, the muscles
act in such a way that the lens bulges out, and becomes thick in the
middle and of the right curvature to focus the near object upon the
screen. When we look at an object several hundred feet away, the
muscles change their pull on the lens and flatten it until it is of
the proper curvature for the new distance. The adjustment of the
muscles is so quick and unconscious that we normally do not experience
any difficulty in changing our range of view. The ability of the eye
to adjust itself to varying distances is called accommodation. The
power of adjustment in general decreases with age.

117. Farsightedness and Nearsightedness. A farsighted person is one
who cannot see near objects so distinctly as far objects, and who in
many cases cannot see near objects at all. The eyeball of a farsighted
person is very short, and the retina is too close to the crystalline
lens. Near objects are brought to a focus behind the retina instead of
on it, and hence are not visible. Even though the muscles of
accommodation do their best to bulge and thicken the lens, the rays of
light are not bent sufficiently to focus sharply on the retina. In
consequence objects look blurred. Farsightedness can be remedied by
convex glasses, since they bend the light and bring it to a closer
focus. Convex glasses, by bending the rays and bringing them to a
nearer focus, overbalance a short eyeball with its tendency to focus
objects behind the retina.

[Illustration: FIG. 79.--The farsighted eye.]

[Illustration: FIG. 80.--The defect is remedied by convex glasses.]

A nearsighted person is one who cannot see objects unless they are
close to the eye. The eyeball of a nearsighted person is very wide,
and the retina is too far away from the crystalline lens. Far objects
are brought to a focus in front of the retina instead of on it, and
hence are not visible. Even though the muscles of accommodation do
their best to pull out and flatten the lens, the rays are not
separated sufficiently to focus as far back as the retina. In
consequence objects look blurred. Nearsightedness can be remedied by
wearing concave glasses, since they separate the light and move the
focus farther away. Concave glasses, by separating the rays and making
the focus more distant, overbalance a wide eyeball with its tendency
to focus objects in front of the retina.

[Illustration: FIG. 81.--The nearsighted eye. The defect is remedied
by concave glasses.]

118. Headach