The eye and the visual pyramid; light and shadow on bodies
Why objects on the visual lines look equal, and why a speck smaller than the pupil hides nothing
A sequence of small optical diagrams, each headed 'Prospettiva', in which an eye at the right views spheres and objects through visual pyramids. The notes argue that an eye placed between light and shadow sees the shadow cross an opaque body through its centre, that objects touching the same visual lines cannot have their intervening distance judged, and that widely separated bodies must differ greatly in size to fill the same pyramid. A final entry gives the sieve test: an object smaller than the pupil hides nothing behind it, but narrowing the eyelids makes the threads of a sieve thicken and cover the view. A short column of numbers stands in the lower right corner.
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Shadow divides the luminous part through a body's centre
An eye placed between the light and the shadow that surround opaque bodies sees the shadow, divided from the luminous part, pass transversally through the centre of that body. The rule ties the eye's position to where the terminator appears.
Distance between objects on the visual lines cannot be judged
Two things seen within the visual pyramids, neither falling short of nor exceeding the pyramid's lines, cannot have the interval between them seen or known by the eye, however great that interval is. The eye reads only the bounding lines, not the depth between the bodies.
Greater separation demands greater difference in size
The greater the distance between bodies enclosed within the pyramidal visual lines, the more necessarily different in size they must be formed. Only so can both continue to touch the same converging lines to the eye.
An object smaller than the pupil hides nothing: the sieve test
If the eye looks at a thing smaller than its pupil, that thing hides no object behind it; but narrowing the eyelids below the object's size lets it cover the second object. Look at something through a sieve with the eye wide, then nearly closed: nearly closed, the sieve's threads seem to thicken and cover the view; wide open, they hide nothing.