(1) 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 of his income under section 22 for any year or to disclose fully and truly all material facts necessary for his assessment for that year, income, profits or gains chargeable to income tax have escaped assessment for that year, or have been under assessed or assessed at too low a rate, or 971 have been made the subject of excessive relief under the Act, or excessive loss or depreciation allowance has been computed, 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 in consequence of information in his possession reason to believe that income, profits or gains chargeable to income tax have escaped assessment for any year, or have been under assessed, or assessed at too low a rate, or have been made the subject of excessive relief under this Act, or that excessive loss or depreciation allowance has been computed, he may in cases falling under clause (a) at any time and in cases falling under clause (b) at any time within four years of the end of that year, serve on the assessee, or, if the assessee is a company, on the principal officer thereof, a notice containing all or any of the requirements which may be included in a notice under sub section (2) of section 22 and may proceed to assess or reassess such income profits or gains or recompute the loss or depreciation allowance; and the provisions of this Act shall, so far as may be, apply accordingly as if the notice were a notice issued under that sub section Provided * * * *" It would be seen that section 34(1) contemplates two categories of cases for re opening the previous assessment (1) where there has been an omission or failure on the part of the assessee to make a return of his income under section 22 or to disclose fully and truly all material facts necessary for his assessment; and (2) where there has been no such omission on the part of the assessee but the Income tax officer on the basis of information in his possession finds that income chargeable to tax has escaped assessment for any year.