the eye and the light which falls upon it from the opposite side.

And its shadows are in the same positions as those were of the
opposite side. Therefore, O Painter! when you do trees close at
hand, remember that if the eye is almost under the tree you will see
its leaves [some] on the upper and [some] on the under side, and the
upper side will be bluer in proportion as they are seen more
foreshortened, and the same leaf sometimes shows part of the right
side and part of the under side, whence you must make it of two
colours.

Of the transparency of leaves (427-429).