When High Voltage was applied between the cathode and anode with pressure, a bright spot of light was formed on the zinc sulphide screen. This is due to: 

When a high voltage (HV) was applied between the cathode and the anode with a small hole at the centre of a partially evacuated tube at a pressure of 0.01 mm of Hg, a bright spot of light was formed on the zinc sulphide screen kept at the opposite end of the discharge tube. This was caused by the rays which originated from the cathode called cathode rays.