Light of window and sky-horizon falling on a shaded sphere
The angle d differs little from c, and the horizon's contribution to the grading of shadow
The text compares the angles at which points of a shadowy body see the window a b and the whole horizon of the sky (m x), noting that angle d differs little from c because the angles bracketing it are less unequal than those lower down. A large pen diagram dominates the leaf: a semicircular arc labelled ORIZONTE spans the top, with a hatched spherical shadowy body below it and long straight lines converging into pyramids that meet at numbered points along the base. The construction visualises how light from the window and the sky's rim grades the sphere's illumination.
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Angle d compared with angle c
The point sees the whole window a b and the whole horizon of the sky m x; the angle d makes little difference from c, because the angles that set it in the middle are not so unequal in proportion as the others below. It lacks only the part of the horizon between y x, and though it gains as much on the opposite side, its line is of little power because its angle is the smaller.
Semicircular horizon, shaded sphere, and converging pyramids
A large diagram fills the page: a semicircular arc lettered ORIZONTE with points marked along it (T at the apex, B and A at the ends, S P M to the right) rests above a hatched sphere. Straight lines drawn from the arc pass the sphere and cross into pyramids that meet at numbered points (6, 8, 7, 4) along the base line.