LENSES AND THEIR ACTION 



[CH. XIV 



FIGS. 320-321. BEAMS OF PARALLEL LIGHT TRAVERSING PLANO-CONVEX 



LENSES TO SHOW MAXIMIUM AND MINIMUM SPHERICAL 



ABERRATION. 



Figure 320 shows the lens in the position to give the maximum aberration. 

 That is, the border rays cross the axis much nearer the lens than the inter- 

 mediate rays. 



Figure 321 shows the lens in the position to give the minimum aV erration. 

 The border and the intermediate rays cross the axis mere nearly in the same 

 place. 



This picture was made in a dark-room (fig. 179). The room was filled with 

 smoke, and the light was partly scattered by the smoke, thus making the rays 

 visible from the side ( 7963). The first element of the triple condenser was 

 covered in the parallel beam, with a perforated metal disc. This permitted only 

 minute cylinders of light to escape along the diameter of the condenser. 



The lens in fig. 320 appears dark as it was clean . The one in fig. 321 appears 

 white because there was some very fine talcum dust on the face. 



