Prof. Ch. V. Zenger on a ?ieiv Spectroscope. 



441 



mirror be given. In the first place, the angle of dispersion 

 produced by the prism itself will be 



equal to the difference of the angle of refraction for the red and 

 the violet rays : t 



sinr,,=w» . sineo ; 



If the angle of the prism is not too large, the sines may be re- 

 placed by the angles : 



r h -n h m- } 



(f)=(n h — n b )(D = cD.An. . . . (1) 



The angle of incidence (i) at a given distance between the 

 mirror and of the prism (fig. 2) is 



i=r— (o + p, 



p being the angle of aperture of the mirror. 



Fig. 2. 



A 



In the tringle m m! o } 



m o : m'o= sin i : sin (r— u>); 

 or approximately 



m o :m'o = i:r — Q> ; r — o)= (n — l)(o. 



