Histology of medicinal plants

Preface

The object of the book is to provide a practical scientific course in vegetable histology for the use of teachers and students in schools and colleges.

The medicinal plants are studied in great detail because they constitute one of the most important groups of economic plants. The cells found in these plants are typical of the cells occurring in the vegetable kingdom.; therefore the book should prove a valuable text-book for all students of histology.

The book contains much that is new. In Part II, which is devoted largely to the study of cells and cell contents, is a new scientific, yet practical, classification of cells and cell contents. The author believes that his classification of bast fibres and hairs will clear up much of the confusion that students have experienced when studying these structures.

The book is replete with illustrations, all of which are from original drawings made by the author. As most of these illustrations are diagnostic of the plants in which they occur, they will prove especially valuable as reference plates.

The material of the book is the outgrowth of the experience of the author in teaching histology at the College of Pharmacy of the City of New York, Columbia University, and of years of practical experience gained by examining powdered drugs in the laboratory of a large importing and exporting wholesale drug house.

The author is indebted to Ernest Leitz and Bausch & Lomb Optical Company for the use of cuts of microscopic apparatus used in Part I of the book.

The author also desires to express his appreciation to Professor Walter S. Cameron, who has rendered him much valuable aid.

William Mansfield.

Columbia University, September, 191 6.

iii

PAGE

Simple microscopes, forms of . . . . 4

CHAPTER II COMPOUND MICROSCOPES Compound microscopes, structure of 7

Compound microscopes, mechanical parts of 7

Compound microscopes, optical parts of 9

Compound microscopes, forms of 12

MICROSCOPIC MEASUREMENTS

Ocular micrometer 19

Stage micrometer 19

Mechanical stage 1 21

Micrometer eye-pieces 21

Camera lucida 22

Drawing apparatus 23

Microphotographic apparatus 24

Illumination 26

Micro lamp 27

Care of the microscope 28

Preparation of specimens for cutting 28

v

Vi CONTENTS

PAGE

Paraffin imbedding oven 30

Paraffin blocks 31

Cutting sections 31

Hand microtome 31

Machine microtomes 32

REAGENTS

Reagent set . 39

Measuring cylinder 40

Temporary mounts 41

Permanent mounts 41

Cover glasses 43

Glass slides 44

Forceps . 45

Needles 46

Scissors 46

Turntable 46

Labeling 47

Preservation of mounted specimens 48

Slide box 48

Slide tray 48

Slide cabinet 49

PART II Tissues, Cells and Cell Contents

CHAPTER I

THE CELL

Typical cell _ . 53

Changes in a cell undergoing division 55

Origin of multicellular plants . 57

Leaf epidermis 59

Testa epidermis 63

Plant hairs . 66

CONTENTS vii

PAGE

Forms of hairs 67

Papillae 67

Unicellular hairs 69

Multicellular hairs 72

Periderm . 80

Cork periderm 80

Stone cell periderm 85

Parenchyma and stone cell periderm 85

Bast fibres . 89

Crystal bearing bast fibres 90

Porous and striated bast fibres 92

Porous and non-striated bast fibres 96

Non-porous and striated bast fibres 96

Non-porous and non-striated bast fibres 96

Occurrence of bast fibres in powdered drugs 103

Wood fibres 104

Ccllenchyma cells 106

Endodermal cells 116

Hypodermal cells 118

ABSORPTION TISSUE

Root hairs 121

Vessels and tracheids 126

Annular vessels 127

Spiral vessels 127

Sclariform vessels 128

Reticulate vessels 131

Pitted vessels 131

Pitted vessels with bordered pores 131

Sieve tubes < 136

Sieve plate . 138

Medullary bundles, rays and cells 138

Medullary ray bundle 139

The medullary ray 139

The medullary ray cell 141 viii

CONTEXTS

PAGE

Structure of the medullary ray cells . 142

Arrangement of the medullary ray cells in the medullary ray . . . 142

Latex tubes

Cortical parenchyma 147

Pith parenchyma 147

Leaf parenchyma 150

Aquatic plant parenchyma 150

Phloem parenchyma . 150

Palisade parenchyma 150

CHAPTER VI AERATING TISSUE

Water pores .151

Stomata 151

Relation of stomata to the surrounding cells 154

Lenticels , 157

Intercellular spaces 158

CHAPTER VII

SYNTHETIC TISSUE

Photosynthetic tissue . 163

Glandular tissue 164

Glandular hairs 164

Secretion cavities 166

Schizogenous cavities . 168

Lysigenous cavities 168

Schizo-lysigenous cavities 168

CHAPTER VIII

STORAGE TISSUE

Storage cells 173

Storage cavities . . . .1 . . . . 176

Crystal cavities 176

Mucilage cavities .176

Latex cavities 176

Oil cavity .". . 178

Glandular hairs as storage organs 178

Storage walls 179

CONTENTS ix

CHAPTER IX CELL CONTENTS

PAGE

Chlorophyll 182

Leucoplastids 183

Starch grains 183

Occurrence . . .... . 184

Outline 185

Size 185

Hilum 185

Nature of hilum 188

Inulin 194

Mucilage . 194

Hesperidin 196

Volatile oil • • 196

Tannin 196

Aleurone grains 197

Structure of aleurone grains 197

Form of aleurone grains ,.197

Description of aleurone grains 198

Tests for aleurcne grains 198

Crystals 200

Micro-crystals 200

Raphides 200

Rosette crystals 202

Cystoliths 210

Forms of cystoliths 210

Tests for cystoliths 215

Part III

Histology of Roots, Rhizomes, Stems, Barks, Woods, Flowers, Fruits and Seeds

CHAPTER I ROOTS AND RHIZOMES

Cross-section of pink root 219

Cross-section of ruellia root 219

Cross-section of spigelia rhizome 223

Cross-section of ruellia rhizome 226

Powdered pink root . 227

Powdered ruellia root 227

X

CONTENTS

PAGE

Herbaceous stems 233

Cross-section, spigelia stem 233

Ruellia stem 235

Powdered horehound 237

Powdered spurious horehound 237

Insect flower stems 241

WOODY STEMS

Buchu stem 242

Mature buchu stem 242

Powdered buchu stem 245

BARKS

White pine bark 248

Powdered white pine bark 250

WOODS

Cross-section quassia 254

Radial-section quassia 254

Tangential-section quassia 258

LEAVES

Klip buchu 260

Powdered klip buchu 262

Mountain laurel 264

Trailing arbutus 264

CHAPTER VII

FLOWERS

Pollen grains 270

Non-spiny- walled pollen grains 273

Spiny- walled pollen grains 273

Stigma papillae . - 274

CONTENTS xi

PAGE

Powdered insect flowers 278

Open insect flowers 280

Powdered white daisies 282

CHAPTER VIII FRUITS

Celery fruit 285

CHAPTER IX

SEEDS

Sweet almonds 289

CHAPTER X

ARRANGEMENT OF VASCULAR BUNDLES

Types of fibro-vascular bundles 292

Radial vascular bundles 292

Concentric vascular bundles 295

Collateral vascular bundles 295

Bi-collateral vascular bundles 298

Open collateral vascular bundles 298

Part I SIMPLE AND COMPOUND MICROSCOPES AND MICROSCOPIC TECHNIC

CHAPTER I

THE SIMPLE MICROSCOPES

The construction and use of the simple microscope (magnifiers) undoubtedly date back to very early times. There is sufficient evidence to prove that spheres of glass were used as burning spheres and as magnifiers by people antedating the Greeks and Romans.

The simple microscopes of to-day have a very wide range of application and a corresponding variation in structure and in appearance.

Simple microscopes are used daily in classifying and studying crude drugs, testing linen and other cloth, repairing watches, in reading, and identifying insects. The more complex simple microscopes are used in the dissection and classification of flowers.

The watchmaker's loupe, the linen tester, the reading glass, the engraver's lens, and the simplest folding magnifiers consist of a double convex lens. Such a lens produces an erect, enlarged image of the object viewed when the lens is placed so that the object is within its focal distance. The focal distance of a lens varies according to the curvature of the lens. The greater the curvature, the shorter the focal distance and the greater the magnification.

The more complicated simple microscope consists of two or more lenses. The double and triple magnifiers consist of two and three lenses respectively.

When an object is viewed through three lenses, the magnification is greater than when viewed through one or two lenses, but a smaller part of the object is magnified.

FORMS OF SIMPLE MICROSCOPES

TRIPOD MAGNIFIER

The tripod magnifier (Fig. i) is a simple lens mounted on a mechanical stand. The tripod is placed over the object and the focus is obtained by means of a screw which raises or lowers the lens, according to the degree it is magnified.

watchmaker's loupe

The watchmaker's loupe (Fig. 2) is a one-lens magnifier mounted on an ebony or metallic tapering rim, which can be

Fig. 1. — Tripod Magnifier Fig. 2. — Watchmaker's Loupe

placed over the eye and held in position by frowning or contracting the eyelid.

FOLDING MAGNIFIER

The folding magnifier (Fig. 3) of one or more lenses is mounted in such a way that, when not in use, the lenses fold up like the

Fig. 3. — Folding Magnifier Fig. 4. — Reading Glass blade of a knife, and when so folded are effectively protected from abrasion by the upper and lower surfaces of the folder.

READING GLASSES

Reading glasses (Fig. 4) are large simple magnifiers, often six inches in diameter. The lens is encircled with a metal band and provided with a handle.

THE SIMPLE MICROSCOPES

STEINHEIL APLANATIC LENSES

Steinheil aplanatic lenses (Fig. 5) consist of three or four lenses cemented together. The combination is such that the field is large, flat, and achromatic. These lenses are suitable

Fig. 5. — Steinheil Aplanatic Lens

for field, dissecting, and pocket use. When such lenses are placed in simple holders, they make good dissecting microscopes.

DISSECTING MICROSCOPE

The dissecting microscope (Fig. 6) consists of a Steinheil lens and an elaborate stand, a firm base, a pillar, a rack and

Fig. 6. — Dissecting Microscope pinion, a glass stage, beneath which there is a groove for holding a metal plate with one black and one white surface. The nature of the object under observation determines whether a plate is used. When the plate is used and when the object is studied by reflected light it is sometimes desirable to use the black and sometimes the white surface. The mirror, which has a concave and a plain surface, is used to reflect the light on the glass stage when the object is studied by transmitted light. The dissecting microscope magnifies objects up to twenty diameters, or twenty times their real size.

The compound microscope has undergone wonderful changes since 1667, the days of Robert Hooke. When we consider the crude construction and the limitations of Robert Hooke 's microscope, we marvel at the structural perfection and the unlimited possibilities of the modern instrument. The advancement made in most sciences has followed the gradual perfection of this instrument.

The illustration of Robert Hooke's microscope (Fig. 7) will convey to the mind more eloquently than words the crudeness of the early microscopes, especially when it is compared with the present-day microscopes.

STRUCTURE OF THE COMPOUND MICROSCOPE

The parts of the compound microscope (Fig. 8) may be grouped into — first, the mechanical, and, secondly, into the optical parts.

THE MECHANICAL PARTS

1. The foot is the basal part, the part which supports all the other mechanical and optical parts. The foot should be heavy enough to balance the other parts when they are inclined. Most modern instruments have a three-parted or tripodshaped base.

2. The pillar is the vertical part of the microscope attached to the base. The pillar is joined to the limb by a hinged joint. The hinges make it possible to incline the microscope at any angle, thus lowering its height. In this way, short, medium, and tall persons can use the microscope with facility. The part of the pillar above the hinge is called the limb. The limb may be either straight or curved. The curved form is preferable, since it offers a more suitable surface to grasp in transferring from box or shelf to the desk, and vice versa.

Fig. 7. — Compound Microscope of Robert Hooke

3. The stage is either stationary or movable, round or square, and is attached to the limb just above the hinge. The upper surface is made of a composition which is not easily attacked by moisture and reagents. The centre of the stage is perforated by a circular opening.

4. The sub-stage is attached below the stage and is for the purpose of holding the iris diaphragm and Abbe condenser. The raising and lowering of the sub-stage are accomplished by a rack and pinion.

5. The iris diaphragm, which is held in the sub-stage below the Abbe condenser, consists of a series of metal plates, so arranged that the light entering the microscope may be cut off completely or its amount regulated by moving a control pin.

6. The fine adjustment is located either at the side or at the top of the limb. It consists of a fine rack and pinion, and is used in focusing an object when the low-power objective is in position, or in finding and focusing the object when the high-power objective is in position.

7. The coarse adjustment is a rack and pinion used in raising and lowering the body-tube and in finding the approximate focus when either the high- or low-power objective is in position.

8. The body-tube is the path traveled by the rays of light entering the objectives and leaving by the eye-piece. To the lower part of the tube is attached the nose-piece, and resting in its upper part is the draw-tube, which holds the eye-piece. On the outer surface of the draw-tube there is a scale which indicates the distance it is drawn from the body-tube.

9. The nose-piece may be simple, double, or triple, and it is protected from dust by a circular piece of metal. Double and triple nose-pieces may be revolved, and like the simple nose-piece they hold the objectives in position.

THE OPTICAL PARTS

i . The mirror is a sub-stage attachment one surface of which is plain and the other concave. The plain surface is used with an Abbe condenser when the source of fight is distant, while the concave surface is used with instruments without an Abbe condenser when the source of light is near at hand.

2. The Abbe condenser (Fig. 9) is a combination of two or more lenses, arranged so as to concentrate the light on the specimen placed on the stage. The condenser is located in the opening of the stage, and its uppermost

surface is circular and flat.

3. Objectives (Figs. 10, 11, and 12).

There are low, medium, and high-power objectives. The low-power objectives have fewer and larger lenses, and they magnify least, but they show more of the object than do the high-power objectives.

There are three chief types of objectives: First, dry objectives; second, wet objectives, of which there are the water-immersion objectives; and third, the oil-immersion objectives. The dry objectives are used for most histological and pharmacognostical work. For studying smaller objects the water ob-

FiG. 9. — Abbe Condenser

Fig. 11.

Objectives.

jective is sometimes desirable, but in bacteriological work the oil-immersion objective is almost exclusively used. The globule of water or oil, as the case may be, increases the amount of light entering the objective, because the oil and water bend many rays into the objective which would otherwise escape.

4. Eye-pieces (Figs. 13, 14, and 15) are of variable length, but structurally they are somewhat similar. The eye-piece consists of a metal tube with a blackened inner tube. In the centre of this tube there is a small diaphragm for holding the ocular micrometer. In the lower end of the tube a lens is fastened by means of a screw. This, the field lens, is the larger lens of the ocular. The upper, smaller lens is fastened in the

Eye-Pieces.

tube by a screw, but there is a projecting collar which rests, when in position, on the draw-tube.

The longer the tube the lower the magnification. For instance, a two-inch ocular magnifies less than an inch and a half, a one-inch less than a three-fourths of an inch, etc.

The greater the curvature of the lenses of the ocular the higher will be the magnification and the shorter the tube-length.

FORMS OF COMPOUND MICROSCOPES

The following descriptions refer to three different models of compound microscopes: one which is used chiefly as a pharmacognostic microscope, one as a research microscope stand, while the third type represents a research microscope stand of highest order, which is used at the same time for taking microphotographs.

PHARMACOGNOSTIC MICROSCOPE

Fig. i6.-Pharmacognostic . The pharmacognosy microscope

Microscope (Fig. 16) is an instrument which embodies only those parts which are most essential for the examination of powdered drugs, bacteria, and urinary sediments. This microscope is provided with a stage of the dimensions 105 x 105 mm. This factor and the distance of 80 mm. from the optical centre to the handle arm render it available for the examination of even very large objects and preparations, or preparations suspended in glass dishes. The stand is furnished with a side micrometer, a fine adjustment having knobs on both sides, thereby permitting the manipulation of the micrometer screw either by left or right hand. The illuminating apparatus consists of the Abbe condenser of numerical aperture of 1.20, to which is attached an iris diaphragm for the proper adjustment of the light. A worm screw, mounted in connection with the condenser, serves for the raising and lowering of the condenser, so that the cone of illuminating pencils can be arranged in accordance to the objective employed and to the preparation under observation. The objectives necessary are those of the achromatic type, possessing a focal length of 16.2 mm. and 3 mm. Oculars which render the best results in regard to magnification in connection with the two objectives mentioned are the Huyghenian eye-pieces II and IV so that magnifications are obtained varying from 62 to 625. It is advisable, however, to have the microscope equipped with a triple revolving nose-piece for the objectives, so that provision is made for the addition of an oil-immersion objective at any time later should the microscope become available for bacteriological investigations.

THE RESEARCH MICROSCOPE

The research microscope used in research work (Fig. 17) must be equipped more elaborately than the microscope especially designed for the use of the pharmacognosist. While the simple form of microscope is supplied with the small type of Abbe condenser, the research microscope is furnished with a large illuminating apparatus of which the iris diaphragm is mounted on a rack and pinion, allowing displacement obliquely to the optical centre, also to increase resolving power in the objectives when observing those objects which cannot be revealed to the best advantage with central illumination. Another iris is furnished above the condenser; this iris becomes available the instant an object is to be observed without the aid of the condenser, in which case the upper iris diaphragm allows proper adjustment of the light. The mirror, one side plane, the other concave, is mounted on a movable bar, along which it can be slid — another convenience for the adjustment of the light. The microscope stage of this stand is of the round, rotating

Fig. 17. — Research Fig. 18. — Special

Microscope Research Microscope and centring pattern, which permits a limited motion to the object slide. The rotation of the microscope stage furnishes another convenience in the examination of objects in polarized light, allowing the preparation to be rotated in order to distinguish the polarization properties of the objects under observation.

SPECIAL RESEARCH MICROSCOPE

A special research microscope of the highest order (Fig. 18) is supplied with an extra large body tube, which renders it of special advantage for micro-photography. Otherwise in its mechanical equipment it resembles very closely the medium-sized research microscope stand, with the exception that the stand is larger in its design, therefore offering universal application. In regard to the illuminating apparatus, it is advisable to mention that the one in the large research microscope stand is furnished with a three-lens condenser of a numerical aperture of 1.40, while the medium-sized research stand is provided with a two-lens condenser of a numerical aperture of 1.20. The stage of the microscope is provided with a cross motion — the backward and forward motion of the preparation is secured by rack and pinion, while the side motion is controlled by a micrometric worm screw. In cases where large preparations are to be photographed, the draw-tube with ocular and the slider in which the draw-tubes glide are removed to allow the full aperture of wide-angle objectives to be made use of.

BINOCULAR MICROSCOPE

The Gre enough binocular microscope, as shown in Fig. 19, consists of a microscope stage with two tubes mounted side by side and moving on the same rack and pinion for the focusing adjustment. Either tube can be used without the other. The oculars are capable of more or less separation to suit the eyes of different observers. In each of the drub-like mountings, near the point where the oculars are introduced, porro-prisms

have been placed, which erect the image. This microscope gives most perfect stereoscopic images, which are erect instead of inverted, as in the monocular compound microscopes. The Greenough binocular microscope is especially adapted for dissection and for studying objects of considerable thickness.

Fig. 19. — Greenough Binocular Microscope

POLARIZATION MICROSCOPE

The polarization microscope (Fig. 20) is used chiefly for the examination of crystals and mineral sections as well as for the observation of organic bodies in polarized light. It can, however, also be used for the examination of regular biological preparations.

If compared with the regular biological microscope, the polarization microscope is found characteristic of the following points: it is supplied with a polarization arrangement. The latter consists of a polarizer and analyzer. The polarizer is situated in a rotating mount beneath the condensing system.

The microscope, of which the diagram is shown, possesses a triple "Ahrens" prism of calcite. The entering light is divided into two polarized parts, situated perpendicularly to each other. The so-called "ordinary" rays are reflected to one side by total reflection, which takes place on the inner cemented surface of the triple prism, allowing the so-called "'extraordinary" rays to pass through the condenser. If the prism is adjusted to its focal point, it is so situated that the vibration plane of the extra-ordinary rays are in the same position as shown in Fig. 20.— Polarization the diagram of the illustration.

Microscope The analyzer is mounted within the

microscope-tube above the objective.

Situated on a sliding plate, it can be shifted into the optical axis whenever necessary. The analyzer consists of a polarization prism after Glan-Thompson. The polarization plane of the active extraordinary rays is situated perpendicularly to the plane as shown in the diagram. The polarization prisms are ordinarily crossed. In this position the field of the microscope is darkened as long as no substance of a double refractive index has been introduced between the analyzer and polarizer. In rotating the polarizer up to the mark 90, the polarization prisms are mounted parallel and the field of the microscope is lighted again. Immediately above the analyzer and attached to the mounting of the analyzer a lens of a comparatively long focal length has been placed in order to overcome the difference in focus created by the introduction of the analyzer into the optical rays.

The condensing system is mounted on a slider, and, furthermore, can be raised and lowered along the optical centre by means of a rack-and-pinion adjustment. If lowered sufficiently, the condensing system can be thrown to the side to be removed from the optical rays. The condenser consists of three lenses. The two upper lenses are separately mounted to an arm, which permits them to be tilted to one side in order to be removed from the optical rays. The complete condenser is used only in connection with high-power objectives. As far as low-power objectives are concerned, the lower condensing lens alone is made use of, and the latter is found mounted to the polarizer sleeve. Below the polarizer and above the lower condensing lens an iris diaphragm is found.

The microscope table is graduated on its periphery, and, furthermore, carries a vernier for more exact reading.

The polarization microscope is not furnished with an objective nose-piece. Every objective, however, is supplied with an mdividual centring head, which permits the objective to be attached to an objective clutch-changer, situated at the lower end of the microscope-tube. The centring head permits the objectives to be perfectly centred and to remain centred even if another objective is introduced into the objective clutch-changer.

At an angle of 45 degrees to the polarization plane of polarizer and analyzer, a slot has been provided, which serves for the introduction of compensators.

Between analyzer and ocular, another slot is found which permits the Amici-Bertrand lens to be introduced into the optical axis. The slider for the Bertrand lens is supplied with two centring screws whereby this lens can be perfectly and easily centred. The Bertrand lens serves the purpose of observing the back focal plane of the microscope objective. In order to allow the Bertrand lens to be focused, the tube can be raised and lowered for this purpose. An iris diaphragm is mounted above the Bertrand lens.

IS

If the Bertrand lens is shifted out of the optical axis, one can observe the preparation placed upon the microscope stage and, depending on its thickness or its double refraction, the interference color of the specimen. This interference figure is called the orthoscopic image and, accordingly, one speaks of the microscope as being used as an "orthoscope."

After the Bertrand lens has been introduced into the optical axis, the interference figure is visible in the back focal plane of the objective. Each point of this interference figure corresponds to a certain direction of the rays of the preparation itself. This arrangement permits observation of the change of the reflection of light taking place in the preparation, this in accordance with the change of the direction of the rays. This interference figure is called the conoscopic image, and, accordingly, the microscope is used as a "conoscope."

Many types of polarization microscopes have been constructed; those of a more elaborate form are used for research investigations; others of smaller design for routine investigations.

MICROSCOPIC MEASUREMENTS

In making critical examinations of powdered drugs, it is frequently necessary to measure the elements under observation, particularly in the case of starches and crystals.

OCULAR MICROMETER

Microscopic measurements are made by the ocular micrometer (Fig. 21). This consists of a circular piece of transparent glass on the centre of which is etched a one- or two-millimeter scale divided into one hundred or two hundred divisions re-

FlG. 21. — Ocular Micrometer Fig. 22. — Stage Micrometer

spectively. The value of each line is determined by standardizing with a stage micrometer.

STAGE MICROMETER

The stage micrometer (Fig. 22) consists of a glass slide upon which is etched a millimeter scale divided into one hundred equal parts or lines: each line has a value of one hundredth of a millimeter.

STANDARDIZATION OF OCULAR MICROMETER WITH LOW-POWER

OBJECTIVE

Having placed the ocular micrometer in the eye-piece and the stage micrometer on the centre of the stage, focus until the lines of the stage micrometer are clearly seen. Then adjust the scales until the lines of the stage micrometer are parallel with and directly under the lines of the ocular micrometer.

Ascertain the number of lines of the stage micrometer covered by the one hundred lines of the ocular micrometer. Then calculate the value of each line of the ocular. This is done in the following manner:

If the one hundred lines of the ocular cover seventy-five lines of the stage micrometer, then the one hundred lines of the ocular micrometer are equivalent to seventy-five one-hundredths, or three-fourths, of a millimeter. One line of the ocular micrometer will therefore be equivalent to one-hundredth

Fig. 23. — Micrometer Eye-Piece of seventy-five one-hundredths, or .0075 part of a millimeter, and as a micron is the unit for measuring microscopic objects, this being equivalent to one one-thousandth of a millimeter, the value of each line of the ocular will therefore be 7.5 microns.

With the high-power objective in place, ascertain the value of each line of the ocular. If one hundred lines of the ocular cover only twelve lines of the stage micrometer, then the one hundred lines of the ocular are equivalent to twelve one-hundredths of a millimeter, the value of one line being equivalent to one one-hundredth of twelve one-hundredths, or twelve tenthousandths of a millimeter, or .0012, or 1.2 u.

It will therefore be seen that objects as small as a thousandth of a millimeter can be accurately measured by the ocular micrometer.

In making microscopic measurements it is only necessary

MICROSCOPIC MEASUREMENTS to find how many lines of the ocular scale are covered by the object. The number of lines multiplied by the equivalent of each line will be the size of the object in microns, or micromillimeters.

MICROMETER EYE-PIECES

Micrometer eye-pieces (Figs. 23 and 24) may be used in making measurements. These eye-pieces with micrometer com-

Fig. 24. — -Micrometer Eye-Piece binations are preferred by some workers, but the ocular micrometer will meet the needs of the average worker.

MECHANICAL STAGES

Moving objects by hand is tiresome and unsatisfactory, first, because of the possibility of losing sight of the object under observation, and secondly, because the field cannot be covered so systematically as when a mechanical stage is used for moving slides.

The mechanical stage (Fig. 25) is fastened to the stage by a screw. The slide is held by two clamps. There is a rack and pinion for moving the slide to left or right, and another rack and pinion for moving the slide forward and backward.

CAMERA LUCIDA

The camera lucida is an optical mechanical device for aiding the worker in making drawings of microscopic objects. The

Fig. 27. — Camera Lucida instrument is particularly necessary in research work where it is desirable to reproduce an object in all its details. In fact, all reproductions illustrating original work should be made by means of the camera lucida or by microphotography.

A great many different types of camera lucidas or drawing apparatus are obtainable, varying from simple-inexpensive to complex-expensive forms. Figs. 26, 27, and 28 show simple and complex forms.

MICROPHOTOGRAPHIC APPARATUS

The microphotographic apparatus (Fig. 29), as the name implies, is an apparatus constructed in such a manner that it may be attached to a microscope when we desire to photograph microscopic objects. It consists of a metal base and a polished metal pillar for holding the bellows, slide holder, ground-glass observation plate, and eye-piece. In making photographs, the small end of the bellows is attached to the ocular of the micro-