Magneto-optic Phenomenon. 



263 



zable, though powerfully absorbent, substances, but without 

 any positive result — sulphide of iron (FeS, amorphous), and 

 oxide of iron (Fe 2 3 , rhombohedr. hemihedr. hexagonal ; 

 mirror normal to principal axis). The fact that a measurable 

 Kerr's effect has hitherto been detected only on the strongly 

 magnetizable opaque substances Co, Ki, Fe, Fe 3 4 , points 

 once more to the important part which magnetization plays 

 in the phenomenon. I am much obliged to Prof. Bucking 

 for kindly supplying me with these minerals. 



§ 19. Absolute constants. — After having considerably im- 

 proved the accuracy of the optical observations, as pointed 

 out in § 16, I could venture to apply them in their turn to 

 magnetic measuring purposes. This application of the law of 

 proportionality I shall describe* in detail; at present I will 

 only mention that by means of it I was able to find the 

 magnetizations corresponding to the rotations e given below. 

 By division the absolute values of the constant K are ob- 

 tained for " red " light in minutes per unit magnetization. 





Cobalt. 



Nickel. 



Iron. 



Magnetite. 



6 o 



-20'-97 



-7'25 



-22'-99 





3 



1060 



453 



1669 





K 



-•0198 



-•0160 



-•0138 



+ •012 



The value for magnetite is approximate ; the others refer 

 to the massive metals free from oxide ; I could not detect any 

 effect due to the polishing material. Thick electrolytic films 

 gave about the same rotation under the same circumstances, 

 but in general this depends on their thickness and surface 

 condition! ; such films are, therefore, hardly suited for absolute 

 measurement. 



§ 20. The rotational dispersion of Kerr's phenomenon is 

 anomalous, but only for iron is it sufficiently developed to 

 interfere with working in white light. For quantitative 

 measurement I applied spectral analysis in connexion with 

 Lippich's X penumbral method. By means of a prism " a 

 vision directe " a solar spectrum was thrown on a small screen. 



* In the April number of this Magazine. 



t See Kundt, Wied. Ann. xxvii. p. 199 (1886). 



I Lippich, he. cit, p. 1070 ; and Wied. Ann. xxxvi. p. 767 (1889). 



