Copy letter, from Sir John Herschel to William Henry Fox Talbot, 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 reports his discovery of a highly sensitive paper. Tint continues to deepen with time, as bromine does not. Not prepared to reveal process at present. Is off to Paris. 27 April 1839