what does mr. nathan radley know about the intruders in his garden? why does miss stephanie refer to a "negro" over whose head mr. nathan has fired?

I think Mr. Radley knows that Scout, Jem and Dill have been poking around his house for a while. He shoots into the sky as to not hurt them, just scare them. Miss. Stephanie only assumes anybody in someone else's yard at night would be a Negro, it's her racial prejudice coming through.