The Eye adjusts itself to Vision at different Distances. 297

which though it be convenient to retain it, must be taken with some modification of its meaning. The cone of rays that enters the pupil from any visible point of an object is said to converge to a focal point upon the retina; it is obvious, that such a focal point is very different from a point in the strict mathematical sense of the word; it signifies only a very small circle.

But there is another cause, why the range of perfect vision has its limits, which has been already in part illustrated. It appears by the phenomena of myopic and presbyopic eyes, that a very nice arrangement of the rays of light upon the retina is necessary for distinct vision; it is not therefore wonderful that we cannot see perfectly at all distances; but on the contrary it ought rather to excite our admiration, that we are able to see objects with any degree of distinctness at more distances than one. When we consider how different the angle must be at which the marginal rays of each cone reach the eye from objects at different distances, it is reasonable to suppose that the focal length of the eye adapted to one case must be essentially unfit for any other. Nevertheless we are not conscious of making an effort to produce a change in the refractive power of the eye, at the time we direct our attention from a near object to one more remote.

A simple experiment, however, serves at once to prove, that when the eye is capable of seeing distinctly objects at one distance, it is unfit to distinguish objects at any other, and that we possess a voluntary power of instantaneously altering the focal length of the eye.

If a clear straight line be drawn with a pen upon a plane white surface from a foot to two feet in length, and the eye be placed just above the level of the white

298 Range of Adjustment, which the Eye possesses.

surface, and be directed along the black line, the latter will appear distinct at one point only, on either side of which it appears confused, and spread over a widening space. If the eye be fixed upon a point nearer than that first looked at, but within the limits of distinct vision, the nearer point becomes defined, and the remote point confused.

In Dr. Young's optometer a single line is seen through several narrow slits in a thin brass plate, two or more of which correspond with the aperture of the pupil. Hence it happens, that except at the point to which the eye is adjusted, the line appears double or triple; or the lines are seen to cross at the point, at which vision is distinct; and the crossing of the lines may be made to appear more or less remote by directing the attention successively to different points of the surface. By means of a convergent lens, the effect of infinite distance is given to the length of a few inches upon the optometer, and a graduated scale shows the true distance at which vision is distinct.

At eighteen or nineteen years of age, a good eye should be capable of adjusting itself to objects situated at any point between five or six inches from the eye and infinite distance, or even of bringing to a focal point upon the retina convergent rays. As life advances, the power of adjusting the eye is continually diminished by an increasing inability to distinguish near objects. Between fifty and sixty, the refractive power of an eye originally perfect is qualified to bring to a focus parallel rays only, or the power of adjustment is wholly lost. A myopic eye does not, as is usually supposed, acquire a long sight in the advance of life; it possesses at first a certain power of adjustment, as for instance, between four and nine inches; and when the power of adaptation is lost, its vision re

Mechanism by which the Eye alters its focal Length? 299

mains perfect at the remotest point to which its power of adjustment originally extended.

The mechanism by which the eye alters its focal length remains in obscurity, notwithstanding the numerous attempts that have been made to explain it.

An experiment made by Dr. Young, is fatal to the supposition that the change produced consists in an alteration of the form of the cornea. A convex lens fixed in a socket, which contained water, and the edges of which were secured with wax, was applied to the eye, so that the cornea entered half way into the socket and was every where in contact with the water: the eye immediately became presbyopic; but upon the addition of another convex lens to make up for the loss of the convexity of the cornea, vision was restored to its natural state, and the eye regained the power of adjustments.

Other experiments, made by Dr. Young, set aside the supposition that a change takes place in the length of the axis of the eye, to fit it for vision at different distances;-if experiments are indeed necessary to disprove the application of any considerable pressure on this delicate organ, of which we have no consciousness at the time when by this hypothesis it should take place.

Dr. Young himself concludes that the means of adjustment consist in a change of form in the crystalline, the fibres of which he describes, and which he supposes to be irritable. But it does not appear from direct experiment that the crystalline possesses irritability; and if faith can be attached to a single well attested observation upon a point so delicate, the instance of Henry

5. Phil. Trans. vol. xci. p. 58.

300

The Diameter of the Pupil changes,

Miles, recorded by Sir Everard Home, proves that the eye may retain its power of adjustment after the removal of this parth.

The only evident change in the eye when adjusting its focal length to different distances, is an alteration in the diameter of the pupil. The pupil enlarges when a distant object is seen, and diminishes when we look at a nearer point. Upon a superficial analogy we might conclude that these changes are sufficient to produce the requisite alterations of the focal length of the eye: for by viewing objects through a series of pinholes in a card, the largest smaller than the aperture of the pupil and each of the rest in succession smaller than the last, the eye is rendered capable of seeing distinctly at the distance of four, of three, and even of two inches. When however the correctness of this hypothetical explanation is put to the test of direct experiment, it proves to be fallacious.

In investigating the point under consideration I availed myself of the assistance of Mr. Robinson of Devonshire-street, a very ingenious artist, who makes the optometer contrived by Dr. Young, and who is very conversant with the use of that instrument.

A room was darkened by half closing the shutters, and I attentively observed the state of the pupil, when Mr. Robinson directed his eye to a definite point upon the optometer: the pupil was of course considerably dilated: the shutters being then opened, the pupil instantly contracted, but the point upon the optometer at which the lines crossed did not shift its place.

When by some practice I had accustomed my own

" Phil. Trans. vol. xcii. p. 8.

when the Focal Length of the Eye is altered. 301

eye to the use of the optometer, I compared its range in the brightest and in the obscurest light in which the lines were visible, and observed no apparent difference in the two cases. Mr. Robinson made a similar observation. Either of these experiments prove that the change in the size of the pupil is not the means by which the adjustment of the eye to distances is effected. But an additional fact may be mentioned. In an old lady of sixty-seven, whose sight in early life was remarkably good, but whose eyes can now only bring to a focus parallel rays, the pupil retains its mobility perfectly under variations of light; and even sensibly moves upon her making ineffectual attempts to read without spectacles a page held at different distances from her.

It deserves remark, that after the eye has had some practice in accommodating itself to exact vision at different distances, it is easy when an object, as for instance a screen, is held at the distance of six or seven inches, and has been for a few seconds distinctly seen, to adjust at pleasure the focal length of the eye for vision at a remoter point: under these circumstances the object held before the eye becomes confusedly seen; the optic axes diverge, and the pupil dilates. In a similar way the eye may be adjusted at pleasure to a shorter distance, at which no visible object is situated: thus a power appears to be acquired of voluntarily influencing the action of the iris.

I have already observed that one part of the retina appears habitually used for accurate vision: I cannot better illustrate this subject than by making the following extract from the Philosophical Transactions.

in

"The visual axis (observes Dr. Young) being fixed any direction, I can at the same time see a luminous