jQuery选择器中空格的有关问题 _Web前端

jQuery选择器中空格的问题
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
 <head>
  <title> New Document </title>
  <meta name="Generator" content="EditPlus">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">

 <script type="text/javascript" src="jquery-1.4.4.js"></script>

<script type="text/javascript">

$(function()
{
	alert($('.test :hidden').length); //选择class为test的元素当中的隐藏子元素
	alert($('.test:visible').length); //选择隐藏的class为test的元素
});
</script>
</head>
<body>
  
 <div class="test">
    <div style="display:none">aaaa</div>
    <div style="display:none">bbbb</div>
    <div style="display:none">cccc</div>
    <div class="test" style="display:none">dddd</div>
 </div>

 <div class="test" style="display:none">eeee</div>
 </body>
</html>