60 ELEMENTARY CHEMICAL MICROSCOPY 



opening or adjustable diaphragm is introduced between the 

 radiant and d. The objective C 3 screws into a tube fitting into 

 the sleeve T and may be slid forward and back for coarse ad- 

 justment. A very sensitive forward and back movement is 

 further provided by the fine adjustment screw Vi. A second 

 fine adjustment to the right and left for accurately centering 

 the illuminating cone of light is obtained by the screw V 2 . By 

 means of these two screws it is possible to adjust the tiny beam 

 entering the material to be studied, in such a manner as to 

 ensure the focal point of the condensing objective C 3 falling in 

 the line of the optic axis of the observing microscope M, and 

 therefore have the whole of the tiny beam lying across the exact 

 center of the field of view. 



FIG. 25. Biltz Cell. 



To the lower end of the body tube of the microscope is at- 

 tached an adapter A with centering screws 0, a, providing a 

 device for accurately centering the objective O (see page 47). 



The liquid containing suspensoids is conveniently placed for 

 examination in a Biltz cell, Fig. 25, or, when the short piece of 



FIG. 26. Biltz-Thomae Cell. 



rubber tubing which is attached to the end of the tube is objec- 

 tionable because of its possible action on the colloids, a Biltz- 

 Thomae cell, Fig. 26, may be substituted. In both of these 

 cells the essential feature is the central dark glass chamber of 



