Stereoscopic images have been in existence since the early 19th century. The
side-by-side images on stereoscopic cards, one image for each eye, give the illusion
of a three-dimensional effect when viewed through a stereoscope. As a result of this
three-dimensional effect, stereoscopics were very popular throughout the 19th and
early 20th centuries. Later viewers, such as the View-Master, replaced the
stereoscope but the technology behind them is largely unchanged.
