It has been supposed that certain electrons revolve like satellites in orbits around the atoms with which they are associated, a view which receives strong support from the phenomena of the Zeeman effect, and on this assumption a theory has been worked out by P. Langevin, 2 which accounts for many, of the observed facts of magnetism.
This subject is treated in the article Magneto-Optics, to which the reader is also referred for John Kerr's discovery of the effect on polarization produced by reflection from a magnetic pole, and for the action of a magnetic field on the radiation of a source - the "Zeeman effect."
The modulation frequency corresponds to the Larmor precession frequency of the spin, or the Zeeman energy splitting of the spin quantum state.
For the study of Zeeman effects (see Magneto-Optics) the echelon seems specially adapted, while the great pliability of 'Fabry and Perot's methods, allowing a clear interpretation of results, is likely to secure them permanently an established place in measurements of precision.