ULTRAMICROSCOPES 111 



After passing through the slit the light rays enter the lens C2, 

 having a focal length of 55 millimeters, whose function is to 

 project a reduced image of the slit into the condenser-objective 

 C3. Since both slit and lens C2 are movable forward and back 

 upon the optical bench, the lens C2 serves a double purpose, 

 projection and adjustment of the magnitude of the light beam 

 entering C3. The objective C3 projects into the preparation 

 contained in the cell of black glass U a tiny conical beam of 

 light at right angles to the optic axis of the microscope M. To 

 prevent any side light from entering the preparation, lenses Ci 

 and C2 are small and are mounted in blackened metal screens; 

 as a further precaution a large metal screen D with tubular 

 opening or adjustable diaphragm is introduced between the 

 radiant and Ci. The objective C3 screws into a tube fitting into 

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

 ment. A very sensitive forward and back movement is further 

 provided by the fine adjustment screw Vi. A second fine adjust- 

 ment to the right and left for accurately centering the illumi- 

 nating cone of light is obtained by the screw V2. 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 C3 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. 



To the lower end 

 of the body tube of 

 the microscope is at- 

 tached an adapter Fig. 52. Biltz Cell. 

 A with centering 



screws a, a, providing a device for accurately centering the 

 objective O. 



The liquid containing suspensoids is conveniently placed for 

 examination in a Biltz cell, Fig. 52, or, when the short piece of 

 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. 53, may be substituted. In both of these 



