mitation, designed
to be temporary, may become permanent.

_Observation._ The vision of a "cross-eye" is always defective. In
general, only one eye is called into action, in viewing the object to
which the mind is directed. This defect can be remedied by a surgical
operation, which also corrects the position of the eye.

937. _Children should be trained to use the eye upon objects at
different distances._ This is necessary, in order that the vision may
be correct when objects at various distances are viewed. Any action
unnatural to the muscles, if frequently repeated, may and will modify
the character and action of the parts so operated upon. If a limb, as
the arm, be kept flexed for a long time, one set of muscles will be
relaxed and elongated, and another will be shortened, and its
contractile power will be increased. The same principle is true of the
eye.

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What causes palsy of the retina? 936. Why should we avoid oblique
positions of the eye in viewing objects? What is said of the practice
of imitating persons thus affected? What is said in reference to the
vision of a "cross-eye"? 937. Why should children be trained to use
the eye upon objects at different distances? What is the effect if an
unnatural action of the muscles is frequently repeated? Does the same
principle apply to the eye?

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938. In viewing objects very near the eye, the ciliary processes are
called into action to produce a proper inclination of the crystalline
lens, so that the rays of light may be properly refracted to form a
perfect image on the retina. In looking at objects at a great
distance, the ciliary processes are called into a different action, to
produce a different inclination of the lens. Let either of these
actions be repeated, again and again, for weeks and months, and they
will become natural, and the acquired inclination will be permanent.

939. From the preceding principle, a person becomes short or long
sighted, as the objects to which the eye is usually directed are near
or remote. This is one reason why scholars, watchmakers, and
artisans, who bring minute objects near the eye to examine them,
are short-sighted, and why hunters and sailors, who are habituated
to view objects at a distance, are long-sighted.

_Observation._ In the management of children, whether in the nursery
or school-room, it is very important that their books, or articles
upon which they may lab