The sun reflected in water and the book of mirrors
How the sun's mirrored image grows with the eye's height, from the water's edge up to a viewer standing on the moon
Two columns discuss the reflection of the sun in water as a case of the science of mirrors. Leonardo argues that the higher the eye rises above the water, the larger the stretch of water the sun's reflection appears to occupy: at r near the surface it is only the size t o, and it grows as the eye moves to f, then m. Pressing the thought experiment, he says that from the moon, or with an eye as large as the world, the reflected sun would fill all the illuminated waters of seas, lakes and rivers; and he notes that, because the eye is small, the sun seems to walk beside a traveller along a river. A diagram at right shows three eyes observing the sun's image in water.
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The reflected sun grows with the eye's height
As shown in the book of mirrors, a body appears within a plane mirror as far behind it as it stands in front. So the sun mirrored in water measures only t o to an eye at r near the surface, but occupies more water as the eye rises to f and then m, because the cut of the visual pyramid by the water widens. Walking along a river, the small eye sees the reflected sun seem to travel alongside the walker.
From the moon or an eye as large as the world
Pushing the reasoning, Leonardo imagines rising to the moon while by day it passes lightless through our hemisphere: the sun reflected in the water would then occupy all the waters it illuminates, and would make to one on the moon what our moon makes as night when it shines. An eye as large as the world would likewise see all seas, lakes and rivers luminous with the sun's likeness.
Three eyes and the sun's image in water
The right-hand figure sets three eyes against the sun's image in water, lettered sole (sun), m, f, c, r, acqua (water), b, o, t, n, a and the simulacro (image) of the sun. If the eye, or point, were as large as the sun and at a like distance from the sea, the reflection that now appears the size of a trencher would seem to take up all the sea that could be seen.