In an electrostatic field the relationship of the variation of the field magnitude along a flux line(dE/dn)+2H(n)E=0 has been derived by the geometrical method. Its exact and approximate solutions of field magnitudes can be obtained in a specific condition. It is noted that when it is used on a boundary of the field
the solution can not be used to obtain any exact formulation for the surface charge density on a coductor.