backward_btn.setMargin( btn_insets );
backward_btn.setToolTipText( "Scroll Backward by half a screen" );
backward_btn.setMnemonic( KeyEvent.VK_LEFT );
// backward_btn.setPreferredSize( btn_dim );
backward_btn.addActionListener(
new ActionVportBackward( time_scrollbar ) );
super.add( backward_btn );
icon_URL = getURL( Const.IMG_PATH + "Forward24.gif" );
if ( icon_URL != null )
forward_btn = new JButton( new ImageIcon( icon_URL ) );