( 1 ) THIS appeal is at the instance of the authorities of the commercial taxes and it is directed against the judgment of Sabyasachi Mukharji, J. , whereby his Lordship made the rule nisi, which was obtained by the respondent Suresh Chandra Bose on his application under Article 226 of the Constitution, absolute.

( 2 ) BOSE is a qualified sanitary engineer and at the relevant time he had been working as a sanitary contractor. In 1950, Bose applied before the Commercial Tax Officer for being registered as a dealer under the Bengal Finance (Sales Tax) Act, 1941, under a mistaken impression that he was a dealer under the said Act and was liable to pay sales tax. Bose was registered as a dealer under the Act. He had submitted returns from 1951 to 1958 and paid a total sum of Rs. 18,812. 45 on account of sales tax along with the returns. The case of the respondent was that the said sum was paid by him on a mistaken notion of the law. After the decision in Dukhineswar Sarkar and Brothers v. Commercial Tax Officer [1957] 8 S. T. C. 478, and the decision of the Supreme Court in State of Madras v. Gannon Dunkerley and Co [1958] 9 S. T. C. 353 (S. C.), the law came to