Fig. 1 Christoph Grienberger's enhanced version of Mario Bettini's scenographic instrument, from Mario Bettini, Apiaria Universae Philosophiae Mathematicae, Bononiae: Io. Baptistae Ferronij; 1645, Apiarium V, Aput VI, p. 45. Engraving. Note the eye, A, and hand, S, of the invisible user. Click on the image for an enlargement.