The steady advances in geometrical optics in the sixteenth and seventeenth centuries fostered the wider application of the principles of the new science to the generation of amazing effects. These included anamorphoses, which became highly popular in the seventeenth century. An anamorphic application generally displays a figure intentionally deformed according to precise rules of perspective; the figure becomes visible again in its proper shape only if the image is observed by reflection in a mirror (in the shape of a cylinder, cone, or other form) or at a particular angle. Another frequent use of the technique was to create optical illusions. An admirable example is the fake dome of the church of Saint Ignatius in Rome. The ceiling is actually flat but, if observed from a precise point indicated on the floor, it gives the impression of being surmounted by a dome. Other spectacular applications of optical illusions are to be found in well-known paintings such as Holbein's "The Ambassadors," which, if observed at a grazing angle, shows an anamorphic image of a skull, and the "Portraits of Frederick III and of the Queen of Denmark" by Gert Dittmers, which show a cylindrical anamorphosis legible with a mirror.