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Regarding the perception of motion, Fechner argues:
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When two impressions arrive too quickly one after the other, they fuse into one uniform sensation, and one might ask how great the interval between them would have to be, so that they could still be perceived as distinct. One cannot give a purely experimental answer to this question for reasons analogous to those which apply to other thresholds of space perception, for each impression leaves an aftereffect, just as each impression is surrounded by an irradiation circle. If this aftereffect of the first impression is still strong enough when the second one does not reach the differential threshold of intensity, then one impression must uniformly fuse with another.62
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Motion requires a directional component in either its stimulus or its percept. This directional component makes motion different from other forms of spatio-temporal modulation. The proximal stimulus for the vast majority of motion experiences is some corresponding movement of the retinal image.63
This correspondence has led some students of perception to explain motion perception in terms of various physical correlates in the stimulus.64
The detection of movement is a primary function of virtually every sort of visual system, and it has obvious biological utility. But, detection of movement, or even of relative velocities, is not sufficient to account for the way we perceive the environment as a stable frame of reference within which we orientate ourselves, in which we move, and in which other objects are observed to be in motion: We must account for the ability to discriminate between the visual effects of self-produced movement and the visual effects of other forms of motion in the environment.65
Again quoting Ibn al-Haytham:
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As for motion, sight perceives it by inference from comparing the moving object with other visible objects. For when sight perceives a moving object together with other visible objects, it perceives the position of the object in relation to the others and its alignment with them. If the object is moving, but those objects do not share in the same motion, then the position of that object will vary in relation to those objects while in motion. And if sight perceives it together with those objects and perceives its position with respect to them, then it will perceive the object's motion. Sight therefore perceives motion by perceiving the varying position of the moving object in relation to other objects. Sight perceives motion in one of three ways: by comparing the moving object with other objects, or with a single object, or with the eye itself.66
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That celestial magnitudes appear larger at the horizon than at higher altitudes is a commonly known phenomenon that has been recorded and investigated since antiquity. Because the phenomenon is particularly noticeable in the case of the moon, it has sometimes been referred to in recent times as the "moon illusion," a designation also reflecting the accepted understanding of the apparent enlargement as a psychological effect. Ibn al-Haytham freed himself from the erroneous view in the Almagest and, setting off from a new level of understanding some of the elements of which he found in Ptolemy's Optica, he offered in his Book of Optics psychological explanations in terms of what modern psychologists have called, with some exaggeration, the size-distance constancy principle.67
The moon illusion refers to the phenomenon that the moon appears larger when it is viewed at the horizon than at the zenith, although the projected images in both cases are identical. In fact, the moon occupies a far smaller fraction of the visible sky than most individuals assume.68
Boring proposed that the apparent size of the moon is affected by the angle of the eyes relative to the head.69
That is, the moon illusion is produced by changes in the position of the eyes in the head accompanying changes in the angle of elevation of the moon. He concluded that the moon illusion depends on raising or lowering the eyes with respect to the head. Mere movements of the neck, head, and body are not causal factors. There is, however, no convincing psychological process to explain Boring's general findings.70
He stated in 1943 that there is no satisfactory theory for explaining this phenomenon. It is not due to physical causes outside the visual mechanism. It is not due to the greater brightness of the moon in elevation, when atmospheric haze is diminished. It depends on raising or lowering the eyes. Movements of the head, neck, and body do not cause it.
Ibn al-Haytham offered his psychological explanation of the moon illusion at the end of the last chapter in Book VII of his Optics, a chapter devoted to the "errors of sight" due to refraction.71
He wrote that sight perceives any star at the zenith to be smaller than in any region of the sky through which the star travels; that the farther the star is from the zenith the larger its magnitude will appear; that the star looks largest at the horizon; and that the same is true of the intervals between the stars. Now this is found to be so in fact, namely, that the stars, and their mutual distances, appear to be smaller in the middle of the sky than when they are far from it, and that the star (or interval between two stars) appears largest at the horizon. Why this is so remains to be shown. In Book II in the discussion of size, it is shown that sight perceives size from the magnitude of the angles subtended at the center of the eye and from the magnitudes of the distances of the visible objects and from comparing the magnitudes of the angles to those of distances.72
Perhaps the most remarkable series of neglected theories and discoveries in Ibn al-Haytham's book are those concerning binocular vision, which are set out in chapter 2 of Book III. His ideas on how the optic nerves combine in the optic chiasma are described in Book I and are derived from Galen. These ideas have been well cited; however, Howard and Rogers found no reference to the ideas on binocular vision contained in Book III, and most of these ideas were not described again until the nineteenth century.73
Ibn al-Haytham mentions that an object appears double when one eye is pushed by the finger and, as Galen pointed out, when the visual axes converge on the object of interest. Ibn al-Haytham writes:
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When one eye moves for the purpose of vision, the other eye moves for the same purpose and with the same motion; and when one of them comes to rest, the other is at rest.74
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Hering cited Ibn al-Haytham on this point, which is now referred to as Hering's Law of Equal Innervation.75
In contemporary psychology, eye movements are classified in a number of ways, depending upon the kinds of analyses being made. We can separate those that are voluntary from those that are involuntary; those in which the eyes move in the socket from those in which the entire head moves; those that result in large displacements of the retinal image from those that result in small displacements; and those in which the two eyes move in parallel from those in which they move in opposition. When the eye is stationary and oriented straight ahead the three sets of muscles are under tension, each member of a pair in balance with its opposite. When a movement occurs along any one of these planes, one member of the pair contracts while the other relaxes.76
Ibn al-Haytham verified experimentally his theory of binocular vision, which involved two eyes.77
There can be no doubt, however, about the importance of Book III for the history of the psychology of vision, if only because of the large amount of material it contains and the experimental and, apparently, original concepts around which this material is organized.78
The experimental examination in these paragraphs consists of five experiments carried out by Ibn al-Haytham.
Psychology
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