When looking through the eye-piece be sure to place the eye in close proximation to the cap, otherwise the whole field will not be perfectly visible; it should appear as an equally well-illuminated circular disc. If the eyelashes are reflected from the eye-glass, the observer is looking upon the eye-piece, and not through it.
_The Mirror._--The working focal distance of the mirror is that which brings the images of the window-bars sharply out upon the object resting upon the stage. In other words, the focus of the mirror is that which brings parallel rays to a correct focus on the object-glass. If employing artificial light, then the flame of the lamp should be distinguishable; a slight change in the inclination of the mirror will throw the image of the lamp-flame out of the field.
The strongest light is reflected from the concave side of the mirror, that from the flat side is more diffuse and less intense. Oblique light can be obtained by turning the mirror on one side and then adjusting it so as to illuminate the field from that position. All the necessary mechanism of the microscope is easily and quickly learned. The object-glasses or objectives are, as previously explained, designated according to the focal distance of a single lens of the same magnifying power. Thus a 2-inch objective is understood to be a combination which has the magnifying power of a single lens whose focal point is two inches from the object, and so on with reference to other powers. By the aid of different eye-pieces an extensive range of magnifying power can be obtained; for example, the 2-inch objective with a deep eye-piece will give the same amplification as the quarter objective with the ordinary eye-piece. Indeed, for certain observations, the combination of a wide-angled low-power objective, with a deep eye-piece, or _compensating eye-piece_, is considered to have an advantage.
It has been already explained that two objectives, one of much greater power than the other, but both having only the same numerical aperture, will show only the same amount of _detail_; the higher power on a larger scale. That is, supposing with a 1/4-inch objective of 1·0 numerical aperture certain structure is resolved, then a 1/8-inch substituted with exactly the same numerical aperture, but with double the magnification, no more _resolving power_ will be found in the latter objective than in the former. For this reason a doubt has been expressed as to whether high-power objectives--especially the more expensive oil-immersions, made to transmit large pencils of light through their larger apertures--are so well adapted for ordinary research as the best series of dry achromatic objectives, or even, in some instances, the medium aperture lenses; undoubtedly, for histological (physiological and pathological) work, the latter will be found to meet the students’ requirements quite as well as the former.