Of the eye of man.

On the great variation that nocturnal animals make from their larger pupil of the eye at night to the smaller pupil by day.

Proportion of the ventricle of the receptive faculty (imprensiva) placed in the brain of animals to their pupil.

It continues concerning the eye of nocturnal animals, which see more by night than by day. And this in great part happens, as was said before, because much greater is the difference in the growing and diminishing of their pupil than there is in diurnal animals. For if the pupil of man doubles its diameter at night, which means four times that of the day, the diameter of the pupil of the eagle-owl, or owl, grows ten times that of the day, which in sum means a hundred times the pupil of the day. Besides this, the ventricle placed in the brain of man, called the imprensiva, is more than ten times the whole eye of man, of which the pupil, whence seeing is caused, is less than the thousandth part of that eye; and in the owl the nocturnal pupil is much larger than the ventricle of the imprensiva placed in its brain. Whence man has a greater proportion of his imprensiva to his light (pupil), the imprensiva being ten thousand times more than the light, than had that of the eagle-owl, which is nearly equal. And this imprensiva of man, compared to that of the owl, is like a great hall that has light through a small hole compared to a small hall standing wholly open; for in the great hall there is night at midday, and in the small open one there is day at midnight, the weather not being cloudy. And with this, more powerful causes will be shown by means of the anatomy of the eyes and imprensiva of these two animals, that is, of man and of the eagle-owl.

How the thing seen by a larger pupil appears larger.

That thing will appear greater in light and in size which is seen by a larger pupil. This can be tested in our own eyes: for if you make a hole as small as can be in a paper and bring it as close to the eye as possible, and through it look at a star, here only a small part of the pupil can work, which sees that star with much space of sky around it, and sees it so small that almost nothing could be smaller. And if you make such a hole near the edge of the said paper, you will be able at the same time to see with the other eye the same star, and it will appear large to you, and so at that time you will see with two eyes a star twice, of which one is very small and the other large. You will also be able to see the whole body of the sun, and with little splendour, because as the magnitude diminishes, so proportionally the splendour diminishes, as was proposed above. And hence it arises that great pupils see little by day, because the excessive light impedes their sight.