166 Dr Esser on the Luminousness of the Eyes. 



being scarcely visible, may be the reason why it is believed that 

 the whole eye of the cat is luminous, although its light is, never- 

 theless, only in proportion to the dilatation of the pupil. 



The dilatation'of the pupil in twilight is, however, not the only 

 cause of the light of the eyes ; but the light surrounding the ani- 

 mal being fainter, also assists us in perceiving with greater dis- 

 tinctness the light as it is more vividly reflected from their eyes ; 

 for, if we suddenly illuminate the chamber in which there is a 

 cat, there remains nothing but a luminous brightness where there 

 was formerly a beautiful yellowish green light. 



The light of my cat's eyes seemed to be more vivid when she 

 opened them wide from apprehension, or looked around her at- 

 tentively ; whence Treviranus observed, that the eyes of cats 

 sparkled most when the animals were in a lurking position, or in 

 a state of irritation. That author says, 



" The light of the cat's eyes appears most conspicuous when 

 she is in a lurking position, — when she is attracted by any 

 unusual appearance, — or when irritated. In the first two in- 

 stances, the light is faint and dull : in the last instances, it darts 

 forth in intermittent scintillations, and at those moments when 

 the light is most vivid, there are accompanying movements of 

 the eyes." That the light of the eyes of animals appears bright- 

 er in a state of irritation^ than in a state of quiescence, seems 

 to originate in this, that the eyes of all animals, as well as those 

 of man, appear brighter in violent rage, and sparkle more, than 

 in a quiescent state. This, in man, seems to arise from an in- 

 creased secretion of the lachrymal fluid on the surface of the 

 eye, by which fluid the light of the eye is undoubtedly 

 rendered more brilliant. Treviranus farther observes, " The 

 eyes of the cat shine also where no rays of light penetrate, 

 and the light must in many, if not in all, cases proceed from 

 the eye itself." Before performing the above experiments, 

 I entertained the same opinion with Treviranus, and made many 

 fruitless experiments with cats in the dark, before I abandoned 

 the position. The light must be brighter in proportion to the 

 darkness of the place where the cat is. I soon renounced this . 

 opinion, when, in all the experiments I made on cats, in places 

 absolutely dark, I did not discover the slightest trace of light in 

 the eyes of these animals, let me irritate them as I could. 



