-curves must not intersect each other, and through each point of the surface one and only one curve must pass. Thus a perfectly definite value of
belongs to every point on the surface of the marble slab. In like manner we imagine a system of
-curves drawn on the surface. These satisfy the same conditions as the
-curves, they are provided with numbers in a corresponding manner, and they may likewise be of arbitrary shape. It follows that a value of
and a value of