1855 John Stull Stereo Viewer Double Dag Daguerreotype Quarter Plate
A stereoscope is a device for viewing a stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image.
Two separate images are printed side-by-side. When viewed without a stereoscopic viewer the user is required to force his eyes either to cross, or to diverge, so that the two images appear to be three. Then as each eye sees a different image, the effect of depth is achieved in the central image of the three.