A PURE SPECTRUM ON A SCREEN. 99 



If fj. = 1-5 and a = 60, then 8 = 22 5 2', that is, the ray within the prism 

 makes an angle of 11 26' with the base, which corresponds to an angle of 

 incidence 82 2 50', or the incident ray is inclined 7 10' to the face of the prism. 



If two lenses'"" are used, of the same material with the prism, we may 

 correct the defects of the prism without turning it so far from its position of 

 least deviation. 



Let a be the distance of the slit from the prism, and b that of the screen 

 from the prism. Let /, be the focal length of the lens between the prism 

 and slit, and f, that of the lens between the prism and the screen, then the 

 condition of a flat image is 



1 di\ 1 dv, _ 1 /i\ v t \ 



7 """ " ~7 " ? ( ~~f ' ? ) 



V.dfJ. V 3 rf/A jll-1 V/ fj 



Let us first find the conditions of a flat spectrum S = o. 

 When 8 = 0, v l = v, a and we obtain the conditions 



1 1 1 _ 1 _ /I 1\ ( 1 + cos a)' {1 - fa* (1 - cos a)} _ A 1\ 



from which f t and /, may be found in terms of known quantities. 



When a, the angle of the prism, is 60, then 



C- * ~ _ / i \ q 7 ~, -i \ 



If we now make /x = 1'5 we find c = '5025, and 



1 _ -4975 _ ^025 

 f=~^~ b 

 1 _ 1-5025 ^025 



7 



/, b a 



If a = ^b, the first lens may be dispensed with, and the second lens will 

 correct the defects of the prism. 



* [These lenses are supposed to be close to the edge of the prism.] 



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