549 



mixtures of aqueous vapour with any of the gases, according to 

 which, each gaseous body is uniformly diffused throughout the 

 whole space, its particles repelling those of its own kind, but exert- 

 ing no pressure on the particles of any other kind. He considers 

 the fact that a given portion of air has its volume expanded by the 

 addition of aqueous vapour, as being of itself a sufficient refutation 

 of that theory. The author then takes occasion to discuss the ques- 

 tion, whether aqueous vapour exists in the atmosphere in the state 

 of mechanical mixture or of chemical solution, and argues in favour 

 of the latter view of the subject. 



In the concluding section, the author enters at large into the in- 

 vestigation of the method of ascertaining heights by barometric ob- 

 servations, and gives various tables to be used for that purpose. 



April 3, 1845. 



The MARQUIS OF NORTHAMPTON, President, in the Chair. 



'Ajucjo^wra, No. 2. " On the Epipolic Dispersion of Light ; being 

 a Supplement to a paper entitled ' On a case of Superficial Colour 

 presented by a Homogeneous Liquid internally colourless.' " By Sir 

 John Frederick William Herschel, Bart., F.R.S. &c. 



The author inquires whether the peculiar coloured dispersion of 

 white light intromitted into a solution of sulphate of quinine, is the 

 result of an analysis of the incident light into two distinct species, 

 or merely of a simple subdivision analogous to that which takes 

 place in partial reflexion, as exemplified in the colours of thin plates. 

 He endeavours to ascertain the laws which regulate this singular 

 mode of dispersion, which for brevity he terms epipolic, on account 

 of the proximity of the seat of dispersion to the intromitting surface 

 of the fluid. It might have been expected that by passiiig the same 

 incident beam successively through many such dispersive surfaces, 

 the whole of the blue rays would at length be separated from it, and 

 an orange, or red residual beam be left : but the author establishes, 

 by numerous experiments, the general fact, that an epipolical beam 

 of light, meaning thereby a beam which has been once transmitted 

 through a quiniferous solution, and undergone its dispersing action, 

 is incapable of farther undergoing epipolic dispersion. 



There were only two liquids, out of all those examined by the 

 author, namely oil of turpentine and pyroxylic spirit, which, when 

 interposed in the incident beam, act like the solutions of quinine in 

 preventing the formation of the blue film : and the only solid in 

 which the author discovered a similar power of epipolic dispersion, 

 is the green fluor of Alston Moor, and which by this action exhibits 

 at its surface a fine deep blue colour. 



