208 BACTERIOLOGY 



Microscopic Examination of Cover-slips. The stained cover- 

 slip is to be examined with the oil-immersion objective, 

 and with the diaphragm of the sub-stage condensing appara- 

 tus open to its full extent. The object gained by allowing 

 the light to enter in such a large volume is that the contrast 

 produced by the colored bacteria in the brightly illuminated 

 field is much more conspicuous 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 differen- 

 tiation from surrounding objects when they are located in 

 tissues. With unstained bacteria and tissues, on the con- 

 trary, 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-contrast, 

 but contrasts which appear as lights and shadows, 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-immer- 

 sion System. Place upon the center of the cover-slip which 

 covers the preparation a small drop of immersion oil. Place 

 the slide upon the center of the stage of the microscope. 

 With the coarse adjustment lower the oil-immersion objec- 

 tive until it just touches the drop of oil. Open the illumi- 

 nating 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 pro- 

 ceed more slowly in the same direction, and, after one or 

 two turns, the object will be in focus. Do not remove the 

 eye from the instrument until this has been accomplished. 



Then, with one hand upon the fine adjustment and the 

 thumb and index finger of the other hand holding the slide 



