Dr. Brewster on the action , &c. 
Table showing the change in the direction of the polarisation in 
different azimuths. 
Azimuth. 
Inclination of inci- 
dent ray to the axis. 
Change in the direction 
of the polarisation. 
O 
0 / 
O / 
0 
0 0 
0 0 
10 
6 54 
40 36 
20 
16 50 
65 6 
3 ° 
23 0 
77 22 
40 
29 24 
88 sz 
45 » 
32 3 8 
94 34 
5 ° sn 
36 29 
100 54 
60 
42 17 
1 10 10 
70 
48 32 
120 0 
80 
54 37 
129 8 
90 
60 0 
137 0 
100 
65 56 
145 48 
1 10 
7 i 4 
153 4 
120 , 
75 42 
*59 4 8 
129 21 
79 28 
16; 3 
*35 
81 41 
169 46 
140 
83 22 
> 7 ° 34 
150 
86 12 
>74 52 
160 
88 18 
1 77 3<5 
170 
8 9 33 
179 28 
180 
90 0 
180 0 
The results in the preceding table enable us to explain the 
phenomenon described in p. 154. As the interposed oil of 
cassia has a prismatic form and a very high dispersive power, 
the blue and the red rays are incident at different angles with 
the axis, and therefore the change in the direction of their 
polarisation must be different. The nearest approximation to 
evanescence in one of the images, belongs to the mean ray of 
the spectrum, and therefore at this point the image that should 
have vanished, must consist of blue and red light, one of which 
will disappear before , and the other after, the mean ray. 
I have the honour to be, & c. &c. See. 
DAVID BREWSTER. 
To the Right Hon. Sir Joseph Banks, Bart. 
G.C.B. P. R.S. &c. &c. &c. 
Edinburgh, Nov. 12, 1818. 
From Ibe Press of 
W. BULMER & Co. 
Cleveland-row, St. James's, 
London. 
