Magneto-optic Phenomenon. 311 



17. Prof. H. A. Lorentz has shown how the Hall pheno- 

 menon leads to an explanation of magnetic reflexion, if we 

 assume that every movement of electricity in a magnetic field 

 is accompanied by a transverse electromotive force. In this 

 theory it is assumed that the constant of magnetic polarization 

 is the same for air and iron ; that is to say, the magnetic iron 

 molecules cannot direct themselves in accordance with the 

 rapidly alternating magnetic forces of visible electric vibrations 

 (light). I have put forward an experimental proof of this, 

 inasmuch as it has been shown that reflexion from iron does 

 not differ in the least from reflexion at the surface of a 

 non-magnetic metal*. The theory farther considers the 

 behaviour of ordinary electric currents and of visible electric 

 vibrations to be the same. This, however, has not yet been 

 experimentally proved. But I have not been able to detect 

 any difference in the constants of reflexion of iron at 120° C, 

 and at ordinary temperatures, although the fairly large in- 

 fluence of temperature on electrical resistance would lead us 

 to expect such a result according to this theory f . 



It is, however, very important to compare the results of 

 observation with theory. In this way we obtain experimental 

 indications of the behaviour of visible electric vibrations in 

 metals %. 



If we have regard to the difference between the positive 

 direction of vibration, which we have assumed in order to 

 completely specify the phase-differences, and that which is 

 derived from theoretical investigations, we find from the 

 electromagnetic theory of magnetic reflexion § that 



cr 3 sin a 2tt , , T 



a is the angle of incidence, cr, p, t, w are auxiliary quan- 



* Sissingh, Dissertation, p. 129 ; also Arch. Neer. xx. p. 213 (1886). 

 t Sissingh, loc. cit. p. 133 ; loc. cit. p. 215 (1886). 

 _ \ Sissingh, loc. cit. p. 139 ; loc. cit. p. 218 (1886). Kundt has ques- 

 tioned the accuracy of the above conclusion. It is, however, borne out 

 by the small influence of temperature which Du Bois found in the case 

 of normal polar magnetic reflexion (VVied. Ann. xxxix. p. 35, 1890). 



§ H. A. Lorentz, Versl. en Mechd. der Acad. Amsterdam, Reeks 2, 

 Deel xix. (1883) ; van Loghem, Theorie der terugkaatsing van het Licht 

 door magneten, Leyden, 1883, Dissertation : also Beibl. viii p. 869 

 (1S84). 



Z2 



