258 Dr . Brewster on the laws of polarisation, &c. 
through the axes. If the third axis is rectangular to a, and |8, 
or placed at A, the effect of it would be to throw the pole G 
farther from A towards F ; but as it is not probable that the 
deviation of « and /3 would be in a plane exactly at right 
angles to A, while A kept its situation ; then if A deviates 
from A, or if while A retains its position, the deviations of 
a. and j 3 are not in the plane BC, an irregular system of 
curves will be produced, differing essentially from those which 
are formed by two axes. 
Here then we have three leading causes of the destruction 
of the equilibrium of the three axes. In consequence of the 
first, tints of a single structure will be developed, as in some 
specimens of diamond and alum. In consequence of the 
second, the opposite tints of a double structure will be de- 
veloped, as in several specimens of diamond, fluor spar 
muriate of soda, semiopal, and analcime; and in consequence of 
the third, a confused system of rings will be exhibited, such 
as I have discovered in several specimens of leucite. 
The coincidence of the preceding deductions, with the ex- 
perimental results, affords a strong presumption, that we have 
been tracing the actual operations of nature. In the phe- 
nomena of crystals both with one and with two axes, we 
have seen that these axes are coincident with some permanent 
line in the primitive or secondary form of the crystal. This 
is the case in every crystal with one axis that I have ex- 
amined ; but it is not easy to establish this coincidence in 
crystals with two axes, for it is impossible to fix the situation 
of the axes by which the phenomena are produced. In almost 
every case, however, the most probable position of the axes is 
