818 THE POPULAR SCIENCE MONTHLY. 



This is the true explanation of the luminous appearance of the eyes 

 of some animals when they are in comparative obscurity. It is simply 

 the light reflected from the bottom of their eyes, which is generally of 

 a reddish tinge on account of the red blood in the vascular layer of 

 the choroid back of the semitransparent retina, and not light that is 

 generated there at all. This reflection is most apparent when the ani- 

 mal is in obscurity, but the observer must be in the light, and some- 

 what in the relative position indicated in the above-described experi- 

 ment that is, the eye of the. observer must be on the same line with 

 the light and the observed eye. The eyes of nearly all animals are 

 hypermetropic, most of them very highly so, so that they send out the 

 rays of light which have entered them in a very diverging manner. 



The circumstances under which the phenomena of luminosity are 

 usually seen are, it will be noted, those most favorable for the success 

 of the experiment. The animal is always in an obscure corner, under 

 a table or chair, as in the case of the cat, while the deer is in the outer 

 darkness of the night. It is well known that the pupils dilate when 

 in the dark, and they often attain an immense size in the eyes of those 

 animals with nocturnal habits, and the size of the cone of light is gov- 

 erned by the size of the pupil, since its circumferential boundary is 

 formed by it. 



In making some experiments on dogs and cats, for the purpose of 

 determining the size of this cone of light, I found that it had actually 

 about twice the diameter it should have theoretically, from the amount 

 of hypermetropia present, as determined by means of the ophthalmo- 

 scope. This I can account for only by the great dispersion of light 

 at the periphery of the lens and cornea, rendered possible by the im- 

 mense dilatation of the pupil ; and this I think, too, is the reason why 

 the phenomenon is not more frequently observed in the higher animals 

 affected with hypermetropia. The pupil in man never attains the size, 

 under the same circumstances, as that of the cat, for example ; and, 

 moreover, it is most likely that the surfaces of the cornea and lens are 

 more regular in their curve, even at their more peripheral parts, and 

 consequently disperse the light in a very much less degree. 



> 



PKEHISTOEIC AET IN AMEEICA. 



By the Marquis de NADAILLAC. 



THE world of science was astonished a quarter of a century ago 

 by the discovery made in the caves of Vezere, France, of works 

 of art executed by the prehistoric troglodytes. The specimens con- 

 sisted of representations of mammals, birds, fishes, and of man him- 

 self, sculptured in relief or engraved upon elephants' tusks, bears' 



