WHY EYES OF ANIMALS SHINE IN THE DARK. 817 



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 



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 sufli- 

 cient number to show the bottom of the eye illuminated. 



Place a child (because the pupils of children are large), and by pref- 

 erence 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 ob- 

 ject 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. 

 vol. xxiv. 52 



