World's simplest background parallax effect

How to apply the World's simplest background parallax effect to you web site background image for the scroll function.

$(window).scroll(function() {
    var x = $(this).scrollTop();
    $('#slide5').css('background-position', '100% ' + parseInt(-x / 3.5) + 'px' + ', 0% ' + parseInt(-x / 5) + 'px');
});