290 Professor 2eeman [March 30, 



Magnetisation of the Spectral Lines. 



In my own case, the thought to submit a source of light to the 

 influence of magnetism occurred to me during a quantitative investiga- 

 tion of the effect discovered by Kerr, concerning the Hght reflected 

 by magnetised mirrors. I was working at the time in Leiden, in 

 Professor Onnes' laboratory. The account of Faraday's negative 

 experiment encouraged me in my endeavours, and also an argument 

 in 1856 by Lord Kelvin, referred to by Maxwell as the "exceed- 

 ingly important remark of Sir W. Thomson." If it might be 

 accepted, that the forces operating during the propagation of light 

 in magnetised substances exist also whenever the source of light is in 

 the magnetic field, we can expect some direct effect of magnetism on 

 radiation. 



My own successful experiments date from 1896 to 1897, whereas 

 three years earlier I also had recorded a negative result, not having 

 then used adequate means. 



As you know, a sodium flame chiefly emits two kinds of yeUow 

 light, and accordingly its spectrum when analysed with one of Row- 

 land's large concave gratings, shows two yellow lines. With a grating 

 of medium size, these lines have a distance of one millimetre ; they 

 are rather narrow as shown in the slide. In August 1896, I found 

 that when a sodium flame is placed between the poles of an electro- 

 magnet, and is looked at with a spectroscope in a direction at right 

 angles to the lines of force, the yellow lines in its spectrum become 

 somewhat wider when the magnetic field is put on.* This fact can 

 be expressed in a different way by saying that besides the original 

 vibrations, a flame in a magnetic field emits other vibrations, of 

 which some have a somewhat greater, and some a somewhat smaller 

 frequency than the original vibrations. 



This observation of a small change in a spectral line was the 

 origin of my subsequent work. I realised that this change, however 

 small, was worth a closer examination. Indeed, it seemed clear at 

 once, that here we had a means of studying the internal vibrations 

 of a molecule by modifying in a simple way the conditions under 

 which they are going on. Of course, the result was verified in all 

 directions. As there is now, I think, no doubt as to the reality of 

 the observed changes, I shall only refer very briefly to this stage of 

 the work. In the first place the widening of the lines was observed 

 in the direction of the lines of force also. Then the fact was estab- 

 lished that to the observed direct effect, there corresponds an immerse 

 one. When white light traverses the incandescent sodium vapour, we 

 observe the al)Sorption lines ; these also are widened when the vapour 

 is subjected to magnetic forces. Secondary influences were discarded 



* Zeeman. Verslagen Kon. Akademie v. Wetenschappen, Amsterdam 

 October and November, 18*J0. Phil. Mag., March, 1897. 



