Leo

This was copied from a sketch of Leonardo da Vinci 

Mike

This was copied from a study of Michaelangelo used to decorate the dusty ceilings of the Cistine chapel.

Rembrandt

It's nice to draw old people.  There are many lines which you could use as basis vectors in order to map all coordinates throughout all dimensions of space-time.

Copied from Rembrandts self portrait..