30. On the Action of the Second Surfaces of 
If we make the experiment in 
the manner shown in Fig. 2 
with a colorless and well anneal- 
ed prism of glass EF'D, in place 
of a plate of glass; and make the 
ray BI enter the surface FD per-_ 
pendicularly at I, we get rid of all 
- sourees 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 x B 
is a Bees off from the pencil which enters the eye. 
. Arago’s form of the experiment, part of the ray BI (Fig. 1.) 
entered 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 pencil AI that enters the plate, under= 
goes partial reflexions, and the part reflected from the first surface 
carries along with it another portion of light polarized 1 in the plane ol of 
reflexion, so that four portions of light polarized in the plane of re- 
flexion reach the eye, while only two portions reach it polarized at — 
right angles to the plane of reflexion, viz. those which are polarized — 4 
: 
ie 
i a 
OST 
by the refraction of each of the surfaces of the plate. Now the part 
of the pencil Al which suffers a first reflexion from each of the sur-_ 
faces of the plate, is, as we shall presently show, defective in polar- 
ized light compared with that which has experienced two refractions, © 
so that it requires the above additional quantities to produce a com- 
pensation with the transmitted pencil BO. If this is not the true” 
cause of the apparent compensation, that is, if M. Arago took means 
to exclude the reflected pencils which seem to have pr the 
compensation, we must then ascribe the equality of the two images . 
to inaccuracy of observation. 
But even if we admit that M. Araco’s experimental results are 
correct with regard to plates, it necessarily follows that they cannot © 
be true with regard to surfaces; for it is obvious from the slightest 
consideration of the subject, that the phenomena of the one cal 
never be interchanigesie With those of the other. 
