//Set the opacity of butterfly and copy it onto
          // the display. Then repaint the display.
          butterfly =
                  setOpacity(butterfly,slider.getValue());
          drawPictureOnPicture(
                  butterfly,
                  display,
                  backgroundWidth/2 - butterflyWidth/2,
                  backgroundHeight/2 - butterflyHeight/2);
          
          display.repaint();
        }//end stateChanged
      }//end new ChangeListener
    );//end addChangeListener
    //--------------------------------------------------//
  }//end constructor