Q. How do those 3-D pictures work, the ones for which you focus your eyes to see hidden images?
A. They work somewhat the way an old-fashioned stereopticon viewer (holding two slightly different photos) works. Each eye focuses on a separate image, but the brain combines them into one image that gives the illusion of depth (the third dimension) in addition to width and height. It does this by matching up the similarities and adding in the small differences. The result looks like what you see when you look at a solid object from the two slightly different angles of your two eyes.
The first pairs of random dot stereograms, with images, embedded in dots, that can be seen only by people with good depth perception, were produced in 1959 by Dr. Bela Julesz; 20 years later, Christopher Tyler, his student, found that patterns could be arranged in a single image to get the same effect.
The ability to see computer-generated 3-D images depends on having healthy vision with both eyes working smoothly together, and the images can be used as a test or exercise, according to Rachel Cooper, author of educational material on stereo vision for the Optometrists Network at www.optometrist.org. There is a link to copyrighted Magic Eye images at www.vision3d.com.
Vision problems that can interfere with depth perception include amblyopia, or ''lazy eye,'' and strabismus, informally called ''wall eyes'' or ''crossed eyes.'' C. CLAIBORNE RAY
Drawing (Victoria Roberts)