Rays Seen Around a Light Arise in the Eye Itself
Experiments with the eyelashes and a stylus, and why the rays lengthen with distance
Leonardo pursues the rays seen around a bright light when the eyelids are narrowed, arguing they arise from the lashes and lids. He describes experiments as proof: draw a stylus crosswise over the lower lid and the upper rays move with it; turn the face slowly left and right and the rays wheel and exchange one into another. He prescribes the conditions, the light at least 25 braccia away, a dark room, the eye kept narrow, and the head steadied against a wall. He also explains why a distant light's rays look long while a near light's look short, since the half-closed lids embrace only a little space above and below the flame, as the lettered a b and c d figures show.
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Rays around a light arise from the lashes and lids
It is the lower lashes that display the upper rays: move a stylus lightly crosswise over the lower eyelid and the upper rays move accordingly, a sign born of reason. Turn the face slowly to right and left and the rays that spring from the shine of the lids turn and exchange too. Leonardo prescribes a light at least 25 braccia off, a dark room, the eye kept narrow, and the head rested against a wall.
Why a light's rays look long from afar and short up close
A luminous object near the eye shows shorter rays than one far off, because the half-closed lids (as one does to see rays around a light) embrace little space above and below it. Since the rays cannot open wider than the space the eye sees, a short space shows short rays and a long space long rays, as a b above and c d demonstrate.