[bes]

строка 25

var str = '.' + 10;
alert(str + '\n' + typeof(str));

<div id="slider">
	<img src="http://javascript.ru/forum/image.php?u=19820&dateline=1334914235" id="first_img">
	<img src="http://javascript.ru/forum/image.php?u=19820&dateline=1334914235" id="second_img">
</div>
<script>
window.onload = function () {
	var first  = document.getElementById("first_img");
	var second = document.getElementById("second_img");
	var n = 1;
	var int = setInterval(function () {
		n++;
		first.style.opacity = n/10;
		second.style.opacity = '.' + (10 - n);
		if (n == 10) {
			clearInterval(int);
		}
	}, 500);
}
</script>