Chapters 17 and 18 are the sections in Harper Lee's novel To Kill a Mockingbird in which the physical evidence of Tom Robinson's innocence is revealed. The revelation concerning Tom's physical condition -- in effect, that his crippled left arm made it impossible for him to have inflicted the bruises on Mayella Ewell's face and similarly made it extremely unlikely that this destitute African American could have raped this large, healthy young woman. It is during Atticus Finch's questioning of Mayella during the trial that Tom's disability is revealed. In the following passage, Scout, the novel's young but observant narrator describes the defendant's condition:
"Tom Robinson’s powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him."
The above passage is from Chapter 18 of Lee's novel. The preceding chapter is the one in which Atticus questions Mayella's father, the bitter and virulently racist 'white trash' who we are led to believe in Atticus' cross-examination is the one who is guilty of beating his daughter. It is in Chapter 17 that Atticus attempts to inject the notion of reasonable doubt into the jury's mind (despite knowing that Tom was doomed the minute he was accused of raping a white woman). The clever attorney, and conscience of this story, knows what the reader does not yet know -- that Tom's left arm is crippled. During his interrogation of Bob Ewell, Atticus asks the witness to sign his name, thereby displaying for the jury the fact that this ignorant, violent racist is left-handed, and so was the likely individual who struck Mayella. Scout provides her observation of her father's tactics in Chapter 17:
". . .Atticus was trying to show, it seemed to me, that Mr. Ewell could have beaten up Mayella. That much I could follow. If her right eye was blacked and she was beaten mostly on the right side of the face, it would tend to show that a left-handed person did it. Sherlock Holmes and Jem Finch would agree. But Tom Robinson could easily be left-handed, too."
It is early in the following chapter, 18, that the earlier passage describing Tom Robinson's disability is provided. It is the revelation of Tom's disability, combined with the reputation of Bob Ewell, that provides the greatest evidence of the former's innocence. That Tom would be convicted anyway, however, is simply a sign of the times depicted in Lee's novel. In the American South of the 1930s (as well as the centuries preceding that period and the decades that followed) simply being accused of raping a white woman was sufficient to ensure the African American male's conviction.