(435 ) 
by means of the hypothetical descending “sap-current’”’, by the newer 
physiologists they were, as a rule, not explained at all. Now that the 
existence of a resulting deseending current could be proved with 
Caulerpa, which shows so many analogous phenomena, it seems to 
me to be probable that on closer investigation it will also be found 
with higher plants, although perhaps in an entirely different form 
than was originally thought. 
Physics. — “Double refraction near the components of absorption 
lines magnetically split into several components’’, according to 
experiments made by Mr. J. Gerst. By Prof. P. Zeman. 
It has already appeared from experiments which | had the honour 
to communicate to the Academy on a former occasion that the 
magneto-optie theory of Vorer'), who established a simple and 
rational connexion between the magnetic splitting up of the spectral 
lines and dispersion, accounts extremely well for all the phenomena 
observed in the region of the absorption lines. 
If light traverses parallel to the lines of force very attenuated 
sodium vapour placed in the magnetic field, the plane of polarization 
is rotated im the positive direction for all periods lying outside the 
components of the doublet, but in the negative direction, and very 
strongly *), for periods intermediate between those of the components. 
If light traverses the vapour normally to the field, there is double 
refraction as predicted by Vorer from theory. When placed in a 
magnetic field, all isotropic bodies should show double refraction, 
but to a measurable degree only in the neighbourhood of the absorption 
lines. Vorer in collaboration with Wiscnert experimentally verified 
this result, using a small grating and a flame with relatively much 
sodium vapour. 
I have extended these results*) by working with sodium vapour 
so dilute that, in a strong magnetic field, there were seen the four 
absorption lines corresponding to the components of the quartet into 
which the line D, is split by the magnetic field. The mode of depen- 
dency of double refraction on the period could, in this special case 
with some reserve, be predicted from Voiert’s theory. Observations, 
in which Mr. Grrsr took part, confirmed the theoretical result. Mr. 
1) Voter, Wrepemann’s Annalen. Bd 67, p. 359, 1899. 
2) Zeeman, Proc. Acad. Amsterdam, May 1902, see also Harro, Thesis for the 
doctorate, Amsterdam, 1902. 
5) Zeeman en Geest, Proc. Acad. Amsterdam, May 1903. 
TOF 
