The beginning of perspective: the visual pyramid
How images reach the eye by pyramids intersected by a screen; the eyes of distance and alignment; the vanishing point at eye level.
This sheet opens Leonardo's treatment of perspective. Diagrams of an eye receiving visual rays from an upright barrier show that all things send their images to the eye in pyramids, and that the nearer the pyramid is intersected by a perspective screen, the smaller the image appears. He distinguishes two viewpoints of a single eye, 'eye f' for distance (how far one stands away) and 'eye t' for straightness (whether one is centered or to the side), which must be kept at the same height and used together. A final demonstration with a square and a perpendicular staff r S concludes that objects receding on a flat plain rise toward, but never above, the level of the eye, meeting at the point of diminution to which all the visual pyramids converge.
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All images reach the eye by pyramids
All things send their images to the eye by pyramids; the nearer to the eye these are intersected, the smaller the images appear than their causes. One intersects the pyramid with the perspective screen touching the base of that pyramid, as shown at screen a n.
The two eyes: distance (f) and straightness (t)
Eye f and eye t are one and the same, but eye f denotes distance (how far you stand from an object) and eye t denotes straightness (whether you stand centered on it or to the side); the two are always situated at the same height.
Using eye f and eye t together
If you raise or lower eye f for distance, do the same to eye t for straightness. Point f shows distance but not alignment, and point t shows alignment but not distance; only by using both together do they indicate the same thing.
Receding objects rise toward the eye's level
With a square whose side equals the distance S to c and a staff fixed perpendicular at r S, the near side strikes the base of the screen and the far side rises to point n. Objects receding on a flat plain rise ever higher, but only up to the level of the eye, meeting at the point of diminution.
The visual pyramid from barrier to eye
Visual rays converge into the eye from an upright barrier whose top and base are labeled c and d, forming within the pyramid a tiny image labeled a and b close to the eye.