The image 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 darkened wall.  The window frame may be plainly
  seen for a moment.

  Glance at the sun for a moment, close the eyes and the image of the sun
  may be seen for a few seconds.