document.getElementById().行26 错误：缺少对象

求高手帮忙！！！

行26： document.getElementById("focusThree1").style.display="block";

整段代码为：<SCRIPT language=javascript>

var nThree;
nThree=1;
setTimeout('change_Three()',6000);
function change_Three()
{
if(nThree>4) nThree=1
setTimeout('setFocusThree('+nThree+')',6000);
nThree++;
Threett=setTimeout('change_Three()',6000);
}
function setFocusThree(i)
{
selectLayerThree(i);
}
function selectLayerThree(i)
{
switch(i)
{
case 1:
document.getElementById("focusThree1").style.display="block";
document.getElementById("focusThree2").style.display="none";
document.getElementById("focusThree3").style.display="none";
document.getElementById("focusThree4").style.display="none";
document.getElementById("foc

document.all. document.all document是什么意思？ document 方法 document.cookie document.getElementById document.open() Stenography Document document.layers document.all是什么意思 document.implementation&&document.implementation.createDocument是什么意思