336. =Light Moves in Straight Lines.=--Light, like heat and sound, radiates in straight lines in all directions from its source. We can see this to be true by admitting rays of light into a darkened room through small openings in the shutters, the rays making straight lines across the darkness, as may be seen by the motes which are flying in the air. The fact is recognized by the marksman in taking aim, and by the engineer in making his levels. The carpenter acts upon it when he tests the smoothness of any surface by letting the light pass along over it to his eye. [Illustration: Fig. 223.] [Illustration: Fig. 224.] 337. =Diffusion of Light.=--As light passes in all directions from any body or point, the farther we go from its source the less will the light be. If we take any two rays of light, the farther we trace them from their source the farther are they separated from each other, and what is true of any two rays is true of all the rays. It follows that the farther removed any surface is from a source of light the less light will there be upon it. This decrease of light in proportion to distance is a perfectly regular decrease, and it is as the square of the distance; or, in other words, the intensity of light is inversely as the square of the distance. Take a screen, Fig. 223, and a candle, placing a square piece of pasteboard between them at one foot from each. The shadow on the screen, you see, covers a space four times as large as the pasteboard. That is, the light that shines on the pasteboard, if allowed to pass on to the screen, would be diffused over four times the space, and therefore would have only one-quarter of the intensity. So if as shown in Fig. 224, the screen be placed at twice the distance from the pasteboard that the light is, the shadow will cover a space nine times as large as the pasteboard, and therefore the light there would have one-ninth of the intensity which it has where the pasteboard is. Again, it is seen by Fig. 225 that if the screen be placed at the distance of three feet the intensity of the light is one-sixteenth of that which it is at the pasteboard. While the distances, therefore, are as 1, 2, 3, 4, etc., the intensity of the light is _inversely_ as the numbers 1, 4, 9, 16, etc., that is _inversely as the squares of the distance_. [Illustration: Fig. 225.]