The income tax was a method of raising revenue which did not produce any considerable returns until after the war was over. Acts passed during the war had levied a tax on all incomes over six hundred dollars and had introduced progressively increasing rates on higher amounts. Incomes above $5,000, for example, were taxed ten per cent. The greatest number of people were reached and the largest returns obtained in 1866 when nearly half a million persons paid an aggregate of about seventy-three million dollars. The entire system was abolished in 1872. Aside from the tariff, the "legal-tender" notes gave rise to the greatest number of political and constitutional tangles. By acts of February 25, 1862 and later, Congress had provided for the issue of four hundred and fifty million dollars of United States paper notes, which were commonly known as greenbacks or legal-tenders. The latter name came from the fact that, under the law, the United States notes were legal tender for all debts, public or private, except customs duties and interest on the public debt. In other words, the law compelled creditors to receive the greenbacks in payment of all debts, with the two exceptions mentioned. Three main questions arose in connection with these issues of paper: whether Congress had power under the Constitution to make them legal tender; whether their volume should be allowed to remain at war magnitude, be somewhat contracted or entirely done away with; and whether the government should resume specie payments--that is, exchange gold for paper on the demand of holders of the latter. The first of these questions was twice decided in the Supreme Court. In 1870, in Hepburn _v._ Griswold, the point at issue was whether the greenbacks could lawfully be offered to satisfy a debt contracted before the legal-tender act had been passed. As it happened, Salmon P. Chase, who had been Secretary of the Treasury during the war, was now Chief Justice of the Supreme Court and delivered its opinion. By a vote of four to three it decided that the greenbacks were not legal tender for contracts made previous to the passage of the law. At the time when the case was decided, however, there were two vacancies on the bench which were immediately filled, and shortly thereafter two new cases involving the legal-tender act were brought before the Court (Knox _v._ Lee, and Parker _v._ Davis). The decision, which was announced in 1871, over-ruled the judgment in Hepburn _v._ Griswold and held by a vote of five to four that the legal-tender act was constitutional as applied to contracts made either before or after its passage.