\documentclass[pstricks]{standalone}
\usepackage{pst-contourplot,pst-math,multido}
\begin{document}
% 4 dip�les de Hertz aux sommets d'un carr�
% �volution du champ au cours du temps
\multido{\rt=0+0.2}{32}{%
\begin{pspicture}(-6.25,-6.25)(6.25,6.25)
\pstVerb{/t \rt\space def /k0 2 PI mul def
/xi1 -1 def /xi2 1 def /xi3 -1 def /xi4 1 def
/yi1 -1 def /yi2 1 def /yi3 1 def /yi4 -1 def}%
%\psframe*(-6.25,-6.25)(6.25,6.25)
\multido{\rc=-1.1+0.2,\n=0.0+0.1}{11}{
\definecolor{HERTZ}{hsb}{\n,1,1}
\psContourPlot[unit=2.5,a=0.025,linewidth=0.02,linecolor=HERTZ,
function=
/r1 x xi1 sub dup mul y yi1 sub dup mul add sqrt k0 mul def
/theta1 y yi1 sub x xi1 sub atan def
/r2 x xi2 sub dup mul y yi2 sub dup mul add sqrt k0 mul def
/theta2 y yi2 sub x xi2 sub atan def
/r3 x xi3 sub dup mul y yi3 sub dup mul add sqrt k0 mul def
/theta3 y yi3 sub x xi3 sub atan def
/r4 x xi4 sub dup mul y yi4 sub dup mul add sqrt k0 mul def
/theta4 y yi4 sub x xi4 sub atan def
r1 t sub COS r1 t sub SIN r1 div add theta1 sin dup mul mul
r2 t sub COS r2 t sub SIN r2 div add theta2 sin dup mul mul add
r3 t sub COS r3 t sub SIN r3 div add theta3 sin dup mul mul add
r4 t sub COS r4 t sub SIN r4 div add theta4 sin dup mul mul add
\rc\space sub](-2.5,-2.5)(2.5,2.5)}%
\end{pspicture}}
