Cathy Conlon, age 11, of Huntsville, Alabama, for her question:
How can we see in three dimensions?
Rays of light bounce off surfaces of the scenery at various angles and carry these images to our eyes. Vision is our ability to see these pictures and to interpret what they mean. Our eyes are sense organs of the nervous system, which includes the brain. Together, these built in biological parts provide a superb communications system with the world around us.
The talents of human vision far surpass the best camera and any other man made visual device. Our eyes create their own colored movies and send them to brain headquarters to be interpreted in a flash. Without waiting for conscious orders, they adjust to bright and dim light and also blink their eyelid windshield wipers to keep their surfaces sparkling bright and clean. Their colored movies are superior to most of those made by man because they are in three dimensions. This is because our living visual equipment is built to see slightly, very slightly around corners.
A flat, two dimensional picture scans only in two directions up and down and from side to side. The third dimension is depth. It scans and estimates the direction from where you are to way off in the distance. You might call it distance vision. Biologists call it stereoscopic vision, or binocular vision. Stereoscopic is coined from an older word for solids which of course have three dimensions. Binoculars are field glasses with distance lenses, one for each eye. And human vision performs its magical 3 D trick by using each of its two eyes to see depth and distance.
The two eyes, of course, are set a short distance apart. But light, carrying its images, arrives in perfectly straight lines from the .scenery to each eye. So each eye gets a slightly different picture. Normally, however, we do not see double. This is because our visual equipment has a built in blinder. It fuses the separate flat pictures from each eye into a single picture that shows depth or three dimensions.
Light triggers pinpoints on the eye's retina screen. This data is scanned and relayed by tiny nerves that join to form the optic nerve, which is the trunk line to the visual headquarters in the brain. Nerves from the right side of each eye reach the right side of the brain center. Nerves from the left side of each eye reach the left side of the center. The brain gets two slightly different images, one from each eye.
The problem is adjusted by optic chiasma, a word that means cross lines. This center of highly complex nerves is near the brain. It fuses the two images into one picture so that the slight overlaps reveal tiny peeks around the corners. This trick adds three dimensional depth to our private colored movies and also helps us to estimate distance.
Your stereoscopic vision is easy to verify with a simple experiment. Close one eye and point your finger in a direct line with a spot on a wall. Keep your finger strictly on target while you open your eye and close the other one. Your finger seems to have jumped to one side. Line it up again and open both eyes. The target is now halfway between the two points because the separate images from both eyes are am fused into one three dimensional picture.