Colour perspective in shadow; lowering a rectangle to a set height
An aerial-perspective note on receding colour above a compass-and-arc area construction
The upper text opens a section on the perspective of colours in dark places, arguing that in a field graded uniformly from light into shadow the colour nearest the eye reads as lightest and grows darker with distance. The lower half is a geometrical problem, illustrated by a quarter-arc and a drawn rectangular box: given the rectangle a b c d, Leonardo swings an arc from c to bring corner a down to the height e f and so recover the figure's new width. The construction relies on the equal radii of a single circle. A parallelepiped block sketched below appears to show the same reduction in three dimensions.
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Lowering the rectangle a b c d to the level e f with compass and arc
Given the rectangular surface a b c d, Leonardo wishes to lower it into the site e f and asks for its new width. He sets the compass at c, opens it to a, and swings the arc a r down to the given height e f, giving the line e r. Drawing the straight line c r yields a segment equal to a c by the definition of the circle, from which a perpendicular is raised to the base c d.
On the perspective of colours in dark places
In places lit uniformly yet grading away into darkness, the colour nearest the eye will be the lightest, and it will grow darker the more remote it is from that eye. The heading frames this as the 'perspective of colours in dark places', an aerial-perspective rule for how tone recedes.