Cathode ray tubes. Artwork of Crookes tubes, a type of cathode ray or vacuum tube. Crookes tubes were used by the British physicist W. Crookes from 1878 to investigate cathode rays (electron beams). The upper five glass tubes are filled with different amounts of air, with the lowest tube the most empty. In this lower tube, the inner glass wall is seen fluorescing with a greenish light. In the large tube at bottom, a Maltese cross has been placed in the tube, and the rays cast a sharp shadow. This led to Crookes theorising that the cathode rays travelled in straight lines. He also found that the beams are bent by magnetic fields, which led to the discovery of the electron.