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. 



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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