Light rays and the gradation of shadow through an aperture
Ray diagrams proving that a moving shadowy body casts a shadow in the opposite sense
A dense sheet of geometric-optics studies covering the striking (percussione) of luminous rays and how shadow acquires degrees of darkness across the illuminated field. Leonardo pairs numerous pen diagrams of crossing rays, apertures and luminous discs with lettered demonstrations that graded light (whole, three-quarters, half and quarter light) produces correspondingly graded shadow. He argues that a shadowy object moved between the light and an aperture (spiraculo) casts its shadow in contrary motion, and observes that a first, purely linear figure fails where experience does not confirm equal brightness. Further faint writing and diagrams run along the cut lower margin.
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Graded darkening across the ray's striking (a e, b e, c e, d e)
The whole light a e reaches i; three-quarters of the light b e reaches h; the half-light c e reaches g; and the quarter-light d e reaches f, so f is less luminous than i by three-quarters. Since a first figure does not conclude this, a second figure is drawn to complete the proof.
Shadow of a body moved between light and aperture descends in contrary motion
Moving a shadowy object from p to r between the luminous body and the aperture makes its cast shadow descend the opposite way, from K to i. Leonardo notes that mere intersecting lines would predict uniform brightness (K f of equal light), but experience does not confirm it, so something further must be at work.