the light may be so placed, as to give all gradations 

 of oblique illumination from the central to the limit of 

 aperture, although the condenser may be centered ; 



With daylight have evenly illuminated field. 



With lamplight attach 2-3 inch objective; open 

 diaphrfigm to full extent and focus upon the minute 

 image of flame; adjust mirror so that the image will 

 be in the center of the 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 propor- 

 tionately long. Owing to the variation in thickness of 

 slides, its focus falls in some slides above and in others 

 below its effective position, in relation to the object, 

 and an adjustment is therefore necessary to make it 

 most effective. 



In all the various forms of mountings the condenser 

 is so mounted that at the uppermost limit of adjustment 

 its upper surface is just below the surface of the stage 

 so that it cannot come in contact with the slide. 



With all objectives having a numerical aperture 

 less than i.o the condenser may be used dry, i. e. with- 

 out oil. 



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