Maxwell. 307 



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 seenf 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 Helrnholtz'js great memoir on vortex- 

 motion ; adopting Helmholtz's results, Maxwell assumed for the 

 additional term introduced into the magnetic force by the dis- 

 placement of the vortices the value 9e/B0, where e denotes the 

 displacement of the medium (i.e. the light vector), and the 

 operator d/dO denotes H^/dx + Hy'dj^y + H z d/dz, H denoting the 

 imposed magnetic field. Thus the luminous motion, by dis- 

 turbing the vortices, gives rise to an electric current in the 

 medium, proportional to curl 



*P. Lebedew, Archives des Sciences Phys. et Nat. (4) viii (1899), p. 184. 

 Ann. d. Phys. vi (1901), p. 433. E. F. Nichols and G. F. Hull, Phys. Rev. 

 xiii (1901), p. 293 ; Astrophys. Jour., xvii (1903), p. 315. t Cf. p. 274. 



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