1. A ray of solar light is composed of an indeterminate number of variously coloured rays, which are distributed into groups, termed red rays, orange rays, yellow rays, green rays, blue rays, indigo rays, violet rays.

2. But all the rays comprised in the same group, the red for example, are not identical in colour; on the contrary, they may be considered as differing more or less among themselves, although we recognise the impression they produce separately, as comprised in that which we ascribe to red light.

3. When light is reflected by an opaque white body, it is not modified in proportion to the variously coloured rays which constitute white light; but, 1. If the body is not polished, every point of its surface is to be considered as dispersing the white light which falls upon it, in all directions, into the surrounding space; so that the point becomes visible to an eye placed in the direction of one of its rays. We may easily conceive that the image of the body, in a given position, is composed of the sum of the physical points, which send to the eye so placed, a portion of the light which each point radiates. 2. If the body is polished, as, for example, the surface of a mirror, a portion of the light is irregularly reflected, as in the preceding case; while another portion is regularly reflected, giving to the mirror the property of presenting to an eye, suitably placed, the image of the body which sends its light to the reflector. One consequence of this distinction is, that if we regard two plane surfaces which reflect white light, and differ from each other only in polish, it will happen that where the unpolished surface is visible, all its parts will be equally, or almost equally, illuminated; while the eye, when in a position to receive only the light which it reflects irregularly, will receive very little light from the polished surface; but it will receive much more light when in a position to receive that which is regularly reflected.

4. If the light which falls on a body is completely absorbed by that body, so that it disappears from sight, as in falling into a perfectly dark cavity, then the body appears to us black; and it becomes visible only because it is contiguous to surfaces which reflect or transmit light. We know of no bodies which are perfectly black, and it is only because they reflect a little white light that we judge they have relief, like other material objects.

5. When light is reflected by an opaque coloured body, there is always a reflection of white light, and a reflection of coloured light; the latter is owing to the fact that the body absorbs or extinguishes within itself some of the coloured rays, and reflects the others. It is evident that the absorbed coloured rays are of a different colour from the reflected coloured rays; and farther, that if these be reunited with the former, white light will be reproduced. It is evident, also, that unpolished opaque bodies reflect irregularly white light, and the coloured light which makes them appear coloured; and that those which are polished reflect irregularly a portion only of these two lights, while they reflect regularly the other portion.

6. It thus appears, by what has been said concerning the physical composition of solar light, that if the whole of the coloured light which is absorbed by a coloured body were reunited with the whole of the light which it reflects, white light would result. Now, it is this property of two variously coloured lights, taken in a certain proportion, to reproduce white light, that we express by the words coloured lights complementary to each other, or complementary colours. It is in this sense that we say,

7. It must not be supposed that a red body, a yellow body, &c. reflects, besides white light, only the red rays or yellow rays, &c., each of these bodies reflects also every sort of coloured rays; but, the rays which cause us to judge it to be red or yellow, &c., being more numerous than the others, produce more effect than they; yet, the latter have an undoubted influence in modifying the action of red and yellow rays upon the organ of sight. This explains the innumerable diversities of colour observable among various red bodies, various yellow bodies, &c. It is difficult not to admit that among these diversely coloured rays reflected by bodies, there are a certain number which, complementary to each other, must reproduce white light upon reaching the retina.