It's time for a second golden age of the stereoscope.
I'm surprised that we haven't yet seen a stereoscope revival. This is a perfect time for it, what with our culture's obsession with Victorian detritus and with the artisanal, as well as new technical developments and the new popularity of 3D cinema.
I first played with a stereoscope myself long before I became a historian. At a party in Minneapolis, a friend dug out her antique store finds, a stereoscope and box of stereograms, and we all gathered round, oohing and aahing, anxious to take our turns to see the faded double photographs pop into sharp 3D. This was exactly how stereoscopes were used in middle-class parlors in the 19th century, as a group leisure activity for parties and rainy days, like showing your friends the latest kitten antics on YouTube. Cute animals were a favorite stereogram subject.
In stereoscopy, two paired 2D photos, of the same scene but taken from a slightly different perspective, are viewed together to trick the brain into seeing depth. Stereo photography and viewing techniques were developed before the Civil War (they were a hit at the Great Exhibition of 1851), and the practice flourished among the newly leisured middle classes and the new mass media culture well into the advent of cinema. Special stereo cameras followed closely on the invention of stereo photography, so photographers no longer needed to move their cameras between shots -- the twin lenses were set within the general range of distance between human eyes (50-80 mm). Color stereograms (anaglyphs) are slightly more complicated, with red/cyan color filters creating the illusion of depth.