jquery 居中设置

<SPAN style="FONT-SIZE: medium">
<script type="text/javascript" src="js/jquery-1.4.2.js"></script>   
  
<script type="text/javascript">   
$(document).ready(function() {   
    jQuery.fn.center = function() {   
       this.css("position", "absolute");   
        this.css("top", ($(window).height() - this.height()) / 2 + $(window).scrollTop() + "px");   
       this.css("left", ($(window).width() - this.width()) / 2 + $(window).scrollLeft() + "px");   
       return this;   
    }   
   $("#id").center();   
});   
</script>   
   
</SPAN>   
  
<SPAN style="FONT-SIZE: medium">//ID为要在屏幕上下左右居中的元素ID，而不是class</SPAN>