Reflected Light Weakens in Proportion to Its Source
The darkest shadow sits between incident and reflected light; harsh cast shadows and matching light to a scene
Leonardo states that reflected light bears the same proportion to the incident light as the incident light bears to it: if the incident light a b has two degrees of power and the object e f has one (subduple to two), then the reflected light c m d falling on the sphere is subduple to the direct light c n d. He adds that in spherical or columnar bodies the darkest part of the shadow lies interposed between the incident and the reflected light. Two further chapters follow: [753] that shadows cast by particular lights such as the sun should be avoided, being graceless and as strong at their end as at their beginning; and [754] that lights must be matched to the site of the thing lit, since a single scene mixes parts in the open universal light of the air with others [elsewhere]. A small diagram of the sphere between object and light illustrates the reflection.
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Reflected light halved when its source is halved
Incident light a b lights the sphere c d in c n d and passes to the object e f, whence it reflects into c m d. If a b has two degrees of power and e f has one, subduple to two, then the reflected light c m d is subduple to the direct light c n d.
Darkest shadow between incident and reflected light
Chapter [752] treats the darkest part of the shadow in spherical or columnar bodies. That part of the shadow is interposed between the body's incident light and its reflected light.
Avoid the harsh shadows of particular lights
Chapter [753] warns that shadows made by the sun or other particular lights should be avoided, being without grace: they leave the parts confusedly divided with a hard boundary between shadow and light, and are as strong at their end as at their beginning.
Matching lights to the site of each thing
Chapter [754] insists on giving the due lights to illuminated things according to their sites, since in one and the same scene some parts stand in the open under the universal light of the air while others are placed elsewhere.