and meet exactly at the macula lutea. The interrupted or dotted lines represent rays coming from a near point. These rays diverge as they approach the eye. Hence, if they are to meet at the macula, they must be more strongly refracted than the parallel rays represented by the solid lines. To accomplish this the ciliary muscle contracts, thus becoming a ring of less diameter. (The dotted lines at the ciliary muscle show the change in its form). This contraction in the diameter of the ciliary ring relaxes the tension upon the capsule, when, by its innate elasticity, the lens assumes a more convex form, as is seen in its dotted outline. This stronger convex lens now refracts more strongly than before, and thus the diverging rays are brought to a focus exactly at the point at which the distant or parallel rays were when the eye was at rest. As soon as the force of contracting the ciliary ring is removed, its diameter is increased, the tension upon the capsule is renewed, and the lens returns to its original state.

In an ideally constituted eye, the distant point of clear vision (punctum remotum) is the horizon or infinite distance. Parallel rays are brought to a focus without effort on the part of the ciliary muscle, and pencils of light from the retina pass out of the eye in parallel rays. Objects situated at about twenty feet from the eye send to it rays which are practically parallel, and hence in ophthalmology objects seen at twenty feet are regarded as at infinite distance.

The distance between the remote point (punctum remotum) and the nearest point (punctum proximum)

of clear vision, representing the extent of accommodative power, is called the range of accommodation. Accommodation is a positive force acting only in producing clear vision as objects approach within finite distance. It can not act to magnify very distant objects by a process of negative accommodation.

The crystalline lens, like every other tissue of the body, becomes less elastic with each year of life. Hence the power of accommodation diminishes and the near point advances toward the distant on account of the constantly increasing difficulty of changing the curvatures of the crystalline lens by the action of the ciliary muscle.

At the age of twenty the near point is at about ten centimetres (eight and a half inches) from the eye, while at the age of forty it has reached to twice that distance, and at seventy-five it has been gradually transferred to the remote point. In other words, the faculty of accommodation is at that age practically lost.

It is evident that in this gradually progressive removal of the near point there must come a time when the normal eye can not clearly see objects within the ordinary distance of reading, and artificial help in the form of glasses becomes necessary. This, to the best eyes, occurs between the ages of forty-five and fifty, and the condition of accommodation demanding such aid is called presbyopia. Presbyopia is not necessarily a failure of visual power, nor is it, as is commonly supposed, an indication of perfect eyes that one is able to read without the aid of glasses after the age of fifty.

People who read without glasses after that age are near-sighted, or have some other defect of the eye.

As the practical treatment of presbyopia is materially modified by errors in the refractive condition of the eye, its further consideration will be resumed after these errors have been discussed.

REFRACTION OF THE EYE.

All eyes are not constructed on the plan which has been shown above. Some eyes are longer and some shorter than in emmetropia, and some have irregular refracting sur

faces. These conditions, varying from emmetropia, are, according to Donders, known as conditions of ametropia.

FIG. 5.-This represents the form of the emmetropic

eye, in which parallel rays are brought to a focus at the back of the eye, without an effort at accommodation.

If the eye is short, and parallel rays, could they

FIG. 6. The hyperopic or short eye. The solid lines represent the course which parallel rays would take were the back of the eye transparent. A convex lens, placed in front of such an eye, gives the rays the directions shown by the dotted lines, which meet at the retina.

pass beyond the back of

the eye, would come to a focus behind the retina, the condition is called hypermetropia or hype

ropia (Fig. 6).

If, on the contrary, the eye is long, and parallel rays come to a focus in front of the retina, the con

dition is known as

myopia (Fig. 7).

An astigmatic eye is one in which there is a difference of refraction in different meridians. Thus, in one meridian of an eye, emmetropia may exist; while in a me

FIG. 7.-The myopic eye. It is too long. Parallel rays, shown by the solid lines, unite before reaching the retina, and must cross and fall upon it in diffusion. A concave lens causes the rays to enter the eye in a diverging manner, and they unite farther back, as shown by the dotted lines.

ridian at right angles to this, myopia or hyperopia may be found.

HYPEROPIA, OR FAR-SIGHT (H.).

Hyperopia (Fig. 6) is one of the most common conditions which the ophthalmic surgeon is called upon to treat. It depends generally upon the form of the eye, which is too short, and dates from birth. It does not increase with age, except in a slight degree after the age of fifty, but, if neglected, may pass into the reverse condition, myopia.* As, in this condition, the rays are not brought to a focus at the retina but behind it, when the eye is at rest, even distant objects are not seen clearly, and objects at near points are still less distinctly seen. But if the faculty of accommodation

* Occasionally, also, hypermetropia may arise from too feeble refracting power, on account of flattening of the cornea, or of the surfaces of the lens, or on account of absence of the lens (aphakia), or the refracting power of the aqueous humor or lens may be insufficient.

is called into exercise, distant, and, with greater effort, even nearer objects are seen clearly. The ability thus to bring the focus upon the retina will, however, depend upon the degree of hypermetropia and the power of the ciliary muscle to effect the accommodation. As this faculty of accommodation is exercised without direct consciousness on the part of the individual, the fact that one has good vision, both for far and near points, does not show that hyperopia does not exist.

It will be seen that even in viewing distant objects the accommodation must be used, and a greater demand for its exercise is made in seeing at near points. Hence hyperopic eyes are seldom at rest during waking hours, and a constant amount of contraction of the ciliary muscle is demanded. It is not surprising, therefore, that hyperopic eyes, especially if required to perform much close work, as in reading or sewing, suffer from a condition of fatigue known as accommodative asthenopia.

The symptoms and results of hyperopia are due largely to this fatigue of accommodation, but the perplexity arising from the absence of harmony between the functions of accommodation and of convergence has already been shown in the first part of this work (page 19). If the degree of hyperopia is slight and the power of accommodation active, little inconvenience may be experienced; but if the vigor of the ciliary muscle is diminished, the eyes become painful, a dull, aching sensation is felt in and about the brows, the patient complains that letters and small objects become, after a short use of the eyes, indistinct. The