Page:On the various forces of nature and their relations to each other.djvu/51

Rh I have here (fig. 11) a lamp A, shining most intensely upon this disc, B, C, D; and this light acts as a sun by which I can get a shadow from this little screen, B F (merely a square piece of card, which, as you know, when I place it close to the large screen, just shadows as much of it as is exactly equal to its own size. But now let me take this card E, which is equal to the other one in size, and place it midway between the lamp and the screen: now look at the size of the shadow B D—it is four times the original size. Here, then, comes the "inverse square of the distance." This distance, A E, is one, and that distance, A B, is two; but that size E being