Lay the object on the stage of the microscope, so as to get its centre exactly under the centre of the object-glass, and illuminate it as you best can. Put on the lowest power, and, without looking through the tube, lower the object-glass until it nearly touches the object. Now look through the tube, and raise the object-glass gradually from the object, until the right focus is obtained. The reason for taking these precautions is, that if you look through the tube and lower it upon the object, you will in all probability push the glass against the object, and damage either the one or the other. When you have thus learned the focus of the lowest power, add another, and repeat the process; and so on until you have made out the focus of each object-glass. If you have more than one eye-piece, try them both with each object-glass.
The proper light is our next point, and upon it rests the chief beauty of the effect. The light which will suit one object will not suit another, and even the same object should be examined under every variety of light. Some objects are best shown when the light is thrown upon them from above, and others when it is thrown through them from below. Again, the direction of the light is of vast importance; for it will easily be seen that an oblique light will exhibit minute projections by throwing a shadow on one side and brilliancy on the other, while a vertical illumination would fail to show them. On the same principle, one object will be shown better with the light in front, and another when it is on one side.
One of the most effective means of attaining this object is by using the “bull’s-eye condenser,” which is sometimes fixed to the stage, but is usually detached, as represented in the [illustration]. As the upright stem is telescopic, the glass can be raised to a considerable height, while the joint and sliding-rod permit the lens to be applied at any angle which promises the most brilliant light.
As for the kind of light that is employed, there is nothing which equals that of a white cloud; but as such clouds are rare, and are at the best extremely transient, and can only be seen by day, various artificial methods of illumination have been invented. Novices generally think that when the sky is bright and blue they will be very successful in their illumination, and feel grievously disappointed at finding that they obtained much more light from the clouds, whose disappearance they had anxiously been watching. Finding that the blue sky gives scarcely any light at all, they rush to the other extreme, turn the mirror towards the sun, and pour such a blaze of light upon the object, that the eye is blinded by the scintillating refulgence, and the object is often injured, because the mirror is capable of reflecting heat as well as light.
In the daytime there is nothing better than the “white-cloud illuminator,” which is made easily enough by means of plaster of Paris. A sheet of thin white paper fastened against a window-pane is also useful; and the simple plan of dabbing the glass with putty will have a beneficial effect in softening the light, when the window has a southern aspect. In default of these conveniences, it will be often sufficient to fix a piece of white letter-paper over the mirror, or even to dull its surface with wax. At all events, he who aspires to be a true microscopist must be ready with expedients, and if he finds himself in a difficulty, he must summarily invent a method of obviating it.
At night a lamp is necessary; candles are useless, because they have two faults—they flicker, and they become lower as they burn. The latter defect can be cured by using a candle-lamp, but no arrangement will cure the flame of flickering; it is peculiarly trying to the eyes, and destructive of accurate definition. An ordinary moderator lamp answers pretty well, and a small one is even better for the microscopist than one of large dimensions. The chief drawback to the moderator lamp is, that the flame cannot be elevated or lowered, so that the only way to procure a light at a higher elevation, is to stand the lamp on a block of wood or a book. Small lamps are, however, made expressly for the microscope, and, if possible, should be procured, and used for no other purpose, and intrusted to no other hands.
If you want a really brilliant, clear, white light, you must trim the lamp yourself. A small piece of pale blue or neutral-tint glass, interposed between the lamp and the microscope, has a wonderful effect in diminishing the yellow hue which belongs more or less to all artificial lights which are produced by the combustion of oil or fat. We have no doubt but that in a few years we shall be rid of the clumsy and dirty machines that we call lamps, and have substituted for them the pure brilliancy of the electric light.