332 History of Luminescence 



etc.) glasses, which transmit only " red making rays " some other 

 principle than light must be involved in producing the various 

 colors and 



that principle must have been the phlogiston which was previously 

 lodged within the shell (during calcination) .... After they have been 

 phlogisticated they can emit no other colored light than what corre- 

 sponds with the thickness and densities of their parts: for example the 

 densest parts . . . cannot emit any other colour than red, no more than 

 those which are next in degree of density can emit any other color than 

 yellow and so on. 



The future importance of phosphors emitting light of various 

 colors was suggested in a statement at the end of Wilson's account: 

 " Why may not chance hereafter, by some lucky combination of 

 circumstances, again produce some discovery, in consequence of 

 those extraordinary appearances, that may turn out to be of far 

 greater importance? Attention and industry seldom fail of produc- 

 ing something worthy the notice of philosophers." Detailed knowl- 

 edge of the effect of traces of various heavy metals, which do deter- 

 mine the color of phosphorescence, was to come later and resulted 

 in the controlled production of phosphors, indispensable for modern 

 living. 



Wilson's (1775) publication led to a number of comments and 

 theories, based largely on the generally accepted fact that blue light 

 has a particularly strong effect in exciting phosphorescence. Leon- 

 hard Euler (1707-1783), the famous mathematician, who revived 

 the wave theory of light ^^ in 1746, published a paper in 1777 ^^ in 

 which Wilson's views were rejected and a vibration theory substi- 

 tuted. Euler held that when white light strikes the surface of an 

 opaque or colored body, the light is not reflected, but characteristic 

 vibrations are set up in the material particles of the surface, prac- 

 tically resonance vibrations, which are transmitted to the ether as 

 the color of the object. In a similar manner the phosphor particles 

 exposed to light were believed to vibrate after the exciting light is 

 cut off, like the continued vibrations of a plucked violin string, with 

 a frequency depending on the phosphor. Blue light can excite an 

 orange phosphorescence because the natural period of the phosphor 

 vibrators corresponds to orange. This point of view was a new one, 

 unique at the time, just as Euler's support of the wave theory was 

 a departure from the thought of the eighteenth century. 



^^ Nova theoria liicis et colorum in opiiscula 1, art. 3, 1746. 



^^ The essential idea was outlined in letter 27 of Euler's Lettres a une princesse 

 d'Allemagne (1770) , written July 12, 1760. This book contains a good account of 

 Euler's ideas on light and other physical subjects. 



