Phosphorescence 353 



the phosphorescent screen on a photographic plate to make the 

 record (after development) permanent and called the print a 

 phosphorograph.^° Draper had published a general paper " On the 

 Phosphorescence of Bodies," in 1851, and another, " On the Pro- 

 duction of Light by Chemical Action," in 1848. His interest in 

 phosphors was no doubt the result of previous work on photo- 

 chemical and phosphorogenic effects of the chemically active ultra- 

 violet light, which he referred to as tithonic ®^ rays in papers pub- 

 lished in 1841-1845. 



In 1852 G. G. Stokes (1820-1903) published the correct inter- 

 pretation of fluorescence and announced what is now called Stokes' 

 law that the exciting light is of shorter wave-length than the emitted 

 light. Becquerel (1859) confirmed the general truth of this law, 

 which was so much discussed in later work on luminescence. 



The testing of methods of excitation other than visible ^^ or 

 ultraviolet light came with the development of vacuum techniques. 

 In the 1839 paper, the Becquerel group had placed phosphorescent 

 substances in a vacuum and exposed them to electrical discharges 

 sent through the evacuated tube. A luminescence was observed but 

 it was much weaker than the excitation from the electric discharge 

 outside the vacuum. However, in 1857 to 1859 E. Becquerel's experi- 

 ments in glass tubes with sealed-in electrodes, highly evacuated, and 

 also containing various gases under different pressures, led to new 

 results. He noticed that double cyanides of platinum or sulphides 

 of calcium and barium placed in the tubes luminesced most brightly 

 in the neighborhood of the cathode. His observation that the glass 

 of the tube fluoresced green when a high tension current was passed 

 through, probably indicating the generation of cathode rays, whose 

 excitation of glass is now well known. In the same year, J. Pliicker 



1891) published more extended papers on " Phosphoro-photographie des ultrarothen 

 spectrums." 



"" Draper's claims to priority for the phosphorograph gave rise to some acrimonious 

 discussion. See W. de W. Abney (1881) and E. Becquerel (1882) . H. Becquere) 

 (1883, 1884) and E. Lommel (1888, 1889, 1890) continued the studies. 



*^ From Tithonus, a beautiful youth with whom Aurora fell in love. 



®^A particularly potent source of light for phosphor excitation was that of lime 

 heated by an invisible flame like burning alcohol. H. F. Talbot (1835) had spread 

 lime on paper and then burnt the paper with an alcohol flame, finding that the lime 

 framework gave oft a very intense light. He thought the heat of the flame caused 

 the lime molecules to vibrate, thereby causing undulations of light in the surround- 

 ing medium. If molecules were able to vibrate when cold they would also produce 

 light, and he therefore proposed this simple explanation of the light of phosphors. 

 The first description of the " lime light," which became a symbol of brilliancy, appears 

 to be that of Wm. Brewster in the Edinburgh Philos. Jour. 3: 343-344, 1820. Draper 

 (1845) called attention to the difference between lime light and electric sparks in 

 exciting Canton's phosphor. 



