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 soon found to be due to thermal effects; and the existence of a true light-pressure was not confirmed experimentally until 1899. Since then the subject has been considerably developed, especially in regard to the part played by the pressure of radiation in cosmical physics.

Another matter which received attention in Maxwell's Treatise was the influence of a magnetic field on the propagation of light in material substances. We have already seen that the theory of magnetic vortices had its origin in Thomson's speculations on this phenomenon; and Maxwell in his memoir of 1861–2 had attempted by the help of that theory to arrive at some explanation of it. The more complete investigation which is given in the Treatise is based on the same general assumptions, namely, that in a medium subjected to a magnetic field there exist concealed vortical motions, the axes of the vortices being in the direction of the lines of magnetic force; and that waves of light passing through the medium disturb the vortices, which thereupon react dynamically on the luminous motion, and so affect its velocity of propagation.

The manner of this dynamical interaction must now be more closely examined. Maxwell supposed that the magnetic vortices are affected by the light-waves in the same way as vortex-filaments in a liquid would be affected by any other coexisting motion in the liquid. The latter problem had been already discussed in Helmholtz's great memoir on vortex-motion; adopting Helmholtz's results, Maxwell assumed for the additional term introduced into the magnetic force by the displacement of the vortices the value &part;e/&part;θ, where e denotes the displacement of the medium (i.e. the light vector), and the operator &part;/&part;θ denotes Hx&part;/&part;x + Hy&part;/&part;y + Hz&part;/&part;z, H denoting the imposed magnetic field. Thus the luminous motion, by disturbing the vortices, gives rise to an electric current in the medium, proportional to curl &part;e/&part;θ.