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.

Plate I

ANATOMY OF THE EYE

The upper illustration shows the six muscles attached to the eye. The Superior Rectus Muscle pulls and directs the eye upward; the Inferior Rectus, downward; the External and Internal Rectus Muscles pull the eye to the right and left; the Oblique Muscles move the eye slantwise in any direction.

Lack of balance of these muscles, and especially inability to focus both eyes on a near object without effort, constitute "eye-strain."

The lower cut illustrates the relation of the crystalline lens to sight. Lens Nearsight Focus shows the lens bulging forward and very convex; Lens Farsight Focus shows it flat and less convex.

This adjustment of the shape of the crystalline lens is called "accommodation"; it is effected by a small muscle in the eyeball.

In the normal eye, the rays of light from an object pass through the lens, adjusted for the proper distance, and focus on the retina.

In the nearsighted eye, these rays focus at a point in front of the retina; while in the farsighted eye these rays focus behind the retina; the nearsighted eye being elongated, and the farsighted eye being shortened.

PLATE I

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 work—there 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,[Pg 30]
[Pg 31] top or the back of the head, or limited to one side. It frequently takes the form of "sick headache" (p. 113). It is perhaps more apt to appear after any unusual use of the eyes in reading, writing, sewing, riding, shopping, or sight-seeing, and going to theaters and picture galleries, but this is not by any means invariably the case, as eye headache may appear without apparent cause.

Nausea and vomiting, with or without headache, nervousness, sleeplessness, and dizziness often accompany eye-strain. Sometimes there is weakness of the eyes, i. e., lack of endurance for eye work, twitching of the eyelids, weeping, styes, and inflammation of the lids. In view of the extreme frequency of eye-disorders which lead to eye-strain, it behooves people,