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XVII. On 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. # E. 
Read February 12, 1829. 
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 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 generality of our assumptions, and tend to establish a relation be- 
tween the attractive and repulsive forces 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 the 
president of the Physical Class of the Royal Society 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 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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