40 
ELEMENTARY CHEMICAL MICROSCOPY 
tive. Only those rays included in a low numerical aperture are 
available. Hence the employment of an objective of high 
numerical aperture and very short working distance yields a 
field which is never dark. Since practically all high-power 
immersion objectives are made with as high numerical apertures 
as possible, it is absolutely essential that some means be used 
to reduce their numerical aperture below i, if they are to be 
employed in dark-field studies. This is accomplished by 
introducing into the objective mount some form of diaphragm: 
or specially constructed objectives of N.A. less than i may be 
(D, D, D, Removable Diaphragms.) 
purchased. Diaphragms for use with objectives in dark-field 
studies are generally supplied by the manufacturers of reflecting 
condensers for introduction into the special objectives to be used. 
These funnel-like diaphragms are not interchangeable and can 
be employed only for the objectives for which they are designed. 
Figs. 16, 17 and 18 show three different types and forms of dia¬ 
phragms employed for this purpose. In the case of Fig. 16 the 
lens mounting is unscrewed just back of the back lens combina¬ 
tion and the funnel diaphragm, provided with male thread, is 
screwed into the opening tapped into the upper half of the objec¬ 
tive mounting. In the case of Fig. 17, the objective is also 
unscrewed just above the back lens combination, but in this 
case the diaphragm is merely dropped into the hole in the lower 
