THE BEST ARRANGEMENT FOR PRODUCING A PURE SPECTRUM ON A SCREEN. 97 



incidence. This effect will increase with the difference of these angles and with 

 the refractive index. 



When the angle of incidence is equal to that of emergence the convergence 

 of the pencil is unaltered, but since the more refrangible rays have the greatest 

 angle of emergence, their convergency or divergency will be greater than that 

 of the less refrangible rays. 



This effect will be increased by making the angle of incidence less, and 

 that of emergence greater ; and it will be diminished by increasing the angle 

 of incidence, that is, by turning the prism round its edge towards the slit. If 

 the angle of the prism is not too great, the convergency or divergency of all 

 the pencils may be made the same (approximately) by turning the prism in this 

 way. 



This correction, however, diminishes the separation of the colours. It is 

 inapplicable to a prism of large angle, and it takes no account of the chromatic 

 aberration of the lens. By altering the arrangement, the lens may be made to 

 correct the prism. The effect of a convex lens is to increase the convergency, and 

 to diminish the divergency, of every pencil ; but the change is greatest on the 

 most refrangible rays. The prism, except when its base is very much turned 

 towards the slit, makes the highly refrangible rays more convergent or more 

 divergent than the less refrangible rays, according as they were convergent or 

 divergent originally. If the rays pass through the prism before they reach the 

 lens, the pencils will be divergent at incidence, and the more refrangible will 

 be most divergent at emergence. If they then fall on the lens, they will be 

 more converged than the rest ; so that by a proper arrangement all may be 

 brought to their foci at approximately the same distance. If the violet rays 

 come to their focus first, we must turn the base of the prism more towards the 

 light, and vice versa. 



We proceed to the numerical calculation of the proper arrangement. 



To find the variation of position of the focus of light passed through a prism' 

 as dependent on the nature of the light. 



VOL. n. 13 



