170 Dr Esser on the Luminousness of the Eyes. 



determine to what extent the remoter parts of the eye contri- 

 buted to its luminousness. 



I attempted to perform the above operation on a cat, but the 

 utter restlessness of the animal rendered it extremely difficult, 

 indeed almost impossible. Having ascertained that eyes of cats 

 shine after death, I resolved to kill the cat, that I might have 

 it in my power to dissect any part of the eye I thought proper. 



First, by means of a pair of scissors I cut away the whole of 

 the cornea, and completely destroyed the anterior chamber of 

 the eye. I now observed, that the light of the eye was not in 

 the least diminished, but somewhat weakened in regard to colour, 

 which was changed from a yellow to a pale green. I then took 

 away the iris, that lay exposed before me, without injuring the 

 conformity of the hinder part of the eye, to discover whether the 

 iris, as Treviranus maintained, really contributed to the light. 

 This, however, was not the case ; for the light still continued. 

 The taking away of the lens was followed by a diffisrent result, 

 which considerably weakened the intensity of the light, and the 

 greenness of its colour. It now struck me that the tapetum in the 

 hinder part of the eye must form a spot^ which caused the reflec- 

 tion of the incident rays of light, and thus produced the shining. 

 This was the more probable, as the light of the eye now seemed 

 to emanate from a single spot. After taking away the vitreous 

 humour, I observed, that, in reality, the entire want of the pig- 

 ment in the hinder part of the choroid coat, where the optic 

 nerve enters, formed a greenish silver coloured changeable ob- 

 long spot, which was not symmetrical, but surrounded the optic 

 nerve in such a manner, that the greater part was above, and 

 only a small part below it ; and, therefore, the greater part lay 

 beyond the axis of vision. It is this spot, therefore, that produces 

 the reflection of the incident rays of light, and, beyond all doubt, 

 according to its tint, contributes to the different colouring of the 

 light, to which, nevertheless, the remaining parts of the eye, 

 when conjoined, seem to be no less necessary. 



The situation of this spot corresponds exactly with the posi- 

 tion in which the shining of the eyes is seen to the greatest ad- 

 vantage. I have before remarked, that the shining is percepti- 

 ble only in a certain position, and, in fact, when the eyes of the 

 observer are almost opposite to the eyes of the animal on which 



