ons from objects had
fallen directly upon it. It is obvious that it would have been exposed
to injury from every floating particle of dust, and you would always
have felt such a sensation as is caused by a burn or scald when the skin
peels off, and leaves the ends of the nerves exposed to the air. The
tender points of the fibers of the optic nerve, too, would soon become
blunted and broken, and the eye, of course, useless. How, then, is the
nerve to be protected, and yet the sight not obstructed? If it were
covered with skin, as the other nerves are, you could not see through
it. For thousands of years after men had eyes and used them, they knew
no substance, at once hard and transparent, which could answer the
double purpose of protection and vision. And to this day they know none
hard enough for protection, clear enough for vision, and elastic enough
to resume its form after a blow. But men did the best they could, and
put a round piece of brittle but transparent glass in a ring of tougher
metal for the protection of the hands of a watch; and he who first
invented the watch crystal thought he had made a discovery. Now, observe
in the eye, that forward part is the watch glass; the cornea, made of a
substance at once hard, transparent and elastic--which man has never
been able to imitate--set into the sclerotica, that white, muscular coat
which constitutes the white of your eye, acts as a frame for the cornea,
and answers another important purpose, as we shall presently see.

[Illustration: Structure of the Human Eye]

But, supposing the end of the nerve protected by the glass, we might
have had it brought up to the glass without any interposing lenses or
humors, as, in fact, is nearly the case with some crustacea. We can not
well imagine all the inconveniences of such an eye to us. If we could
see distinctly at all, we could not see much farther or wider than the
breadth of the end of the nerve at once. Our sight would then be very
like that faculty of perceiving colors by the points of the fingers,
which some persons are said to possess. In that case, seeing would only
be a nicer kind of groping, and our eyes would be more conveniently
fixed on the points of our fingers; or, as with many insects, on the
ends of long antennae. Such a form of eye is precisely suited to the
wants of an animal which has not an idea beyond its food, which has no
business with any object too large for its mouth, and whose great
concern is