If it shall appear necessary before the close of the case for the prosecution, the commissioners may in their discretion allow the prosecutor to exhibit evidence not included in the list given to the person accused, or may themselves call for new evidence; and in such case the person accused shall be entitled to have, if he demands it, an adjournment of the proceedings for three clear days, before the exhibition of such new evidence exclusive of the day of adjournment and of the day to which the proceedings are adjourned.