The device dates back to 1907. Various inventors had been playing with stereoscopic photos for years before then, and the stereoscope was apparently a fairly popular item for what passed for photo nerds at the turn of the 20th century. This one, found on the indie-seller site Etsy, is called a Holmes Stereo Viewer after Oliver Wendell Holmes, who first started selling them in 1881. Like most 3D tech, the effect is created by putting two images of the same object, each slightly offset, side by side. Each of the photos included with our ‘scope was curved slightly forward. The curve wasn’t brought on by age — each pic is mounted in a curved frame to help create the 3D effect. That makes creating the pics fairly cumbersome, which is probably the main reason the stereoscope was eventually succeeded by the Viewmaster.