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tion, so this is supposed to be a too small refractive power in the same condition. As the myopic eye throws the focus of parallel rays in front of the retina, so it is supposed the presbyopic eye throws the focus of parallel rays behind the retina, because the retina is too near the lens, at S' S', Fig. 7, page 21. It is further supposed that the change which takes place with age is a flattening, and therefore a loss of refractive power, of the lenses of the eye. It is constantly asserted, therefore, that the myopic eye may be expected to become normal with age.
Now this view of the nature of presbyopy is wholly wrong. The presbyopic eye sees distant objects perfectly well, and precisely like the normal eye. Its passive structure is therefore unaltered. It makes a perfect image of distant objects on the retina, like the normal eye. Its focus of parallel rays is on the retina, not behind it. It is therefore normal in its passive state, or in its structure. The defect, therefore, consists not in a change of the structure which originally adapted it to the imaging of distant objects, but in the loss of power to adjust for near objects. And this loss of adjusting power is, again, probably the result of loss of the elasticity of the crystalline lens. In the normal young eye, when the ciliary muscle pulls forward the lens curtain, and thus relaxes its tension, the lens by its elasticity swells and thickens, and becomes more refractive. In the presbyopic eye, the ciliary muscle pulls, and the curtain or capsule relaxes its tension, in vain ; the lens, for want of elasticity, does not swell out. Therefore the remedy for presbyopy is the use of convex glasses, not habitually, not in looking at distant objects, but only in looking at or imaging near objects. The putting on of convex glasses does not make the
presbyopic eye normal, as the use of concave glasses makes the myopic eye, or convex glasses the hyperopic eye; therefore they cannot be worn habitually. In looking at near objects, it uses glasses; in looking at distant objects, the glasses are removed. Myopy is a structural defect; presbyopy is a functional defect. One is a defect of prearrangement of the instrument; the other is a loss of power to adjust the instrument. To compare with the camera again: the presbyopic eye is like a camera which was originally arranged for distant objects, and by drawing the tube could be adjusted for near objects also, but, through age and misuse and rust, the draw-tube has become so stiff that the apparatus for adjustment no longer works. It still operates well for distant objects, but can not be adjusted for nearer objects. If we desire to image a near object in such a camera, obviously we must supplement its lens with another convex lens.
From what has been said it is evident that the myopic eye does not improve with age, and finally become normal, as many suppose. Myopic persons continue to wear glasses of the same curvature until sixty or seventy years of age. I have never known a strongly myopic person who discontinued the use of glasses as he grew older. The same change, however, takes place in the myopic as in the normal eye-i. e., the loss of adjustment. In all young eyes there is a range of adjustment between a nearer and a farther limit; in the normal eye it is between five inches, near limit, and infinite distance, the farther limit (if limit it can be called); in the myopic eye the nearer limit may be two inches, the farther limit four inches, or it may be between three and six inches, or four inches and one foot, according to the degree of myopy. Now, with advancing age,
the nearer limit-i. e., the limit of adjustment-recedes. In the normal eye it is first eight inches, then one foot, then three feet, etc., until, when adjustment is entirely lost, it reaches the farther limit, and there is but one distance of distinct vision; but the farther linit-i. e., structural limit, does not change. So also in the myopic eye, with advancing age, the nearer limit or limit of adjustment recedes, but not the farther limit or structural limit. This remains the same. But, as this was always too near for useful vision, glasses must still be worn. The same glasses, however, will no longer do for all distances. An old myopic speaker will lift up his glasses to read his notes. Thus it is evident that myopy and presbyopy may exist in the same individual.
In extreme old age, when the tissues begin to break down, it is probable that some flattening of the eye may take place. To such persons it would be necessary to wear weak convex glasses, even for distant objects. But this is not ordinary presbyopy. In fact, it is probable that in most of such cases there has been slight hyperopy. There is another possible explanation, however, viz., a progressive flattening of the lens by age, but corrected by permanent accommodation, until at last the lens becomes too flat to be accommodated even for distant objects, and therefore two glasses must be used.
4. Astigmatism-Dim-sightedness.--In all the other defects there is clear sight at some distance, although it may not be a convenient distance. In this there is no perfect image, and therefore no clear sight at any distance. In the perfect eye, and also in the cases of imperfect eye thus far explained, the form of the lenses is that of a spheroid of revolution about the visual axis -the curvature and the refraction is the same in all
directions-i: e., on all meridians. This is necessary in order to bring all the rays from any radiant to a single focal point. But eyes are found in which the horizontal curvature of the cornea or of the lens, or of both, is different from the vertical curvature-the curvature is ellipsoid, with long diameter at right angles to the optic axis. Such eyes are said to be astigmatic, because the rays from any radiant are brought, not to a single focal point, but to two focal lines, a horizontal and a vertical, which are shorter or longer, and at a less or greater distance apart, according to the degree of astigmatism. A slight astigmatism is very common, and often exists unknown to the subject.
Test for Astigmatism.--This defect may be detected by looking at a cross of considerable size
the eye is astigmatic, the vertical and horizontal lines are not equally distinct at every distance. At a certain distance the vertical, and at another the horizontal, line is most distinct.
Explanation.-The cause of this defect is difficult to explain in popular language. I have used the following method in my classes. Observe: 1. In a perfect lens, with curvature equal in all directions, the emergent pencil of all the rays from a single radiant is a cone with apex at the focus. The successive sections of this cone will be circles growing smaller until it becomes a point at the focus. Beyond this the circle again enlarges without limit. This is one extreme. 2. In a cylindrical lens, in which there is no curvature at all in one direction, the emergent beam will be a wedge, sections of which will be a parallelogram becoming narrower and narrower until it becomes a focal line as long as the cylinder if the radiant is a distant one. This is
the other extreme. 3. Now, the lens of an astigmatic eye is neither a lens of equal curvature in all directions on the one hand, nor a cylindrical lens on the other, but a mean between these extremes. Its emergent beam is a complex solid, the successive sections of which are shown in Fig. 21, B. As it would be difficult to represent this solid except by a model, I have taken from the whole emergent beam two planes of rays, a vertical and a horizontal. Supposing the vertical curvature the greatest (the most usual case), the vertical plane of rays a b will meet and cross at ƒ (Fig. 21, 4), while those of the horizontal plane c d will meet at some more distant point, f. Now, since the rays of the vertical plane will meet and cross at f, while those of the horizontal plane have not yet come together, it is evident that the sec
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DIAGRAM SHOWING THE FORM OF THE FOCAL CONE IN THE ASTIGMATIC EYE.
tion of the emergent beam here will be a horizontal line. On the other hand, since the horizontals cross at f', but the verticals have already crossed and again spread into a plane, the section here of the emergent beam will be a vertical line.
For the sake of simplicity I have taken only two rectangular planes of rays. If, now, we consider all the rays, the form of the solid emergent beam is shown by the series of sections beneath (Fig. 21, B), and the cross