EXAMINATION OF COVER-SLIPS. 181 



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 

 tliis 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 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 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. 



Microscopic Examination of Cover-slips. — The 

 stained cover-slip is to be examined with the oil-immer- 

 \ sion objective, and with the diaphragm of the sub-stage 

 condensing apparatus open to its full extent. The object 

 gained by allowing the light to enter in such a large vol- 

 ume is that the contrast produced by the colored bacteria 

 \ in the brightly illuminated field is much more conspic- 

 uous than when a smaller amount of light is thrown upon 



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