X. On the action of the second surfaces of transparent plates upon light. 
By David Brewster, LL.D. F.R.S. Lond. 8$ Edin. 
Read February 25 , 1830. 
In - a paper on the Polarization of Light by Reflexion, published in the Philo- 
sophical Transactions for 1815, I showed that the Law of the Tangents was 
rigorously true for the second surfaces of transparent bodies, provided that the 
sine of the angle of incidence was less than the reciprocal of the index of 
refraction. The action of the second surfaces of plates at angles of incidence 
different from the maximum polarizing angle, was studied by M. Arago, who 
conducted his experiments in the following manner. 
“ With respect to this phsenomenon,” says M. Arago, “ a remarkable result 
of experiment may here be noticed ; that is, that in every possible inclination 
A = A' * 
Let us suppose that a plate of glass E D 
(Fig. 1.) is placed in the position that the figure 
represents before a medium A B of a uniform tint ; 
for instance, a sheet of fine white paper. The eye 
placed at O, will receive simultaneously the ray 
I O reflected at I, and the ray BIO transmitted 
at the same point. Place at m n an opaque dia- 
phragm blackened, and perforated by a small hole at S. Lastly, let the eye 
be furnished with a doubly refracting crystal C, which affords two images of 
the aperture. 
“ If now, by means of a little black screen placed between B and I, we 
stop the ray BI which would have been transmitted, the crystal properly 
placed will give an ordinary image = A -j- | B, and an extraordinary image 
* A is the light polarized by reflexion, and A' that polarized by refraction. 
Fig. i. 
MDCCCXXX. 
U 
