A Treatise on Painting/Chapter 295
Chap. CCXCV.—Of the Perspective of Colours.
The principal colours, or those nearest to the eye, should be pure and simple; and the degree of their diminution should be in proportion to their distance, viz. the nearer they are to the principal point, the more they will possess of the purity of those colours, and they will partake of the colour of the horizon in proportion as they approach to it.