On Kerr's Magneto-optic Phenomenon. 253 



obviated by the use of the wire-gauze tray suspended just 

 below the surface of the water. 



When dealing with liquids, where dissolution takes place 

 almost instantaneously, the " uncertain interval " when the 

 temperature is between the initial and final points is practi- 

 cally nil, and may be overlooked. The process in making 

 the determination therefore consists in ascertaining the rate 

 of cooling at the u initial temperature," and correcting the 

 last reading of this so as to give its value at the moment of 

 the addition of the solution (15 seconds generally after this 

 last reading), and dating the commencement of the rate of 

 cooling at the " final temperature " from this moment. Three 

 or four readings at intervals of one minute generally suffice 

 to determine this latter rate satisfactorily. 



I now invariably read the thermometers without stopping 

 the stirrers and tapping-apparatus, and I believe that greater 

 accuracy in the results is thereby obtained. 



XXVII. On Kerr's Magneto-optic Phenomenon. 

 By H. E. J. G. du Bois, of the Hague*. 



§ 1. INTRODUCTORY.— T)r. Kerr has shown, in 1877, 

 that the mode of vibration of light is in general 

 affected by reflexion from a magnet. Hitherto this phenome- 

 non has been almost exclusively studied from an optical point 

 of view f ; in particular, the complicated behaviour of light, 

 obliquely reflected from magnets, has been much discussed. 

 A magnetic curve was, however, given by Prof. Kundt, from 

 which I drew the following conclusion J : for light normally 

 reflected, the rotation is probably proportional to the magneti- 

 zation, as it is for light transmitted through magnets. Start- 

 ing from this, I have now further investigated the phenomenon 



* Translated by the Author from Wied. Ann. xxxix. p. 25 (1890) ; a 

 synopsis of results had been communicated to the British Association, 

 Newcastle. See Proceed- of Sect. A, Sept. 17, 1889 Report. 



t Literature :— Kerr, Phil. Mag. [5] iii. p. 321 (1877), and v. p. 161 

 (1878) ; Kaz, Dissert., Amsterdam (1884) ; Kundt, Wied. Ann. xxiii. 

 p. 228 (1884), and xxvii. p. 198 (1886) ; Righi, Ann. de Chim. et Phys. [6] 

 iv. p. 433 (1885), ix. p. 65 (1886), and x. p. 200 (1887). 



Theoretical papers: — Fitzgerald, Proc. Roy. Soc. xxv. p. 447 (1876) ; 

 Phil. Mag. [5] iii. p. 529 (1877) ; Phil. Trans, clxxi. p. 691 ( 1 880) ; Wied. 

 Ann. xxv. p. 136 (1885). Rowland, Phil. Mag. [5] ix. p. 432 (1880), and 

 xi. p. 254 (1881). H. A. Lorentz, Versl. en Mededeel. Amsterdam, xix. 

 p. 217 (1883), and Arch. Neerl. xix. p. 123 (1884). van Loghem, Dis- 

 sert., Leiden, 1883, and Wied. Beibl. viii. p. 869 (1884). Yoigt, Wied. 

 Ann. xxiii. p. 493 (1884). Ketteler, Theor. Optics, Brunswick, 1885. 



| du Bois, Wied. Ann. xxxl pp. 965, 974 (1887). 



