Transparent Plates upon Light. 29° 
ye Bigs 2.2 
“Let us suppose that a plate of 
glass ED (Fig. 1.) is placed in the "ze 
position that the figure represents 
before a medium AB of a uniform 
tint; for instance, a sheet of fine 
white paper. The eye placed at O, 
will receive simultaneously the ray 
10 reflected at I, and the ray BIO 
transmitted at the same point. Place at mn an opaque diaphragm _ 
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 crys- — 
tal properly placed will give an ordinary image =A-+3B, and an 
extraordinary image =4B. But if the screen were placed between 
A and I, and the ray AI were intercepted, we should still have two 
images of the hole, and their intensities would be 4B’ and A’+3B’ 
respectively. Consequently, without any screen, if the whole of the 
reflected light AIO, and the transmitted BIO, are allowed to arrive 
at the eye, we shall have for the ordinary image A+4B-+4B,, and 
-for the extraordinary image $B+ A’+4B% 
‘“‘ Now it appears, from actually making the experiment, that the 
two images are perfectly equal, whatever may be the angle formed by 
the ray- Al with the plate of glass, which can only be because A is 
always equal to A’. Consequently, 
“The quantity of polarized light contained in the pencil transmit- 
ted by a transparent plate, is exactly equal to the quantity of light 
polarized | at. right mee, which is found in the pencil saben by 
the same — 
We have no doubt that M. Arago obtained these icles partion. 
larly 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 Jauded for its elegance and ingenuity, we shall see reason to 
pronounce its results as nothing more than coarse estimates, in which 
the apparent equality of the two images is the effect either of im- 
perfect observation or of some unrecognized compensation. 
“ 
