Analysis of information sources in references of the Wikipedia article "Ferronnière" in English language version.
The allusion is to a tour de tete, also known as a ferroniere, an oval or circular brow-ornament held in place by a thin chain. This fashionable ornament - which was said to give the wearer a look of 'erudition' - was copied from a painting by Leonardo da Vinci...
La confusion a été facilitée par le bijou, appelé ferronnière, que le modèle porte sur le front.