What’s unique about this particular device is that it can convert from a graphoscope to a stereoscope. Graphoscope, really, is nothing more than a glorified magnifying glass, which allowed users to look at photographs, drawings, and text up close.
But a stereoscope, also called a stereoview, was something special. Using two similar images placed side-by-side, the dual lenses created the illusion of 3D imagery—quite a thrill in the days before IMAX theaters. Just ask Brian May of Queen.