The Stereoscope: How Two Flat Pictures Make Depth

Read the episode (Narration script)

Welcome to Origins. In this program we explore the technological and scientific roots of the modern world. Let's begin.

Hold up one finger, about a hand's length in front of your face. Close your left eye. Now open it and close your right. The finger jumps sideways against the room behind it.

Each eye has its own view. The two sit roughly two and a half inches apart, so they look at the world from slightly different positions. For a distant hill, the difference is tiny. For a finger, a teacup, or a branch within reach, the two views disagree noticeably.

Yet with both eyes open, there is one finger. And it seems to stand out in space.

The geometry is not difficult. For a very long time, though, almost no one treated the mismatch as a clue to how depth is seen. Leonardo da Vinci came close. He noted that a painting could never show relief equal to natural objects, unless those objects were viewed from a distance and with a single eye. A canvas offers both eyes the same picture. The world does not.

The person who turned that remark into an experiment was Charles Wheatstone, a London physicist now better remembered for telegraphs and for an electrical measuring circuit that carries his name.

In June of eighteen thirty-eight, the Royal Society heard his paper, modestly titled Contributions to the Physiology of Vision, Part the First. Its central claim was that when both eyes converge on a nearby object, the pictures formed in the two eyes are obviously different. That difference, Wheatstone wrote, had to be regarded as a new fact in the theory of vision.

To test it, he built an instrument.

Two flat mirrors, each about four inches square, were mounted with their backs at a right angle to each other. An observer put their face up to the point where the mirrors met. The left eye looked into the left mirror, the right eye into the right. Off to each side, facing its mirror, stood a drawing on a sliding panel, so each eye saw only its own picture. The panels slid until the two reflections overlapped where the eyes were aimed.

The drawings were plain outlines of simple solid figures, each drawn twice, as it would appear from the left eye's position and from the right eye's. Wheatstone chose outlines on purpose. With no shading and no color, nothing on the paper could suggest depth except the difference between the two drawings.

On its own, each drawing looked flat. Through the mirrors, a single solid figure appeared to stand in the air.

Wheatstone named the device a stereoscope, from Greek words for solid and looking, to indicate, as he put it, its property of representing solid figures.

He also tried swapping the drawings, so that each eye received the picture meant for the other. The figure came out inverted. Points that belonged nearest now appeared farthest away. He called this the converse figure.

That small trick carries the main result. Depth depended on which eye received which picture. The mind was not reading solidity off either drawing. It was working it out from their disagreement. Two flat images go in, and one solid scene comes out.

Wheatstone had to draw his pairs by hand, because photography had not yet been announced. That came from Paris about a year later. Once cameras could take two pictures from slightly separated positions, a stereoscope could hold a face or a street.

The Scottish physicist David Brewster made the instrument small. His version replaced the mirrors with a pair of lenses in a compact box. Unable to interest British opticians, he carried one to Paris in eighteen fifty, where the optician Jules Duboscq began making them for sale, along with paired daguerreotypes of people, statues and flowers. Crowds came to look.

At the Great Exhibition of eighteen fifty-one, in London's Crystal Palace, Duboscq displayed a lens stereoscope that caught the particular attention of Queen Victoria. Brewster presented her with one, and orders followed.

In eighteen fifty-six, Brewster wrote that an estimated half a million stereoscopes had been sold, and that photographers were taking paired pictures among the ruins of Pompeii and on Swiss glaciers. The same book spends many pages disputing how much credit Wheatstone deserved.

Across the Atlantic, the physician and writer Oliver Wendell Holmes became the stereoscope's most eloquent admirer. In an eighteen fifty-nine essay for The Atlantic Monthly, he described the experience precisely. "The mind feels its way into the very depths of the picture." Tree branches in the foreground "run out at us as if they would scratch our eyes out."

He went further. "Form is henceforth divorced from matter." A cathedral or a mountain could now be gathered as a pair of paper images and kept in a drawer.

Holmes also designed a simpler handheld viewer, with two lenses, a hood for the eyes, and a sliding holder for a card. He deliberately did not patent it. A Boston dealer in musical instruments and umbrellas, Joseph Bates, put it into production. Cheap viewers and boxes of stereo cards remained a fixture of parlors for half a century.

One question remained open. Perhaps the brain first recognizes things, a face or a column, and only then works out their depth.

A century later, an answer came from Bell Labs. Béla Julesz, an engineer who had left Hungary after the Soviet invasion of nineteen fifty-six, used a computer to print fields of random dots. He made two identical copies, then shifted a square region in one copy very slightly sideways.

Each image, alone, looks like television static. There is nothing to recognize. Viewed through a stereoscope, after a moment, a square separates from the speckled background and sits at its own depth.

His paper appeared in nineteen sixty. Depth could be computed with no identifiable objects at all. The comparison between the eyes could happen first, before anything was recognized.

The descendants are everywhere. Three-dimensional films send a slightly different picture to each eye. The hidden-image posters of the nineteen nineties descend from Julesz's random dots. And a virtual reality headset places one small screen in front of each eye, which is Wheatstone's arrangement of eighteen thirty-eight with electronics in place of mirrors.

So hold up that finger once more. Two views, slightly at odds. One finger, standing in space. That solid world is something the brain builds, moment by moment, out of a disagreement between two eyes.

Thanks for listening to Origins.