4. It appeared possible to keep stationary each of the stages des- 
eribed in (8) during a considerable time. Excellent photographs could be 
secured with plates which were sensitised for yellow light with erythro- 
sine silver. Instead of the gasflame fed with oxygen it was easier, 
in the ease of greater distances between the poles, to use a Bunsen 
burner wherein common salt was introduced. 
5. If the density of the vapour was maintained as constant as 
possible and if it and the fieldintensity corresponded to the eireum- 
stances represented in fig. 3 (3) then an merease of the field gave 
a motion of the arrow (fig. 3) (3) upwards, corresponding to a decrease 
of the negative rotation and reciprocally. It was possible to observe 
by eye observation very clearly this decrease when the field was 
changed e.g. from 18000 to 25000. If the circumstances were more 
in accordance with fig. 2 (3) then the same change of field produced 
a change only just perceptible of the negative rotation but in the 
same sense as mentioned in the case of fig. 3. 
An enlarged reproduction of one of the photographs is shown 
in fig. 2 of the plate. The distance between the poles in this experi- 
ment was 6,3 mM., the field intensity about 14000 °). The negative 
rotation in the case of D, is somewhat less than 90°. In the case 
of D,- yet only some traces of the interior fringes can be seen (3). 
The negative rotation is ‘about 180°. In the photograph are seen 
also the reversed very narrow J,-line and the broader D-line, 
which are due to the are itselfand have nothing to do with our subject. 
6. The observations (3, 4, 5) agree qualitatively in an excellent 
manner with the conclusions from Vorer’s theory. According to it, 
the negative rotation must be of the same order of magnitude as the 
positive one. This last was known from Macatuso’s and CorBrxo’s 
experiments to be very great. The enormous value and the sign of 
the negative rotation given in (38) may thus be regarded as a beautiful 
confirmation of the theory. 
As much is this the case with the direction (5) of the change of 
the negative rotation with increasing field. In order to see this we 
| cht 
9 
( = fieldintensity, c and 9 parameters of the absorptionband), for 
must know the value of the quantity occurring in the theory ? = 
which the comparison must take place. It was possible to assign a 
value to P by comparison of the phenomenon with Vorert's figure 1 *). 
1) The intensities of the field were measured by means of a bismuth spiral in 
the centre of the field. Probably the values given are somewhat too high. Measure- 
ments of the magnetic change of the spectral lines give lower values, 
2) Annalen der Physik, 6 p. 789, 1901. 
