Abstract: As early as 1858, Plücker observed that, when a tube in which an electric discharge is passing is placed longitudinally in a strong magnetic field, a band of light is seen, which has the cathode for its section, and for its direction the line of magnetic force passing through the cathode. Until comparatively recently the usually accepted explanation of this light was as follows: Under the influence of the magnetic field the cathode rays are bent into the form of a spiral or helix round the lines of magnetic force. Now these primary cathode rays produce secondary rays by collisions with the molecules of the gas, and both the primary and secondary rays produce fluorescence in the gas. The former alone would show the spiral nature of the path, the latter cause a decrease in the definition of the light, which now appears as a band. In the meantime, other physicists have observed the same phenomenon. Attempts have been made to detect an electric charge on the rays, but without success. This lack of charge and the necessity of a magnetic field for their production, have led Villard and Righi, among others, to regard them as a new kind of rays, and the name magneto-cathodic rays (Villard) or magnetic rays (Righi), has been assigned to them to indicate their origin.