Cathode ray tubes. Artwork of 3 Crookes tubes, a type of cathode ray or vacuum tube, showing effects caused by exposure to cathode rays. In the upper image a screen painted with platinum cyanide and barium fluoresces. In the centre image two minerals that appear colourless under normal light are fluorescing blue & orange. In the lower image Geissler tubes filled with fluid are fluorescing. Crookes tubes were used by British physicist W. Crookes from 1878 to investigate cathode rays. He found that the rays (which are made of electrons) made glass fluoresce, travelled in straight lines and could be bent by applying a magnetic field. Image by H. Kraemer in the 19th century.