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 unfavourably received by the most distinguished of Maxwell's contemporaries. Helmholtz indeed ultimately accepted it, but only after many years; and W. Thomson (Kelvin) seems never to have thoroughly believed it to the end of his long life. In 1888 he referred to it as a "curious and ingenious, but not wholly tenable hypothesis," and proposed to replace it by an extension of the older potential theories. In 1896 he had some inclination to speculate that alterations of electrostatic force due to rapidly-changing electrification are propagated by condensational waves in the luminiferous aether. In 1904 he admitted that a bar-magnet rotating about an axis at right angles to its length is equivalent to a lamp emitting light of period equal to the period of the rotation, but gave his final judgment in the sentence :—"The so-called electromagnetic theory of light has not helped us hitherto."

Thomson appears to have based his ideas of the propagation of electric disturbance on the case which had first become familiar to him—that of the transmission of signals along a wire. He clung to the older view that in such a disturbance the wire is the actual medium of transmission; whereas in Maxwell's theory the function of the wire is merely to guide the disturbance, which is resident in the surrounding dielectric.

This opinion that conductors are the media of propagation of electric disturbance was entertained also by Ludwig Lorenz (b. 1829, d. 1891), of Copenhagen, who independently developed an electromagnetic theory of light a few years after the publication of Maxwell's memoirs. The procedure which Lorenz followed was that which Riemann had suggested in 1858—namely, to modify the accepted formulae of electrodynamics by introducing terms which, though too small to be