crystallized bodies on homogeneous light. 83 
and, by the intervention of the red glass above mentioned, 
observed the maxima and minima of the extraordinary pencil 
between the poles. As these observations, when repeated, 
seldom agreed unless within a few minutes, ten were taken 
of each maximum and minimum. The angles of incidence, 
deduced from a mean of similar observations on each side of 
the perpendicular, are set down in the 2d column of the 
following table, each number in which is therefore a mean 
result of 20 observations. The 1st column contains the va- 
lues of n, or the order of the ring observed ; the 3d, the angles 
of refraction, to obtain which I used the index 1*300, em- 
ployed by M. Biot.* The 4th and 3th columns contain the 
values of 0,0' thence computed, and the 6th, values of the 
coefficient h , deduced from the formula h = sin 6 - ~ sin 6 - 
71 . COS <p 
in. 
Table X. Mica. Thickness=o-02goj8. 
Values 
of n 
Angles of 
Incidence. 
Angles of 
Refraction. 
Values of f). 
Values of Q' 
Values of h. 
Excesses above 
mean. 
O / // 
O 
/ 
// 
O 
/ 
// 
O 
/ 
// 
0 
35 3 3 ° 
22 
3 i 
0 
O 
0 
0 
45 
2 
O 
0-5 
32 55 20 
21 
J 4 4 ° 
I 
16 
20 
43 
45 
40 
0-032952 
—0-000195 
1 
30 34 40 
l 9 
49 
3 ° 
2 
4 i 
3 ° 
42 
20 
3 o 
0-033622 
-f 0-000475 
!'5 
28 15 40 
18 
24 
0 
4 
7 
0 
40 
55 
0 
0-033035 
— 0-000 I 12 
2 
z 5 34 20 
l6 
43 
30 
5 
47 
30 
39 
14 
3 ° 
0-033327 
+ o-ooo 1 80 
2 5 
22 46 20 
•4 57 
*5 
7 
33 
45 
37 
28 
15 
0-033148 
+ 0-00000 I 
3 
l 9 35 4 ° 
12 
55 
10 
9 
35 
50 
35 
26 
10 
0033058 
— 0-000089 
3 5 
15 48 40 
10 
27 
5 ° 
12 
3 
10 
32 58 50 
0-033026 
— 0-000121 
4 
10 48 50 
7 
1 1 
10 
!5 
19 50 
29 42 
10 
0-033010 
— 0-000137 
I 
0-033147 
* Recherches sur les Mouvemens des Molecules de la lumiere, &c. p. 482. He 
takes it equal to that of glass —' “ ce qui ne doit pas etre fort eloigne de la verite.” I 
have attempted, without success, to measure its value. What has satisfied M. Biot 
and Dr. Brewster (for the latter has evidently used this index, or one very near it, 
Phil. Trans. 1818, p. 230) ought to satisfy every one: yet it is fortunate that in 
the present instance, a slight variation in the refractive index will produce but a very 
trifling change in the relative values of h. 
