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. (See Plate I, p. 30.),

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. 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