204 ; Dr. Brewster on the laws of polarisation, &c. 
only one axis of double refraction from which the polarising* 
force emanates ; that all crystals are divisible into two 
classes, namely, attractive and repulsive ; and that the laws 
investigated for sulphate of lime, abstracting the effects of 
certain secondary forces, are the general laws of polarisation 
for all other crystals.* 
This view of the laws of polarisation, though deduced from 
ingenious investigations, and by methods which exhibit the 
finest talents for physical research, is incompatible with the 
experiments and observations contained in the following pa- 
per. No fewer than seven out of the twelve minerals employed 
by M. Biot, have two or more axes of double refraction. 
Sulphate of lime itself belongs to this number ; and all 
the irregularities of its action, which M. Biot has repre- 
sented by empirical formulae, are the legitimate and calcu- 
lable results of two rectangular axes. The division of 
crystals into attractive and repulsive, and the reference of 
the phenomena of mica to two repulsive axes, whose inten- 
sities are as 100 to 677, will also be found to be entirely 
hypothetical. 
From this slight sketch of the present state of the science, 
it will be readily seen that the process of generalisation, 
however ingeniously it has been managed, has been carried 
on too rapidly, and has far outrun the progress of observa- 
tion and experiment. In attempting, therefore, to establish 
new and more general laws, I began my researches by laying 
a broad foundation of facts. One hundred and sixty-five 
crystals were subjected to examination. In 165 of these, I have 
observed the property of double refraction. In about 100, I 
* See Biot’s Traitd de Physique, tom. iv. p. 377. 
