180 BAOTEBIOLOaY. 



The ooular or eye-pieoe (a) is the lens at which the eye 

 is placed in looking through the instrument. It serves 

 to magnify the image projected through the objective. 



The objective (b) is the lens which is at the distal end 

 of the barrel of the instrument, and which serves to 

 magnify the object to be examined. 



The stage (c) is the shelf or platform of the micro- 

 scope on which the object to be examined rests. 



The diaphragms are the perforated stops that fit in 

 the centre of the stage. They vary in size, so that dif- 

 ferent amounts of light may be admitted to the object 

 by using diaphragms with larger or smaller openings. 



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 direct the light to the object to be ex- 

 amined. 



The coarse adjustment (f) is the rack-and-pinion ar- 

 rangement 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 

 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 



