60mm bronze medal of Jean Le Rond D'Alembert (1717-1783) by by N. Gatteaux, signed and dated 1786. God alone knows where the `smudge' on the rhs field has come from. it's certainly not on the medal itself.

The reverse is the same design as used on jetons of the L' Academie Francaise which D'Alembert was Secretary.

Here's what wiki has to say about him :

D'Alembert

Very nice medal. I wonder if the smudge is just a reflection of some kind.