dispersion and play of colors. To avoid to the 

 greatest possible extent the spherical and chro- 

 matic aberration, has been for many years the 

 study of opticians. 



Fig-. 10. 



If in Fig-, in the beam of lights b is followed 

 it will be found to separate at the first surface of 

 the first prism and still more at its emergence from 

 the second surface. The violet ray undergoing 

 the greatest amount of refraction, takes the direc- 

 tion of v r', and the red ray being refracted to 

 a lesser extent, follows the course of r r . To 

 neutralize or correct this dispersion a second 

 prism, identical with the first, but in a reversed 

 position, may be placed to intercept these rays, so 

 that they will be recombined,. as shown in r c and 

 v c and finally emerge as white light. Now while 

 the dispersion can be corrected in this manner, it 

 will be noticed that the emergent ray c d from 

 the second prism takes the same direction as the 

 incident ray a b and that this course corresponds 



