VIII. Botany for Painters and Elements of Landscape Painting: 440.
440.
ON PAINTING.
In the position of the eye which sees that portion of a tree illuminated which turns towards the light, one tree will never be seen to be illuminated equally with the other. To prove this, let the eye be c which sees
the two trees b d which are illuminated by the sun a; I say that this eye c will not see the light in the same proportion to the shade, in one tree as in the other. Because, the tree which is nearest to the sun will display so much the stronger shadow than the more distant one, in proportion as one tree is nearer to the rays of the sun that converge to the eye than the other; &c
You see that the eye c sees nothing of the tree d but shadow, while the same eye c sees thè tree b half in light and half in shade.
When a tree is seen from below, the eye sees the top of it as placed within the circle made by its boughs[23].
Remember, O Painter! that the variety of depth of shade in any one particular species of tree is in proportion to the rarity or density of their branches.