faces would be readily manifested by an approach or recession of the two images belonging to it. In accommodation for remote objects (Fig. 200, 4), the two images from the anterior surface of the lens are of considerable size and somewhat widely separated; in accommodation for near objects (B), they diminish in size and approach each other. The double reflections from the cornea and the posterior surface of the lens, remain at sensibly the same distance from each other in both states of accommodation.

Fig. 200.

CHANGE OF POSITION IN DOUBLE CATOPTRIC IMAGES during accommodation.-4. Position of the images in accommodation for distant objects. B. Position of the images in accommodation for near ob

The advance of the iris and pupil, in consequence of the protrusion of the anterior face of the lens, as remarked by Helmholtz, can also be observed directly, by looking into the eye from the side. If the observer look from this direction so as to obtain a profile view of the cornea and part of the sclerotic between the opening of the eyelids, he will see the dark pupil in perspective under the form of an upright elongated oval, a little in front of the edge of the sclerotic. The person under observation fixes his sight upon a distant object, and the observer places himself steadily in such a position that the hither edge of the iris is just concealed by the anterior border of the sclerotic. If the sight be now shifted from the distant to a near object, in the same linear range, the pupil visibly advances toward the cornea, and the edge of the iris shows itself a little from behind the edge of the sclerotic. If the sight be again directed to the distant object, the pupil recedes and the edge of the iris disappears, as before, behind the sclerotic.

jects. a. Corneal image. b. Image from anrior surface of lens. (Helmholtz.)

terior surface of lens. c. Image from poste

The accommodation of the eye for near objects is therefore produced by an increased refractive power of the lens, from the greater bulging

of its anterior face. This has the effect of increasing the rapidity of convergence of rays passing through it, and consequently compensates for their greater divergence before entering its substance. In the ordinary condition of ocular repose, when the eye is directed to distant ob

jects, the rays coming from any point of such an object arrive at the cornea in a nearly parallel position, and are then refracted to such a degree that they meet in a focus at the retina (Fig. 201). When the eye is directed to a nearer point (Fig. 202), the lens increases its anterior convexity; and the divergent rays, being more strongly refracted, are still brought to a focus at the retina, as before. It thus becomes possi-' ble to fix alternately, in distinct vision, objects at various distances in front of the eye.

Mechanism of the Change in Figure of the Lens in Accommodation.The mechanism by which the lens is rendered more convex, in vision for near objects, is far from being completely demonstrated. The reasons have already been given which lead to the conclusion that it is accomplished, in some way, by muscular action; and the two muscles which, separately or together, undoubtedly produce this change, are the iris and the ciliary muscle.

The iris certainly contracts in accommodation for near objects. This is easily observed on examining by daylight the pupil of an eye which is alternately directed to near and remote objects. The pupil visibly diminishes in size when the eye is fixed upon a point near by, and again enlarges when the sight is accommodated for the distance. The movements of the ciliary muscle, on the other hand, are not subject to observation; but the attachments and position of this muscle have led many writers to attribute to it an important, if not the principal, part in causing a change of form in the crystalline lens.

So far as we are at present able to form a judgment on this question, it may be said that the diminution in size of the pupil is not by itself an efficient cause of accommodation; since, according to Helmholtz, if the observer look through a perforated card, the orifice of which is smaller than the pupil, near objects still appear indistinct when the sight is directed to the distance, and vice versa, notwithstanding the invariable dimensions of the artificial pupil thus employed. The contraction of the circular fibres of the sphincter pupillæ must, therefore, have for its probable object to fix the inner border of the iris, thus affording an internal point of attachment for the radiating fibres of the same muscle. These fibres have for their external attachment the elastic tissue at the inner wall of the canal of Schlemm (Fig. 189); and from this circle also arise the fibres of the ciliary muscle, which radiate outward and backward to their final attachment at the surface of the choroid membrane. If the circular and radiating fibres of both these muscles contract together, they will form a connected system, which may exert a pressure upon the borders of the lens, sufficient to cause the protrusion of its anterior face at the pupil, where alone its advance is not resisted. The aqueous humor, displaced by the protrusion of the lens, may find room in the external parts of the anterior chamber, where the outer border of the iris recedes, under the traction of the ciliary muscle. These are the general features of the mechanical action in accommodation, as it is generally supposed to take place. At the same

zime, its details are by no means clearly understood; and explanations, varying more or less from that given above, have been proposed by observers of very high authority. The direction and degree in which pressure would be exerted, by muscular fibres attached like those in the interior of the eye, are too imperfectly known to warrant a positive statement in this respect.

Limits of Accommodation for the Normal Eye.-The normal eye is so constructed that rays emanating from a single point, though coming from an indefinite distance, and therefore sensibly parallel to each other, are brought to a focus at the retina (Fig. 203). Vision is accordingly distinct, even for the heavenly bodies, provided their light be neither too dim nor too excessive in brilliancy. For bodies situated nearer to the eye, the convexity of the lens increases with the diminution of the distance, and vision still remains perfect. But there is a limit to the change of shape which the lens is capable of assuming; and when this limit is reached, a closer approximation of the object necessarily destroys the accuracy of its image. For ordinary normal eyes, in the early or middle periods of life, accommodation fails and vision becomes indistinct, when the object is placed at less than 15 centimetres (6 inches) from the eye. Between these two limits, of 15 centimetres and infinity, the amount of accommodation required is by no means in simple proportion to the variation of the distance. The change of accommodation necessary for objects situated respectively at 15 and 30 centimetres from the eye (6 inches and 12 inches), is much greater than that corresponding to the distances of one yard and two yards. The farther the object recedes from the eye, the less difference is produced, in the sensible divergence of the rays, by any additional increase of distance; and consequently less variation is required in the refractive condition of the eye to preserve the accuracy of its image. It is generally found that no sensible effort of accommodation is required for objects situated at any distance beyond fifty feet from the observer; while within this limit the amount of accommodation necessary for distinct vision increases rapidly with the diminution of the distance.

An eye which is capable of accommodating for distinct vision, throughout the whole range included between 15 centimetres and an indefinite distance, is, in this respect, a normal eye, and is said to be emmetropic; that is, its powers of accommodation are placed within the natural limits or measurements of this function.

Presbyopic Eye.-The power of accommodation diminishes naturally with the advance of age; and observation shows that this diminution dates from the earliest period of life. Infants often examine minute objects at very short distances, in a manner which would be quite impracticable for the healthy adult eye; and the minimum distance of distinct vision at twenty years of age is placed by some writers at ten centimetres instead of fifteen. The power of increasing the convexity of the lens to this extent is soon lost; and, as it continues to diminish, a time arrives, usually between the ages of 40 and 50 years, when the incapacity

of accommodation for near objects begins to interfere with the ordinary occupations of life. When this condition is established the eye is said to be presbyopic. Its vision is still perfect for distant objects, but it can no longer adapt itself for the examination of those in close prox imity to the eye. To remedy this defect the patient employs a convex eye-glass, which replaces for him the increased convexity of the crys talline lens, in accommodation for near objects; and by the aid of such a glass he is able to read or write at ordinary distances and in characters of the ordinary size.

The use of a convex eye-glass does not restore the perfection of sight as it existed beforehand. In the normal eye, the degree of accommodation varies for every change of distance within fifty feet; and the organ is thus adjusted, by an instantaneous and unconscious movement, for the most delicate variations of refractive power. But an eye-glass, the curvatures of which are invariable, can give perfect correction only for a single distance. A glass is, therefore, usually selected of such a strength as to serve for the most convenient distance in the ordinary manipulation of near objects.

MYOPIC EYE, in vision at long distances. (Wundt.)

Myopic Eye. In many instances, where the eye is otherwise of normal configuration, its antero-posterior diameter is longer than usual, thus placing the retina at a greater distance behind the lens. The consequence of this peculiarity is that while the luminous rays are brought to a focus at the usual distance from their point of entrance into the eye, this focus is situated within the vitreous body; and the rays reach the retina only after they have crossed and suffered a partial dispersion.

(Fig. 204.) This produces an indistinct image for all remote objects. Within, however, a certain distance from the eye, the rays enter the pupil under such a degree of divergence, that their focus behind the lens falls at the situation of the retina, and the object is distinctly seen. Such an eye is said to be myopic, or, in ordinary language, "near sighted," because its range of distinct vision is confined to objects situated comparatively near the eye. The flexibility of the lens, and its en pacity for increasing its convexity, may be, in the myopic eye, fully up to the normal standard, and consequently its power of accommodation may be as great in reality, though not in distance, as that of the normal eye. In the emmetropic condition, a certain degree of variation in the curvature of the lens produces the necessary change of accommodation for any distance between, 15 centimetres and infinity. In the myopic eye the same amount of accommodating power may be present, though perfectly distinct vision be confined between the distances of 8 and 20 centimetres. The myopic eye consequently has distinct vision at shorter distances than a natural one, but gives an imperfect image for remote objects.

The remedy adopted for the myopic eye is to employ a concave eyeglass, which increases the divergence of the incident rays. This enables the eye to bring parallel or nearly parallel rays to a focus situated farther back than it would otherwise fall, and at the actual position of the retina; thus giving distinct vision for remote objects. As the accommodative power is normal in amount, this contrivance restores completely the perfection of sight, in a myopic eye which is otherwise well-formed; and the patient can then accommodate accurately for all distances within the natural limits of distinct vision.

Apparent Position of Objects, and Binocular Vision.-The apparent position of an object is determined by the direction in which the luminous rays pass from it to the interior of the eye. The perception of the light itself necessarily marks the direction from which it has arrived, and therefore the apparent position of its source. It is difficult to understand fully the precise physiological conditions which cause this appreciation of the path followed by a luminous beam; although there seems reason for the belief that it is in some way connected with the position of the rods and cones which stand perpendicularly to the curved surface of the retina, and thus receive the impression of a ray, if at all, in the direction of their longitudinal axes. But whatever may be the optical or physiological mechanism of the process, its plain result is that a ray coming from below attracts attention to the inferior part of the field of vision; and one coming from above is referred to its point of origin in the upper part of the same field. Thus if two luminous points appear simultaneously in the field of vision, they present themselves in a certain position with regard to each other, above or below, to the right or the left, according to the direction in which their light has reached

the eye.