At about 10 a.m. on the morning of Oct. 14, 1852, John M.W. Lace, 35, was one of several people looking at a display of photographs in the window of a bookstore on Wisconsin Avenue when Mary Ann Wheeler, 23, a seamstress, walked up behind him, drew a double-barreled pistol from under her shawl and shot him in the back of the head at point-blank range. Yet jurors in two separate trials refused to convict her.