Page:Popular Science Monthly Volume 24.djvu/837

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817
WHY EYES OF ANIMALS SHINE IN THE DARK.

the focus, they are rendered convergent, or brought toward another focus.

In accordance with these laws, therefore, we must expect the rays of light to take a different course in coming out of an eye according as it is near-or far-sighted. The course of the rays coming from the far-sighted or hypermetropic eye is shown in Fig. 3.

If the retina lay in the focus of the refracting surfaces of the eye at E, then the light from the inverted image c d of the flame would travel back, in the same direction in which it came, to the flame a b

PSM V24 D837 Illuminating the back of the eye.jpg

itself. If, however, it meets the reflecting surface of the retina within the focus at H, then the rays from the confused image e i would come out in a divergent manner, and form a cone of light, F G, like that from the head-light of a locomotive.

It is now easy to see that if an observing eye is placed anywhere in the vicinity of the source of illumination, as at o, it will take in some of the rays coming from e i, and see it illuminated. There are very few human eyes so accurately adjusted as to their focus that all the rays come back to the source of light; some of them are scattered, and by a very simple arrangement it is possible to catch them in sufficient number to show the bottom of the eye illuminated.

Place a child (because the pupils of children are large), and by preference a blonde, at a distance of ten or fifteen feet from a lamp which is the only source of light in a room, and cause it to look at some object in the direction of the lamp, turning the eye you wish to look at slightly inward toward the nose. Now, put your own eye close behind the lamp-flame, with a card between it and the flame. If you will then look close by the edge of the flame covered by the card into the eye of the child, you will see, instead of a perfectly black pupil, a reddish-yellow circle. If the eye happens to be hypermetropic, you will be able to see the red reflex when your own eye is at some distance to one side of the flame.