A bird flying level appears to lose height as it recedes
Degrees of motion read as degrees of descent in the eye
Leonardo states that a bird flying along the "line of equality" (a level horizontal path) while moving away from the eye will appear, degree by degree of its motion, to gain degrees of lowness, i.e. to sink. The claim is an optical illusion of perspective rather than any real descent of the bird. A single diagram at the upper left shows an eye enclosed in a circle at the bottom, with rays running up to a horizontal line carrying a bird-like mark at either end.
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Level flight away from the eye reads as descent
A bird that flies along the line of equality while separating from the eye shows itself, in the degrees of its motion, to acquire degrees of lowness. The level path is perceived as a gradual dropping the farther the bird goes.
Eye, sightlines and the bird's level path
The lower-left diagram places the eye within a small circle and draws rays climbing to a horizontal line, with a bird sketched at each end of that line. It illustrates how the same level flight subtends changing angles at the eye.