Experiment: a moving object seen through a pinhole card
Fine-pierced paper near the eye; eyelashes beyond the hole seem to move against the eye's motion
Leonardo describes an optical experiment: pierce a paper with the finest holes and move a thin object—like the tip of a straw—between the eye and the card, held about a quarter-braccio away, while looking at the open air through the holes. If the paper is instead brought close enough almost to touch the eyelashes and the face is moved left and right in a short motion, the lashes seen beyond the hole appear to move in the direction opposite to the eye's own motion. The page carries no diagram, only two paragraphs of mirror script.
On this page
Fine-perforated card and a moving straw-tip
Make the perforations with the smallest holes and move a slender object, like the tip of a straw, touching the eyelid as you move it. Hold the card a quarter-braccio from the eye and look at the open air through the holes.
Eyelashes seem to move against the eye
Bring the card close enough almost to touch the lashes of the eyelid and move the face right and left with a short motion. The hairs seen beyond the hole will appear to move in the direction contrary to the motion made by your eye.