How the eye discerns a body against its background
Binocular vision sees behind a small object; the moon appears joined to the sun in eclipse
Headed "Perspective," the page joins a discussion of vision to a large diagram of visual cones and fields (labelled d, m n a b c K, o, p t, r S, g), where r S are the two eyes. Leonardo argues that no body can be well judged except by the variety of the fields against which its edges terminate, and cites the eclipse: the moon, though far from the sun, appears joined to it because it is silhouetted against it. He then shows that an object narrower than the gap between the pupils cannot fully hide the wall behind it — the left eye sees the wall segment n a while the right eye sees the part a c hidden from the left — so the two eyes together see behind a small body.
On this page
A body is judged only against the field around it
No evident body can be well comprehended and judged by human eyes except through the variety of the fields where its edges terminate and border; nothing appears divided from those fields except by the outline of its edges.
The two eyes see behind a small object
With r S the two eyes, p t the observed body, and m K the wall, the left eye sees the wall segment n a and finds a c hidden by the body, while the right eye sees a c (or the part b c of it) that the left eye could not; so a body narrower than the interval between the pupils never hides as much wall as its own size.
The moon appears joined to the sun in eclipse
The moon, although very distant from the body of the sun, appears to human eyes joined and attached to it during an eclipse, because it is silhouetted against the sun when it comes between our eyes and it.