Why winter landscapes must not be painted blue
Aerial perspective, the blue of distant mountains, and tree shadows
Headed "Pittura," this leaf sets out rules of aerial perspective, with a small marginal diagram of the sun, three variously lit plants, and the eye. Leonardo argues that a winter landscape should not show its mountains as blue as a summer one: bare, brownish trees are lighter than green foliage, and since the darker a distant mountain the bluer it looks, summer greenery appears more blue than winter's bare branches. A second note treats how the shadows of plants differ on the right and left sides of a scene when the sun stands to one side, and a marginal line refers the whole to the definition of the blue colour of the air.
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Winter mountains should be less blue than summer ones
Leonardo cites his own rule that among mountains seen at long distance the one darker in itself appears more blue. Because bare plants are of a brownish colour and leaves are green, and the green is darker than the brownish, summer greenery shows as more blue than winter's bare branches. Hence the painter must not give a winter landscape mountains as blue as those of summer.
Leafy plants cast darker shadows than bare ones
Leonardo invokes the rule that the shadows of leafy plants are darker than those of leafless plants, in proportion as the leafy are less sparse than the bare. This links the colour of the tree to the density of its foliage across the seasons. The reasoning supports the argument about the changing blueness of the wooded landscape.
Plant shadows shift with the sun's position
In the diagram of the sun, three lit plants, and the eye, labelled a; b c; d e; f g; h, Leonardo notes that shadows do not clothe plants on the right in the same way as those on the left, especially when the sun stands to one side. He cites propositions that an opaque body between light and eye shows wholly shadowed, an eye between body and light sees it wholly lit, and eye and body between darkness and light show half shadowed and half luminous.
The blue of the air governs seasonal landscape colour
A marginal note refers the whole argument to the definition of the blue colour of the air. That definition, Leonardo says, gives judgement on why landscapes appear more blue in summer than in winter. It anchors the seasonal difference in an optical property of the atmosphere itself.