Matt Verner, age 14, of Indianapolis, Indiana, for his question:

How do the eye's rods and cones differ?

All the delicate parts of the eye are arranged to cast rays of light onto the rods and cones. There are about 130 million of these light sensitive cells, some shaped like mini rods and others like mini cones. The cones translate and transmit pictures during the bright light of day. The rods take over when the light grows dim.

Light rays bounce from the scenery and enter the eye through a window called the pupil. The transparent lens behind the pupil focuses them onto the curved retina screen that lines most of the eyeball. There they strike the remarkable tissues that translate the assorted bits of information and relay them to the vision center of the brain.

The sensitive tissues of the retina are arranged in layers. Near the surface are networks of super fine nerve cells. Behind them are the light sensitive nerve cells called the rods and cones. The cone¬ shaped cells are concentrated in and around a central dimple shaped area called the fovea.

The cones are stimulated to action by bright light rays. They also contain chemicals that are changed by light and these changes are translated as colors. Some researchers suspect that three types of cone cell respond to separate groups in the red, green and blue color ranges. In any case, the millions of cones concentrated in and around the fovea give clear sharp color pictures during the day and under bright artificial lights.

But the cones are useless in dark and dim surroundings. Then they cease to function and the miraculous business of vision is taken over by the more sensitive rods. The rods are unable to sense colors and unable to define sharp details. This is why we see the night scenery as a rather blurry picture of light and dark grays.

At night there also is an extra blurry patch in the very center of the picture. This matches the fovea area in the center of the retina. This region is crowded with cones, with few if any rods. During the day, its crowded cones give us the brightest part of the color picture. At night, the fovea is almost sightless.

This is why during the day we get the clearest picture by looking straight at an object. At night, we can see an object more clearly by looking slightly to one side of it. A star looks brightest when we catch a glimpse of it from the corner of the eye.

The two systems do not turn on and off as fast as a light switch. In a brightly lit room, the cones are at work giving us sharp color images. If we step outdoors into the dark night, they cease to work and the rods take a while to get going. It takes from ten to twenty minutes for vision to adjust from bright light to darkness and some time, though it's not so, obvious, from darkness to bright light. The two visual systems are a little slow about changing roles.