–The sight may be affected in many different ways. It may be impaired by inflammation, or the consequences of inflammation. Or the transparent parts of the eye, as the lens, may become opaque, which is “cataract.” But the commonest causes of defective sight are peculiarities in the shape of the eye. Of these there are three principal ones–short-sightedness (myopia), long-sightedness (hypermetropia), and old-sightedness (presbyopia). In addition to these is another defect caused by irregularities in the curves of the transparent part of the front of the eye (the cornea). This is called “astigmatism.” All these defects can be corrected with glasses.
But it must not be forgotten that defective structure often depends on imperfect constitutional health, and it is always advisable to attend to the general health and give the patient, if a child, a prolonged course of constitutional treatment before putting on glasses. In many cases the necessity for them is removed.
Near-sightedness depends on the eye being too long from before backward. The image of all objects, except those quite close to the eye, is formed in front of, instead of upon the retina (the membrane which is the actual organ of sight) at the back of the eye. This condition may be inherited, or it may be brought on by over-use of the eyes, especially in children. For this concave glasses are required.
Long-sightedness is due to the opposite defect; the eyeball is too short from front to back, and the image is formed behind the retina. This is always an inherited defect. It is corrected by convex glasses. What is commonly called “weak sight” is in a general way due to this defect. In early life it is not so much noticed, as the accommodating powers of the lens of the eye are great enough to overcome it. The first thing that is noticed is that distant objects are seen clearly, but the vision of near objects is blurred. If not corrected early this condition may led to squint.
Old Sight.–Persons who have always had good sight, when they come to the age of forty-three to forty-five find that they must hold books and newspapers they are reading farther off than previously. This is due to changes in the crystalline lens of the eye, and requires the use of glasses.
The choice of spectacles is not to be entrusted to ignorant persons. For simple defects an experienced optician will be able to suit the patient.
But for defects that are at all complicated it will be necessary to consult an oculist, who will examine the eye and prescribe the glasses needed.
Persons who work near fires, or travellers is snow regions, or where there is dazzling, sunlight, should wear light blue spectacles.
For the preservation of good sight, care must be taken to have a good light for using the eyes–the light to fall on the work, no on the eyes. The light should come from the side, and a little behind. Close work should never be carried on continuously without a rest. Reading should not be done whilst riding, unless the carriage is very steady.
See also BLINDNESS, EYES, INFANTS.