The stereoscope was the first device to show photographs with the illusion of three dimensions. Ordinarily, a camera only shows images in two dimensions, but two different cameras used simultaneously can simulate two eyes, if the lenses of the cameras are the right distance apart. Thus, when the resulting photographs are viewed together, the image appears to have depth.

The first stereoscopes were developed in France in the early nineteenth century as an offshoot of the intense research on light and optics that was happening. The stereoscope was modified in 1849 by Sir David Brewster, who had invented the kaleidoscope some thirty-five years earlier. The improved stereoscope featured a system of lenses and prisms that would overlap the two photographs without straining the viewer's eyes.

Like the kaleidoscope, the stereoscope was a popular success; marketed as "refined amusement," it found its way into the drawing rooms of high-minded audiences...