MICROSCOPICAL EXAMINATION. 209 
sharp. It is essential, therefore, that the microscope be 
provided with an oil immersion system and a substage 
condensing apparatus. In using the oil immersion lens 
a drop of oil of the same index of refraction as the 
glass is placed upon the face of the lens, so as to con- 
nect it with the cover-glass when the bacteria are in 
focus. There is thus no loss of light through deflection, 
as is the case in the dry system. 
The Substage Condensing Apparatus (H) is a system of 
lenses situated beneath the central opening of the stage. 
It serves to condense the light passing through the re- 
flector to the object in such a way that it is focussed 
upon the object, thus furnishing the greatest amount of 
luminosity. Between the condenser and the reflector is 
placed an adjustable diaphragm, the aperture of which 
can be regulated, as circumstances require, to permit 
of either a very small or a very large amount of light 
passing to the object. 
The Examination of Bacteria in the Hanging Drop. 
It is often valuable to observe bacteria alive, so as to 
study them under natural conditions. We can thus 
note the method and rate of their multiplication, 
Fig, 19. 
eS 
Hollow slide with cover-glass. 
whether they move and produce spores, and whether 
or not they clump or disintegrate with specific serums. 
For this special slides and methods are desirable. The 
usual form is one in which there is ground out on one 
surface a hollow having a diameter of about half an 
inch. (Fig. 19.) According to the purpose for which 
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