yn/iTOT nr^fHT^-c-; -« [ 187 ] 



XVII. Oil the reflection and decomposition of light at the separating surfaces of 

 media of the same and of different refractive powers*. By David Brewster, 

 L.L.D. F.R.S. L. 8s E. 



Read February 12, 1829. 



It is a necessary result of the Newtonian theory of lightj'^and one which 

 Newton himself deduced, that when white light is incident on the separating 

 surfaces of different media, it preserves 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 philosopher 

 indeed who, in as far as I know, has made any experiments 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 genei-ality of our assumptions, and tend to establish a relation be- 

 tween the attractive and repulsive foi-ces to which the refraction 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 



* The principal experiments contained in this paper were made in 1816, and were signed by thie 

 president of the Physical Class of the Royal Society of Edinburgh. A brief notice of them was 

 pubhshed in the Quarterly Journal for July — October, 181C, and a more extended paper was read 

 at the Royal Society of Edinburgh on the 4th of January 1819. The difficulties of the subject, however, 

 prevented me from pursuing it but at distant intervals ; and the more fertile topic of polarisation 

 afterwards required all the time I could devote to such inquiries. 



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