48 



PKACTICAL PHOTO-MICKOGKAPHY 



small in relation to the whole. The aplanatic cone, therefore, 

 which is the largest cone of light that the lens will bring to one 

 focal point, is in this case a small one. 



Fig. 13 b shows the lens system corrected for spherical 

 aberration ; the size of the aplanatic cone now approaches 

 the total aperture of the condensing system, and the marginal 

 rays are focussed either exactly or very nearly in the same 

 position as the central rays. On the degree to which this is 

 accomplished depends the size of the aplanatic cone. 



A-~.--.rj 



FIG. 13 a, b. Diagram showing Correction for Spherical Aberration. 



As the primary object of the sub-stage condenser is to collect 

 light from the illuminant and to project as much of it as 

 possible on to the object, it is obvious that it is important 

 for the aplanatic cone to bear as large a relation as possible 

 to the total aperture. With condensers of the very best type 

 the aplanatic cone is little less than the total aperture, 

 and in some cases is claimed to be equal to it. Where the 

 cone is not aplanatic, considerable loss of light occurs ; a 

 certain amount of light becomes diffused and is not projected 

 into the object-plane as it should be. 



Illumination may be effected by two different methods : 

 either by what is called a solid, or by a hollow, cone of light. A 



