Changing alpha and stopping

Hi all

I’m looking for a simple way increase the alpha of an object, but then stop it before it gets to 100
I was thinking somehting like

two_mc._alpha = 0;
one_btn.onRelease = function() {
     _root.onEnterFrame = function() {
          two_mc._alpha += 10;
          if (this._alpha == 50) {
               this._alpha = 50;
               delete this.onEnterFrame;

two_mc is a movieClip, I want to increase its alpha but stop it when it gets to 50
I’m sure its simple, but I tryed a few methods without any results.