The pupil widens in the dark; a wider pupil enlarges the image
Notebook D: pupil adjustment, the growth of the image, and refraction through spectacle lenses
Leonardo observes that a hollow looks darker from outside than from within because the eye in open air contracts its pupil, while in a dark place the pupil dilates and gains power, so faint darkness then reads as illuminated; excess light injures the eye, which shields itself as a man closes part of a window against glare. Two right-hand diagrams argue that a wider pupil makes objects look larger, because the object's rays reach the enlarged pupil along parallel lines rather than as a narrowing pyramid. A third demonstration holds that the pupil takes in images by way of the surface of the light and not the object itself, proved by how spectacle lenses magnify or shrink according to their curvature and by the equality of the incident angles at the flat pupil, which sits with the crystalline humour.
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The pupil adapts to light and dark
The eye in open air strongly contracts its pupil, while in a dark place the pupil grows and its power increases with it; a weak pupil finds small darkness gloomy, a stronger one finds great darkness illuminated. Excess light injures the eye, which shields itself as a man closes part of a window against the sun's glare.
A wider pupil enlarges the image
Let r be the pupil, receiving the object g by a pyramid whose width is measured at the line a b in the part o. Enlarge the pupil by the whole diameter m f: then g no longer makes a pyramid, but its species arrive by parallel lines and cut at the same line a b, so the image of g grows with the growth of the pupil.
Vision comes from the surface of the light, tested by lenses
The pupil receives images by way of the surface of the light, not the object. This is proved by spectacle lenses, which enlarge or shrink the passing images according to the greater or lesser curvature of their outer glass surfaces, and by the exterior angles r o being equal of necessity to the two interior angles t v.
The flat pupil and the crystalline humour
Because the pupil is in itself flat, together with the surrounding part occupied by the crystalline humour, the interior and exterior angles made by the penetrating rays must be right angles; otherwise the angles could not be generated in the pupil but only far from it.