But all this time we have been speaking of light, and yet the subject of this chapter is color. Well, the reason is, that color is light and light is color. If we were shut up in a cellar from which all light was excluded, we should see no color. Our eyes would experience no sensations of sight whatever, and, if we were left there a long time, our eyes, not being used, would probably lose their sense of sight. But, if after we had been in a cellar a little while surrounded by “thick darkness” as the old English expression is--meaning a darkness so opaque that the eye cannot penetrate it--the window shutter should be opened a trifle, then immediately our eyes would experience a sensation of color. The shaft of light, cutting across the darkness, would look white; but, if it hit upon a shelf of apples, our eye would receive a sensation of green or red or yellow. If light is color, why should it seem white in one case and some other hue in another? It is because in the whiteness of light are contained all the colors of which we are conscious. Very likely you know the experiment by which the truth of this is shown. Supposing you are still in the cellar and place in the pathway of the shaft of light a prism--that is to say, a bar of glass not round or square, but triangular--what will happen? The glass being transparent, the light will pass through it. But not in a straight line; for, as it hits one of the sloping surfaces of the prism, it will be bent out of its course; and then, as it reaches the opposite sloping side, it will again be bent into another direction. So the light in its passage through the prism will have been twice bent out of its original direction; and, when it emerges, it will be no longer a single shaft of white light, but will appear as a broad band of many colored lights; red, orange, yellow, green, blue, violet. We may call this succession, a scale of color lights. They correspond in hue and order to the bands or scale of colored lights in the rainbow, for the latter is the result of an act of nature, which on a very large scale is like our experiment with the prism. Only nature’s prism is formed by a bar of rain on which strikes a shaft of light through a slit in the thick upper clouds.