如何取得div内被选中的文字

<HTML>
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
<!--
function Test()
{
///////////////ie///////////////////
        var selecter=document.selection.createRange();
        alert(selecter.text);
//////////ff////////////////
       //var selecter=window.getSelection();
      //alert(selecter);}
//-->
</SCRIPT>
</HEAD>
<BODY>
<div id="sel">abcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefg</div>
<INPUT TYPE="button"  value="click" onclick="Test();">
</form>
</BODY>
</HTML>