In the law every word is significant. The words on and of mean in reference to or respect to or relating to. So tax on income means a tax in reference to, respect to or relating to income. Taxes being direct taxes. Duties and Imposts relate to Imports into the country and specifically taxed in the statutes.
Excises are on privileged revenue taxable activities.
The subject of direct taxes are people and property. The subjects of duties, imposts and excises are happenings or activities. Income however defined under the constitution can never be the subject of any constitutional tax. The Revenue Tax Act of 1916 attempted to tax cash and stock dividends as income, thereby attempting to define income. This decision said cash dividends are income and can be taxed as income but rejected the notion that stock dividends are not income but did classify stock dividends as property which is not the subject of an income tax.