In Child Labor Tax Case, 259 U.S. 20 (1922), the Supreme Court, in holding unconstitutional the provisions of the Revenue Act of February 24, 1919, imposing a tax upon the employment of child labor, fully recognized the foregoing limitations upon the judicial authority; but declared that the act constituted an attempt to regulate a matter exclusively within the control of the state, and that, although the exaction was called a tax, it was, in fact, not a tax but a penalty exacted for the violation of the regulation. "Taxes are occasionally imposed," it was said (p. 38), "in the discretion of the legislature on proper subjects with the primary motive of obtaining revenue from them and with the incidental motive of discouraging them by making their continuance onerous. They do not lose their character as taxes because of the incidental motive. But there comes a time in the extension of the penalizing features of the so-called tax when it loses its character as such and becomes a merely penalty with the characteristics of regulation and punishment. Such is the case in the law before us."