Why a luminous aperture seems to narrow near an interposed object
The eye's visual power spread across the whole pupil, demonstrated with rays a b … m S
Leonardo argues that a luminous aperture (spiraculo) viewed from a dark place, though of uniform width, appears to constrict wherever an object is interposed between eye and aperture, because the edges of any interposed object are never seen sharply but blurred by the air that darkens near them. A second demonstration holds that the eye employs its visual power throughout the whole size of the pupil, less so as the operating part is smaller. A lettered eye diagram (a b c d e o f t, p q, r g h i K n m S) traces how different parts of the pupil's light see the object p q terminate at successive points of the field r S. The page carries a hatched aperture sketch at upper right and the ray diagram at lower right.
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A uniform luminous aperture appears to constrict near an interposed object
Seen from a dark place, the aperture seems to narrow beside any object placed between the eye and it. This follows because the edges of any interposed object are never seen distinctly but confused by the air that darkens near those edges, an obscurity that grows stronger the closer it approaches the edges.
Visual power distributed across the whole pupil (a b c d e o f t; p q; r g h i K n m S)
The eye's visual power is spread through the whole pupil, so the object's edges cannot be seen sharply in the field. Part f o of the light sees the upper field at g h, o e sees it occupied by the object h i, d sees the object's top at i, c at K, b at n and a at m, and from m downward nothing is seen.