function flipBook() {
  var imgOld = jQuery('div#imgStack img.topLevel');
  var imgNew = imgOld.next().length ? imgOld.next() : jQuery('div#imgStack img:first');

  // console.log('old image, id: ' + imgOld.attr('id') + ', new image, id: ' + imgNew.attr('id'));
  imgOld.addClass('belowTopLevel');
  imgOld.removeClass('topLevel');
  imgNew.css({opacity: 0.0});
  imgNew.addClass('topLevel');
  imgNew.animate({opacity: 1.0}, 1800, function() {
    imgOld.removeClass('belowTopLevel');
  });
}

jQuery(document).ready(function() {
  setInterval('flipBook()', 4000 );
});