PARABOLIC REFLECTOR LIEBERKUHN. 



79 



For the illumination of opaque objects several kinds of 

 apparatus are used, and of these we may consider first, the 

 parabolic reflector as made by several opticians. This is 

 most useful for low powers, from the 2-inch to the , and is 

 made to fasten on to the objectives by means of a spring 

 clip, so that it may slide up or down or be turned round to 

 secure the best illumination. 



When this reflector is used with a lamp, the bull's-eye 

 condenser must be placed between them with the flat surface 

 towards the lamp, and its focal point 

 so adjusted that the rays emitted from 

 the bull's-eye shall be parallel. The 

 chief use of this reflector is to reflect 

 side light from all points of its surface 

 to a point situated on the plane of the 

 base in the focus of the objective. The form as made by 

 Messrs. Beck is illustrated in Fig. 61. 



FIG. 61. 



FIG. 62. 



At A, B, C, and D, Fig. 62, are shown the accessories 

 called Lieberkuhns, or Lieberkuhn's reflectors, which are 

 used occasionally for the vertical illumination of opaque 

 objects. They are small silvered concave mirrors mounted 

 on short tubes, to admit of adjustment on the objective, 

 and thereby yielding the maximum illumination. A 

 separate Lieberkuhn is required for each object-glass, as 

 the focus of the concave mirror has to be adjusted to that 

 of the objective. 



In using this appliance, parallel rays of light are thrown 



