The variation of Verdet's constant with temperature has been determined for carbon bisulphide and water by Rodger and Watson (_loc. cit._). They find if R_t, R0 are the values of Verdet's constant at t°C and 0°C. respectively, then for carbon bisulphide R_t = R0 (1 - .0016961), and for water R_t = R0 (1 - .0000305t - .00000305t²). For the magnetic metals Kundt found that the rotation did not increase so rapidly as the magnetic force, but that as this force was increased the rotation reached a maximum value. This suggests that the rotation is proportional to the intensity of magnetization, and not to the magnetic force. The amount of rotation in a given field depends greatly upon the wave length of the light; the shorter the wave length the greater the rotation, the rotation varying a little more rapidly than the inverse square of the wave length. Verdet[11] has compared in the cases of carbon bisulphide and creosote the rotation given by the formula c² / di \ [theta] = mc[gamma] --------- ( c - [lamda] --------- ) [lambda]² \ d[lambda]/ with those actually observed; in this formula [theta] is the angular rotation of the plane of polarization, m a constant depending on the medium, [lambda] the wave length of the light in air, and i its index of refraction in the medium. Verdet found that, though the agreement is fair, the differences are greater than can be explained by errors of experiment. Verdet[12] has shown that the rotation of a salt solution is the sum of the rotations due to the salt and the solvent; thus, by mixing a salt which produces negative rotation with water which produces positive rotation, it is possible to get a solution which does not exhibit any rotation. Such solutions are not in general magnetically neutral. By mixing diamagnetic and paramagnetic substances we can get magnetically neutral solutions, which, however, produce a finite rotation of the plane of polarization. The relation of the magnetic rotation to chemical constitution has been studied in great detail by Perkin,[13] Wachsmuth,[14] Jahn[15] and Schönrock.[16]