The substance of these Lectures was written early in 1872: they were originally intended only for a select audience of friends at Harrow, but on the suggestion of some of those who heard them they were afterwards considerably enlarged, and were delivered before the Royal Instritution of Great Britain in the months of February and March 1874. Воспроизведено в оригинальной авторской орфографии издания 1874 года (издательство "Smith, Elder, & CO.").