( 46 ) 
This figure gives ny, (y, angle of rotation, 7 a mean value of the 
index of refraction) as function of a certain variable 4, whereas our 
phenomenon is a representation of %, as a function of 2. Reducing 
the abseis of the mentioned fig. 1 to */,, or */,, we obtain diagrams 
resembling in the main features fig. 2 of the Plate. To the 
greater observed negative rotation (3) correspond values of , which 
can be estimated at 5 or 8. The smallest easily observed rotations in 
the used strong field are probably in the vicinity of the critical value 
Pe tid. 
7. The slope of the exterior interference fringes is greater towards 
the side of the greater wavelengths than towards the violet, at least 
so far as the rotation due to one band does not influence visibly the 
rotation due to the other. At the same distances, if not very small, 
of each of the two YD lines the rotation at the side of the violet is 
greatest. The interior fringes also show a slight asymmetry, so e. g. 
the point of the arrow in fig. 3 (8) ought to be asymmetrical. The 
part at the side of the violet is predominating. 
It is clear that these phenomena depend upon an asymmetry of 
the dispersion curve. 
8. With very dense sodium vapour, hence under circumstances 
which are beyond the last stage of (5), 1 observed phenomena very 
probably identical with those observed by CorBiNo. In my first expe- 
riments with those dense vapours I thought it absolutely necessary 
for securing sufficient intensity to widen the slit beyond the width 
used in the already given- experiments. I now see however that 
this is unnecessary. 
Using these very dense vapours one sees in the absorption banda 
horizontal part of an interference fringe, which seems to have under- 
gone a very small displacement wpwards by the action of the field. 
These horizontal parts are more ill-defined and broader and the 
whole phenomenon in the bands is darker than under the circum- 
stances described in (3), (4), (5). 
Figs. 3 and 4 of the Plate will give a clearer, impression of the 
change in the phenomenon than a long description. 
Fig. 3 was obtained with a field of 4500 units and much sodium. 
I have made some measurements, according to a method not to be 
given here, concerning the displacement of the central (in horizontal 
and vertical direction) part of the interference fringe, and | have 
found a displacement, which would correspond to a positive rotation 
of about 8° with both D-lines. Fig. 4 was taken with a field of 10700 
and much sodium. The exterior interference fringes are very clear 
and much deformed; the rotation in the parts adjacent to the absorp- 
