Page:Popular Science Monthly Volume 4.djvu/688

668 the flame; but, if the eye, the candle, or either screen, be slightly displaced to the right or left, the ray is interrupted, and the flame becomes invisible.

This propagation of light in straight lines, though the first condition of the production of images, is not the only condition; for, in that case, the images of illuminated objects would be repeated everywhere, and, when the blinds were opened, a picture of the landscape might be thrown, through the window, upon the opposite wall of a room. For the formation of an image, the rays of an object must be collected and passed through an aperture. This is shown by a simple experiment illustrated in Fig. 2. A card is pierced with a large pin-hole and held between a candle and screen, when an image of the candle will be formed upon the screen in an inverted position. That the image must be upside-down is evident, if the rays take a straight course. A line from the top of the candle-flame through the puncture is prolonged to the bottom of the image, and another, from the bottom of the flame, crosses the first at the aperture, and strikes the top of the



image. A line from the centre of the candle passes straight through and strikes the centre of the image, while lines from the two sides of the flame cross again, and are prolonged to the opposite sides of the image. Thus, as each point upon the screen receives only the light from a corresponding point of the flame, the image repeats the object in outline, color, and brightness, though in a reversed position.

This principle may be applied on a larger scale. Let a room be made quite dark, and a white screen be placed opposite a small hole