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It will be remembered that the lens is invested by a thin, transparent membrane (lens-capsule), which extends outward from its edge as a circular curtain, and is attached all around to the sclerotic, thus dividing the interior of the eye into two chambers—the anterior, filled with the aqueous, and the posterior, with the vitreous humor. It will be remembered, further, that this membrane is naturally drawn tight by the elastic rigidity of the sclerotic, and presses gently on the elastic lens, flattening it slightly. This is the normal passive condition, as when gazing at a distance. Now there are certain muscular fibers (ciliary muscle, Fig. 20, d) which, arising from the exterior fixed border of the iris just where it is attached to the sclerotic, run backward, radiating, and take hold upon the outer edge of the lens curtain. When these fibers contract, they pull forward the tense curtain to a smaller portion of the globe, and thus relax its tension. The relaxing of the tension of the curtain relaxes also the pressure of the capsule on the lens, which therefore immediately swells or thickens in proportion to the degree of relaxation.* According to Helmholtz, then, we adjust the eye to near objects by contraction of the ciliary muscle. There are other views on this subject, but this seems the most probable.
The normal eye in a passive state is adjusted to infinitely distant objects. By change of the form of the lens, it can adjust itself to all distances up to about five inches. The range of adjustment or of distinct vision is, therefore, within these limits. It is only at comparatively near distances, however, that the change is great. Between twenty feet and infinite distance the adjustment is almost imperceptible.
* It is probable, also, that there are certain circular fibers which, by contraction, draw together the lens curtain and thus relax the capsule. (Fuchs.)
We see, then, that the mode of adjustment of the eye is somewhat like that of the microscope-i. e., the change is in the lens, not in the position of the receiving screen. Like the microscope, but how infinitely superior! The microscope has its four-inch lens, its two-inch lens, its one-inch lens, its half-inch lens, its quarter-inch, its tenth-inch, and even its fiftieth-inch lens. It changes one for another, according to the distance of the object. But the eye changes its one lens, and makes it a five-inch lens, a foot lens, a twenty-foot lens, a mile lens, or a million-mile lens; for at all these distances it makes a perfect image.
SECTION II.-THE ABNORMAL EYE, OR DEFECTS OF THE
EYE AS AN INSTRUMENT.
In the preceding section we have attempted to bring out in a clear and intelligible form, the beautiful structure of the eye, by comparing it with the camera and showing its superiority. But the eye of which we have been speaking is the normal or perfect eye. This normal condition is called emmetropy. The eye, however, is not always a perfect instrument. There are certain defects of the eye which are quite common. The principles involved in the construction of the normal eye may be still further enforced and illustrated by an explanation of these defects. Let it be observed, however, that these defects must not be regarded as the result of imperfect work on the part of Nature, but rather as the effects of misuse of the eye, accumulated by inheritance for many generations. They do not occur in animals, nor in the same degree in savage races; and most of them are also very rare in persons living for many generations in the country.
Emmetropy, or Normal Sight.—The normal or emmetropic eye adjusts itself perfectly for all distances, from about five inches to infinity. It makes a perfect image of objects at all these distances. This is called its range of distinct vision. It has but one limit, viz., the nearer limit of five inches. Now in the passive state of the eye, as, for instance, in gazing on vacancy, or when the eye is taken out of the socket as a dead instrument, it is prearranged for perfect image of objects at an infinite distance. Its focus of parallel rays in a passive state is on the retina. For all nearer objects, a voluntary effort is necessary to throw the image on the retina, which effort is greater as the object is nearer, until it is limited at the distance of about five inches. The normal eye, therefore, is like a camera, which, when pushed up as much as possible, is arranged for making a perfect image of sun, or moon, or a distant landscape, but can by drawing the tube be adjusted to shorter and shorter distances up to five inches, but not
This is the standard. Any considerable departure from this is a defect.
The most important defects are myopy, hyperopy, presbyopy, and astigmatism.
1. Myopy.—The myopic eye is not prearranged for perfect image of distant objects. Its focus for distant objects (focus of parallel rays) is not on the retina, but in front of it. The refractive power of the lenses in their passive state is too great, or else the receiving screen (retina) may be regarded as too far back from the lens, viz., at S" S", Fig. 7, page 21. The rays (Fig. 8, page 22, r") have already reached focus, crossed, and again spread out before they reach the retina. An
object must be brought much nearer before its perfect image will be thrown on the retina. Within this farther limit of perfect image, however, it has its own range of adjustment, like the normal eye. The range of the normal eye is from infinite distance to five inches. In the myopic eye the range may be from a yard to four inches, or from a foot to three inches, or from six inches to two inches, or even from three inches to one inch, according to the degree of myopy. The amount of ocular adjustment or change in the lens to effect these ranges is as great as for the normal range from infinite distance to five inches, but the latter is a far more useful range. The myopic eye, therefore, is like a camera which was never intended to be used for taking distant objects, and which, therefore, when shortened to the greatest degree, is still too long in the chamber for distant objects, but is adapted only for near objects within a certain limited range.
It is evident, then, that, the defect of the myopic eye being too great refractive power of the lens in a passive state, this defect may be remedied by the use of concave glasses, with concavity just sufficient to correct the excess of refractive power, and therefore to throw the image of distant objects back to the retinal screen in the passive state of the eye. The eye then adjusts itself to all nearer distances, and becomes in all respects a normal eye. From the nature of the defect (structural defect), it is evident that the glasses must be worn habitually.
2. Hypermetropy-Hyperopy-oversightedness.—Hyperopy is the 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, or r', Fig. 8) 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 very near at hand, and therefore it is apt to be confounded with slight presbyopy. The reason is, that a slight adjustment adapts the eye for perfect rctinal image of distant objects; but the near limit of its range of adjustment is somewhat 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 the tube, it may be adjusted for distant objects, but not for near objects. This defect is very common, and is peculiarly distressing. The eye never rests, but is always under the strain of accommodation even for distant objects.
3. Presbyopy, or Old-sightedness. This defect is often called long-sightedness, or farsightedness; but this is a misnomer, based on a misconception of its true nature. It is obviously impossible to have an eye more longsighted than the normal eye, for this defines with perfect distinctness the most distant objects, such as the moon or the sun when the dazzling effect is prevented by smoked glass. It is usually regarded as a defect the reverse of near-sightedness. As near-sightedness is the result of too great refractive power in a passive condi