THE UEBEHKUHN. 



181 



revolved till the two pencils of light are thrown most 

 suitably across the object." 



The Lieberkuhn. — The concave speculum termed a 

 " Lieberkuhn," from its celebrated inventor, was formerly 

 much in use as a reflector, but is now almost abandoned, 

 or rather replaced by other and better contrivances. The 

 Lieberkuhn is generally attached to the object-glass, in 

 the manner represented at fig. 105, where a exhibits th > 

 lower part of the compound body, b, the object-glass, ov« 

 which is slid a tube and the 

 Lieberkuhn, c, attached to it ; 

 the rays of light reflected from 

 the mirror are brought to a focus 

 upon an object d, placed between 

 it and the mirror. The object 

 may either be mounted on a slip 

 of glass, or else held in the for- 

 ceps,/; and when too small to 

 fill up the entire field of view, 

 or when transparent, it is neces- 

 sary to place behind it the dark 

 well, e. 



Each Lieberkuhn being 

 mounted on a short piece of 

 tube, can be slid up and down on the outside of the 

 object-glass, so that the maximum of illumination may be 

 readily obtained. In the higher powers the end of the 

 object-glass is turned small enough to pass through the 

 aperture in the centre of the Lieberkuhn ; but in the 

 lower powers, where a great amount of reflecting surface 

 would be lost on account of the large size of the glasses 

 employed if this plan were adopted, the aperture in the 

 centre of the Lieberkuhn is made to admit as many rays 

 as will fill the field of view, and no more. 



As the flood of light thrown back by the Lieberkuhn is 

 sometimes so great as to obliterate much of the beauty and 

 delicacy of structure of certain objects, it was proposed by 

 Mr. Bridgeman, of Norwich, to cover up a portion of its 

 reflecting surface by gumming a piece of dull-black paper 

 over one half of the reflector ; then, by rotating the 

 Lieberkuhn upon the object-glass, any proportion of oblique 



Fig. 100. 



