Optical illusions were all the rage back in the 1990s, long before the internet became an indispensable part of our lives and *The Dress* made headlines last year.

If you were growing up two decades ago, or were parents to the kids who were, then you’ll probably remember those mind-boggling Magic Eye puzzles that some people could see while others couldn’t.

These puzzles, known as stereograms or autostereograms, are pictures within pictures that give a three-dimensional representation of a solid object or surface. When viewed correctly, the hidden image in each Magic Eye illusion will appear in 3D.

So are you good at spotting them?

You should be able to see a dinosaur shape in the last one but are you struggling to find the hidden images? Then give your eyes a little break (you might need one) and read on.

Here’s how to “see” them

Some stereograms are designed for divergent viewing, meaning instead of looking directly at an image, you’re supposed to look right through it by “over diverging” your eyes.

Other illusions are meant to be viewed cross-eyed.

While some people are able to see the 3D image hidden in a stereogram naturally, others will have to train their eyes to see the illusion.

To see it, you need to hold up the image close to your face at first so the nose touches it and slowly pull the image away. As easystereogrambuilder explains: “Bring the stereogram image really close to your eyes (until you touch it with your nose).

“At this distance your eyes cannot focus on the image and they look somewhere behind the image. Now, slowly push the image away from you, while trying to keep the eyes off focus. At some point you will see the hidden image.

“Another method is to take an object and put it behind the image (about half of meter behind it). Now, focus on the object behind the image while keeping the eyes looking at the image.”

Why some people can’t see them

Unfortunately, stereograms don’t work on everyone and depth perception requires both eyes to work in conjunction.

That means people with impaired depth perception (binocular vision or stereo vision), or people who have one eye which is extremely dominant (amblyopia or lazy eye) will find it more difficult to spot the image.

How they become so popular

Scientists have been using stereograms to study human depth perception for decades.

According to Magic Eye: “The first random dot stereogram was invented by Dr Bela Julesz in 1959 as an experiment to test stereopsis, the ability to see in 3D.”

Twenty years later, Christopher Tyler, a student of Julesz, discovered that the offset scheme could be applied to a single image.

“Using this new program in combination with state of the art 3D modeling software and colorful art techniques, a totally new patented art form was developed… Magic Eye,” the website continues.

“In 1991, engineer Tom Baccei and 3D artist Cheri Smith collaborated to improve on the research of Julesz and Tyler. With the assistance of programmer Bob Salitsky, the group developed the first sophisticated, full-color stereogram program.”

And if you are fascinated by it, here‘s how you can build your own stereogram.