Phosphorescence 325 



The brighter the exciting light the brighter the luminescence, the 

 longer it lasted, and the more noticeably reddish it became. Very 

 weak excitingr liorht resulted in a whitish luminescence. Although 

 these men took all precautions to dark-adapt their eyes, it was not 

 realized at that time that the color change was a peculiarity of the 

 eye, that all dim lights appear whitish, no matter what their spectral 

 distribution may be. 



The group observed that i£ one part of the phosphor was illumi- 

 nated only that part luminesced, without propagation of light to 

 other regions, and they decided that phosphors which had been 

 most used imbibed the light better than when first tested and better 

 than others not much used. 



In 1713 Zanotti (1748) , together with Count Francesco Algarotti 

 (1712-1764) , the Italian writer and connoisseur, repeated and con- 

 firmed Zucchi's experiment on colored light, using sunlight and a 

 prism instead of colored filters, and found that their sample emitted 

 the same whitish luminescence after exposure to either blue or red 

 light. This showed that the phosphor " shines by its own natural 

 light, which is only kindled by foreign light." When the phospho- 

 rescence was observed through a prism, no spectrum appeared but 

 the luminescence was monochromatic, like that of dim coals, un- 

 doubtedly because too faint for the eye to perceive colors. Despite 

 poor apparatus, Zanotti appears to have been the first to investigate 

 the spectrum. The Italian group in 1719 also found that the Bono- 

 nian phosphor appeared to be less bright in a vacuum but this effect 

 may have been due to poor glass of the receiver. 



Somewhat later, Beccari observed that many other substances, 

 both organic and inorganic, were phosphors. According to Priestley 

 (1772; 368) , as a physician he " was visiting a lady after her lying-in, 

 when the room being pretty dark, she casually observed that a 

 diamond which he wore in a ring appeared exceedingly bright." 

 Beccari soon discovered that his own diamond was truly phospho- 

 rescent and decided to test a large number of substances. He had 

 built for himself a special light-tight box, large enough to enclose a 

 man and equipped with a window into which fitted a special rotating 

 cylinder. The material to be examined was placed in this cylinder 

 and by a quick rotary movement could be first exposed to sunlight 

 and then examined in complete darkness. After using this improved 

 device, Beccari wrote: ^° " It followed from this accuracy that some 

 things were observed which had before escaped not only me, who 

 am a pretty diligent observer, but likewise other persons of great 

 sagacity and discernment." 



*■> Quotation from the B. Wilson (1775) translation. 



