Measuring the Diminishing Visual Pyramid with a Rod
The convergence point found in another's eye; a rod gauging a tower's foreshortening
The folio continues Leonardo's study of the visual pyramid. He argues that the point to which sight converges can be grasped by looking into another person's eye and imagining lines from your own ears narrowing toward your reflected image. A diagram of tower m n and a rod e f, held at successive positions c d and a b, shows a practical method for measuring how a body's image diminishes through the air. Two divided triangles and a closing problem establish that a pyramid's half-base equals its width at mid-length.
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Finding the point of convergence in another's eye
Look into someone's eye and you will see your own image there. If you imagine two lines running from your ears to the ears of that reflected image, you will see them narrow so sharply that, a little beyond the image, they would meet at a single point. This makes the convergence of the visual pyramid visible.
Gauging a tower's diminishment with rod e f (tower m n)
Adjust a rod e f until its ends just cover the extent of tower m n; then bring it closer to the eye, to c d, and the tower's image reads smaller at r o; closer still, to a b, and the rod overruns the tower's image by a to l and t to b. From this the lines are seen to converge to a point shortly beyond the eye.
A pyramid's half-base equals its mid-length width
Two triangles set on their bases, each divided by a vertical line at mid-length, show that every pyramid's half-base is as wide as the pyramid is at the middle of its length: a b equals d e, and m n equals p q. A closing problem asks the distance between the two lines at the middle of a pyramid built on line S t.