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be demonstrated by what is known as Scheiner's experiment, which is carried out in the following way.
Two pin holes are made in a card at a distance from each other not wider than the diameter of the pupil. The card is then brought close to the eye, so that a small object-such as the head of a bright pin-can be seen through the holes. The dioptric media being fixed, moving the object
To illustrate Scheiner's experiment; for exnearer to or further from the
planation, see text. have the same effect as changing the relation of the retina to m n or p q in Fig. 225, by means of which we may explain the following observations: (1) The eye being fixed upon the object (of which only one image is seen), move the pin rapidly away ; two objects now appear, showing that the rays coming through the holes have met before they reach the retina, as at p 9. (2) Move the pin near the eye; again two very blurred objects are seen, for the rays have not met when they strike the retina, as at m n. (3) Keeping the object in the same position, alter the gaze, as if to look first at distant and then at near objects; in both extremes two images are seen. (4) When the object is in exact focus, as at c, the closure of one of the holes does not affect the single image. (5) When two images are seen, closing the right-hand hole at & causes the right or left image to disappear, according as the focus c falls short of m n or is beyond P9, the retina. (6) By moving the pin's head nearer the eye, a point is reached at which the object cannot be brought to a focus as a single image. This limit of near accommodation marks the near point. A little attention teaches us that looking at the near object requires an effort which looking at the distant one does not; in fact, we have to do something to see things near us distinctly. This act is the voluntary adjustment of the eye which we call its accommodation for near vision.
ACCOMMODATION. The difference of distance for which we can adjust our eyes is great, so that our range of distinct vision is
extensive. As already stated, the normal eye is considered to be constructed so that parallel rays of light, i. e., those coming from practically infinite distance, are brought to a focus on the retina. This is why we see the stars—which are practically infinitely remote from us-as mere luminous points. It is, therefore, impossible to fix a far limit to our power of distant vision. The nearer an object is brought to our eyes, the more effort is required to see it distinctly, until at last a point is reached where we cannot get a clear outline, no matter how we “ strain our eyes.” For a normal eye, called the emmetropic eye, this near limit is about 12 cm. or 5 inches, but it varies in different individuals.
For objects that are over 10 metres distance, very little change in the eye is required to see each distinctly, and the nearer the object approaches, the more frequently the adjustment of the eye has to be altered to see it clearly. When the eye is focused for any point within the limits of distinct vision, a certain range of objects at different distances from the eye can be recognized without moving the adjustment. The range of this power is measured on the line of vision, and called the focal depth. In the distance we can take in a great depth of landscape, without effort or fatigue ; but when looking at near objects the focal depth is less, and we must constantly accommodate our eyes afresh in order to see clearly objects at slightly different distances because of the shallowness of the focal depth in the nearer parts of visual distance.
The method by which the accommodation of the eye is effected differs from anything that can be applied to an artificial optical instrument, and is more perfect.
The following alterations are observed to occur in the eye during active accommodation, i.e., when looking at near objects : (1) The iris contracts so that the pupil becomes smaller ; (2) the central part of the anterior surface of the crystalline lens moves slightly forward, pushing before it the pupillary margin of the iris, so that the lens becomes more convex; (3) the posterior surface of the lens also becomes more convex, owing to the general change of shape of the lens, but the centre of this surface does not change its position ; (4) both eyes converge.
These changes can be seen in the accompanying diagram, showing a section of the lens, cornea and ciliary region (Fig. 226), in the left-hand side of which the lens is drawn in the position it assumes when accommodated for near objects. These movements can be seen in life by observing the changes in relative positions, etc., of the reflections of a candle flame thrown from the cornea and the two surfaces of the lens. On the cornea is seen a bright upright flame : next comes a large diffused reflection from the anterior surface of the lens, and at the other
Diagram showing the changes in the lens during accommodation. The muscle on the right
is supposed to be passive as in looking at distant objects, the ligament (L), is, therefore, tight, and compresses the anterior surface of the lens (A) so as to flatten it. On the left the ciliary muscle (M) is contracting so as to relax the ligament, which allows the lens to be
This contraction occurs when looking at near objects.
come more convex.
side of this a small, inverted image of the flame reflected from the posterior surface of the lens. When the adjustment is changed by looking from a far to a near object, the image on the front of the lens becomes smaller and moves toward the centre of the pupil. The image on the back of the lens also becomes smaller, but does not change its position. The amount of movement has been accurately measured by a special instrument called an ophthalmometer. The motions can be more exactly studied by means of the phakoscope, a dark box, in which prisms are placed before the observed eye, and each image is made double. The change in relative position of the two is more readily recognized than a mere change of size of the one.
Muscular Mechanism of Accommodation.—The alteration in the shape of the lens is accomplished by the action of the muscular layer, already named, which radiates from the edge of the cornea to the ciliary region of the choroid coat, where it is attached. When the ciliary muscle contracts, it draws the choroid coat and the connections of the suspensory ligament of the lens slightly forward, the junction of the cornea and sclerotic being its fixed point. Under ordinary circumstances, the eye being at rest, the suspensory ligament is tense and exerts a radial traction on the anterior part of the capsule of the lens, tending to stretch it fat; this affects the shape of the soft lens and reduces its convexity. When the ciliary muscle shortens, it draws forward the attachment of the suspensory ligament, relaxes it, and removes the tension of the capsule, so that the unconstrained elastic lens bulges into its natural form. The posterior surface cannot extend backward, because there it is in contact with the vitreous humor, which is held more firmly against it by the increased tension of the hyaloid membrane during the contraction of the ciliary muscle.
Some circular muscular fibres help to relax the ligament and relieve it from the increased pressure which the contraction of the radiating fibres must indirectly cause on the vitreous humor.
The act of accommodation is a voluntary one, the nerve bearing the impulse to the ciliary and iris muscles, coming from the 3d nerve by the ciliary branches of the lenticular ganglion. The local application of the alkaloid of the belladonna plant (atropin) causes paralysis of the ciliary muscle and wide dilatation of the pupil; and the alkaloid of the Calabar bean (physostigmin) produces contraction of the muscle of accommodation and extreme contraction of the pupil.
DEFECTS OF ACCOMMODATION. Myopia. It has been said that the “ near limit” of distinct vision differs in many persons from the twelve centimeters of the normal emmetropic eye, and it is found that the power of accommodation varies very much in different individuals. Thus, in "short-sighted" people, who have myopic eyes, i. e., in which
parallel rays are focused short of the retina, the near limit may only be half the normal, i. e., five centimeters, and the far limit, which is normally indefinite, is found to be within a comparatively short distance of the eye. They, therefore, cannot see distant objects clearly, since the rays are focused before the retina is reached, and then diverging, cause diffusion circles and a blurred picture. The work of their accommodation is also much more laborious, since they can only see in that part of the range of accommodation where the adjustment has to be altered for slight variations of distance. The defect can be made much less distressing by the use of concave glasses, which make parallel rays strike the cornea as divergent ones, and thus allow them to be focused on the retina.
Hypermetropia.—Another abnormality is “ long sight.” In the
Showing the course of the rays of light from two luminous points to the retina. The rays
from the point a on passing through the cornea, lens, etc., are collected on the retina at b. Those from a' meet b', and thus the lower point becomes the upper.
hypermetropic eye, parallel rays of light are brought to a focus at a point beyond the retina, so that divergent or parallel rays cause diffusion circles and a blurred image. This may be corrected by means of convex glasses, which make the rays convergent before they strike the corneal surface, and thus enable them to be sooner brought to a focus by the dioptric media of the eye.
Presbyopia is the name given to a change in the perfectness of accommodation frequently accompanying old age. The lens probably becomes less elastic and the ciliary muscle weaker, so that the change in form required to see near objects is difficult or impossible to attain. Biconvex lenses help to overcome the difficulty.