Eye strain is commonly due to either astigmatism, nearsightedness, farsightedness, or weakness of the eye muscles. The farsighted eye is one in which parallel rays entering the eye, as from a distance, come to a focus behind the retina. The retina is the sensitive area for receiving light impressions in the back of the eyeball. Sight is really a brain function; one sees with the brain, since the optic nerve endings in the back of the eye merely carry light impressions to the brain where they are properly interpreted.
In order that vision be clear and perfect, it is essential that the rays of light entering the eye be bent so that they strike the retina as a single point. In the farsighted or hyperopic eye, the eyeball is usually too short for the rays to be properly focused on the sensitive nerve area in the back of the eye.
This defect in vision is, however, overcome by the act of “accommodation.” There is a beautiful transparent, double convex body, about one third of an inch thick, which looks very much like an ordinary glass lens, and is situated in the eye just back of the pupil. This is what is known as the crystalline lens, and the rays of light are bent in passing through it so as to be properly focused on the retina.
The foregoing statements have been made as though objects were always at a distance from the eye, so that the rays of light coming from them were almost parallel. Yet when one is looking at an object within a few inches of the eye the rays diverge or spread out, and these the normal eye (if rigid) could not focus on the retina much less the farsighted eye. But the eye is adaptable to change of focus through the action of a certain muscle, situated within the eyeball about the lens, which controls to a considerable extent the shape of the lens. When the muscle contracts it allows the lens to bulge forward by virtue of its elasticity, and, therefore, become more convex. This is what happens when one looks at near objects, the increased convexity of the lens bending the rays of light so that they will focus as a point on the retina.
Now in the farsighted eye this muscular control or “accommodative action” must be continually exercised even in looking at distant objects, and it is this constant attempt of nature to cure an optical defect of the eye which frequently leads to nervous exhaustion or eye strain. The nerve centers, which animate and control the nerves supplying the eye muscles to which we have just alluded, are in close proximity to other most important nerve centers in the brain, so irritation of the eye centers will produce sympathetic irritation of these other centers, leading to manifold and complex symptoms which we will describe under this head. But these symptoms do not necessarily develop in everyone having farsightedness or astigmatism, since both are often present at birth.
The power of accommodation is sufficient to overcome the optical defect of the eye, providing that the general health is good and the eye is not used much for near work. If, on the other hand, excessive use of the eyes in reading, writing, figuring, sewing, or other fine work is required, and especially if the health becomes impaired, it happens that the constant drain on the eye center in the brain will result in a group of symptoms which we will consider later. Failure of accommodation comes on at about forty, and gradually increases until all accommodation is lost at the age of seventy five.
For this reason it is necessary for persons over forty five years of age, having normal or farsighted eyes, to wear convex glasses in reading or doing near work, and these should be changed for stronger ones every year or two. These convex glasses save the eyes in their attempt to make the lens more convex when looking at near objects in farsightedness, and also prove serviceable in the same manner when accommodation begins to fail in the case of what is called “old sight.” The neglect to provide proper glasses for reading any time after the age of forty five, and the failure to replace them by stronger lenses when required, distinctly favor the occurrence of cataract in later life.
In the act of accommodation, in addition to the muscular action by which the lens is made more convex, there is the tendency for the action of another group of muscles outside the eyeball, which turn the eyes inward when they are directed toward a near object. Here then is another source of trouble resulting from farsightedness, i. e., the not infrequent occurrence of inward “squint” occasioned by the constant use of the muscles pulling the eyes inward during accommodation for near objects. Again, inflammation of the eyelids, and sometimes of deeper parts of the eyeball, follows untreated hyperopia. Early distaste for reading is often acquired by farsighted persons, owing to the strain on the accommodative apparatus. The convex lens is that used to correct farsightedness.