162 BACTERIOLOGY. 



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. 



MicEoscopic Examination of Cover-slips. — 

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

 immersion 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 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 

 diflPerentiation from surrounding objects when they are 

 located in tissues. With unstained bacteria and tissues, 

 on the contrary,, the structure can best be made out by 

 reducing the bundle of light-rays to its smallest 

 amount, and in this way favoring, not color contrast, 

 but contrasts which appear as lights and shadows due to 

 the differences in permeability to light of the various 

 parts of the material under examination. 



Steps in Examining Stained Preparations 

 WITH the Oil-immersion System. — Place upon the 

 centre of the cover-slip which covers the preparation a 

 small drop of immersion oil. Place the slide upon the 

 centre of the stage of the microscope. With the coarse 



