function pop_up(mypage, w, h, scroll, resizable) {
  var winl = (screen.width - w) / 2;
  var wint = (screen.height - h) / 2;
  winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',scrollbars='+scroll+',resizable';
  win = window.open(mypage, 'nw', winprops);
  if (parseInt(navigator.appVersion) >= 4) { win.window.focus(); }
}