For instance, if the ray A B is bent during its passage to the eye at B, and consequently reaches it in the direction B C, it is at A´, and not at A, that we shall see the object from which it proceeds. Every ray of light which passes out of a medium of a certain density into another of a different density is bent from its primary course, or, in scientific language, it is refracted. The experiments we made in a former chapter on the properties of the prism are founded on this principle. 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. [Illustration: FIG. 16.—The Effects of Plane Mirrors. ] 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 below the horizon. Our eyes consequently are still deceiving us, no matter what part of the domain of optics we may enter. There are two kinds of mirrors—plane and curved. We will first examine the properties of the former sort, being those which are ordinarily applied to the usages of every-day life. [Illustration: FIG. 17.—Reflection from the Surface of Water. ] In the figure in the preceding page we have a young lady looking at her reflection in a tall cheval glass. Every point upon the surface of her clothes and face is reflected back to her eye from the surface of the tin amalgam which has been applied to the back of the mirror by the looking-glass maker, for the purpose of rendering the image of the object more brilliant than if the glass alone were used. The rays which proceed from every one of these points strike upon the surface of this metallic layer, are stopped by its opacity, and are reflected back to the eye at an angle equal to that at which they strike the surface. The image seen by the eye is formed, consequently, by the reflection of every one of these rays; and as we always see objects in the direction taken by the luminous ray at the moment it enters the eye, we fancy we see objects before us that are really behind, or on each side of us. For instance, the ray starting from the left foot of the young lady in the figure is reflected from the point indicated on the surface of the glass, but the eye does not stop here, but sees the foot at an equal distance beyond the mirror. The same thing takes place, not only with glass, but with all substances having polished surfaces. Still water, which to all intents and purposes has a polished surface, reflects the objects within its range as perfectly as a mirror. The preceding observations apply to all plane reflecting surfaces; but there are other sorts of mirrors, whose effects are of a more interesting nature, and which we must hasten to describe—we allude to those whose surfaces are either convex or concave.