In To Kill a Mockingbird what does Atticus lead Sheriff Tate to remember about Mayella's black eye?

He wants the jury to see that if it was her right eye, naturally a left handed person would attack her, leading to the fact that Tom Robinson's left arm is useless and Mr. Ewell is left handed.
Atticus makes him remember in which eye it was. Atticus make the sheriff doubt about his response. Was it on the right eye or on your right side. At the end the sheriff confessed that it was on her right eye, indicating she was hit by a left handed man. Tom Robinson couldn't have done this because he only had one good arm, his left.