The muscles upon which the form of the eye and the size of the pupil
depend are subject to the general laws of muscular action. Their
strength and flexibility, which are increased by healthful exercise, are
impaired by disuse. Hence students who have neglected this rule, and
have accustomed themselves for a long time to view objects near by,
lose the power of adjusting the eye so as to view things at a distance.
As a consequence, they become near-sighted, and put on glasses, when, by
a proper use of the eye, their vision might have been preserved
unimpaired many long years. I know some students upon whom this habit
became so firmly fixed before they were twenty years of age, that they
felt compelled to put on glasses, but who, unwilling to contract so
pernicious a habit in early life, commenced a course of discipline in
accordance with the suggestions here given. By perseverance, their eyes
not only recovered their former healthful action, but became so improved
that they now possess the sense of vision unimpaired not only, but in a
very high state of cultivation.

_Persons become near or long sighted_ as the objects to which they are
accustomed to direct the eye are near or remote. This is illustrated in
the case of students, watch-makers, and engravers, who are accustomed to
examine minute objects near the eye, and, as a consequence, become
near-sighted; and of surveyors, hunters, and sailors, who, being
accustomed to view objects at a distance, become long-sighted. By a
proper discipline of the eye, persons may attain and retain the power of
viewing objects near by and at a distance, as is illustrated in the case
of those gunsmiths who are accustomed to manufacture guns, and to try
them in shooting at a mark at a great distance. The preceding principles
being borne in mind in their various applications. I need, perhaps,
state but one more rule.

He who would secure clear and distinct vision, must observe all those
rules which are necessary to keep the body in health. The sympathy of
the eyes with all the other organs of the body is wonderful and
intimate. There is no other organ whose strength depends so much on the
general vigor of the system. Strict temperance in eating and drinking
may be regarded as an indispensable requisite for the preservation of
healthy eyes. To this may be attributed the clear heads of the ancient
philosophers, who, unlike most students of the present day, exercised
their bodies and limbs