Universal and Particular Light, and the Middle Shadow
A point is lit in proportion to how much sky and sun it can see
Continuing Book Five, this page states that a surface is more illuminated the greater the quantity of luminous source it can see, illustrated by point e (which sees more sky) being brighter than a. A chapter on universal light mixed with the particular light of the sun proves that a point o which sees the whole span of sky n a m and the sun a is far more lit than a point r that sees only the sky segment m e and no sun. A final chapter introduces the 'middle shadow' interposed between a body's lit and shadowed parts, whose boundaries shift as it turns to shadow on one side and to light on the other. Two diagrams accompany the text, a rayed circle and a domed sky n a m over a small shaded sphere, and the page bears a running head and folio number.
On this page
A point lit by all the sky and sun it can see
The part of a shadowed body seen by less of the universal and particular light is the less illuminated. The point o, which sees and is seen by the whole span of the universal light n a m and the whole sun a turned toward it, is more illuminated than the point r, which is seen only by the sky segment m e and sees no part of the sun.
The middle shadow between light and shade
Between the illuminated part and the shadowed part of bodies the middle shadow is interposed, and it varies its boundaries greatly. Where it ends against the shadow it turns into shadow, and where it ends against the illuminated part it takes on the brightness of that lit region.