MICROSCOPIC EXAMINATION OF PREPARATIONS 189 



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 oil4mmersion 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 

 hmnogeneotis. 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. 

 Between the condenser and reflector is placed the "iris" 

 diaphragm, the aperature 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 such a way that by 

 simply rotating the nose-piece the various lenses of different 

 power may be conveniently used in succession. 



