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

The Mind's Eye 221But here Wheatstone ran into a problem, one that still challenges allstereoscopic gadgets. The brain physically adjusts the eyes to the depthof a surface in two ways. First, though I have been describing the pupilas if it were a pinhole, in fact it has a lens to accumulate many rays oflight emanating from a point in the world and to focus them all at a pointon the retina. The closer the object, the more the rays have to be bent forthem to converge to a point rather than to a blurry disk, and the fatter thelens of the eye has to be. Muscles inside the eyeball have to thicken thelens to focus nearby objects and flatten it to focus distant objects.near objectneeds fat lensfar objectneeds skinny lensThe squeezing is controlled by the focusing reflex, a feedback loop thatadjusts the shape of the lens until the fine detail on the retina is at amaximum. (The circuit is similar to the one used in some autofocus cameras.)Poorly focused movies are annoying to watch because the brainkeeps trying to eliminate the blur by accommodating the lens, a futilegesture.The second physical adjustment is to aim the two eyes, which areabout two and a half inches apart, at the same spot in the world. Thecloser the object, the more the eyes must be crossed.near object needshighly crossed eyesfar object needsless crossed eyes