As I've explained, there are basically 2 types of portrait miniatures: 1. a living person who sat for their portrait in real life, and 2. those done "après" famous works of art in the museums, fueled by the souvenir hungry Grand Tour travelers. This is a Grand Tour portrait miniature, after the work The Emperor Napoleon In His Study at the Tuileries (1805, detail), by Jacques-Louis David, which is on display in The National Portrait Gallery in London. The miniature painting dates sometime after that, likely (from the frame) mid-late 1800. A typical tourist work, the painter has signed. Measurements noted on the photos.
Very good condition for age and type. Souvenir quality after a very famous Napoleon painting, it would have been a treasured reminder of someone's own Grand Tour to the museums and cities of UK and Europe. Frame is wood, shows some wear. Broad worked heavy brass frame mount sets the painting off well.