alloy (2j8 Mn, 13 Al, 61 Cu). The first two were kindly prepared by 
Dr. Hilpkrt in the metallurgical Laboratory of the “Technische 
Hochschule” in Chariot ten burg; the natural magnetite is from the 
collection of the Bosscha-Laboratory, and is the same specimen, 
possessing a polished octahedral surface, which was formerly examined 
by du Bois 1 ). A very fine-formed crystal of ilmenite was kindly 
lent by Prof. Liebisch. The Heusler alloy was supplied by the de 
Haen chemical factory in Seelze; its interior was full of bubbles, 
but its surface was capable of polish and supplied a very good 
mirror. The “invar” contained about 36 # / 0 Nickel and came from 
France (Societe de Commentry-Fourchambault). For the chromic 
oxide I am indebted to Dr. Koppel. I desire to express my obli¬ 
gations to all the above mentioned gentlemen. 
Throughout this paper I shall denote as usual by: the field 
intensity in kilogausses, 3 the magnetization, 3* its saturation 
value, e single rotation of the plane of polarization in minutes, 
K, Kerr’s constant. In the tables, the column under AT shows 
the number of readings in each series of measurements, which depend¬ 
ed upon the polish of the mirror and the variable brightness, X 
denotes the wave-length in w, A the direct scale-reading in mm. of 
the double rotation produced by reversal of the current. The average 
values of the single rotations are given in the fourth column and 
in the fifth and sixth the average errors in minutes and in percentages. 
The sense of the rotation is referred as usual to that of the 
magnetizing current; e.g. in the case of iron the polar err 
rotation is negative. 
Results. The results obtained with the various substances were 
as follows: 
1. Cuprifemte. Measurements were made on twom.rrorsof this 
material with similar results. Tbe relation between the *>»ahon and 
. ivr* »hi,h JZ >«,. 
- maximum occurring in the 
In the violet the rotation is positive, 
blue; withincreasing wave-length the rotation gradual* d~J 
and in the neighbourhood of 587 W go^ through rero ltecommg 
negative for longer wave-lengths. Between 640 »d» a rather 
flat minimum is Exhibited, thecurve then gradually P™^ 
The rotations are small throughout, the max,mum value not bemg 
l ) H. DU Bois, Wied. Ann. 39, p. 25, 1890. 
Proceedings Royal Acad. Amsterdam. Vol. XU. 
56 
