The Sun's Image Reflected in Rippling Water
Why the sun glitters brighter on small waves, with a spiral vortex in a circular pool
This transmitted-light plate gathers Leonardo's optics of sunlight on water. He argues that waves enlarge the reflected image of the sun, spreading it from the water c d over the wider stretch c e as the eye recedes, citing the fourth proposition of his Perspective. He explains that small, closely-set waves make the sun's image brighter than large waves, and that waves broken like a pine-cone's rind give the greatest splendour because their shadows become imperceptible. A separate opening note treats water entering a circular pool at right angles to its diameter, which spins into a spiral (snail-like) vortex diminishing toward the centre.
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Spiral vortex in a circular pool
Water entering a circular pool, striking it at a right angle to the end of its diameter, necessarily generates a revolving motion that goes diminishing toward the centre in the manner of a snail, that is, a spiral.
Waves enlarge the reflected image of the sun
The waves enlarge the mirrored image of a thing. With a the sun and f the eye, the sun that occupied only the water c d on a still surface now occupies the whole stretch c e when the water is ruffled, and occupies more the farther the image lies from the eye.
Small and pine-cone waves give the brightest glitter
The sun's image is brighter on small waves than on large ones because its likenesses are more frequent there, and denser splendours give more light. Waves broken like a pine-cone's rind shine most, since the shadows between their humps are small and become imperceptible.
The image spreads wider as the eye recedes
With a the sun, p q its image, a b the water surface, r a near eye and c a remote eye: the image occupies the space o m to the near eye but a greater stretch by the amount n o to the more distant one.