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metals, were extensively studied ^[1] in the years following the publication of Rowland's memoir: but it was not until the modern theory of electrons had been developed that a satisfactory representation of the molecular processes involved in magnetooptic phenomena was attained.

The allied phenomenon of rotary polarization in naturally active bodies was investigated in 1892 by Goldhammer.^[2] It

1. The theory of Basset (Phil. Trans. clxxxii (1891), p. 371) was, like Rowland's, based on the idea of extending Hall's phenomenon to dielectric media. An objection to this theory was that the tangential component of the electromotive force was not continuous a cross the interface between a magnetized and an unmagnetized medium; but Basset subsequently overcame this difficulty (Nature, lii (1895), p. 618; liii (1895), p. 130; Amer. Jour. Math. xix (1897), p. 60)—the effect analogous to Hall's being introduced into the equation connecting electric displacement with electric force, so that the equation took the form

   $\mathbf {E} =(4\pi c^{2}/\epsilon )\mathbf {D} +\sigma [\mathbf {K.{\dot {D}}} ]$.

   
   

   Basset, in 1893 (Proc. Camb. Phil. Soc. viii, p. 68), derived analytical expressions which represent Kerr's magneto-optic phenomenon by substituting a complex quantity for the refractive index in the formulae applicable to transparent magnetized substances.
   The magnetic rotation of light and Kerr's phenomenon have been investigated also by R. T. Glazebrook, Phil. Mag, xi (1881), p. 397; by J. J. Thomson, Recent Researches, p. 482: by D. A. Goldhammer, Ann. d. Phys. xlvi (1892), p. 71; xlvii (1892), p. 345; xlviii (1893), p.740; 1 (1893), p. 772: by P. Drude, Ann. d. Phys. xlvi (1892), p. 353; xlviii (1893), p. 122; xlix (1893), p. 690; lii (1894)) p. 496: by C. H. Wind, Verslagen Kon, Akad, Amsterdam, 29th Sept., 1894: by Reiff, Ann. d. Phys. lvii (1896), p. 281: by J. G. Leathem, Phil. Trans. exe (1897), p. 89; Trans. Camb. Phil. Soc. xvii (1898), p. 16: and by W. Voigt in many memoirs, und in his treatise, Magneto- und Elektro-optik. Larmor's report presented to the British Association in 1893 has been already mentioned.
   In most of the later theories the equations of propagation of light in magnetized metals are derived from the two fundamental electromagnetic equations

   ${\text{curl }}\mathbf {H} =4\pi \mathbf {S} ,\qquad {\text{curl }}\mathbf {E} =\mathbf {\dot {H}}$;

   the total current S being assumed to consist of a part (the displacement-current) proportional to Ė, a part (the conduction-current) proportional to E, and a part proportional to the vector-product of E and the magnetization.
   Various mechanical models of media in which magneto-optic phenomena take place have been devised at different times. W. Thomson (Proc. Lond. Math. Soc. vi (1875)) investigated the propagation of waves of displacement along a stretched chain whose links contain rotating fly-wheels: cf. also Larmor, Proc. Lond. Math. Soc. xxi. (1890), p.423; xxiii (1891), p. 127; F. Hasenöhrl, Wien Sitzungsherichte cvii, 2a (1898), p. 1015; W. Thomson (Kelvin), Phil. Mag. xlviii (1899), p. 236, and Baltimore Lectures; and FitzGerald, Electrician, Aug. 4, 1899, FitzGerald's Scientific Writings, p. 481.

2. Journal de Physique (3) i, pp. 205, 345.