Johann Caspar Lavater advocated for the phenomenon of physiognomy, by which one could judge character from facial characteristics. Lavater claimed, for instance, that heavy eyebrows grown together meant criminality; while a sloping forehead might show that one was less smart. The work had a large impact throughout Europe and as Lavater’s ideas spread, people naturally wanted to have more professional and accurate depictions of other people and themselves, making the art of the silhouette portrait very popular. He is credited with developing the silhouette machine. The silhouette was, according to Lavater, the ‘drawing of the essence’ of a face, because it focused on its solids. This "device" he used in a standardized way to carefully draw someone’s profile, which was then analyzed psychologically. Lavater’s work may represent the earliest book to reproduce portraits made by the action of light, a precursor to the physionotrace.