The seas make the world shine like the moon; light and the eye
Earthshine and the moon's cold moisture; why a distant light keeps its size — paper windows and mirrors
Leonardo announces that his book will show how the Ocean and other seas, lit by the sun, make the earth shine like the moon and appear as a star to distant observers, pairing the moon's cold moisture with that of water. He then turns to the optics of light and the eye: distant lights throw out rays that seem to enlarge them, yet a luminous object does not diminish in size with distance, only in brightness. The main column explains this through translucent paper windows and sunlight on mirrors, whose image reflects outward because lead behind the glass blocks it from passing through, just as the eye receives the light within. A small burst of rays is drawn at the upper right, labelled a b and c.
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The seas make the earth shine like a moon
Leonardo states that his book will show how the Ocean and the other seas, lit by the sun, make our world shine after the manner of the moon, appearing as a star to the most distant observers, and promises to prove it.
The moon cold and moist, like the sea
A marginal note pairs the moon, called cold and moist, with water, also cold and moist, concluding that our sea gives the moon the same influence the moon gives to us.
Distant lights throw out enlarging rays
Every light far from the eye throws out rays that seem to enlarge the figure of the luminous body, from which a further consequence is begun but broken off. A small star-like burst of rays is drawn at the upper right, labelled a b and c.
Why a luminous object keeps its size: mirrors and paper windows
The eye does not diminish a light at any distance because the light's likeness, imprinted on the eye's surface, illuminates within, as translucent paper windows spread the light they receive. Likewise sunlight on a mirror reflects outward as real light; were the lead removed, the sun's image would pass through and illuminate behind the glass, as it does within the eye.