96 PHOTO-MICROGRAPHY 



So far as relates to the various "cones" of light given by the condenser, terms 

 which should be remembered as they are often referred to later on, it yet remains 

 to briefly describe, sufliciently for the purpose of the photo -micrographer, the general 

 optical construction and other considerations concerning the condenser as well as to 

 show how to critically examine one. 



It has been already said that condensers are manufactured of different foci, which 

 is true, and some are made so that by unscrewing the top lens the lower portion can 

 be used for lenses of lower N. A. Personally speaking we prefer those specially 

 made for low angles ; it is certainly too often true that by unscrewing the top lens 

 the remainder only becomes a makeshift, and, as Dr. Dalhnger remarks : " when the 

 highest class of work has to be done it is needful to have condensers suited to the 

 power of the objective used!' 



The leading points to consider in a condenser are : 



1. The focus. 



2. The numerical aperture. 



3. The size of the aplanatic cone. 



4. The definition. 



1. With respect to the length of focus, this is usually given by the makers, but 

 may be roughly obtained by allowing parallel rays, say from the sun, to fall upon the 

 back lens of the combination, and having focussed the sun's image on a piece of paper, 

 by ascertaining the distance this point of focus is from a plane midway between the 

 front and back lens. The method is rough, but accurate enough for the purpose. 

 But the question is, what are the best foci for different lenses. This is much a matter 

 of opinion. Some consider it should be approximately the same as that of the 

 objective whereas advocates can be found that think it should be less ; anyhow its 

 working distance must be such as to allow a slip of reasonable thickness to be employed. 



(In testnig condensers for N. A., the thickness of the shp for which the condenser 

 is designed must be carefully known, for if not a mistaken quantity may be obtained 

 by the apertometer, and manufacturers may have misrepresentations made about 

 their work which are not deserved.) 



The focal lengths mostly chosen are for immersion condensers about i inch : for high 

 powers with N. A. about i 'O between t and I inch : for medium powers about N. A. '6 

 say i inch : and for quite low powers about | to i inch focus. 



2. To ascertain the N. A. it is only necessary to employ the Abbe apertometer in 

 the manner already described (see page 89, under note). 



