Why Two Eyes Rob Painting of Relief; One Vanishing Point
Binocular vision and the mirror; the folly of scenes stacked with many points
Chapter 118 concludes that a painting can never match the relief of things seen in a mirror, and that the fault lies in the painters' ignorance, not in reason. Leonardo explains that because two eyes see a body from two points, the near body cannot wholly hide the one behind it, whereas one eye alone makes a single point and true occlusion; a lettered diagram sets an eye a-b viewing bodies n-m against a single eye s viewing f and r. Chapter 119 then condemns the common practice of stacking several stories one above another on chapel walls, each with its own vanishing point, since the true point belongs to the beholder's eye.
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Two eyes prevent relief; one eye alone gives true occlusion
A painting cannot show the relief of things in a mirror, both being on a surface, unless it is viewed with a single eye. With two eyes, a and b, seeing bodies n and m, the near body m cannot wholly cover n, because the base of the visual lines is wide enough to see the second body past the first. But close one eye, s, and the body f will occupy r, because the visual line springs from one point; so a second body of equal size is never seen.
Chapter 119: do not stack scenes with different vanishing points
The common usage on chapel walls of setting one story on a plane with its landscape and buildings, then raising a step for another with a shifted point, and so a third and fourth, is roundly blamed. A single facade thus ends with four points, which Leonardo calls the utmost folly of such masters. The true point is set at the eye of the beholder of the story.