ILLUMINATORS. 



207 



The bar, C, forms the centre upon which the diaphragm 

 turns ; it is provided with a spring and catch, the latter of 

 which not only shows when an aperture is in its right place, 

 but also indicates the number of the same. The condenser 

 fits upon a square bar attached to the under surface of the 

 stage by the screw D ; it can be moved up and down and be 

 made to traverse from side to side by the screw E. Beneath 

 the cone of diaphragms are two milled heads, F G ; the one 

 at F is for the purpose of adjusting the front combination 

 for thickness of glass upon which the object is placed, and 

 that at G for clamping the entire condenser to the parts 

 concerned in its adjustment. 



Pig. 135. 



This instrument requires some considerable practice to 

 bring out its powers, and at night is employed with a cam- 

 phine lamp and reflector of peculiar construction; a more 

 detailed description of it will therefore be given in the chapter 

 on test objects. 



