function popup( ww, wh, wu, wn )
{
   var xc = ( screen.width - ww ) / 2; var yc = ( screen.height - wh ) / 2;
   window.open( wu, wn, 'width=' + ww + ',height=' + wh + ',left=' + xc + ',top=' + yc + ',screenX=' + xc + ',screenY=' + yc + ',resizable=yes,scrollbars=yes,toolbar=no,location=no,directories=no,status=no,menubar=no,copyhistory=no' );
}