Light of the Hemisphere and the Color of Shadows
Why the shadow on a face borrows the color of the objects set against it
Text page of the Treatise on Painting explaining how the total light of the sky-hemisphere wraps around the lowest parts of a shadowed body, unless its horizon blocks it, and especially when the body is raised above the ground. A small semicircular diagram labels the shadowed body f, the earth, the hemisphere a b c d, and its horizon a d, with the earth's darkness u x. A new chapter argues that true shadows rarely keep the color of the lit surface, since every surface takes on the color of the objects facing it — a black object casts blackish shadow into a face, and likewise yellow, green, or blue.
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The sky-hemisphere's light reaches the lowest parts of a shadowed body
The great sum of light of our hemisphere surrounds even the lowest parts of shadowed bodies, provided it is not impeded by its horizon, and especially when the body is suspended above the earth. Where the darkness of the ground can see a body, it darkens as much of it as it sees; where the bright horizon sees the same parts, it illuminates them and cancels the dark cast the ground would otherwise throw beneath the object.
Diagram of the shadowed body f within the hemisphere a–d
A small semicircular figure drawn into the text stands for the celestial hemisphere. Its rim is lettered a b c d and its base line a d is named the horizon of that hemisphere; the shadowed body f sits at the center above the earth, whose darkness is marked u x.
A lit surface takes the color of the object facing it
True shadows that keep the color of their own lit part are very rare in opaque bodies. Because the surface of every shadowed body partakes of the color of its object, the lit color of faces set before a black object will take on black shadows, and so with yellow, green, blue, and every opposing color, since every body sends its likeness through all the surrounding air.