Steven Pinker -- How the Mind Works - Hampshire High Italian ...

The Mind's Eye 225The principle was discovered a century and a half ago by David Brewster,the Scottish physicist who also studied polarized light and inventedthe kaleidoscope and the Victorian-era stereoscope. Brewster noticedthat the repeating patterns on wallpaper can leap out in depth. Adjacentcopies of the pattern, say a flower, can each lure one eye into fixating onit. That can happen because identical flowers are positioned at the sameplaces on the two retinas, so the double image looks like a single image.In fact, like a misbuttoned shirt, a whole parade of double images canfalsely mesh into a single image, except for the unpaired members ateach end. The brain, seeing no double image, is prematurely satisfiedthat it has converged the eyes properly, and locks them into the falsealignment. This leaves the eyes aimed at an imaginary point behind thewall, and the flowers seem to float in space at that distance. They alsoseem inflated, because the brain does its trigonometry and calculateshow big the flower would have to be at that depth to project its currentretinal image.what the eyes should dowhat the repeating patterntricks the eyes into doingAn easy way to experience the wallpaper effect is to stare at a tile walla few inches away, too close to focus and converge on comfortably.(Many men rediscover the effect as they stand at a urinal.) The tiles infront of each eye easily fuse, creating the surreal impression of a verylarge tile wall a great distance away. The wall bows outward, and as thehead moves from side to side the wall rocks in the opposite direction.Both would have to happen in the world if the wall were really at thatdistance while projecting the current retinal image. The brain createsthose illusions in its headlong attempt to keep the geometry of the wholehallucination consistent.