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. 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? -=-=-=-=-=-=-=-=-=-=-=-= _Note._ Let the anatomy and physiology of the eye be reviewed from figs. 139 and 143, or from anatomical outline plate No. 10. 936. _Long-continued oblique positions of the eye should be avoided, when viewing objects._ If the eye is turned obliquely for a long time in viewing objects, it may produce an unnatural contraction of the muscle called into action. This contraction of the muscle is termed _stra-bis´mus_, or cross-eye. The practice of imitating the appearance of a person thus affected, is injudicious, as the imitation, designed to be temporary, may become permanent. _Observation._ The vision of a "cross-eye" is always defective. In general, only one eye is called into action, in viewing the object to which the mind is directed. This defect can be remedied by a surgical operation, which also corrects the position of the eye. 937. _Children should be trained to use the eye upon objects at different distances._ This is necessary, in order that the vision may be correct when objects at various distances are viewed. Any action unnatural to the muscles, if frequently repeated, may and will modify the character and action of the parts so operated upon. If a limb, as the arm, be kept flexed for a long time, one set of muscles will be relaxed and elongated, and another will be shortened, and its contractile power will be increased. The same principle is true of the eye. -=-=-=-=-=-=-=-=-=-=-=-= What causes palsy of the retina? 936. Why should we avoid oblique positions of the eye in viewing objects? What is said of the practice of imitating persons thus affected? What is said in reference to the vision of a "cross-eye"? 937. Why should children be trained to use the eye upon objects at different distances? What is the effect if an unnatural action of the muscles is frequently repeated? Does the same principle apply to the eye? -=-=-=-=-=-=-=-=-=-=-=-=