Introduction 7 



In the case of fluorescence, the displaced electrons return to their 

 original level within 10"^ or 10"^ seconds. If they fail to return in 

 this time, they may be constrained, with the delay lasting from frac- 

 tions of a second to several hours, as in phosphorescence. Sometimes 

 they may become actually trapped, and only return to their original 

 position after release by application of additional energy. This 

 energy may be heat in the case of thermoluminescence, more prop- 

 erly called thermostimulation, or light in the case of photostimula- 

 tion, when a phosphor which has ceased to luminesce may again 

 emit if exposed to infrared radiation. 



With recent improvement in light-detecting photocells and elec- 

 tronic circuits, the individual flash induced in various substances 

 by a single quantum of light (a photon) can be detected, and the 

 incident radiation measured with a scintillation counter. This 

 device is a modern and more sensitive development of the old spin- 

 thariscope of Crookes (1903) which made use of the flash of light 

 from a zinc sulphide phosphor to render visible the alpha particles 

 (charged helium atoms) from radium, which struck the phosphor. 



The completely dark-adapted eye is a very sensitive instrument, 

 but the scintillation-counter can now detect and measure lights in 

 the visible region of the spectrum which are completely invisible to 

 the most sensitive human eye. The historian of the future will have 

 a wholly new world of unseen luminescences with which to contend. 



