In the absence of any specification to the contrary, the board may deposit the principal sum, in cash, with the Treasurer of the United States as a permanent loan to the United States Treasury, and the Treasurer shall thereafter credit such deposit with interest at a rate which is the higher of the rate of 4 per centum per annum or a rate which is 0.25 percentage points less than a rate determined by the Secretary of the Treasury, taking into consideration the current average market yield on outstanding long-term marketable obligations of the United States, adjusted to the nearest one-eighth of 1 per centum, payable semi-annually, such interest, as income, being subject to disbursement by the Librarian of Congress for the purposes specified: , That the total of such principal sums at any time so held by the Treasurer under this authorization shall not exceed the sum of $10,000,000.Provided, however