21 9 
Dr. Brewster on the laws of polarisation , &c. 
electricity present, we may explain the phenomena of both 
classes, by supposing each crystal to be endowed both with 
positive and negative axes. 
In marking, therefore, this difference of action, I have em- 
ployed the terms positive and negative , as denoting merely 
the opposition , and not the nature of the polarising forces. 
Hitherto we have considered the system of coloured rings 
as produced only near the axes of crystals, or as capable of 
being developed at any distance from the axis, merely by 
diminishing the size of the sphere ; but there are two modes 
by which the rings can be rendered visible at any distance 
from the axis, and with any thickness of crystals. 
If we take a prism of flint glass, with a large refracting 
angle, and examine through it the system of rings, we shall 
find that instead of nine or ten rings, which are visible with- 
out the prism, we may reckon, by estimation, from 80 to 100 
on that side of the axis towards which the refraction is made. 
This observation is analogous to that of Newton upon the 
rings formed by thin plates, and decidedly proves that the 
coloured tints are actually produced at distances from the 
axis where the phenomena of fixed polarisation are exhibited. 
The other mode of developing the rings at any distance 
from the axis, and with any thickness of crystal, consists in 
crossing the tints with plates of rock crystal cut parallel to its 
axis of double refraction, or with laminas of sulphate of lime. 
If the axis AB, PI. XV. fig.i, of a plate of rock crystal, having 
such a thickness as to produce no coloured tints, even at a con- 
siderable obliquity, is placed in the direction of a diameter of 
the rings given by calcareous spar, a new system of rings 
will be produced at A and B, having their centres at O. The 
