from Bradley to FresneL 123 



still always interpreted by the analogy with the vibrations of 

 sound in air, for which the direction of vibration is the same as 

 that of propagation. It was therefore necessary to give some 

 justification for the new departure. With wonderful insight 

 Fresnel indicated* the precise direction in which the theory of 

 vibrations in ponderable bodies needed to be extended in order 

 to allow of waves similar to those of light : " the geometers," he 

 wrote, " who have discussed the vibrations of elastic fluids hitherto 

 have taken account of no accelerating forces except those arising 

 from the difference of condensation or dilatation between conse- 

 cutive layers." He pointed out that if we also suppose the 

 medium to possess a rigidity, or power of resisting distortion, such 

 as is manifested by all actual solid bodies, it will be capable of 

 transverse vibration. The absence of longitudinal waves in the 

 aether he accounted for by supposing that the forces which oppose 

 condensation are far more powerful than those which oppose 

 distortion, and that the velocity with which condensations are 

 propagated is so great compared with the speed of the oscillations 

 of light, that a practical equilibrium of pressure is maintained 

 perpetually. 



The nature of ordinary non-polarized light was next discussed. 

 " If then," Fresnel wrote,f " the polarization of a ray of light 

 consists in this, that all its vibrations are executed in the same 

 direction, it results from any hypothesis on the generation of 

 light-waves, that a ray emanating from a single centre of dis- 

 turbance will always be polarized in a definite plane at any 

 instant. But an instant afterwards, the direction of the motion 

 changes, and with it the plane of polarization ; and these 

 variations follow each other as quickly as the perturbations of 

 the vibrations of the luminous particle : so that even if we could 



*Annales de Chiinie, xvii (1821), p. 180; (Eiwres, i, p. 629. Young had 

 already drawn attention to this point. " It is difficult," he says in his Lectures on 

 Natural Philosophy, ed. 1807, vol. i, p. 138, "to compare the lateral adhesion, or 

 the force which resists the detrusion of the parts of a solid, with any form of direct 

 cohesion. This force constitutes the rigidity or hardness of a solid body, and is 

 wholly absent from liquids." 



t Loc. cit, p. 185. 



