facet creates refraction with its consequent dispersion 

 and play of colors. 



With the light as refracted by a lens or prism, on 

 its emergence the violet ray, being the more refrang- 

 ible, will be principally affected and be brought to a 

 focus within the principal focus, and the red ray will 

 be brought to a focus beyond that of the violet. 



To avoid spherical and chromatic aberrations to 

 the greatest possible extent has been for many years 

 the study of opticians. 



Fig. 12. 



How this was successfully accomplished is explained 

 by reference to Fig. 1 2 where it will be seen that the 

 ray of white light is separated into the various colored 

 rays, from red to violet, in passing through the first 

 prism which is made of crown glass, and that the rays 

 would continue to diverge in the direction r r' for the 

 red, and v v' for the violet. If now a second prism of 



12 



