172 BA CTERIOL OGY. 



microscopically, a few definitions of the terms 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. It serves to 

 magnify the image projected through the objective. 



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 diaphragms are the perforated stops that fit in the 

 centre of the stage. They vary in size so that different 

 amounts of light may be admitted to the object by using 

 diaphragms with larger or smaller openings. 



The " Iris " diaphragm 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 arm in one or another direction. 



The reflector is the mirror placed beneath the stage, 

 which serves to direct the light to the object to be exam- 

 ined. 



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 

 transparent media through which the light passes in enter- 

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



