Common Perspective: Why Distant Air Appears Blue
The physics of aerial blue: light through the air's thickness, with the eye at p viewing the lines p r and p s.
This chapter of the Treatise on Painting, headed 'Common perspective, and on the diminution of colors at long distance,' gives Leonardo's physical explanation of the blue of the sky and distant air. Solar rays lighting the countless motes suspended in the air make its whiteness read as blue against the darkness beyond, and the blue looks brighter the greater the thickness of air interposed. A small diagram set into the column illustrates an eye placed at p that sees the air's thickness overhead along p r and, looking lower, a greater thickness along the oblique p s.
On this page
Diminution of color toward the horizon
The air partakes the less of blue the nearer it lies to the horizon, and is the darker the more remote it is from that horizon. This governs how a painter must fade colors of distant landscape.
Why the air appears blue
Because air is less rare than fire, it is more lit by the solar rays that penetrate it, kindling the infinite motes diffused through it. The whiteness of the lit air seen against the darkness beyond necessarily appears blue, and the more so the greater the thickness of air interposed.
The eye at p and the lines p r and p s
A small figure demonstrates the argument: an eye set at p looks overhead through the air's thickness p r, then, declining, sees a greater thickness along the oblique line p s, which therefore appears brighter. Inclined toward the horizon, the eye finds the air almost wholly deprived of blue.