230 Dr. P. Zeeman on the Influence of Magnetism 



qualitative kind *. In the case of a band-spectrum the mole- 

 cules are complicated, in the case of a line-spectrum the 

 widely separated molecules contain but a few atoms. Further 

 investigation has shown that the representation I had formed 

 of the cause of the widening in the case of a line-spectrum 

 in the main was really true. 



13. A glass tube, closed at both ends by glass plates with 

 parallel faces and containing a piece of iodine, was placed 

 between the poles of the JRuhmkorff electromagnet in the 

 same manner as the tube of porcelain in § 7. A small flame 

 under the tube vaporized the iodine, the violet vapour filling 

 the tube. 



By means of electric light the absorption-spectrum could 

 be examined. As the temperature is low this is the band- 

 spectrum. With the high dispersion used, there are seen 

 in the bands a very great number of fine dark lines. If the 

 current round the magnet is closed, no change in the dark 

 lines is observed, which is contrary to the result of the expe- 

 riments with sodium vapour. 



The absence of the phenomenon in this case supports the 

 explanation, that even in the first experiment, with sodium 

 vapour (§ 7), the convection-currents had no influence. For 

 in the case now considered, the convection-currents origi- 

 nated by magnetism, which I believed to be possible in that 

 case, apparently are insufficient to cause a change of the 

 spectrum ; yet, though I could not see it in the appearance of 

 the absorption-lines [of. § 7), the band-spectrum is, like the 

 line-spectrum, very sensible to changes of density and of 

 temperature. 



14. Although the means at my disposal did not enable me 

 to execute more than a preliminary approximate measurement, 

 I yet thought it of importance to determine approximately 

 the value of the magnetic change of the period. 



The widening of the sodium lines to both sides amounted 

 to about Jq of the distance between the said lines, the inten- 

 sity of the magnetic field being about 10 4 C.Gr.S. units. 

 Hence follows a positive and negative magnetic change of 



4u^oo of tne P eriod - 



15. The train of reasoning mentioned in (1), by which 

 I was induced to search after an influence of magnetism, was 

 at first the following: — If the hypothesis is true that in a 

 magnetic field a rotatory motion of the sether is going on, the 

 axis of rotation being in the direction of the magnetic forces 

 (Kelvin and Maxwell), and if the radiation of light may be 

 imagined as caused by the motion of the atoms, relative to 



* Kayser in Winkelmann's Handbuch, ii. 1, p. 421, 



