There is a piece of white paper of the same size glued to the disk. The centre of the paper (diameter 8.5 cm) and a strip around at its edge (width 2.5 cm) are painted black. The remaining circular crown is divided into five equal sectors, each of which subdivided into seven others. Each of these sectors is painted with one of the seven basic colours in an attempt to produce five identical successions of seven colours as close as possible to the spectrum of solar light.