function altern ()
{
if (document.getElementById("div1").style.visibility=="visible") 
	{document.getElementById("div1").style.visibility="hidden"
	 document.getElementById("div2").style.visibility="visible"}
else {document.getElementById("div1").style.visibility="visible"
	 document.getElementById("div2").style.visibility="hidden"}
setTimeout("altern()",3000)  
	
}