Wood and brass viewfinder, or stereoscope, patented in 1866 and manufactured during the 1880s. Stereoscopes combine the twin images of stereoptical slides within one plane of vision, producing a three-dimensional effect. The viewfinder was grasped at the bottom and a slide inserted into the slot at one end, as shown in the picture below.
Example of a stereoscopic slide:
For details, view the stereoscope in our online collections database.