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VIII.

RADIA TIOIV.

1. Visible and Invisible Radiation.

BETWEEN the mind of man and the outer world are in-
terposed the nerves of the human body, which translate,
or enable the mind to translate, the impressions of that
world into facts of consciousness and thought.

Different nerves are suited to the perception of different
impressions. We do not see with the ear, nor hear with
the eye, nor are we rendered sensible of sound by the
nerves of the tongue. Out of the general assemblage of
physical actions, each nerve, or group of nerves, selects and
responds to those for the perception of which it is specially
organized.

The optic nerve passes from the brain to the back of
the eyeball and there spreads out, to form the retina, a web
of nerve filaments, on which the images of external objects
are projected by the optical portion of the eye. This nerve
is limited to the apprehension of the phenomena of radia-
tion, and, notwithstanding its marvellous sensibility to
certain impressions of this class, it is singularly obtuse to
other impressions.

Nor does the Optic nerve embrace the entire range even
of radiation. Some rays, when they reach it, are incom-
petent; to evoke its power, while others never reach it at
all, being. absorbed by the humors of the eye. To all rays

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