1318 
when the horizontal field is excited. Whereas after the breaking up of 
the perpendicular field the extinction is much larger than in the 
virginal state, the extinction is much smaller than before after the 
removal of the horizontal field. This condition of diminished extinction 
seems to be stable, it holds during some hours; the condition of 
increased extinction, as is left after removal of the perpendicular 
field, is of a transitory nature, gradually the virginal state being 
restored. . 
We expected from theoretical considerations which will be treated 
more fully hereafter that the effect of the temporary increase of 
the extinction when a vertical field is excited would appear more 
clearly in the case of weaker fields (compare the temporary drop of 
the curve between B and C in-fig. 2). At the same time we expected 
that in the case of a sufficiently weak perpendicular field it might 
occur that the extinction of the field is on, would remain larger than 
before. 
The experiments confirmed our expectation completely as is shown 
by the following figures 
Fig. 4. Perpendicular field of 300 Gauss. 
B | 
a 
Fig. 5. Perpendicular field of 100 Gauss. 
We have also examined whether commutation of the magnetic field 
has any influence on the extinction, but we could not state such an 
influence. *) 
1) Considering the very different character which the influence of a magnetic 
field on the same preparation may have, it is easily understood that various 
observers, by visual examination of the extinction have come to quite contradictory 
results. Moreover, small impurities of the preparation and especially the state of 
the surface of the glass are of decisive influence. A further research on the 
influence of the surrounding surfaces is being prepared; we only want to remark 
that we performed these measurings with plates of glass which were not chemic- 
ally cleared. 
/ 
