Perspective, diminution, and shadow on opposed bodies
The wall as guide to perspective; an eye among the light rays sees no shadow
Repeated 'Prospettiva' entries with eye-and-pyramid diagrams argue that perspective supplies the judgement the eye lacks in things that diminish. The eye cannot truly measure how much one quantity shrinks beside another except through the wall (pariete), the mistress and guide of perspective; the same holds for the three divisions of a face when painter and sitter share an eye-level (n, e f, a b c d). Two further notes turn to shadow: an eye set among the natural light rays sees no shadow on the bodies opposite it, and a sphere lying between the light-centre and the eye-centre appears wholly dark ringed by an equal circle of light.
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Perspective supplies what judgement lacks in diminishing things
Perspective is defined as what supplies where judgement fails in things that diminish. It sets up the page's theme: the eye alone cannot reliably measure apparent shrinkage with distance.
The wall (e f) as guide; the three divisions of the face
The eye cannot judge how much a quantity diminishes below and near another similar one except through the wall (pariete), the guide of perspective. With eye n midway on equal lines, a b appears as large as b c while the lower, farther c d looks smaller; the same appears in the three divisions of the face when the painter's eye is level with the sitter's.
An eye among the light rays sees no shadow
An eye that sends out its visual pyramids in the same direction as the natural luminous rays, if it sits in the midst of those rays, can see no shadow on the shadowy bodies set opposite it. Vision aligned with the light meets only lit faces.
A sphere between light-centre and eye-centre: dark with a ring of light
A spherical body lying between the centre of the natural light and the centre of the visual pyramids is seen by the eye wholly shadowy, surrounded by an equal luminous circle. The offset of the two centres controls how much of the sphere reads as lit.