Multiple Shadows: A Body Casts One for Each Surrounding Light
When light and body match in size the cast shadow keeps the primitive's shape; many lights make many coloured shadows
A text page from the fifth part of the Treatise on Painting continuing the science of shadow. It states that when the luminous body equals the shadowed body in length and breadth, the cast (derived) shadow keeps the same figure as the primitive shadow, and that the borders of cast shadows are tinged with the colour of the illuminated surroundings. It then argues that a shadowed body generates as many shadows, of as many colours, as there are lit surfaces around it, and that the shadow made by the strongest light is the darkest.
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Cast shadow keeps the primitive's shape when light equals body
If the light is thinner and longer than the shadowed body, the derived shadow strikes thicker and shorter than the primitive; but when the length and breadth of the luminous body equal those of the shadowed body, the derived shadow's bounds match the primitive shadow's figure exactly.
Coloured borders around a cast shadow
The bounds of simple derived shadows, where they strike, are always surrounded by the colour of the illuminated things that send their rays from the same side as the light which lights the body casting the shadow. Reflected local colour tinges the shadow's edge.
As many shadows as there are surrounding lights
A shadowed body generates as many kinds of shadow around its base, and of as many colours, as there are opposite illuminated colours surrounding it; one is stronger than another as its facing light is more splendid, and of all these encircling shadows the darkest is that made by the most powerful light.