ON PAINTING.
The comparison of the various qualities of shadows and lights not infrequently seems ambiguous and confused to the painter who desires to imitate and copy the objects he sees. The reason is this: If you see a white drapery side by side with a black one, that part of the white drapery which lies against the black one will certainly look much whiter than the part which lies against something whiter than itself. [Footnote: It is evident from this that so early as in 1492 Leonardo’s writing in perspective was so far advanced that he could quote his own statements.—As bearing on this subject compare what is said in No. 280.] And the reason of this is shown in my [book on] perspective.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.