Please help, simple (i guess) code problemo

Hello everyone!
I am using this code, but for some reason it doesnt work…
I have all the movie clips (mc_effect_1 to mc_effect_750) on my stage.
Why it doesnt work?
Any clues?
Thanks.
Cheres.
Leo.


var i =0;
while (i < 751)
{
    _root.mc_list_streets["mc_effect_"+i].onRollOver = function ()
	     {
  		_root.mc_list_streets["mc_effect_"+i]._alpha = 30;
     };
	i++;
}