the plan of the microscope. It was formerly thought that, in looking at a near object, the straight muscles, acting all together, squeezed the eye about the equatorial belt, and increased its axial diameter in other words, made it egg-shaped—and thus carried the retinal screen farther back from the lens. But now it is known that the retinal screen remains immovable, and the lens changes its form so as to throw the image to the same place.

Experiment. This is proved in the following manner: A person is chosen with good, normal young eyes. The experimenter stands in a dark room, in front of

f

FIG. 17.

B

n

A, eye observed; B, eye of observer; c, section of candle flame; f, a distant point of sight, and n a near point of sight. (After Helmholtz.)

the patient, A, with a lighted candle in his hand, a little to one side, as in Fig. 17, C, while his own point of observation is on the other side, B. If the observer now looks carefully, he will see in the eye of the patient three images of the candle-flame: first, one reflected from the surface of the cornea, which is by far the brightest (Fig. 18, a); second, one from the anterior surface of the crystalline, much fainter (Fig. 18, b); third, one from the posterior surface of the crystalline, the faintest of all, and very small (c). Further, it will be observed that the first and second are erect images,

FIG. 18.

because reflected from a convex surface, while the third is inverted, because reflected from a concave surface. Now directing the patient to gaze on vacancy, or a distant point, f, Fig. 17, we observe carefully the position and size of these several images. Then, if by direction the patient transfers the point of sight to a very near point, n, without changing the direction, we observe that the images a and c do not change, but the image b changes its position and grows smaller. image is reflected from the anterior surface of the crystalline. The anterior surface of the crystalline, therefore, changes its form. Again, the nature of the change of the image, viz., that it becomes smaller, shows that this anterior surface becomes more convex. By careful examination the iris, too, may be seen to protrude a little

FIG. 19.

This

F, lens adjusted to distant objects; N, to near objects; a, aqueous humor; d, ciliary muscle; e, ciliary process.

in the middle. Evidently, therefore, in adjusting the eye to very near objects, the crystalline becomes thicker in the middle, and pushes the pupil a little forward. In the accompanying diagram, Fig. 19, the crystalline lens is divided by a plane through the center. The right side, N, is adapted to near objects; the left, F, to distant objects.

Theory of Adjustment. Thus much may be considered certain. It is certain that in adjusting the eye for looking at very near objects, the lens becomes more convex. But the question, "How is this done?" is more difficult to answer. Helmholtz thinks it is done in the following manner:*

It will be remembered that the lens is invested by a thin, transparent membrane, 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. 19, 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

* "Optique Physiologique," p. 150.

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.

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.

CHAPTER III.

DEFECTS OF THE EYE AS AN INSTRUMENT.

In the preceding chapter 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.

The most important of these defects are myopy and presbyopy.

Myopy, Brachymetropy, or Near - Sightedness.-The normal or emmetropic eye adjusts itself perfectly for all distances, from about five inches to infinity. It