MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
39 
q' tan 2 (0, — fl) 
q Tan 2 (0 ; + 0) 
q, 4 sin 20 ( .sin 20 
q 1-i-s (sin 20 ; + sin 20) 2 
cos (0, + 0 ) 
cos (0,-0) 
2 sin (0, + 0) 
tan y — — tan y 
tan y { — tan y 
sin 20, + sin 20 
p' q' 
For photometrical experiments, it should be remembered that whilst^, f express 
the relative brightness of the reflected and incident rays, primary and secondary, ^ 
ind - must be multiplied by -—j to obtain the proper photometrical ratios for the 
cos 
cos 
comparison of the refracted and incident rays. 
7 r 
39. For the ‘ critical angle’ at which sin 6,=-$, we have sin #='A, 
P'=P> ( / = ( h 
P t - 0, 5f/=°- 
Both rays are therefore reflected without loss of intensity, and this is true notwith- 
standing any absorptive tendency of the medium, which in fact the rays do not enter. 
Compare No. 26 and No. 2/. 
40. The general value of the polarizing angle for which f — 0, will be determined 
by the equations 
( 1 — j~sj sin 2$— (1 sin 2$,, 
( 1 +a‘) sin d cos 6— ( 1 +/) sin 0, cos 0 t , 
or 
and 
whence 
If 
we have 
or 
sin sin $ ; =y— sin 0 t , 
. 1 +$ . fJ. . 
cos cos COS 0. 
-t 
^-l + / 
cos 0=f cos 0 
1 — sin 2 1 — sin 2 0) 
1 — sin 2 0=f 2 —f 2 sin 2 0, 
1-fVF 
w he nee 
