| FIG. 2.—Hyaline casts, one containing renal cells; properly subdued illumination (from Greene's "Medical Diagnosis"). | FIG. 3.—Same as Fig. 2; strong illumination. The casts are lost in the glare, and only the renal cells are seen (from Greene's "Medical Diagnosis"). |
If the reflection of the window-frame or other nearby object is seen in the field, the condenser should be lowered a little.
Focusing.—It is always best to "focus up," which saves annoyance and probable damage to slides and objectives. This is accomplished by bringing the objective nearer the slide than the proper focus, and then, with the eye at the eye-piece, turning the tube up until the object is clearly seen. The fine adjustment should be used only to get an exact focus with the higher power objectives after the instrument is in approximate focus. It should not be turned more than one revolution.
There will be less fatigue to the eyes if both are kept open while using the microscope, and if no effort is made to see objects which are out of distinct focus. Fine focusing should be done with the fine adjustment, not with the eye. An experienced microscopist keeps his fingers almost constantly upon one or other of the focusing adjustments. Greater skill in recognizing objects will be acquired if the same eye be always used. To be seen most clearly, an object should be brought to the center of the field.
Magnification.—The degree of magnification should always be expressed in diameters, not times, which is a misleading term. The former refers to increase of diameter; the latter, to increase of area. The comparatively low magnification of 100 diameters is the same as the apparently enormous magnification of 10,000 times.
Magnification may be increased—(a) by using a higher power objective, which is the best way; (b) by using a higher eye-piece; or (c) by increasing the length of tube.
Eye-pieces and Objectives.—The usual equipment consists of one- and two-inch eye-pieces, and two-thirds, one-sixth, and one-twelfth inch objectives. These are very satisfactory for clinical work. It is an advantage to add a one-half-inch eye-piece for occasional use with the two-thirds objective. The one-sixth should have an especially long working distance, otherwise it cannot be used satisfactorily with the Thoma-Zeiss blood-counting instrument, which has a very thick cover-glass. Such a "special one-sixth for blood work" is made by most of the microscope manufacturers.
Objectives are "corrected" for use under certain fixed conditions, and they will give the best results only when used under the conditions for which corrected. The most important corrections are: (a) For tube length; (b) for thickness of cover-glass; and (c) for the medium between objective and cover-glass.
(a) The tube length with which an objective is to be used is usually engraved upon it—in most cases it is 160 mm.
(b) The average No. 2 cover-glass is about the thickness for which most objectives are corrected. Low powers do not require any cover-glass. A cover should always be used with high powers, but its exact thickness is more important in theory than in practice.