It may be shown that the distance c b' corresponds fo the distance of the retinal vessels from the layer of rods and cones. If the light enter

the cornea, as la

the figure to the

right, and if the

light be moved,

the image v.-i ll

be displaced in

the same direction

as the light,

if the movement

does not extend

beyond

the

middle of the

j, -y

cornea, but in the

Fig. 10.— Purkinje's Figures.

opposite direction

In the eye to the right the illumination is to the light through the sclerotic, and in the one to the when the latter left through the cornea.

js moved up and

down. Thus, if a be moved to a', d will be moved to d', the shadow on the retina from c to c', and the image b to h'. If, on the other

hand, a be moved above the plane of the paper, d will move below, consequently c wUl move above, and b' will appear to sink. (2) The retinal vessels may also be seen by looking at a strong light through a minute aperture, in front of which a rapid to-and-fro movement is made. Such experiments prove that the sensitive part of the retina is its deepest and most external layer (Jacob's membrane).

•

4. Accommodation, or the Mechanism of Adjuslmcnl for Different Distances.— When a camera is placed in front of an object, it is necessary to focus accurately in order to obtain a clear and distinct image on the sensitive plate. This may be done by moving either the lens or the sensitive plate backwards or forwards so as to have the posterior focal point of the lens corresponding with the sensitive plate. For similar reasons,

a mechanism of adjustment, or accommodation for different distances, is necessary in the human eye. In the normal eye,

any number of parallel rays, coming from a great distance, are focused on the retina.

Such an eye is termed em metro pic (fig. II, A). Another form of eye (B) may be such that parallel rays are brought to a focus in

front of the retina.

This form

of eye is myopic or shortsighted, inasmuch as, for distinct

vision, the object must be

brought near the eye, so as to

catch the divergent rays, which are then focused on the retina. A third form is seen in C, where the focal point, for ordinary

distances, is behind the retina, and consequently the object

must be held far off, so as to

allow only the less divergent or parallel rays to reach the eye. This kind of eye is called hyper metro pic, or far-sighted. For

ordinary distances, at which

objects must be seen distinctly in everyday life, the fault of

the myopic eye may be corrected by the use of concave and of

the hyper metro pic by convex

glasses.

In the first case, the

posterior focal point a little

Fig.

A, Emmetropic or normal eye;

B, Myopic or short-sighted

eye; C, Hypermetropic or

long-sighted eye.

cohcave glass will move the

farther back, and in the second the convex glass will bring it farther forwards; in- both cases, however, the glasses may be so adjusted, both as regards refractive index and radius of curvature, as to bring the rays to a focus on the retina, and consequently secure distinct vision. From any point 65 metres distant, rays may be regarded as almost parallel, and the point will be seen without any effort of accommodation. This point, either at this distance or in infinity, is called the punctum rtmotum, or the most distant point seen without accommodation. In the myopic eye it is much nearer, and for the hyper metro pic, there is really no such point, and accomm.odation is always necessary. If an object were brought too close to the eye for the refractive media to focus it on the retina, circles of diffusion would be formed, with the result of causing indistinctness of vision, unless the eye possessed some power of adapting itself to different distances. That the eye

has some such power of accommodation is proved by the fact that, if we attempt to look through the meshes of a net at a. distant object, we cannot see both the meshes and the object with equal distinctness at the same time. Again, if we look

continuously at very near objects, the eye speedily becomes fatigued. Beyond a distance of 65 metres, no accommodation is necessary; but within it, the condition of the eye must be adapted to the diminished distance until we reach a point near the eye which may be regarded as the limit of visibility for near objects. This point, called the punctum proximiim, is usually 12 centimetres (or 4-8 inches) from the eye. The range of

accommodation is thus from the punctum remotum to the punctum proximum.

The mechanism of accommodation has been much disputed, but there can be no doubt it is chiefly effected by a change in the curvature of the anterior surface of the crystalline lens. If we hold a lighted candle in front and a little to the side of an eye to be examined, three reflections may be seen in the eye, as represented in fig. 12 . The first, a, is erect, large and bright, from the anterior surface of the cornea; the second, 6, also erect, but dim, from the anterior surface of the cry^stalline lens; and the third, c, inverted, and very dim, from the posterior surface of the lens, or perhaps the concave surface of the vitreous humour to which the convex surface of the lens is adapted. Suppose the three images to be in the position shown in the figure for Fig. 12 . — Reflected distant vision, it will be found that the middle Images in the Eye. image b moves towards a, on looking at a near object. The change is due to an alteration of the curvature of the lens, as shown in fig. 13. The changes occurring during accommodation are: Fig. 13. — Mechanism of Accommodation. A, The lens during accommodation, showing its anterior surface advanced; B, The lens as for distant vision; C, Position of the ciliary muscle.

(1) the curvature of the anterior surface of the crystalline lens increases, and may pass from 10 to 6 mm.; (2) the pupil contracts; and (3) the intraocular pressure increases in the posterior part of the eyA. An explanation of the increased curvature of the anterior surface of the lens during accommodation has been thus given by H. von Helmholtz. In the normal condition,

that is, for the em metro pic eye, the crystalline lens is flattened anteriorly by the pressure of the anterior layer of the capsule; during accomm.odation, the radiating fibres of the ciliary muscles pull the ciliary processes forivard, thus relieving the tension of the anterior layer of the capsule, and the lens at once bulges forward by its elasticity.

By this mechanism the radius of curvature of the anterior