Stereoscope viewer, on display in the <i>Schulhistorische Sammlung</i> (School Historical Museum), Bremerhaven, Germany.  By putting a picture in the viewer and looking at it through the viewer, a three-dimensional (stereo) image of the object can be seen.   The stereo pictures shown consist of two different images of the same scene taken by a special stereo camera with two lenses an eyes-distance apart.
Dieter Albers
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>