HOW to create an IF ELSE statemtents for scrolling

Question

I am looking to create an IF ELSE statement that says IF You reach the end of the UL LI, unbind click function from scroll button, ELSE bind click function

$('#down').bind('click', function() {
 $(".project_thumbs").stop().animate({"top": "-=510px"});


});


 $('#up').bind('click', function() {
$(".project_thumbs").stop().animate({"top": "+=510px"});