Histology of Medicinal Plants

CHAPTER I. The Simple Microscopes

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. 1) 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 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 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.

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 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 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.

CHAPTER II. Compound Microscopes

COMPOUND MICROSCOPES

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 tripod-shaped 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.

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 Abbé 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 Abbé 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

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

Eyepiece Draw Tube Body Tube Coarse Adjustment Revolving Nosepiece for three Objectives Fine Adjustment Stage Objectives Limb Abbi Condenser Iris Diaphragm Hinge for Inclining Substage Attachment Mirror Pillar Foot]

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

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