A magic eye is a simplistic triode tube that was used as a tuning indicator in radio receivers around the time of world-war II. It has a bowl shaped anode coated with a phosphorescent material that produces a green glow when electrons strike on it. This makes possible to measure the curvature of electrons’ trajectory when the tube is placed in a magnetic field. By measuring the radius of curvature, the applied voltage between cathode and anode and the magnetic field, charge-to-mass ratio of the electron can be measured.