EXAMINATION OF COVER-SLIPS. 173 



homogeneous. This is accomplished by interposing be- 

 tween 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 no loss of light from deflection, as is the case in 

 the dry system. 



The sub-stage condensing apparatus 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 an adjustable diaphragm, the aperture of which 

 can be regulated, as circumstances require, to permit of 

 either a very small or very large amount of light pass- 

 ing to the object. 



MICROSCOPIC EXAMINATION OF COVER-SLIPS. 

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

 immersiou objective, and with the diaphragm of the 

 sub-sta^e condensing apparatus open to its full extent. 

 The object gained by allowing the light to enter in such 

 a large volume is that the contrast produced by the 

 colored bacteria in the brightly illuminated field is 

 much more conspicuous than when a smaller amount of 

 light is thrown upon them. This is true not only for 

 stained bacteria on cover-slips, but likewise for their 

 differentiation from surrounding objects when they are 

 located in tissues. With unstained bacteria and tissues, 

 on the contrary, the structure can best be made out 



