sbo Dr. Brewster on the laws of polarisation, &c. 
exhibited during the propagation of heat along plates of 
glass and other uncrystallized bodies ; and of the method of 
communicating permanently to these substances, all the pro- 
perties of regular crystals. Since that paper was written, 
I have completed a series of new experiments on the distri- 
bution of the polarising force in prepared glass, which led me 
to the determination of the laws by which all the phenomena 
can be calculated. These laws are precisely the same as those 
which we have already deduced in the preceding sections ; 
and there is not one phenomenon belonging to regular crys- 
tals which I have not been able to imitate in glass. 
In these artificial crystals, however, the centre of the 
sphere is infinitely distant, and therefore the sine of the 
angle which the refracted ray makes with the axis in the 
regular crystals, is equivalent in the artificial ones to the 
distance of the ray from the axis, and is replaced by this 
quantity in the new formulae. Thus if T is the tint corres- 
ponding to any given distance D from the axis, then the 
tint t at any other distance, d is t = a formula which 
represents accurately the progress of the tints whether the 
axis is negative or positive. 
If in rectangular plates of glass D represents the distance 
of any of the black fringes, or the lines of no polarisation 
MN,OP, PI. xvi. fig. 13, from the axis ab of the plate, and 
T the maximum central tint ; then if w r e conceive an axis 
situated perpendicular to the plate in every point of the line 
a b , and an axis in the plane of the plate perpendicular to the 
former axis,T will be the tint produced in every part of the plate 
by the axis in its plane when polarised light is transmitted 
through it perpendicularly. But as this tint, whose value is 
