Copy letter, from Sir John Herschel to [William Whewell], dated at Slough Contemporary copy in sender's hand, signed by sender. Wishes to have mentioned to the Physical Section of the B.A.A.S. that John Frederick William Herschel has discovered that the extreme red rays [infra-red?] of the sun do not darken photographic paper, but tend to whiten it. 28 August 1839