1 4(5 Dll. BREWSTER ON THE ACTION OF THE SECOND SURFACES 
= h B. But if the screen were placed between A and I, and the ray A I 
were intercepted, we should still have two images of the hole, and their inten- 
sities would be ^ B' and A' + ^ B' respectively. Consequently, without any 
screen, if the whole of the reflected light A 10, and the transmitted BIO 
are allowed to arrive at the eye, we shall have for the ordinary image 
A -f i B + ^ B', and for the extraordinary image ^ B -f- A' + \ B'. 
“ N ow it appears from actually making the experiment, that the two images 
are perfectly equal, whatever may he the angle formed by the ray A I with the 
plate of glass which can only be because A is always equal to A'. Conse- 
quently 
“ The quantity of polarized light contained in the pencil transmitted by a 
transparent plate is exactly equal, to the quantity of light polarized at right 
angles, which is found in the pencil reflected by the same plate.” 
We have no doubt that M. Arago obtained these results, particularly near 
the polarizing angle, at which limit they are rigorously true ; but at all other 
angles of incidence they are wholly incorrect. When we consider, indeed, the 
nature of the experiment which has been lauded for its elegance and ingenuity, 
we shall see reason to pronounce its results as nothing more than coarse esti- 
mates, in which the apparent equality of the two images is the effect either of 
imperfect observation or of some unrecognized compensation. 
If we make the experiment in the manner F! s- 2 - 
shown in Fig. 2. with a colourless and well 
annealed prism of glass E F D, in place of 
a plate of glass ; and make the ray B I enter 
the surface F D perpendicularly at I, we get 
rid of all sources of error, and we obtain, what 
is really wanted, the result for a single surface. 
In this case the experiment is not disturbed by 
the light reflected from the inner surfaces of 
the prism, which is all thrown off from the pencil 
which enters the eye. A B 
In M. Arago’s form of the experiment, part of the ray B I (Fig. 1.) undergoes 
reflexions within the plate, and there comes along with it to the eye, at O, a 
portion of light polarized in the plane of reflexion : in like manner the part of the 
