Copy letter, from Sir John Herschel to William Henry Fox Talbot, dated at Slough Copy letter in John Herschel's hand. John Frederick William Herschel will as William Henry Fox Talbot requests stop mentioning John Frederick William Herschel's hyposulfite fixing process and await publication of William Henry Fox Talbot's process. Reports in detail on some of John Frederick William Herschel's experiments relating to photography and to spectrum analysis. [In a P.S. dated 1839-2-13,] John Frederick William Herschel states: 'I have discovered your secret of fixing [or its] equivalent.' 12 February 1839