Testing a Shadow with the Finger; Light in the Countryside
Sun behind the eye shows bodies without shadow
Chapter [761] gives a practical test for a figure's shadow: cast a shadow with your own finger over the lit area, and if that accidental shadow matches the natural shadow the finger throws on your painting, the tone is right, neither too red nor too yellow. Chapter [762] turns to lights and shadows of things seen in open country: when the eye looks along the sun's rays it sees every body wholly lit and without shadow, proved by the ninth proposition that an opaque surface partakes of the colour of its object. A further heading [763] begins the case of the sun in the east with the eye to the north or south. The page is clearly written with no diagram.
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[761] Testing a figure's shadow with your own finger
To check that a shadow is companion to the light and not too red or yellow, cast a shadow with your finger over the lit part; if this accidental shadow matches the natural shadow the finger throws on your work, it is correct, and moving the finger nearer or farther yields darker or lighter shadows to compare.
[762] Looking along the sun's rays: bodies without shadow
When the eye sees all the parts of bodies lit by the sun, it sees them without shadow. By the ninth proposition, every opaque surface partakes of the colour of its object, so surfaces facing the sun share its brightness, and the eye can see no primitive or derivative shadow on such bodies.
Positions of light and shade for things seen in the countryside
The chapter frames the placement of lights and shadows on bodies viewed outdoors in relation to the sun's position, opening the sequence of cases that begins with the sun in the east and the eye to the north or south.