THE STRUCTURE OF 



be placed a little out of the axis of the tube of the 

 Microscope. By 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 obtained in 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 



hemispherical lens 

 with a central black 



Fig. HB. Diagram illustrating 

 the action of a " Lieberkiihn." 

 a, object-glass ; b, a concave 

 silver reflector ; c, a black spot 

 ("dark well") ; d, d, rays of light; stop, called a " spot 

 e, e, the same reflected and lens," is placed be- 

 brought to a focus at /. neath ^ ob j ect The 



accompanying figure will explain its action : 

 a is " spot lens " ; 6, 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 obtained) ; c, 



