A very delicate experiment, yet a very natural one, which Buffon appears to have first noticed, led in all probability to the invention of the monochromatic mode of painting, or painting with a single colour. If, at the moment which precedes sunset, at the close of a cloudless day, a body is placed near a wall, or against another polished body, or on a smooth chalky soil, the shadow carried by this body is blue, instead of being black or colourless. This effect is produced by the light of the sun being so weakened, that the blue rays which are reflected from the sky—which has always this colour on a clear day—fall, and are again driven back or reflected on that part of the wall which the dying light of the sun cannot strike; for even at its last moment, the light which falls straight and direct, is sufficiently strong to destroy that of the heavens, which is only reflected, wherever they meet.