Section 147 of the 1961 Act provides : If (a) the Income tax Officer has reason to believe that, by reason of the omission or failure on the part of an assessee to make a return under Section 139 for any assessment year to the Income tax Officer or to disclose fully and truly all material facts necessary for his 'assessment for that year, income chargeable to tax has escaped assessment for that year, or (b) notwithstanding that there has been no omission or failure as mentioned in clause (a) on the part of the assessee, the Income tax Officer has 616 consequence of information in his possession reason to believe that income chargeable to tax has escaped assessment for any assessment year, he may, subject to the provisions of Sections 148 to 153 assess or re assess such income or re compute the loss or the depreciation allowance, as the case may be, for the assessment year concerned (hereinafter in Sections 148 to 153 referred to as the relevant assessment year)".