Then he again sang out, sharply, "Come in." He was deaf, and had, along with his insensibility to sounds, that occasional abnormal perception of them which the deaf seem sometimes to possess.

He often heard sounds when none were recognizable to other people.
Now, evidently having perceived no result from his first response, he had heard this second knock, which did not exist except in his own supposition and the waiting woman's intent.

She had, indeed, just at this point said to herself that she would slip out and knock again if he did not look around.

She had not the courage to speak.

It was almost as if the deaf lawyer, piecing out his defective ears with a subtler perception, had actually become aware of her intention, which had thundered upon him like the knock itself.
Mrs.Field made an inarticulate response, and took a grating step forward.