You stare at waterfall for a minute or two, and then shift your gaze to its surroundings. What you now see appears to drift upward. These optical illusions occur because the brain is constantly matching its model of reality to signals from the body's sensors and interpreting what must be happening--that your brain must have moved, not the other; that downward motions is now normal, so a change from it must now be perceived as upward motion. The sensors that make this magic are of two kinds. Each eye contains about 120 million rods , which provide somewhat blurry black and white vision. These are the windows of night vision; once adapted to the dark, they can detect a candle burning ten miles away. Color vision in each eye comes from six to seven million structures called cones . Under ideal conditions, every cone can "see" the entire rainbow spectrum of visible color1s, but one type of cone is most sensitive to red, another to green, a third to blue. Rods and cones send their messages pulsing an average 20 to 25 times per second along the optic nerve. We see an image for a fraction of a second longer than it actually appears. In movies, reels of still photographs are projected onto screens at 24 frames per second, tricking our eyes into seeing a continuous moving picture. Like apparent motion, color1 vision is also subject to unusual effects. When day gives way to night, twilight brings what the poet T.S. Eliot called "the violet hour." A light levels fall, the rods become progressively less responsive. Rods are most sensitive to the shorter wavelengths of blue and green, and they impart a strange vividness to the garden's blue flowers. However, look at a white shirt during the reddish light of sunset, and you'll still see it in its "true" color1--white, not red. Our eyes are constantly comparing an object against its surroundings. They therefore observe the effect of a shift in the color1 of illuminating on both, and adjust accordingly. The eyes can distinguish several million graduations of light and shade of color1. Each waking second they flash tens of millions of pieces of information to the brain, which weaves them incessantly into a picture of the world around us. Yet all this is done at the back of each eye by a fabric of sensors, called the retina , about as wide and as thick as a postage stamp. As the Renaissance inventor and artist Leonardo da Vinci wrote in wonder, "Who would believe that so small a space could contain the images of all the universe?" Visual illusions often take place when the image of reality is   _  .
matched to six to seven million structures called cones. confused in the body's sensors of both rods and cones. interpreted in the brain as what must be the case. signaled by about 120 million rods in the eye.