Keith Damm, Age 10, Of San Diego, Calif., for his question:
What causes our eye pupils to dilate?
Your head is fitted with two little built in cameras. They are, of course, your two bright eyes, and they are far better than the best of man made cameras. They can outdo a whole studio full of cameras made to take miles and miles of colored movies indoors and out.
Man made cameras can take photographs only if the scenery is flooded with bright light. Your eyes can take pictures of the scenery in bright light, under cloudy skies and even at dusk when the light is very dim. They adjust themselves to different amounts of light by changing the size of the pupils, those two round black holes in the center of the colored part of the eyes. 11hat's more, the pupils adjust to different amounts of light by themselves without waiting for you to order them to do so.
You can watch how this happens by studying your eyes in a small mirror. If you are outdoors in the bright noonday sun, you will bee that the pupils are small black dots. If you stare for a while into a dark corner, the mirror will reflect large black pupils. The pupils will stay large only for a short while; then light reflected from the shiny mirror will cause them to shrink again.
The pupil of the human eye acts like the shutter of a camera. It is actually a hole in the iris, the colored part of the eye. Light, carrying a picture of the scenery, passes through the pupil, through the center of the eyeball, and falls upon a screen ca11ed the retina, which is on the back inside wall of the eye. The retina has about a million tiny nerve ce11s that sort out the picture into patches of color, light and dark. These cells flash their information to the brain which assembles the pieces and shows you the scenery.
The colored iris is a movable muscle. It can squeeze tight, making the pupil small. It can relax, making the round dark pupil larger. When the light is normal, the pupil is about half open half its full size. ?,when the light is very strong, the retina screen must be protected, for it can be damaged by vivid brilliance and some of its cells destroyed forever. So the pupil contracts to its smallest size. In the darkness the pupil dilates to its fullest size to catch as much of the glimmering light as possible.
Just behind the pupil is a lens somewhat like a small magnifying glass. Certain eye muscles can make the lens thin or thick in order to bring objects into focus. When you look at a close object the lens is thick. For distant viewing the lens is flat and thin. By focusing the lens and by adjusting the size of the pupil, the human eye can improve the quality of its pictures a million times, and all without any instructions from the conscious mind.