Exclusion of evidence to contradict answer to question testing accuracy, previous conviction or impartiality

When a witness under cross-examination has been asked and has answered any question referred to in section 68, no evidence can be given to contradict him, except in the following cases– (a) if a witness is asked whether he has been previously convicted of any felony or misdemeanour, and denies or does not admit it, or refuses to answer, evidence may be given of the previous conviction; and (b) if a witness is asked any question tending to show that he is not impartial and answers it by denying the facts suggested, he may, by permission of the judge, be contradicted by evidence of those facts. 41