Dr Esser on the Ltiminousness of the Eyes. 165 



animal, by moving its eyes to and fro, brought them into a 

 different position. In a situation wherein I could best observe 

 the eyes of my cat, I caused the room to be slowly darkened, 

 by gradually closing the window-shutters. The light of its eyes 

 became weaker, and vanished entirely as soon as the room, on 

 the place where the cat was situated, became absolutely dark. 

 .'Incident rays of light were always necessary to produce the 

 luminousness of the eyes. 



I wrapped another cat in a cloth, but left the head uncovered, 

 whereby I was able to handle the animal as I had a mind^ and 

 place it in any situation I chose. In this cat what I have just 

 stated was confirmed. I placed it in such a position that its head, 

 at the distance of a few steps, was directed towards the window, 

 by which means I could lighten or darken the room at pleasure. 

 I now permitted a few rays of light to fall through the window 

 into the room, in such a manner, that the place where the cat 

 was present was illuminated; and I placed myself in such a di- 

 rection towards the window, that my eyes were in a straight 

 line with those of the animal, so that I saw the hght of its eyes 

 very distinctly, which light, as in the former experiment, sud- 

 denly vanished when I turned my head, or the cat turned its 

 eyes. At the moment when my eyes were directed in the manr 

 ner just mentioned, I observed a most beautiful green light ; 

 but when they were out of this direction, the caf s eyes had their 

 usual appearance. By the turning of my head, or by any 

 other arrangement I chose, by which I intercepted the inci- 

 dent rays, I could at pleasure cause sometimes the one eye of 

 the cat, sometimes the other, and sometimes both together, to 

 shine. If I intercepted the incident rays of light from the left 

 half of the head, the right eye became luminous, and conversely. 

 In these experiments, I observed quite distinctly that the light 

 of my cat''s eye emanated from the pupil, the eye itself being 

 lightened only in proportion to the dilatation of that part of it. 

 By suddenly admitting a strong glare of light into the room, I 

 produced a contraction of the pupil ; and when I suddenly ren- 

 dered the room somewhat dark, a small round luminous point 

 first appeared in the eye, and that point enlarged according as 

 the pupil was dilated. The pupil of the eye of these animals 

 being thus dilated in imperfect darkness, so that the iris seems 

 to encircle the pupil as a small ring, and the sclerotic in cats 



