Maxwell. 291 



of ponderable matter, and that the amount of dispersion 

 depends on the ratio of the wave-length to the distance 

 between adjacent molecules. This hypothesis was, however, 

 seen to be inadequate, when, in 1862, F. P. Leroux* found that 

 a prism filled with the vapour of iodine refracted the red rays 

 to a greater degree than the blue rays; for in all theories 

 which depend on the assumption of a coarse-grained lumini- 

 f erous medium, the refractive index increases with the frequency 

 of the light. 



Leroux's phenomenon, to which the name anomalous dis- 

 persion was given, was shown by later investigators-)- to be 

 generally associated with " surface-colour." i.e., the property of 

 brilliantly reflecting incident light of some particular frequency. 

 Such an association seemed to indicate that the dispersive 

 property of a substance is intimately connected with a certain 

 frequency of vibration which is peculiar to that substance, and 

 which, when it happens to fall within the limits of the visible 

 spectrum, is apparent in the surface-colour. This idea of a 

 frequency of vibration peculiar to each kind of ponderable 

 matter is found in the writings of Stokes as far back as the 

 year 1852 ; when, discussing fluorescence, he remarked: 

 " Nothing seems more natural than to suppose that the incident 

 vibrations of the luminiferous aether produce vibratory move- 

 ments among the ultimate molecules of sensitive substances, 

 and that the molecules in turn, swinging on their own account, 

 produce vibrations in the luminiferous aether, and thus cause 

 the sensation of light. The periodic times of these vibrations 

 depend on the periods in which the molecules are disposed to 

 swing, not upon the periodic time of the incident vibrations." 



The principle here introduced, of considering the molecules 

 as dynamical systems which possess natural free periods, and 

 which interact with the incident vibrations, lies at the basis of 



* Comptes Rendus, Iv (1862), p. 126. In 1870 C. Christiansen (Ann. d. Phys. 

 cxli, p. 479 ; cxliii, p. 250) observed a similar effect in a solution of fuchsin. 



r Especially by Kundt, in a series of papers in the Annalen d. Phys., from 

 vol. cxlii (1871) onwards. 



j Phil. Trans., 1852, p. 463. Stokes's Coll. Papers, iii., p. 267. 



U 2 



