Lectures on Natural and Experimental Philosophy, Considered in Its Present State of Improvement. Describing in a Familiar and Easy Manner the Principa

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated.1799 Excerpt: ... OF THE FORCE AND INTENSITY OF LIGHT. /// a free medium the force and intensity of light propagated in parallel rays is always the same. For in such a medium there is nothing to impede its progress, hinder its action, retard its velocity, or change its direction. In a free medium the force and intensity of the light, U'/iich is prop agated by rays proceeding from the same point, are in an inverse ratio of the squares of the distances from this point. This is manifest from the principles of geometry, and I shall soon render it evident to you by experiment. It scarcely, however, wants any experimental proof; for light radiating from a center, its intensity must decrease, as all rays do that proceed from a center; /. e. must necessarily become thinner and thinner, continually spreading themselves, as they pass along, into a larger space, and that in proportion to the squares of their distance from the body; that is, at the distance of two spaces, they are four times thinner, or less intense, than they are at one, and so on, because they spread themselves in a twofold manner, upwards and downwards, and sideways. If the light, which flows from a point, and passes through a square hole, as be d e, plate 1, fig. 3, be received upon a plane, BCDE, placed parallel to the plane of the hole; and the distance, AB, be double of A b; then the length and breadth of the shadow, BD, will each be double the length and breadth of the plane, b d, and treble when AB is treble of A b, and so on; as you may easily prove by the light of a candle placed at A. Hence luminous bodies, or those that shine with their own light, are considerably brighter than opake bodies illuminated by them; for opake bodies disperse the light falling upon them in all manner of ways: whence supposing all ...