The rooms and space recently occupied by the Library of Congress in the Capitol building shall be divided into three stories, the third story of which shall be fitted up and used for a reference library for the Senate and House of Representatives, and that portion of the other two stories north of a line drawn east and west through the center of the Rotunda shall be used for such purpose as may be designated by the Senate of the United States, and that portion of the first and second stories south of said line shall be used for such purpose as may be designated by the House of Representatives.