Maxwell was impressed, as Kirchhoff had been before him, by the close agreement between the electric ratio c and the velocity of light[1]; and having demonstrated that the propagation 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:[2] "Is there any electric fluid sui generis at all, distinct from the aether?"
The presence of the dielectric constant ε in the expression cε-12, which Maxwell had obtained for the velocity of propagation 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 μ has been supposed to have the value unity. If this is not the case, the
- ↑ He had "worked out the formulae in the country, before seeing Weber's result." Cf. Campbell and Garnett's Life of Maxwell, p. 244.
- ↑ Priestley's History, p. 488.
2·98 * 1010 cm./sec. Subsequent determinations by Michelson in 1879 (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. 682), the value obtained for the velocity of light by the methods of Fizesu 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 nondispersive 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.
