perceived through an extensive intervening medium. But our judgment
is largely guided by the transparency of this medium, since the
medium itself is not perceived with much accuracy, except when it
is transparent. Accordingly, as the lucidity of air is greater than
that of water, an object looks more distant through air than through
water."

"Why does not a single object appear double, inasmuch as we have two
eyes?" To this he replies: "From the anterior part of the brain two
optic nerves pass to the two eyes. But these two nerves unite at a
certain point into one. Now, since the two nerves are of equal length,
two images proceeding from a single object do not make the object
seem double, but single, since the two images are united into one, and
accordingly one object is seen as one image."

Other physiological speculations are introduced by the questions: "May
one see an object not actually present?" "Why do some animals see best
objects at a distance, others those near at hand?" "Why are objects
seen in their proper position?" All these questions are answered in
accordance with the scholastic formulae, and, not infrequently, with
considerable acuteness.

A chapter entitled "_De signis oculorum_" also introduces us to a
curious discussion of ocular physiognomy. Thus:

"When we see a man with large eyes, we argue that he is indolent."

"If his eyes are deeply situated in his head, we say that he is crafty
and a deceiver."