Basic Microscope Usage for Histology Table of Contents
When the microscope instrument is ready for use it should be placed upon a firm and rather low table, near a window, which does not receive the direct rays of the sun. A thin sheet or filter of blue glass may sometimes be found to assist the eye when artificial illumination is used, as the light is made white. Some microscope makers furnish with their instruments a set of blue glasses varying in color from very light to dark blue. They are rarely needed, as the eye soon becomes accustomed to continuous work for long sittings, even when strong light is employed. Those who work much with the microscope keep both eyes open, and use first one and then the other.
It is presumed that students engaging in histological work are more or less familiar with the mechanism of the microscope. For this reason the subject of optical principles and the description of the different parts of a microscope are omitted here. Some find it a great assistance to direct the unengaged eye upon a dark object, such as a blackened card, which they fasten to the tube of the instrument near its top.
When the pencil of light has been reflected from the mirror upon the opening in the stage, it is plain that a larger or smaller amount of light will pass, according to the size of the opening. The appliances that regulate this matter are called stage diaphragms sometimes they are simply cylindrical tubes with capped upper extremities, each tube being provided with caps of varying aperture. The tubes are pushed into the stage from beneath. When polished they undoubtedly aid in converging the light upon the aperture.
Other diaphragms are simply round holes in a circular revolving plate which is set into the stage.
The diameters of the apertures vary from that of a pin's point to about three-fourths of an inch or even more. The revolving diaphragms have now come into general use, because they work simply and efficiently. Some have the advantage of a cylindrical diaphragm, in so far as it converges the pencil of light upon the diaphragmatic opening, while the size of the opening is regulated by the action of a single thumb-screw. It acts as the iris does in enlarging or diminishing the pupil, from which its name, the iris diaphragm.
Of these there should be two, one plane when a diffuse light is needed ; the other concave for a concentrated beam. The latter is frequently used, the former seldom.
Direct and oblique light. Thus far the descriptions have applied to direct light, and it is the only kind much used in histological work. In testing a lens, however, as with a diatom, it is often necessary to use oblique light in order to resolve a line or series of lines. In such cases the aperture in the stage should be made as large as possible, and the mirror, concave or plane, is to be carried well up under the stage, to the left or right so that the pencil of light may be thrown across the object. By this means, little inequalities of the surface which would be invisible under direct light are clearly demonstrated.
The poorer lenses, however, are those which necessitate oblique light. When reference is made to the definition of the lens, direct light is intended.