206 BACTERIOLOGY 



The "iris" diaphragm (D) opens and closes like the iris 

 of the eye. It is so arranged that its opening for admission 

 of light can be increased or diminished by moving a small 

 lever in one or another direction. 



The reflector (E) is the mirror placed beneath the stage, 

 which serves to illuminate the object to be examined. 



The coarse adjustment (F) is the rack-and-pinion arrange- 

 ment by which the barrel of the microscope can be quickly 

 raised or lowered. 



The fine adjustment (G) 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 of an 

 objective so constructed that it can only be used when the 

 transparent media through which the light passes in enter- 

 ing 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 refraction as the glass is placed upon 

 the face of the lens, and the examinations are made through 

 this oil. There is thus little or no loss of light from deflec- 

 tion, as is the case in the dry system. 



The sub-stage condensing apparatus (H) 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, thus furnishing the greatest amount of illumination. 



