252 
SIR DAVID BREWSTER ON CERTAIN PHENOMENA, ETC. 
resemblance to those of the original spectrum, so that the spectrum has actually been 
analysed by dissection. This effect is so decided, that even by a single subdivision 
of a banded spectrum I have succeeded in insulating a band nearly white, and of 
course incapable of being decomposed by the prism. 
Hence we deduce from the phenomena of thin plates, and polarized tints, the exist- 
ence of a new property of light, in virtue of which the reflecting force selects, as it 
were, out of differently coloured rays of the same refrangibility rays of a particular 
colour, allowing the others to pass into the transmitted beam ; or to use the language 
of the undulatory theory, the colour produced by the interference of homogeneous 
pencils reflected from the first and second surfaces of thin plates, is different from 
the colour produced by the interference of the transmitted light with that which 
has suffered two internal reflexions within the plate. If, for example, we use the 
greenish yellow light of the spectrum between the lines D and E, the system of re- 
flected rings will be more yellow than the transmitted rings towards E, and more 
green than the same rings towards D, a result which in so far as the transmitted 
tints are concerned, is seen in the colours of smalt blue glass. 
Here then we have a principle not provided for in either of the theories of light to 
which the phenomena of absorption, produced by nacrite, by decomposed films of 
glass and by polarizing plates are distinctly referable. Here also we have the pro- 
bable cause of certain remarkable phenomena of dichroism in doubly refracting 
bodies, in which rays of the same refrangibility but of different colours pass into the 
ordinary and extraordinary pencils, 
Allerly by Melrose , 
May 5th, 183 7> 
