So much of the look of the 19th century as we picture it today - serious, solid, severe - comes courtesy of the camera. Early photographs required long exposure times, and lent themselves to monumental subjects: ancient ruins, mighty trees, palaces, the sea. All those subjects figure in the 35 photographs that make up “Silver, Salt, and Sunlight: Early Photography in Britain and France.’’ The show runs through Aug. 12 at the Museum of Fine Arts.