JavaScript ロールオーバー時に画像の透明度を変更する

<!-- 宣言 -->
<script language="Javascript">
<!--
function over(object){
    document.all(object.id).filters(0).Opacity = 100;
}

function out(object){
    document.all(object.id).filters(0).Opacity = 50;
}// -->
</script>

<!-- 呼び出し -->
<img src="back.gif" id="back"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">
<img src="top.gif"  id="top"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">
<img src="home.gif" id="home"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">
<img src="mail.gif" id="mail"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">
<img src="menu.gif" id="menu"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">
<img src="next.gif" id="next"
    style="filter:alpha(opacity=50);" onMouseOver="over(this)" onMouseOut="out(this)">