ty of the lens
produced by the action of this muscle, and it is conceivable that the
axis of the lens may be tilted one way or another by the same agency,
and for the same purpose. But, as we have already pointed out, this
continual muscular action entails great strain on the nerve centers
which animate the muscle, and if constant near work is requisite, or
the health is impaired, the nervous exhaustion becomes apparent. The
lesser degrees of astigmatism often give more trouble than the
greater.

[Illustration: PLATE I

=Plate I=

=ANATOMY OF THE EYE=

The upper illustration shows the six muscles attached to the eye. The
=Superior Rectus Muscle= pulls and directs the eye upward; the
=Inferior Rectus=, downward; the =External= and =Internal Rectus
Muscles= pull the eye to the right and left; the =Oblique Muscles=
move the eye slantwise in any direction.

Lack of balance of these muscles, and especially inability to focus
both eyes on a near object without effort, constitute "eye-strain."

The lower cut illustrates the relation of the crystalline lens to
sight. =Lens Nearsight Focus= shows the lens bulging forward and very
convex; =Lens Farsight Focus= shows it flat and less convex.

This adjustment of the shape of the crystalline lens is called
"accommodation"; it is effected by a small muscle in the eyeball.

In the normal eye, the rays of light from an object pass through the
lens, adjusted for the proper distance, and focus on the retina.

In the nearsighted eye, these rays focus at a point in front of the
retina; while in the farsighted eye these rays focus behind the
retina; the nearsighted eye being elongated, and the farsighted eye
being shortened.]

=WEAKNESS OF THE EYE MUSCLES.=--There are six muscles attached to the
outside of the eyeball which pull it in various directions, and so
enable each eye to be directed upon a common point, otherwise objects
will appear double. Weakness of these muscles or insufficiency,
especially of those required to direct the eyes inward for near work,
may lead to symptoms of eye-strain. When reading, for example, the
muscles which pull the eye inward soon grow tired and relax, allowing
the opposing muscles to pull the eye outward so that the eyes are no
longer directed toward a common point, and two images may be perceived
or, more frequently, they become fused together producing a general
blurring on the page. Then by a new effort of will the internal
muscles pu