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Chapter XII.
The Theory of Aether and Electrons in the Closing Years of the Nineteenth Century.
The attempts of Maxwell[1] and of Hertz[2] to extend the theory of the electromagnetic field to the case in which ponderable bodies are in motion had not been altogether successful. Neither writer had taken account of any motion of the material particles relative to the aether entangled with them, so that in both investigations the moving bodies were regarded simply as homogeneous portions of the medium which fills all space, distinguished only by special values of the electric and magnetic constants. Such an assumption is evidently inconsistent with the admirable theory by which Fresnel[3] had explained the optical behaviour of moving transparent bodies; it was therefore not surprising that writers subsequent to Hertz should have proposed to replace his equations by others designed to agree with Fresnel's formulae. Before discussing these, however, it may be well to review briefly the evidence for and against the motion of the aether in and adjacent to moving ponderable bodies, as it appeared in the last decade of the nineteenth century.
The phenomena of aberration had been explained by Young[4] on the assumption that the aether around bodies is unaffected by their motion. But it was shown by Stokes[5] in 1845 that this is not the only possible explanation. For suppose that the motion of the earth communicates motion to the neighbouring portions of the aether; this may be regarded as superposed on the vibratory motion which the aethereal particles have
