Letter, regarding postscript for paper on solar rays acting upon vegetable colours from J F W [John Frederick William] Herschel to unknown recipient Letter by Herschel requesting the addition of a postscript to his paper 'On the action of the rays of the solar spectrum on vegetable colours', found at PT/26/12. Postscript itself found at PT/26/13. The letter also includes specimens of various photographic experiments that were not published as part of the paper - chryostypes, a cyanotype and paper treated with Papaver orientale. Some prints have faded completely, with the images no longer visible. Includes some undeveloped photographic paper. Subject: Chemistry Written by Herschel at Collingwood [Kent, England]. 23 July 1842