(1) Where the terms of a transfer of property direct that the income arising from the property shall be accumulated either wholly or in part during a period longer than--
(a) the life of the transferor, or
(b) a period of eighteen years from the date of the transfer, such direction shall, save as hereinafter provided, be void to the extent to which the period during which the accumulation is directed exceeds the longer of the aforesaid periods, and at the end of such last- mentioned period the property and the income thereof shall be disposed of as if the period during which the accumulation has been directed to be made had elapsed.