first() next() last()

first() next() last()

hi, every body. help me
why my this code not run????
<html>
<head>
  <style>
  .box{
     background-color : gray ;
     height : 200px ;
     width : 500px ;
  }
  </style>
  <script src="js/jquery-latest.js"></script>
  <script language="javascript">
  $(document).ready(function(){
        $("#btn1").bind('click',function(){
           var $p1=$("#div1 div").first();
           $p1.css('color','yellow');
           var $p2=$("#div1 div").last();
           while($p1 != $p2){
                $p1=$p1.nextAll();
                $p1.css('color','yellow');
           }
        });
  });
  </script>
</head>
<body>
<div id="div1" class="box">
    <div>1</div>
    <div>2</div>
    <div>3</div>
    <div>4</div>
</div>
<button id="btn1">test</button>

</body>
</html>