159 



flame ; adjust mirror so that the image will be in 

 the center of field. 



To Focus Condenser. Since the condenser is 

 nothing more than a combination of lenses similar 

 to those in an objective, but used in a reversed 

 position, it has the same properties of angular 

 aperture, focus and working distance. But, as it 

 must work through the thickness of slide, its 

 working distance is proportionately large to the 

 angular aperture and owing to the variation in 

 thickness of slides, its focus is variable, falling in 

 some slides above and in others below its effective 

 position. In the simple mountings the condenser 

 is generally so placed that its upper surface is just 

 below the surface of the stage and for the low and 

 medium powers this will be generally found suf- 

 ficiently close, in fact at this point it may often be 

 found that the illumination is too intense and may 

 then be reduced either by reducing the aperture 

 of the diaphragm, or by removing the condenser 

 farther from the object. 



In all objectives with a numerical aperture less 

 than 1.0 the condenser may be used dry, without oil. 

 In immersion objectives the top of condenser 

 should be brought in fluid contact with the slide. 

 The same phenomena take place by refraction of 

 the light from condenser through the slide, as 

 from an object through the cover glass to the 

 objective. To make contact, place a drop of oil 



