Once upon a time all barber shops were marked with a "barber pole." I've seen two types: with alternating red and white bands, and with alternating red, white and blue bands. The illusion is probably familiar to you. The bands are printed diagonally on a cylindrical card which rotates inside a transparent glass cylinder. But when you view the cylinder from any point, the bands seem either to be continuously rising or continuously falling, depending on direction of rotation. The same illusion is seen whenever a moving flat piece of art showing diagonal bands of contrasting colors is viewed through a narrow slit. This makes the illusion a natural to display with java applets, flash, shockwave or other standard animations. It is easy to construct the animation to be interactive, so that the viewer can control various aspects that give rise to the illusion. You can find such interactive pages here, or here, or here, or here.

In this animation, the same moving block of diagonal stripes is seen to be moving horizontally as seen through the horizontal part of the plus-shaped slit, and vertically as seen through the vertical part of the slit! Concentrate on the very center, where the two slits intersect... what do you make of that?

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One last, very interesting version of the illusion.