The café wall

---

---

Are these lines straight and parallel?

Although barely believable, all these lines are straight, and parallel to one another.

This illusion comes about because the black and white rectangles do not sit directly above one another.

You clearly see the horizontal grey space between the black and the black, and accentuate this difference.

However, the horizontal grey between the white and the black rectangles is not so clear.

Your visual system decides this space is not real, so shrinks it.

This has the effect of producing lots of wedges, which your brain puts together and decides you are looking at non-parallel wavy lines.

---

Spotted in a café

Walking past a café here in Bristol, Professor Richard Gregory and members of his lab spotted this illusion created by the tiling.

After several months of theories, discussion and the building of interactive models, they had understood how our visual systems create the café wall illusion.

This café is on the corner at the bottom of St Michael’s Hill, Bristol (pictured).

Sadly, Professor Richard Gregory passed away on 17 May 2010. Professor Richard Gregory was founder of the Exploratory, the first hands-on science centre in the UK.