Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. The method is also extended to cover the case of one surface deviating slightly from a plane. The only difference in this problem is that the boundary conditions of continuity are now to hold on a surface which is not quite plane. The method adopted is to consider the boundary conditions on the plane from which the actual surface deviates slightly. The effect of this approach is that the components tangential to this plane will now have known discontinuities across the plane; the formulation will otherwise be the same as for plane surfaces.