260 NOTICES OF THE MEETINGS [ Feb. 18, 
same effect; that is, they do not exhibit any blue stratum when they 
are incident a second time on a solution of quinine. To express the 
modification which the transmitted light had undergone, the further 
nature of which did not at the time appear, Sir John Herschel made 
use of the term ‘‘ epipolized.” 
Sir David Brewster had several years before discovered a re- 
markable phenomenon in an alcoholic solution of the green colouring 
matter of leaves, or, as it is called by chemists, chlorophyll. This 
fluid when of moderate strength and viewed across a moderate thick- 
ness is of a fine emerald green colour; but Sir David Brewster 
found that when a bright pencil of rays, formed by condensing the 
sun’s light by a lens, was admitted into the fluid, the path of the rays 
was marked by a bright beam of a blood red colour.* This singular 
phenomenon he has designated internal dispersion. He supposed it 
to be due to suspended particles which reflected a red light, and 
conceived that it might be imitated by a fluid holding in suspension 
an excessively fine coloured precipitate. A similar phenomenon was 
observed by him in a great many other solutions, and in some solids ; 
and in a paper read before the Royal Society of Edinburgh in 1846 
he has entered fully into the subject.f In consequence of Sir John 
Herschel’s papers, which had just appeared, he was led to examine 
a solution of sulphate of quinine; and he concluded from his ob- 
servations that the ‘‘ epipolic” dispersion of light exhibited by this 
fluid was only a particular instance of internal dispersion, distin- 
guished by the extraordinary rapidity with which the rays capable 
of dispersion were dispersed. 
The Lecturer stated, that, having had his attention called some 
time ago to Sir John Herschel’s papers, he had no sooner repeated 
some of the experiments than he felt an extreme interest in the phe- 
nomenon. The reality of the epipolic analysis of light was at once 
evident from the experiments; and he felt confident that certain 
theoretical views respecting the nature of light had only to be fol- 
lowed fearlessly into their legitimate consequences, in order to explain 
the real nature of epipolized light. 
The exhibition of a richly coloured beam of light in a perfectly 
clear fluid, when the observation is conducted in the manner of Sir 
David Brewster, seemed to point to the dispersions exhibited by the 
solutions of quinine and chlorophyll as one and the same phe- 
nomenon. The latter fluid, as has been already stated, disperses 
light of a blood red colour. When the transmitted light is subjected 
to prismatic analysis, there is found a remarkably intense band of 
absorption in the red, besides certain other absorption bands, of less 
intensity, in other parts of the spectrum. Nothing at first seemed 
more likely than that, in consequence of some action of the ultimate 
molecules of the medium, the incident rays belonging to the absorp- 
* Edinburgh Phil. Trans. Vol. XII. p. 542. 
+ Vol. XVI. Part 2, and Phil. Mag. June, 1848. 
eA 
