Why the Outlines of Opaque Bodies Are Never Sharp
The visual power spreads across the whole pupil, blurring the edges of bodies on the wall behind
This page explains why the true outlines of opaque bodies can never be seen with perfect distinctness. Because the visual power is infused throughout the whole pupil rather than gathered at a single point, each part of the pupil sees the body's edge projected onto a different place on the wall behind. A diagram in the text column shows the pupil a b c viewing the body n, whose extremity m occupies the whole space d e f on the wall g h. A closing heading restates that outlines seen by one and the same pupil never fall in a single place on the body.
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Surfaces have no body, so give no distinct perception
What is lost at short distance is the surface of bodies, otherwise called the boundary of dense bodies; having no substance of its own, it gives no distinct perception, and the less the more remote it is from the observer.
The visual power is spread across the whole pupil
True outlines of opaque bodies are never seen with distinct cognition because the visual power is not produced at a single point but is infused throughout the whole pupil, as proved in the 3rd of the 5th book of perspective.
Each part of the pupil projects the edge differently
Pupil a b c sees the outline of body n at its extremity m fill the whole space d e f on wall g h: the upper part a sees the edge m at point d, the middle b sees a lower edge at e higher than d, and the lower part c sees another edge carried higher on the wall. This is the cause of the confusion of the outlines of shadowed bodies.
Diagram of pupil, body and projection wall
A small diagram in the text column shows the pupil marked a, b, c at right, the body n with extremity m, and the wall g, h with the divided space d, e, f, tracing how the single edge is spread across the wall.