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, Red is complementary to Green, and _vice versâ_. Orange ” ” Blue, ” ” Greenish-Yellow ” Violet, ” ” Indigo ” ” Orange-Yellow, ” 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. PART THE FIRST. SECTION THE FIRST. OF THE LAW OF SIMULTANEOUS CONTRAST OF COLOURS, AND OF ITS DEMONSTRATION BY MEANS OF EXPERIMENT. CHAPTER I. _Manner of observing the Phenomena of the Simultaneous Contrast of Colours._—_Definition of Simultaneous Contrast._ 8. If we look at the same time at two stripes of unequal tints of the same colour, or at two stripes of equal tints of different colours, in juxtaposition, that is to say, contiguous by one of their edges, the eye will perceive, if the stripes be not too wide, certain modifications; in the first case affecting the intensity of the two tints, in the second, the optical composition of the two colours so placed. Now as these modifications cause the colours to appear, when looked at together, more different than they really are, I have given to them the name of the _simultaneous contrast of colours_. The modification which affects the intensity of colour, I term _contrast of tone_; and the modification which affects the optical composition of the contiguous colours, I term _contrast of colour_. The twofold phenomena of contrast of colour and contrast of tone may be readily shown by the following. _Experimental Demonstration of Contrast of Tone._ Plate A Aʹ.