Page:Popular Science Monthly Volume 4.djvu/325

Rh Phosphorus, the light-bearer, as its name implies, has the property, long supposed to be peculiar to it, of faintly shining in the dark. But, if a diamond is exposed to sunshine, and then withdrawn into darkness, it continues feebly luminous for a considerable time, and is, therefore, said to be phosphorescent. Other substances, as sulphuret of calcium, and sulphuret of barium, have also been long noted for this property, and recent researches have shown that, so far from being any thing peculiar, the same property is manifested in a much lower degree by a vast number of substances. The differences are in the time the phosphorescence continued after withdrawal from the sun's rays. It was found, in most instances, extremely short, only the small fraction of a second, and it became necessary to devise some means of measuring the time in different cases. A contrivance was necessary which should expose an object to the sun, and then jerk it quickly into total darkness, where it could be seen by the observer if it dragged any light along with it, for even the thousandth of a second.



A contrivance for this purpose was made by Edmund Becquerel, and called the phosphoroscope. It consisted of a train of wheels and pinions (Fig. 1) for producing rapid revolving motion. There was a