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Page:Popular Science Monthly Volume 24.djvu/835

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815
WHY EYES OF ANIMALS SHINE IN THE DARK.

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

PSM V24 D835 Explanation of the law of conjugate foci.jpg

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 conjugate 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

PSM V24 D835 Demonstrating the law of conjugate foci pertaining to the eye.jpg

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 illuminated 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 0, for instance, to get any of the light that is constantly coming from the bot-