106 BACTERIOLOGT. 



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 to be 

 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 System consists in an objective so 

 constructed that it can only be used when the media 

 tlirough 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 ex- 

 amined 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 refraction as the glass 

 is placed upon the face of the lens, and the examina- 

 tions are made through this oil. There is thus no loss 

 of light from deflection, as is the case in the dry systems. 



The sub-stage condensing apparatus is a system of 

 lenses situated beneath the central opening of the stage. 

 They serve to condense the light passing from the 

 reflector to the object in such a way that it is focussed 

 upon the object. Between the condenser and reflector 



