§ 703. Employment of counsel. With the written approval of the county judge of a county in which an indictment has been found for a capital or other crime which presents unusual difficulty upon the trial thereof, the district attorney may employ counsel to assist him upon such trial. Such approval shall be filed in the office of the county clerk. The board of supervisors shall audit as other county charges a reasonable compensation and the necessary expenses of such counsel as certified by the presiding judge or justice.