Each commissioner shall receive a salary, payable in the same manner as the salaries of the judges of the courts of the United States. The commission shall appoint a secretary, who shall receive a salary, and it shall have authority to employ and fix the compensation of such attorneys, special experts, examiners, clerks, and other employees as it may from time to time find necessary for the proper performance of its duties and as may be from time to time appropriated for by Congress. With the exception