Phosphorescence 357 



1893) , F. E. Kester (1899) , and many others have noted deviations 

 from any simple law over the whole period of decay, and proposed 

 decay equations of their own. The problem is greatly complicated 

 in some phosphors by the emission of several bands of light which 

 decay at different rates. The decay equations have become important 

 in testing various theories of phosphoresence. 



SPECTRUM OF PHOSPHORESCENCE 



Another question which was to occupy the research time of Bec- 

 querel and many later investigators, and which was to lead to im- 

 portant theoretical interpretations of phosphor light was the nature 

 of the emission spectra. Although Zanotti (1748) observed the light 

 of phosphors through a prism in 1717, the introduction of the slit 

 (by Wollaston in 1802) was necessary before accurate studies could 

 be made. Again, Becquerel (1843-1859) led the way. In his most 

 important work, the second paper of 1859, he described the bands 

 observed when the material to be studied was mounted as a narrow 

 rod (essentially a slit) in his phosphoroscope, followed by colli- 

 mator, prism, and telescope. Sunlight, electric lights, or sparks were 

 used for excitation. He found that the bands remained the same 

 with different wave-lengths of exciting light although the intensity 

 varied. The bands of the phosphorescent light occupied different 

 spectral regions in different substances; in the uranium salts, they 

 became a series of narrow bands, related in a definite way to the 

 wave-lengths of exciting light. 



In 1852 Stokes had published his announcement that fluorescent 

 light is always of longer wave-length than the exciting light (Stokes' 

 law) , and the same principle was found to apply in most cases to 

 phosphorescence. During the years 1859 and 1872 Becquerel amassed 

 a large amount of data on spectra, some of which appeared in his 

 book (1867) and in his 1872 paper. During this time there was 

 practically no study of phosphorescence spectra in other laboratories, 

 with the exception of a note by Kindt (1867) on fluorspar. 



With the eighteen-eighties a new era began with a flood of papers 

 on luminescent spectra— Ca and Sr sulphide phosphors exposed to 

 light by W. de W. Abney (1882) and by E. Lommel (1886, 1887) , 

 aluminum and rare earth salts exposed to cathode rays by Crookes 

 (1881, 1887) and Lecoq de Boisbaudran (1886-1888), calcium sul- 

 phide by Klatt and Lenard (1889) , as well as many other isolated 

 observations. The importance of spectral measurements had been 

 demonstrated but the results are too detailed and specialized to be 

 considered here. 



