6 7 
crystallized bodies on homogeneous light. 
According to the theory of the polarised rings if extended 
to crystals with two axes, the number of periods performed 
in a given space (= 1 ) by a molecule of a given colour, 
transmitted in a direction making angles 9,9' with the axes, 
can only be a function of the form k. $ (9,9'),k depending 
on the intensity of the polarising force ; or as before, being a 
function of c, the nature of the ray, and of the intrinsic energy 
of the molecules of the crystal. Now if we call t the thick- 
ness of the plate, and $ the angle of refraction, is the 
length of the path described, and therefore we must have for 
the number of periods 
B 4, (M') ; 
so that the value of M must be which must be a 
COS (p ’ 
function of c. Now t is obviously independent of it ; and if 
we neglect at present the very trifling effect at moderate in- 
cidences of the ordinary dispersive powers of the media ex- 
amined,* <p is so also. It is therefore in the form of the func- 
tion -J/ (9, 9') that we must look for the cause of the pheno- 
mena ; and since, we have 9' = 9 -j- 2 a, 2 a being the angle 
between the axes (because the observations are made in the 
principal section) we see that \J/ (0, 9 2 a) must involve c, 
and consequently, 9 being arbitrary and independent, a must 
be a function of c. In order then to render the theory of 
alternations applicable, we must admit the angle between the 
* It is easy to see that in the two classes of crystals above described, the effects 
of the dispersive powers will be opposite to each other, in one opposing, and in the 
other conspiring with the causes which produce the deviation of tints. In the 
tables. Nos. V, VI, VII, where the virtual poles were observed almost at a per- 
pendicular incidence, the influence of the dispersive power is quite insensible. 
