Found in the mathematics section of the Science Museum, this model was used in the late ninteenth century to illustrate lectures on equations. It is the surface for: Z=3a(x2-y2)-(x3+y3)
The blue line is a straight line, the green lines are ellipses, the red lines are parabolas, the black and yellow lines - cubic equations and the dark red line - a simple contour line.
Models like this inspired artists like Henry Moore and Max Ernst.