Crookes tube. Illustration of a Crookes tube, 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. Objects placed in the evacuated, low pressure tubes formed distinct shadows on the glass, proving that the rays travelled in straight lines. 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.