Fascinating desktop illusions! Spin the holographic disk on the mirror. Does it speed up? Listen to its rhythmic sounds. Why does the humming get louder as the speed of the disk increases? This patented 3" Diameter chrome-plated steel disk creates a hypnotic display of light and sound as it rotates. Includes Euler's disk, 9" Concave mirror base and nine pieces of magnetized holographic foil. Complete with instructions and the scientific answers!