224 
PROFESSOR POWELL ON A NEW CASE OF 
“ In this manner we obtain the following table 
(42.) Calc-spar and Oil of Cassia. Extraordinary ray. 
Ray. 
Calc-spar 
do - 
Calc-spar 
i“'e- 
s. 
d - 
Oil of 
Cassia, ij .. 
e — e - 
X 
d - H - „ 
— x-0 
Dilf. 
B. 
1-6531 
1-4839 
47 42-0 
1-5628 
1-5945 
— -0317 
— 1248 
- 49-9 
D. 
1-6585 
1-4863 
44-8 
1-5665 
1-6073 
--0408 
— 1876 
— 75-0 
- 25-1 
F. 
1-6680 
1-4907 
49-5 
1-5731 
1-6358 
--0627 
-3495 
-139-8 
- 64-8 
G. 
1-6762 
1-4945 
53-6 
1-5787 
1-6671 
— -0884 
— 5570 
— 222-8 
— 83-0 
H. 
1-6833 
1-4978 
57-0 
1-5837 
1-7025 
--1188 
-8115 
-324-6 
-101-8 
^75 
‘‘The direction considered being about 45° from the axis, ought to be nearly 
equal to the mean of f^'o (^'e- Now the values of /a' found above, fall short of the mean 
of (Jj'q and ^'e by the following quantities: — 
For B . . 
. . -0057 
D . . 
. . -0059 
F . . . 
. . -0062 
G . . 
. . -0066 
H . . 
. . -0068 
“The smallness and regularity of these numbers is a test of the correctness of the 
arithmetic.” 
(43.) Quartz and Oil of Sassafras. 
The angle of the prism being 60° and the ray F at the minimum deviation, and (p, 
p' being the angles of incidence and refraction for the first surface, we have for either 
pencil, 
^'p=30°, sin (Pe=(^f sin 
which gives (pp, and therefore p for the other rays ; 
also 
. , sin <2 
sm p = — - 
gives p' for the other rays. 
If i be the angle of incidence on the plate then in position (1) (see figs. 2, 3) 
I = 60 °— 9 ', and in position ( 2 ) i=p' . 
Then (as in Airy’s Tract, Art. 151) if i' be the angle of refraction referring to the 
normal of the wave, we have 
tan i' = 
a sin i 
V siu^ i 
a fj-s, 
where 
