How to apply simple parallax scrolling

function parallax(){
    var scrolled = $(window).scrollTop();
    $('.bg').css('top', -(scrolled * 0.2) + 'px');
}

$(window).scroll(function(e){
    parallax();
}); 

How to apply parallax effect (changing active class on menu on scroll)

************* HTML**************

<header>
  <nav>
    <span class="movetext">Company Name</span>
    <img class="" src="https://www.matchbook.com/assets/images/icons/white/Twitter_logo_white.png">
    <a href="#portion1" class="active">Portion1</a>
    <a href="#portion2">Portion2</a>
    <a href="#portion3">Portion3</a>
    <a href="#portion4">Portion4</a>
  </nav>
</header>

<div class="portion">
  <h2>Portion 1.</h2>
</div>

<div id="portion1" class="info">
  <h3>Heading for Portion 1</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Debitis nobis ut dicta quisquam officia deleniti, amet. Obcaecati atque, cumque quae culpa ad hic commodi provident pariatur, iure nobis voluptatibus laborum amet temporibus quia, dolores veritatis, repellendus consectetur. Ut ullam officiis minus quod assumenda facere quaerat, esse, magni, aliquid culpa vero.</p>
  <div class="button">
    <a href="#">Example Button</a>
  </div>
</div>

<div class="portion two">
  <h2>Portion 2</h2>
</div>

<div id="portion2" class="info">
  <h3>Heading for Portion 2</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Tempora pariatur voluptate laboriosam impedit praesentium sed, nihil, dignissimos et minima recusandae quaerat enim consectetur. Molestiae assumenda distinctio, rem nostrum dolores repellendus.</p>
  <div class="button">
    <a href="#">Example Button</a>
  </div>
</div>

<div class="portion">
  <h2>Portion 3.</h2>
</div>

<div id="portion3" class="info">
  <h3>Heading for Portion 3.</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Odio accusamus in vitae saepe harum soluta reiciendis eaque repellat distinctio quos voluptate nemo error ratione numquam, nisi quibusdam veritatis repudiandae ea possimus rerum pariatur dolores? Optio inventore iusto, voluptatibus numquam aperiam, harum maxime beatae minima aliquam quas sapiente totam cumque unde..</p>
  <div class="button">
    <a href="#">Example Button</a>
  </div>
</div>

<div class="portion two">
  <h2>Portion 4.</h2>
</div>

<div id="portion4" class="info">
  <h3>Heading for Portion 4.</h3>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Magni incidunt quos iste voluptates, tenetur ad repudiandae ea fuga eveniet quam, unde iure suscipit rem odit in, sint nulla itaque? Labore beatae, est voluptatibus saepe rerum illum repudiandae quasi perspiciatis, molestiae quidem fugiat voluptates voluptate neque totam earum enim mollitia iure quod tempora veritatis quam optio. Error odit laudantium eum voluptate.</p>
  <div class="button">
    <a href="#">Example Button</a>
  </div>
</div>
<footer class="info">
  <div id="toTop"><a href="#"><img src="http://www.superdruglookatme.com/wp-content/themes/superdrug/images/top.png"></a></div>
  <p>&copy; 2014 Arif Hussain Bangash</p>
 
</footer>

********************************CSS**************************************

body {
  margin: 0;
  padding: 0;
  font-family: "Source Sans Pro", sans-serif;
  font-size: 20px;
}

header nav {
  text-align: right;
  background: rgba(0,0,0,0.6);
  color: white;
  font-size: 1em;
  position: fixed;
  width: 100%;
}
header nav a {
  color: inherit;
  text-decoration: none;
  display: inline-block;
  padding: 2em 1em .5em;
  -webkit-transition: all .3s ease-in;
}
.small {
  padding: .5em 1em;
}
header nav a:hover {
  text-decoration: underline;
}

.active {
  background: rgba(255,255,255,0.3);
}

.portion {
 
  background-image:  url('http://p1.pichost.me/640/60/1842352.jpg');
  background-position: center center;
  background-attachment: fixed;
  background-size: cover;
  text-align: center;
}
.portion.two {
  background-image: url('http://cdn.desktopwallpapers4.me/media/thumbs_400x250/4/31677.jpg');
}
.portion h2 {
  color: white;
  font-weight: normal;
  font-size: 3em;
  padding: 2em 0;
  margin: 0;
  text-shadow: 0 0 10px black;
  background: rgba(0,0,0,0.6);
}

.info {
  background: #FFFAF0;
  color: #777;
  padding: 3em 20%;
}

.button {
  text-align: center;
}
.button a {
  display: inline-block;
  border: 2px solid #777;
  padding: .75em 1.5em;
  color: #777;
  text-decoration: none;
  text-transform: uppercase;
  border-radius: 5px;
  letter-spacing: .15em;
  word-spacing: .25em;
  font-weight: bold;
  transition: all 0.3s ease-in-out;
}
.button a:hover {
  color: #FFFAF0;
  background: #777;
}
footer.info {
  color: #FFFAF0;
  background: #777;
  padding: 0 auto;
  text-align: center;
}

#toTop img {
  width: 50px;
}
#toTop img:hover {
  cursor: pointer;
}
nav img {
  z-index: 1000;
  height: 2.5em;
  position: absolute;
  left: 40px;
  top: 10px;
  transition: all 0.5s linear;
}

.move {
  top: -60px;
}

nav span {
  position: absolute;
  left: 40px;
  top: 10px;
  font-weight: bold;
  transition: all 0.5s ease-in;
}

.movetext {
  top: -30px;
}


*******************************jQuery****************************


// Smooth Scroll on clicking nav items
$('nav a').click(function () {
  var $href = $(this).attr('href');
  $('body').stop().animate({
    scrollTop: $($href).offset().top
  }, 1000);
  return false;
});

// back to top
$('#toTop a').click(function () {
  $('body').animate({
    scrollTop: 0
  }, 1000);
  return false;
});

// Parallaxing + add class active on scroll
$(document).scroll(function () {
 
  // parallaxing
  var $movebg = $(window).scrollTop() * -0.3;
  $('.portion').css('background-positionY', ($movebg) + 'px');

  // add class active to nav a on scroll
  var scrollPos = $(document).scrollTop() + 100;
  $('nav a').each(function () {
    var currLink = $(this);
    var refElement = $(currLink.attr("href"));
    if (refElement.position().top <= scrollPos && refElement.position().top + refElement.height() > scrollPos) {
      $('nav a').removeClass("active");
      currLink.addClass("active");
    }
  });
 
  // changing padding of nav a on scroll
    if (scrollPos > 250) {
      $('nav a').addClass('small');
      $('nav img').addClass('move');
      $('nav span').removeClass('movetext');
    } else {
      $('nav a').removeClass('small');
      $('nav img').removeClass('move');
      $('nav span').addClass('movetext');
    }
 
});