MouseOver Speeds

I have an image of a twitter icon in black and white and a second image of it in color. When the mouse hovers over it changes to the colored one. However I can only choose between 'fast' and 'slow' speeds. I tried putting in numbers but it didn't work. Is there a way to make the change even faster.


<script type='text/javascript' src='../jquery.js'></script>
<script type='text/javascript'>
$(document).ready(function(){

$("img.a").hover(
function() {
$(this).stop().animate({"opacity": "0"}, "fast");
},
function() {
$(this).stop().animate({"opacity": "1"}, "fast");
});

});
</script>
<style>
div.fadehover {
position: relative;
}

img.a {
position: absolute;
left: 0;
top: 0;
z-index: 10;
}

img.b {
position: absolute;
left: 0;
top: 0;
}
</style>



<div class="fadehover">
<img src="../../../twitter1.png" alt="" class="a" />
<img src="../../../twitter2.png" alt="" class="b" />
</div>