234 Dr. P. Zeeman on the Influence of Magnetism 



period. In the third place there are ions which seem to move 

 in rotating elliptical orbits. 



If one desires to know the state of the aether originated by 

 the moving ions one may nse the following rule, deduced by 

 Prof. Lorentz from the general theory: — Let us suppose that 

 in a molecule an ion P — of which the position of equilibrium 

 is P — has two or more motions at the same time, viz. let the 

 vector P P always be obtained by adding the vectors P P 

 which should occur in each of the component motions at that 

 moment ; then the state in the aether at a very great distance 

 in comparison with P P will be obtained by superposing the 

 states which would occur in the two cases taken separately. 



Hence it follows in the first place that a circular motion of 

 an ion gives circularly polarized light to points on the axis of 

 the circle. 



Further, one may choose instead of the above-considered 

 elliptical orbits a resolution more suited to our purpose. One 

 may resolve the motion of the ion, existing before the putting 

 on of the magnetic force, into a rectilinear harmonic motion 

 parallel to the axis of z and two circular (right-handed and 

 left-handed) motions in the plane of (#, y). 



The first remains unchanged under the influence of the 

 magnetic force, the periods of the last are changed. 



By the action of the grating the vibrations originated by 

 the motion of the ions are sorted according to the period, and 

 hence the complete motion is broken up into three groups. 

 The line will be a triplet. At any rate one may expect that 

 the line of the spectrum will be wider than in the absence of 

 the magnetic field, and that the edges will give out circularly- 

 polarized light *. 



20. A confirmation of the last conclusion may be certainly 

 taken as a confirmation of the guiding idea of Prof, Lorentz's 

 theory. To decide this point by experiment, the electromagnet 

 of § 2, but now with pierced poles, was placed so that the 

 axes of the holes were in the same straight line with the centre 

 of the grating. The sodium lines were observed w T ith an eye- 

 piece with a vertical cross-wire. Between the grating and 

 the eyepiece were placed the quarter-undulation plate and 

 nicol which I formerly used in my investigation of the light 

 normally reflected from a polarly magnetized iron mirror f. 



The plate and the nicol were placed relatively in such a 



* I saw afterwards that Stoney, Trans. Roy. Soc. Dublin, iv., en- 

 deavours to explain the existence of doublets and triplets in a spectrum 

 by the rotation of the elliptical orbits of the " electrons " under the in- 

 fluence of perturbing forces. 



t Zeeman, ' Communications of the Leyden Laboratory,' no. 15. 



