Cathode Ray, a high-speed electron emitted by the negative electrode of a vacuum tube when an electric current is passed through it. Cathode rays were first generated by means of the Crookes tube, an invention of the British physicist Sir William Crookes. While conducting research, the German physicist Wilhelm Conrad Roentgen in 1895 accidentally discovered that cathode rays striking a metal target produce X rays. Cathode rays can be deflected and focused by magnetic fields or by electrostatic fields. These properties are utilized in the electron microscope, in the cathode-ray oscilloscope, and in the image tube of a television receiver. See Electron; Electronics; Television.