ULTRAMICROSCOPES 
125 
an adjustable slit-diaphragm, thus permitting the cutting down 
of the field until only a certain selected portion is visible. 
The mutual arrangement of the two objectives is shown in 
the diagram. These objectives embody several unique ideas in 
mounting, construction and in the component lenses themselves; 
the end, or front, lenses are of quartz. An examination of the 
diagram will show that ^ drop of liquid brought into contact 
