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 and virtual reality headsets of today, that allowed people to resolve a pair of flat, two-dimensional images into one astonishingly three-dimensional scene. First invented by Sir Charles Wheatstone in 1838, it proved immensely popular and went through many iterations by 1904. Stephen's comment seems to reflect awareness that these devices take advantage of a natural feature of human vision.

John Hunt 2017

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