Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/384

The old question as to whether the light-vector is in, or at right angles to, the plane of polarization^[1] now presented itself in a new aspect. The wave-front of an electric wave contains two vectors, the electric and magnetic, which are at right angles to each other. Which of these is in the plane of polarization? The answer was furnished by FitzGerald and Trouton,^[2] who found on reflecting Hertzian waves from a wall of masonry that no reflexion was obtained at the polarizing angle when the vibrator was in the plane of reflexion. The inference from this is that the magnetic vector is in the plane of polarization of the electric wave, and the electric vector is at right angles to the plane of polarization. An interesting development followed in 1890, when O. Wiener^[3] succeeded in photographing stationary waves of light. The stationary waves were obtained by the composition of a beam incident on a mirror with the reflected beam, and were photographed on a thin film of transparent collodion, placed close to the mirror and slightly inclined to it. If the beam used in such an experiment is plane-polarized, and is incident at an angle of 45°, the stationary vector is evidently that perpendicular to the plane of incidence; but Wiener found that under these conditions the effect was obtained only when the light was polarized in the plane of incidence; so that the chemical activity must be associated with the vector perpendicular to the plane of polarization—i.e., the electric vector.

In 1890 and the years immediately following appeared several memoirs relating to the fundamental equations of electro-magnetic theory. Hertz, after presenting^[4] the general

1. Cf. pp. 168 et sqq.
2. Nature, xxxix (1889), p. 391.
3. Ann. d. Phys. xl (1890), p. 203. Cf. a controversy regarding the results; Comptes Rendus, cxii (1891), pp. 186, 325, 329, 365, 383, 456; and Ann. d. Phys. xli (1890), p. 154; xliii (1891), p. 177; xlviii (1893), p. 119.
4. Gott. Nach. 1890, p. 106; Ann, d. Phys. xl (1890), p. 577; Electric Waves (English ed.), p. 195. In this memoir Hertz advocated the form of the equations which Maxwell had used in his paper of 1868 (cf. supra, p. 287) in preference to the earlier farm, which involved the scalar and vector potentials.