Dr. Brewster on the laws of polarisation , &c. 233 
will afterwards be noticed, (relative principally to the tints, 
and the coincidence between some of the hypothetical axes and 
the principal lines in the crystal,) may lead us to prefer these 
axes to others ; but however plausible the grounds of our 
preference may be, we must take care not to admit these 
hypothetical deductions among the number of demonstrated 
physical truths. 
We have already stated that M. Biot has divided crystals 
into two classes, attractive and repulsive ; and that this di- 
vision is quite hypothetical, even when applied to crystals 
with one axis. To the case of crystals with two axes, the 
classification is wholly inapplicable, for it has been shown, 
that there are no means of ascertaining either the nature, the 
number, or the position of the axes. Still, however, the 
terms positive and negative may, in most cases, be conve- 
niently employed to mark the principal resulting force which 
the crystal exhibits. Thus, if we suppose O, PI. x v. fig. 4, which 
is the middle point between the twb nearest poles of no polari- 
sation to be the position of the principal axis, then mica will 
be negative, because the tints from O to A, from O to B, from 
P to C, and from P' to D, have all a negative character ; and 
topaz will be positive, because the same tints have a positive 
character. In the case of crystals, however, where PP is go 0 , 
even this limited application of the terms negative and posi- 
tive entirely fails, and these crystals cannot be considered 
as belonging to one class more than to another. 
H h 
MDCCCXVIU. 
