What is the test for cross-examination of a witness?

20. Browne v. Dunn, supra, is a relatively short decision. At pages 70-71 of the decision, Lord Herschell, L.C. said that: ... it seems to me to be absolutely essential to the proper conduct of a cause, where it is intended to suggest that a witness is not speaking the truth on a particular point, to direct his attention to the fact by some questions put in cross-examination showing that that imputation is intended to be made, and not to take his evidence and pass it by as a matter altogether unchallenged, and then, when it is impossible for him to explain, as perhaps he might have been able to do if such questions had been put to him, the circumstances which it is suggested indicate that the story he tells ought not to be believed, to argue that he is a witness unworthy of credit ... if you intend to impeach a witness, you are bound, whilst he is in the box, to give him an opportunity of making any explanation which is open to him; and, as it seems to me that is not only a rule of professional practice in the conduct of a case, but it is essential to fair play and dealing with witnesses. Sometimes reflections have been made upon excessive cross-examination of witnesses, and it has been complained of as undue; but it seems to me that a cross-examination of a witness which errs in the direction of excess may be far more fair to him than to leave him without cross-examination, and afterwards to suggest that he is not a witness of truth, I mean upon a point on which it is not otherwise perfectly clear that he has had full notice beforehand that there is an intention to impeach the credibility of the story which he is telling ... there are cases in which the notice has been so distinctly and unmistakeably given, and the point upon which he is, impeached, and is to be impeached, is so manifest, that it is not necessary to waste time in putting questions to him upon it ... it will not do to impeach the credibility of a witness upon a matter on which he has not had the opportunity of giving an explanation by reason of there having been no suggestion whatever in the course of the case that his story is not accepted. (emphasis added)