174 BACTERIOLOGY. 



by reducing the bundle of light-rays to its 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 perme- 

 ability 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, 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 lightly by its end, the slide may be moved 

 about under the objective. At the same time the screw 

 of the fine adjustment must be turned back and forth so 

 that the different levels of the preparation may one after 

 the other be brought into focus. In this way the whole 

 section or preparation may be inspected. When the 

 examination is finished, raise the objective from the 

 preparation by turning the screw of the coarse adjust- 

 ment toward you. Remove the preparation from the 

 stage, and, with a fiue silk cloth or handkerchief, wipe 

 very gently and carefully the oil from the face of the lens. 



