As you surely know, the great French artist Jacques-Louis David painted the very famous Death Of Socrates, along with many other brilliant neo-classical works.  

What is less well known is that at one point he thought about painting  Diotima Instructing Socrates.  He made a sketch, which is now held by the National Gallery of Art in Washington.  Apparently the sketch isn't on display, but here it is, linked from their web site: