26 



THE STRUCTURE OF 



this method an oblique beam of light is thrown on 

 the Lieberkuhn^ and the light from it is reflected 

 unequally upon the object; thus producing the light 

 and shade so necessary for the proper definition of an 

 object. (The cost of a Lieberkuhn varies from 6s. 

 to 15s. ; those for low powers costing more than 

 those for the higher.) 



The details of many 

 transparent objects 

 are much more dis- 

 tinctly seen when 

 examined by light 

 transmitted by the 

 object only. This is 

 called black ground 

 illumination, and can 

 be obtainedin several 

 ways. With a very 

 low power the light 

 can be reflected with 

 sufficient obliquity if 

 the mirror is thrown 

 out of the axis ; but 

 much better effects 



are obtained when a 

 Diagram illustrating hem i 8pherica ] ] ens 



with a central black 



Fig. HB 



the action of a " Lieberkiihn." a, 

 object-glass ; 6, a concave silver 

 reflector; c, a black spot ("dark stop, called a " spot 

 well '') ; d, d, rays of light ; e, e, Jens," is placed be- 

 the same reflected and brought to a 

 focus at /. 



neath the object. 



The accompanying 

 figure will explain its action : a is " spot lens " ; 

 >, a brass tube in which it is mounted (this is fitted 

 into a larger tube fitted to the short tube attached 

 to the lower surface of the stage : by sliding this up 

 or down, the proper distance from the object is ob- 

 tained) ; e, parallel lines from mirror ; </, the same 



