The Da Vinci Notebooks at sacred-texts.com
A leaf with a concave surface seen from the under side and up-side-down will sometimes show itself as half in shade, and half transparent. Thus, if o p is the leaf and the light m and the eye n, this will see o in shadow because the light does not fall upon it between equal angles, neither on the upper nor the under side, and p is lighted on the upper side and the light is transmitted to its under side. 216
216:216 : See Pl. XXVIII, No. 2, the upper sketch on the page. In the original they are drawn in red chalk.