function PopupCentrata() {
   var w = 600;
   var h = 300;
   var l = Math.floor((screen.width-w)/2);
   var t = Math.floor((screen.height-h)/2);
      window.open("http://www.riccilegnami.eu/credits.html","","width=" + w + ",height=" + h + ",top=" + t + ",left=" + l);
 }