404 Mr. W. J. M. Rankine's General View of an 



ference of light, of its propagation in crystalline and uncrystal- 

 line substances, of diffraction, of single and double refraction, of 

 dispersion by refraction, and of partial and total reflexion. 



It has hitherto been always assumed, that the kind of motion 

 which constitutes light is a vibration from side to side, trans- 

 mitted from particle to particle of the luminiferous medium, by 

 means of forces acting between the particles. In order to account 

 for the transmission of such transverse vibrations, the lumini- 

 ferous medium has been supposed to possess a kind of elasticity 

 which resists distortion of its parts, like that of an elastic solid ; 

 and in order to account for the non-appearance in ordinary cases 

 of effects which can be ascribed to longitudinal vibrations, it has 

 been found necessary to suppose further, that this medium resists 

 compression with an elasticity immensely greater than that with 

 which it resists distortion ; the latter species of elasticity being, 

 nevertheless, sufficiently great to transmit one of the most pow- 

 erful kinds of physical energy through interstellar space with a 

 speed in comparison with which that of the swiftest planets of 

 our system in their orbits is appreciable, but no more. 



It seems impossible to reconcile these suppositions with the 

 fact, that the luminiferous medium in interstellar space offers no 

 sensible resistance to the motions of the heavenly bodies. 



A step towards the solution of this difficulty was made by 

 Mr. MacCullagh. The equations which he used to express the 

 laws of the propagation of light, when interpreted physically, 

 denote the condition of a medium whose molecules tend to range 

 themselves in straight lines, and when disturbed, to return to 

 those lines with a force depending on the curvature of the lines 

 into which they have been moved. But even this hypothesis 

 requires the assumption, that the elasticity of the luminiferous 

 medium to resist compression is immensely greater than the 

 elasticity which transmits transverse vibrations. 



The difficulty just referred to arises from a comparison of the 

 hypothesis of transverse vibrations with the observed phenomena 

 of the world. 



Another difficulty arises within the hypothesis itself. Fresnel 

 originally assumed, that in crystalline media, where the velocity 

 of light varies with the position of the plane of polarization, the 

 direction of vibration is perpendicular to that plane. This is 

 equivalent to the supposition, that the velocity with which a state 

 of rectilinear transverse vibration is transmitted through such a 

 medium, is a function simply of the direction in which the par- 

 ticles vibrate. From this hypothesis he deduced the form of 

 that wave-surface which expresses completely the law of the pro- 

 pagation of plane-polarized light through crystalline media, and 

 he obtained also a near approximation to the laws of the inten- 



