It was on June 19, back in 1905, that the very first movie theater opened for business. At that time, it was quite common to call a concert hall an "odeon" (or "odeum"), after the ancient Greek word for a public theater, but this first movie theater was special in that it charged only 5 cents - a nickel - for each admission. The name of this theater in Pittsburgh, Penn., was the Nickelodeon.

Even though your children are part of the "video age," it is good for them to understand the science behind "moving pictures" because the same principle will explain some of the wonders and mysteries they see going on around them every day.A strip of movie film is actually a series of many separate, individual, still photographs, each of which shows the subject in a slightly different position. These photographs, or frames, are passed through the projector so rapidly (about 24 frames each second) that the human eye cannot distinguish them as separate images. Before the image from one frame fades from our brain, another image is presented, then another and another, and so the brain blurs them together and tells us we are seeing continuous, smooth movement.

This process goes on all the time, not just with movies. The light we take in through our eyes excites some light-sensitive cells in the retina, which then send messages to the brain. But it takes about one-tenthof a second for these cells to calm down again and send a different message. So, the first message, or image, "persists" in the brain, a phenomenon that is called "persistence of vision."

Have you ever seen someone waive a glowing stick or a flashlight around in a circle on a dark night? It appears that they have "drawn" that circle in mid air because your brain retains the image it received of the light in one position, and blurs it with images of the light in other positions around the circle.

Persistence of vision explains why we see the moving spokes of a bicycle wheel or wagon wheel as a blur instead of individual spokes. But sometimes the timing of our brain's recognition is such that it will "see" the spokes in the very same position every tenth of a second, and so the spokes will appear to be standing still. Or, the brain may register their image just an instant before they return to their original position, and so the spokes will appear to be turning backwards!

There are several interesting activities that you can do with your children to demonstrate persistence of vision to them. You might, for example, cut a 2-inch square of card stock and, positioning a corner at the top and bottom, draw the outline of a fishbowl on one side, and the outline of a fish on the other. Now tape or pin the bottom corner of the card to the eraser end of a pencil (held vertically) and spin the card by rolling the pencil between the palms of your hands. Presto! The fish appears to be swimming inside the bowl.

You can also make a "flip-book" by drawing stick figures in successively different positions on each page of a small tablet, then bending the tablet back and releasing the pages with your thumb to make the stick figures appear to move.

You'll find more activities to help you demonstrate persistence of vision in the 507 and 778 sections of the children's books in your local library. Particularly good is one called "Gee, Wiz!" by Linda Allison and David Katz.