Having struggled to see the world in Bishop Berkeley's way, 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 two-dimensional images as one more or less three-dimensional scene. First invented by Sir Charles Wheatstone in 1838, the device had gone through many iterations by 1904.

JH 2017

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

Illustration of two kinds of binocular vision in mammals, showing the rabbit's small sliver of stereoscopic depth perception and the monkey's larger one. Source:

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