(42) 
The long since known phenomenon of the rotation of the plane of 
polarisation and the magnetic separation of the spectral lines were 
closely connected *). 
One result however of Voier’s*) theory relating to the rotation 
of the plane of polarisation in the interior of an absorption band 
seemed to be in contradiction with the results of CorBINO *) or at 
least were not confirmed by the experiments of Scumauss*) The 
theory of Vorer requires a negative °) rotation of the plane of polarisation 
in the interior of an absorption band, CorBrxo however only succeeded 
in observing a very small positive rotation. 
It would be very remarkable however, if there existed a disagreement 
between theory and observation in this special field so closely con- 
nected with other well understood phenomena. 
I have been experimenting already some time on this subject. In 
executing these experiments I have been aided in an excellent manner 
by Mr. Harro. 
I have succeeded in observing a negative rotation in the interior 
of an absorption band, the results of my observations being in perfect 
qualitative agreement with Vorer’s theory. 
2. The method used in the following observations on the rotation 
in sodium vapour is principally the same as that which has been 
used by Vorer®) in his demonstration of the double refraction of 
sodium vapour placed in a magnetic field. Already Hvussen’) used it 
in a determination of the naturel rotation of the plane of polarisation 
in quartz, and also Corpino in his first experiments on sodium. 
By means of a system of quartz prisms (as has been used by 
FresneL in his experiment on the division of a plane-polarised ray 
into two circularly polarised rays) a number of horizontal interference 
fringes are formed in a spectrum. The light traverses the prism in 
the direction of the axis and the edges are horizontal and perpendi- 
cular to the slit of the spectroscope. The prism system (length 50 mm.) 
was placed in my experiments as near as possible before the slit of 
spectral apparatus and a small Nicol, used as analysator, behind the 
slit. The polarising Nicol was placed, of course, before the eleetro- 
1) cf. also Larnwor. Aether and Matter, p. 203. 
2) Vorer. Ann. der Physik. (4), 6, p. 784, 1901. 
5) Corpino. Atti R. Ace. dei Lincei. Vol. 10 p. 137, 1901, Nuovo cimento 
Febbraio 1902. 
') Scumauss. Ann. d. Phys. 2 p. 280, 1900. 
5) The magnetic rotation in the vicinity of the band is positive in sodium vapour, 
6) Vorer. Wied. Ann. 67, p. 360, 1899. 
1) Husser. Wied Ann. Bd. 43, p. 498, 1891, 
