MICROSCOPIC EXAMINATION OF PREPARATIONS 191 



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 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 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 

 lightly by its end, it may be moved about under the objec- 

 tive. 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 the examination is finished raise the 

 objective from the preparation by tiu'ning the screw of the 

 coarse adjustment toward you. Remove the preparation 

 from the stage, and, with a fine silk cloth or handkerchief, 

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



During work, of course, the lens need not be cleaned and 



