STEREO SUE.(Stereoscopic vision)

When Galen, in the second century, and Leonardo, thirteen centuries later, observed that the images received by the two eyes were slightly different, neither of them appreciated the full significance of these differences. It was not until the early eighteen-thirties that the English scientist and inventor Charles Wheatstone began to suspect that the disparities between the two retinal images were in fact crucial to the brain's mysterious ability to generate a sensation of depth--and that the brain somehow fused these images automatically and unconsciously.

Wheatstone confirmed the truth of his conjecture by an experimental method as simple as it was brilliant. He made pairs of drawings of a solid object as seen from the slightly different perspectives of the two eyes, and then designed an instrument that used mirrors to insure that each eye saw only its own drawing. He called it a stereoscope, from the Greek for "solid vision." If one looked into the stereoscope, the two flat drawings would fuse to produce a single three-dimensional drawing poised in space.

(One does not need a stereoscope to see stereo depth; it is relatively easy for most people to learn how to "free-fuse" such drawings, simply by diverging or converging the eyes. So it is strange that stereopsis was not discovered centuries before: Euclid or Archimedes could have drawn stereo diagrams in the sand, as David Hubel has remarked, and discovered stereopsis in the third century B.C. But they did not, as far as we know.)

A few years after Wheatstone's discovery came the invention of photography, and stereophotographs, with their magical illusion of depth, became immensely popular. Queen Victoria herself was presented with a stereoscope after admiring one at the 1851 Great Exhibition, at the Crystal Palace, and soon no Victorian drawing room was complete without one. With the development of smaller, cheaper stereoscopes, easier photographic printing, and even stereo parlors, there were few people in Europe or America who did not have access to stereo viewers by the end of the nineteenth century.

With stereophotographs, viewers could see the monuments of Paris and London, or great sights of nature like Niagara Falls or the Alps, in all their majesty and depth--with an uncanny verisimilitude that made them feel as if they were hovering over the actual scenes. (By the mid-eighteen-fifties, a subspecialty of stereophotography, stereopornography, was already well established, though this was of a rather static type, because the photographic processes used at the time required lengthy exposures.)

In 1861, Oliver Wendell Holmes (who invented the popular handheld Holmes Stereo Viewer), in one of several Atlantic Monthly articles on stereoscopes, remarked on the special pleasure people seemed to derive from this magical illusion of depth:

The shutting out of surrounding objects, and the concentration of the whole attention . . . produces a dream-like exaltation . . . in which we seem to leave the body behind us and sail into one strange scene after another, like disembodied spirits.