"The income tax is, therefore, not a tax on income as such. It is an excise tax with respect to certain activities and privileges which is measured by reference to the income which they produce. The income is not the subject of the tax: it is the basis for determining the amount of the tax." F. Morse Hubard, U.S. Congressional Record, March 27, 1943 (page 2580)

1928 TAX ACT
GROSS INCOME DEFINED: NONRESIDENT ALIEN INDIVIDUAL.
[Src. 213.3 (c) In the case of a nonresident alien individual, gross income means only the gross income from sources within the United States, determined under the provisions of section 217.

ART. 92. Gross income of nonresident alien individuals.-In the case of nonresident alien individuals " gross income " means only the gross income from sources within the United States, determined under the provisions of section 217. See articles 316-329. As to the gross income of foreign corporations see section 233 (b) of the statute and article 550 ; also section 217 and articles 316-329. The items bf gross income from sources without the United States and therefore not taxable to nonresident aliens or foreign corporations are’described in section 217(c) and article 322. As to who are nonresident alien individuals.

ART, 311. Definition.-A " nonresident alien individual " means an individual (a) whose residence is not within the United States and (6) who is not a citizen of the United States. An alien actually present in the United States who is not a mere transient or sojourner is a resident of the United States for purposes of the income tax. Whether he is a transient or not is determined by his intentions with...

Internal Revenue Code of 1954

SEC. 451. GENERAL RULE FOR TAXABLE YEAR OF INCLUSION.
(a) GENERAL RULE. — The amount of any item of gross income shall be included in the gross income for the taxable year in which received by the taxpayer, unless, under the method of accounting used in computing taxable income, such amount is to be properly accounted for as of a different period.