...the brightness given to it by the splendour of the air; by which increase of shadows and lights the face has great relief, and in the illuminated part the shadows are almost imperceptible, and in the shadowed part the lights are almost imperceptible; and from this manner of representing and increasing shadows and lights the face gains much beauty.

[94] Manner of portraying, and of simple and composite shadow. B.
Do not portray a figure indoors with a particular light, feigning it to be the universal light of the open country without sun, because the open country makes simple shadow, while the particular light of a window or of the sun makes composite shadow, that is, mixed with reflections.

[95] On the light in which one portrays the flesh tones of faces, or nudes. C. (Book A, folio 38.)
This dwelling should be open to the air, with walls of flesh colour, and the portraits should be made in summer, when clouds cover the sun; or else you shall make the southern wall so high that the sun's rays do not strike the northern wall, so that their reflected rays do not spoil the shadows.

[96] On the manner of portraying figures for narrative scenes (istorie). D.
The painter must always consider, on the wall he has to fill with a story, the height of the place where he wishes to set his figures; and whatever he portrays from nature for that purpose, he should hold his eye as much lower than the thing he portrays as that thing will be set high in the work...