Stereoscope, << STEHR ee uh `skohp,` >> is an optical viewing device that makes photographs seem to have three dimensions. An ordinary camera sees things only in a flat plane, and not in the round, the way our eyes usually see things. But a camera with two lenses set a small distance apart can work like our eyes. It can be used to take two photographs of an object at the same time. These photographs are then mounted side by side and viewed through a combination of lenses and prisms inside the stereoscope. To the user, the two photographs seem to blend into a single three-dimensional image.
If two stereo photographs are mounted side by side, a person can view them without prisms or lenses and still see an image that appears to have depth. However, viewing images in this way may cause eyestrain.
During the late 1800’s and early 1900’s, many families enjoyed looking at pictures with a stereoscope, sometimes called a stereopticon. The old-style stereoscope consisted of a rack and handle, special slides, and a set of lenses or prisms. The present-day stereoscope is a plastic box with two viewing holes. One popular type, a toy called the View-Master, has picture slides mounted in a cardboard or plastic disk.
Today, mapmakers and geographers use stereoscopes in aerial surveys to map out land elevations. The images are produced from photographs of the ground taken at two slightly different times from a camera mounted in the belly of an airplane. Botanists and forest rangers also use stereo images and viewers to help them identify plants and trees. Astronomers use a special type of stereoscope for finding comets, asteroids, and other heavenly bodies.
