Convert a string into a movie clip

i’m looping through and wanting to store button names for use with an onRelease function. my code is this in the loop:

var invis = “gallery_mc.thumbs_mc.invis” + i + “_btn”;

however, if i try using this:

invis.onRelease = function() {

it doesnt work. i’m almost positive its because the invis variable renders out a string. if i hardcode it, it does work. so my question is, how can i convert the invis string into a movie clip or button so i can use it with the invis.onRelease function?