Note 193 — LETTERS ON SCIENCE.

It must always be remembered that there are two kinds of reflection,—one from polished bodies, giving back rays of light unaltered; the other from unpolished bodies, giving back rays of light altered. By the one reflection we see the images of other objects on the surface of the reflecting object; by the other we are made aware of that surface itself. The difference between these two kinds of reflection has not been well worked by writers on optics; but the great distinction between them is, that the rough body reflects most rays when the angle at which the rays impinge is largest, and the polished body when the angle is smallest. It is the reflection from polished bodies exclusively which I usually indicate by the term; and that from rough bodies I commonly distinguish as “positive light;” but as I have here used the term in its general sense, the explanation of the distinction becomes necessary. All light and shade on matter is caused by reflection of some kind; and the distinction made throughout this paper between reflected and positive light, and between real and pseudo shadow, is nothing more than the distinction between two kinds of reflection.

I believe some of Bouguer’s[194] experiments have been rendered inaccurate—not in their general result, nor in ratio of quantities, but in the quantities themselves—by the difficulty of distinguishing between the two kinds of reflected rays.