The compound microscope will often prove useful for the examination of very minute objects, as well as for the study of the structure of the principal tissues of the larger species; but since detailed instructions for the management of the microscope, and for the preparation of objects for microscopic examination would occupy much more space than we can spare, we shall content ourselves with nothing more than a few general hints on this portion of the young naturalist’s work, dealing more particularly with those points which commonly present difficulties to the amateur.
If it is desired to examine some minute living object, such as a protozoon, place the object in a drop of the water in which it lived just in the middle of a clean glass slip, and cover it with a cover-glass. The quantity of water should be just sufficient to fill the space between the two glasses. If less than this has been used, a little more applied to the edge of the cover by means of a glass rod will immediately run in between the glasses; while if an excessive amount was employed, the surplus may be removed by the application of a strip of blotting paper. Place the glass slip on the stage of the microscope, and reflect light through it from the mirror below.
Examine it first with a low power; and, after having observed as much as possible of the creature’s movements and structure with this aid, repeat with a higher power. This rule applies not only to such small objects as we have now under consideration, but to all objects, and parts of them, in which minute details are to be observed.
Beginners with the microscope often find prolonged examination very tiring to the eyes, but this, we believe, would seldom be the case if right methods were followed. Both eyes should always be open, and the microscopist should train himself to use both eyes equally for the actual observation.
The higher the magnifying power used, the nearer must the objective (the lower combination of lenses) be brought to the object itself, and it is no uncommon thing for the amateur, in his attempts to focus his object, to lower the body of the microscope beyond its proper position, causing the objective to crush the object, break the thin cover-glass, and become wetted with the liquid, if any, in which the object was being examined. All this may be avoided by lowering the body of the microscope until it nearly touches the cover-glass before attempting to view the object through it, and then, with the eye above the object-glass, to gradually raise the body until the object is in focus.
Fig. 47.—Cell for small Living Objects
The top of the cover-glass should always be perfectly dry; and if by any chance the objective becomes wet it should be wiped perfectly dry with a piece of old silk or with chamois leather. Also, if permanent mounting is attempted, and the preservative liquid is allowed to come in contact with the objective, such liquid must, of course, be washed off with some suitable solvent before any attempt is made to wipe the lens dry.
If the object under examination is of such dimensions that the cover-glass has a tendency to rock on it, or if it is a living object of such a size that it is unable to move freely in the exceedingly thin film of water between the cover and the slip, it should be placed in a cell. The cell may be made by cementing a ring of glass or vulcanite to the middle of a slip, or it may be a little circular cavity prepared in the slip itself. In either case the cell must be quite full of water before the cover-glass is applied, so that no air-bubbles are included.