112 PHOTOMICROGRAPHY 



live. For most purposes the difference between the aper- 

 tures 1-15 and 0-95 is unimportant, but the use of an 

 Abbe condenser sets a limit to the performance of an 

 objective that is rather low for critical work. A J" 

 objective of high N.A. (0-9 for instance) could be used at 

 full aperture with an achromatic condenser, but an Abbe 

 would restrict its working aperture to (-9+ -5) -f 2=0-7, 

 which is a considerable drop. 



If it is decided to be content with an Abbe condenser 

 it is best to get that with the larger N.A., as it is adaptable 

 to a wider range of objectives than the lower power ; 

 with front lens removed it is a very good low-power 

 illuminator. But if an achromatic condenser is decided 

 upon the choice is wider. The advantages of a large 

 aperture in objectives have been described, and to use this 

 aperture as fully as possible a large illuminating cone is 

 required, but the cost of the oil-immersion condensers is 

 so great that the worker who dees not aspire to such 

 difficult feats as the resolution of Amphipleura into dots 

 may be well content with one of the dry condensers. 

 Those with a wide back lens and a N.A. of 1-0 are the 

 most generally used. They constitute in themselves 

 almost a complete battery ; they can be used with high- 

 power oil-immersion objectives without great sacrifice of 

 aperture, with dry objectives at their full aperture, and 

 with low powers down to about 1-J" or 2". They are, in 

 fact, practically universal condensers, and can be used for 

 almost any work, even to illuminate such a large field as 

 is covered by a short- focus planar lens. But they fail 

 with the longer -focus planars, for which one of the 

 low-power illuminators must be chosen, if a condenser is 

 to be employed ; and they are also inadequate for the 

 photomicrography of many diatoms and other objects 

 which call for the highest resolving power obtainable. 

 Still, it is between these two extremes that by far the 

 greatest amount of photomicrography is done, and for 



