Colour is therefore the procreatrix of design. Or, colour being simply the irradiation of light, it follows that all colour is composed of the same elements as sunlight, namely the seven tones of the spectrum. It is known, that these seven tones appear different owing to the unequal speed of the waves of light. The tones of nature appear to us therefore different, like those of the spectrum, and for the same reason. The colours vary with the intensity of light. There is no colour peculiar to any object, but only more or less rapid vibration of light upon its surface. The speed depends, as is demonstrated by optics, on the degree of the inclination of the rays which, according to their vertical or oblique direction, give different light and colour. The colours of the spectrum are thus recomposed in everything we see. It is their relative proportion which makes new tones out of the seven spectral tones. This leads immediately to some practical conclusions, the first of which is, that what has formerly been called _local colour_ is an error: a leaf is not green, a tree-trunk is not brown, and, according to the time of day, _i.e._ according to the greater or smaller inclination of the rays (scientifically called the angle of incidence), the green of the leaf and the brown of the tree are modified. What has to be studied therefore in these objects, if one wishes to recall their colour to the beholder of a picture, is the composition of the atmosphere which separates them from the eye. This atmosphere is the real subject of the picture, and whatever is represented upon it only exists through its medium. [Illustration: DEGAS THE GREEK DANCE--PASTEL.] A second consequence of this analysis of light is, that shadow is not absence of light, but light of _a different quality_ and of different value. Shadow is not a part of the landscape, where light ceases, but where it is subordinated to a light which appears to us more intense. In the shadow the rays of the spectrum vibrate with different speed. Painting should therefore try to discover here, as in the light parts, the play of the atoms of solar light, instead of representing shadows with ready-made tones composed of bitumen and black.