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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 27. 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; therefore they can not 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 presby
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 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 limit, 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. 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 convex glasses, even for distant objects. But this is not ordinary presbyopy. In fact, it is probable that most of such cases belong to the next category.
Hypermetropy. We have dwelt on the two most common defects of the eye, but there are others less common, which must be briefly characterized. Hypermetropy is the true opposite of myopy. Like the latter, it is a structural defect, but in the opposite direction. In this case the lens is not sufficiently refractive for the length of the chamber, or the receiving screen is too near (at S' S', Fig. 7) for the refractive power of the lens. Therefore the focus of parallel rays is behind the retina in a passive state of the eye. The hypermetropic eye when young usually sees well at a distance, but not near at hand, and therefore it is apt to be confounded with presbyopy. The reason is, that a slight adjustment adapts the eye for perfect retinal image of distant objects; but the near limit of its range of adjustment is much farther off than in the normal. When, however, the hypermetropic eye loses its power of adjustment with age, then even distant objects can not be seen distinctly. Such persons, therefore, while young, should habitually wear slightly convex glasses,
which make their eyes normal. When they grow old, they are compelled to have two pairs of glasses, one for distant objects and one for near objects; one for walking and one for reading. The hypermetropic eye may be compared to a camera which, when entirely pushed up, is too short for the imaging of any objects whatever. By drawing, it may be adjusted for distant objects, but not for near objects.
Astigmatism. The form of a perfect eye is that of a spheroid of revolution about the optic axis. Its refraction in a horizontal and a vertical plane will be equal. This is necessary to bring all rays to a perfect point at the same distance. But eyes are found in which the horizontal curvature of the cornea or of the crystalline, or both, is different from the vertical curvature. Such eyes are said to be astigmatic, because the rays from any radiant are brought to a focal line, instead of a focal point. A very slight degree of astigmatism is not uncommon, and often exists unknown to the patient. CHAPTER IV.
EXPLANATION OF PHENOMENA OF MONOCULAR VISION.
SECTION 1.-STRUCTURE OF THE RETINA.
We have thus far treated of the eye, and compared it with the camera, purely as an optical instrument, contrived to form an image upon a receiving screen suitably placed. We have also treated of the defects of the eye, as much as possible, from the same physical point of view as defects of an instrument. But in both the camera and the eye the image is only a means to accomplish a higher purpose, viz., to make a photographic picture in the one case and to accomplish vision in the other. We have thus far spoken as much as possible only of an insensitive screen, the ground-glass plate in the one case and the dead retina in the other. But in both, when accomplishing their real work, we have a sensitive screen, in which wonderful changes take place, viz., the iodized plate in the one and the living retina in the other. In order to understand the real function of the eye in the living animal, it is necessary that we study the structure and functions of the retina.
Structure of the Retina.—The retina, as already stated, page 22, is a thin membranous expansion of the