In "The Case for the Defence," the author says that Mrs. Salmon is the ideal witness. She is testifying in the murder case against Adams.
There are a few reasons why she is an ideal witness. First, Greene cites her slight Scotch accent. Second, he says that she looks completely honest and kind and caring. As Mrs. Salmon's testimony comes out, we can see why she is ideal as well. She gives her testimony very clearly and she openly admits her limitations (like correcting her age from 55 to 56 and admitting her eyes are not great).