Crookes tube. Invented by William Crookes (1832 - 1919) in the late 19th century, this apparatus was used to investigate the path taken by cathode rays. On the application of an electric current, electrons are emitted from the cathode. They cause fluorescence on the back wall. By placing a Maltese Cross in front of the cathode, Crookes showed from the resulting shadow that cathode rays travel in straight lines.