same purpose 291, suggests 
 " mikron ' 246, sight uninjured by micro- 
 scopical research 133, measurement of focal 
 distance 50, observations with Leeuwenhoeck's 
 microscope 338, testimony to Janssen as 
 inventor of compound microscope 343, arrange- 
 ment of polarising apparatus 95. 
 
 Harting and Van der Kolk, researches as to 
 niuscae volitantes 29: 
 
 Hartnack (E., optician) 162, continental model 
 162, microscope No. 4, 164, new microscope 
 No. s, 165, objectives 165-167, dissection eye- 
 piece with 102 micrometer 89 
 
 Hartnack and Prazmowski's polarising apparatus 
 95 
 
 Heliostat [ Gk. >'i\iog the sun, araroq fixed from 
 "icsT>]]ii I stand], for photographing 265, Mr. 
 Comber's 288, as to Col. Woodward's and Dr. 
 Maddox, see M.M.J. 1869, pp. 27-31 ; as to a 
 simple form by Mr. Comber, see J.R.M.S. 
 1890, pp. 423, 429-434 ; for Prazmowski's 
 see J.R.M.S., 1879, p. 44. 
 
 Helmholtz (Prof. Hermann L. F. von), experi- 
 ments in optics 23, theory of magnifying 
 power 357, resolving power increased by 
 photography 361 
 
 Hooke (Robt., b. 1635, invented barometer 1656, 
 Gresham professor 1664, Secretary R.S. 1677, 
 d. 1703) published Micrographia 1656, 338. 
 344, studies in light 21, invents lenses of cast 
 glass 338, his microscope 344 
 
 Homogeneous [Gk. o^oc like ykvoQ kind] 
 defined 1, objective 46, see Prof. Abbe on 
 Stephenson's system of homogeneous immer- 
 sion for microscope objectives (J.R.M S. 1879, 
 p. 256) ; on the origin and essence of homo- 
 geneous immersion (J.R.M.S., 1881, p. 131). 
 
 Hughes and Lewis' microtome 321 
 
 Huygen's (Hugenms) van Zuylichem [b. 1629; 
 observed double refraction, 1678 ; suggested 
 undulatory theory ; d. 1695J undulatory 
 theory 21, ocular 70 
 
 I. 
 
 Iceland spar [CaC ; CaC0 3 ] , a transparent 
 rhomboidal variety of calcite, or calcareous 
 spar, or carbonate of lime. It occurs massive, 
 and in high crystals, in a trap rock in Iceland, 
 It possesses the property of double refraction 
 and is valuable for experiments on the double 
 refraction and polarization of light (index of 
 refraction of ordinary ray 1 '658), 94 
 
 Illumination by transmitted light 78-87, see Prof. 
 Abbe on the effect of illumination by means 
 of wide-angled cones of light, (J.R.M.S., 1889, 
 p. 721). 
 
 Illumination of opaque objects 87 
 
 Illuminating apparatus 78-88, mirrors 78, diaph- 
 ragms 79, condensers 80, Abbe condenser 81, 
 Powell and Lealand's condensers 84-86, rules 
 for using condenser 86, Wenham's radial 87, 
 
376 
 
 INDEX. 
 
 condensing lenses 87, Lieberkuhn's speculum 
 87, vertical illuminator 87, lamps 108, electric 
 illumination 109, see Dr. H. E. Fripp(J.R. M.S., 
 
 !879> P- 5°3 i i 88 °> P- 74 2 )- 
 Image [L. imago, from imitor, I imitate], false, 
 39, formation of 24, diffraction 26, of object 27, 
 of periodic structure 30, of striation 30, with 
 concave lenses 13, with convex 11, 12, how 
 reflected in binocular microscopes 103 
 Image of the flame, observations in, 86 
 Immersion [L. im-mergo, immersum to plunge 
 into] illuminators 85, see Mayall, on immersion 
 illuminators (J.R.M.S., 1879, pp. 27,837); J. 
 W. Stephenson, on a catoptric immersion 
 illuminator (J.R.M.S. , 1879, p. 36) ; on a cata- 
 dioptric illuminator (J.R.M.S., 1885, p. 207); 
 J. Edmunds, on a revolver immersion prism 
 for sub-stage illuminator (J.R.M.S. 1879, 
 P- 3 2 )- 
 
 Immersion liquids 359, 360 
 
 Immersion objectives 46, 47, advantage of 46, 
 see F. H. Wenham (J.R.M.S., 1879 p. 394). 
 
 Inch [A. Sax. ince, ynce ; L. uncia, a twelfth 
 part. " Ounce " is same word in another 
 form] , 1-12A of a foot = 25-3998 millimetres 
 = 2 '53998 centimetres = '253998 metres. 
 
 Incandescent [L. in and candesco to become red 
 hot] electric lamps 109 
 
 Incidence [L. incido, I fall upon] , angle of 
 incidence, 2, 3, 4, 5, 6 
 
 Index [L. digitus, a finger] of cedar oil 46, of 
 flint glass 49, of fluorite 18, of refraction 
 defined 6, of iodide of methyl 304, of violet and 
 red in slightly refractive and dispersive glass 
 16, of yellow medium 305, 306, of monobromide 
 of naphthalin 304, of styrax 302. As to 
 index of refraction of mounting media, see 
 Nelson (J.Q.M.C., 1882, p. 8). 
 
 Initial magnifying power, see Magnification. 
 
 Inostranzeffs comparer 105 
 
 Instruments for preparing objects, 311 
 
 Interference of light 22, effect of interference of 
 diffracted pencils 40 
 
 Internal reflection in tube of microscope, how 
 avoided, 229, see J. W. Groves (J.R.M.S., 
 1880, p. 225). 
 
 Iodide of methyl 304, refractive index and use 
 3°4 
 
 Iodized chloride of zinc 307, 308 
 
 Iodine green 311, 3T3, solution of iodine 307, 308 
 
 Iridescence [L. iris, Gk. Ipig the rainbow] , 
 
 having or giving out colours like the rainbow 
 
 290 
 
 Iris diaphragm 79 
 
 Isochromatic [Gk. laoQ equal ^pupa, xP^I^'toc 
 
 colour] plates for photography 282 
 Isotropes [Gk. i'o-oc equal rpoirl) a turning] 95 
 
 J. 
 
 Janssen (Zaccharias) 341, 342, inventor of micro- 
 scope 41, how discovered 337, Harting's testi- 
 mony to 343, microscope at Middleburg 343, 
 Mayall's copy, 343, description of 343, 344, 
 see further hereon Queckett on Microscope p. 2, 
 Brewster's Treatise on Microscope p. 2, 
 Mayall's Cantor Lectures ; Carpenter, 4th ed., 
 p. 122 
 
 Jones' microscope 347, 352 
 
 K. 
 
 Kagenaar's electric lamp 113 
 
 Kaiser's gelatine = glycerine jelly 299 
 
 Kircher's solar microscope 123 
 
 Kitton (Frederick, Hon., F.R.M.S.) on styrax 
 
 302 
 
 Koch ( R. ) as photo-micrographer 249 
 Koristka (F., optician) 168, medium petrological 
 stand 169, model No. III. 170, model No. VI, 
 171, achromatic objectives 172, apochromatic 
 objectives 173 
 
 L. 
 
 Labels for microscopical preparations 330, see 
 Prof. Gage (J R.M S. 1883, p. 924). 
 
 Lamps 1 08, electric lamp carriers in, incandescent 
 lamps no 
 
 Lantern microscope 127, see also Lewis Wright 
 (J.R.M.S., 1885, p. 196). 
 
 Latitudinal or lateral aberration, the distance of 
 any ray from its geometrical focus when 
 measured at right angles to the axis 15 
 
 Latteux (Dr. P.) Manuel de Technique Micro- 
 scopique 333 
 
 Lebaillif, stage micrometer 91, divides millimetre 
 into 500 parts 91, makes globules 338, material 
 for globules 340 
 
 Leitz (E. , optician) 174, large inclining micro- 
 scope 175, slow movement 176, Stativ la. 177, 
 Stativ II. (medium) 178, medium upright micro- 
 scope 179, Stativ III. 180, objectives 181 
 
 Lens [L. tens a lentil] defined 7, cleaning 295, 
 different series 7, focus in lens formed of different 
 lenses 50, formation of foci 10, 13, frontal 46, 
 manufacture 334-337, manufacture of cut and 
 cast glass 338, optical centre 8, principal and 
 secondary axes 10, production of images by n, 
 13, properties 7-14 
 
 Leeuwenhoeck (Anton Van, Dutch naturalist, b. 
 1632, discovered rotifers and animalculre by 
 microscopes made by himself; published Arcana 
 naturae 1695-9, d. 1723), microscopes 338, 
 Harting's observations with 338, observations 
 at 90 years of age 133 
 
 Lepidocyrtis curvicollis 58 
 
 Light [A. Sax. liht, leohl\, defined 1, reflection 2, 
 
 refraction 5, how modified in work room, 129 
 Limit of resolving power 35, of visibility 36, see 
 
 F. Crisp (J.R.M.S., 1885, p. 968), Royston— 
 
 Piggott (J.R.M.S., 1879, p. 9). 
 Lines resolvable by oblique light, Prof. Abbe's 
 
 rule as to 36 
 Liquid No. 28 as a preservative medium 300 
 Liquidambar 301, 329 
 
 Litre [Fr. from Gk. \irpa a silver coin = L. 
 libra a pound] the French standard measure 
 of capacity in the decimal system. It is a cubic 
 decimetre (a cube each of whose sides is 3-937 
 inches). It contains 61-028 English cubic 
 inches, and is equal to 1-75953 imp. pint. 
 
 Longitudinal aberration, the distance of any ray 
 from its geometrical focus when measured along 
 the axis 15 
 
 Logwood (Hoematoxyli Lignum) 309, preparation 
 of 310 
 
 Lieberkuhn (Dr. Johann Nathanael, Berlin, b. 
 1711, d. 1756), his speculum 87, 88, 131, intro- 
 duces solar microscope 123 
 
 Lost time, see Back lash. 
 
INDEX. 
 
 377 
 
 M. 
 
 u = the symbol employed (I.) in problems to 
 denote the refractive index of the medium, (II.) 
 in measurements to denote a micron (one- 
 thousandth of a millimetre). 
 
 Macroglossa stellatarum 354 
 
 Maddox (Dr. R. L., inventor of gelatino-bromide 
 plates, Hon. F. R.M.S. ), remarks on photo- 
 micrography 283 
 
 Magnification, unit of, 246, how denoted 248, 
 does not necessarily disclose more detail 132, 
 of projected image 73, see E. M. Nelson, 
 virtual images and initial magnifying power, 
 (J.R.M.S., 1892, p. 180) 
 
 Magnifying glasses, principle of 13 
 
 Magnifying power, measurement of 247, relation 
 of aperture to 56, see Aperture ; J. Gorham, 
 (Q.J. M.S., 1856, pp. 27-44 ; '854, PP- 218-234; 
 
 l8 5S. PP- I " I S) 
 Maltwood (T.) finder 98, see also T.M.S., 1858, 
 
 P- 59 
 
 Mandl on danger of reckless use of microscope 
 *33 
 
 Manufacture of lenses 334-337, of globules 339 
 Marie-bath 299, small 328, the Naples 328 
 Marktanner-Turnertscher — Die Mikrophoto- 
 graphie 333 
 
 Marshall (John, optician, London) microscope 
 345 
 
 Martin microscope 347, 351 
 Mawson and Swan's electric microscope 1 10 
 Maximum number of lines to inch visible 37 
 May's diatom finder 99 
 
 Mayall, John (late secretary R.M.S. ), Cantor 
 Lectures 332 
 
 Measurement of microscopical objects 245 of 
 magnifying power 247, of focal distance 50 of 
 increase of power of microscope 357, see Sir 
 Walter Sendall on an improved method of 
 making microscopic measurements with the 
 camera lucida (J. R.M.S,, 1891, p. 705). For 
 new measuring apparatus, see J. R.M.S., 1891, 
 p. 252 
 
 Mechanical stage 75, Zeiss' 76, Reichert's 202 
 
 Medium (L. the middle) denned 1 
 
 Media (pi. of medium) for preserving objects 299, 
 aqueous and oily 299, resinous 301, chemical 
 304, arsenical medium (refractive index 2^4) 305 
 for immersion with high power objectives 360 
 
 Meniscus [Gk. p.i]inaKOQ a little moon] convergent 
 and divergent lens 7 
 
 Methyl [Gk. /ietu with v\r] wood] green 310, 311, 
 violet 311, iodide of 304, refractive index and 
 use 304 
 
 Metre [Fr. from Gk. fi'sTpov a measure] the 
 French standard measure of length in the 
 decimal system = 39 '3704 inches = 10 deci- 
 metres = 100 centimetres = 1000 millimetres 
 = 1,000,000 microns. 
 
 Micrographia of Hooke 339, 344 
 
 Micron [Gk. fiiicpoQ smallj one thousandth of a 
 millimetre = 001 mm. = '00003937 inch, 
 usually denoted by Greek p.. The size and 
 striaj of diatoms are generally referred to this 
 measure. See J. R.M.S., 1888, pp. 502, 526; 
 Nature, vol. xxxvii., pp. 388-9,438; xxxviii., 
 pp. 221, 244. 
 
 Micrometer [Gk. pl K p6e small p'trpov a measure] 
 89-91, eye-piece micrometer 89, stage 91, used 
 
 in measuring magnifying power 247, and focal 
 distances 50, Ramsden's 90, Zeiss' 90. 
 
 Micro-photography, a photographic process by 
 which an object is reduced in size, while its 
 exact form is retained ; not to be confounded 
 with photo-micrography, a process by which 
 the magnified image of a microscopical object 
 is photographed. 
 
 Microscope [Gk. plicpoi; small oKowiw I view] 
 history of the 334, the future of the 357, 
 inventor of 41, 343, parts of 41-43, objectives 
 44-70, stage 75-76, adjustments 77, illumi- 
 nating apparatus 78-88, accessory apparatus 
 89-117 
 
 Binocular 103, books on 331, choice of 138 
 
 Compound microscope defined 19, magnifi- 
 cation of image in 19, 20, invented by 
 Janssen 41, 341,342, Janssen's 343, Hooke's 344, 
 Campani's 344, Cherubin's 345, Grindelius's 
 345, Bonanni's 345, Marshall's 345, Culpepper 
 and Scarlet's 346, Cuff's 348, 349, Jones' 347, 
 Adams' 348, Martin's 351, Dellebarre's 347, 350, 
 Van Deyl's 353, Amici's 354, Chevalier's 354 
 
 Early 338, future of 357 
 
 Gas 126, Cooper's 126, Chevalier and Galy- 
 Cazalat's 126 
 
 Leeuwenhoeck 338, past history 334, photo- 
 electric 126, Chevalier's 126, Donn6 and 
 Foucoult 116, Duboscq 126 
 
 Preservation of 294, projection 122-128, 
 Lieberkuhu's 123, Kircher's 123 
 
 Radial 206 
 
 Seed 338 
 
 Simple 118-121, history of 337, Chevalier's 
 119, Nachet's 119, 120, Zeiss' 120, Bausch and 
 Lomb's 120, 121, Leenwenhoeck's 338, Wilson's 
 340, Cuff's 341, Raspail's 342 
 
 Solar, Ch. Chevalier's 123-126, stereoscopic 
 io 3 
 
 Use of 127-138, vitrum pulicanum 337 
 Microscopic vision, Prof. Abbe's theory of 21-31, 
 
 experiments on application of Prof. Abbe's 
 
 theory 32-40 
 Microscopical measurements, see Measurements 
 Microscopical observation, example of a, 135 
 Microscopical preparations, see Preparations 
 Microscopical research need not injure sight, 133 
 Microtome [Gk. pTicp6<; small Ttpi>M I cut] 311 
 
 Topping's by Chevalier 314, by Nachet 315, 
 
 Reichert's large 315, 316, Leitz 317, Cathcart 
 
 by Watson 317, Erbe's 318-321 
 Millimetre, a thousandth part of a metre = '3,937 
 
 inch, ten millimetres = one centimetre, 1,000 
 
 microns = one millimetre. 
 Mirror 2, how used 78, 130, concave 3, object of 
 
 43, various kinds 87 
 Moitessier (Dr. A.), author of La Photographie 
 
 appliquee aux recherches micrographiques, 333, 
 
 as photo-micrographer 249 
 Molecular [Fr. molt'cule diminutive, from L. moles 
 
 a mass] attraction in microscopical objects 292 
 Moller (of Wedel, Holstein) test-plate 66 
 Monobromide of naphthalin 304, refractive index 
 
 and use 304, used as immersion fluid 49 
 Monochromatic [Gk. fiovog single ^pw/ta, 
 
 Xpiop-aroc colour] light 48, 132. For a new 
 
 apparatus for producing monochromatic light, 
 
 see Nelson (J. R.M.S., 1891, p. 443 ; 1892, p. 1). 
 Mounting preparations 326, slips and cover-glasses 
 
 298, aqueous and oily media 299, resinous 
 
 media 301, chemical media 304, varnish for 
 
378 
 
 INDEX. 
 
 cells 307, styrax 301, yellow medium 305. As 
 to Father Thompson's high refractive medium 
 see J.Q.M.C., 1892, p. 123. 
 Movements 77, see Rack and pinion ; Fine adjust- 
 ment. 
 
 Muscae volitantes [L. musra a fly, volitans flying 
 about] researches on 291, how produced 291, 
 how avoided 291, no cause for anxiety 292 
 
 Myope [Gk. fivu) I close uty, ojttoc an eye] a 
 short-sighted person 19 
 
 N. 
 
 Naegeli und Schwendener, Das mikroskop, 333 
 Nachet, (A., optician, F. R. M.S. ), as photographer 
 249, his adapter 100, erecting ocular 103, 
 revolving nose-piece 100, camera lucida 92, 
 moveable carriers 75, large perfected model 
 microscope 183, grand microscope stand 186, 
 new inclining model microscope 187, small 
 modified erect 188, objectives 188-190, invents 
 an achromatic condenser 80, French screw- 
 thread used by 49, also English 49 
 Naples marie-bath 328 
 
 Naples Zoological Station, object clamp for 
 
 microtome 315 
 Needles 312 
 
 Neuhauss (Dr. R.), author of Lehrbuch der 
 mikro-photographie 333, as photo-micrographer 
 
 249 
 
 New York Microscopical Society, the journal of, 
 33 1 
 
 Newton (Sir Isaac, mathematician, b. 1642, 
 discovers composition (prismatic colours) of 
 sunlight, 1666 ; published his theory of light 
 and colours, 1676 ; on refraction of light, 1687 ; 
 d. 1727), emission theory of light 21 
 
 Neyt (A., of Ghent) 249 
 
 Nicol prism 94 
 
 Nigrosine 309-310 
 
 Nitric acid [HNO3 ; Mol. wt. 63], effect of, 308 
 Nobert (J. A., optician, Barth) test groups 64, 
 
 instrument used by 56, tests 54, 55, 5b 
 Normal [L. normalis, from norma a square] 
 
 defined 2 
 Normal vision 248 
 Nose-piece, rotating 100 
 
 Nuclein (= chromatine) 311, how to stain 310 
 Nucleus [L. nux, nucis a nut] staining 310-311 
 Numerical aperture 29, 52, see J. W. Stephenson 
 (J.R.M.S., 1878, p. 51); Prof. Abbe (J.R.M.S., 
 1881, pp. 388-423); Frank Crisp (J.R.M.S., 
 1881, pp. 303-360) ; Hockin (J.R.M.S., 1883, 
 P- 337)- f or a table of equivalent angles of 
 dry, water immersion, and oil (or homogeneous) 
 immersion objectives, with their respective 
 illuminating powers and theoretical resolving 
 power for every 0"2 of numerical aperture 
 from '40 to i"S2, see J.R.M.S., 1879, p. 839; 
 1881, p. 325 ; 1884, p. 972 ; 1889, pp. 292, 454 
 Nuremberg microscopes 347, Vervliet's 347 
 
 o. 
 
 Oberhauser's camera lucida 91 
 Object clamp for microtome 315 
 Object glass, see Objective. 
 
 Objects, see Media for preserving ; Preparation 
 of objects ; Slip or slide. 
 
 Objective or object glass 19, 42, 44-70, achroma- 
 tic 44, Amici and Ross discover interference by 
 cover-glass 45, Amici corrects for cover-glass 
 thickness 45, angle of aperture 50, 51, angle 
 should vary as power 51, apertometer for 
 examination of, 53, apochromatics 44, 47, 366, 
 battery of 56, 57, Beelsnyder's 352, Chevalier's 
 first 44, changer 101, classification of 50, 
 correction collar 46, defining power 56, dry 46, 
 essentials for higher apertures 55, focal distance 
 50, frontal distance 51, immersion 46, number 
 of required 56, numerical aperture 52, optical 
 qualities of good 51, penetrating power 56, 
 resolving power 54-56, Ross invents correction 
 collar 45, screw thread for 49, testing 337, tests 
 f° r 57-7°. with fixed mounting 45, with correction 
 collar 45, Zeiss' graduated scale on correction 
 collar 46 
 
 Oblique illumination 79, when used, advantages, 
 objections answered 131 
 
 Ocular, or eye-piece [L. ocularis, from oculus an 
 eye] 19, 42, 70-74, classification 73 (see also 
 Abbe, J. R. M.S., 1887, p. 25), compensating 71, 
 dissecting 102, field glass 20, Huygen's 70, 
 orthoscopic 71, projection 72, Ramsden's 71, 
 searcher 71, stereoscopic 104, Van Heurck's 
 comparing ocular 105, working 71, micrometer 
 89, 90, see also Nelson on a new eye-piece 
 (J.R.M.S., 1887, p. 928). 
 
 Ohm, George Simon, German electrician, b. 
 1787, d. 1854. 
 
 Ohm [from Ohm as above] the electro-magnetic 
 unit of resistance (sym. R.) 114 
 
 Oil as a preservative medium 300 
 I Oil immersion compared with dry and water 
 objectives 52 
 
 ! Ongania (optician, Antwerp), old microscopes 
 346, sells Nuremberg microscopes 347 
 Opaque objects, illumination of, 87, 88 
 Optics [Gk. oTrrtKog from root 6tt — to see], 
 elementary optics, pp. 1-20. For fuller explana- 
 tion of optical laws applied to the microscope, 
 see Naegeli and Schwendener, The microscope 
 I 333 
 
 Optical centre of lens 8, how formed 9 
 Optical planes 77 
 
 Optical projection, principle in apparatus for 12 
 
 Ordinary ray in polarization 94 
 
 Orthoscopic [Gk. 6p96g straight, (tkotteu) I view] 
 
 ocular 71 
 Oxy-hydrogen gas microscope r26 
 
 P. 
 
 Pancratic [Gk. ttAv all Kparoq strength] ocular. 
 An ocular capable of adjustment so as to obtain 
 a variable magnifying power. Pancratic micro- 
 scope defined 102 
 i Pantocsek (Dr. Josef), his finder 98, see also 
 Zeitschr f. Wiss. Mik., vol. v., 1888, 39-42 
 (J.R.M.S., 1889, p. 121). 
 
 Parabolic [Gk. irapajioXr) a placing beside. 
 A conic section, so called because its axis is 
 parallel to the side of the cone] illumination, 
 see Mayall (J.R.M.S., 1879, p. 27). 
 
 Parallel rays from sun assumed 5 
 
 Parallel surfaces, light passing through media 
 with 5 
 
 Parallelopiped [Gk. 7rapa\\j;\oc parallel 
 eTriTTtdov a plane surface] a solid figure con- 
 
INDEX. 
 
 379 
 
 tained by six quadrilateral figures whereof 
 every opposite two are parallel (Eucl. 6, xi.) 
 94 
 
 Pedesis or Brownian movement 292, see also Prof. 
 
 Jevons (Q.J. M.S., 1878, p. 172); Rev. f. 
 
 Delsaulx (M.J. M.S., 1877, p. 1); Naegeli— 
 
 Nature, 1880, p. 350 ; Dr. Win. M. Ord 
 
 (J.R.M.S., 1879, pp. 656-662), 
 Pelletan (Dr. Jules, Paris), editor of Journal de 
 
 micrographie 331, author of Le microscope et 
 
 son application 333. on numerical aperture 52 
 Pencil [L. penicellus dim, of penis a tail] defined 1 
 Penetrating power 56, see Nelson (J.R.M.S. 1892, 
 
 P- 33* )• 
 
 IVragallo (H. ), Histoire sommaire du microscope 
 332 
 
 Petrographical microscope 168, 182, see Nachet, 
 (J.R.M.S., 1880, p. 227). 
 
 Phenic acid = Carbolic acid. 
 
 Photo-electric microscope 126 
 
 Photography [Gk. <ftaiQ, 0iuroc light, ypaQio I 
 write] with collodion 249, increases resolving 
 power of microscope 362, advantage over 
 direct vision 363, processes in 270 
 
 Photo-micrography 249, installations 250, Wood- 
 ward's 250, ordinary 250, the Author's 251, 
 apparatus 252, Nachet's 252, Reichert's 258, 
 Zeiss' 259, Leitz 261, the Author's 262, Pringle's 
 269, the illuminant 264, sunlight 265, heliostat 
 265, electric light, 267, oxy-hydrogen 268, lamp- 
 light 268, white cloud light 270, photographic 
 processes 270, production of a negative 270, 
 developer 271, normal and over-exposed 
 negatives 272, instantaneous and under-exposed 
 negatives 273, intensifi«afeion of negatives 273, 
 fixing solution 274, printihfe; ph pap'ev 
 production of lantern- sli°dHs .2,77, Ipi'ocess! of 
 obtaining micrographs 'intended fdr" prdjeCUdii 
 277, general instructions, 280, Dr. Maddox 
 on 283, Mr. Andrew Pringle "oh 287, Mr. 
 Comber on 288, books on 333 ; ' 1,' ' ) 
 
 Photographing with piojection ocul;1r7'i, anirllifi-,' 
 cation calculated 73 with comparing ocular 106 
 
 Photophore [Gr. <pMc, light <popho I bear] Trouve's 
 112, Engelmann's 112, Kagenaar's 113 
 
 Phosphoius [Gk. 0wc light <ptpu I bring] Sym. 
 P. At. wt 31 Sp. Gr. 1 '826, used inSchott glass 
 17. See}. W. Stephenson (J.R.M.S., 1882, p. 
 .163). 
 
 Pince Suisse, adapter 100 
 Pipettes [Fr. a small pipe] 311, 313 
 Picrocarminate of ammonia 309, 310 
 Planchon (Prof.), use of Van Heurck's gas micro- 
 scope 127 
 Plano-concave lens 7 
 Plano-convex lens 7 
 
 Pleurosigma angulatum. Formation of appear- 
 ance similar to structure of 39, 40, as test 60 
 
 Podura plombea as test 58 
 
 Polarizer 94, ^J.R.M.S, 1891, p. 810. 
 
 Polarisation described 94, 95 ordinary and extra- 
 ordinary rays 94. As to polarising prisms, 
 see J.R.M.S., 1891, p. 253. 
 
 Polariscope [Eng. polar and Gk. dKoiriix) to 
 view] apparatus 94, Hartnack & Prazmowski's 
 95, Swift's 95, Ross' 95, Darker's revolving 
 selenite stage 95. See Ahrens, (J.R.M.S., 
 1884, p. 533 ; 1886, p. 397). 
 
 Pope Leo X. using magnifying glass 337 
 
 Positive ocular. An ocular by which the vertical 
 image of an object can only be obtained by 
 
 using the ocular as a simple microscope because 
 its focus is exterior to itself 90 
 Potash, caustic (hydrate of potassium) [KHO.] 
 
 3°9 . 
 
 Potassium chlorate [KC10 3 ] 308, 309 
 Potassium iodide [KI] 308 
 
 Preparations. — The objects prepared for examina- 
 tion. As to medium for preparing objects 299, 
 instruments used in preparing objects 311, 
 ' mounting 326, preserving preparations 329, 
 effect of damp and cold 329, how stored 329, 
 how labelled 330 
 
 Presbyte [Gk. irpzafivTriQ an old man] sight 
 (long sight) 19 
 
 Preservation of microscope 294, of microscopical 
 objects 299, of preparations 329 
 
 Principal axis of mirrors 3, of lenses 10 
 
 Principal focal length 10 
 
 Principal focus 3, 10, 4, 5, of bi-convex lens 10, of 
 bi-convex lens 13 
 
 Pringle (Andrew, F.R.M.S.), President of Photo- 
 graphic Convention of U.K., 1882, author of 
 Practical Photo-micrography, 333, as photo- 
 micrographer 249, photo-micrographic appara- 
 tus 268, 269, remarks on photo-micrography 
 287, 288 
 
 Prism [Gk. 7rptafia from irpiZui I saw] 6, as re- 
 flector 7, refraction by 6, reversing 102, com- 
 parison 97, nicol 94 
 
 Projection oculars 72 
 
 Protoplasm [Gk. irpwroc first irXaafia form] 
 
 studies in 311 
 Powell and Lealand 190, condensers 84-86, 195, 
 chromatic oil 84, apochromatic dry 85, achro- 
 matic oil 85, apochromatic immersion 85, 
 >large >Umroscop&, Nd. projection 
 > '.iocujard ^..objectives^ici-i^' 
 Projection (bp'tical), 'principle of apparatus 12 
 Pygidium of rlea, as test, 58 
 
 > , j - .... >R, 
 
 Rack and pinion. The coarse adjustment form- 
 ing the rapid movement of a microscope, by 
 which the whole body of the microscope is 
 moved. It should be so accurate, that without 
 shake, all low powers can be perfectly focussed 
 without the assistance of the fine adjustment 
 42, 77, rack and pinion in Van Heurck micro- 
 cope 230 
 
 Radiguet's battery 114, 115, Siphon 116, 117 
 Radius of curvature 3, 5 
 Ramsden ocular 71, micrometer 90 
 Ranvier(L. Paris, Hon. F.R.M.S.), his double 
 stain 310 
 
 Raphael's picture of Pope, Leo X. with lens, 337 
 
 Raspail's simple microscope 341 
 
 Ray [O.Fr. ray a sunbeam, from L. radius a 
 ray] defined 1, parallel 5, ordinary and extra- 
 ordinary 94, acting only on sensitized photo- 
 graphic plate 249. For diagram of ray through 
 an objective, see Prof. Keith, (J.R.M.S., 1878, 
 p. 142 ; 1879, p. 269). For the same through 
 an apochromatic objective 366 
 
 Razors 311, 312, razor strops 312 
 
 Reagents 309-311 
 
 Real image, formation of in convex lenses 12, in 
 
 compound microscope 19 
 Realgar [As 2 S 2 ] one of the three sulphides of 
 
 arsenic used by Prof. H. L. Smith in his yellow 
 
 medium 305 
 
3 8o 
 
 INDEX. 
 
 Reflection (L. re back, flecto, flexum to bend) 
 defined 2, described 2-5, angle of reflection 2, 
 total reflection 6, see Internal reflection. 
 
 Ruling lines upon glass, see Prof. Rogers, 
 (J.R.M.S., 1886, p. 16). 
 
 Refraction [L. refrattgo, refractum to break up] 
 defined 2, angle of 5 ; double refraction 94, index 
 of 6, see Index of refraction. 
 
 Refractive index, see Encly. Britt. "Optics," 
 XVII. 200, see Index of refraction. 
 
 Refrangibility [L. re and, frango I break] aberra- 
 tion of 16, of various colours 16 
 
 Reichert (Carl, optician) 195, his mechanical 
 stage 75, 202, projection oculars 72, large 
 microtome 315, 316, largest model microscope 
 196, smaller 197, substage and condenser 198, 
 stand No. III. 200, smaller stand 201, 202, 
 achromatic objectives 202, apochromatic objec- 
 tives 203, new semi-apochromatic objectives 
 205, constructs comparing ocular 105 
 
 Resolving power, formula of 31, limit o*" function 
 of aperture 36, why affected little by excessive 
 aperture 37, tables of 54, 55, exhibits detail 65, 
 of microscope increased bv photography 361, 
 see J. W. Stephenson (J. R.'M.S., 1878, p. 186; 
 1886, p. 37). 
 
 Reversing prism 103 
 
 Rheostat [Gk. pem I flow, qtutoq standing still] 
 electric light regulator 112, 113 
 
 Rivet designs Reichert's microtome 315 
 
 Robin (Ch.) Traite" du Microscope 333 
 
 Room for microscopical work 129, for micro- 
 scopical preparation 329 
 
 Ross (Andrew, optician) invents correction collar 
 objectives 45 1 * 1 
 
 Ross & Co. (Opticjans), 2C'b,Lpo'arizi;ng v jpparatns.-, 
 95, Wcnhun's .radial &7i 207, '209,. Rqs> 
 Zentmayet No. t. microscope^, '210, objectives 
 211 
 
 Rotary stage 75 
 Rotating nose-piece 100 
 
 Royal Microscopical Society (20, HanD.ve r TSqEe,,. 1 
 W. , Estb. 1839, incorporated by Royal Char- 
 ter, 1866,) Journal 331, screw-thread 49, table 
 of resolving power 54, see Numerical aperture 
 
 Rules for using condensers 86, for microscopical 
 research 133 
 
 s. 
 
 Saccharine [C& H 4 CO NH] produced from toluene 
 
 (C6 H 5 CH 3 ) one of the components of coal 
 
 tar naptha by Fahlberg 300 
 Saccharinated water as a preservative medium 300 
 Salvino d' Armato degli Armati of Florence 
 
 makes glass spectacles 334 
 Scale in spectroscope, measuring wave lengths 97 
 Scalpels 311, 313 
 
 Schacht (H. ) author of Le Microscope et son ap- 
 plication 333, sight uninjured by microscopical 
 research 133 
 
 Schwendener (Prof. S., Berlin, Hon. F.R.M.S.) 
 Das mikroskop 333 
 
 Schott (Dr., chemist) investigates vitreous com- 
 pounds 17 
 
 Schott & Co. (Drs. Schott, Abbe and Zeiss) 
 manufacture various kinds of glass 17 
 
 Schott glass 17, with index of 2*0 358. For re- 
 fractive index, medium dispersion and specific 
 
 gravity of these glasses, see J.R.M.S., 1890, 
 
 p. 398 
 
 Schleusner (Henri, Antwerp) introduces isochro- 
 matic plates 282, use of coloured screens 282 
 
 Schultze (F.E., Berlin, Hon. F.R.M.S.) on pre- 
 paration of iodized chloride of zinc 308, macera- 
 ting process 308 
 
 Schumann, photographs ultra-violet of spectrum 
 364 
 
 Schwarzer Maskenlack No. 3, for cells, 307 
 Screw thread of tube and objective 49, various 
 
 kinds of 49 
 Searcher oculars 71 
 
 Sections, hand and microtome 313, 314, prepara- 
 tion of objects for microtome 322, by freezing 
 322, by embedding 323 
 
 Secondary axis of lenses, 10; foci. 11. 
 
 Secondary spectrum. The colour or colours left 
 outstanding in a corrected combination of 
 lenses. Suppressed in apochromatic objectives 
 47. Final upper lens (triplet) to destroy 370 
 
 Seed microscope 337 
 
 Seibert (W. & H. , opticians) 212, large stand 213, 
 stand No. 2, 214; No. 4, 215; No. 5, 216; 
 objectives 216, rotating nose-piece 100 
 
 Selenite [foliated crystallized sulphate of lime] 
 stage, Darker's revolving 96 
 
 Selligue microscope 353, 354 
 
 Sights, long and short in microscopical observa- 
 tions 19, protected in microscopical research 
 
 r ... 
 
 Silica [ Si0 2 ] oxide of silicon, principal ingre- 
 dient in flints, quartz, glass and diatom valves, 
 &c. Refractive index of silica of diatoms 304 
 
 Silicon [L. si/ex, silieie-a flint. Sym. Si. At. wt. 
 ■'.H] nYetiillic .element 
 
 Sine . TV an nnHe'fL. -sinus, a bending] Dej. 
 Iii 6ne of the "'lines bounding an acute angle 
 take any point and from it let fall a perpendi- 
 cular on-t(5 the other bounding line, then the 
 r;'t!o.'o> thrs perpendicular to that side of the 
 tiiarfgle dtiwn above, which is opposite the 
 right is called the sine of that acute angle. For 
 a complete list of the series of angles see Cham- 
 ber's Mathematical Tables. The following, 
 however, may be useful : — 
 
 Angle. 
 
 Sine. 
 
 Angle. 
 
 Sine. 
 
 
 ■2588 
 
 6S 
 
 •9063 
 
 3° 
 
 '5 
 
 70 
 
 "9397 
 
 45 
 
 •7071 
 
 75 
 
 •9659 
 
 5° 
 
 •7660 
 
 80 
 
 •9848 
 
 55 
 
 ■8191 
 
 8S 
 
 •9962 
 
 60 
 
 •8660 
 
 90 
 
 I 'OOOO 
 
 Simple microscope, principle of 13, magnifying 
 glass as 18, Wollaston doublet 18, Leeuwen- 
 hoeck's 338, Wilson's 340, Cuff's 340, Raspail's 
 341. As to amplification varying as distance 
 between lenses, see Robin, Traits on Microscope 
 (2nd Ed.) p. 33, 34 See also Microscope. 
 
 Sliding objective changer 101 
 
 Slip or slide, sizes 298, thickness 298 
 
 Smith (H. L. Prof, physics, Hobart College, 
 Geneva, N.Y., Hon. F.R.M.S.) vertical illumi- 
 nator 88, yellow medium (index 2'4) 305 
 
 Society's screw thread 49 
 
 Sodium [Na. At. wt. 23] sulphindigotate 309, 310 
 Solar microscope 123 
 
INDEX. 
 
 38l 
 
 Solar spectrum, colours of, 16, see further on this 
 subject, J. Norman Lockyer, Studies in spectrum 
 analysis. 
 
 Sorby (H.C.) his spectroscope 96 
 
 Spar, see Fluor spar ; Iceland spar 
 
 Spectrum [L. spectrum, an image or appearance], 
 the several coloured and other rays of which 
 light is composed, separated by the refraction 
 of a prism or other means, and exhibited either 
 as opened out on a screen or by direct vision. 
 Spectrum in spectroscope 96, 97 
 
 Spectra, one row of admitted, object indetermi- 
 nate 38, 40, see Secondary spectrum 
 
 Spectroscope [L. spectrum an image, and Gk. 
 aKoirku} I see] 95, Sorby's 95, see Ward, 
 (J.R.M.S., 1879, p. 326) 
 
 Speculum [L. a mirror, speccre to look] of Lieber- 
 kuhn 87, 88, 131 
 
 Spencer (H. R., optician) 218 
 
 Spherical aberration, defined 14, avoidance of, 
 15,16, corrected in apochromatic objectives 47, 
 368, 369, and cover glass correction 45. For 
 formula, see Encyclopaedia Brittanica (9th Ed.) 
 XVII., p. 803; see Abbe (]. R.M.S., 1879, 
 p. 812) 
 
 Spirogyra preserved in camphorated water 299 
 Spirit lamp 311 
 
 Spurious lines, formation of, 37, 38 
 
 Stage [O. Fr. estage a story of a house, L. stare 
 to stand] 41, 42, qualities required 75, rotary 75, 
 mechanical 75, Reichert's 75, Nachet's move- 
 able carriers 75, Zeiss' mechanical 76, Reichert's 
 mechanical 202, moveable stage as finder 97 
 
 Stage micrometer 91 
 
 Staining 309-311, double staining 311, staining 
 tissues 323, with single colour 324, with double 
 colour 325, with iodine green 325, with picro- 
 carminate 326. For methods employed at 
 Naples histological station, .kv (J.R.M.S., 1882, 
 pp. 866-883), see Richardson on Multiple stain- 
 ing (J.R.M.S., 1881, p. 868). Gibbes on 
 Double and treble staining (J.R.M.S., 1880, 
 P- 39°) 
 
 Steam's electric lamps 109 
 
 Stephenson's (J. W., formerly Treas. R.M.S.) 
 experiments on the application of Dr. Abbe's 
 Theory of microscopic vision 32-40, see also 
 J. R.M.S., 1878, p. 186; and on an erecting 
 binocular microscope, M.M.J. , 1870, p. 61 ; 
 1872, p. 167, 215 ; 1877, p. 82, 1873, 
 p. 41 
 
 Stereoscopic [Gk. crept of solid aKOTriw I view] 
 microscope 103 ; eye-piece (Abbe's) 104. As to 
 Wenham's, see T.M.S. 1866, p. 103; as to 
 stereoscopic vision with non-steroscopic arrange- 
 ments, see J.R.M.S., 1881, p. 203; 1882, 
 p. 271 
 
 Stop, see Diaphragm 
 
 Strasburger (Prof. E., Iena, Hon. F.R.M.S.) 
 recommends styrax 302 ; staining chlorophyl 
 3" 
 
 Stria [L. stria a streak], the lines visible on the 
 outside of the silicious valves of many diatoms. 
 As tests 54, micrometrical measurement of 90 
 
 Styrax from liquidambar orientalis 301, how pre- 
 pared 303, refractive index 302, use 311, see 
 J. R.M.S., 1884, pp. 318,475, 655, 827; 1885, 
 pp. 166, 744 ; 1886, p. 171 ; 1887, pp. 359, 692 ; 
 1888, p. 1057; J.Q.M.C., 1884, p. 116; 1886, 
 p. no ; 1891, p. 395 
 
 Substage condensers 80, Abbe's 81, in the Van 
 
 Heurck microscope 230, Powell and Lealand's 
 84, Reichert's 199, rules for using 86, see Nelson 
 (J.Q.M.C., 1890, p, 116 ; J.R.M.S., 1891, p. go). 
 Sulphur, bromide of [S 2 Br 2 ] in the yellow 
 medium 306 
 
 Sulphuric acid [H = S0 4 molecular wt. 98, sp. gr. 
 
 1 -8] effect of 308 
 Superposed lenses 353 
 
 Surfaces, passage of light through media with 
 parallel 5, with non-parallel 6 
 
 Surirella gemma as test 62 
 
 Swan Company's electric lamps 109 
 
 Swift (James and Son, opticians) 219, polarizing 
 apparatus 95, challenge microscope 219, sea- 
 side microscope 220, small pocket microscope 
 221 
 
 T. 
 
 Table for workroom 129, turn table 327, of nu- 
 merical apertures, see Numerical apertures 
 Testing lenses 337 
 
 Test plate (Abbe's) described 68, use in testing 
 apochromatic objectives 47, 48 
 
 Tests for objectives 57-70, Abbe's 68, amphi- 
 pleura pcllucida 63, Moller's 66, Nobert's 64, 
 pleurosigma angulatum 60, podura plumbea 
 58, pygidium of ajlea 58, surirella gemma 62, 
 Van Heurck's photographic 67, vanheurckia 
 rhomboides 62 
 
 Thomas' microtome 318 
 
 Thum (Ed. Leipzig) tests 67 
 
 Thury, (Prof., Geneva) invents "pince Suisse" ico 
 Torre (Pere del la, Naples) constructs globules 339 
 Total reflection defined 6, in Nicol prism 94 
 Transformer, Van Heurck's, 106 
 Translucent [L. trans across, luceo I shine], 1 
 Transparent [L. trans across, pareo I appear], 1 
 Trouve's Photophore 112, portable battery 114. 
 
 "5 , , 
 
 ! Trutat (E.) Traits elementaire du microscope 333 
 Triplet in apochromatic objectives 370, object of 
 270, first suggested by Holland, see Tr. Soc. 
 of Arts xlix. 
 
 Tube of microscope 42, English length 42, 
 Continental length 42, see Crisp (J. R.M.S. , 
 1883, p. 816). 
 
 Turn-table 327 
 
 u. 
 
 ; Undulatory [N. Lat. undulatio, motion resem- 
 bling waves], theory of light 21 
 Universal microscope of Chevalier 355, Martin's 
 351 
 
 j Van Beneden (Edward, Dr. of Sciences, Louvain) 
 
 249 
 
 Van der Kolk, Schrceder, researches on musca; 
 volitantes 291 
 I Van Deyl, achromatic lens 352, microscope 352 
 
 Van Heurck (Ferdinand) sale of reagents, media 
 j and tests 307, agent for Reichert, for Belgium 
 i and Holland 206 
 
 Van Heurck (Dr. Henri), author of Synopsis 
 des Diatomees de Belgique 333, La lumiere 
 electrique io9,acquiresMouchet's collections 80, 
 
3«2 
 
 INDEX. 
 
 introduces Styrax 301, his comparing ocular 
 105, microscope 224-232, photo-micrographic 
 installation 251, and apparatus 262, 263, trans- 
 former 106, photographing with comparing 
 ocular 106 
 
 Vanheurckia crassinervis as test 62. 
 
 Vanheurckia rhomboides as test 62. 
 
 Van Rysselberghe and electric light 117 
 
 Varnish for cells 326 
 
 Vernier (Pierre, mathematician) b. at Ornans, 
 near Besancon, 1580, published at Brussels 
 "Construction et usage de Quadrant," 1631, 
 d. at Ornans, 1637. 
 
 Vernier [after inventor as above] a small move- 
 able scale or sector running parallel with the 
 fixed scale of a graduated instrument, and used 
 for measuring a fractional part of one of the 
 equal divisions on the graduated fixed scale. 
 Vernier on mechanical stage 98. 
 
 Vertical illuminator, Prof. Smith's 88, see 
 Stephenson (J.R.M.S., 1879, p. 266), Rogers on 
 Tolles (J.R.M.S., 1880, p. 754). 
 
 Violet methyl 311 
 
 Virtual focus of mirrors 4, of convex lenses 11, 
 
 of concave 13 
 Virtual image, defined 3, explained 13, in 
 
 apochromatic combinations 369, see Nelson, 
 
 (J.R.M.S., 1892, p. 180) 
 Visibility, limit of 36, as to visibility of minute 
 
 objects,^ Stephenson (J.R.M.S., 1880, p. 564). 
 Vision, see Double vision. 
 
 Volta, Alesseuoro, b. 1745, d. 1827, Italian 
 electrician. 
 
 Volt [from Volta], unit of tension or electro- 
 motive force no 
 
 w. 
 
 Watch glasses 311, 313 
 Warwick, gas microscope 126 
 Water, index ot refraction 46 
 Water immersion objective compared with oil 
 and dry 152 
 
 Watson and Sons (opticians) 221, slow move- 
 ment to Van Heurck microscope 77, 229, 
 Scientist's microscope 222, Edinburgh micro- 
 scope 224, Van Heurck microscope 224-232, 
 fine adjustment 229, and rack and pinion in 
 
 same 230, objectives 232, projection microscope 
 128, Cathcart microtome 317, lamp 108 
 
 Wave lengths in spectroscope 97 
 
 Weigert introduces ammonium picrocarminate 
 310 
 
 Wenham radial 87, 206-209, binocular microscope 
 103 
 
 Wilson (J.) simple microscope 340 
 
 Wollaston (William Hyde, M.D., b. 1766, 
 d, 1828) invents doubtlet 18 
 
 Woodward (Lieut.-Colonel J. J., M.D., U.S. 
 Army Medical Department, Washington) 249 
 his photo-micrographic apparatus 250, photo- 
 graph of 19th band Nobert test 66. 
 
 Working oculars 71 
 
 Work room, situation and arrangement of, 129, 
 for solar microscopes 125 
 
 Y. 
 
 Yellow medium (Index 2-4) 305, amphipleura 
 photographed in 63 
 I Young (Thomas, M.D., secretary to Board of 
 Longitude, b. 1773, discovered interference of 
 light 1801, d. 1829), studies in light 21 
 
 z. 
 
 Zeiss, Carl (opticians, Iena)232,Abbe'sapertometer 
 1 53 ; apochromatic objectives, first sold by 47, 
 described 239, prices of 243 ; compensating 
 oculars 71, 244; condensers 81-84; diffraction 
 plate, 33 ; English screw thread used by 49 ; 
 figures on correction objectives 46 ; manu- 
 facture frontal with dense flint glass 47 ; 
 mechanical stage 76 ; micrometer 90 ; micro- 
 scopes (stand I. a) 233, (stand for photo-micro- 
 graphy) 235, (stand II. a) 236, (stand IV. a) 237, 
 (stand VI.) 238, objectives 239-243 ; objective 
 of N.A. 163, 49, 240; projection oculars 71, 
 244 ; spectroscope 97 ; testing objectives 337 ; 
 thin plates of glass for lenses 334, transformer 
 106 
 
 Zeiss (Dr. Roderick, F.R.M.S.), as photo-micro- 
 grapher 249, special-catalog iiber apparate fiir 
 niikrophoto-graphie 333, his sliding objective 
 changer 101 
 
 Zinc, chloride [ZnCL. ] iodized 307, 308 
 
 ERRATA. 
 
 Before luminous insert the (p. 11, line 5 from bottom). — For spectra trays read spectral 
 rays (p. 35, line 21). — For visability read visibility (p. 36, line 15). — For slip xead glass (p. 46, 
 sec. 3). — Insert in before the microscope (p.47, line 21). — Y ox give read gave (p. 47, line 22).— For 
 adapted read adopted (p. 49, line 20). — For admissable read admissible (p. 131, line 9). — For 
 Pleurosigna read Pleurosigma (p. 190, line 18). — For object lead objective (p, 218, line 12 from 
 end). — For megalocephalus read megalocephala (p. 249, line 17). — For Leon read Leo (p. 337, 
 line 20). — For Grindelins read Grindelius (p. 344, line 11 from end). 
 
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