Dr, Brewster on the laws of polarisation, &c, 231 
In examining the preceding table, we cannot fail to be 
struck with the uniform distribution of the angles over the 
quadrant ; a circumstance which contributes to render this 
new property of crystals a most distinct and valuable phy- 
sical character in mineralogy. 
Having thus determined the angles of the resultant axes, 
we must now proceed to ascertain the position, the number, 
and the relative intensities of the real axes, by the combination 
of whose action the various tints are produced ; and in doing 
this we shall take mica as an example. 
M. Biot has announced it as a demonstrated physical 
truth, that mica has two repulsive axes, one in the plane of 
the laminae, and the other perpendicular to the laminae ; and 
that the polarising force of the first is to that of the second 
as 100 to 677.* As the experiments from which he deduced 
this system of forces, consist of observations on thetints, merely 
in the two rectangular directions,. AOB, COD, PL. XV. fig. 4, 
we cannot admit them as any thing approaching to evidence 
for the conclusion which he has drawn ; for it is possible that 
the tints developed in the four quadrants, AGO, ADO, CBO, 
DBO, may be quite incompatible with such an arrangement- 
I have observed the tints produced by mica in almost every 
part of the four quadrants ; and it follows from the general 
law of polarisation which I have discovered, that there is no 
particular system of forces indicated by the phenomena ; but 
* “ Tous ces resultats etant exactement conformes a ce qu’on observe, il est bien 
vraisemblable que le systeme des forces polarisantes du mica est ainsi combine ; mais 
pour changer cette presomption en certitude, il fiut exprimer par le calcul les effets 
d’un pareil systeme, et voir si la marche des teintes, observees sous chaque incidence, 
s’y confonxie exactement.” Biot Traite de physique , tom. iv. p. 557. 
