"The stereoscope provided 19th-century viewers with the illusion of 3D depth, in much the same way as the virtual reality headsets of today. In the pre-television, pre-movie days, families would gather round the stereoscope instead."

 ...

 "The stereoscope allowed viewers to see a pair of two-dimensional images taken from slight different angles – one for the right eye, one for the left. Viewed through a stereoscope, the brain combines the two offset photographs into one, giving the impression of a 3D image."

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Source : The Guardian