WHY EYES OF ANIMALS SHINE IN THE DARK. 815 



whether going from the image to the object or from the object to the 

 image. 



Let us now apply this law to the case of the eye. We will suppose 

 the eye to be in a normal optical condition ; that is, that the retina 

 on which the image is formed is to be found exactly at the focus of 



the lenses by which the light is refracted. By consulting Fig. 2, we 

 can follow the course of the rays of light in both directions. We 

 have rays going from a in the flame a b, which after refraction by 

 the lenses of the eye are brought to a focus at c, and form the lower 

 end of the inverted image ; whereas, these going from b are united 

 again at d. But, since the bottom of the eye is a reflecting surface, 

 and sends back a part, at least, of the light which falls on it, some of 

 these rays pass out again, but, in accordance with the law of conju- 

 gate foci, they must follow the same lines as in entering ; therefore, 

 the rays from c will come back to a, and those from d will come 



back to b. If we could place our eye at a b, then we would catch 

 some of these rays, and the bottom of the eye would appear illu- 

 minated just as any other surface from which light was reflected. 

 But our eye and the candle can not occupy the same place at the 

 same time, and if we place it behind the candle, the flame itself 

 cuts off the rays of light, and if we place it in front, our head 

 obstructs the passage of the light to the eye to be observed. So, 

 under these circumstances, it is impossible for an eye, at o, for in- 

 stance, to get any of the light that is constantly coming from the bot- 



