By Alex Fradera

A short paper in the journal i-Perception presents a disconcerting visual illusion spotted “in the wild”: how stackable chairs, viewed from a certain angle, mess with your head. This is an unedited image, but your mind resists accepting it could be real. The illusion was first noticed in the office of lead author Nick Scott-Samuel at the University of Bristol, who notes in the paper that “it obtains in real life as well as in images, even when sober”.

The cause of the trick appears to be the two “edges” seen coming up from the near-base of the stack – marked AD and BC in the annotated version of the image below – which “suggests a change in depth along those lines which does not actually exist” and makes it seem as if the bars that run from one “edge” to another compose an outward face.

It misleads us a little like the Penrose triangle (see right), but whereas the triangle is actually impossible, the chairs only appear to be so – as demonstrated in this video where you can see the effect dip in and out. Scott-Samuel and his colleagues experimented with the chair stack and it appears that you need at least four chairs to create the effect.

You know the angles, you know the number: turn up to the meeting room early next time and use that recipe to astound even the most jaded eyes.

—Stacking Chairs: Local Sense and Global Nonsense

Alex Fradera (@alexfradera) is Staff Writer at BPS Research Digest