182 BAQTEBIOLOGY. 



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 shadows, 

 due to the differences in permeability to light of the 

 various parts of the material under examination. 



Steps in Examining Stained Peeparations 

 WITH THE OiL-iMMBRSiON SvsTEJt. — 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 

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

 remove the eye from the instrument until this ha^ 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 cod, 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 planes of the preparation may be 

 brought into focus one after the other. In this way the 

 whole section or preparation may be inspected. When 



