ILLUMINATING APPARATUS. 
If the object be opaque, it may be illuminated from above by 
several expedients ; being placed upon a blackened plate resting 
on the stage s s, light proceeding from a window or a lamp may 
be condensed upon it by a concave reflector m' m', or by a convex 
lens L l. These arrangements, however, are only applicable 
when the object is at such a distance from the object-piece that the 
light proceeding from m' i' or i l shall not be wholly or partially 
intercepted by the object-piece. This would always be the case, 
however, when very high powers are used, and when, conse¬ 
quently, the object must be brought very close to the object- 
piece. In that case, the object is supported upon a small piece 
of blackened cork, or in a dark cell of the form represented at 
w w; this support is placed in the centre of the opening of the 
stage, so as not to intercept any but the central rays reflected 
from m m ; upon the end of the object-piece a conoave reflector, 
having a hole in its centre, through which the object-piece passes, 
is fixed; the light proceeding from it M, and falling upon this 
reflector, is reflected by it, so as to converge upon the object, and 
thus to illuminate it. 
A concave illuminator thus mounted is called, from its inventor, 
a lieberkuhn . 
29. In the illumination of objects it is frequently necessary to 
limit, to a greater or less extent, the diameter of the pencil of 
light thrown from the reflector, ii ir, upon the object. Although 
this may partly be accomplished by varying the distance of the 
reflector from the object, or by the interposition of a convex lens, 
such expedients are not always the most convenient, and a much 
more ready and effectual method of attaining this end is supplied 
by providing below the stage, s s, a circular blackened disc, 
capable of being turned upon its centre in its own plane. This 
disc is pierced with a number of 
holes of different diameters, as shown 
in fig. 14, and it is so mounted, that 
the openings in it, by turning it 
round its centre, may be brought 
successively under the object. This 
is easily done by fixing the centre 
of this disc at a distance from the 
centre of the stage, equal to the 
distance between the centre of the 
disc and the centres of the holes 
made in it. 
This appendage is called the disc 
of diaphragms, and is of great use in the illumination of objects, 
as will appear hereafter. 
Fig. 14. 
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