30 



On the Action of the Second Surfaces of 



If we make the experiment in Fig. 2. 



the manner shown in Fig. 2. 

 with a colorless and well anneal- 

 ed prism of glass EFD, 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 

 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. 



In M. Arago's form of the experiment, part of the ray BI (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 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 in the plane 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 

 by the refraction of each of the surfaces of the plate. Now the part 

 of the pencil AI 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 produced the 

 compensation, we must then ascribe the equality of the two images 

 to inaccuracy of observation. 



But even if we admit that M. Arago'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 phsenomena of the one can 

 never be interchangeable with those of the other. 



