Trouble fading element on scroll

Question

I have a fixed-position article element that I would like to fade out when scrolling the page. I'm not very experienced in Javascript, but after some research I have put together this script;

<script type="text/javascript">

        //when the DOM has loaded
        $(document).ready(function() {

            //attach some code to the scroll event of the window object
            //or whatever element(s) see http://docs.jquery.com/Selectors
            $(window).scroll(function () {
                  var height = $('body').height();
                  var scrollTop = $('body').scrollTop();
                  var opacity = 1;

                  // do some math here, by placing some condition or formula
                  if(scrollTop > 400) {
                      opacity = 0.5;
                  }

                  //set the opacity of div id="someDivId"
                  $('#instructions').css('0', opacity);
            });
        });
    </script>

This doesn't seem to be working, and the element remains at full opacity when scolling (the website is here http://edharrisondesign.com/pocketpictograms/ ).

Any ideas why? Thanks in advance.

Answer 1

You may need to specify the height of the body if you are using var height = $('body').height(); . However, make the following change and it should work:

$('#instructions').css('opacity', opacity);

DEMO: http://jsfiddle.net/SLGdE/20/