Depreciation determination remains tied to acquisition-time tax rates, allowing companies to use those rates for accounting. The specified period for depreciation, determined at acquisition under section 205(5) read with section 350 with reference to income-tax rates then in force, need not be changed when income-tax depreciation rates later change; companies may provide depreciation under clause (b) of section 205(2) on the basis of the rates in force at the time of acquisition.