/// <summary>
        /// Open PictureWindow, with the given picture.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void PictureLink_Click(object sender, RoutedEventArgs e)
        {
            Hyperlink     link = e.OriginalSource as Hyperlink;
            PictureWindow pw   = new PictureWindow(link.NavigateUri.OriginalString);

            pw.Show();
        }
        /// <summary>
        /// Show the PictureWindow with the current picture uri, while window is open disable ShowButton and NextButton.
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ShowButton_Click(object sender, RoutedEventArgs e)
        {
            PictureWindow pw = new PictureWindow(formulas.ElementAt(counter).FormulaImageFileName);

            pw.Closed           += Pw_Closed;
            ShowButton.IsEnabled = false;
            NextButton.IsEnabled = false;
            pw.Show();
        }