The Sun's Direct Rays and the Brightest Part of a Lit Body
Book Five of the Treatise on Painting: rays striking at equal angles light a body most
This page from Book Five of Leonardo's Treatise on Painting first finishes a note that minute lights and darks sent together to the eye merge into a confused, fog-like mixture, as happens in veils and cobwebs. It then opens a chapter on the particular light of the sun, arguing that the part of a body struck by rays at more equal angles shows the most intense brightness, while the part met by more oblique, dissimilar angles is darkest. A diagram of a lit sphere over a ground plane marks the angle n as brightest (struck at the equal angles a n o and b n r) and the point c as least lit. The page also bears the Book Five running head and a folio number.
On this page
Rays striking at equal angles give the brightest light
The part of an illuminated body struck by the luminous ray at more equal (similar) angles shows the most intense brightness, while the least lit part is met by more dissimilar angles. The angle n, on the side facing the sun and struck at the equal angles a n o and b n r, is illuminated more powerfully than any other part; the point c, struck at the most oblique angles, is the least lit.
Minute lights and darks merge into a fog-like blur
When minute lights and darks are sent together to the eye they form a confused mixture in which nothing can be discerned but confusion, after the manner of fog. The same happens when one looks through veils, cobwebs and similar thin screens.