function janela (width, height, myname, url)
{
  var left, top;

  if (screen.width < width)
    left = 0;
  else
    left = (screen.width  - width) / 2;

  if (screen.height < height)
    top = 0;
  else
    top = (screen.height - height) / 4;

  if ( navigator.appName == "Netscape" )
  {  var w = window.open (url, myname, "height="+height+",width="+width+ 
    ",screenX=" + left + ",screenY=" + top + ",scrollbars=yes,status=no,toolbar=0,menubar=no,location=0,directories=0,resizable=yes");}
  else
  {  var w = window.open (url, myname, "height="+height+",width="+width+ 
    ",left=" + left + ",top=" + top + ",scrollbars=yes,status=no,toolbar=0,menubar=no,location=0,directories=0,resizable=yes");}

  if (w)
   w.focus ();

}