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Further complications in the derived shadows (183-187).The greatest depth of shadow is in the simple derived shadow because it is not lighted by either of the two lights a b, c d.
The next less deep shadow is the derived shadow e f n; and in this the shadow is less by half, because it is illuminated by a single light, that is c d.
This is uniform in natural tone because it is lighted throughout by one only of the two luminous bodies . But it varies with the conditions of shadow, inasmuch as the farther it is away from the light the less it is illuminated by it .
The third degree of depth is the middle shadow 86 . But this is not uniform in natural tone; because the nearer it gets to the simple derived shadow the deeper it is 87 , and it is the uniformly gradual diminution by increase of distance which is what modifies it 88 : that is to say the depth of a shadow increases in proportion to the distance from the two lights.
The fourth is the shadow k r s and this is all the darker in natural tone in proportion as it is nearer to k s, because it gets less of the light a o, but by the accident [of distance] it is rendered less deep, because it is nearer to the light c d, and thus is always exposed to both lights.
The fifth is less deep in shadow than either of the others because it is always entirely exposed to one of the lights and to the whole or part of the other; and it is less deep in proportion as it is nearer to the two lights, and in proportion as it is turned towards the outer side x t; because it is more exposed to the second light a b.
102:86 15: We gather from what follows that q g r here means ombra media (the middle shadow).
102:87 18: Compare lines 10-13
102:88 20: See Footnote 18
103:89 : The diagram to this section is given on Pl. V. To the left is the facsimile of the beginning of the text belonging to it.