Why the Eye Reads a Riverbed Better in Shadow
The pupil is dazzled by lit water but sharpened by shadow; still water shows its bed
A leaf of the Trattato della pittura, header PARTE, continuing the demonstration of gravel seen through water. It reasons that gravel beneath a dark mirrored image is more visible than gravel beneath a bright one (the image of the air, lettered b c s m, marking the bright zone), because the pupil is dazzled by the lit water yet sharpened by the dark, so the effect lies in the eye, not the water. A closing chapter states that clear, slow-moving water without waves shows the true shapes of the gravel and sand on its bed. Text only, in a clear scribal hand.
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Gravel is clearer under a dark reflection than a bright one
Where the dark image is mirrored, the gravel is more visible than where the bright image of the air (lettered b c s m) lies, because the visual power is offended by the lit water and strengthened by the dark. The pupil is thus of unequal power, dazzled on one side and sharpened on the other.
The effect lies in the eye, not the water
Leonardo concludes that this difference arises from no cause in the waters or their images, but only from the eye, which is offended by the splendor of the air's image and strengthened on the other side by the dark image.
Still water shows the true figures of its bed (chapter 507)
Water whose bottom shows through its transparency reveals that bottom more distinctly the slower it moves, for slow water has a surface without waves; through that flat surface the true figures of the gravel and sand on the bed are seen.