Phosphorescence 341 



infrared wave-lengths on phosphors cannot be attributed to any one 

 man. Apparently Johann Wolfgang von Goethe (1749-1832) in col- 

 laboration with Thomas Johann Seebeck (1770-1831), a physician, 

 and discoverer of thermoelectric junction potentials, noticed at some 

 time between 1786 and 1792 that the blue and violet end of the 

 spectrum was much more effective in exciting luminescence of vari- 

 ous phosphors than the red end. The account appeared in Goethe's 

 ZuT Farbenlehre (1810) , and contains the statement, " beyond the 

 violet, where scarsely any color can be observed, it [the phosphor] 

 gave a vivid brilliance, as in the violet region." According to 

 R. Winderlich (1936) the observation was made on Goethe's re- 

 turn to Weimar, after a visit to Bologna in October 1786, where 

 he procured specimens of the heavy spar. Goethe wrote from 

 Weimar of this discovery in a letter dated July 2, 1792 to S. T. 

 von Sommerring (1755-1830) , a physician of Mainz, interested in 

 physics and natural history. 



Careful testing of phosphors in the ultraviolet region of the sun's 

 spectrum was made by Ritter (1803) and by Christian Ernst Wunch 

 (1744-1828) , a professor of mathematics and physics at Frankfurt 

 a. O. in 1803 and also by Dessaignes (1808) , Heinrich (1811) , and 

 Grotthus (1815). Somewhat later, T. J. Pearsall (1830) noted the 

 bright green luminescence of chlorophane excited by the ultra- 

 violet light in electric sparks, and made an extended study of thermo- 

 phosphorescence of various materials after such exposure (see 

 Chap. IX) . 



The identification of infrared radiation is usually attributed to 

 the eminent English astronomer. Sir William Herschel, in 1800, 

 but it was von Ritter (1803) , who put the infrared region to a 

 thorough test with phosphors, finding that these wave-lengths 

 diminish the phosphorescence. His explanation was based on an 

 incorrect theory. He believed that the light of phosphors came 

 from a kind of combustion, an oxidation, and that short wave- 

 lengths had a reducing action and long wave-lengths an oxidizing 

 effect on substances they illuminated.^^ When exposed to violet or 

 better to ultraviolet, a phosphor luminesced well because the short 

 wave-lengths reduce and prepare the phosphor for new oxidation. 

 Long wave-lengths like red and infrared, which have an oxidizing 

 effect, actually decrease the light intensity. 



Again the question of priority is difficult to decide. Seebeck may 

 have noticed the quenching action of less refrangible, long-wave 



*^ This idea arose from Ritter's studies on light and silver chloride. Similar views 

 will be found in the work of W. Herschel, E. Becquerel and J. W. Draper on photo- 

 chemical action. 



