(M>) 
5. The Heusler alloy, supposed to contain 61°/, On, 26'/ 0 Mn and 
13% A1 is rather strongly magnetisable. Different portions of two 
well-polished mirrors were carefuly examined in various parts of the 
spectrum but proved to be magneto-optically ineffective. It is of 
course possible that the Kerr- Effect might be less than 0,3' in this 
case. Quite recently there appeared a communication by Tokmatschew 
recording similar experiments on the Heusler alloy No. 32 (58,9 Cu, 
26,5 Mn, 34,6 Al). From theoretical considerations the author arrives 
at the conclusion of the probability of an effect capable of measurement 
occurring in the neighbourhood of 450 pp. I have carried out a series 
of readings at this wave-length but no rotation could be observed. 
Ingersoll also failed to notice any measurable effect either in the 
visible spectrum or in the infra-red. 
The discussion of the theoretical signification of the above partially 
positive and partially negative results I reserve for a future occasion; 
further experiments are in preparation, and the determination of the 
purely optical properties of the investigated substances is already in 
progress. 
ERRATA. 
In the Proceedings of the Meetings of Jan. and Febr. 1910: 
p. 872 Table III for 5050 read 8050. 
p. 675 Table VII for 102.58 read 102.85. 
p. 676 Table VIII for 71.75 read 71.95. 
(May 26, 1910). 
