100 THE BEST ARRANGEMENT FOR PRODUCING A PURE SPECTRUM ON A SCREEN. 



If a is greater in proportion to b, a concave lens must be placed in front 

 of the prism. The most convenient arrangement will be that in which tl it- 

 prism ia placed in the position of least deviation, and the lens placed between 

 the prism and the screen, while the distance from the slit to the prism is to 

 that between the prism and the screen as 1 c is to c. For quartz, in which 



p. = 1*58 4 for the ordinary ray, - = 2'53, so that the best arrangement is 



C 



i=l '53 6, or the lens should be placed on the side next the screen, and the 

 distance from the slit which admits the light to the prism should be about 

 one and a half times the distance from the prism to the screen. 



