354 History of Luminescence 



(1859) had also observed the green fluorescence of the glass of 

 vacuum tubes. These two men were apparently the first to observe 

 cathodoluminescence. To Wm. Crookes (1879) belongs the credit 

 for exhaustive experimentation and observation of a variety of 

 luminescences excited by cathode rays, canal rays, X-rays, and radium 

 rays and other kinds of radiation, as described in Chapter XII on 

 radioluminescence. 



DURATION OF PHOSPHORESCENCE 



Early workers may have presumed that a phosphor emitted light 

 during illumination, but actual proof of the emission cannot be 

 obtained by observation in sunlight, which obscures the feeble lumi- 

 nescence. However, the phosphorescence can be seen during illumi- 

 nation if the exciting light is of a different wave-length from the 

 emitted light. The most striking demonstration comes from excita- 

 tion with completely invisible ultraviolet light, as in the previously 

 mentioned spectral studies. Nevertheless, because a short time in- 

 terval cannot be estimated by the eye, the question remains as to 

 how quickly after excitation phosphorescence appears and how soon 

 it ceases after the exciting light is cut off.^^ 



This duration of phosphorescent light appears to have aroused 

 much less interest among early observers than its color, or the 

 methods of excitation. The very early workers noticed only the 

 long lasting phosphorescences. However, the improved technique 

 for observation introduced by Beccari and his associates indicated 

 that many materials would phosphoresce for a very short time after 

 exposure. The question as to whether a phosphor luminesced the 

 instant it was illuminated or stopped the instant it was cut off, or 

 how short an exposure was necessary to excite a phosphorescence 

 could only be answered by new experimental techniques for meas- 

 urement of short time intervals. 



Probably the first attempt to measure the duration of a fluores- 

 cence, that of uranium glass, was made by E. Esselbach in 1856, 

 although his method was not published and only the result, about 

 1/2000 second, appeared in 1863.^* Credit for invention of the 

 phosphoroscope ^^ belongs entirely to Edmond Becquerel. His in- 



"^ The present term, fluorescence, was not applied to a phosphorescence which 

 lasted only as long as the material was exposed to light until after 1852. At that time 

 the word was coined by G. G. Stokes for light emission in fluorspar (hence the word) 

 and also for material in solution (see Chapter XI) . 



<"■ Rep. Brit. Assoc. Adv. Sc. for 1862, 32: 22, 1863. 



"^ Becquerel (Corii. Rend. Acad. Sci. 46: 969-975, 1858) , as early as 1854, considered 

 there was no difference between fluorescence and phosphorescence and set out to 

 develop the phosphoroscope to test this. (See his book, 1: 320, 1867.) 



