A typical viewer for stereoscopic views about 1900.
This kind of construction was used since 1860 and still until the flag of interest in stereoscopic photography shortly after the end of the 19th century.

A box made of wood [a] contents the stereo viewer and an area to store about 40 stereoscopic views.
The hold of the stereo viewer has to be turned down to put the viewer into the box. A bridge on the middle axis [b] prevents to see the picture for the other eye. The part of the viewer that holds the stereoscopic view can be shift on distinct distances.