244 Dr. Brewster on the laws of polarisation, &c. 
the laminae as the line which M. Biot calls the axis,* when the 
other axis is perpendicular to the plane of the laminae. These 
positions are indicated in PL xv. fig. 4, by O and A, and in 
fig- 9 > by C and A. Since GC, therefore, or half the distance of 
the resultant axis is 30°, we shall have, the relative intensity of 
the axis C, to that of A, as c . Att- : or as 4 to 1. With these 
Sin. OC. ^ 
data the following table has been calculated. 
Angle 
of re- 
fraction. 
Angulardis- 
tance from 
the axis. 
Azimuth 
of o°. 
Azimuth 
of 22 0 30'. 
Azimuth of 
the resultant 
axis, 30°. 
Azimuth 
of 45° 
Azimuth 
of 67° 30' 
Azimuth | 
of 90 9 . j 
o° 
go Q 
1,0000 
1/0000 
1,0000 
1,0000 
1,0000 
1,0000 
10 
80 
°>9775 
0,9796 
0,9811 
0,9848 
0,9903 
0,9925 
20 
70 
0,9124 
0,9202 
0,9258 
0,9401 
0,9614 
0,9707 
3° 
60 
0,8125 
0,8272 
0,8385 
0,8660 
0,9160 
o,9375 
40 
S° 
0,6901 
0,7093 
0,7254 
0,7729 
0,8573 ■ 
0,8967 
5° 
40 
0,5600 
O.S75S 
0,5936 
0,6593 
0,7901 
0,8533 
60 
3° 
°»437S 
0,4356 
0,4507 
0,5339 
0,7221. 
0,8125 
70 
20 
0.3378 
0,2980 
0,3018 
0,4071 
0,6615 
0,7792 
80 
10 
0,2726 
0,1238 
0,1511 
0,2983 
0,6190 
o,7575 
90 
0 
0,2500 
0,1035 
0,0000 
0,2500 
0,6036 
0,7500 
The agreement between the numbers in this and the 
preceding table, will convey an idea of the accuracy with 
which the law accommodates itself to all the capricious 
changes of the tints. I have compared it also with numerous 
experiments made on nitre , mica, topaz, sulphate of iron, &c. 
embracing all the varieties in the inclination of the resultant 
axes, and have found it equally accurate in all these crystals. 
The projection of the calculated results, indeed, compared 
with the system of rings where their curvature is the most 
variable, may be considered as an ocular demonstration of the 
correctness of the law. 
* It is a curious fact, that sulphate of lime is the only crystal, out of a great 
number, in which the principal axis does not coincide with any of the prominent 
lines in its primitive form, as ascertained by Hau y. Does not this prove incontestibly 
that its primitive form is at present undetermined ? 
