Stacy Huber, age 15, of Baton Rouge, La., for her question:

HOW DOES A STEREOSCOPE WORK?

A stereoscope is an optical viewing device that makes photographs seem to have three dimensions.

An ordinary camera sees things only in a flat plane, and never in a rounded way as our eyes usually see them. But two cameras set slightly apart can work like our eyes. They can photograph the same object at the same time. These two photographs are then mounted side by side and viewed through a combination of lenses and prisms called a stereoscope.

Without strain, the eyes see the two views through the stereoscope, and the resulting image appears to have three dimensions.

A stereoscope was once a common household item. The old style model had a rack and handle, a slide and a pair of screened lens prisms. The present day stereoscope is a plastic box with two viewing holes. One popular type has picture slides mounted in a cardboard or plastic disks.

Today, stereoscopes are also employed extensively in aerial surveys to map out land elevations.