light gave interest to a short paper of a speculative character which Faraday published[1] in 1846, under the title "Thoughts on Ray-Vibrations." In this it is possible to trace the progress of Faraday's thought towards something like an electro-magnetic theory of light.
Considering first the nature of ponderable matter, he suggests that an ultimate atom may be nothing else than a field of force—electric, magnetic,and gravitational—surrounding a point-centre; on this view, which is substantially that of Michell and Boscovich, an atom would have no definite size, but ought rather to be conceived of as completely penetrable, and extending throughout all space; and the molecule of a chemical compound would consist not of atoms side by side, but of "spheres of power mutually penetrated, and the centres even coinciding."[2]
All space being thus permeated by lines of force, Faraday suggested that light and radiant heat might be transverse vibrations propagated along these lines of force. In this way he proposed to "dismiss the aether," or rather to replace it by lines of force between centres, the centres together with their lines of force constituting the particles of material substances.
If the existence of a luminiferous aether were to be admitted, Faraday suggested that it might be the vehicle of magnetic force; " for," he wrote in 1851,[3] "it is not at all unlikely that if there be an aether, it should have other uses than simply the conveyance of radiations." This sentence may be regarded as the origin of the electro-magnetic theory of light.
At the time when the "Thoughts on Ray-Vibrations" were published, Faraday was evidently trying to comprehend everything in terms of lines of force; his confidence in which had been recently justified by another discovery. A few weeks after the first observation of the magnetic rotation of light, he noticed[4] that a bar of the heavy glass which had been used in
