Why the Air Is Blue; The Definition of Painting
Colour never seen pure; sky-blue from light, darkness and moisture; painting in universal light
Leonardo explains that colour is never seen simple, because every surface — even transparent air or water — partakes of the colour of its object, and air tinges itself with whatever colours lie between the eye and the thing seen. He accounts for the blue of the sky as light and darkness compounded: the moist lower air, thickened and struck by the sun's rays, is illuminated while the upper air stays dark, so the blue deepens or pales with the air's moisture. The leaf then gives his definition of painting as a composition of light and darkness mixed with all simple and compound colours, and opens a chapter on painting in universal light, urging that the parts of figures be made darker the lower they are and the nearer to the centre of a crowd.
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Why the air appears blue
Air in itself has no more colour than water, but the moisture that mixes with it from the middle region downward thickens it, so the solar rays striking it illuminate it while the upper air remains dark. Because light and darkness compose the colour blue, this is the blue in which the air is tinted. The blue grows darker or paler as the air is mixed with less or more moisture.
The definition of painting and painting in universal light
Painting is defined as a composition of light and darkness mixed together with the various qualities of all the simple and compound colours. In universal light the painter should make the parts of figures in a crowd darker the lower they lie and the nearer they are to the middle of the multitude, even where the bodies are of uniform colour.