eye is sometimes turned up or down,
but this is of less frequent occurrence.

It would be interesting to notice the protecting organs of the eye,
consisting of the _orbit_, which is a deep bony socket, in which the
eye securely rests; of the _eye-brows_, which are two projecting arches,
covered with hair, and so arranged as to prevent the moisture that
accumulates upon the forehead, in free perspiration, from flowing into
the eye; of the _eye-lids_, which are two movable curtains for the
protection of the eye, and which secrete a fluid that moistens and
lubricates it; of the _lachrymal gland_, with its ducts, which keeps the
eye constantly moist, and whose secretions go on while we wake and when
we sleep, etc., etc.; but the preceding must suffice.

With this brief description of the apparatus of vision, we proceed to
the consideration of the means of preserving and improving this sense,
and of rendering it tributary to intellectual and moral culture.

The rule requiring that _action should alternate with rest_, which has
been so often stated, and which applies to all the organs of both body
and mind, should be especially observed in relation to the eye. This
organ requires exercise, and light is its appropriate stimulus; but
injury is the inevitable consequence of keeping it too constantly
employed, or too intently fixed for a long time on any object. Whenever
the eye is fixed for any length of time upon an object which it
distinguishes with difficulty, it experiences a painful sensation, which
is a sure indication that it has been overtaxed. The sight is also
impaired when the eye is too little used, or when its natural stimulus
is shut out, as is strikingly illustrated in the case of persons
confined in dungeons. A distinguished oculist has said that many men
daily impair or destroy their eyes by immoderate use, and that not a few
have done the same by too little use of them.

The exposure of the eyes to _sudden transitions from weak to strong
light_ is very injurious. This may be regarded as one of the must
prolific causes of weakness of sight. The injury is generally gradual,
it is true, but it is none the less fatal on that account. The immediate
sensation of pain, when a strong light is brought into a dark room,
should be a sufficient warning to avoid such sudden extremes. The iris
dilates and contracts, and thus enlarges or diminishes the size of the
pupil as the light that fails upon the eye is faint or stro