Copy of a letter, from John Frederick William Herschel to Edward Sabine, 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. Has discovered a flaw in the theory of actinometer observations. Realizes that John Frederick William Herschel's actinometer observations at the Cape may be useless and that observations made at Government and East India observatories should be suspended. 20 February 1847