Visual disparity can still be exploited over greater distances by using optical devices that magnify the parallax distance separately for each eye. The phenomenon of binocular disparity functions primarily in near space because the angular difference between the two retinal images diminishes when viewing objects at a distance. The greater the parallax difference between the two retinal images, the closer the object is perceived to be. Thus, in looking at the indicator needle on a pressure gauge, for example, the effects of parallax will cause a person to make slightly different readings when using first the left eye alone and then the right eye. The degree of disparity between the two retinal images-a phenomenon known as binocular parallax-depends on the difference between the angles at which an object is fixed by the right eye and by the left eye. The two retinal images of the same object are apparently perceived by the brain as a three-dimensional experience. Because the eyes are imbedded at different points in the skull, they receive slightly different images of any given object. Perhaps the most important perceptual cues of distance and depth depend on so-called binocular disparity.
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