Window opacity isn't being affected?

        private void Button_Click_2(object sender, RoutedEventArgs e)
        {
            if (overlay.thisOverlay != null)
            {
                overlay.thisOverlay.makeOpaque();
            }    
        }

// This function is in the "overlay" class
        public void makeOpaque()
        {
            this.Opacity = 1;
            Trace.WriteLine("Works");
        }