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You stare at waterfall for a minute or two, then shift your gaze to its surrounding. What you now see appears to drift upward. You __1
are board a train in a busy station when suddenly another train next __2
to your starts moving forward. __3
For a fraction of a second you feel that your train has lurched backward. These optical illusions occur because the brain is constantly matching its model of reality to signals from the body’s sensors and interpret what must be happening – that your train __4
might have moved, not the other; that downward motion is now __5
normal, so a change from it must be perceived as upward motion.
The sensors that make this magic are two kinds. __6
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 burnt __7
ten miles away. Colorful vision in each eye comes from six to seven __8
million structures called cones. Under ideal conditions, every cone can “see” the entire rainbow spectrum of visible colors, but one type of cone is most sensitive to red, another to green, the third to __9
blue. By monitoring how many wavelength of light affects the different cones, a connected ganglion cell can determine its “color” and relay that data brainwork. Rods and cones send their messages pulsing an __10
average 20 to 25 times per second along the optic nerve.
