Letter, from William Henry Fox Talbot to Sir John Herschel, dated at London Autograph letter signed by sender. John Frederick William Herschel is indisposed, so William Henry Fox Talbot will come to Slough to review paper [see William Henry Fox Talbot's 1839-1-25]. Asks John Frederick William Herschel about appropriate curve on lenses for camera obscura for 2-foot focus. 28 January 1839