HERMANN VON HELMHOLTZ 



animals, more especially those of birds of prey, 

 glisten or sparkle in the dark. If a cat is observed 

 entering a room in shadow, her eyes may be seen 

 like little balls of fire, and the red eyes of white rabbits 

 and other albinos are familiar to everyone. In 1811, 

 Pallas suggested that perhaps one saw in such cases the 

 naked -electricity of the retina (forte nudum electricum 

 retina nervts}. Johannes Miiller proved the truth of 

 the view first suggested by Hassenstein, that such eyes 

 do not really emit, but only reflect light, and it 

 was found that those eyes glistened most which were 

 furnished with a special structure, called a tapetum, 

 adapted for the reflection of light. Briicke was the 

 first to show that all eyes could be made to glisten by 

 throwing into the eye the beam of a lantern while 

 the rest of the room was dark ; and it is said that at 

 night he went to the Zoological Gardens and found 

 that, by taking up a suitable position, he could 

 illuminate the eyes of all animals. He then tried 

 the experiment with the human eye, guided by the 

 curious bit of information, that a servant of his father 

 had been dismissed from his situation because it was 

 * uncanny ' to see his eyes shining in the dark ! 

 The first eyes that were illuminated by Briicke, so 

 as to cause them to shine, were those of his friend 

 Du Bois Reymond. Soon afterwards von Erlach, 

 of Bern, who happened to wear concave spectacles, 

 found that by placing his glasses at a particular angle, 

 and thus reflecting light into the eyes of his patient, 

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