teristic glow. For example, if a screen coated with, zinc sulphide is placed within a discharge tube in the manner shown in Fig. 3, the cathode rays which pass through the slit in the mica diaphragm just opposite the cathode, light it up brilliantly in the parts along which they graze, and thus trace a distinct outline of their path from one end of the tube to the other.

The nature of these rays was the subject of much dispute between the years 1880, when they first began to be studied, and 1898. Some thought them to be streams of minute negatively charged particles shot off with enormous velocities from the cathode C, while others maintained that they did not consist of Fig. 2. Showing Cathode Beam of Rays.Fig. 3. Showing Deflection of Cathode Rays by a Magnet. particles at all, but were waves in the ether, just like light waves. The dispute was finally ended by two very conclusive experiments performed, the first by Perrin, a Frenchman, and the other by J. J. Thomson, professor of physics in Cambridge University, England. Perrin's experiment consisted in proving that under all circumstances a body which was placed along the path OP, so that the cathode rays could fall upon it, became charged with negative electricity, just as would be expected if the cathode rays consisted of negatively charged particles. J. J. Thomson's experiment consisted in showing that if a charge of positive electricity were placed upon the plate E (see Fig. 1), and a charge of negative electricity upon the plate D, the rays were deflected out of the line OP and into the path OP'. This, too, was to have been expected if the rays consist of negatively charged particles, for these particles would be repelled by the negative electricity upon D and attracted by the positive electricity upon E.

There is a further property of the rays, which, although it had long been known, adds powerful support to the projected particle theory. It is that when a magnet is brought near the cathode beam in the manner shown in Fig. 3, the beam is deflected by it also, just as would be expected if it consisted of a stream of negatively charged particles. These three experiments settled the question in favor of the projected particle theory, so that physicists are now all agreed in regarding the cathode rays as streams of minute, negatively charged corpuscles shot off in straight lines from the surface of the negative electrode and in a direction at right angles to this surface.