Maxwell. 283 



Maxwell was impressed, as Kirchhoff had been before him, 

 by the close agreement between the electric ratio c and the 

 velocity of light* ; and having demonstrated that the propaga- 

 tion of electric disturbance resembles that of light, he did not 

 hesitate to assert the identity of the two phenomena. "We 

 can scarcely avoid the inference," he said, " that light consists 

 in the transverse undulations of the same medium which is the 

 cause of electric and magnetic phenomena." Thus was answered 

 the question which Priestley had asked almost exactly a hundred 

 years before :f "Is there any electric fluid sui generis at all, 

 distinct from the aether ? " 



The presence of the dielectric constant e in the expression 

 ct -i, which Maxwell had obtained for the velocity of propaga- 

 tion of electromagnetic disturbances, suggested a further test 

 of the identity of these disturbances with light: for the velocity 

 of light in a medium is known to be inversely proportional to 

 the refractive index of the medium, and therefore the refractive 

 index should be, according to the theory, proportional to the 

 square root of the specific inductive capacity. At the time, 

 however, Maxwell did not examine whether this relation 

 was confirmed by experiment. 



In what has preceded, the magnetic permeability //, has been 

 supposed to have the value unity. If this is not the case, the 



2-98 x 10 10 cm./sec. Subsequent determinations by Michelson in 187'.) (Ast. 

 Papers of the Amer. Ephemeris, i), and by Newcomb in 1882 (ibid., ii) depended 

 on the same principle. 



As was shown afterwards by Lord Rayleigh (Nature, xxiv, p. 382, xxv, p. 52) 

 and by Gibbs (Nature, xxxiii, p. 582), the value obtained for the velocity of light 

 by the methods of Fizeau and Foucault represents the group-velocity, not the wave- 

 velocity ; the eclipses of Jupiter's satellites also give the group-velocity, while the 

 value deduced from the coefficient of aberration is the wave- velocity. In a non- 

 dispersive medium, the group- velocity coincides with the wave- velocity ; and the 

 agreement of the values of the velocity of light obtained by the two astronomical 

 methods seems to negative the possibility of any appreciable dispersion in free 

 aether. 



The velocity of light in dispersive media was directly investigated by Michelson 

 in 1883-4, with results in accordance with theory. 



* He had "worked out the formulae in the country, before seeing Weber's 

 result." Cf. Campbell and Garnett's Life of Maxwell, p. 244. 



f Priestley's Eistory, p. 488. 



