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' M' or L 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 concave reflector, 

 having a hole in its centre, through which the object-piece passes, 

 is fixed ; the light proceeding from M :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, M if, 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 Fi - 14 - 



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. 



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