on; one sees with the brain,
since the optic nerve endings in the back of the eye merely carry
light impressions to the brain where they are properly interpreted.

In order that vision be clear and perfect, it is essential that the
rays of light entering the eye be bent so that they strike the retina
as a single point. In the farsighted or hyperopic eye, the eyeball is
usually too short for the rays to be properly focused on the sensitive
nerve area in the back of the eye.

This defect in vision is, however, overcome by the act of
"accommodation." There is a beautiful transparent, double-convex body,
about one-third of an inch thick, which looks very much like an
ordinary glass lens, and is situated in the eye just back of the
pupil. This is what is known as the crystalline lens, and the rays of
light are bent in passing through it so as to be properly focused on
the retina.

The foregoing statements have been made as though objects were always
at a distance from the eye, so that the rays of light coming from them
were almost parallel. Yet when one is looking at an object within a
few inches of the eye the rays diverge or spread out, and these the
normal eye (if rigid) could not focus on the retina--much less the
farsighted eye. But the eye is adaptable to change of focus through
the action of a certain muscle, situated within the eyeball about the
lens, which controls to a considerable extent the shape of the lens.
When the muscle contracts it allows the lens to bulge forward by
virtue of its elasticity, and, therefore, become more convex. This is
what happens when one looks at near objects, the increased convexity
of the lens bending the rays of light so that they will focus as a
point on the retina. (See Plate I, p. 30.)

Now in the farsighted eye this muscular control or "accommodative
action" must be continually exercised even in looking at distant
objects, and it is this constant attempt of nature to cure an optical
defect of the eye which frequently leads to nervous exhaustion or
eye-strain. The nerve centers, which animate and control the nerves
supplying the eye muscles to which we have just alluded, are in close
proximity to other most important nerve centers in the brain, so
irritation of the eye centers will produce sympathetic irritation of
these other centers, leading to manifold and complex symptoms which we
will describe under this head. But these symptoms do not necessarily
develop in everyone havi