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. Comments on several approaches to fixing images on paper, especially using pure water as a fixing agent. Has developed a new sensitive paper but is keeping it secret until perfected. Is leaving for Continent on 1 May. 22 April 1839