Reflected Light Between Two Mirror Surfaces
How the strength of reflected light falls off in proportion across surfaces
Beneath a small sketch of an object on a stand, headed "Mirror," Leonardo sets out a rule for light reflected between two facing surfaces. He claims that when the illuminated surface equals in size both the illuminating body and the surface that receives the reflection, the quality of the reflected light stands to the intermediate light in the same proportion as that second light stands to the first, provided the bodies are flat and white.
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Proportional diminution of light through successive reflections
If the illuminated surface matches in size both the luminous body and the surface on which the light is reflected, then the reflected light bears the same ratio to the intermediate light as that second light bears to the first. Leonardo restricts the rule to flat, white bodies, treating reflected brightness as diminishing in a fixed proportion from stage to stage.