Trattato pratico sull'illuminazione a gas

Le prime righe

[3] Philosophical Journal, 4to Series, Vol. I. p. 70. METHOD OF ASCERTAINING THE ILLUMINATING POWER OF CANDLES, LAMPS, GAS-LIGHTS, AND OTHER LUMINOUS BODIES. Though the eye is not fitted to judge of the proportional force of different lights, it can distinguish, in many cases with great precision, when two similar surfaces, presented together, are equally illuminated. But as the lucid particles are darted in right lines, they must spread uniformly, and hence their density will diminish in the duplicate ratio of their distance. From the respective situations, therefore, of the centres of divergency, when the contrasted surfaces become equally bright, we may easily compute their relative degrees of intensity. For this purpose it is assumed as a principle, that the same quantity of light, diverging in all directions from a luminous body, remains undiminished in all distances from the centre of divergency. Thus we must suppose, that the quantity of light falling on every body, is the same as would have fallen on the places occupied by the shadow; and if there were any doubt of the truth of the supposition, it might be confirmed by some simple experiment. Therefore, it follows, that, since the shadow of a square inch of any surface occupies at twice the distance of the surface from the luminous point the space of four square inches, the intensity of the light diminishes as the square of the distance increases. If, consequently, we remove two sources of light to such distances from an object that they may illuminate it in equal degrees, we may conclude that their original intensities are inversely as the squares of the distances. Hence, if two lights of unequal illuminating powers shine upon the same surface at equal obliquities, and an opake body be interposed between them