132 The Luminiferous Medium, 



but, as Stokes observed*: "If we reflect on the state of the 

 subject as Fresnel found it, and as he left it, the wonder is, not 

 that he failed to give a rigorous dynamical theory, but that a 

 single mind was capable of effecting so much." 



In a second supplement to his first memoir on Double 

 Eefraction, presented to the Academy on November 26th, 1821,-]- 

 Fresnel indicated the lines on which his theory might be 

 extended so as to take account of dispersion. " The molecular 

 groups, or the particles of bodies," he wrote, " may be separated 

 by intervals which, though small, are certainly not altogether 

 insensible relatively to the length of a wave." Such a coarse- 

 grainedness of the medium would, as he foresaw, introduce into 

 the equations terms by which dispersion might be explained ; 

 indeed, the theory of dispersion which was afterwards given by 

 Cauchy was actually based on this principle. It seems likely 

 that, towards the close of his life, Fresnel was contemplating a 

 great memoir on dispersion^ which was never completed. 



Fresnel had reason at first to be pleased with the reception of 

 his work on the optics of crystals : for in August, 1822, Laplace 

 spoke highly of it in public ; and when at the end of the year a 

 seat in the Academy became vacant, he was encouraged to hope 

 that the choice would fall on him. In this he was disappointed. . 

 Meanwhile his researches were steadily continued ; and in 

 January, 1823, the very month of his rejection, he presented to- 

 the Academy a theory in which reflexion and refraction] | are 

 referred to the dynamical properties of the luminiferous media. 



*Brit. Assoc. Rep., 1862, p. 254. 



t (Euvres, ii, p. 438. 



J Cf. the biography in (Euvres de Fresnel, i, p. xcvi. 



Writing to Young in the spring of 1823, he says : " Tous ces memoires, 

 que dernierement j'ai pre'sentes coup sur coup a 1'Academie des Sciences, ne 

 m'en ont pas cependant otivert la porte. C'est M. Dulong qui a ete nomine 

 pour remplir la place vacante dans la section de physique. . . Vous voyez, 

 Monsieur, que la theorie des ondulations ne m'a point porte honheur : mais cela 

 ne m'en degoute pas : et je me console de ce malheur en m* occupant d'optique 

 avec une nouvelle ardeur." 



|| The MSS- was for some time believed to be lost, but was ultimately found 

 among the papers of Fourier, and printed in Mem. de 1'Acad. xi (1832), p. 393 : 

 (Euvres, i, p. 767. 



