Letter, from Negretti & Zambra to Sir John Herschel, dated at London Autograph letter signed by sender. Have made several of John Frederick William Herschel's actinometers but have always experienced difficulty with the fluid, ammonia sulphate of copper. Can some other fluid be used? They are now making two actinometers for a foreign observatory and would like to make them as accurate as possible. 3 June 1863