362. =Recomposition of Light.=--After decomposing light by passing it through a prism we can bring the separated colors together again and form from them white light. The manner in which this is done is represented in Fig. 251. The beam of light, after passing through the prism S A A', instead of proceeding in the direction indicated by the dotted lines to form the spectrum, is made to pass through the prism S' B B', placed in a reversed position, and its rays are refracted so as to assume their original relation, making a white beam, M. Here the second prism counteracts the effect of the first, because its position is exactly the reverse. [Illustration: Fig. 252.] Newton very justly considered the decomposition and the recomposition of light as affording the most positive proof that white light contains all the seven colors. He tried various experiments to prove the same thing. Thus he mingled together intimately seven powders having the seven prismatic colors, and found that the mixture had a grayish-white aspect. He also painted a circular board with these colors, and found that on whirling it so rapidly that the colors could not be distinguished the whole board appeared to be white. In order to have this succeed perfectly the proportion between the colors must be observed, as in Fig. 252. A very pretty way of illustrating the composition of light is to have a top painted in this way. When the top is whirling rapidly it is white, but as it slackens its motion the seven colors appear. 363. =Colors of Objects.=--The color of any object depends upon the manner in which it reflects light. Thus, if it be red, it reflects the red rays of the spectrum, absorbing the other rays; and if it be green, it reflects the green rays, etc. If it reflect all the colors together, it is white; and if it reflect none, or almost none, of the light, it is black. You can readily see why the color of an object varies with the kind of light that shines upon it. If an object which is red in sunlight be exposed to a yellow light, as a yellow flame, or sunlight that has passed through a yellow-colored glass or curtain, it loses its red color, for there are no red rays in the light to be reflected by it into our eyes. A person exposed to such a light has a deathlike paleness, the lips and skin losing entirely their red color. This effect can be witnessed at any time by mixing alcohol with a little salt on a plate and setting fire to it. You see in what has been said the reason that, in examining goods in the evening, especially by candle-light, we find the colors often differ somewhat from those which they have in the day. In some substances the colors are changeable with varying positions, though the light be the same. We see this often in shells and minerals. We see it also in some fabrics, as changeable silk. This is owing to the arrangement of the particles, it being such as to occasion variety in reflection with changes of position.