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GRAVITATION.
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
Fig. 11.
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 one, this size b d of shadow is four instead of
