Observation. If the eye is fixed for a time on some object which is distinguished with difficulty, there is a painful sensation, similar to that experienced by other muscles of the body when used too long. This is called “straining the eye.”
932. When the refraction of the rays of light is too great, as in over-convexity of the cornea, or the crystalline lens, or the vitreous humor, or all of them, the image is formed a 410 little in front of the retina. Persons thus affected cannot see distinctly, except at a very short distance. This infirmity is called near, or short-sightedness. This defect is in a great measure obviated by the use of concave glasses, which scatter the luminous rays, and thus counterbalance the too strong refracting force of the eye.
What does fig. 143 represent? 931. Why can we see objects at the side as well as in front of the eye, without turning the body? What is the effect when the eye is fixed on an object that is indistinctly seen?
933. When the different parts of the eye are not sufficiently convex, the image is formed beyond the retina, and thus only distant objects are distinctly seen. This defect is called long-sightedness. The feebleness in the refracting power of the eye may be caused by disease; but usually it is a consequence of old age, and is remedied by wearing spectacles with convex glasses.
HYGIENE OF THE ORGANS OF VISION.
934. The eye, like other organs of the body, should be used, and then rested. If we look intently at an object for a long time, the eye becomes wearied, and the power of vision diminished. The observance of this rule is particularly needful to those whose eyes are weak, and predisposed to inflammation. On the contrary, if the eye is not called into action, its functions are enfeebled.
935. Sudden transitions of light should be avoided. The iris enlarges or contracts, as the light that falls upon the eye is faint or strong; but the change is not instantaneous. 411 Hence the imperfect vision in passing from a strong to a dim light, and the overwhelming sensation experienced on emerging from a dimly-lighted apartment to one brilliantly illuminated. A common cause of am-aur-o´sis, or paralysis of the retina, is, using the eye for a long time in a very intense light.
932. What is short-sightedness? How is the defect remedied? 933. What is long-sightedness? How is the defect remedied? 934–942. Give the hygiene of the organs of vision. 934. Do the same principles apply to the use of the eye as to other organs? What is the effect if the eye is fixed intently on an object for a long time? What results if the eye is not called into action? 935. Why should sudden transitions of light be avoided?