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phenomenon in (he arc is dependent in a high degree on the quantity 

 of soda, the temperature varying very little as long as the equilibrium 

 in the light aro. is not disturbed. 



In my opinion besides the density also the temperature can have 

 influence, though the latter will be slight. Tf the ate lines were 

 exclusively dependent on the density of the vapour in the arc, the 

 horizontal dotted lines in the figure would have to cut the curves 

 for Z)i and D^ in points for which the abscissae are equal. As this 

 seems to be almost the case, the influence of the temperature carmot 

 be very large. 



Also with regard to the degree of the absorption — so the value 

 of the absorption index — , there can very well be difference between 

 higher and lower temperatures; I have, however, not examined this. 

 And at last, the absorption lines of low temperature are somewhat 

 sharper than those in the light arc. 



Returning to what Miss v. Ubisch derived from her experiments, 

 viz. a displacement of the absorption maxima of sodium vapour on 

 change of temperature, we may question whether this result can be 

 brought into connection with the observations described just now. 



In the experiments under consideration the main point was (he 

 absorption to which the components of the two magnetically split 

 sodium lines (briefly called ZEEMAN-components) were subjected in 

 the Wood tube filled with sodium vapour. 



The measurements were made of the relative intensities of the 

 differently polarized beams of light both normal to the lines of force 

 of the magnetic field (transversal), and iu the direction of the lines 

 of force (longitudinal) (in the transversal observations the beams 

 polarized parallel to and normal to the lines of force, in the longi- 

 tudinal observations both the circularly polarized ones.) These beams 

 of light were emitted by a sodium flame which was placed between 

 the poles of a magnet; the undispersed sodium light was subjected 

 to absorption in a Wood tube. This tube was every time heated to 

 a definite temperature, and the temperature being kept constant, 

 the magnetic field was varied till the difference of the intensities 

 of the differently polarized beams of light was a maximum; this 

 measurement took place by rotation of a glass plate, which served 

 as a compensator. F'or every temperature of the Wood tube the field 

 was determined, in which the difference of the intensities became a 

 maximum. Miss v. Ubisch makes the supposition that this difference 

 will be a maximum when one ZEEMAN-component coincides with the 

 maximum of absorption, and the other is not absorbed at all. By the 

 aid of this the writer deduces that at 270^ the displacement of the 



