Dr. Brewster on the laws of polarisation, &c. 267 
the same phenomena by transmitting polarised light through 
any of its diameters.* 
In the very same manner we may suppose the elementary 
spheroid of crystals with two axes to be formed by elliptical 
plates, bent into spheroidal strata, and the spheroid itself may 
be actually constructed by means of spheroidal strata of 
glass. When this is done, it will exhibit all the complicated 
phenomena produced by the simultaneous actions of two 
unequal axes. 
Section VII. On the laws of double refraction in crystals with 
any number of axes. 
From the coincidence which I have invariably observed 
between the axes of polarisation and extraordinary refraction, 
in all crystals that have only one apparent axis, we may 
consider it as an established fact, that the Huygenian law 
represents, within the limits of experimental errors, not only 
the phenomena of calcareous spar, but those of all other 
crystals with a single axis of double refraction. 
But since it is the general character of crystals to have 
more than one axis of polarisation, it becomes interesting to 
ascertain, if they also have two axes of extraordinary refrac- 
tion ; and if they have, to investigate, by direct experiment, 
the general laws of double refraction, for crystals with any 
number of axes. Such an investigation presents difficulties of 
no ordinary kind. The want of a transparent mineral with 
two powerful axes, which, like calcareous spar, could be 
obtained in large pieces, and cut with facility in every direc- 
See Phil . Trans . 1816, pp. 314, 315. 
