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IX. On the laws of the polarization of light by refraction. 
LL.D. F.R.S. L. 8s E. 
By David Brewster, 
Read Feb. 25, 1830. 
J-N the autumn of 1813 I announced to the Royal Society the discovery which 
I had then made of the polarization of light by refraction * ; and in the No- 
vember following I communicated an extensive series of experiments which 
established the general law of the phenomena. During the sixteen years which 
have since elapsed, the subject does not seem to have made any progress. 
From experiments indeed stated to have been performed at all angles of inci- 
dence with plates of glass, M. Arago announced that the quantity of light which 
the plate polarized by reflexion at any given angle was equal to the quantity 
polarized by transmission ; but this result, founded upon incorrect observa- 
tion, led to false views, and thus contributed to stop the progress of this 
branch of optics. 
I had shown in 1813, from incontrovertible experiments, that the action of 
each refracting surface in polarizing light, produced a physical change on the 
refracted pencil, and brought it into a state approaching more and more to that 
of complete polarization. But this result, which will be presently demonstrated, 
was opposed as hypothetical by Dr. Young and the French philosophers ; and 
Mr. Herschel has more recently given it as his decision, that of the two con- 
tending opinions, that which was first asserted by Malus, and subsequently 
maintained by Biot, Arago, and Fresnel, is the most probable, — namely, that 
the unpolarized part of the pencil, in place of having suffered any physical 
change, retains the condition of common light. 
I shall now proceed to apply to this subject the same principles which I 
* In this discovery I was anticipated by Malus. 
