Dr Esser mi the Luminousness of the Eyes. 171 



he is performing the experiment. This is easily explained. 

 Only those rays of light are reflected which fall on that part of 

 the choroid where the pigmentum is wanting ; but as this spot 

 occupies rather the upper wall of the concavity of the choroid, 

 the reflection caused by it will not be perceived, if the eye of 

 the observer is not in a nearly straight direction to the eye of the 

 animal, and at some distance ; and hence it is why, in living 

 cats, we observe the light only when their eyes are directed to- 

 wards our own ; in which case, the upper wall of the eye-ball 

 becomes more the hinder and under, and the point of reflection 

 stands in almost a straight line with our own eyes. 



From these experiments, it is abundantly evident that there 

 is no light or shining in the eyes in places absolutely dark, and 

 that the opinion of many authors is, in this respect, completely 

 erroneous. These experiments, at the same time, prove what 

 has been doubted by some physiologists, the transparency of tlie 

 retina ; for it must naturally be transparent, if reflection takes 

 place from behind it. The transparency of the retina may also 

 be proved from our seeing the image upon the choroid, or ra- 

 ther upon its pigment, while the retina has not the least share 

 in producing the efi^ect ; since it appears when the retina on be- 

 ing taken away, brings the vitreous humour, or the lens, to the 

 coats of the eye. 



I remarked above, that the light of the eyes of animals was 

 stronger when they were irritated than when they were in a 

 quiescent state ; and I attributed this phenomenon to the great- 

 er projection of the eyes, but particularly to the increased se- 

 cretion of the lachrymal fluids. This was rendered still more 

 probable by my last experiment, when I destroyed the convexi- 

 ty of the eye, by taking away the cornea and the lens. By this 

 it appears, that a shining substance is better fitted for reflection 

 than a dull one, which is proved by the gradual fading away 

 of the light after death, from the cornea becoming duller, and 

 by the gradual increase of light, when the cornea is moistened. 

 I further remarked, that the different colours of animals, parti- 

 cularly of the cat, probably tend to strengthen or weaken the 

 light ; which may be thus explained, that, in beasts, as well as 

 human beings, the greater or smaller size of the pigment may 

 usually be in conformity with the colour of the hair, which is 

 the common covering. 



