REPORT OX PHYSICAL OPTICS. 855 



elusion 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 

 equations of the motion of fluids, and therefore also the conse- 

 quences deduced from them, will probably require modification 

 in the case of very rapid motions, such as those of the lumi- 

 niferous ether ; there being a finite interval of time, whose 

 magnitude depends on the nature of the fluid, during which the 

 pressure is not the same in all directions. In the case of very 

 rapid motions this time must be taken into account, and the 

 equations of motion of fluids will no longer be those furnished 

 by the principle of D'Alembert *. 



M. Poisson has shown also that a disturbance produced in a 

 limited portion of a solid hody will give rise to tivo ivaves, 

 which will be propagated with different velocities. He proves 

 further that whatever be the initial motions of the disturbed 

 particles, the vibrations in one of these waves will finally be 

 radial, or in the direction of the motion propagated ; while 

 those of the other are perpendicular to that direction, or trans- 

 versal. The first are attended with dilatations proportionate 

 to the absolute velocities of the molecules, and the waves thus 

 propagated are similar to those which take place in fluids. The 

 transversal vibrations, on the other hand, are unaccompanied 

 by any change of density in the medium. M. Poisson does not 

 seem to think that this result can justify the hypothesis of 

 transversal vibrations in the ethereal fluid ; though he admits 

 that the properties attributed to the ether are in some respects 

 analogous to those of a solid body. 



The propagation of transversal vibrations appears to be now 

 established as a necessary consequence of dynamical principles 

 by the able researches of M. Cauchy f. I shall shortly have 

 occasion to allude more particularly to the important conclu- 

 sions arrived at by this mathematician, on applying the general 

 laws of the propagation of motion in elastic media to the case of 

 light. For the present it will be sufficient to observe that the 

 form of the wave- surface, obtained in the course of these in- 

 vestigations, is a curved surface of three sheets ; and that con- 

 sequently a ray of light on entering any medium will be, in 

 general, subdivided into three rays, the directions of the vibra- 

 tions being determined in each. When the elasticity of the 

 ether, in this medium, is the same in all directions, these three 



* Annales de Chimie, torn. xliv. 



t " Memoire sur laTheorie de la Lumi^re," Mhn. List., torn. x. 

 2 A 2 



