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140 



duced by elevations on the object. For " show " objects the 

 Lieberkuhn is unequalled in its effects, and there is no doubt 

 that in certain investigations it may be made to do good ser- 

 vice. 



The Parabolic Reflector. This accessory was first 

 made by Messrs. Beck for Mr. Sorby, who employed it to ex- 

 amine the microscopical structure of iron and steel. As ordin- 

 arily constructed, it consists of a parabolic mirror attached to 

 the end of a rod furnished with universal joints, so that it may 

 be placed in any position as regards the object and the illumin- 

 ation. It answers admirably for condensing the light on the 



surface of objects, and by throw- 

 ing the rays in any particular di- 

 rection across the surface, the 

 observer is enabled, by means of 

 the shadows, to determine the 

 nature of irregularities upon some 

 objects in a very satisfactory man- 

 ner. In this it resembles the or- 

 dinary side reflector, which, how- 

 ever, is formed to a spherical in- 

 stead of a parabolic curve. Like 

 the side reflector, this illuminator 

 should always be made to receive 

 parallel rays, and condense them 

 upon the object. To obtain par- 

 Fig. 44. PARABOLIC REFLECTOR, allel rays, place the lamp in the 



focus of the bulls-eye condenser. 



The side and the parabolic reflectors cannot be used with ob- 

 jectives which have a short working distance, since the rays 

 from the lamp must reach the reflector from the opposite side 

 of the objective. 



Since many stands have no conveniences for attaching this 

 accessory, some opticians furnish it with an adapter, whereby 

 it may be fastened between the objective and the nose-piece. 

 Such a contrivance, however, must have originated in a want of 

 knowledge of the principles which control this method of illum- 

 ination. It is evident that the object should always be in the 



