542 
Prof. Thomson, On the velocity of 
was a window made of wire gauze and in the middle of the box, 
insulated from it, was a horizontal piece of wire gauze divided into 
two pieces D and E which were insulated from each other. 
Electrodes came from the box and from the two pieces of gauze 
to the outside of the tube. A window formed of a piece of tube 
in the side of the box allowed observations of the luminosity in 
the box to be made from the outside. The distance between the 
top and bottom of the box was 7 mm. When the cathode rays 
passed the gauze at the top, the bluish haze spread through the 
box, filling it with uniform luminosity. If the sides of the box 
were connected to the positive pole of a battery and the gauze to 
the negative pole, the negative particles sent through the window 
at the top were acted upon by a force tending to stop them, and 
if the potential difference between the gauze and the top of the 
box exceeded a certain value, the luminosity instead of filling the 
box stopped short of the gauze and was confined to a layer at the 
top of the box, the thickness of this lnyer diminishing as the 
potential difference between the gauze and the box increased. 
If the particles lost their power of producing luminosity long 
before being stopped, so that some of them travelled through the 
dark space at the top of the box, then the electric field below the 
gauze would increase their energy, restoring to the particles the 
energy lost in the upper part of the box, and the luminosity ought 
to appear again against the lower surface of the box. 
