rays are continually traversing the immense spaces of creation, or from some other sources to us unknown. 6. _The intensity of light is diminished in proportion to the square of the distance from the luminous body._ Thus, a person at two feet distance from a candle, has only the fourth part of the light he would have at one foot, at three feet distance the ninth part, at four feet the sixteenth part, at five feet the twenty fifth part, and so on for other distances. Hence the light received by the planets of the Solar system decreases in proportion to the squares of the distances of these bodies from the sun. This may be illustrated by the following figure, [Illustration: _Figure 1._] Suppose the light which flows from a point A, and passes through a square hole B, is received upon a plane C, parallel to the plane of the hole--or, let the figure C be considered as the shadow of the plane B. When the distance of C is double of B, the length and breadth of the shadow C will be each double of the length and breadth of the plane B, and treble when AD is treble of AB, and so on, which may be easily examined by the light of a candle placed at A. Therefore the surface of the shadow C, at the distance AC--double of AB, is divisible into four squares, and at a treble distance, into nine squares, severally equal to the square B, as represented in the figure. The light, then, which falls upon the plane B being suffered to pass to double that distance, will be uniformly spread over four times the space, and consequently will be four times thinner in every part of that space. And at a treble distance it will be nine times thinner, and at a quadruple distance sixteen times thinner than it was at first. Consequently the quantities of this rarified light received upon a surface of any given size and shape when removed successively to these several distances, will be but one-fourth, one-ninth, one-sixteenth, of the whole quantity received by it at the first distance AB.