Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/388

 light, FitzGerald proceeded to extend it so as to take account of a closely related phenomenon. In 1876 J. Kerr had shown experimentally that when plane-polarized light is regularly reflected from either pole of an iron electromagnet, the reflected ray has a component polarized in a plane at right angles to the ordinary reflected ray. Shortly after this discovery had been made known, FitzGerald had proposed to explain it by means of the same term in the equations which accounts for the magnetic rotation of light in transparent bodies. His argument was that if the incident plane-polarized ray be resolved into two rays circularly polarized in opposite senses, the refractive index will have different values for these two rays, and hence the intensities after reflexion will be different; so that on recompounding them, two plane-polarized rays will be obtained—one polarized in the plane of incidence, and the other polarized at right angles to it.

The analytical discussion of Kerr's phenomenon, which was given by FitzGerald in his memoir of 1879, was based on these ideas; the most essential features of the phenomenon were explained, but the investigation was in some respects imperfect.

A new and fruitful conception was introduced in 1879–1880, when H. A. Rowland suggested a connexion between the magnetic rotation of light and the phenomenon which had been discovered by his pupil Hall. Hall's effect may be regarded