function preview_window(file_name)
{
  var mywin;
   W = 590;
   H  = 440;
   var myW = W ? W : screen.availWidth;
   var myH = H ? H : screen.availHeight;
   mywin = window.open(file_name,"",'width='+myW+',height='+myH+', top=2,left=2,scrollbars=yes');
   mywin.resizeTo(myW,myH);
}