Each of these points will impress a retinal element, and the result will be the perception of a transverse line; but this will not be the same for the points c, d, e, f, g, situated in space in a linear series, in the sagittal direction, only one of those points, c, will impress the corresponding retinal element, and we can see only one point at a time in the line c g. By accommodating successively, however, for the various points at different and considerable distances along the line c g, we may excite retinal elements in rapid succession. Thus, partly by the fusion of the successive impressions on the retina, and partly from the muscular sensations caused by repeated accommodations and possibly of ocular movements, we obtain a notion of depth in space, even with the use of only one eye. It is, however, one of the chief effects of binocular vision to give precision to the notion of space in the sagittal direction.
(d) Visual Sensations are Continuous.—Suppose the image of a luminous line falls on the retina, it will appear as a line although it is placed on perhaps 200 cones or rods, each of which may be separately excited, so as to cause a distinct sensation. Again, on the same principle, the impression of a superficial surface may be regarded as a kind of mosaic, made up of individual portions corresponding to the rods or cones on which the image of the surface falls. But in both cases the sensation is continuous, so that we see a line or a surface. The individual images are fused together.
2. Notions derived from Visual Perceptions.—When we look at any object, we judge of its size, the direction of its surfaces (unless it be a point), its distance from the eye, its apparent movement or fixedness and its appearance of solidity.
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Fig. 26.—Diagram to illustrate Illusions of Size and Distance. | ||||||||||
(a) Apparent Size.—This, so far as regards a comparatively small object, depends on the size of the retinal image, as determined by the visual angle. With a very large object, there is an appreciation of size from the muscular sensations derived from the movements of the eyeball as we “range” the eye over it. It is difficult to appreciate the distance separating two points between which there are other points, as contrasted with an apparently similar distance without intermediate points. For example, the distance A to B appears to be greater than from B to C, in fig. 26.
(b) Direction.—As the retina is a curved surface, a long
straight line, especially when seen from a distance, appears
curved. In fig. 27 a curious illusion of direction, first shown
Fig. 27.—Zoellner's Figure showing an
Illusion of Direction.
by J. K. F. Zoellner,
is depicted. If these
lines be looked at
somewhat obliquely,
say from one corner,
they will appear to
converge or diverge,
and the oblique lines,
on each side of the
vertical lines, will
appear not to be
exactly opposite each
other. But the
vertical lines are parallel,
and the oblique lines
are continuous across
them. The effect is
evidently due to an
error of judgment,
as it may be
controlled by an intense
effort, when the lines will be seen as they really are.
(c) Apparent Distance.—We judge of distance, as regards large objects at a great distance from the eye—(1) from their apparent size, which depends on the dimensions of the visual angle, and (2) from the interposition of other objects between the eye and the distant object. Thus, at sea, we cannot form, without great experience, an accurate estimate of how many miles we are off the coast, and all know how difficult it is to estimate accurately the width of a river. But if objects be interposed between the eye and the distant object say a few vessels at different distances at sea, or a boat in the river, then we have certain materials on which to form a judgment, the accuracy of which, however, even with these aids, will depend on experience. When we look at a near object, we judge of its distance chiefly by the sense of effort put forth in bringing the two lines of regard to converge upon it.
(d) The Movement of a Body.—If the eye be fixed, we judge
Fig. 28.—Illustrating
Stereoscopic Vision.
of movement by successive portions
of the retina being affected, and
possibly also by a feeling of an
absence of muscular contractions
necessary to move the eyeballs.
When the eye moves, so as to
“follow” the object, there is a
sense of muscular effort, which is
increased when, in addition, we require to move the head.
(e) The Apparent Solidity of an Object.—If we look at an object, say a cube, first with the right eye and then with the left, it will be found that the two images of the object are somewhat different, as in fig. 28. If, then, by means of a stereoscope, or by holding a card between the two eyes, and causing a slight convergence of the eyes, the two images are brought upon corresponding points of the two retinae, the image will at once be seen in relief.
See also article “Vision” by W. H. R. Rivers in Schäfer's Text-Book of Physiology, vol. ii. p. 1026. (J. G. M.)
7. Errors of Refraction and Accommodation and their Curative Treatment
The following is a classification of the diseases of vision, from a medical point of view (see also Eye: diseases):—
- a. Errors of refraction: hyperopia, myopia, astigmatism, anisometropia, aphakia.
- b. Errors of accommodation:—
- (1) Loss of accommodation
- (a) From advancing years (presbyopia), or from debility.
- (b) From paralysis (cycloplegia) due to—
- 1. Drugs such as atropine.
- 2. Systemic poisons: diphtheria, influenza, syphilis, &c.
- 3. Diseases of the nervous system, concussion of the brain.
- (2) Spasm of accommodation.
- (3) Meridional asymmetrical accommodation by means of which low errors of astigmatism are corrected, producing eye-strain.
- (1) Loss of accommodation
Hyperopia or Hypermetropia (H.) (Far-sight; German = Uebersicht).—This is a condition of the refraction of the eye in which, with the eye at rest, parallel rays of light focus beyond the retina, which means that the image of a distant object is not in focus when it meets the retina, because the eye is too short antero-posteriorly. Most eyes at birth are hyperopia, but as the child grows the eye also grows; when, however, this does not take place, or does not take place sufficiently, normal development is thus arrested. There are other conditions that cause hyperopia, but this shortening of the antero-posterior axis is by far the commonest.
Hyperopia is corrected by convex glasses (fig. 29), and the measurement of the hyperopia is that convex glass which enables the hyperopic eye, at rest, to see distinctly objects at a distance. When the hyperopia is not too high it can also be corrected by the eye itself by means of the ciliary muscle (muscle of accommodation) which causes the crystalline lens to become more convex, and thus brings about the same result as placing a convex glass before the eye.
In young people when the error is not too high this work is done unconsciously, vision appears to be perfect, and it is only by placing the eye under the influence of atropine that
