The Secretary of the Treasury is authorized and directed to receive into the Treasury from time to time as a permanent loan by the Board of Trustees of the National Gallery of Art to the United States sums in cash of not to exceed $5,000,000 in the aggregate, and to pay interest on the principal amount of such loan at a rate which is the higher of the rate of 4 per centum per annum or a rate which is .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 semiannually. Such interest is permanently appropriated for payment to the Board of Trustees of the National Gallery of Art.