Page:Popular Science Monthly Volume 24.djvu/838

818 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 animal is in obscurity, but the observer must be in the light, and somewhat in the relative position indicated in the above-described experiment—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 governed 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 ophthalmoscope. This I can account for only by the great dispersion of light at the periphery of the lens and cornea, rendered possible by the immense 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.

 

HE world of science was astonished a quarter of a century ago by the discovery made in the caves of Vézère, France, of works of art executed by the prehistoric troglodytes. The specimens consisted of representations of mammals, birds, fishes, and of man himself, sculptured in relief or engraved upon elephants' tusks, bears' 