1906.] on Recent Progress in Magneto -Optics. 291 



by suitable modifications of the experiments. In one case no change 

 was observed. The spectra of fluted bands, such as those of iodine, 

 carbon, or nitrogen did not show any effect ; nor could Becquerel and 

 Deslandres using increased power discover it. 



Before I could answer the different questions which presented 

 themselves, I had the advantage that the beautiful theory of the 

 electromagnetic and optical phenomena, developed by my friend Prof. 

 Lorentz, gave its quickening influence on my experimental work. 



In this theory, it is supposed that the material world is built up 

 of three things : ponderable matter, ether, and electrons. I think it 

 is rather superfluous to remind you here in the land of Maxwell, 

 Kelvin, Crookes, J. J. Thomson, Schuster, Larmor, Heaviside, and 

 Johnstone Stoney, that electrons or corpuscles are exceedingly small, 

 electrically charged particles, which are supposed to be present in all 

 material bodies. 



These electrons can perform oscillations, under the influence of 

 the forces which attract them to their position of equilibrium. Be- 

 cause they are electrified, they have sufficient hold on the ether to 

 excite in it the electromagnetic vibrations, which, according to 

 Maxwell's theory, constitute light. 



The oscillatory periods of the electrons determine the position of 

 the lines in the spectrum, and with every change in the period of 

 oscillation we observe a displacement of the coiTesponding fine. 



In Lorentz's theory the explanation of the effect of a magnetic 

 field is as simple, as it is beautiful. 



The forces operating on the vibrating electron in a magnetic field 

 are fairly well known. These forces are the same which curve the 

 path of the kathode rays in a vacuum tube which is acted on by a 

 magnet. All motions of the electrons in the molecules of a flame 

 may be supposed to be made up of three particular motions, chosen 

 in such a manner that the action of the magnetic field on each of 

 them can be easily foreseen. The light of the flame is exactly the 

 same as it would be, if the flame contained three groups of electrons, 

 vibrating in these simple ways. 



In this model, the electrons are represented by red balls, the black 

 arrow indicates the direction of the magnetic force. (Fig. 1.) 



As a first simple motion we choose a vibration parallel to the lines 

 of force. On the group of electrons which possess this motion, the 

 magnetic force has no influence ; the period, which we call T, remains 

 unmodified. The other two simple motions are circular motions, clock- 

 wise or anti-clockwise, in planes perpendicular to the fines of force. 



An electron performing either of these rotations, will be acted on 

 by a force which is directed towards or from the centre, dependent 

 on the direction of the rotation. The magnetic field must, therefore, 

 cause the speed of the electron either to increase or to decrease, and 

 so will either increase or diminish the period. Therefore, instead of 

 one motion with period T, we get under the influence of the field 



IT 2 



