Where Shadow Is Darkest; Shadow and Light Interchange
Derived shadow darkens near its cause; the lesser light is the shadow of the greater, so shade and light convert into each other
A text page from the fifth part of the Treatise on Painting continuing the science of shadow. It restates that a derived shadow is darkest nearest its cause and lighter farther off, and that a body's shadow enlarges either as the light shrinks or as the space between light and body increases. It then frames light and shadow as relative: a place shadowed from the sun stays lit by the air once the sun departs, since the lesser light is always the shadow of the greater, and conversely a place lit only by air turns dark when ringed by sunlight.
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Derived shadow is darkest nearest its cause
That part of the derived shadow will be darkest which lies nearest to its cause; the contrary follows, that the part more remote from its cause will be of less darkness.
Shadow grows as the light shrinks or the gap widens
The shadow of one body grows as much as the light that generates it diminishes, without any change of position. And it likewise grows and shrinks as much as the space between the light and the shadowed object grows or shrinks, the object being smaller than the luminous body.
Shadow that turns to light, and light that turns to shadow
A place shadowed from the sun stays illuminated by the air after the sun departs, because the lesser light is always the shadow of the greater. Conversely, a place lit by the air becomes shadowy when surrounded by the strike of sunbeams, since the greater light makes the lesser appear dark.