A stereoscope is a viewing device which allows users to create a three-dimensional image from a set of two-dimensional photographs or drawings. Original stereoscopes were popular around the turn of the 20th century, and featured a clip for holding special stereoscopic cards in place. The viewer would peer through a rudimentary binocular system, which forced each eye to see only one of the two images. By either crossing or diverging one's eyes, a third image would eventually appear in the middle, and this image would provide the illusion of depth for as long as the viewer maintained proper concentration and focus.
The secret behind the stereoscope was the relative perspective of the photographs or drawings. When both eyes are trained on the same two-dimensional picture, the viewer's brain does not receive enough divergent information to form a binocular or three-dimensional image. The photograph will remain two-dimensional, with no sense of depth. However, if two images are taken at the same distance apart as the viewer's eyes, the brain can merge the images into one stereoscopic mental picture. A stereoscope helps accomplish this visual trick.
When looking through a stereoscope, each eye only sees one of the two images on the stereoscope card. In order to create a perceived three-dimensional image, the viewer either crosses his or her eyes or diverges them outwards. Each image, the left and the right, should meet together to form a third image. Since the two photographs were spaced an eye's length apart, the brain now has enough information to create a sense of depth in this new image. Some people can intuitively view a stereograph card and achieve the same results without a stereograph, but the device does help viewers to focus each eye on a separate image.
The stereograph concept was further refined during the early to mid-20th century, including the development of special stereoscopic cameras which could be used by amateur photographers to create their own stereoscopic cards. These cameras used two separate lenses and film stocks set approximately two and a half inches apart to capture stereoscopic views of popular natural sites and tourist destinations. Stereoscopes and stereoscopic cards could often be purchased at gift shops near popular sites.
Perhaps the most recognizable example of a modern stereoscope is the children's toy known as a View-Master®. A View-Master® features a binocular personal viewer which accommodates special stereoscopic disks. These images appear in the separate lenses of the viewer and the user sees a three-dimensional image without the need to cross or diverge his or her eyes unnaturally. This is a vast improvement over the visual gymnastics often demanded by the original stereoscope system.
While the original stereoscope may have fallen out of favor, the idea of creating three-dimensional images from two-dimensional sources has not. Recently, a new form of artificial stereoscopic imagery called an autostereogram or "Magic Eye" has completely eliminated the need for a special viewing device. The original two-dimensional image, which often appears to be a random set of dots and lines, contains enough stereoscopic information for the viewer's mind to create a three-dimensional image. Although it may take several attempts to achieve the ideal focus, those who have learned to view these autostereograms can see a number of images in the foreground and a surprising amount of depth in the background.