Leonardo was fascinated by the atmosphere and by its effects on the colors and distinctness of distant objects. Though other artists had already begun to create some of these effects in their work, Leonardo was the first to make careful measurements and suggest rules for applying them realistically in painting. He called the subject aerial perspective.
In morning light Leonardo observed that distant objects such as mountains look bluer and less distinct than nearby mountains. He also noted that the more distant the mountain, the more its color approached that of the surrounding atmosphere.
His experiments suggested that to correctly color objects at different distances, artists should do as follows: Paint the nearest one its true color. Paint the one behind proportionately bluer, and the one behind that bluer still.
