By the terms of the Constitution, the power of Congress to levy taxes is subject to but one exception and two qualifications. Articles exported from any State may not be taxed at all. Direct taxes must be levied by the rule of apportionment and indirect taxes by the rule of uniformity. The Court has emphasized the sweeping character of this power by saying from time to time that it "reaches every subject,"[216] that it is "exhaustive"[217] or that it "embraces every conceivable power of taxation."[218] Despite these generalizations, the power has been at times substantially curtailed by judicial decision with respect to the subject matter of taxation, the manner in which taxes are imposed, and the objects for which they may be levied. DECLINE OF THE FORBIDDEN SUBJECT MATTER TEST In recent years the Supreme Court has restored to Congress the power to tax most of the subject matter which had previously been withdrawn from its reach by judicial decision. The holding of Evans _v._ Gore[219] and Miles _v._ Graham[220] that the inclusion of the salaries received by federal judges in measuring the liability for a nondiscriminatory income tax violated the constitutional mandate that the compensation of such judges should not be diminished during their continuance in office was repudiated in O'Malley _v._ Woodrough.[221] The specific ruling of Collector _v._ Day[222] that the salary of a State officer is immune to federal income taxation also has been overruled.[223] But the principle underlying that decision--that Congress may not lay a tax which would impair the sovereignty of the States--is still recognized as retaining some vitality. THE RISE AND FALL OF COLLECTOR _v._ DAY