Copy letter, from Sir John Herschel to [Aimé] Guinand, dated at Slough Contemporary copy in sender's hand, signed by sender. Tells Aimé Guinand that John Frederick William Herschel is a member of a committee looking for ways of making better glass, so Aimé Guinand should not reveal any secrets to John Frederick William Herschel. John Frederick William Herschel provides the names of the leading telescope-makers in London, whom Aimé Guinand may wish to contact. 20 October 1827