Cathode ray tube. Circular Crookes tube, a type of cathode ray or vacuum tube. 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 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. This discovery that cathode rays were charged led directly to J.J. Thompson's discovery of the electron in 1897. Image of a Crookes tube from the 19th century.