HERMANN VON HELMHOLTZ 



is easy to understand that a source of light so placed 

 had escaped all observers. It was also held that the 

 eyes of white rabbits and of albinos had a light 

 of their own. Prevost, in 1810, was the first to show 

 that the light of the eyes of animals was never seen 

 in complete darkness, that neither an effort of will 

 nor irritation caused it, and that it was always due 

 to the reflection of an incident light. Gruithuisen 

 found the same result, and further showed that the 

 cause of the phenomenon was in the tapetum along 

 with the ' refraction extraordinaire ' of the lens. He 

 also saw the light in the eyes of dead animals. These 

 facts were confirmed by Rudolphi, Johannes Miiller, 

 Esser, Wiedemann and Hassenstein. Rudolphi ob- 

 served that we must look at the eye in a certain 

 direction to perceive the light ; and Esser gave a 

 good explanation of the changes of colour by the 

 appearance of different coloured parts of the retina 

 which were presented successively behind the pupil. 

 Hassenstein, finally, found that light was produced 

 when the eye was compressed along its axis, and 

 supposed that in the living animal luminosity might 

 be produced voluntarily by a shortening of the axis by 

 pressure of muscles. The earlier observers recognised, 

 then, luminosity of the eye as a phenomenon of re- 

 flection without giving an account of the conditions 

 that determined it. 



In the human eye luminosity was observed in 

 certain rare diseases, in particular when tumours occu- 

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