MICROSCOPIC EXAMINATIONS. 107 



is placed an adjustable diaphragm, the aperture of which 

 can be regulated, as circumstances require, to permit of 

 either a very small or very large amount of light passing 

 to the object. 



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 apparatus 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 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 its smallest amount, 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-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 appa- 

 ratus to its full extent. Then, with the eye to tire micro- 

 scope 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, 



