A sheet of colour-coated glass hangs down from the ceiling. An electric motor turns it at the rate of half-a-revolution per minute. A spotlight illuminates it, so that the purple reflections from the glass move through the room at a corresponding speed. The light shining through the pane creates a yellow shape on the opposite wall, which changes from spherical to elliptical, depending upon its position. The reflected light and the light from the spotlight complement each other. The shades of colour change, depending upon the angle.