In what way is Atticus optimistic?

Atticus shows optimism in a few parts of the story.

The most obvious of course is the way in which he defends Tom Robinson.  He puts everything he has into it - perhaps not assuming he will win, but trying nonetheless.  His entire closing in the courtroom in Chapter 20 could be categorized as optimistic.

He shows other signs of optimism though.  In the lessons he teaches Scout about reading in school (chapter 3) - he comes across as optimistic that not only will everything be okay at school - but it has been okay forever.

Later, in chapter 15, Atticus is debriefing the incident of the gathering of men outside the Finch home (presumably "friend" warning Atticus to be careful about going through with the case) with Jem.  He makes this statement: "The Ku Klux's gone... it'll never come back." Historically we know the Ku Klux was neither gone at this time nor that it stayed in hiding for long.  I think here Atticus is mixing optimism with reassurance for his children.