The vertebrate single-lens eye.

(a) The structure of the human eye. (b) Changes in lens shape during accommodation. When an object is near, the ciliary muscles contract and the lens becomes rounder, causing light to bend more. When the object is far away, the ciliary muscles relax and the lens flattens. (c) Demonstration of the blind spot. First, hold the picture up in front of your face. Next, close your left eye and stare at the black spot with your right eye while you move the picture toward and away from your face. At some point, light reflecting off the plus (+) sign will fall directly on your blind spot, and it will seem to disappear.