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. 44.) 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.

The

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.

WEAKNESS OF THE EYE MUSCLES.There are six muscles attached to the outside of the

eyeball which pull it in various directions, and so enable each eye to be directed upon a common point, otherwise objects will appear double. Weakness of these muscles or insufficiency, especially of those required to direct the eyes inward for near work, may lead to symptoms of eye-strain. When reading, for example, the muscles which pull the eye inward soon grow tired and relax, allowing the opposing muscles to pull the eye outward so that the eyes are no longer directed toward a common point, and two images may be perceived or, more frequently, they become fused together producing a general blurring on the page. Then by a new effort of will the internal muscles pull the eyes into line again, only to have the performance repeated, all of which entails a great strain upon the nervous system, and may lead to permanent squint, as has been pointed out. In addition to these symptoms caused by weakness of the eye muscles-seeing double, blurred vision, and want of endurance for close workthere are others which are common to eye-strain in general, as headache, nausea, etc., described in the following paragraph.

Symptoms of Eye-strain.-Headache is the most frequent symptom. It may be about the eyes, but there is no special characteristic which will positively enable one to know an eye headache from that arising from other sources, although eye-strain is probably the most common cause of headache. The headache resulting from eye-strain may then be in the forehead, temples,

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