CXCV. Of sight.
Finally, the extremities of the optic nerves, composing the coat in
the eyes called the retina, are not moved by the air nor by any
terrestrial object, but only by the globules of the second element,
whence we have the sense of light and colours: as I have already at
sufficient length explained in the Dioptrics and treatise of
Meteors. [Footnote: In the French this section begins, "Finally,
sight is the most subtle of all the senses," etc.]
CXCVI. That the soul perceives only in so far as it is in the
brain.
It is clearly established, however, that the soul does not perceive
in so far as it is in each member of the body, but only in so far as
it is in the brain, where the nerves by their movements convey to it
the diverse actions of the external objects that touch the parts of
the body in which they are inserted. For, in the first place, there
are various maladies, which, though they affect the brain alone, yet
bring disorder upon, or deprive us altogether of the use of, our
senses, just as sleep, which affects the brain only, and yet takes
from us daily during a great part of our time the faculty of
perception, which afterwards in our waking state is restored to us.
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