All other objects which do not produce light, such as an apple, a stone, the Earth and other planets and moons are called _non-luminous bodies_. Yet these non-luminous bodies can send out light if they are lighted up by some self-luminous body. It is well that this is so, or else we could never see anything that was not in itself giving out light. If you will hold an apple or a stone in your hand and let the light of a candle or the Sun fall on it you will be able to see the apple or stone, and, although you will hardly be able to notice it, you will see them by the light which strikes them and is turned back, or _reflected_ from their surfaces, as shown in Fig. 141. If a rubber ball is thrown on the sidewalk it will bounce back and this is just the way light acts when it strikes most objects—it bounces back, or, to use the right word, it is reflected. When we look at the surface of the Earth by daylight we see the sand and stones, grass and trees, houses and other objects by the light which is reflected, or thrown back from the surface of these things by the Sun. When we look at the surface of the Earth by the light of the Moon we also see the objects by reflected light, but in this case the light is twice reflected, for moonlight is the light of the Sun falling on the Moon and which is then reflected to the Earth, where it is again reflected to our eyes from the objects it falls on. This is the reason moonlight is so pale when compared with sunlight. [Illustration: FIG. 141.—LIGHT REFLECTED BY AN APPLE.] [Illustration: FIG. 142.—LIGHT REFRACTED. SPOON IN GLASS OF WATER.] =Refraction of Light.=—When a beam of light passes through glass, water and other transparent substances, and is bent out of a straight line it is said to be _refracted_. Place a spoon in a glass of water, as shown in Fig. 142 and it will look as if the spoon is broken in two at the point where it touches the water. The bending of the beam of light will be more clearly understood from the drawing in Fig. 143. If you will look at a star squarely through a thick piece of glass and the star seems to change its position a little you will know that the sides of the glass are not quite parallel. A _prism_ is a three-sided piece of glass, if we except the ends, as shown in Fig. 144. When a beam of light passes through a prism the prism affects the light in two ways: first, it bends the beam, and second, it separates the ether waves, or light waves, as they are called, according to their lengths, and as color depends on the length of the waves in the ether a prism will show the different colors on a screen and this is called the _spectrum_. It is shown in Fig. 145. [Illustration: FIG. 143.—HOW LIGHT IS REFRACTED.] [Illustration: FIG. 144.—PRISM.] Lenses are pieces of glass having curved surfaces. When a beam of light passes through a lens it is also bent out of its original direction, or refracted.