CONCERNING EYES 187 



death still weighs on my conscience, albeit by 

 killing it I bestowed on it that dusty immortality 

 which is the portion of stuffed specimens in a 

 museum. 



The question as to the cause of this fiery ap- 

 pearance is one hard to answer. We know that 

 the source of the luminosity in owls' and cats' 

 eyes is the tapedum lucidum the light-reflecting 

 membrane between the retina and the sclerotic 

 coat of the eyeball; but the mystery remains. 

 "When with the bird, I particularly noticed that 

 every time I retired the nictitating membrane 

 would immediately cover the eyes and obscure 

 them for some time, as they will when an owl is 

 confronted with strong sunlight; and this gave 

 me the impression that the fiery, flashing appear- 

 ance was accompanied with, or followed by, a 

 burning or smarting sensation. I will here quote 

 a very suggestive passage from a letter on this 

 subject written to me by a gentleman of great at- 

 tainments in science : ' ' Eyes certainly do shine in 

 the dark some eyes, e.g. those of cats and owls; 

 and the scintillation you speak of is probably an- 

 other form of the phenomenon. It probably de- 

 pends upon some extra-sensibility of the retina 

 analogous to what exists in the molecular consti- 

 tution of sulphide of calcium and other phosphor- 

 escent substances. The difficulty is in the scintilla- 

 tion. We know that light of this character has its 

 source in the heat vibrations of molecules at the 

 temperature of incandescence, and the electric 



