Blue sky itself, for example. Why is the sky blue? To explain that, we must state a few preliminary facts concerning light, and beg pardon of any one whose wisdom may be outraged by the elementary character of our information. There are some among our readers, no doubt, who may find it useful. In the first place, then, we will begin with the erection of a pole upon a play-ground, and, like boys and girls, we will go out to play about it with an india-rubber ball. The pole being planted upright, is said to be planted at right angles to the surface of the ground. Now, if we climb the pole, and throw our ball down in the same line with it, it will run down the pole and strike the ground, and then jump back again by the same road into our fingers. The bouncing back is called in scientific phrase, Reflection; and so we may declare about our ball, that if it strike a plane surface at right angles, it is reflected immediately back upon the line it went by, or, as scientific people say, "the line of incidence." Now, let us walk off, and mount a wall at a short distance from the pole. We throw our ball so that it strikes the ground quite close to the spot at which the pole is planted in the earth, and we observe that the said ball no longer returns into our hand, but flies up without deviating to the right or left (in the same plane, says Science) beyond the pole, with exactly the same inclination toward the pole on one side, and the surface of the ground on the other, as we gave it when we sent it down. So if there were a wall on the other side of our pole, exactly as distant and as high as our own, and somebody should sit thereon directly opposite to us, the ball would shoot down from our fingers to the root of the pole, and then up from the pole into his hand. Spread a string on each side along the course the ball has taken, from wall to pole, and from pole to wall. The string on each side will make with the pole an equal angle: the angle to the pole, by which the ball went, is called, we said, the angle of incidence; the angle from the pole, by which it bounced off, is called the angle of reflection. Now, it is true not only of balls, but of all things that are reflected; of light, for example, reflected from a looking-glass, or a sheet of water, that "the angle of reflection is equal to the angle of incidence."