ng; but this
dilation and contraction are not instantaneous. There are numerous
instances on record in which total blindness has resulted from a sudden
transition from darkness to the brilliancy of day. The habit of looking
at a bright light of any kind, and especially of watching flashes of
lightning, which is practiced by many, is exceedingly dangerous. The
practice which many students and others indulge in, of resting their
eyes as the twilight of evening advances, and allowing the pupil to
dilate until it is quite dark, and then suddenly introducing a bright
light, is a palpable violation of this rule, and one that is sure,
sooner or later, sensibly to injure the eyes. The exposure of the eyes
suddenly to a strong light upon waking from sleep, and all sudden
changes of whatever kind from darkness to intense light, should be
carefully avoided by persons who would preserve their sight unimpaired.

The strength of light used should be regulated _according to the powers
of the eye_. This is a general, though a very important rule. Both the
amount and the distribution of light should be such as to produce no
unpleasant sensations. The eye possesses a certain degree of adaptation
to light, according as it is intense or feeble. Some eyes require a
stronger light than others, but all eyes are injured by being used in
light that is too intense or too feeble. Reading by a strong sunlight,
and by moon or star light, may be adduced as illustrations which are
alike painful and injurious.

Too little light is well-nigh as injurious as too much, as he can not
fail to have noticed who has had occasion to travel a difficult road in
a dark night. The injury, in such cases, is two-fold; for while, on the
one hand, the radiated muscle of the iris is unduly contracted for a
length of time, in order sufficiently to enlarge the pupil to render
objects visible, the sensitive retina, on the other hand, is overtaxed
to gain a knowledge of them in too feeble light. The pain which the
strained eye thus experiences is only an indication and a warning to the
individual of the permanent injury he is inflicting upon this delicate
organ.

_Rooms should be well and evenly lighted._ The irregular and flickering
light of common lamps and candles is very injurious, and should be
avoided in the study, and in all mechanical pursuits where the eye is
much taxed. The best oculists concur in the opinion that reflected and
concentrated light are highly in