Pulfrich Phenomenon
Place a neutral density (gray) filter or a dark sunglass lens (either polarized or unpolarized is fine) over one eye, and look at the pendulums. Now place the filter over the other eye. Use the slider to vary the speed of the pendulums. The button at the lower left toggles the sound.
Pick a pleasant image from the choices at the lower right. (Pictures from B.U. Photo Services).