Quite a while back I read a story where the Devil comes to an astronomer (?) and wants his soul. The astronomer agrees with the condition that he gets one thing from the Devil and he will tell him what it is when he's ready. Being greedy, the Devil agrees and thru the years keeps asking the astronomer what it is that he wants but never gets an answer. Finally, on his death-bed, the astronomer tells the Devil he wants a "personally guided tour of the entire Universe by the Devil himself". What is the name of that story?