The Glass-Pane Rule and the Law of Aerial Perspective
Matching drawn to real trees, and making distant buildings paler and bluer
Completing the glass-pane exercise, Leonardo says to color each drawn tree until it matches the real one seen with one eye, keeping the glass at a fixed distance, and repeats it for the second and third trees; he adds the measured rule that the second tree diminishes four-fifths from the first at twenty braccia. He then defines 'aerial perspective': in somewhat thick air the different distances of buildings ranged along a single wall-line are read by keeping the nearest building its own color and making the farther ones progressively less outlined and more blue, like mountains seen through much air. A small pen sketch of buildings standing on a wall is set into the text to illustrate the rule. Headed SECVNDA, foliated 78.
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Matching drawn trees and the four-fifths rule
Color your drawn tree until, with one eye closed, it matches the real one in color and form, keeping the glass at one fixed distance, and do the same for the second and third trees. Leonardo adds a measured rule: the second tree diminishes four-fifths from the first when it is twenty braccia distant from it.
The definition of aerial perspective
He names a second perspective, the 'aerial': by the variety of the air one can tell the distances of buildings ranged on a single line above a wall, all appearing the same size. In somewhat thick air the farthest things, like mountains, look blue; so make the nearest building its own color and each farther one less outlined and more blue.
Sketch of buildings on a wall
A small pen diagram set into the text shows several roofed buildings with towers standing in a row on top of a horizontal wall-line, illustrating how equal-seeming structures on one baseline are distinguished by graded blueness.