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 sHt 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 
hght at right angles to the optic axis of the microscope M. To 
prevent any side hght 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 
