4:88 



Dr. C. V. Drvsdale on the 



Retardation by Oblique Parallel Plate. — In fig. 9, PABQ 

 is the course of a parallel beam of light, AB being the 



Fig-. 9. 



Oblique Refraction by a Parallel Plate. 



distance travelled through the plate ; we have D as the position 

 the wave-front would have reached in the absence of the 

 plate such that 



and retardation 



AD =fi.AB 



s = fi. AB-AC. 



But AB = — and AC = ABcos(<r_ 1 -o- 1 ). 



COSO"! 



Hence ^ ~ cos (o-_i — <n) . 



s = — -*• 



cos a x 



Putting sin &-i=p sin a x 



and simplifying we have 



8 = { \Zf^' — S1U 2 CT—i — cos O"! } t 



accurately, or to a second approximation 



=(-- 1 )( 1+ ^) 



