The visual pyramid and the circular spread of images
Parallel rays down a cylinder, pyramids narrowing with distance, and light spreading like ripples
These optical studies analyse how the eye receives the forms of things. Bodies of equal size at greater distances send their species to the eye by ever narrower pyramids, whose lines converge to a point within the eye along straight paths, while a cylinder viewed end-on shows rays that run parallel and cannot converge. Leonardo compares the spread of visual images to a stone thrown into water, whose impact becomes the centre of expanding circles, and to a sound that spreads circularly through air, so that every body in the luminous air fills the surrounding space with countless images of itself, whole and entire at every point. A final figure of two windows flanking a small hole begins to prove this by closing a west-facing window and making a hole, the note breaking off; the diagrams carry the letter-labels a, b, c, d, e, f.
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Parallel rays along a cylinder
Two eyes sighting along the length of a cylinder placed against them find that the rays running along its outer sides are parallel and cannot converge; a cross-section is drawn beside the word parallel.
Pyramids narrowing with distance
Bodies of equal size at different distances are seen by different pyramids, which grow narrower as their cause is farther away, and send their species to the eye accordingly. The figure shows an eye viewing three objects, the limits lettered a, b, c, d, e, f, the farther pairs converging within the eye.
The visual line converges in the eye
The convergence of the pyramidal lines caused by objects and terminating in the eye is necessarily along straight lines. An eye views through a narrow cylinder, the rays meeting at a point within it.
Images spread circularly, like ripples
Just as a stone thrown into water becomes the centre and cause of expanding circles, and a sound spreads circularly through air, every body in the luminous air spreads circularly and fills the surrounding parts with infinite images of itself, whole and entire at every part.
The window-and-hole experiment (unfinished)
The claim is to be proved by experience: closing a window that faces west and making a hole. The note breaks off; the figure shows two windows on either side of a small circular hole with rays crossing to opposite extremes of the circle.