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Monday, 2d February 1846. 

 Sir T. M. BRISBANE, Bart., President, in the Chair. 



The following Communications were read : — 



1. On the Decomposition and Dispersion of Light within 

 Solid Bodies. By Sir David Brewster, K.H. 



After noticing various cases of solid and liquid bodies, which dis- 

 perse light of a colour quite different from the transmitted tint, and 

 Sir John Herschel's explanation of the phenomenon in the solution 

 of acid sulphate of quinine, which, although itself colourless, dis- 

 perses light of a bright blue, the author considers — 



1. The internal dispersion of fluor-spar. By employing a more 

 intense light than Sir J. Herschel did — namely, a condensed beam 

 of the sun's light — he conceives that he has proved that the disper- 

 sion is not, as Sir J. Herschel supposes, superficial or epipolic, but, 

 on the contrary, belongs, in some specimens at least, to every part 

 of the crystal. The phenomenon is only seen in the green fluor of 

 Alston, and in some pink and bluish-yellow varieties from Derby- 

 shire. Some specimens are formed of strata alternately dispersing 

 and non- dispersing. The author has not found the same appear- 

 ances in any other mineral ; but he has observed similar pheno- 

 mena of dispersion in several kinds of glass, and even in some co- 

 lourless glasses, which disperse a fine green tint. 



2. On the internal dispersion of the acid solution of sulphate of qui- 

 nine. Here, also, by using a more intense hght than Sir J. Herschel, 

 the author establishes that the peculiar dispersion occurs, not only 

 in a strattim l-50th of an inch thick at the surface of the liquid, 

 but at all parts of it ; and he concludes —First, That a beam of 

 light, epipolised by the action of a solid or a liquid, is capable of 

 further dispersion, provided the thickness of the medium has not 

 been sufficient to disperse all the dispersible rays. Secondly, \Mieu 

 such a medium is thus rendered incapable of dispersing moi'e light, it 

 is not, as Sir J. Herschel supposes, because the light has lost a pi'o- 

 perty which it previously possessed, but because it has been deprived 

 of all the dispersible rays it contained. 



The rays thus dispersed are few in number, and, by their mixture, 

 yield blue light, but they extend over a great range of refi"angibility ; 

 while other rays, equally refrangible, are either less dispersible, or not 

 dispersible. But this appears less surprising when we advert to the 

 phenomena of absorption. 



