A richly decorated wooden base supports a column carrying a circular table. Along the edges of the table are inserted 36 ivory sectors, each of 10°. A diaphragm with three circular holes with shutters slides on the edge. A metal mirror is set in a brass frame at the center. Projecting a beam of light through the diaphragm onto the mirror, we can observe that it is reflected by an angle equal to the angle of incidence.