Can't get square brackets to target Movie Clip

Hiya, I’m stuck with trying to target a movieclip dynamically from a loop. I’m using the square brackket syntax but I can’t figure out why it’s not working.
Here’s what I’m trying to do:

this.t1.t1._alpha=20;

and here’s how I’m trying to do it:

this["t"+1+".t"+1]._alpha= 20;

If someone could point me in the right direction it would be mucho appreciated.
Thanks :slight_smile:
Schm