DIFFERENT PARTS OF THE MICROSCOPE. 203 



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 

 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 little or no loss of light from deflection, 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. Between the condenser and 

 reflector is placed the " iris " diaphragm, the aperture 

 of which can be regulated, as circumstances require, to 

 permit of either a very small or a very large amount of 

 light passing to the object. 



The nose-piece (i) consists of a collar, or group of 

 collars joined together (two or more), that is attached to 

 the distal end of the tube of the microscope. It enables 

 one to attach several objectives to the instrument in 



