John Ruskin,
Modern Painters, Volume 1 (of 5…
“ It is perhaps one of the most difficult lessons to learn in art, that the warm colors of distance, even the most glowing, are subdued by the air so as in no wise to resemble the same color seen on a foreground object; so that the rose of sunset on clouds or mountains has a gray in it which distinguishes it from the rose color of the leaf of a flower; and the mingling of this gray of distance, without in the slightest degree taking away the expression of the intense and perfect purity of the color in and by itself, is perhaps the last attainment of the great landscape colorist. ”

