How Greater Light Lights a Greater Part of the Sphere
Leonardo rebuts the painter who scorns theory, then opens the study of reflected light
By the converse of the preceding proposition, Leonardo argues that the greater the sum of sky-light, the larger the illuminated shape on the sphere: if a small light b c, d e lights only a small part of sphere a h, the maximal light of the hemisphere lights the maximal part, and a portion b c d f lighting n m r also reaches k n on one flank and l r on the opposite side. He then answers an imagined adversary who says he needs no such science, only the practice of copying nature: nothing deceives us more than trusting our own judgment without reasoning, as experience proves, that enemy of alchemists, necromancers and other simple minds. A semicircular sky-and-sphere diagram accompanies the proof, and chapter [751] opens the theme of the proportion between the luminous parts of bodies and their reflections.
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More light, a larger lit shape on the sphere
By the converse of the antecedent, the greater the sum of the luminous, the larger the illuminated figure on the sphere. If b c d f lights only the part n m r, the rest of the hemisphere joined to it lights the remainder of the sphere; and b c d f, besides n m r, also lights k n on one flank and l r on the opposite side.
Reply to the adversary who wants only practice
An imagined adversary says he wants no such science, that the practice of copying natural things suffices. Leonardo answers that nothing deceives us more than trusting our own judgment without reasoning, as experience always proves, that enemy of the alchemists, necromancers and other simple minds.
Opening the study of reflected light
Chapter [751] begins on the proportion the luminous parts of bodies have with their reflections. It states that the part lit by direct light will bear a proportion to the part lit by reflected light.