Atticus's glasses serve
- as a sign of his intelligence;
- as a necessary tool that comes with age and wisdom;
- and, in the case of the mad dog, as an added edge that provides him with an unfair advantage for marksmanship--and killing.
Although it could be argued that the glasses only got in Atticus's way as he took aim at Tim Johnson, and that he removed them order to avoid the hindrance, this would not really make sense. Atticus's glasses helped him to see more clearly, and his aim would have been better had he kept them on. But Atticus hated the idea of killing, and he was not proud of the marksmanship skills that made him the "deadest shot in Maycomb County" as a youth. He gave up this skill, and never mentioned it to his children, because Atticus considered it in conflict with his peaceful nature. The pair of glasses seem to take on a life of its own when he picks up the rifle for the first time in 30 years. He deliberately raises them to his forehead--probably in an attempt to give the dog a chance--but they slip down, signifying the necessity of the crucial shot being on target. When he drops them to the ground, it is his way of conveying that he is breaking the promise he made to himself to never shoot again--a sign of wisdom and peace--in order to save his neighbors from the possible harm the dog could inflict. It is also Atticus's way of showing a sense of fair play: The dog is no match for his skill with the gun, and when he shoots, sans glasses, it is not a perfect kill.
"You were a little to the right, Mr. Finch," he (Sheriff Tate) called.
Tim Johnson also serves as a symbol of racism in the novel, and how rabies--like racism--can spread quickly. Author Harper Lee also uses this action to show that only a significant event could cause Atticus to again pick up a gun. The death of the rabid dog--and the symbolic death of racism--proved to be an appropriate occasion.
The glasses come to life once again during the trial when Atticus is forced to question Mayella Ewell in a forceful manner. When he ends his background questions--"a good visit," he calls it--and begins to attack her credibility, his glasses slip again. When he really gets down to business, and "rained questions on her," he is forced to take them off his head entirely: He is no longer the peaceful, loving Atticus, but the ruthless attorney trying to prove his client innocent.