Reveal Module Pattern Counter Not Updating

I am struggling with the Modular Reveal Pattern. This small example illustrates the issue I am having. When the 'Add' button is clicked the 'counter' value increases by 1. When the 'Show' button is clicked Module_2 logs the change to Module_1's counter value. Unfortunately Module_2 doesn't detect the change.

Is there a way to get Module_2 to 'see' the change to the 'counter' in Module_1?

  var Module_1 = (function() {
    $("#test1").on("click", add);
    var counter = 0;
    function add() {counter += 1;}
    return {counter : counter};
  })();

  var Module_2 = (function() {
    $("#test2").on("click", Show);
    function Show() {console.log(Module_1.counter);}
  })();