(4)The deduction to be allowed under sub-section (1) for any relevant previous year shall be-(a)an amount equal to one-tenth of the expenditure specified in sub-section (2) (such one-tenth being hereafter in this sub-section referred to as the instalment); or(b)such amount as is sufficient to reduce to nil the income (as computed before making the deduction under this section) of that previous year arising from the commercial exploitation [whether or not such commercial exploitation is as a result of the operations or development referred to in sub-section (2) of any mine or other natural deposit of the mineral or any one or more of the minerals in a group of associated minerals as aforesaid in respect of which the expenditure was incurred, whichever amount is less:Provided that the amount of the instalment relating to any relevant previous year, to the extent to which it remains unallowed, shall be carried forward and added to the instalment relating to the previous year next following and deemed to be part of that instalment, and so on, for succeeding previous years, so, however, that no part of any instalment shall be carried forward beyond the tenth previous year as reckoned from the year of commercial production.
