INTRODUCTION 



ONE OF THE PROBLEMS o£ the historian of luminescence is to sift 

 the true from the false report. Stories of luminous jewels and 

 luminous stones have been handed down from earliest times, but 

 in most cases the light has been due to reflection rather than light 

 emission. The instances of luminous human beings are fascinating 

 to read of, but it is quite certain that man has never acquired the 

 ability to produce a light like that of the firefly. 



Of the many types of reflection ^ which have been confused with 

 true light emission the most common is the glow of the eye. The 

 glowing eyes of cats are well known, and glowing eyes have also 

 been observed in human beings. They were particularly mentioned 

 by classic writers as characteristic of warriors during the heat of 

 battle. The Greeks and Romans thought the ability to see at night 

 was connected with this glowing of the eye. Another phenomenon 

 and a purely subjective one, the phosphene or apparent light that 

 appears when the eyeball is pressed or struck, was known to Aris- 

 totle ^ and has frequently been confused with real luminescent 

 phenomena. 



Among the true luminescences recognized by the ancients may be 

 placed the aurora borealis, the light of the sea, luminous animals of 

 various kinds, phosphorescent wood and flesh, and ignis lambens, a 

 silent electrical discharge in air. Discovery of the Bolognian stone 

 in 1603, a true phosphor which glows after exposure to light, and 

 isolation of the element phosphorus in 1669 added two more im- 

 portant examples to those already known. Much later, passage of 

 electricity through " vacuum tubes " reproduced the silent electric 

 discharge in the laboratory. It is interesting to note that the term 

 " electric light " did not originally refer to the electric incandescent 

 bulb or arc light but to a luminescent glow in partially evacuated 

 vessels when brought near an electrical machine. These " globes " 

 were very thoroughly studied at the beginning of the eighteenth 

 century by Francis Hauksbee. Despite the manifold examples of 

 luminescence available to anyone who troubles to make the search, 

 the remarkable brightness of firefly light with no perceptible rise 

 in temperature has always stood as a symbol of light without heat 



* An animal (Sapphirina) , an alga (Chromophyton) , and a moss (Schistostega) 

 reflect the light from cells or surfaces in such a way as to appear luminous and 

 irridescent; see " General Statement " of Part III. 



" See De sensu, in Parva naturalia, trans, by J. I. Beare, 437a, Oxford Univ. Press, 

 1908. 



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