physiological inquiries, and many suppositions and speculations were from time to time accepted. The first to discover and to demonstrate the actual changes which occur in the exercise of this important function was Dr. Thomas Young.

From the era of Kepler until the time of Dr. Young's contributions to the "Philosophical Transactions" in 1801, much had been written and but little had been known of the nature of this faculty possessed by the normal eye of adapting itself to bring to a focus rays of light emanating from points at different distances. Young, by experiments, and by what, had they been properly understood, should have been regarded as conclusive arguments, showed that the change of focal adjustment of the eye in accommodation depends upon alteration in the degree of convexity of the crystalline lens. A similar hypothesis had previously been held, but no demonstrations had been adduced.

Little attention was paid to Young's theory until Helmholtz and Cramer, working independently, proved by mathematical and ocular demonstrations the truth of the theory. This important physiological problem having been solved, it remained to others, and notably to the illustrious Professor Donders, to develop the theories of accommodation and refraction in respect to individual defects. The result of Professor Donders's labors in this direction were given to the world in his great work, "On the Anomalies of Accommodation and Refraction of the Eye," published in

1864.

According to the present knowledge of the function of accommodation, the ciliary muscle, a small muscular ring situated in the interior of the eye and surrounding the border of the crystalline lens, acting upon the lens in such a manner as to modify its curvatures, and hence its refracting power, is the seat of the faculty of accommodation.

According to the investigations of Cramer and Helmholtz, it is shown that in the act of accommodating the eye for near points the lens becomes convex, its anterior surface advancing toward the cornea, while the posterior surface remains nearly stationary, a change produced by the contraction of the ciliary muscle. When this contraction is discontinued, the lens resumes its original form, and the eye is adjusted for distance. The modification of the convexity of the lens, when accommodated for distance and near points, is well shown in the accompanying diagram :

FIG. 4.

In Fig. 4 parallel rays are shown by the solid lines which enter the eye, where they undergo refraction

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, va

rying 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 hypermetro

pia or hype

ropia (Fig. 6).