a method of making a glass die for moulding progressive concave - convex ophthalmic lenses of organic glass , i . e . of synthetic polymeric material , which have their convex surface of aspheric shape , wherein a glass blank for the die is formed with its convex surface of aspheric shape and is then supported on a support member having a concave spherical support surface while being heated to a temperature at which the glass deforms without flowing , so that the convex surface sags to conform with the spherical support surface and the aspheric character is transferred to the concave surface .