And if you wish to see a better and more palpable demonstration of what has been said, put and remove the hand before the light of the candle set opposite. And keep the eye fixed on one of the larger stars, and such a star will appear to you at once to grow, and at once to diminish. The growing happens when you cover the light; the diminishing arises when the light is uncovered. And this happens because, in covering the light, the pupil widens, and in uncovering that light, the pupil diminishes. And thus, by what has been said, everything seen by that pupil diminishes its image together with the diminution of the aforesaid pupil.

All the pupils of the eyes grow together with the increase of the brightness of the day, and thus diminish as that brightness diminishes.

It is seen that nocturnal animals are endowed by nature with a very great pupil, which grows and diminishes according to the increase or decrease of the splendour set before it. And the day not being able to diminish enough to be sufficient for them to endure anything illuminated by the daytime splendour, they are compelled at such a time to dwell in dark places. And at night their pupil multiplies to so great a size that every great darkness appears to them a place strongly illuminated.

If the size of the pupil of the owl in the night-time be considered, in proportion to the pupil of man, the pupil of the owl will be found capable of more than 40 times that of man. And if, beyond this, the proportion of the one and the other pupil be considered in comparison of their perceptions, the perceptive power of man will be found to be more than 40 times that of the owl.

The small perceptive power and large pupil render equal objects four times larger than the double perceptive power and small pupil do. Among these perceptions and pupils, objects equal to them are rendered equal.

Unequal perceptive powers, in equal pupils, render equal objects greatly unequal.