On the eyes of animals.

Make anatomy of various eyes and see which are the muscles that open and close the said pupils of the eyes of animals.

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The eyes of all animals have their pupils, which of themselves grow and diminish according to the greater and lesser light of the sun or other brightness. But in birds it makes a greater difference, and most of all in the nocturnal ones, such as owls, barn owls and tawny owls, which are of the species of the little owl. In these the pupil grows so that it almost occupies the whole eye, and diminishes down to the size of a grain of millet, and it always keeps a circular figure. But the leonine species, such as panthers, leopards, lynxes, tigers, wolf-lynxes, Spanish cats and other like ones, diminish the light from the perfect circle to the biangular figure, that is this one, and as is shown in the margin. But man, having weaker sight than any other animal, is less harmed by excessive light and his [pupil] increases less in dark places. But in the eyes of the said nocturnal animals, the owl, that horned bird which is the greatest of the species of nocturnal birds, the visual power increases so much that in the least nocturnal light (which we call darkness) there is far more vigour than in us amid the splendour of midday, in which such birds stay hidden in dark places; and if they are indeed compelled to go out into the air lit by the sun, they diminish their pupil so much that the visual power diminishes together with the quantity of that light.