The eye accustomed to darkness that suddenly sees the light receives injury, whence it at once closes again, unable to bear that light. And this happens because, when the pupil, in the accustomed darkness, wishes to make out something, it grows in size, exerting all its power to send to the imprensiva [the receiving faculty] the likeness of the shadowy things. And the sudden light reaching within it, too great a quantity of the already darkened pupil is harmed by the supervening splendor, directly contrary to the darkness already grown habitual and settled in the eye, which seek to keep themselves in their being and do not depart from their place without injury to the eye.

One could also say that the pain the darkened eye receives from the sudden light comes about through the sudden closing of the pupil, which is not without sudden contact and rubbing of the sensitive parts of the eye. And if you wish to see proof of this, look well and consider the size of the pupil of one who gazes in a dark place, and then have a candle brought before him, moving it quickly nearer to the eye, and you will see the sudden diminution of its pupil.

Of reflected motions.

I desire to define why corporeal and spiritual motions, after the percussion they make upon the object, rebound backward between equal angles.

Of corporeal motions.

The voice of an echo, I say, is reflected by the percussion to the ear, as to the eye are the percussions made in mirrors by the species [images] of objects. And just as the likenesses fall from the thing to the mirror and from the mirror to the eye between equal angles, so between equal angles will the voice fall and rebound in the concavity of the first percussion to the ear.