THE PROOF AND REASON WHY AMONG THE ILLUMINATED PARTS CERTAIN PORTIONS ARE IN HIGHER LIGHT THAN OTHERS.
Since it is proved that every definite light is, or seems to be, derived from one single point the side illuminated by it will have its highest light on the portion where the line of radiance falls perpendicularly; as is shown above in the lines a g, and also in a h and in l a; and that portion of the illuminated side will be least luminous, where the line of incidence strikes it between two more dissimilar angles, as is seen at b c d. And by this means you may also know which parts are deprived of light as is seen at m k.
Where the angles made by the lines of incidence are most equal there will be the highest light, and where they are most unequal it will be darkest.
I will make further mention of the reason of reflections.
[Footnote: See Pl. XXXII. The text, here given complete, is on the right hand side. The small circles above the beginning of lines 5 and 11 as well as the circle above the text on Pl. XXXI, are in a paler ink and evidently added by a later hand in order to distinguish the text as belonging to the Libro di Pittura (see Prolegomena. No. 12, p. 3). The text on the left hand side of this page is given as Nos. 577 and 137.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.