Model of a Cubic Surface, c 1875.
© Science Museum / Science & Society Picture Library
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.