QUICKER THAN LIGHTNING. 



311 



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 if a diamond is exposed to sunshine, and then withdrawn into dark- 

 ness, it continues feebly luminous for a considerable time, and is, there- 

 fore, said to he phosphorescent. Other substances, as sulphuret of cal- 

 cium, and sulphuret of barium, have also been long noted for this prop- 

 erty, and recent researches have shown that, so far from being any 

 thing peculiar, the same property is manifested in a much lower de- 

 gree 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. 



Fig 1. 



Becquerel's Phosphoroscope. 



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 



