Model of a Cubic Surface, c 1875.

Model made by Olaus Henrici. Henrici taught mechanical engineering at the City and Guilds College, South Kensington, London, in the late 19th Century. This model follows the formula: xyz=k cubed, (x+y+z-1) cubed. On it are drawn the 27 straight lines that can be found on a cubic surface, but in this case the lines form three groups of nine so appear as three.