Any person, otherwise competent, who will make oath that he is impartial in the case, shall be competent as a juror in any criminal case, notwithstanding the fact that he has an impression or an opinion as to the guilt or innocence of the accused, if it appear to the satisfaction of the court that he has no bias or feeling or prejudice in the case, and no desire to reach any result in it, except that to which the evidence may conduct. Any juror shall be excluded, however, if the court be of opinion that he cannot try the case impartially, and the exclusion shall not be assignable for error.