The Painter's Own Defects and the Error of Studio Light
Why artists mirror their own bodies in their figures, and misjudge shadow
Two chapters of Leonardo's Treatise on Painting. The first warns the painter that his soul, mistress of his body, inclines his judgement to reproduce in his figures the very proportional defects found in his own person, so he must measure himself and guard against them. The second, headed as chapter 110, attacks painters who model a thing in relief by a single indoor light and then set it outdoors under the universal light of the open air, producing dark shadows and reflections where none can exist.
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The soul reproduces its own body's defects in the figures a painter makes
The painter should measure himself and see where his person departs from praiseworthy proportion, then study to avoid those same defects in his figures. Leonardo says this vice is hard to fight because it is born with one's judgement: the soul, mistress of the body, delights in works resembling the body she composed. Hence no woman's figure is so ugly that she finds no lover, unless she be monstrous.
Chapter 110: a thing lit by one indoor light set under the open sky
It is a great error to portray something in relief by a particular light indoors and then display that portrait under the universal light of the air in the countryside, where the air embraces and illuminates every part alike. Such a painter makes dark shadows where no shadow can be, or where a shadow is so bright it is imperceptible. Likewise he places reflections where it is impossible for them to be.