NATURAL PHILOSOPHY. 173 



ON THE CHANGE OF REFRANGIBILITY OF LIGHT, AND THE 

 EXHIBITION THEREBY OF THE CHEMICAL RAYS. 



The following is an abstract of a lecture recently delivered before 

 the Royal Institution, England, by Prof. Stokes. When a weak acid 

 solution of quinine is prepared, by dissolving, suppose one part of the 

 commercial disulphate in 200 parts of water acidulated with sulphuric 

 acid, a fluid is obtained which appears colorless and transparent when 

 viewed by transmitted light, but which exhibits nevertheless in certain 

 aspects a peculiar sky-blue color. This color of course had frequently 

 been noticed ; but it is to Sir John Herschel that we owe the first 

 analysis of the phenomenon. He found that the blue light emanates 

 in all directions from a very thin stratum of fluid adjacent to the sur- 

 face (whether it be the free surface or the surface of contact of the 

 fluid with the containing glass vessel,) by which the incident rays 

 enter the fluid. His experiments clearly show that what here takes 

 place is not a mere subdivision of light into a portion which is dis- 

 persed and a portion which passes on, but an actual analysis. For 

 after the rays have once passed through the stratum from which the 

 blue dispersed light comes, they are deprived of the power of produc- 

 ing the 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 remarkable phenomenon in an alcoholic solution 

 of the green coloring matter of leaves, or, as it is called by chemists, 

 chlorophyll. This fluid, when of moderate strength and viewed across 

 a moderate thickness, is of a fine emerald green color ; but Sir David 

 Brewster found that when a bright pencil of rays, formed by condens- 

 ing 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 color. This 

 singular phenomenon he has designated internal dispersion. He sup- 

 posed it to be due to suspended particles which reflected a red light, 

 and conceived that it might be imitated by a fluid holding in suspen- 

 sion an excessively fine colored precipitate. Prof. Stokes stated, that, 

 having had his attention called some time ago to Sir J. Herschel's papers, 

 he had no sooner repeated some of the experiments than he felt an 

 extreme interest in the phenomenon. 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 followed fearlessly into their legitimate conse- 

 quences, in order to explain the real nature of epipolized light. The 

 exhibition of a richly colored beam of light in a perfectly clear fluid, 

 when the observation is conducted in the manner of Sir David Brew- 

 ster, seemed to point to the dispersions exhibited by the solutions of 

 quinine and chlorophyll as one and the same phenomenon. The 

 latter fluid, as has been already stated, disperses light of a blood red 



