Plaster Surface Model by Alexander Crum Brown, c 1900.
© Science Museum / Science & Society Picture Library
Alexander Crum Brown (1838-1922), mathematician and chemist, was profesor of mathematics at Edinburgh University and a prolific maker of mathematical models. This surface represents the equation: z=3a(x squared - y squared) - (x cubed + y cubed). Every section made by a plane pasing through the blue line and cutting the surface is an ellipse.