Why an arc appears of different sizes as the eye moves
On apparent magnitude and the distance of the viewing eye, against an adversary
A perspective-and-optics note built around the large diagram at the right, where straight rays fan out from a single point (the eye) toward a set of nested rings and an arc placed at increasing distances. The text argues that the arc a b appears in as many different sizes as there are different distances of the eye that sees it, refuting an 'adversary' who assumes the arc keeps a single size and merely claims that the more distant eye sees it smaller. Faint geometric sketches (small squares and construction lines) also appear toward the left and are not transcribed.
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The arc a b seen from different distances
The arc labelled a b will appear at as many different sizes as there are different distances between it and the eye that observes it. Leonardo sets this against an opponent who treats the arc as fixed in one place and concludes only that the eye placed farther away sees it smaller.
Cone of visual rays reaching nested rings
Straight lines diverge from a single point at the left to a series of concentric rings and an arc set at increasing distances. The construction shows how one and the same arc subtends the eye differently according to how remote it is, the geometric ground of the argument in the text.