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.
Experiment 152. Take a round piece of white cardboard the size of a saucer, and paint it in alternate rings of red and yellow,—two primary colors. Thrust a pin through the center and rotate it rapidly. The eye perceives neither color, but orange,—the secondary color.
Experiment 153. To note the shadows cast upon the retina by opaque matters in the vitreous humor (popularly known as floating specks, or gossamer threads), look through a small pin-hole in a card at a bright light covered by a ground-glass shade.
Experiment 154. To illustrate accommodation. Standing near a source of light, close one eye, hold up both forefingers not quite in a line, keeping one finger about six or seven inches from the other eye, and the other forefinger about sixteen to eighteen inches from the eye. Look at the near finger; a distinct image is obtained of it, while the far one is blurred or indistinct. Look at the far image; it becomes distinct, while the near one becomes blurred. Observe that in accommodating for the near object, one is conscious of a distinct effort.
In many cases near-sightedness becomes a serious matter and demands skillful advice and careful treatment. To remedy this defect, something must be done to throw farther back the rays proceeding from an object so that they will come to a focus exactly on the retina. This is done by means of concave glasses, properly adjusted to meet the conditions of the eyes. The selection of suitable glasses calls for great care, as much harm may be done by using glasses not properly fitted to the eye.
