CHAPTER IV 



STEUCTUEE OF LUMINOUS OEGANS 



The production of light is the converse of the detection 

 of light. In the first case chemical energy is converted 

 into radiant energy ; in the second case radiant energy is 

 converted into chemical energy. The lantern of the firefly 

 is an organ of chemi-photic change; the eye is an organ 

 of photo-chemical change. While it is theoretically prob- 

 able that all reactions which proceed in one direction under 

 the influence of light, will proceed in the opposite direction 

 with the evolution of light, the formation of lucif erin from 

 oxyluciferin (described in Chapter VI) is the only one 

 definitely known. Perhaps we may place in this category 

 also the instances of photoluminescence, but the chemical 

 reaction involved cannot be pointed out. 



We know of no animal whose eyes, the organs, pa7' 

 excellence^ of photochemical change, give off light in the 

 dark. All cases of luminous eyes have been conclusively 

 shown to be purely reflection phenomena. The eyes of a 

 cat only glow if some stray light is present which may 

 enter and be reflected out again. Photochemical reactions 

 and chemiluminescent reactions do have this in common, 

 however, that they are largely but not exclusively oxida- 

 tions. Whether all photochemical changes in the eyes in 

 animals require oxygen or not, is unknown, but all animal 

 light-producing reactions, without exception, are oxida- 

 tions, and light is only produced if oxygen is present. 

 Some material is oxidized. 



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