function pop(url, width, height)
   {
   if (document.all)
      {
      var x = window.screenLeft;
      var y = window.screenTop;
      var w = window.document.body.offsetWidth;
      var h = window.document.body.offsetHeight;
      }
  else
   {
   var x = window.screenX;
   var y = window.screenY;
   var w = window.outerWidth;
   var h = window.outerHeight;
   }
  var cntx = x + Math.round((w - width) / 2);
  var cnty = y + Math.round((h - height) / 2);
  window.open (url, null, 'left=' + cntx + ',top=' + cnty + ',width=' + 
width + ',height=' + height);
}