var alpha = 0;
function play(){
timer = setInterval(function(){
alpha += 2;
alpha > 100 && (alpha = 100);
aImg[index].style.opacity = alpha / 100;
aImg[index].style.filter = "alpha(opacity = " + alpha/100 + ")";
alpha == 100 && clearInterval(timer);
},40)
}

function css(obj,attr,val) {
switch(arguments.length) {
case 2:
if(typeof arguments[1] == "string"){
return obj.currentStyle ? obj.currentStyle[attr] : getComputedStyle(obj,null)[attr];
}else{
for(var i in attr) {
obj.style[i] = attr[i];
}
}
break;
case 3:
obj.style[attr] = val;
break;
default:
alert("错误参数");
}
}