我照着老师写的为什么没有效果

#box {width:200px;height:200px;opacity:0.3;margin:0 auto;background-color:#F00;}

</style>

<script type=text/javascript>

window.onload=function(){

var bo1=document.getElementById(box);

bo1.onmouseover=function(){

boc(100);

}

bo1.onmouseout=function(){

boc(30);}

}

var timer=null;

var va=30;

function boc(ita)

{

clearInterval(timer);

var bo1=document.getElementById(box);

    timer=setInterval(function(){

var speed=0;

if (va>ita)

{

speed=-10;}

else

{

speed=10;}

if (va==ita)

{

clearInterval(timer);

}

else{

va+=speed;

bo1.style.opacity=va/100;

}},30)

}

</script>