How light and shadow fall on trees
The eye, the sun, and the shading of trees by their own branches
Headed "Pittura," this leaf pairs mirror-script notes with small diagrams: an eye-and-two-plants scheme drawn at upper right and, along the right margin, three studies of a tree (one blotted, one illuminated, one in shadow). Leonardo reasons that an eye can never see two trees lit alike, because the tree nearer the sun's rays looks the more shadowed, so that of tree d only shadow is seen while tree b appears half lit and half shaded. He reminds the painter that within a single species the darkness of a tree's shadow varies with how sparse or dense its branching is.
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Eye c seeing trees b and d lit by sun a
Given the eye c looking at two plants b and d illuminated by the sun a, Leonardo argues that the eye never sees the two lit alike. The tree nearer the sun appears more shadowed, in proportion as it lies nearer the meeting of the solar rays that reach the eye. Thus of tree d the eye c sees nothing but shadow, while tree b is seen half illuminated and half in shadow.
Shadow darkness varies with the density of branching
Leonardo tells the painter that within one and the same species of plant the darkness of the shadows varies as widely as the thinness or density of the branchings varies. A note on a tree seen from below adds that the eye sees its crown lying within the circle formed by its own branches. The three marginal tree studies illustrate a blotted, an illuminated, and a shaded tree.
A precept for painting trees
The whole discussion is offered as painting practice, under the heading Pittura. It teaches the painter to judge, for any given tree, how much of it will read as lit and how much as shadow according to the eye's position relative to the sun. Only the surfaces facing the light show as illuminated.