On the eye. The eye is an instrument of spherical surface, which stands midway between the object and the common sense; and this surface takes into itself all the likenesses of the things set opposite it; and those that are found in the middle of the others are grasped by the sense with least error, while those farther off and less understood are less so. Double figure of the eye and radiant lines, with: a–b n f–c; a b c. Clearly it is understood that the eye does not perceive objects unless their species come to it by a straight line, and a thing is the less understood the more it departs from that line. Therefore if, in the depth of the eye, f is the first degree of judgment and the surface n the last, then whatever is found in line with f–n will be better judged by the sense, as b–n; and so n–c and n–a are poorly understood, because the straightness of the lines of their species ends at n, which is an obtuse angle and the last degree of the judgment of the perceptive faculty f–n.

a–b n f–c; a b c. Clearly it is understood that the eye does not perceive objects unless their species come to it by a straight line, and a thing is the less understood the more it departs from that line. Therefore if, in the depth of the eye, f is the first degree of judgment and the surface n the last, then whatever is found in line with f–n will be better judged by the sense, as b–n; and so n–c and so n–a are poorly understood, because the straightness of the lines of their species ends at n, which is an obtuse angle and the last degree of the judgment of the perceptive faculty f–n.

a–c b–d. If the eye has to see a thing that is too close, it cannot judge it well, as happens to one who wishes to see the tip of his nose. Whence as a general rule nature teaches that a thing will never be seen perfectly unless the interval between the eye and the thing seen is at least as great as the size of the face. On the air. The air is all full of the species caused by the objects that surround it. Take the case of the stars that appear at night: there will be no part of this our hemisphere, where the radiant lines caused by them are ended, that does not gather into itself all the objects that cause those radiant lines; and thus they are all in all the air and all appear in the part, and each star is entire in the part and entire in all the air. I say the objects fill the air with their likenesses. This second reason is proved in the sun, which sends out of itself species of two kinds: the first luminous, the other of heat. That luminous one, all the... That the air draws to itself, like a magnet, all the likenesses of the things that surround it, and not only the forms of bodies but also their natures, is clearly seen in the sun, which is a hot and luminous body. All the air that is opposite to it becomes wholly incorporated with light and heat and receives into itself the form of the cause of the heat and splendour, and does the like in its every least part. The lodestar shows, through the magnet, that it does this same thing; and the moon and other planets do the like without diminution of themselves. Among earthly things the same is done by musk and other odours.