The apparatus invented by Newton for the recomposing of white light decomposed into the seven basic colours by means of a prism, consists of seven small, flat, round mirrors (diameter 3.2 cm) with frames of burnished brass. They can rotate around two orthogonal axes on top of a brass staff (length 36 cm) on an articulated joint on a column that can rotate (height 30 cm), it too brass, held by a heavy round base.
Each of the seven beams emerging from a prism illuminated by a beam of white light is made to fall at the centre of one of the mirrors which, when appropriately rotated, make them all converge on the same point, thus recomposing the white light. It is obvious that the task of regulating the mirrors is one of great difficulty.