OPTICS. 129 
simple microscope. The shorter the focus, or the greater the convexity of 
the lens, the greater the magnifying power. The apparent increase in the 
size of an object by a lens, depends upon its enabling us to see the object 
distinctly at a much less distance than with the naked eye, the angle of 
vision increasing with this proximity. The magnifying power of a lens is 
obtained by dividing five inches, the limit of distinct vision, by the focal 
length of the lens. If this be 3, 1, 2 inches, the magnifying power will be 
10, 5, 2. 
The compound microscope, in its simplest form, consists of two convex 
lenses; one of short focus, called the objective or object-glass, the other of 
longer focus, called the ocular or eye-glass. To the latter the eye is applied. 
The object being placed in the focus of the objective, an image of it is 
formed in the ocular. This first image is magnified by the ocular, the 
second image being painted on the retina. The object of course appears 
considerably larger after the second magnifying than after the first. 
Fig. 61 represents an improved form of the instrument as constructed by 
Cuevauier, of Paris. Here the objective is at 6, the ocular at c. Rays 
from the object, placed on the stage. f, of the microscope, after passing 
vertically through the objective, strike on the inside of the hypothenuse of 
the three-sided glass prism, and are reflected horizontally along the axis of 
the ocular tube. | 
To remedy the deficiency of light which always exists in the use of 
higher powers, the stage, f, has a central perforation. A plane or concave 
mirror, m, is so adjusted as to reflect rays of light through the perforation 
of the stage on the object. The light may be derived from the sun, from a 
white cloud, from the sky, or from a lamp. 
The stage, f, is attached to a slide or socket, d, which, by means of a 
rack and pinion arrangement, can be moved up and down the rectangular 
bar, g. This adjustment is necessary to enable objects placed on the stage 
to be brought nearer to, or more remote from the objective, as the different 
focal lengths may require. ‘T'wo other screws, & and q, serve to bring all 
the different parts of the object successively into the focus, by communi- 
cating to the stage a backward, forward, or lateral motion. The instrument 
may be rendered vertical by removing the part containing the prism, and 
attaching the objective directly to the tube containing the ocular. The 
objective may consist of a single lens, or of two, and even three: they 
should be achromatic, however. The ocular, also, in addition to the simple 
eye-glass, comprises a field glass, a second lens, generally plano-convex, 
whose object is to increase the field of view. For further information on 
this extended and interesting subject, we must refer our readers to professed 
treatises on the microscope, as those of Vogel, Pritchard, Goring, Ross, 
Chevalier, Quekett, and others. 
The Solar Microscope—a simple microscope in principle—is represented 
in fig. 59. The mirror, m, reflects the light of the sun to the tube, ¢, in a 
direction parallel to its axis. The lens, ir, causes the rays to converge 
somewhat; a second lens, f, brings them toa focus, in or near which the object 
to be magnified is placed. This second lens can be moved backwards and 
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