The Adapting Pupil, Angle of Incidence, and Water on Mountains
The eye in light and dark; why springs rise on mountain summits
Leonardo notes how the pupil contracts before luminous things so that dimmer objects then look dark, while an eye kept in darkness finds middling light dazzling, to its own detriment. A central geometric diagram (labelled a r v, o b n, m f) studies the angle of incidence n o f, showing that even between the equal angles v o r and f o m the central ray does not fall symmetrically, since angle n o b is acute and n o a obtuse. A separate note asks why water is upon the mountains: reckoning a braccio of fall per mile over the 3,500 miles from Gibraltar to the Don, and observing that no European mountain rises a mile above sea level, he suggests summit water descends from higher seas and rivers.
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The pupil's adaptation to bright and dark
The eye narrows its pupil so much before luminous things that objects of lesser light then appear dark; an eye kept in a dark place finds middling brightness dazzling. This happens because the power of sight grows in darkness, so it then sees moderate light to its own detriment.
The angle of incidence and the central ray
Although the angle of incidence n o f is made between the equal angles v o r and f o m, the central line does not fall between equal angles, because angle n o b is not equal to angle n o a: the first is acute, the second obtuse.
Why water is upon the mountains
From the Strait of Gibraltar to the Don is 3,500 miles, a drop of about a mile and a sixth at one braccio of fall per mile of moderately moving water; the Caspian is much higher, and no European mountain rises a mile above the sea. One might therefore say the water on mountain summits comes from the greater height of those seas, or of the rivers that pour into them.