When these rays meet with a substance which they cannot go through, and which is therefore called opaque, such as a man or a house, they are turned back or reflected, and strike the eye of the person who may be looking in that direction, so as to cause vision.
But when they meet with a body which they can pass through, called therefore transparent, such as water or glass, they are turned aside out of their original course or refracted; and this refraction takes place in different degrees, according to the density of the substance.
Thus, in looking at objects through air, you do not see anything exactly in its real position; but in looking at them through water, the variation between the reality and the appearance is still greater, because the density of water being greater than that of air, it will refract more. To prove this you need only put a stick obliquely into water, and it will look as if it were broken or bent at the surface of the water.
The true position of the stick is marked by A B, and the apparent position, by A C.
We see the rising sun some minutes before he has risen above our horizon, and the setting sun after he has sunk below it, because his rays are refracted by our atmosphere.
If the sun be at A, he will appear to a spectator on the surface of the earth at C, as if he were at B, because the rays will be refracted at D, which represents the limit of the atmosphere, towards C.
Perhaps you knew all this before. Well, the knowledge of these laws will very nearly enable you to understand the wonders of which I have told you. I will try to explain to you the manner in which it seems that the appearances are produced by the operation of the laws.