232 Dr. Brewster on the laws of polarisation, &c. 
that various systems may be proposed which are equally 
consistent with the observed tints. The number and position 
of the axes of mica, the rectangularity of the axes, and the 
nature of the forces which emanate from them, as given by 
M. Biot, are therefore entirely hypothetical results. 
The phenomena of mica may be explained b) the follow- 
ing hypothetical arrangements of the axes. 
1. Rectangular axes. 
1st, By two inequal negative axes , O 0, AB, PL XV. fig. 4, 
whose relative intensities are as 100 to 677. 
2d, By a negative axis O 0, and a positive axis CD, whose 
intensities are as 100 to 173. 
3d, By two inequal positive axes , CD, AB, whose intensities 
are as 100 to 117. 
2. Oblique axes. 
4th, By two equal positive axes , RS, TV, inclined to each 
other at an angle of 85° 26' and 
5th, By two equal negative axes X, Z, inclined to each other 
at an angle of 42 0 2'. 
Hitherto we have supposed that the axes from which the 
forces emanate, are only two in number; but it will appear 
from a subsequent part of this paper, that any one axis may 
be resolved into any even or odd number of axes, by which 
the phenomena may be explained. All that we know, in 
short, is that a certain polarising force is exerted at a parti- 
cular point, but we have no means of ascertaining either the 
number or the direction of the forces of which it is the re- 
sultant. Certain physical circumstances, however, which 
