I'm not sure I understand your question concerning Chapter 17 of To Kill a Mockingbird, but I will give it a shot.

Atticus' cross-examination of Bob Ewell has left many of his statements tainted with doubt. Atticus has established that only a left-handed person could have inflicted the injuries to the right side of Mayella's face. Bob is left-handed, and Tom Robinson's left hand is crippled and incapable of doing the damage that was caused. Atticus also points out that Bob had done a lot of running that night:

"... you ran to the house, you ran to the window, you ran inside, you ran to Mayella, you ran for Mr. Tate. Did you, during all this running, run for a doctor?"

Bob answered that there was no need; he saw what happened. But Atticus showed that Bob's own concern for his daughter's injuries was seemingly unimportant to him. Atticus' questions left the jurors and those in attendance with some doubt about his testimony. When Mayella took the stand in the next chapter, more questions arose about the conflicting testimony of the two Ewells.