396 History of Luminescence 



the fluorspar and quinine sulphate solutions, and red within a chloro- 

 phyll solution, undoubtedly similar phenomena. He used the term 

 " internal dispersion " in order to contrast these effects with the 

 " superficial reflexion " of Herschel. Brewster also noted that the 

 internally dispersed beam became fainter the farther it penetrated 

 into the solution and hence behaved as if absorption were occurring. 

 In fact, that was true, the only difference between Herschel's blue 

 light at the surface and Brewster's blue in the interior was one of 

 absorption. Light absorbed near the surface becomes extinguished 

 and hence more and more invisible in the interior. 



Brewster again referred to the sample of fluorspar in which dif- 

 ferent layers emitted different colored thermoluminescences on heat- 

 ing, and showed corresponding colors when traversed by a beam of 

 light, and pointed out that aesculin solutions showed blue disper- 

 sion and uranium glass gave a green " dispersion." He had pre- 

 viously (1815) found that light reflected from a surface at a certain 

 angle is polarized. There was some polarization of the whitish beam 

 of light in quinine and aesculin solutions, but the blue light was 

 unpolarized, and the green beam in uranium glass was also unpo- 

 larized. He concluded that the small amount of polarization must 

 have come from scattering, i. e., from reflection by colloidal par- 

 ticles ^° in the solutions, since such scattered light is always polarized. 

 Althousfh these observations were not in accord with the view that 

 the absorbed and dispersed light were of the same wave-length, 

 nevertheless Brewster still had this idea in mind rather than an 

 emission of light from within the liquid. 



The subject was reconsidered four years later by George Gabriel 

 Stokes (1820-1903) , professor of mathematics at Cambridge, a physi- 

 cist and a recipient of many honors during his life. Stokes' (1852) 

 paper in the Phil. Trans, was one hundred pages in length, entitled 

 " On the Change of Refiangibility of Light," and was followed by a 

 postscript in 1853.^^ It is one of the most thorough in the history 



*" Later observers (Lallemand, 1869, 1870), who claimed polarized fluorescence in 

 solution, were led astray by polarization from reflection; see Soret (1869) . 



^^ It should be noted that A. J. Angstrom published similar views in his " Optische 

 Untersuchungen " in the Swedish Academy Transactions for February 16, 1853, a paper 

 dealing with dispersion, absorption, and diffusion of light in relation to vibration of 

 molecules, written in Swedish. He later submitted the article to the Annalen der 

 Physik for 1855, which was translated in the Philosophical Magazine (1855: 341) and 

 contains the following statement by Angstrom: " I have only recently had an oppor- 

 tunity of becoming fully acquainted with the interesting memoir of Stokes ' On the 

 change of refrangibility of light,' I see with satisfaction that Stokes' explanation of 

 the remarkable phenomena of dispersion in the green colours of plants, in sulphate 

 of quinine, and in an infusion of horse-chestnut bark, namely, that the medium, 

 when illuminated by the sun, becomes itself luminous, is exactly the same as that 

 which I have given in the foregoing pages of the same phenomenon." 



