116 THE LUMINIFEROUS .ETHER. 



upon condensation and rarefaction, it was supposed naturally that 

 light was propagated in a similar nianuer, by virtue of the forces brought 

 into play by the condensation and rarefaction of the tether. But there 

 IS one whole class of phenomena which have actually no counterpart 

 in those of sound: I refer to polarization and double refraction. 



The evidence for the truth of the theory of undulatums as regards 

 the phenomena of common light depends in great measure upon the 

 fact of interference and the explanation which the theory gives of the 

 complicated phenomena of diffraction. But in studying the interfer- 

 ence of polarized light, additional phenomena presented themselves 

 which ultimately pointed out that the vibrations with which we are 

 concerned in the case of the aether differ altogether in their character 

 from those which belong to sound. The phenomena of the interference 

 of polarized light prove incontestably that there exists in light an 

 element t)f some kind having relation to directions transverse to that 

 of propagation, and admitting of composition and resolution in a plane 

 perpendicular to the direction of transmission according to the very 

 same laws as those of the composition and resolution of forces, or 

 velocities, or displacements in such a plane. This reijuires us to 

 attribute to the a'ther a constitution altogether different from that of 

 air. It points out the existence of a sort of elasticity whereby the 

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

 no extimple of such a force in the case of ponderal)Ie matter ? We have. 

 We know that an elastic solid, which for simplicity 1 will suppose to 

 be uncrysttilline, and alike in all directions, has two kinds of elasticity, 

 by one of which it, like air, tends to resist compression txnd 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 

 mtignitude 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 thtxt in the tether there is really an elasticity tending to 

 restore it to its primitive condition when one Itiyer tends to glide over 

 another, an elasticity which it ap])ears to be tibsolutely necessary to 

 admit in order to account for the observed laws of interference of polar- 

 ized light, the (piestion arises, can we thereby explain double refrac- 

 tion ? 



The earliest attempts to explain it in txccordiince with the theory of 

 transverse vibrations were made by attributing to the tether a moleciihir 

 constitution more or less aimlogous to that which we believe to exist in 

 ponderable mtitter. Following out speculations founded upon that 

 view, the celebrated Fresnel was led to the discovery of the actual 

 laws of double refraction ; the theory however which he gtive was by 

 no means complete, inasmuch as the results were not rigorously deduced 

 from the premises, Oauchy and Neumann, independently and about 



