- Scientists still don't understand exactly how some optical illusions play tricks on people's eyes.
- Johann Karl Friedrich Zöllner, a German astrophysicist, discovered the Zöllner effect that makes parallel lines look curved in 1860.
- One of the first ambiguous images (images that show two things at once) was published in a German magazine in 1892.
- The McCollough effect makes black and white stripes look colored, discovered by Celeste McCollough in 1965.
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Optical illusions often go viral on social media, but they've been around long before Twitter.
From straight lines that appear slanted to ambiguous images that can be interpreted a number of ways, these illusions have been stumping scientists and laypeople alike for decades (and in some cases, centuries).
Here are 10 historical optical illusions that still leave people puzzled all these years later.
The optical illusion hidden in this oil painting from 1533 has been confusing people for almost 500 years.
Hans Holbein the Younger painted "The Ambassadors" in 1533. Looking at the painting head-on, it seems like there's a giant blob on the floor. But from a different angle, it becomes clear that the blob is a skull. Art historians believe that the skull is a "memento mori"— a reminder of mortality.
Read more: This optical illusion has been confusing people for almost 500 years — can you figure it out?
What animal do you see? (Hint: there are two right answers.)
The image shows two animals: a duck and a rabbit. Optical illusions that display two different things depending on how you look at them are known as ambiguous images or reversible figures.
The duck-rabbit illusion was first published in a German magazine in 1892, then used by Joseph Jastrow in 1899 in his research on perception. He found that how fast the viewer sees both the duck and the rabbit can indicate how creative they are.
There are also two possible ways to interpret this image.
Some people see the black silhouettes of two heads facing each other, and others see a white vase. It depends on what your brain interprets as the "figure" to focus on and what it interprets as the "ground" that seems further away in a two-dimensional setting.
This optical illusion is known as Rubin's vase, created by psychologist Edgar Rubin around 1915.
These black lines might not look parallel, but they are.
Johann Karl Friedrich Zöllner, a German astrophysicist, discovered the phenomenon he called the Zöllner effect in 1860. It may have something to do with the way our brains overestimate acute angles and underestimate obtuse angles, or the shorter lines that cross through the longer diagonal lines creating an appearance of depth.
Fraser's spiral is named for Sir James Fraser, the British psychologist who discovered it in 1908.
Fraser published his findings in the British Journal of Psychology in 1908.
It's not actually a spiral.
Fraser's spiral isn't actually a spiral at all. The overlapping concentric circles and spiral background just make it look like one. The illusion is a little bit easier to see when the circles are outlined in a different color.
Kanizsa's triangle is a contour illusion discovered in 1955.
It looks like there's a white triangle pointing downwards that's brighter than the rest of the background, but it's just an illusion. Our brains create an outline that isn't there in what's known as the phantom edge phenomena.
The nonexistent triangle is named for Italian psychologist Gaetano Kanizsa, who published his research on the subject in an Italian psychology journal in 1955.
The Jastrow illusion makes it look like the bottom toy train track is longer than the one above it.
The two tracks are actually the same length. Joseph Jastrow discovered the illusion in 1889. Scientists still aren't exactly sure why the brain perceives one object as longer or shorter than the other when arranged this way.
To experience the McCollough effect, alternate between looking at the red and black stripes and green and black stripes for a few minutes. (Warning: this optical illusion could affect your vision for over three months if you look at it for too long.)
The phenomenon was discovered by Celeste McCollough in 1965.
Read more: This optical illusion could affect your vision for over 3 months if you look at it for too long
Now, look at the black and white stripes. The white horizontal stripes should appear green, and the white vertical stripes should look red.
This phenomenon is a kind of afterimage, which Encyclopedia Britannica defines as a " visual illusion in which retinal impressions persist after the removal of a stimulus, believed to be caused by the continued activation of the visual system." In other words, the strong colors make an impression on the visual system and cause it to think that it's still looking at the colors even when it's not, making the black and white stripes appear tinged with red and green.
The same way a camera's bright flash can linger in your vision, a study in the Journal of Experimental Psychology found that the McCollough effect can last more than three months if the subject looks at the colorful gratings for extended periods of time.
This illusion is known as the "cafe wall illusion" since it was first observed outside a cafe in the 1970s.
A neuropsychology lab worker in Richard Gregory's lab at the University of Bristol discovered this illusion in tiles on a cafe wall in 1979. Hugo Munsterberg also wrote about a similar illusion in 1897.
The gray lines appear to be slanted, but if you cover up the black and white tiles, you'll see that the lines are actually straight. The effect works because of the way neurons interact in the brain, interpreting a brightness contrast between tiles as a small wedge to make the lines appear slanted.
The Müller-Lyer illusion makes it look like all of these lines are different lengths.
A German psychiatrist and sociologist named Franz Carl Müller-Lyer first published the illusion in 1889.
It's not exactly clear why our brains trick our eyes this way, but one theory is that our brains are used to perceiving angles as corners that are near or far away and sees the inward-facing corners as more distant and therefore smaller.
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