convex will not aid this condition, but only
make it worse, so that it is not attempted.

Eye-strain in this optical defect is brought on by constant use of the
eye muscles (attached to the outside of the eyeball) in directing both
eyes inward so that they will both center on near objects; the only
ones which can be seen. Outward squint frequently results, because the
muscular efforts required to direct both eyes equally inward to see
near objects are so great that the use of both eyes together is given
up, and the poorer eye is not used and squints outward, while the
better eye is turned inward in the endeavor to see. Nearsighted
persons are apt to stoop, owing to the habitual necessity for coming
close to the object looked at. Their facial expression is also likely
to be rather vacant, since they do not distinctly see, and do not
respond to the facial movements of others.

Nearsightedness, or myopia, is not a congenital defect, but is usually
acquired owing to excessive near work which requires that the eye
muscles constantly direct both eyes inward to see near objects. In so
acting the muscles compress the sides of the eyeballs and tend to
increase their length, interfere with their nutrition, and aggravate
the condition when it is once begun. (See Diagram.) Concave lenses are
used to correct myopia, and they must be worn all the time.

=ASTIGMATISM.=--This is a condition caused by inequality of the outer
surface of the front of the eyeball, and rarely by a similar defect in
the surfaces of the lens. The curvature of the eyeball in the
astigmatic eye is greater in one meridian than in the opposite. In
other words, the front of the eyeball is not regularly spherical, but
bulges out along a certain line or meridian, while the curvature is
flattened or normal in the other meridian. For instance, if two
imaginary lines were drawn, one vertically, and the other horizontally
across the front of the eyeball intersecting in the center of the
pupil, they would represent the principal meridians, the vertical and
the horizontal. As a rule the meridian of greatest curvature is
approximately vertical, and that of least curvature is at right angles
to it, or horizontal.

Rays of light in passing through the different meridians of the
astigmatic eye are differently bent, so that in one of the principal
meridians rays may focus perfectly on the retina, while in the other
the rays may focus on a point behind the retinal field.