An experiment enabled this prevision to be verified.  It was made, as is well known, as early as 1896 by Zeeman; and the discovery produced a legitimate sensation.  When a flame is subjected to the action of a magnetic field, a brilliant line is decomposed in conditions more or less complex which an attentive study, however, allows us to define.  According to whether the observation is made in a plane normal to the magnetic field or in the same direction, the line transforms itself into a triplet or doublet, and the new lines are polarized rectilinearly or circularly.

These are the precise phenomena which the calculation foretells:  the analysis of the modifications undergone by the light supplies, moreover, valuable information on the electron itself.  From the direction of the circular vibrations of the greatest frequency we can determine the sign of the electric charge in motion and we find it to be negative.  But, further than this, from the variation of the period we can calculate the relation of the force acting on the electron to its material mass, and, in addition, the relation of the charge to the mass.  We then find for this relation precisely that value which we have already met with so many times.  Such a coincidence cannot be fortuitous, and we have the right to believe that the electron revealed by the luminous wave which emanates from it, is really the same as the one made known to us by the study of the cathode rays and of the radioactive substances.

However, the elementary theory does not suffice to interpret the complications which later experiments have revealed.  The physicists most qualified to effect measurements in these delicate optical questions—­M.  Cornu, Mr Preston, M. Cotton, MM.  Becquerel and Deslandres, M. Broca, Professor Michelson, and others—­have pointed out some remarkable peculiarities.  Thus in some cases the number of the component rays dissociated by the magnetic field may be very considerable.

The great modification brought to a radiation by the Zeeman effect may, besides, combine itself with other phenomena, and alter the light in a still more complicated manner.  A pencil of polarized light, as demonstrated by Signori Macaluzo and Corbino, undergoes, in a magnetic field, modifications with regard to absorption and speed of propagation.

Some ingenious researches by M. Becquerel and M. Cotton have perfectly elucidated all these complications from an experimental point of view.  It would not be impossible to link together all these phenomena without adopting the electronic hypothesis, by preserving the old optical equations as modified by the terms relating to the action of the magnetic field.  This has actually been done in some very remarkable work by M. Voigt, but we may also, like Professor Lorentz, look for more general theories, in which the essential image of the electrons shall be preserved, and which will allow all the facts revealed by experiment to be included.