What: The Zöllner Illusion (pronounced TSOLL-ner, or just “zollner” if you’re not feeling fancy) is one of those delightfully old illusions that still gets me every time. It was discovered way back in 1860 by Johann Zöllner, a German astrophysicist who was probably just trying to look at fabric patterns and got thoroughly distracted [1].

Once you start playing around with the controls, the effect will be pretty clear… and it’s at least a little bit more fun to me knowing that the diagonal lines are causing my brain to glitch out.

Another cool thing, is you can look here [2] and see the original drawing amongst a bunch of, what I can only call “space-y stuff”

How To: Try following a single line from left to right with your finger, or better yet grab something with a straight edge. You’ll confirm it’s perfectly horizontal, and then the moment you look at the whole thing again… tilt city.

Play around with the hatch angle in the controls, for me the illusion hits hardest around 45 degrees (makes sense). You can also turn on the animation to watch the hatches slide along, which makes the lines look like they’re full on wavey. Different thicknesses and angles can increase/decrease the intensity.

Explain it: The going theory for this effect is something called “angle expansion.” Basically, your visual system tends to overestimate acute angles, making them look bigger than they are. The hatches create these acute angles with the main lines, and your brain expands them, which makes the whole line appear tilted.

The fact that the hatches alternate direction on each line just amplifies everything, each line tilts the opposite way from its neighbor, creating that wild herringbone effect.

Fun fact: Zöllner was an astronomer! He stumbled onto this while examining a fabric pattern. Some of the best discoveries happen when you’re looking at something completely unrelated.

Cites and Extras:

• [1] The Zöllner Illusion Wiki - Good overview of the history and the man himself
• [2] The VERY original amongst his other musings from 1800!
• [https://en.wikipedia.org/wiki/Poggendorff_illusion](An illusion within an illusion) If you’re looking closely, you may also accidetnally stumble into this illusion when your hatches are densley set.
• You might also enjoy the Café Wall illusion, one I built last year, another classic case of parallel lines misbehaving, though it is also interesting that one uses perpendicular lines to create the illusion, and the zolner diagonals.

I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via toymaker@toms.toys, be kind!