Having struggled to see the world as Bishop Berkeley thought it should be seen, as colors on an essentially "flat" background, Stephen allows his mind to snap back into its normal perception of depth: "Ah, see now! Falls back suddenly, frozen in stereoscope. Click does the trick." The stereoscope was a 19th century invention, precursor to the 3D movie glasses of today, that allowed people to resolve a pair of identical two-dimensional images as a single three-dimensional scene.

JH 2017

The Brewster stereoscope, a type demonstrated at the Great Exhibition of 1851, where it impressed Queen Victoria. Source: Wikimedia Commons.

A cheaper type invented, patent-free, in 1861 by the American Oliver Wendell Holmes, photographed by Davepape (2006). Source: Wikimedia Commons.