( 836 ) 
latter, the rotatory dispersion, according to the same author, shews 
certain regularities. For iron, cobalt, and nickel the rotations visually 
observed were always negative; for iron the dispersion-curve seems 
to indicate a numerical minimum in the ultraviolet and thence ascends 
from violet towards red; in the case of cobalt the minimum occurs 
between blue and green, and for nickel in the yellow. These 
numerical minima of negative rotation may be considered algebraic 
maxima, their wave-length increasing as the metal’s position in the 
periodic system advances. For magnetite the observed rotations were 
in every case positive, though the curve appeared directed towards 
negative values beyond the blue; a distinct maximum occurred in 
the yellow, corresponding to the above algebraic, maxima. 
More recently Ingersoll *) has contributed important papers relative 
to this subject; he was able to supplement du Bois’ curves in the 
infra-red up to about 3 p. According to this author the complete 
rotatory dispersion-curves thus obtained shew a marked resemblance 
to a typical dispersion-curve in the region of an exceedingly broad 
band of resonance-absorption. The particular cases of nickel and 
magnetite are notable, for the rotation appears to vanish between 
1 and 1,5 p and then to change in sign. 
Further progress in this subject was difficult in view of the fact, 
that as yet the only ferromagnetic substances suitable for a study of 
the KERR-Effect were the four above-mentioned bodies. Several attempts 
to study with reflected and transmitted light the magneto-optic pheno¬ 
mena connected with the KERR-Effect were made with partially 
transparent films of metals prepared electrolytically, after the manner 
of Kundt, or by cathodic discharge. Although the latest investigations ’) 
on the optical properties of these films of magnetic metals brought 
to light further interesting but confusing results, yet the conditions 
in the films can obviously depend on their structure and on their 
mode of preparation in a very complex way. As a reflecting surface 
such a film is certainly inferior from a physical point of view to a 
mirror polished on a compact and massive block of metal. 
An attempt to add to the number of substances which exhibit the 
KERR-Effect was thus of some interest. I entertained some hopes in 
this respect, since several chemists of late have synthetically prepared 
new ferromagnetic substances. In the first place, a number of com¬ 
binations of different oxides with iron oxide, so called metaferrites. 
') L. R. Ingersoll, Phil. Mag. (6) 11 p. 41, 1906 & 18 p. 74, 1909. 
2 ) C. A. Skinner & A. Q. Tool, Phil. Mag. (6) 16 p. 883, 1908. H. Behrens 
Inaug. Diss. Munster i. W. 1908. L. R. Ingersoll, loc. cit. 
