It may be shown that the distance c b′ corresponds to the distance of the retinal vessels from the layer of rods and cones. If the light enter the cornea, as in the figure to the right, and if the light be moved, the image will be displaced in the same direction as the light, if the movement does not extend beyond the middle of the cornea, but in the opposite direction to the light when the latter is 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 b′. If, on the other hand, a be moved above the plane of the paper, d will move below, consequently c will 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).
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Fig. 10.—Purkinje's Figures. |
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In the eye to the right the illumination is through the sclerotic, and in the one to the left through the cornea. |
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Fig. 11. |
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A, Emmetropic or normal eye; B, Myopic or short-sighted eye; C, Hypermetropic or long-sighted eye. |
4. Accommodation, or the Mechanism of Adjustment 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 emmetropic (fig. 11, 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 short-sighted, 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 hypermetropic, 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 hypermetropic by convex glasses. In the first case, the concave glass will move the posterior focal point a little 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 remotum, or the most distant point seen without accommodation. In the myopic eye it is much nearer, and for the hypermetropic, there is really no such point, and accommodation 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 proximum, 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,
Fig. 12.—Reflected
Images in the Eye.
from the anterior surface of the cornea;
the second, b, also erect, but dim, from the
anterior surface of the crystalline 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
distant vision, it will be found that the middle
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:
(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 eye. 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 emmetropic eye, the crystalline lens is flattened
anteriorly by the pressure of the anterior layer of the capsule;
during accommodation, the radiating fibres of the ciliary muscles
pull the ciliary processes forward, thus relieving the tension
of the anterior layer of the capsule, and the lens at once bulges
forward by its elasticity.
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Fig. 13.—Mechanism of Accommodation. |
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A, The lens during accommodation, showing its anterior surface advanced; B, The lens as for distant vision; C, Position of the ciliary muscle. |
By this mechanism the radius of curvature of the anterior
