Originally published at: Check out this trippy optical illusion | Boing Boing
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Déjà vu:
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First discovered in the 13th century!
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No, I came here to pedant that there’s no illusion here. You can make a mechanical model of this, and the ring of dots really does roll around the large circle; in fact you can use this to couple a rolling motion to a linear motion(s).
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A former colleague of mine, who researches visual perception and so ends up making a lot of optical illusions, was telling me about Tusi couples a while back. An interesting thing is how the brain goes from seeing one or a few points going back and forth to seeing a circle turning. There are other interesting effects that you can see when the oscillations are out of phase. (I’d point you to his website, but I only seem to be able to find the old flash based one.)
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Arguably the motions of the ‘Trammel of Archimedes’ has the same projection of circular motion onto lines. This works with any number of tracks, not just two ( Trammel of Archimedes - Do Nothing Machine part 2 // Homemade Science with Bruce Yeany - YouTube ). There’s nothing actually associating Archimedes with the device, or that he used more than two tracks.
The illusion (if there is one) may lie in the perception of the dots as separate motions along lines, or as a rolling circle, but not both at once. In that way, it is similar to the Kaninza triangle, which is commonly classed as an illusion.
Here, the Pac-Man shapes at the corners give a strong impression of an overlaying white triangle. Rotate one Pac-Man sightly and the sense of the triangle disappears. We could argue that the Kaninza triangle is objectively ‘there’ or ‘not there’ and there is no illusion. Or the cat in the photo is actually a cat and not a crow. Or the dress is actually blue and black.
Here, any number of points could be sat within holes in a rigid circle. But for me, about when the fifth dot is added, I get the impression of a tumbling ‘house shape’ rather than five moving dots. ‘Illusion’ is a loose term, applied both to physical effects such as mirages, and also to perceptual effects that may tell us something about how we see.
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Of course, timing of movement is critical. If they were not synchronised it would hardly matter that the balls are on straight paths, as it would all end up in a sad mess
This is an optical illusion as much as a TV is: watch what happens when you use just three colors, RGB, and then make a 1920x1080 grid of them illuminate in a specific pattern!
Witchcraft maybe, but not an optical illusion.
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This reminded me of Margaret Livingstone’s book on how our visual system works and why we perceive illusions and artwork like we do. She’s a neurobiologist but the book is intended for a general audience for anyone interested in the topic
We’re getting some complete deconstructions of murine visual cortex and hippocampus; it’s only a matter of time until ‘we’ smash the stack and add the feeling of smelling a lush rosemary bush and of having stopped believing in the good of voter suppression. Or a stable, and trading in BTC$ in the Dominican Republic. Lavender and a distant memory of reading market news without an investor-heavy angle. Maybe context switch-ups for various media outlets, second tellings, etc.
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Machine Design 101 
The missing link to the interactive version referenced in the post :
… is this an illusion?
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