Section 39
SEC. 39. An officer of the United States Army or Navy, when detailed to perform the duties of any office created or provided in this Act, shall receive in lieu of the salary authorized for said office the difference between the pay actually received by him from the Army or Navy during said detail and the amount of said salary: Provided, That the per diems heretofore authorized to be paid Army and Navy officers now performing such civil duties shall be continued during the period of their respective details.