He also engaged in work on magnetism, the polarization of light, phosphorescence and the absorption of light in crystals.
The phosphorescence produced by friction has been known since the time of Robert Boyle (1663); the diamond becomes luminous in a dark room after exposure to sunlight or in the presence of radium; and many stones phosphoresce beautifully (generally with a pale green light) when subjected to the electric discharge in a vacuum tube.
An authority on precious stones, and especially the diamond, he succeeded in artificially making some minute specimens of the latter gem; and on the discovery of radium he was one of the first to take up the study of its properties, in particular inventing the spinthariscope, an instrument in which the effects of a trace of radium salt are manifested by the phosphorescence produced on a zinc sulphide screen.
Michelson's experiments therefore argue in favour of the view that the luminescence in a vacuum tube is similar to that produced by phosphorescence where the translatory energy does not correspond to the oscillatory energy - but further experiments are desirable.
Formerly the sparkand absorption-spectra were the sole methods available; a third method was introduced by Crookes, who submitted the oxides, or preferably the basic sulphates, to the action of a negative electric discharge in vacuo, and investigated the phosphorescence induced spectroscopically.
The phosphorescence of the sulphide obtained by heating the thiosulphate is much increased by adding uranium, bismuth, or thorium before ignition pr.
Characteristic, also, is their action on a photographic plate, and the phosphorescence which they occasion when they impinge on zinc sulphide and some other salts.