202 Dr. Brewster on the Affections of Light 
complement of the angle OCr, at which light is polarised by 
reflection at C.* 
V. On the elliptical coloured rings produced by depolarising 
Crystals, 
In a former work, to which I have already had occasion to 
refer, I have given some account of the colours which accom- 
pany the depolarisation of light, and I have particularly noticed 
the remarkable fact, that when a beam of white depolarised 
light is transmitted through a doubly refracting crystal, the 
red rays go to the formation of one image, while the bluish 
green rays go to the formation of the other image. In re- 
peating and extending these experiments, I have been led into 
a new field of inquiry which has already afforded a series of 
instructive results deduced from a class of phenomena un- 
questionably the most brilliant within the whole range of 
optics. 
The plate of topaz which was used in these experiments, is 
* Since the preceding section was written, I have performed a very extensive series 
of experiments on the depolarisation of light, and have thus been led to a satisfactory 
generalisation of the phenomena. In this theory the phenomena are referred to the 
general principle of polarisation ; such bodies as have neutral and depolarising axes 
are supposed to form two images polarised in an opposite manner, and either pro- 
duced by the same or by different refractive powers ; while those which depolarise 
light in every direction, like gum Arabic, caoutchouc, &c. are composed of films or 
lajers, each of which is a doubly polarising crystal, the neutral and the depolarising 
axes of one film not being coincident with the neutral and depolarising axes of the 
rest. In a separate memoir, which I have drawn up for the consideration of the Royal 
Society, I have given a full account of this theory, of the experiments on which it is 
founded, and of the new views to which it leads respecting the formation and struc- 
ture of organised matter. 
