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.
NEARSIGHTED EYE.—In the nearsighted eye the eyeball is too long for parallel rays entering the eye to be focused upon the retina; they are bent, instead, to a point in front of the retina, and then diverge making the vision blurred. ([Plate I], p. 30.) The act of accommodation in making the lens more convex will not aid this condition, but only make it worse, so that it is not attempted.
Eye-strain in this optical defect is brought on by constant use of the eye muscles (attached to the outside of the eyeball) in directing both eyes inward so that they will both center on near objects; the only ones which can be seen. Outward squint frequently results, because the muscular efforts required to direct both eyes equally inward to see near objects are so great that the use of both eyes together is given up, and the poorer eye is not used and squints outward, while the better eye is turned inward in the endeavor to see. Nearsighted persons are apt to stoop, owing to the habitual necessity for coming close to the object looked at. Their facial expression is also likely to be rather vacant, since they do not distinctly see, and do not respond to the facial movements of others.
Nearsightedness, or myopia, is not a congenital defect, but is usually acquired owing to excessive near work which requires that the eye muscles constantly direct both eyes inward to see near objects. In so acting the muscles compress the sides of the eyeballs and tend to increase their length, interfere with their nutrition, and aggravate the condition when it is once begun. (See [Diagram].) Concave lenses are used to correct myopia, and they must be worn all the time.
ASTIGMATISM.—This is a condition caused by inequality of the outer surface of the front of the eyeball, and rarely by a similar defect in the surfaces of the lens. The curvature of the eyeball in the astigmatic eye is greater in one meridian than in the opposite. In other words, the front of the eyeball is not regularly spherical, but bulges out along a certain line or meridian, while the curvature is flattened or normal in the other meridian. For instance, if two imaginary lines were drawn, one vertically, and the other horizontally across the front of the eyeball intersecting in the center of the pupil, they would represent the principal meridians, the vertical and the horizontal. As a rule the meridian of greatest curvature is approximately vertical, and that of least curvature is at right angles to it, or horizontal.
Rays of light in passing through the different meridians of the astigmatic eye are differently bent, so that in one of the principal meridians rays may focus perfectly on the retina, while in the other the rays may focus on a point behind the retinal field. In this case the eye is made farsighted or hyperopic in one meridian, and is normal in the other. Or again, the rays may be focused in front of the retina in one meridian, and directly on the retina in the other; this would be an example of nearsighted or myopic astigmatism. Farsightedness and nearsightedness are then both caused by astigmatism, although in this case not by the length of the eyeball, but by inequality in the curvature of the front part (cornea) of the eyeball. For example, in simple astigmatism one of the principal meridians is hyperopic (turning the rays so that they focus behind the retina) or myopic (bending the rays so that they focus in front of the retina), while the other meridian is normal. In mixed astigmatism, one of the principal meridians is myopic, the other hyperopic; in compound astigmatism the principal meridians are both myopic, or both hyperopic, but differ in degree; while in irregular astigmatism, rays of light passing through different parts of the outer surface of the eyeball are turned in so many various directions that they can never be brought to a perfect focus by glasses.
It is not by any means possible for a layman to be able always to inform himself that he is astigmatic, unless the defect is considerable. If a card, on which are heavy black lines of equal size and radiating from a common center like the spokes of a wheel, be placed on a wall in good light, it will appear to the astigmatic eye as if certain lines (which are in the faulty meridian of the eyeball) are much blurred, while the lines at right angles to these are clear and distinct. Each eye should be tested separately, the other being closed. The chart should be viewed from a distance as great as any part of it can be seen distinctly. All the lines on the test card should look equally black and clear to the normal eye.
Astigmatism is corrected by a cylindrical lens, which is in fact a segment of a solid cylinder of glass. The axis of the cylindrical lens should be at right angles to the defective meridian of the eye, in order to correct the astigmatism. Eye-strain is caused by astigmatism in the same manner that it is brought about in the simple farsighted eye, i. e., by constant strain on the ciliary muscle, which regulates the convexity of the crystalline lens. For it is possible for the inequalities of the front surface of the eyeball or of the lens to be offset or counterbalanced by change in the convexity of the lens produced by the action of this muscle, and it is conceivable that the axis of the lens may be tilted one way or another by the same agency, and for the same purpose. But, as we have already pointed out, this continual muscular action entails great strain on the nerve centers which animate the muscle, and if constant near work is requisite, or the health is impaired, the nervous exhaustion becomes apparent. The lesser degrees of astigmatism often give more trouble than the greater.