William Crookes' famous 1887 Maltese cros experiment, c 1990s.
© Science Museum / Science & Society Picture Library
Modern demonstration of William Crookes' famous 1887 Maltese cros experiment. When electric current pases through a glas tube containing a small amount of air, the air glows. The invisible radiation causing the fluorescence is called cathode rays, or electrons. The English physicist and chemist, Crookes (1832-1919), conducted investigations into cathode rays, demonstrating that they usually travelled in straight lines by placing an obstacle, a Maltese cros, in the path of the rays, so a shadow was cast in the fluorescence at the end of the tube.