The dial forming the subject of this paper, acquired by the British Museum in 1923, is of gilt copper, made in the form of a book, along the edges of which are inscribed in capitals the words: Lucerna instrumentalis | intellectus directiva | sive instrumentum sciendi. The dial-plate which is fixed in the interior has a compass and two very short gnomons. It is for use in the latitudes of 42 and 45, and would serve for Rome and one of the large towns in the North Italian plain, perhaps Milan or Venice. It was made at Rome in the year 1593, as shown by the inscription on the dial-plate. On the cover is a shield of arms, barry, and in chief the letters I H S surmounted by a cross, a feature perhaps indicating that the owner was a member of the Society of Jesus; a fuller device, in which the three nails of the Passion are seen below the sacred monogram and cross, occupies the centre of the figure on the outside of the lower cover. The identification of the arms presents difficulties. They might be those of the Caraffa (gules, three bars argent), a member of which family, Vincenzio Caraffa, was general of the Jesuits in 1645.