(ii)the amount of the investment allowance, to the extent to which it has not been allowed as aforesaid, shall be carried forward to the following assessment year, and the investment allowance to be allowed for the following assessment year shall be such amount as is sufficient to reduce the total income of the assessee assessable for that assessment year, computed in the manner aforesaid, to nil, and the balance of the investment allowance, if any, still outstanding shall be carried forward to the following assessment year and so on, so, however, that no portion of the investment allowance shall be carried forward for more than eight assessment years immediately succeeding the assessment year relevant to the previous year in which the ship or aircraft was acquired or the machinery or plant was installed or, as the case may be, the immediately succeeding previous year.