ge P' of a point P falls behind the retina in the unaccommodated
eye. By means of a convex lens it may be focused on the retina without
accommodation (dotted lines). (To save space P is placed much too near the
eye.)]

Hence, when a person about forty-five years of age complains of dim light,
poor print, and tired eyes, the time has come to seek the advice of an
optician. A convex lens may be needed to aid the failing power to increase
the convexity of the lens, and to assist it in bringing the divergent rays
of light to a focus.

In "long sight," or hypermetropia both the near and far point of
vision are concerned, and there is no distinct vision at any distance
without a strain. It is a defect in the focus, dependent upon the form of
the eyes, and exists in childhood. The axis of the eyeball is too short,
and the focus falls beyond the retina, which is too near the cornea. In
childhood this strain may pass unnoticed, but, sooner or later it
manifests itself by a sense of fatigue, dizziness, and a blurred and
indistinct vision. The remedy is in the use of convex glasses to converge
parallel rays of light before they enter the eye. The muscles of
accommodation are thus relieved of their extra work.

"Short sight," known as myopia, is one of the commonest defects of
vision. In this defect the axis of the eye, or the distance between the
cornea and the retina, is too long and the rays of light are brought to a
focus in front of the retina. The tendency to short-sightedness exists in
many cases at birth, and is largely hereditary. It is alarmingly common
with those who make a severe demand upon the eyes. During childhood there
is a marked increase of near-sightedness. The results of imprudence and
abuse, in matters of eyesight, are so disastrous, especially during school
life, that the question of short sight becomes one of paramount
importance.

Experiment 150. With a hand-mirror reflect the sunlight on a white
wall. Look steadily at the spot for a full minute, and then let the
mirror suddenly be removed. The "complementary" color--a dark spot--will
appear.

Experiment 151. _To show that impressions made upon the retina do
not disappear at once_. Look steadily at a bright light for a moment or
two, and then turn away suddenly, or shut the eyes. A gleam of light
will be seen for a second or two.

Look steadily at a well-lighted window for a few seconds, and then turn
the eyes suddenly to a darken