This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Note. For very critical work, where the relative color values are of utmost importance, the new Autochrome plate may be employed, which will give a transparency of the object in actual colors. For instruction in the manipulation of this process, see Chapter LIX (Russian Oil Coloring. A Method For Coloring Photographs, Prints, Drawings On Paper, China, Etc., With Oil Colors Without Paint, Brush Or Pencil), Color Photography.
The Objectives And Focusing Attachment. The most important parts of the microscope are the objectives and focusing attachments. The objectives, or lenses as they are sometimes called, are the vital parts of the microscope. According to their magnifying power they are termed low power, medium power and high power. All high-grade microscopes, such as the BB8 shown in Illustration 120, are supplied with oil-immersion lenses, also three objectives - low power, marked two-thirds; medium power, marked one-sixth; high power, marked one-twelfth.
698. Students, or beginners, as a rule, are apt to use too much magnification, and often attempt to view a large surface with an objective which will show but a small part of it. It must not be forgotten that the apparent field of view is decreased as the high powers are used, and that a low power objective will give a better impression of a large coarse object and its relative parts, because it includes a larger area or surface in the view.
699. In all known objects it is safe to adopt the following rule: Never use a higher power than is necessary to properly study the object. In fact, for the first experiments it is best to begin with a low power (two-thirds) objective, remembering that the higher the power of the objective, the less space there is between the objective and the slide, which necessitates more careful adjustment. When there is more space between objective and slide, the latitude for focusing is greater.
Focusing Adjustment. The focusing adjustment for the cheaper instruments consists only of a diagonally cut rack and pinion, with no fine adjustment, while the focusing adjustment for the higher-grade instruments consists of a coarse adjustment by standard rack and pinion (see Q, Illustration No. 124) and a fine adjustment by standard micrometer screw movement, with extra large graduated milled head and pointer for measuring thicknesses of objects (see R, Illustration No. 124). The coarser adjustment on higher-grade instruments serves the purpose of obtaining a rough or approximate focus, and the fine adjustment for getting the more accurate focus. For all ordinary purposes the coarse adjustment will enable the student to do all his focusing, except when using a high-power objective. The fine adjustment, by the aid of which an almost imperceptible movement may be imparted to the optical system of the microscope, is one of its most indispensable adjuncts. So sensitive are high-power objectives that the slightest alteration of their distances from the object would result in failure. Therefore, high-power work will be quite impossible without the fine adjustment.