In 1786 a man named Gilles Louis Chretien adapted a popular drawing device, the pantograph (you'll be familiar with them if you look one up on google images), so that it would create engravings. His new device was designed to trace an individual’s physical appearance in profile, often in the form of a silhouette, and it was capable of producing multiple identical engravings and allowed colour to be added. He called it a phsyionotrace, presumably because it could trace people's physiognomy.

The silhouette portraits created by phsyionotraces became popular amongst the growing middle classes, because they were a form of expression that distinguished them from the aristocracy, who historically had an attachment to painting. It therefore fed into early ideas of how biology and physical appearance was intimately related to your moral character and early photography's attachment to ideas of eugenics.

My source for this is Mary Warner Marien's 'Photography: A Cultural History', particularly pages 4 and 5. Hope this helps!

Edit: Spelling mistake