96 SIR G. OtABRIEL STOKES; BART., F.R.S., ON 



ultimately pointed out that the vibrations with which we 

 are conccnied in the case of the ether differ altogether in 

 their character from those which belong to somul. The 

 phenomena of the interference of polarized light prove 

 incontestably that there exists in light an element of some 

 kind having relation to directions transverse to tliat of pro- 

 pagation, and admitting of composition and resolution in a 

 phme perpendicular to the direction of transmission accord- 

 ing to tlie very same laws as those of the coinposition and 

 resohition of forces, or velocities, or displacements in such a 

 plane. This requires us to attribute to the ether a con- 

 stitution altogether difterent from that of air. It points out 

 the existence of a sort of elasticity whereby the ether tends 

 to check the gliding of one layer over another. Have we 

 no example of such a force in the case of ponderable matter? 

 We have. We know that an elastic solid, which for simpli- 

 city I will suppose to be uncrystalline, and alike in all 

 directions, luis two kinds of elasticity, by one of which it, 

 like air, tends to resist compression and rarefaction, while 

 by the other it tends to resist a continuous gliding of one 

 portion over another, and to restore itself to its primitive 

 state if such a gliding has taken place. There is no direct 

 relation between the magnitude of these two kinds of 

 elasticity, and in the case of an elastic solid such as jelly 

 the resistance to compression is enormously great compared 

 to the resistance to a gliding displacement. 



If we assume that in the ether there is really an elas- 

 ticity tending to restore it to its primitive condition when 

 one layer tends to glide over another, an elasticity which 

 it appears to be absolutely necessary to admit in order to 

 account for the observed laws of interference of polarized 

 light, the question arises. Can we thereby explain double 

 refraction ? 



The earliest attempts to explain it in accordance with 

 the theory of transverse vibrations were made by attributing 

 to the ether a molecular constitution more or less analogous 

 to that Avhich we believe to exist in ponderable matter. 

 Following out speculations founded upon that view, the cele- 

 brated Fresnel Avas led to the discovery of the actual laws 

 of double refraction; the theory, however, which he gave 

 was by no means complete, inasmuch as the results were 

 not rigorously deduced from the premises. Cauchy and 

 Neumann, independently and about simultaneously, took 

 up Fresnel's view of the (M)nstitution of the ether and 



