Crookes tube. A diagram showing the apparatus being used to X-ray a hand, 1912. Wilhelm Rontgen discovered X-rays using the Crookes tube in 1895. The Crookes tube was a forerunner to the modern cathode ray tube and television. Crookes tubes were used by British physicist Sir William Crookes from 1878 to investigate cathode rays. He found that the rays, which are actually beams of electrons, made glass fluoresce. Crookes also found that the cathode rays could be bent by applying a magnetic field. The discovery that cathode rays were charged led directly to J.J. Thompson's discovery of the electron in 1897.