FOUR
This shows the trails of some specks of colour as they emanate from four moving masses. Each mass attracts one of the other masses and all the specks of colour that emanate from it with a force that diminishes with the square of the distance: the mass that emits specks of green attracts the specks of violet and the mass that emits them, which in turn attracts the specks of blue and the mass that emits them, which in turn attracts the specks of ochre and the mass that emits them, which in turn attracts the specks of green and the mass that emits them. Each speck of colour fades as it moves.