( 842 ) 
previous one given by du Bois (dotted line), which he obtained with 
the same specimen (a holoedric regular crystal, possessing a natural 
octahedral surface) we see, that with the exception of a displacement 
throughout the whole range of wave-lengths amounting to about 
10 to 30 (i[i — which is explained by the fact that 20 years ago 
only an imperfect method of spectral decomposition was available — 
the curves are in agreement in the region between 486 and 671 pp. 
The rotation attains a maximum value of 4,45' in the yellow and 
decreases rapidly with decreasing wave-length. Du Bois 1 2 ), who was 
unable to proceed further than the blue on account of insufficient 
intensity of light, observed that the rotation probably vanished in 
the blue; he also considered that a change of sign possibly might 
occur in the ultraviolet. I have located this zero-point in the visible 
part of the violet at 464 p,u. For smaller wave-lengths the rotation 
has rather a large negative value, which seems to approach a mini¬ 
mum. TTnfortunately it was impossible to carry the investigation 
beyond 436 pp since the light at that point becomes too feeble. 
At all events, the existing observations establish satisfactorily the 
fact that the dispersion-curve obtained with natural crystalline 
magnetite (FeO. F,O s ) is of the same type as that obtained above 
with cupriferrite. Without entering into theoretical considerations it 
may be seen at once that in both cases the curve passes through 
a maximum, goes through zero and probably also through a mini¬ 
mum. Experiments are being carried out to see whether the course 
of these curves depends on the optical constants of the substances 
investigated, viz. their ordinary absorption- and dispersion-curves. 
In the same way as in the case of cupriferrite the relation between 
the rotation and the field was also investigated. The results are 
shown in Table 4 and Fig. 5. They give > 358 C. G. S., which 
agrees with that obtained by du Bois 5 ) (350). 
The magnetic properties of magnetite crystals have been recently 
investigated by Quittner 3 * ), adopting Weiss’ methods. From his 
measurements it follows that the component magnetization parallel 
to the field, which in this case is alone of interest, reaches a 
saturation value of about 475 C. Gr. S.; this subject and the cause of the 
discrepancy ought to be investigated in greater detail. One remark, 
however, may be made at -once. In many cases the natural magnetite 
slightly departs from the simple structural formula (Fe 0, Fe, 0,); 
3 ) H. du Bois 1. c. p. 38. 
2 ) H. du Bois, Phil. Mag. (5' 29, p. 801, 1890. 
8) P. Weiss, Joum. de Phys. (8) 5 p. 435, 1896 and (4) 9, p. 873, 1910. 
¥. Quotser, Dissertation, Zurich, 1908. 
