24 History of Luminescence 



The word " fungus " must refer to the mushroom or fruiting 

 body, many species of which are luminous. It is possible that Aris- 

 totle also realized that the mycelium of the fungus, growing on 

 rotten wood, was responsible for the luminescence of the latter. 



The head, scales and eyes of fishes are in a different category. 

 They could refer to bacterial luminescence of these substances, 

 although Aristotle may have noted only light reflection from the 

 head scales and eyes of fishes. 



The word " horn " is somewhat in doubt. Placidus Heinrich in 

 his Die Phosphorescenz der Korper, 1815 (p. 415) , believed that 

 keras (meaning horn) should have been " kreas " (meaning flesh) , 

 in which case Aristotle would have referred to luminescent meat. 

 Ehrenberg in Das Leuchten des Meeres (1834) is inclined to follow 

 this interpretation which is also adopted in the J. A. Smith transla- 

 tion of De Anima. It is highly probable that the ancients, living 

 near the sea, must have observed the luminescence of dead fish re- 

 sulting from luminous bacteria, and probably observed the lumi- 

 nescence of flesh, also due to luminous bacteria. 



That there is light in the eyes of fishes, ^^ and in eyes in general, 

 has been noted by many subsequent writers. Such a light could be 

 merely reflection of external light, as from the eye of a cat or the 

 eye of a man. Homer, in describing Achilles rushing into battle, 

 wrote: 



Grief and revenge his furious heart inspire 

 His glowing eyeballs roll with living fire. 



Human experience also shows that pressure on the eyeball or a 

 blow to the eye gives the sensation of light. Theophrastus (374- 

 286 B. c.) quoted Alcmaeon (sixth century b. c.) as stating that " the 

 eye obviously has fire within, for when one is struck (the fire) 

 flashes out." As already pointed out, Aristotle wrote: ^" 



But they [inquirers] hold the organ of sight to consist of fire, being 



visible but only enough to free them from darkness. " Their small light perishes, as 

 it were, during the day, and is swallowed up by a large one " (Gesner, De lunariis, 

 1555) . Ammonius (fifth century a. d.) distinguished two kinds of visible objects. The 

 first can be seen in daylight and are called colors while the second lack a common 

 name and might be called shining. Aristotle thought of light as an activity (energeia) 

 in a medium called the pellucid or the transparent (diaphanes) . 



=^ See the discussion in Chapter IV under Thomas Bartholin. A recent account is 

 given by R. Dubois in the Com. Rend. Ac. Sci. 178: 1030-1033, 1924. 



"De sensu in Parva naturalia, 437, translated by J. I. Beare, 1908. This phenomenon, 

 the phosphene, is due to mechanical stimulation of the visual reception. David 

 Brewster (1831) referred it to "a singular property [in the retina] of being phospho- 

 rescent by pressure," but the light is purely subjective, like the colors seen during 

 bilious attacks or fever, or the after images of the eye, which were once called 

 " ocular spectra " or " accidental colors." 



