Copy letter, from Sir John Herschel to John William Draper, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Comments on John Henry Dallmeyer's work in photography; John Frederick William Herschel goes on to talk about John Frederick William Herschel's ideas of 'chemical' rays of light. 5 December 1842