I 



Dr Brewster on reflected light 209 



Art. III. — On the reflection and decomposition of light at the 

 separating surfaces of media of the same and of different re- 

 fi-active pozvers^. By David Brewster, LL. D. F. R. S. 

 L. & E. 



It is a necessary result of the Newtonian theory of light, and 

 one which Newton himself deduced, that when white light is 

 incident on the separating surfaces of different media, it pre- 

 serves its whiteness after reflection, excepting in those cases 

 where the thickness of one of the media is beneath the 80 mil- 

 lionth part of an inch. 



When the discovery of the different dispersive powers of 

 bodies was made, it should have been obvious that reflected 

 light never could be perfectly white under any circumstances, 

 though such a modification was not likely to be detected in 

 the usual routine of optical experiments. The only philoso- 

 pher indeed who, in as far as I know, has made any experi- 

 ments on the subject is Mr Herschel ; and as his opinions 

 may be considered as representing those of the present period, 

 I shall make no apology for quoting them. 



" The phenomena which take place when light is reflected 

 at the common surface of two media are such as from the 

 above theory we might be led to expect, with the addition, 

 however, of some circumstances, which lead us to limit the ge- 

 nerality of our assumptions, and tend to establish a relation 

 between the attractive and repulsive forces to which the refrac- 

 tion and reflection of light are supposed to be owing. For it 

 is found that when two media are placed in perfect contact, 

 (such as that of a fluid with a solid, or of two fluids with one 

 another,) the intensity of reflection at their common surface 



* The principal experiments contained in this paper were made in 1816, 

 and were signed by the president of the Physical Class of the Royal Socie- 

 ty of Edinburgh. A brief notice of them was published in the Quarterly 

 Journal for July — October 1816, and a more extended paper was read at 

 the Royal Society of Edinburgh on the 4th of January 1819. The diffi- 

 culties of the subject, however, prevented me from pursuing it but at dis- 

 tant intervals ; and the more fertile topic of polarisation afterwards re- 

 quired all the time I could devote to such inquiries. 



NEW SERIES. VOL. I. NO. II. OCT. 1829- O 



