var i = 1;
var interval = 6000;
var direction = 1;
var duration = 3;
var to = 100;
var from = 0;

function fade_images() {
  
  opacityTween = new OpacityTween(document.getElementById('photo_' + i),Tween.regularEaseOut, from, to, duration);
  opacityTween.start();
  
 
  if (i == num_of_images && direction == 1) {
    direction = 0;
    i++;
  }
  if (i == 2 && direction == 0) {
    direction = 1;
    i--;
  }
  
  if (direction == 1) {
    i++;
    to = 100;
    from = 0;
  }
  if (direction == 0) {
    i--;
    to = 0;
    from = 100;
  }
  
  setTimeout('fade_images()', interval);
}