function fenster( url){

                  // Berechnung der Mitte
                  windowwidth = 450;
                  windowheight = 500;

                  if (document.all){
                      var breite = screen.width;
                      var hoehe = screen.height;
                  }
                  else {
                      if (document.layers){
                           var breite = window.outerWidth;
                           var hoehe = window.outerHeight;
                      }
                  }

                  var left = (breite - windowwidth) / 2;
                  var top = (hoehe - windowheight) / 2;

                  my_new_window = window.open (url, "fenster", "menubar=no,toolbar=no,status=no,resizable=no,scrollbars=yes,width="+windowwidth+",height="+windowheight+", screenX="+left+",screenY="+top+",top="+top+",left="+left+"");

}
function bild( url){

                  // Berechnung der Mitte
                  windowwidth = 350;
                  windowheight = 500;

                  if (document.all){
                      var breite = screen.width;
                      var hoehe = screen.height;
                  }
                  else {
                      if (document.layers){
                           var breite = window.outerWidth;
                           var hoehe = window.outerHeight;
                      }
                  }

                  var left = (breite - windowwidth) / 2;
                  var top = (hoehe - windowheight) / 2;

                  my_new_window = window.open (url, "fenster", "menubar=no,toolbar=no,status=no,resizable=no,scrollbars=yes,width="+windowwidth+",height="+windowheight+", screenX="+left+",screenY="+top+",top="+top+",left="+left+"");

}