Why evening shadows on white walls turn blue
Distant trees darken and go blue; the sun's red and the sky's blue tint shadows on a white wall
The page first explains that at distance the small lights of a body are consumed into a middle shadow, so trees and every body grow darker than when near; the intervening air then brightens them and inclines them to blue, more in the shadows than in the lit parts where color reads true. A new chapter [467] asks why, toward evening, the shadows cast by bodies on white walls appear blue. Leonardo argues from his rule that an opaque surface partakes of the color of its object: a white wall, deprived of color, is tinged by the reddening sun and the blue sky, and since a shadow cannot see the sun it is lit only by the sky, so the derivative shadow strikes the wall blue. A small marginal diagram shows the sun casting rays onto a wall.
On this page
Trees darken and turn blue with distance
In the long run the quality and quantity of the principal lights and shadows are consumed and confounded into a middle shadow, so trees and every body at distance grow darker than when near the eye. The air that interposes then brightens the thing and inclines it toward blue, turning blue in the shadows rather than in the luminous parts where the truth of the colors shows more.
Blue shadows cast on a white wall at sunset
Shadows generated by the redness of the sun near the horizon are always blue, because the surface of an opaque body partakes of the color of its object. The white wall, void of color, is tinged by its objects the sun and sky; since the shadow does not see the reddening sun it is seen only by the blue sky, so the derivative shadow strikes the white wall blue.