function popup_window( url , width , height )
{
    var x = (screen.width - width)/2;
    var y = (screen.height - height)/2;
    var props = "screenX="+String(x)+",screenY="+String(y)+",height="+String(height)+",width="+String(width)+",scrollbars";
    window.open( url , "" , props );
    return false;
}