The first American woman to work as a professional astronomer (1818-1889) and who used a telescope to discover what became known as 'Miss Mitchell's Comet.' ALS, one page, 5 x 8, personal monogram letterhead, June 12, 1863. Handwritten letter to "Mrs. Upton," in part: "If any invitation could tempt us into Boston for an evening, it would be yours, but my father rarely leaves the house after sunset, and I feel it my duty to stay with him." In very good to fine condition, with light show-through, and associated corner loss, from old mounting residue on the reverse.