producing light. By the 1870s, Crookes (among other researchers) was able to evacuate his tubes to a lower pressure, 10−6 to 5x10−8 atm, using an improved Sprengel mercury vacuum pump invented by his coworker Charles A. Gimingham. He found that as he pumped more air out of his tubes, a dark area in the glowing gas formed next to the cathode. As the pressure got lower, the dark area, now called the Faraday dark space or Crookes dark space, spread down the tube, until the inside of the tube was totally dark. However, the glass envelope of the tube began to glow at the anode end. What was happening was that as more air was pumped out of the tube, there were fewer gas molecules to obstruct the motion of the electrons from the cathode, so they could travel a longer distance, on average, before they struck one. By the time the inside of the tube became dark, they were able to travel in straight lines from the cathode to the anode, without a collision. They were accelerated to a high velocity by the electric field between the electrodes, both because they did not lose energy to collisions, and also because Crookes tubes were operated at a higher voltage. By the time they reached the anode end of the tube, they were going so fast that many flew past the anode and hit the glass wall. The electrons themselves were invisible, but when they hit the glass walls of the tube they excited the atoms in the glass, making them give off light or fluoresce, usually yellow-green. Later experimenters painted the back wall of Crookes tubes with fluorescent paint, to make the beams more visible. This accidental fluorescence allowed researchers to notice that objects in the tube, such as the anode, cast a sharp-edged shadow on the tube wall. Johann Hittorf was first to recognise in 1869 that something must be travelling in straight lines from the cathode to cast the shadow. In 1876, Eugen Goldstein proved that they came from the cathode, and named them cathode rays (Kathodenstrahlen). At the time, atoms were the smallest particles known and were believed to be indivisible, the electron was unknown, and what carried electric currents was a mystery. During the last quarter of the 19th century, many ingenious types of Crookes tubes were invented and used in historic experiments to determine what