The law may be easily illustrated by allowing a ray of light to fall upon the surface of a vessel of water, as shown in the preceding figure.

The light of the stars and planets undergoes a similar deviation when passing in its course through the earth's atmosphere; and at the moment we see the rising of the sun, the moon, or a star, they are in reality still
Fig. 16.—The Effects of Plane Mirrors.

below the horizon. Our eyes consequently are still deceiving us, no matter what part of the domain of optics we may enter.