For a doublet at S, of moment m, the Stokes' function is M f cos PSx = - m p s3; and for its image at H the Stokes' function is m f cos PHx =m f 3 PH" (6) so that for the comnation _ a3 I I 2 4)-myb12 (f 3 PH PS 3) =m f 3 (pa ll 3 P53)' 3 and this vanishes over the surface of the sphere.