139 
iron sulphide from the laboratory collection were investigated. A 
piece of cerium-iron was also subjected to observation. 
Metals. For electrolytic iron and also for pure cobalt and nickel I 
am again indebted to Prof. Werss *), who has investigated their 
saturation values of magnetisation. 
The dispersion of the Kerr-effect in the metals has been moreover 
previously determined by pv Bors *). 
I beg here to express my best thanks to those gentlemen who 
have assisted me by supplying the materials. 
In the following tables are given: /, the wave-length of the observed 
light in uu. A, the double rotation as observed in mm. on the scale 
after reversal of the current. ¢, the simple rotation in minutes, +de, 
the mean error in minutes and percent respectively. VV the number 
of readings taken for each direction of the current. 
MANGANESE COMPOUNDS. 
Manganese boride*’ (Mn B). In this case, my attempts to observe 
any rotation gave but negative results. Although the material was 
porous and on that account the mirror not very bright I was able 
to convince myself that if a rotation existed it was less than 0,93’, 
Manganese-tin. (Mn 65, Sn 35= Mn, Sn). The dispersion curve 
here remains entirely in the region of negative values (Fig.1). The 
Gao 450 EREN TRR ta 
Fie: E. 
curve, which in the violet falls rather steeply, reaches a numerical 
minimum in the blue and then gradually rises again. The rotation 
always remains of a small order as one would expect from con- 
sidering tbe small magnetisability of the material. 
Two mirrors on the same piece, obtained by grinding ai right 
angles to one another, gave results in good agreement as is shown 
by tables 1 and 2. 
1) P. Weiss, Journ. de Physique (4), 9, p. 373, 1910. 
2) H. ru Bots, Wied. Ann. 89, p. 25, 1890. Phil. Mag. (5) 29 p. 253, 1890. 
3) E. WepexKinp. Zeitschr. für Physik. Chemie 66, p. 614, 1909. 
10 
Proceedings Royal Acad. Amsterdam. Vol. XV. 
