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.
The technical challenges were mostly solved in the mid-19th century, allowing for a flourishing of the stereogram medium. Science, travel, news photography, storytelling tableaux, sentimental scenes -- all were popular stereogram subjects. Stereograms could be viewed on stereoscopes at home but were also available on stereoscopic viewers at fairs and other public spaces. (Stereoscopes were also used in experimental psychology to study depth perception.) 3D still photography has had a number of cultural ups and downs since its Victorian heyday. It peeked back into the mainstream, for instance, in the post-war 1940s, when Viewmaster reels of fairytales and tourist attractions were popular. And the Magic Eye illusions popular in the 1990s are examples of autostereograms, stereo illusions visible without a special viewer by focusing the eyes behind the image.
History repeats itself, first as tragedy, second as animated gif. Stereograms, the photos themselves, have had a new life on the web since a developer had the brilliant idea of turning them into animated gifs. This in turn inspired the New York Public Library's "Stereogranimator," which allows visitors to convert stereograms into gifs by altering degree of overlap of the two photos and amount of time of perspective flip. (The NYPL is now accepting partners to contribute stereograms to the site.) I have no stomach for 3D films (they move too fast for those prone to vertigo) but with traditional stereo viewers and this new gif process, the viewer can control how slowly she sees the illusion of depth. Time has always been an element of photography, but stereo gifs literally incorporate it.
Whither the stereoscope as parlor amusement? New and historic stereoscopes are widely available online. (I have a cheap one I picked up at the 3D museum in Portland.) Historic stereograms are also widely available online and at flea markets and antique stores. As with most interesting technologies that fall in and out of the mainstream, stereo photography has a passionate user community, organized through local clubs and the National Stereoscopic Association. Some of the most astounding contemporary stereo photography is botanical: compelling 3D zooms into the hearts of plants. There are (of course!) stereoscope attachments for iPhones, as well as apps to help you take and view stereo pictures. (If you don't have a 3D attachment you will have to practice cross-eyed autostereogram viewing). The fascination with Victorian technology is at a crest, and the amount of digitized historic stereograms and born-digital stereo photos grow every day. I eagerly await the new golden age of the stereoscope.