MICROSCOPIC EXAMINATIONS. 161 



used in referring to the microscope may not be out of 

 place. 



The ocular or eye-piece is the lens at which the eye is 

 placed in looking through the instrument. 



The objective is the lens which is at the distal end of 

 the barrel of the instrument, and which serves to mag- 

 nify the object to be examined. 



The stage is the shelf or platform of the microscope 

 on which the object rests. 



The reflector is the mirror placed beneath the stage, 

 which serves to direct the light to the object examined. 



The coarse adjustment is the rack-and-pinion arrange- 

 ment by which the barrel of the microscope can be 

 quickly raised or lowered. 



The fine adjustment serves to raise and lower the 

 barrel of the instrument very slowly and gradually. 



For the microscopic study of bacteria it is essential 

 that the microscope be provided with an oil-immersion 

 system and a sub-stage condensing apparatus. 



The Oil-immersion or Homogeneous System consists in 

 an objective so constructed that it can only be used 

 when the media through which the light passes in 

 entering it are all of the same index of refraction — i. e., 

 are homogeneous. This is accomplished by interposing 

 between the face of the lens and the cover slip covering 

 the object to be examined a body which refracts the 

 light in the same way as do the glass slide, the cover- 

 slip, and the glass of which the objective is made. For 

 this purpose a drop of oil of the same index of refrac- 

 tion as the glass is placed upon the face of the lens, and 

 the examinations are made through this oil. There is 

 thus no loss of light from deflection, as is the case in 

 the dry systems. 



