The first stereoscope
Charles Wheatstone shows London’s Royal Society a mirror device that gives each eye its own drawing. It is the first scientific explanation of how two eyes create depth.
The magic behind it
The Secret of Hidden 3D Pictures
A hidden 3D picture looks like a flat, repeating pattern. Look through it the right way, and a hidden shape appears in depth. No glasses, no special light: just your two eyes and a brain that is remarkably good at finding depth.
Here is how it works, where it came from, and a few surprises along the way.
What it is
What is a stereogram?
A stereogram creates the impression of depth by showing each of your eyes a slightly different view. The oldest kind is a pair of pictures side by side, one for each eye, usually looked at through a viewer.
An autostereogram, the kind in our books, packs both views into a single image, so you need nothing but your eyes. You will also hear them called single-image stereograms, or simply hidden 3D pictures.
- stereo
- from the Greek for “solid”
- gram
- from the Greek for “something drawn”
How it works
Three steps from flat to 3D
Two eyes, two views
Your eyes sit about 2½ inches apart, so each one sees the world from a slightly different spot. Try it: hold up a finger in front of something far away, then close one eye, then the other. Your finger seems to jump against the background. Your brain uses that difference to judge distance: the bigger the jump, the closer something is. Scientists call this stereopsis.
A pattern that repeats
An autostereogram repeats the same strip of pattern across the page. When you look through the page, each eye lines up a different copy, one repeat apart. Because the pattern repeats, every piece still matches, so your brain fuses the two views into one pattern floating behind the paper.
Tiny shifts make the shape
Where matching parts of the pattern are closer together, your brain places them nearer than the surrounding pattern. Where they are farther apart, it places them deeper. These small changes create the hidden shape.
Why it takes practice
Normally your eyes aim and focus at the same distance. A stereogram asks them to aim beyond the page while still focusing on it. That is an unusual skill, which is why it can take a few tries, and then suddenly click.
History
From Victorian parlors to your bookshelf
People have been delighted by pictures with depth for almost two centuries.
- 1838
- 1844
The wallpaper effect
Sir David Brewster notices that repeating wallpaper patterns can seem to float in front of or behind the wall when he lets his eyes drift. Hidden 3D pictures use the same effect.
- 1851
A royal hit
Brewster’s handheld stereoscope, with lenses instead of mirrors, is admired by Queen Victoria at the Great Exhibition in London. A stereoscope craze follows.
- 1861
A viewer for everyone
Oliver Wendell Holmes designs a simple, inexpensive viewer and deliberately leaves it unpatented. Versions of it stay in production for about a century.
- 1959
Depth in pure noise
Béla Julesz shows that the brain can find depth in two images of random dots, with no shape visible in either one. Depth, it turns out, is built in the brain.
- 1979
Both views in one picture
Christopher Tyler and Maureen Clarke make the first single-image random-dot stereogram on a home computer.
- 1990s
Color and a craze
Hidden 3D pictures move into full color. Books of them top bestseller lists around the world, and they appear in magazines, posters and puzzle books.
- Today
Still magic
Computers combine colorful patterns and carefully designed depth maps to create hidden scenes. Discovering them still feels like magic.
Fun facts
Surprising things about seeing in 3D
Insects in 3D glasses
In 2016, scientists fitted praying mantises with tiny 3D glasses, fixed on with beeswax. The mantises struck at bugs that seemed to float off the screen, proving that they see in 3D.
Cuttlefish too
In 2020, researchers gave cuttlefish 3D glasses and showed them shrimp on a screen. They also use differences between their two eyes’ views to judge distance.
3D came before photos
Wheatstone’s first stereoscope pictures were drawings: photography had not yet been announced to the world.
Turn it inside out
Look at a hidden 3D picture cross-eyed instead of through it, and the depth flips: shapes that should rise up sink into the page.
The shape lives in your brain
In a random-dot stereogram, neither eye sees any shape at all. The shape exists only once your brain puts the two views together.
The same trick as 3D movies
3D movie glasses send a slightly different picture to each eye. A hidden 3D picture does the same with no glasses at all: each eye picks its own view out of one pattern.
Now see it for yourself
Knowing the secret doesn’t spoil the magic. Try a practice picture, then find your next book.