ABSTRACT
Euclid observed that wheu two objects move at equal speed, the more remote one seems to move more slowly, This is motion parallax, It is comparable with thc perspective elfect whereby the remoter object seems smaller, in that either elfect can reveal the comparative distances of the objects, It is dilferent from the stereoscopic elfect. or binocular parallax, for it is j ust as informative to a one-eyed as to a two-eyed observer. It operates equally whether the objects or the ob server are moving, Mere head movement can produce motion perspective, even where objects are almost too distant to be seen stereoscopicaIly, When a stalking cat. or a snake, waves its head to and fro, it is nqt 'hypnotizing its prey', but range-finding by motion parallax; perhaps also separating its prey from a background that tends to camouflage it. This is not stereoscopic vision, but a good substitute for it.
