Simplify the following text for a Grade 8 native English-speaking person.
Original: He worked as scientific assistant to the professor of public law and political science at the GieÃ en university , until he obtained his doctorate of law in 1991 .
Simplified: Between 1986 to 1991 , he worked as assistant to the Doctor of Public Law and Political Science at the GieÃ en University . He obtained his Doctorate of Law in 1991 .