Of the edges [outlines] of shadows. Some have misty and ill defined edges, others distinct ones.
No opaque body can be devoid of light and shade, except it is in a mist, on ground covered with snow, or when snow is falling on the open country which has no light on it and is surrounded with darkness.
And this occurs [only] in spherical bodies, because in other bodies which have limbs and parts, those sides of limbs which face each other reflect on each other the accidental [hue and tone] of their surface.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.