Firstly is a vacuum tube a completely evacuated tube? In other words is the interior of it a real vacuum? Secondly what exactly is the visible cathode ray in a Crookes or Geissler tube? Sometimes they say the tube is coated with a phosphorescent material which glows when hit by electrons, other times they say the tube contains a low concentration of noble gas atoms and the cathode ray is the light emitted by excited gas atoms returning to their ground state. Which is it, is phosphorescence or is it noble gas emissions?