Editing Cathode ray (section)

===Spectral shift===
[[Eugen Goldstein]] thought he had figured out a method of measuring the speed of cathode rays. If the [[glow discharge]] seen in the gas of Crookes tubes was produced by the moving cathode rays, the light radiated from them in the direction they were moving, down the tube, would be shifted in [[frequency]] due to the [[Doppler effect]].<ref name="Brona" /> This could be detected with a [[spectroscope]] because the [[emission line]] [[spectrum]] would be shifted. He built a tube shaped like an "L", with a spectroscope pointed through the glass of the elbow down one of the arms. He measured the spectrum of the glow when the spectroscope was pointed toward the cathode end, then switched the power supply connections so the cathode became the anode and the electrons were moving in the other direction, and again observed the spectrum looking for a shift. He did not find one, which he calculated meant that the rays were traveling very slowly. It was later recognized that the glow in Crookes tubes is emitted from gas atoms hit by the electrons, not the electrons themselves. Since the atoms are thousands of times more massive than the electrons, they move much slower, accounting for the lack of Doppler shift.