On Painting: three relations of eye, light and shadow
The three ways the eye and the light source may stand to a lit body
In red chalk, a passage headed 'On Painting' sets out the three aspects under which shadows and lights meet the eye. In the first the eye and the light lie on the same side of the body that is seen; in the second the eye faces the object while the light stands behind it; in the third the light stands to one side, so that the lines running from the object to the eye and from the object to the light meet at a right angle. The page is filled with mirror-script and carries no diagram.
On this page
The three aspects of shadow and light seen by the eye
Three configurations are distinguished by where the light stands relative to the eye and the lit body. When the eye and light share a side, the body is seen fully lit; when the light is behind the object, the eye meets shadow; in the third case the line from the object to the eye and the line from the object to the light meet at a right angle. The account treats the geometry of illumination rather than any single picture.