In Brief

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.

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Binocular vision, which uses slightly different angles of sight to determine distance to nearby objects, has operated unconsciously in animal brains for millions of years. (The trick is not confined to vision. Owl hearing accomplishes the same thing with ears set at slightly different levels, and the forked tongues of many snakes are thought to detect smells with similar divided perception.) Human eyeballs lie in a plane because we are primates who evolved jumping between tree branches, where correctly estimating distance was essential to survival. The same selective pressures helped predators gauge distances to fleeing prey animals––whose eyes typically face in opposite directions, giving them a larger total field of vision to warn of danger. As primates and as predators, human beings found it highly advantageous to have their eyes face in the same direction, where slightly different impressions on the retinas could provide depth perception across a broad field. In the illustration reproduced here, the purple section shows this broad binocular field in monkey vision. The grey section shows the downside: a large blind spot, which rabbits do not suffer from. (The narrow sliver behind the rabbit should be colored grey.)

Stereoscopes reproduce the depth-perception effect by mounting two images, drawn or photographed from very slightly different perspectives, on a card and viewing them with different eyes. The human brain melds the two images into a single one that appears to have roundness and solidity. If it seems a bit odd for Stephen to describe the normal binocular vision that all human beings effortlessly possess as something produced with the aid of a clever artificial device, the reader may reflect that Proteus began with him "walking into eternity along Sandymount strand" by closing his eyes, only to decide upon opening them again that the physical world was "There all the time without you." His experiment with seeing reality as flat is of a piece with his metaphysical effort to pierce through the veil of appearances to the underlying truth. Stereocopic vision may be natural, but that does not mean that it is inevitable or that it fundamentally corresponds to reality. In Circe Stephen voices the paradox of single eyes seeing flat images and pairs of eyes seeing depth: "Distance. The eye sees all flat."

Some people saw great educational potential in the stereoscope, while others scorned it as a cheap trick. The French poet Charles Baudelaire lamented the spectacle of "a thousand hungry eyes...bending over the peep-holes of the stereoscope, as though they were attic-windows of the infinite." Joyce was no less interested in the infinite than Baudelaire, but he was fascinated by the new possibilities of perception offered by technologies like the gramophone, the photograph, the mutoscope, and the cinema, and in some instances he clearly looked into the science behind the new technologies. Parallax, a method of determining astronomical distances which operates by the same principles as the stereoscope, became a central organizing theme in his novel.

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.