871.
In proceeding to consider this subject, we assume that the painter is generally acquainted with our sketch of the theory of colours, and that he has made himself well acquainted with certain chapters and rubrics which especially concern him. He will thus be enabled to make use of theory as well as practice in recognising the principles of effect in nature, and in employing the means of art.
COLOUR IN GENERAL NATURE.
872.
The first indication of colour announces itself in nature together with the gradations of aërial perspective; for aërial perspective is intimately connected with the doctrine of semi-transparent mediums. We see the sky, distant objects and even comparatively near shadows, blue. At the same moment, the illuminating and illuminated objects appear yellow, gradually deepening to red. In many cases the physiological suggestion of contrasts comes into the account, and an entirely colourless landscape, by means of these assisting and counteracting tendencies, appears to our eyes completely coloured.
873.
Local colours are composed of the general elementary colours; but these are determined or specified according to the properties of substances and surfaces on which they appear: this specification is infinite.
874.