Transparent Plates upon Light. 



29 



"Let us suppose that a plate of '^' 



glass ED (Fig. 1.) is placed in the 

 position that the figure represents 

 before a medium AB of a uniform 

 lint ; 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 A B 



transmitted at the same point. Place at m n 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 aifords 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-|-^B, and an 

 extraordinary image = JB. 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 ^W and A'+JB' 

 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+^B-f-^B', and 

 for the extraordinary image ^B-[-A'+ JB'. 



" Now it appears, from actually making the experiment, that the 

 two images are perfectly equal, whatever may be the angle formed by 

 the ray AI ivith 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 angles, which is found in the pencil reflected by 

 the same plate." 



We have no doubt that M. Arago obtained these results, particu- 

 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 lauded 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. . 



