84 REPORT ON PHYSICAL OPTICS. 



received without much discussion ; and even to this hour the- 

 opinion of the mathematical world is not entirely at rest upon the 

 subject. In a memoir on the propagation of motion in elastic 

 fluids, read before the Academy of Sciences in the year 1823, 

 M. Poisson arrived at the conclusion that the vibratory motions of 

 the particles finally become normal to the wave, whatever be the 

 direction of the original disturbance * To this Fresnel replied 

 that the equations of motion of elastic fluids employed by M. 

 Poisson are but a mathematical abstraction, which do not apply to 

 anything actually existing ; that, in fact, these fluids are assumed 

 to be composed of contiguous elements, capable of compression in a 

 degree proportionate to the pressure exerted ; that this hypothesis 

 is untrue ; and that, although it may accord with the statical pro- 

 perties of these fluids, it can never lead to the discovery of their 

 dynamical laws.f 



M. Poisson seems to have felt the full force of this objection ; 

 for in his memoirs on the same subject, read to the Academy in 

 the years 1828 and 1830, he has resumed the whole theory, and 

 reared it upon its firmer basis. In the former of these memoirs he 

 has formed the differential equations of equilibrium and motion of 

 elastic bodies, these bodies being supposed to consist of molecules 

 attracting or repelling one another according to some function of 

 the distance.^: In the latter he proceeds to integrate these equations 

 generally, and to deduce the laws of propagation of waves at a consi- 

 derable distance from the origin of disturbance. In the case of fluids 

 he arrives at the conclusion which he had before obtained, namely, 

 that when the distance from the origin of disturbance is very great 

 compared with the length of a wave, the motion of the particles, in 

 any fluid, is normal to the surface of the wave, whatever be the 

 initial motions. He admits, however, that the fundamental equa- 

 tions of the motion of fluids, and therefore also the consequences 

 deduced from them, will probably require modification in the case 

 of very rapid motions, such as those of the luminif erous ether ; 

 there being a finite interval of time, whose magnitude depends on 



* Annales de Chtmie, torn. xxii. 

 t Ibid., torn xxiii. 



J "Memoire sur 1'Equilibre et le Mouvement des Corps Elastiques." J/e'. Insf.,. 

 torn. viii. 



$ " Memoire sur la Propagation du Mouvement dans les Milieux' Elastiques." 

 M<im. In*t., torn. x. 



