Composite vs. Simple Perspective; a Note on Joints
Why peephole (composite) perspective fails for many viewers, with a marginal reminder on the bending of joints
Under 'Perspective', with two diagrams of objects and converging rays lettered a b c, d e, f g, h, Leonardo contrasts nature's perspective (distant things look smaller) with an artificial 'composite' perspective that only works when a single viewer keeps one eye at a fixed peephole. Because many eyes look at one work at once and only one sees it correctly while the rest are confused, he advises avoiding composite perspective and holding to the simple kind, which shows surfaces in their proper form rather than steeply foreshortened. A marginal note, unrelated to the perspective column, reminds him to write a separate treatise on how the joints bend and how flesh grows over them in bending and extension, within a study of the movements of four-footed animals, among which he counts man, who in infancy also goes on four feet.
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Avoid composite perspective, hold to the simple
Composite perspective forces the viewer to keep one eye at a single peephole; only then does it read correctly. Since many eyes view one work together and only one sees its effect while the others are left confused, such perspective is to be shunned in favour of the simple kind, which shows walls in their proper form and cuts the visual pyramids equally distant from the visual power.
The bending of the joints and the flesh over them
A marginal note plans a particular treatise on how the joints bend and in what manner the flesh grows over them in their bendings and extensions. This is to sit within a description of the movements of four-footed animals, among which he numbers man, who in infancy likewise goes on four feet.