function toggle_visibility(id) {
  var e = document.getElementById(id);
  if(e.style.visibility == 'visible')
  {
    e.style.visibility = 'hidden';
  }
  else
  {
    document.getElementById('historia').style.visibility = 'hidden';
    document.getElementById('kuvat').style.visibility = 'hidden';
    e.style.visibility = 'visible';
  } 
}