Arthur G. Shapiro: “Helix Rotation”. American University. USA
The display consists of two columns of dots that move back and forth horizontally. What to do: Put a dark filter over one eye, and keep both eyes open. What to notice: 1. The dots look like a three-dimensional helix. 2. When the dark filter is over the left eye, the helix rotates clockwise; when the filter is over the right eye, the helix rotates counterclockwise. 3. When the viewer uses no filter but blurs the image by squinting both eyes or by using a defocusing lens, motion is upwards for one helix and downwards for the other.