334 History of Luminescence 



same thing, as a material substance absorbed by the phosphor. The 

 Bolognian variety gave off only a very faint greenish white lumi- 

 nescence under red light but a bright reddish luminescence under 

 the blue light of a spectrum. The emitted light appeared to be 

 always of a different color from the absorbed, but Herbert failed to 

 take into account the different intensities of the emitted lumines- 

 cence, which was to confuse interpretation. Herbert explained the 

 behavior by the affinity of different particles of the phosphor for 

 the red and blue light, -^vhereas the true explanation would appear 

 to depend on the inability of the human eye to detect color in weak 

 lights. At least the color of luminescence in Herbert's experiments 

 was not the same as the exciting light which Beccaria had claimed 

 to be the case. 



The experiments of W. L. Krafft (1777) are of special importance 

 because he used the Canton phosphor exposed to the various colors 

 of a solar spectrum, and found the phosphor light to be of one color. 

 In addition to Krafft, many other writers, M. de Grosser (1777) , 

 M. de Magellan (1777) , von Scherer (1795) , H. C. Englefield (1802, 

 1803), C. E. Wunsch (1803, 1807), Goethe and Seebeck (1810)," 

 T. von Grotthuss (1815) , and G. W. Osann (1825) , have failed to 

 confirm Beccaria's results. There can be no doubt that the spectral 

 emission of phosphors remains essentially the same when excited 

 with different wave-lengths, provided these wave-lengths can excite 

 at all. Variation in results can be explained by the fact that colored 

 glasses are not usually monochromatic, and by the inability of the 

 eye to estimate color in the weak phosphor light which is excited 

 by certain colors. Since Krafft's and de Grosser's (1777) time, it 

 has been recognized that the blue end of the spectrum is far more 

 effective in exciting phosphorescence than the red, a fact particularly 

 emphasized by Osann (1834) . 



Phosphorescence and Electric Sparks 



The great interest in electricity during the eighteenth century 

 led to a study of the effect of electric sparks, usually referred to as 

 the electric light, on many materials. Canton (1768) made a casual 

 statement that they excited his phosphor. Apparently he was antici- 

 pated by G. B. Beccaria of Turin, Avho tested ^^ a number of sub- 



■*^ There is an extended discussion of the behavior of phosphors when excited with 

 various colored lights in Goethe's Zur Farberxlehre (1810) , because of its bearing on 

 the Newtonian and Cartesian theories of light and Goethe's anti-Newton bias. For 

 the effect of ultraviolet and infrared light, see a later section. 



^^ The work was carried out before 1753, and appears in an English translation of 

 his book, Dell elettricismo artiftciale e naturale, Turin, 1753, as A treatise upon arti- 

 ficial electricity, London, 1776; sec. 763, " On phosphoreity." 



