The various phenomena of fluorescence and phosphorescence undoubtedly explain some at least of the legends regarding luminous stones, superstition or fantasy having here as in most other cases a certain substratum of fact. This class of physical phenomena has been made the subject of special investigation by the author, as many as 13,000 specimens of various minerals having been subjected to the most searching tests in order to determine their qualities in this respect.[255] His interest in this field of research was greatly stimulated by a fortuitous happening. In 1891 his wife, while hanging up a gown in a closet one evening, saw that the diamond in a ring she was wearing gave off a faint streak of light which was very noticeable in the dark, and this fact led to a long series of experiments on the fluorescence, phosphorescence, and triboluminescence of the diamond.[256] More than two centuries before, Robert Boyle made a similar set of experiments at night with a diamond which must have been an Indian stone, and which he describes as table cut, about one-third of an inch long and somewhat less in width; he remarks that it was a dull stone of very bad water, having a blemish with a whitish cloud covering nearly a third of the stone.[257] The “Journal des Sçavans” for 1739 gives certain tests of the luminous quality of diamonds made by Mons. Du Fay. In order successfully to observe this phenomenon, he prescribes that the experimenter shall remain in a darkened room for fifteen minutes, taking the additional precaution of closing one or both of his eyes. The diamond to be tested should be exposed to the sun’s rays, or to strong daylight, for less than a minute, and when taken into darkness the luminosity, if observable, lasts twelve or thirteen minutes at longest. Not all diamonds show this quality, and nothing in their form or appearance serves to determine their possession of it. However, Mons. du Fay observed that the yellow diamonds, of which he tried a considerable number, were luminous. A single emerald, out of twenty that were tested, proved to be luminous.[258] [Illustration: 1. Self-print of upper diamond of No. 4 by phosphorescence, produced by rubbing briskly with stick covered by woolen cloth. Exposure one-half minute. 2. Self-print, both diamonds, after one minute’s exposure to ultra-violet light, electric action eliminated. 3. Self-print, upper diamond. Exposure one-fourth minute. 4. Upper: blue-white Tiffanyite diamond, 14.86 carats; Bagagem Mine, Brazil. Lower: purple-black diamond, 13.35 carats; Brazil. 5. Self-print, both diamonds; different position. 6. Aspect of both diamonds (No. 4), one minute’s exposure, ultra-violet light; blue-white phosphorescing white, purple-black having red glow.]