468 J. W. Gibbs — Elastic and Electrical Theories of Light. 



Let us first consider the facts to which a correct theory must 

 conform. 



It is generally admitted that the phenomena of light consist 

 in motions (of the type which we call wave-motions) of some- 

 thing which exists both in space void of ponderable matter, 

 and in the spaces between the molecules of bodies, perhaps 

 also in the molecules themselves. The kinematics of these 

 motions is pretty well understood; the question at issue is 

 whether it agrees with the dynamics of elastic solids or with 

 the dynamics of electricity. 



In the case of a simple harmonic wave-motion, which alone 

 we need consider, the wave-velocity (Y) is the quotient of the 

 wave-length (/) by the period of vibration (p). These quantities 

 can be determined with extreme accuracy. In media which 

 are sensibly homogeneous but not isotropic the wave-velocity 

 Y, for any constant value of the period, is a quadratic function 

 of the direction cosines of a certain line, viz : the normal to 

 the so-called " plane of polarization." The physical character- 

 istics of this line have been a matter of dispute. Fresnel con- ' 

 sidered it to be the direction of displacement. Others have 

 maintained that it is the common perpendicular to the wave- 

 normal and the displacement. Others again would define it as 

 that component of the displacement which is perpendicular to 

 the wave-normal. This of course would differ from Fresnel's 

 view only in case the displacements are not perpendicular to 

 the wave-normal, and would in that case be a necessary modi- 

 fication of his view. Although this dispute has been one of 

 the most celebrated in physics, it seems to be at length sub- 

 stantially settled, most directly by experiments upon the scat- 

 tering of light by small particles, which seems to show deci- 

 sively that in isotropic media at least the displacements are 

 normal to the " plane of polarization," and also, with hardly 

 less cogency, by the difficulty of accounting for the intensities 

 of reflected and refracted light on any other supposition.* It 

 should be added that all diversity of opinion on this subject 

 has been confined to those whose theories are based on the 

 dynamics of elastic bodies. Defenders of the electrical theory 



* "At the same time, if the above reasoning be valid, the question as to the 

 direction of the vibrations in polarized light is decided in accordance with the 

 view of Fresnel. ... I confess I cannot see any room for doubt as to the result 

 it leads to. . . I only mean that if- light, as is generally supposed, consists of 

 transversal vibrations similar to those which take place in an elastic solid, the 

 vibration must be normal to the plane of polarization." Lord Rayleigh " On the 

 Light from the Sky, its Polarization and Color;" Phil. Mag. (4), xli (1871), p. 109. 



" Green's dynamics of polarization by reflexion, and Stokes' dynamics of the dif- 

 fraction of polarized light, and Stokes' and Rayleigh's dynamics of the blue sky, 

 all agree in, as it seems to me, irrefragably, demonstrating Fresnel's original con- 

 clusion, that in plane polarized light the line of vibration is perpendicular to the 

 plane of polarization." Sir Wm. Thomson, loc. citat. 



