Mathematical model of a geometrical surface with two apices, whose equation, referred to a suitable system of co-ordinates, may be written (x-p/a1)2 = 1/k6(c-y)3 (c+y), where p=1/f (c-y0 (c+y), q=1/g3(c-y)(c+y) and a,b,c,f,g & k are all constants.