Go back a couple of centuries and there were no chains of shops or malls. In the high street in the UK you would have found the type of shop you were after by looking out for a sign hanging out. There were signs for pharmacists, tobacconists, pawnbrokers, whatever. Nowadays there’s just one traditional sign still sometimes to be seen – the barber’s pole, as left in the animation.
The barber’s sign shows a famous illusion. The cylinder is rotating horizontally around a vertical axis, but the stripes look as if they are rising – which would be impossible, unless you had some long pole sliding through the cylinder.
You can begin to see why in the demo on the right: focus on the vertical slot and the grating seems to be moving vertically (as in the barber’s pole). But focus on the horizontal slot and in a moment the grating may seem to move horizontally. Behind the round hole, for me it tends to look as if moving obliquely.
Want to know more about what’s going on?