LAWS OF DECREMENT. 



349 



RIGHT OBLIQUE-ANGLED PBISM. 



case, as to give the particular values of the indices 

 of each individual plane. 



The following are the units of comparison for de- 

 termining the laws of decrement on the edges. 



Fig. 352. 



For those which produce the planes 0, the unit is 

 the ratio of the perpendicular g b, fig. 352, to an 

 edge b /; and for those which produce the planes cf, 

 the unit is the ratio of a perpendicular b h, to an edge 



To find b g : bf, we have 



ba : bg::R :sm.A 2 : 



H 



and as . . . . b a : b f : : I : 



n 



we must have b g : bf :: n sin. A z : o R. 

 To find b h : bf> we have 



R 



be :bf:i 1 : - 

 m 



b h\ bf :: m sin. A t : o R. 



