352 History of Luminescence 



day in 1859. Becquerel's results will be taken up under these head- 

 ings, together with the important discoveries of some of his early 

 contemporaries. In this way the type of research, and some of the 

 results of the recent period can be presented. 



WAVE-LENGTHS FOR EXCITATION 



When Becquerel started his study of phosphors, the varying effect 

 of different regions of the spectrum was well recognized. His inge- 

 nuity in demonstrating these relations visually is attested by his 

 method (1843) of powdering a phosphor, spreading the powder 

 mixed with gum arable as an adhesive on paper, and then exposing 

 it to the sun's spectrum. The exciting effect of ultraviolet and the 

 quenching of infrared show beautifully under these conditions. He 

 continued the studies in 1848, and in the first paper of 1859 investi- 

 gated a large number of different phosphors, recording the wave- 

 lengths of the exciting light. After G. G. Stokes' (1852) paper, the 

 phosphorescent sheet was replaced by a fluorescent one, and solar 

 spectra in the ultraviolet observed by W. Eisenlohr (1856) , using 

 quinine sulphate paper, and by Stokes (1862) , using uranium glass. 



In 1843 Becquerel noticed a streak in the infrared quenched 

 region which remained bright, a dark infrared line, and he later 

 (1873, 1876) made this observation the basis for a method of study- 

 ing the infrared spectrum. ^^ He (1848) had also noticed that infra- 

 red quenching was preceded by an increased intensity, before the 

 phosphorescence faded. 



In 1881 John William Draper, M. D. (1811-1882), professor of 

 chemistry and physiology in New York University, for whom the 

 Grotthus-Draper law of photochemistry is named, refined the method 

 by adding photography. He had obtained a photograph of the solar 

 spectrum in 1842, showing the region to Avhich silver iodide is sensi- 

 tive. On exposing powdered phosphorescent material to the sun's 

 spectrum in the dark, the exciting action of the blue, violet, and 

 ultraviolet regions were visible, essentially as in his previous photo- 

 graph. A second exposure in a light sufficient to cause moderate 

 phosphorescence of this phosphor allowed the quenching effect of 

 the red and infrared region to become effective.^^ He then placed 



^* The modern method is the use of an infrared sensitive photographic plate. 



^' Extinction by infrared and excitation by blue wave-lengths were responsible for 

 some peculiar effects when phosphor layers were exposed to various mixed light 

 sources, phenomena investigated by Stokes (1882) about the same time as Draper's 

 experiments (1881) , and by G. LeBon (1900) . In 1874 J. L. Soret described a fluo- 

 rescing ocular of uranium glass. In 1883 E. Lommel introduced the phosphorescing 

 ocular (a thin layer of Balmain's paint on glass) for the spectroscope and later (1888- 



