Dr. Brewster on the laws of polarisation, &c. 235 
do not at all represent the actual phenomena, for M. Biot 
never made a single observation on the tints in the direction 
of the laminae, the only direction in which they could be in- 
vestigated below 30°, from the supposed pole. Had the ex- 
periments been made in this way, he would have found that 
all the observations, instead of being owing, as he supposes, 
to secondary forces arising from the inequal superposition of 
the laminae, are the legitimate results of two axes of double 
refraction. The experiments of this philosopher upon sul- 
phate of lime must therefore be set aside as incompetent to 
determine the accuracy of Sin. 2 p, as the expression of the 
tints in crystals with one axis. 
As rock crystal possesses secondary forces which interfere 
with the action of the principal axis, and as there is reason 
for believing that its apparent axis is only the resultant of 
two equal negative axes, the value Sin.*p cannot be deduced 
from the valuation of the tints which it developes. 
The experiments made by M. Biot upon calcareous spar, 
previous to his knowledge of the coloured rings, were better 
fitted to afford an expression of the variation of the tints ; 
but when we consider that his mode of observation was 
such that, when applied to arragonite, it gave Sin. “pas the 
law of the tints, and that this mineral has two axes, and 
cannot therefore have its tints regulated by such a law, we 
are forced to conclude, that this mode of observation is insuf- 
ficient even when applied to calcareous spar. 
Some new method, therefore, of studying the phenomena 
of double refraction and polarisation was wanting, in order 
to determine with certainty, whether any crystal had one or 
more axes ; and what is the law according to which the tints 
vary in crystals with one axis. By the old mode of observation, 
