After the first experiments of the first named of the authors of 
this paper, the change of absorption lines in a magnetic field was 
studied by Konig : ) and Cotton -) ; Righi 3 ) gave an elaborate study 
of the subject., to which we have to return later on. It contains the 
only investigation of the magnetic effect in a direction inclined to 
the lines of. force. Closely connected with our subject are finally 
some observations by Lodge and Davies 4 ) on the influence of a 
magnetic field on flames, emitting “reversed” lines. 
The consideration’ of the inverse effect forms the basis of Yoigt’s 
magneto-optical theories 5 ); and it is considered also by Lorentz") 
in his investigation of the magnetic separation in a direction inclined 
to the line of force. 
Theory indicates different points, which may be tested by expe¬ 
riment. The inverse effect has become of supreme interest in solar 
physics, since Hale’s 7 ) discovery that the dark lines of the sun-spot 
spectrum exhibit the characteristic phenomena of magnetic separation. 
The experiments we intend to describe in the present communi¬ 
cation relate to the division of the sodium lines D x and D 2 . Some 
of our results may already be found in the work of the cited authors. 
In order to present the subject in a connected form it seemed 
necessary not to exclude these. 
The facts now ascertained in combination with former results 
appear to be of some value in explaining peculiarities observed in 
sun-spot spectra. Some instances will be given later on. 
2. Type and relative amount of the magnetic division of the 
sodium emission lines, D t and D„ are 
given in Fig. 1. 
a represents the observations when the 
line of sight is at right angles to . the - 
magnetic field, b when it is parallel to 
the field. 
In a weak magnetic field D a exhibits 
the triplet type, at right angles to the 
M I | | | 
I I 
I I 
* I 
II II 
Fig. 
1 ) Konig. Ann. d. Phys. Bd. 62. 240. 1897. 
2 ) Cotton. Eelairage Electrique. 5 et 26 mars. 1898. 
*) Righi. Sul fenomeno di Zeeman nel caso generale d’un raggio luminosa 
comunque inclinato sulla direzione della forza magnetica. Mem- di. Bologna. 
17 Dicembre 1899. 
*) Lodge and Davies. Proc. R. Soc. 61 413. 1897. 
5 ) W. Voigt. Magneto- und Elektrooptik. Chapter IV and the papers there cited. 
6 ) H. A. Lorentz. These Proceedings, Vol. XII, p. 321, 1909. 
7 ) G. E. Hale. On the probable existence of a magnetic field in sun-spots. 
Contributions from the Mount Wilson Solar Observatory Nr. 30. 1908. 
