with the Aid of a Grating, 



433 



slightly wedge-shaped plate, the one at a (fig. 5) being white, 

 that at c yellowish, the distances apart being the same as in 



Kir. 



the preceding case. Besides this there are two images of 

 the slit at b, figs. 6, 7, which result from double diffraction 



Ficr. 6. 



Fig. 7. 



at the lower face, the case shown corresponding to 6<i first 

 with 6>i thereafter, while the other image corresponds to 

 6>i and 6<i in the two successive diffractions. These slits 

 show strong chromatic aberration, but are nevertheless linear 

 and useful. The optical paths for the three lower rays are : 



transmitted, I, 2y n -f- £/u/cos R ; 



transmitted-refracted, II, 2y n + efi/cos R + 2e/x cos R ; 

 transmitted-diffracted, III, 2y n 4- <?/x/cos R + 2efi cos L . 



These three equations determine the three stationary inter- 

 ferences 



n\=2eficos'R; n\ = 2efxcos6 l ; nX = 2efjb(cos Q\ — cos R), 



discussed in the preceding paper *. If the grating is between 

 two identical plates of glass, these stationary fringes may 



* C. & M. Barus (/. c). 



