That the salary or other emoluments of office of federal officials may not be taxed by the States has not been questioned since the doctrine was first declared in Dobbins v. Commissioners.18 "The powers of the National Government," the court say, "can only be executed by officers whose services must be compensated by Congress. The allowance is in its discretion. The presumption is that the compensation given by law is no more than the services are worth, and only such in amount as will secure from the officer the diligent performance of his duties. . . . The compensation of an officer of the United States is fixed by a law made by Congress. It is in its exclusive discretion to determine what shall be given.....Does not a tax, then, by a State upon the office, diminishing the recompense, conflict with the law of the United States, which secures it to the officer in its entire-ness? It certainly has such an effect," 19