A multi-focal contact lens wherein the lens is manufactured at least partially from a responsive polymer gel. The piston cooperates according to a support zone with the pad facing it. This preserves the electrical, optical, and mechanical properties of the plurality of epitaxial layers. Then, it is determined whether the amplitude of the third order harmonic component is sufficiently large compared with the obtained noise level. The grinding fluid supply-nozzle 50 is made from a curve portion 53, an opening 51 and a straight portion 52 between the curve portion 53 and the opening 51.