MICROSCOPIC METHODS 



79 



ing into the eye-piece, no change of focus is noticed 1>\ turning the 

 micrometer head, or if the micrometer head ceases to turn, the adjust- 

 ment has reached its limit. Turn the micrometer back to bring the 

 fine adjustment midway within its range. When the fine adjustment 

 head stops do not force it. For the microscopic study of bacteria 

 it is essential that we magnify the bacteria as much as possible and 

 still have their definition clear and sharp. It is essential, therefore, 

 that the microscope be provided with an oil-immersion system and a 

 substage condensing apparatus. In using the oil-immersion lens a 

 drop of oil of the same index of refraction as the glass is placed upon 

 the face of the lens, so as to connect it with the cover-glass when the 

 bacteria are in focus. There is thus no loss of sight through deflection, 

 as is the case in the dry system. If the lenses become dirty they should 

 be wiped gently with Japanese lens paper or a clean, soft, old-linen 

 handkerchief. If necessary breathe on the lens before wiping, and if 



FIG. 55 



FIG. 56 



Anthrax bacilli and blood cells. 

 X 1000 diameters. 



Colonies of diphtheria bacilli. 

 X 200 diameters. 



this does not succeed use a little xylol or chloroform. These substances 

 are not to be used unless necessary. An immersion objective should 

 always be cleaned immediately after using. The objective should 

 always be kept covered so as to prevent dust dropping in. 



Light. The best light is obtained from white clouds or a blue sky. 

 Avoid direct sunlight. If necessary use white shades to modify the 

 sunlight. Artificial light has one advantage over daylight in that it is 

 constant in quality and quantity. The Welsbach burner and a whitened 

 incandescent bulb give a good light. A blue glass between the artificial 

 light and the lens is often of value. An eye-shade is often helpful. 



Substage Condensing Apparatus // is a system of lenses situated 

 beneath the central opening of the stage. It serves to condense the 

 light passing through the reflector to the object in such a w r ay that it is 

 focused upon the object, thus furnishing the greatest amount of lumi- 

 nosity. Between the condenser and the reflector is placed an adjustable 



