Vol. I: VIII. Botany for Painters and Elements of Landscape Painting: 423.
The Da Vinci Notebooks
The proportions of light and shade in a leaf (423-426).Sometimes a leaf has three accidents [of light] that is: shade, lustre [reflected light] and transparency [transmitted light]. Thus, if the light were at n as regards the leaf s, and the eye at m, it would see a in full light, b in shadow and c transparent.