Dr Brewster on the Phenomena of Apeyphyllite. 143 
but that it acts much more energetically upon the Red extremi- 
ty, and much less energetically upon the Blue extremity of the 
spectrum. The yellow rays being thus solicited by equal and 
opposite forces, the crystal will exercise over them no polarizing 
energy. Inhered rays being subjected to a greater polarizing ener- 
gy from the Negative than from the Positive axis, will give rings 
corresponding to the difference of their opposite actions, and the 
characters of these rings will be Negative. The blue rays, on 
the contrary, being much less energetically acted upon by the 
Negative than by the Positive axis, will form rings proportional 
to the difference of their actions, and these rings will be Positive, 
from the predominating influence of the positive axis. In this 
way, a particular crystal of Apophyllite may exercise over the 
red rays of the polarized beam a negative influence ; over the 
blue rays a positive influence ; and over the yellow rays no in- 
fluence at all * ; while it is the general character of the mineral 
to exert an attractive doubly refracting force over all the rays 
of the spectrum. The ray of compensation, in place of being 
yellow, may have any position in the spectrum, and those on 
each side of it will afford positive or negative tints, according 
as the positive or the negative axis exercises over them a predo- 
minating influence. 
‘‘ This view of the polarising structure of Apophyllite, affords 
a complete explanation of the singular tints which surround its 
resultant axis. Each order of colours is as it were a residual 
spectrum*}*, arising from the opposite actions of the negative 
and the positive axis, and the tints of which these orders are 
composed, will consequently vary, according to the locality of 
the ray of compensation.” 
• This partial equilibrium of polarising forces is analogous to the paradoxical! 
phenomena of a compound lens, which, as I have elsewhere shewn, may be con= 
structed so as to converge the Blue rays, diverge the Red rays, and exercise no action 
at all upon the Yellow ones. That is, the same compound lens is a Plane lens in 
T/ellow light, a convex one in 6lue light, and a concave one in red light. 
•f* Among the various residual or secondary spectra which I have examined in 
the course of my experiments “ On the Action of Transparent Bodies upon the 
differently coloured Rays of Light,” there are many among the polarised rings 
which have exactly the same tint ; and there are some which resemble as nearly as 
possible those in Apophyllite. — See Edinburgh Transactions, voK viii. p. 1. 
