one, this size B D of shadow is four instead of two, which is the square of the distance; and, if I put the screen at one-third of the distance from the lamp, the shadow on the large screen would be nine times the size. Again, if I hold this screen here, at B F, a certain amount of light falls on it; and if I hold it nearer the lamp at E, more light shines upon it. And you see at once how much—exactly the quantity which I have shut off from the part of this screen, B D, now in shadow; moreover, you see that if I put a single screen here, at G, by the side of the shadow, it can only receive one-fourth of the proportion of light which is obstructed. That, then, is what is meant by the inverse of the square of the distance. This screen E is the brightest, because it is the nearest; and there is the whole secret of this curious expression, inversely as the square of the distance. Now, if you cannot perfectly recollect this when you go home, get a candle and throw a shadow of something—your profile, if you like—on the wall, and then recede or advance, and you will find that your shadow is exactly in proportion to the square of the distance you
