private void Light_Click(object sender, EventArgs e)
 {
     if (null != _dark)
     {
         _dark.Close();
         _dark = null;
     }
     if (null == _light)
     {
         this.Visibility = Visibility.Collapsed;
         _light = new QuartzClock();
         _light.Show();
         _light.Activate();
     }
     else
     {
         _light.Show();
         _light.Activate();
     }
  
 }