Crookes tube. Invented by William Crookes (1832 - 1919) in the late 19th century. This apparatus was used to investigate the path taken by electrons, or cathode rays as they were called then. In this experiment the electrons are emitted from a central disc towards the glass. As the electrons collide with the glass they fluoresce. The metal star pattern blocks the electrons causing a shadow on the glass. Crookes showed from the resulting shadow that electrons travel in straight lines. The overall glow of the apparatus is caused by the excitation of the remaining gas molecules in the tube.