f from unreality. All we
know is, that our vision has formed the habit of discerning in the
universe two notions: form and colour; but these two notions are
inseparable. Only artificially can we distinguish between outline and
colour: in nature the distinction does not exist. Light reveals the
forms, and, playing upon the different states of matter, the substance
of leaves, the grain of stones, the fluidity of air in deep layers,
gives them dissimilar colouring. If the light disappears, forms and
colours vanish together. We only see colours; everything has a colour,
and it is by the perception of the different colour surfaces striking
our eyes, that we conceive the forms, _i.e._ the outlines of these
colours.

The idea of distance, of perspective, of volume is given us by darker or
lighter colours: this idea is what is called in painting the sense of
values. A value is the degree of dark or light intensity, which permits
our eyes to comprehend that one object is further or nearer than
another. And as painting is not and cannot be the _imitation_ of nature,
but merely her artificial interpretation, since it only has at its
disposal two out of three dimensions, the values are the only means that
remain for expressing depth on a flat surface.

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 th