pay, he may send to the Income tax Officer an esti mate of the tax payable by him calculated in the manner laid down in sub section (1 ) on that part of his income 1 for such period, and shall pay such amount as accords with his estimate in equal instalments on such of the dates specified in sub section (1)(a) as have not expired or in one sum if only the last of such dates has not expired: (3) Any person who has not hitherto been assessed shall, before the 15th day of March in each financial year, if his total income of the period which would be the previous year for an assessment for the financial year next following is likely to exceed the maximum amount not chargeable to tax in his case by two thousand five hundred rupees, send to the Income tax Officer an estimate of the tax payable by him on that part of his 25 income to which the provisions of section 18 do not apply of the said previous year calculated in the manner laid down in sub section ( 1 ), and shall pay the. amount, on such of the dates specified in that sub section as have not expired, by instalments which may be revised according to the proviso to sub section (2). ' "42.