In chapter seventeen of To Kill a Mockingbird what does Atticus attempt to establish through Bob Ewell`s testimony?

Atticus' main purpose with Bob Ewell's testimony is actually
two-fold.

First of all, Atticus works to prove that Bob Ewell didn't
care enough about his daughter's health to seek the help of a doctor. This is an odd behavior for
a parent, especially if he is so excited to get Mayella to testify in court against her supposed
attacker. You would think Bob Ewell would want as much evidence as
possible.

Secondly, Atticus tries to prove that Bob is left-handed.
Bob clearly shows he doesn't understand what it means to have strength in both hands or one more
than the other because on the last page of the chapter, he confused the question while proving
himself to be left-handed.