24,0 Dr, Brewster on the laws of polarisation, &c. 
axis separately , and whose angle is double of the angle formed 
by the two planes passing through that point of the sphere and 
the respective axes. 
If the crystal has three or more axes, the resulting tint 
produced from any two of them may, in like manner, be 
combined with the third, and this resulting tint with the 
fourth, till the general resultant of all the forces is ob- 
tained. 
If the number of axes with given intensities exceeds two, 
they may be combined by the methods explained in the next 
section, till they are reduced to two axes, with new relative 
intensities ; and the resultant of all the axes will be obtained 
by the calculation of the diagonal of a single parallelogram. 
The law which we have now explained is obviously 
deduced from no empirical data, but is rigorously physical, 
and is founded upon the same principles which regulate the 
combination of all other mechanical forces. The accuracy 
with which it represents the complicated system of tints is 
very wonderful, and cannot fail to recommend it to the recep- 
tion of philosophers as a true law of nature. In establishing 
its conformity with the actual phenomena, I shall not content 
myself with examining it by means of my own experiments. 
I shall submit it to the severe ordeal of M. Biot’s measure- 
ments of the tints of sulphate of lime , taken long before the 
discovery of the law, and which he considered as the result 
of irregular action depending upon imperfect crystallization. 
To those who may desire farther evidence, a still more deci- 
sive trial may be offered ; a trial too, in which the eye itself 
is capable of recognising the perfect identity between the 
observed and calculated results. If we compute all the tints. 
