On the color of the eyes.
Further one inquires into the color of the eyes, namely, whether blackness arises from an abundance of moisture and grayness arises from a scarcity of it.
1. And it seems not. For in phlegmatic individuals sometimes the eyes appear to be gray, and in melancholic ones they appear black, although in phlegmatic ones nevertheless moisture abounds and in melancholic ones it is lacking.
2. In addition, the visual power consists in the glacial humor. If, then, blackness were caused by an abundance of moisture and grayness by a scarcity of it, then black eyes would see better at night than during the day.
The Philosopher says the opposite.
One must reply that diverse eye colors arise from a diversity of humors. When there is an abundance of moisture in the eye, a black color appears; and when there is a little, a gray color tending to white appears. And when the moisture varies in different parts [of the eye], a varied color appears. This is also evident in things. For where water is deep, it appears black; and where it lacks depth, there the water appears white. And it is the same for the eye, that the eye colors vary according to the diversity of the moisture.
1. On to the arguments. To the first, one must reply that although moisture abounds in phlegmatic people, it nevertheless does not do so in every part. For then it would also be abundant in bone. Therefore, although a phlegmatic complexion is moist and a melancholic complexion is dry, nevertheless nothing prevents a melancholic person's eye from having more glacial humor than does a phlegmatic's eye. And this is why, etc.
2. To the second argument one must reply that a small motion is not perceived and ceases in the presence of a larger one. This is evident in many cases, because stars do not appear in the day the same way they do at night because their modest light is concealed and softened in the presence of the greater light of the sun. Now, however, there is less moisture in gray eyes than in black eyes and more visual light, and this is why that light is hidden in the presence of a greater external light. Therefore, gray eyes see better at night and worse during the day, like a cat's. But black eyes, owing to the abundance of moisture, receive the [visible] species only when accompanied by great motion, and this is why they see only in the presence of a great external light. Therefore, they see more poorly at night and better during the day.
Nevertheless, one must understand that good or sharp vision consists in two things, namely, in the ability to distinguish between two visible objects and in the perception of something visible at a distance. The first is caused by the humor's purity and by the skin's softness surrounding the eye. For just as on a clean cloth or piece of parchment a small-sized spot is apparent, but is not apparent on a dirty cloth, so too a faint image and a distinguishing feature of that image are perceived immediately in a clean eye. And the lightness and softness of the skin contribute to this, because a wrinkle in the eye's tunic causes a sort of shadow on its inside. The second is caused by the eyes' location and by the disposition of the skin that closes the eyes. For those [animals] that have eyelids see from a distance better than those who do not, because those who lack eyelids have hard eyes, and the ones that have eyelids have soft eyes, and this is why they have eyelids, so that the eyes will not be injured by things that are outside it. Likewise, deep-set eyes see better from a distance than prominent eyes, because when an eye stands out it is surrounded on all sides by an external light and its ray (that is, its sight) is better dispersed. But sight is more concentrated in deep-set eyes, and a unified power is stronger than one that is dispersed. And according to this argument, if someone were in a deep well during the day, so that he did not see the brightness of the sun, he would see the stars in the heaven, which he would not see while on the earth's surface because his sight would not be diffused by an external light but would rather be better concentrated.
-  It must be remembered that the Latin for "black," niger, also carries a sense of "dark." Likewise, "white," albus, can often merely imply "light."