Copy of draft letter, from Sir John Herschel to David Brewster, dated at Slough Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. John Frederick William Herschel's theory of colors. Edward Troughton has David Brewster's instrument one-third completed. When will David Brewster want John Frederick William Herschel's encyclopedia article on 'Variation'? 31 January 1824