The microscopy of technical products

Front matter

Clhp S. % Brill library

Tex.i'ib'

UIVESSITV DM MILL LIBRAS i

Author's Preface

Since the appearance of J. Wiesner's admirable work, " Einf iihrung in die technische Mikroskopie" (Vienna, 1867), a work which laid the foundation for the scientific study of commercial products, no suitable handbook covering the whole field of technical microscopy has been placed before the public.

In the "Lehrbuch der technischen Mikroskopie," the author has sought to fulfil two important tasks essential in such a work. On the one hand, the book is designed to serve as a reliable scientific guide to the student entering the field of technical microscopy, serving to familiarize him with the methods of investigation, the fundamental principles and the literature of the subject, and preparing him to undertake independent investigations in this branch of applied natural science. The student who uses the book for this purpose should have a general knowledge of the natural sciences, particularly the morphology and histology of organisms, and should also be familiar with the principles of chemistry.

On the other hand, the book is designed to aid in the solution of purely practical problems. It teaches the technical worker how to investigate microscopically commercial raw materials with reference to their composition and suitability for technical purposes, thus enabling him to reach practical conclusions. It should be remembered, however, that the book is neither a treatise on raw materials nor an economic natural history, but, as it were, a precursor of works on these subjects; it brings together the typical raw materials of each natural group and points out the characters common to each. In cases where the origin, harvesting, preparation, and utilization of the materials are briefly treated, thus leading the reader into the realm of economic natural history, the purpose is not so much to make the presentation complete as to show

iii

106287

»V AUTHOR'S PREhACb.

the influence which methods of preparation have on the structure of the raw material or to determine how far the characters of the raw material permit its use for technical purposes.

The difficulties encountered in combining two purposes, apparently so different, in limiting the subject matter and in selecting types for study, are not inconsiderable. This handbook is the result of many years' experience as a teacher and expert. May it prove practicable and useful.

Dr. T. F. Hanausek.

Vienna, March, 1901.

TRANSLATOR'S PREFACE.

Dr. Hanausek's "Lehrbuch der technischen Mikroskopie" is unique in that it teaches the microscopic identification of technical products and at the same time the fundamental principles of vegetable histology and the histology of certain animal materials. The author is distinguished alike as an investigator, a teacher, and a technical expert, and his work is characterized by its scientific accuracy, its clearness, and its utility as a guide in diagnosis.

The translation has been carried out with the cordial cooperation of the author, who has made numerous changes in the original text. Much new matter has been added to the chapters on textile fibers, and the number of practical examples increased from eight to eighteen. The analytical key for woods has been revised so as to include the most important North American species. In this latter work a set of authenticated specimens of American woods, kindly furnished by Prof. Geo. B. Sudworth of the U. S. Forest Service, has proved invaluable. The article on papr'ka has been omitted.

Twenty seven cuts, included in the German edition, have been dropped, but forty-seven others have been added, so that the total number has been increased by twenty. The cuts new to the work include reproductions of drawings by the author, Professors P.'ccioli and Tschirch, Dr. Barber, and the translator, some published here for the first time, and a number of illustrations of microscopic apparatus kindly loaned by the well-known houses of Ernst Leitz, Carl Zeiss, and The Bausch and Lomb Optical Company. Acknowlegment for the use of cuts is due the Connecticut Agricultural Experiment Station and also t%e following publishers: Ferdinand Enke of Stuttgart (publisher of the German edition), Urban and Schwarzenburg of Vienna and vi TRANSLATOR'S PREFACE.

Berlin, Paul Farcy of Berlin, Alfred Holder of Vienna, \V. Braumuller of Vienna, G. Masson of Paris, and H. Hassel of Leipzig.

The translation could not have been undertaken but for the able assistance of Dr. Kate G. Barber, who from the first has been identified with the work and is well entitled to a place as collaborator.

It is hoped that this handbook will bring to the attention of English* speaking scientists the importance of analytical histology in the examination not only of foods but of libers, woods, paper, and other technical

products.

A. L. Wlnton.

Chicago, May 15, 1907.

Chapter I. The Microscope

PAGE

Simple Microscope 4

Compound Microscope 5

Theories 5

Objectives 6

Oculars 7

Illuminating Apparatus 8

Microscope Stand 8

Base or Foot 8

Standard 8

Stage 8

Arm 8

Tube 9

Nose Piece 9

Illuminating Apparatus 9

Mirror 9

Diaphragm n

Substage Condenser II

Optical Parts 11

Objectives n

Eyepieces (Oculars) „ 13

MICROSCOPY OF THE MOST IMPORTANT TYPES OF TECHNICAL

RAW MATERIALS.

Chapter I

STARCH. INULIN.

General Characters. Potato Starch 27

Formation 29

Chemical Properties 33

Wheat Starch 38

Maize Starch 4^

Rice Starch 42

Buckwheat Starch 42

Oat Starch 43

Maranta Starch, or Bermuda Arrowroot 43

Cassava Starch 45

Sweet -potato Starch 4"

Curcuma Starch, or East India Arrowroot 46

("anna Starch, or Queensland Arrowroot 47

Yam Starch, or Guiana Arrowroot 47

Banana Starch 47

Saco Starch 47

Leguminous Starches 48

Inulin 5°

Chapter II. Vegetable Fibers

I. Hairs (Trichomes) S1

Cellulose 54

Cotton 58

Mercerized 65

CONTENTS. IX

PAGE

Vegetable Down 68

Vegetable Silk 70

II. Fibers of Stems (Bast Fibers, Fibro-vascular Bundles, etc.) 72

(a) Fibers of Dicotyledonous Plants 73

Flax 73

Hemp 77

Jute 81

Gambo Hemp 83

Sunn Hemp 84

Hop Fiber 87

Nettle Fibers 88

Ramie 00

Paper Mulberry Fiber 92

(b) Fibers of Monocotyledonous Plants 94

Manila Hemp 94

Pita and Sisal Hemp 96

New Zealand Flax 99

III. Microscopic Examination of Paper 100

Lignin 101

Sizing 103

Identification of Fibers 104

1. Linen-rag Paper 107

2. Hemp Paper 108

3. Cotton-rag Paper 109

4. Jute Paper 109

5. Straw Cellulose no

6. Mechanical Wood Pulp 116

7. Wood Cellulose, or Chemical Wood Pulp 118

8. Sugar-cane Paper 119

9. Japanese and Chinese Papers 120

10. Miscellaneous Papers 122

CHAPTER III. ANIMAL fibers, mineral fibers, textiles

L Animal Hairs (Wool, etc.) 123

Sheep's Wool 124

Goat's Wool 130

1. Angora Wool or Mohair 130

2. Cashmere or Thibet Wool 131

3. Common Goat's Wool 132

Cow Hair 13?

Distinction between Sheep's Wool. Goat's Wool, and Cow Hair 134

Camel's Hair 137

Alpaca, Vicuna, Llama, Huanaco 139

Shoddy 141

H. Silk and Silk Substitutes 144

Common Silk 144

Wild Silks 148

X CONTENTS.

MM

Sea Silk. 149

Sinew Fiber 150

Artificial Silk 151

III. Mineral FIBERS 156

IV. Microscopic Examination of Textile Fabrics 157

1'r.u tical Examples 159

Chapter IV. Stems And Roots

I. Wood of Dicotyledons and Gymnosperms 174

A. Structure of Wood 175

Coniferous Woods 178

Burgcrstcin's Analytical Key for Spruce and Larch Woods 192

Broad-leaved Woods of the Temperate Zone 196

Tropical Broad-leaved Woods 206

B. Physico-tei hni; al Properties of Woods; Notes on Cell Contents 207

1 . Color 207

2. Hardness 208

C. Analytical Key for the Identification of the Most Important Economic Woods,

with Descriptions of the Species 211

D. Dye woods 243

Yellow Woods 244

Old Fustic. 244

Young Fustic 246

Distinction of Old Fustic from Young Fustic 249

Barberry 250

Blue Woods 251

Logwood 251

Red Woods 252

E. Cork Woods 253

II. MONOCOTYLEDONOUS STEMS 255

Rattan 255

Fibro-vascular Bundles 25S

Other Stems of Monocotyledons 261

III. Subterranean Organs 262

Monocotyledonous Roots and Rhizomes 262

Turmeric 262

Orris Root 266

Dicotyledonous Roots and Rhizomes 268

Alkanna Root 269

Soapwort Root 269

Chicory 271

IV. Barks 274

Cork 274

Oak Bark 281

Quillai Bark 288

V. Final Comments 290

VI. Practical Examples 293

CONTENTS. xi

Chapter V. Leaves

PAGE

Sumach 300

1. Sicilian 301

2. Venetian 303

3. Myrtle-leaved 304

Miscellaneous Leaves 309

Chapter VI

flowers and parts of flowers.

Insect Powder 310

Chapter VII. Fruits And Seeds

I. Kinds of Fruits. Morphology 324

II. The Seed 329

III. Microscopic Investigation of Typical Examples of Technical Fruits and

Seeds 334

The Examination of Flour with Reference to its Identity and Purity. . . . 342

1. Color 343

2. Mechanical Condition 343

4. Microscopy 346

Barley — Chaff of Rice and Millet 349

Barley and Malt Coffee 354

Weinzierl's Method of Qualitative and Quantitative Mechanico-Micro-

scopical Analysis 354

Oil Cakes 358

Cottonseed Cake and Cottonseed Meal 361

Kapok Cake 368

Pumpkin-seed Cake 369

Ground Olive Stones 371

Detection in Pepper 374

Niger Cake 375

Madia Cake 377

Sunflower Cake 377

Sesame Cake 380

Linseed Cake and Linseed Meal 384

Peanut Cake 387

Rape Cake 392

Poppy Cake 393

Hemp Cake 395

Beechnut Cake 397

Palm-nut Cake 400

Xli CONTENTS.

PAGB

Cocoanut Cake 403

Miscellaneous Cakes 406

Myrobalans 406

Ivory Nut or Vegetable Ivory 412

Polynesian Ivory Nut 414

Chapter VIII. Teeth, Bone, Horn, Etc

I. True Bones 418

IL Teeth 421

True Ivory 422

Hippopotamus Ivory 426

Walrus Ivory 427

Narwhal Ivory 428

Sperm-whale Ivory 428

III. Horn, Tortoise-shell, and Whalebone 429

Horn 429

Tortoise-shell 433

Whalebone 435

Chapter IX

microchemical analysis.

Comparative Crystal Analysis 440

Fusion Tests 44 1

Solution Tests 442

Individual Crystal Analysis 443

Barium 444

Calcium 444

Gold 446

Potassium 447

Sodium 447

Formic Acid 448

Citric Acid 448

PART I.

APPARATUS AND METHODS.

HANDBOOK

OF

TECHNICAL MICROSCOPY.

Chapter I. The Microscope.1

According to the definition of Dippel,2 the microscope is an optical instrument that enlarges an object (or part of an object) which of itself subtends too small a visual angle (less than 0.5 minute) to be clearly visible, so that the object itself and its details are distinctly evident to the observer.

This enlargement, known as "magnification," is brought about in various ways.

1 The construction, manipulation, and theories of the microscope are here briefly treated; more detailed descriptions will be found in the following works:

Behrens: Guide to the Microscope in Botany (Trans, by Hervey). Boston, 1885. Carpenter and Dallinger: The Microscope and its Revelations. London, 1901. Chamo"-: Chemical Microscopy. New York, 1915. Czapski: Theorie der optischen Instrumente. Breslau, 1893. Dippel: Handbu.h der allgemeinen Mikroskopie. 1882. I. Theil: Das Mikroskop und seine Anwendung; II. Theil: Anwendung auf die Histologic der Gewachse. 2. Aufl. 1896. Idem: Grundziige der allgemeinen Mikroskopie. Braunschweig. 1885. Frey: Das Mikroskop und die mikroskopische Technik. Leipzig, 1881. Gage: The Microscope. Ithaca, 1914. Hager u. Mez: Das Mikroskop und seine Anwendung. Berlin, 1899. Harting: Das Mikroskop. Braunschweig, 1866. Nageli u. Schwendener: Das Mikroskop. Leipzig, 1877. Schacht: Das Mikroskop. Berlin, 185 S- Strasburger: Das bot. Practicum. Jena, 1897. Strasburger and Hillhoitse: Handbook of Practical Botany. London, 1900. v. Thanhoffer: Das Mikroskop und seine Anwendung. Stuttgart, 1880. Vogl: Commentar zur Osterreich. Pharmakopoe. II. Band: Allgemeiner Theil. Wien, 1892. Wiesner: Einleitung in die technische Mikroskopie. Wien, 1867. Winslow: Elements of Applied Microscopy. New York, 1905.

2 Realenzyklopadie der gesammten Pharmacie. 1. Aufl., 6, 699.

D. H. HILL LIBRARY

APPARATUS -1MD METHODS.

Simple Microscope.

If an object is placed between a biconvex lens and Its focal point, an enlarged image is obtained in a position further removed from the

B

FlG. i. Formation of Image by a Simple Lens. (Thanhoffer.)

lens than the object itself. Jn Fig. i, dc represents the lens, ab the object, and bd and ac two rays parallel to the optical axis. The rays are refracted by the lens and collected in the focal point C, beyond which they diverge toward g and //, where they meet the retina of the eve; while the rays ad and be are refracted toward / and e and there also meet the retina. The eye finds the enlarged image at AB, where the lines formed by extensions of the refracted rays (JA-gA and <B-hB) meet. A lens (or a combination of lenses) which brings about this result is known as a simple microscope. Lenses of this description are usually mounted in metal, hard rubber, or horn, and are cften so arranged that two or more can be used in conjunction. A Pocket Lens with a swinging cover, which

also serves, when open, as a handle, is shown in Fig. 2. Three adjustable Lens Holders. with standards, arc shown in Fig. ^. In the more convenient form known as the Dissecting Microscope | Fig. 4), the lens is carried on an arm which is raised or lowered for focusing by a rack and pinion, while the object is placed on a glass slide or watch glass supported by a stage and may be illuminated by light reflected upward from a mirror beneath.

Fig. 2. I'<" ket Lens. 1 BAUSi u and Lomb.)

Compound Microscope.

When magnification is effected by two lenses or two combinations of lenses, one (objective) producing an enlarged image, the other (ocular or eyepiece) enlarging this image, the instrument is known as a compound microscope or simply as a microscope in contradistinction to a lens or simple microscope.

THEORIES OF THE COMPOUND MICRO- SCOPE.— According to the older theories 1 (which are sufficient

FlG. 3. Lens Holders. (Leitz.)

for our purpose), magnification by a compound microscope is explained as follows:

Fig. 4. Bausch and Lomb Dissecting Microscope.

1 Prof. Abbe's more recent theories as to the formation of in. ages are not here considered.

APPARATUS AND METHODS.

The Objective (Fig. 5, 0) produces an enlarged and inverted image (6V) of the object (lib), which is still further enlarged by the ocular

(C), and is seen by the eye in the position /'".;". This geometric result is, however, never a< tually secured in practice, first because the lenses of the objective increase in refractive power toward the margin, and second because tiny, like a series of prisms, decompose the rays of white light with the formation of variously colored rings.

The first error, known as Spherical

Aberration, blurs the image e.g., at

\ the focus) by the formation of small

1 ones of divergence. In Fig. 6 the

1™ \ --/---- -a" marginal rays (c and d) 1 ross at c, while

the two rays a and & situated nearer the optical axis cross at a', that is, the first two unite to form an image before the second two and as a consequence the image, being surrounded by circles of a h divergence, is blurred. So-called dia-

- it ,• r T u c: phragms serve to cut off the marginal

Fig. 5. Formation <>f Image by Sim- 1 & t>

plesi Form of Compound Microscope, rays and allow only such to enter the eye as have practically the same foci.

The second error, known as Chromatic Aberration, causes the forma tion of colored rings which greatly obscure the detail. The lens acts like c

a

FlG. 6. Spherical Aberration. (Thanhoffer.)

a series of prisms arranged about the optical axis, decomposing the rays

of light by refraction into their color elements, the red rays being the

THE MICROSCOPE.

least, the violet rays the most refracted. In Fig. 7, v is the focus of the violet rays and r of the red rays. If we place a piece of paper in the

Fig. 7. Chromatic Aberration. (Thanhoffer.)

plane c-d no color is evident in the center, since here rays of all colors

unite to form white light, but a violet ring appears at the borders. In the

plane a-b the image is surrounded by

a red ring. These color defects are

largely obviated by employing an

achromatic double lens, which consists

of a biconvex crown-glass lens and a

plano-concave flint-glass lens, the latter,

because of its lead content, having a

greater dispersion than the former, or

else a system of such lenses (Fig. 17).

An aplanatic lens is one in which both forms of aberration are corrected so far as is possible. If the flint glass is in excess and the object has a blue border, the lens is "over-corrected," x' while if the crown glass is in excess, and the object has a red border, the lens is " under-corrected."

The Ocular, shown in Fig. 5, as has been stated, serves to enlarge the image formed by the objective. Its defects are, first, that it magnifies all the errors of the image produced by the objective; and, second, only a part of the image is visible, unless the ocular is made very large, which is both expensive and inconvenient. To obviate these defects a "collective lens" (Fig. 8) is used in con-

FlG. 8. Formation of Image by Compound Microscope with Two-lens Eyepiece. (Thanhoffer.)

s

APPARATUS ASU METHODS jjnction with the ocular lens. The image formed by the objective is brought within the Celd of vision by the collective lens and is Gnally magnified by the ocular proper.

Illuminating Apparatus. —The microscope, through its two systems of lenses the objective and the ocular), performs thru' optical functions: (i) concentration of rays, (2) formation of an image, and (3) magnification. For this, however, sufficient light to illuminate the 1 bject is 1 ssential.

According to Dippel, the illuminating apparatus must fulfil the following conditions: (1) it must be so constructed as to permit the projection of a rone of light on the object at will, either in the direction

of the axis of the instrument or in any oblique direction; (2) it must have an aperture for the passage of light, the size of which can be altered so as to regulate the quantity of light according to the nature of the object. These rest Its are secured by means of the mirror and the diaphragm, supplemented often by a substage eondenser i]). [5 .

Microscope Stand (Figs. 9-15).— The es- ^|jf,,^l?&j2| "J sential parts are the base, the standard, the

stage, the arm, with fine and coarse adjustment the tube, and the illuminating apparat s (substage).

The Base or Foot is usually horseshoe- or Y shaped. Jt is made of cast brass or iron, often weighted with lead, and should be of considerable weight so as to bring the 1 enter of gravity as low as possible, thereby giving the instrument stability.

The Standard consists of a single or double cylinder or prism and in the b< tl ade of instrument- is jointed below the stage.

The Stage is a square or round shelf, with a hole in the middle to admit light from below, on which is placed the glass slide with the object under examination, li the instrument is bent at the joint SO that the stage is considerably 01 t of a horizontal position, the slide is held in place by spring clips.

The Arm, which carries the tube of the microscope with its optical part-, is provided with two adjustment- operated by thumb screws; one

Fig. 9. S< •< lion of C< pound Mil ros ope.

(Leitz.)

(the coarse adjustment) consists of a rack and pinion, the other (the fine adjustment) of a micrometer screw for accurate focusing.

The Tube is provided at the lower end with a thread to which are attached the objectives. An inner or draw tube carries the eyepiece at the upper end. This draw tube is graduated so that the length of the combined tubes, in any given position, may be readily determined.

Although a Nose Piece is not an absolute necessity, it is an inexpensive addition to the microscopic equipment that greatly facilitates work and

Fig. io. Zeiss Microscope, Stand VA. Fig. ii. Leitz Microscope, Stand D.

saves wear and tear of the objectives. It consists of a simple revolving device by means of which any one of the two to four objectives attached to it may be instantly brought into service.

The Illuminating Apparatus, consisting of mirror, diaphragm, and, in the better instruments, condenser, is borne wholly or in part on the substage which is attached either directly to the bottom of the stage or to an arm projecting downward from it.

The Mirror is usually plane on one side and concave on the other. It is so arranged that it can be turned in any direction so that light may be projected against the object whatever the position of the instrument.

JO

.-irr.4R.JTUS AND METHODS.

i Wf 1

Tig. 12.

Leitz Microscope, Stand II C.

Fig. 13.

Bausch & Lomb Microscope, Stand BB 8.

FlG. 14. Leitz Microscope, Stand B.

Fig. 15. Zeiss Microscope, Stand 1\' C.

THE MICROSCOPE.

II

Fig. 16. Iris Diaphragm. (Leitz.)

In its simplest form the Diaphragm consists of a series of differentsized apertures for regulating the amount of light. The apertures may be in the center of detachable discs or arranged in a circle about the1 center of a single revolving disc. A more convenient form is the Iris Diaphragm (Fig. 16) consisting of a series of sickle-shape pieces of thin steel so arranged that they form a central opening which may be enlarged or reduced in size by means of a lever. In some instruments there are two of these diaphragms, one above, the other below the condenser.

The Substage Condenser is described under the head of Accessories

Figs. 10 and n show inexpensive stands suitable for ordinary work,

Figs. 12 and 13, somewhat better instruments meeting the requirements of the advanced worker. The instruments shown in Figs. 14 and 15 represent a high grade of excellence.

OPTICAL PARTS.— The Objectives (Fig. 17) consist of a system of lenses mounted in brass, with a thread for attachment to the lower

end of the tube either directly or by means of a nose piece. The magnification is increased by increasing the number of double lenses (of crown and flint glass) and at the same time the illuminating power is increased. This increased magnification could also to some extent be secured by increasing the curvature of the lens, but this would be impracticable because of the loss of light if for no other reason.

Objectives are designated either by their focal lengths, which range from 2 inches to 1/16 inch or by arbitrary numbers (1-9, etc.). The focal length is an optical value of no real significance to the worker. It is a common error to mistake the working distance, that is the distance between the lower lens of an objective and the slide, for the focal length.

Angular Aperture. — The number of rays which enter an objective and are carried to the plane of the image depends on the light cone whose

Objectives. (Leitz.)

A I ' TARATUS A SO METHODS.

apex lie— at tin- object (or rather the point of tin- objeel that emits the light) ami whose base corresponds with the breadth of tin- lens, or rather

with that of the light opening of the system of lenses formed by the mounting. The angle of this tone is thf angular aperture i Fig. iS i.

Numerical Aperture. — Formerly it was supposed that the power of an objective to collect a greater or lesser amount of light was dependent on the angular aperture, hut more recent

[8 Lens showing. n, Angular Aperture and. F, Focal Point. (Thanhoffkk.)

investigation has shown that tin- true measure is the sine of half of the angular aperture times the refractive

index of the medium. Abbi': designates this product tn sine u=a)

the numerical aperture.

The numerical aperture of ordinary dry objectives (with air between

Fig. 19. Showing Comparative Merits of Oil Immersion and Dry Systems. (Leitz.)

the mounted object and the objective) tan not exceed unity, hut in the so-called immersion lenses is considerably greater. In these latter a drop of water or, in the case of homogeneous immersion objectives, a drop of concentrated cedar oil, which has the same refraction and dispersion as the crown glass, fills the space between the cover glass and the objective, and the loss of light due to refraction in passing from glass to air and again to glass is avoided (Fig. 19).

The following examples given by Dippel illustrate the significance of the term "numerical aperture" as applied to dry and immersion objectives: "If we have two dry objectives with angular apertures respectively 6o° and 1200, the first will not have a numerical aperture half that of the second but in the ratio of the sine of 300 (=0.5) to the sine of 6o° ( =0.87) or as 1 :i-74. If we have a dry and a water immersion objective both with an angular aperture of 1200, the numerical apertures are not the same, but that of the water immersion is n ( =1.33) times as great as the other, or 1.33X0.87 = 1.14. From this it is clear that certain oblique rays which play an important role in the formation of the image are utilized in the immersion objectives, which are lost in the dry objectives, even in those with the highest possible numerical aperture (=1) and angular aperture ( = 180°); and further, that immersion objectives under like conditions are much more efficient than dry, the numerical apertures reaching in practice about 1.20 in water immersions and about 1.35 in oil immersions or in theory 1.33 and 1.52 respectively."

Eyepieces (Oculars). — The theory of the eyepiece has already been explained (pp. 6 and 7). The optical parts are mounted in metal cylinders which are introduced into the upper end of the microscope tube. Eyepieces are designated either by their focal lengths (J inch-2 inches) or by Roman or Arabic numbers.

Chapter II. Microscopic Accessories

Micrometer. — The simplest method of measuring a microscopic object is to plate it on a graduated scale etched on a slide 'Stage Micrometer) and read the dimensions through the microscope. It is, however, seldom practicable to use a stage micrometer directly since it is exceedingly difficult and inconvenient to so place the object that it will coincide with the scale, to say nothing of the danger of injuring the line divisions (often .01 mm. apart).

A more practicable device, which is now almost universally employed, is the Eyepiece Micrometer, consisting of a scale mounted in an eyepiece so that il> divisions can be read by the- eye lens. By its use any object or part of an object can instantly be made to coincide with the scale and measured. Since, however, it is the magnified object, not the object itself, that is compared with the scale, the direct readings are not the actual dimensions, although they are readily a averted into such by the use of a factor which is constant for the same objective and tube length.

Calibration. — To determine this fat tor a stage micrometer with, for example, the m ale of 1 mm. divided in 100 divisions | 1 division =0.01 mm.) is placed on the stage and so adjusted that a certain number of divisions of the eyepiece micrometer cover a certain number on the stage micrometer, both of whi< h are carefully counted, [ffive divisions of the stage scale (=0.05 mm.) cover 20 divisions of the- eyepiece scale then the value of each division of the latter is 0.05 mm. -5-20= 0.0025. ^ an object, examined with the same objective and tube length as above, is covered by 50 divisions of the eyepiece scale, its true size is 50x0.0025 =0.125 mrn- In the- same manner the fac tors for the other objectives may be d( termined, using always a definite tube length. Mieromeasurements are commonly expressed in micron-, also known as micromillimcters, one

MICROSCOPIC ACCESSORIES.

is thousand of which equal one millimeter (1000^ = 1 mm. or ip. =0.001 mm.). In the foregoing example 0.125 mm» = I25tt-

Determination oj Magnification. — The stage micrometer can also be used to determine (with sufficient accuracy for practical purposes) the magnification secured by means of a given objective and eyepiece. This may be accomplished in several ways, but the following are the most convenient:

First Method. Measure the diameter of the field (the illuminated circle seen in the microscope) by means of a stage micrometer. Look with the left eye in the microscope and with the right on a piece of paper raised to the same height as the stage. Make two marks on the paper showing the end points of a diameter of the apparent field. Measure this diameter and divide by the diameter of the actual field, thus obtaining the magnification.

Second Method. Observe the stage micrometer with the left eye and with the right draw on the paper, at the height of the stage, several divisions of the apparent scale. Measure the total width of these divisions and divide by the actual width.

The Abbe Substage Condenser (Fig. 20), although not a necessity, is of great value in securing adequate illumination, especially on dark days and when high-power objectives are employed. It consists of a series of two or three lenses (not achromatic), the upper one of which is plano-convex, suitably mounted in brass. The condenser is attached to the substage above the diaphragm and gives the best results when the upper lens is a little below the object and the light is reflected from the plane mirror.

Polarization Apparatus. — It is often desirable to examine organic tissues and cell contents, as well as inorganic objects, in polarized light for the purpose of bringing out certain physical and chemical properties and characteristic details of structure which are evident in no other way. The polarizing apparatus consists of two Nicol prisms. One, the Polarizer (Fig. 21), is mounted in the substage, the other, the Analyzer, in the tube or above the eyepiece. A Nicol prism consists of two wedge-shaped pieces of Iceland spar cemented together with Canada balsam (Fig. 21, right). A ray of ordinary light on entering the lower wedge is separated into the ordinary and the extraordinary rays; the former is totally

Fig. 20. Abbe Substage Condenser. (Leitz.)

i6

APPARATUS AND METHODS.

reflected by the balsam and is lost, while the latter passes through the upper wedge. The ray which emerges is polarized and differs from an ordinary ray in that the vibrations are Dot in all directions perpendicular to the line of projei tion but are parallel to a plane passing through that line. By means of the analyzer the peculiar properties of the polarized 1 i t_rii t are brought out. If lx>th the analyzer and the polarizer are in such a position that the diagonal surfaces, cemented with balsam, arc parallel, the polarized light from the polarizer is not altered by the analyzer and appears to be the same as ordinary light; if, however, either prism is revolved, the light diminishes in intensity and is entirely extinguished at a position oo° from the first. If a doubly refractive body is placed between the crossed prisms, it appears luminous and displays interference