204 BA CTERIOLOG Y. 



such a way that by simply rotating the nose-piece the 

 various lenses of different power may be conveniently 

 used in succession. 



MICROSCOPIC EXAMINATION OF COVER-SLIPS. The 

 stained cover-slip is to be examined with the oil-immer- 

 sion objective, and with the diaphragm of the sub-stage 

 condensing apparatus open to its full extent. The object 

 gained by allowing the light to enter in such a large vol- 

 ume is that the contrast produced by the colored bacteria 

 in the brightly illuminated field is much more conspicu- 

 ous than when a smaller amount of light is thrown upon 

 them. This is true not only for stained bacteria on 

 cover-slips, but likewise for their differentiation from 

 surrounding objects when they are located in tissues. 

 With unstained bacteria and tissues, on the contrary, the 

 structure can best be made out by reducing the bundle 

 of light-rays to the smallest amount compatible with 

 distinct vision, and in this way favoring, not color-con- 

 trast, but contrasts which appear as lights and shado/r*, 

 due to the differences in permeability to light of the 

 various parts of the material under examination. 



STEPS IN EXAMINING STAINED PREPARATIONS 

 WITH THE OIL-IMMERSION SYSTEM. Place upon the 

 centre of the cover-slip which covers the preparation a 

 small drop of immersion oil. Place the slide upon the 

 centre of the stage of the microscope. With the coarse 

 adjustment lower the oil-immersion objective until it 

 just touches the drop of oil. Open the illuminating 

 apparatus to its full extent. Then, with the eye to the 

 ocular and the hand on the fine adjustment, turn the 

 adjusting-screw toward the right until the field becomes 

 somewhat colored in appearance. When this is seen 

 proceed more slowly in the same direction, and, after 



