Full power and authority are hereby given and granted to the general court to impose and levy a tax on income in the manner hereinafter provided. Such tax may be at different rates upon income derived from different classes of property, but shall be levied at a uniform rate throughout the commonwealth upon incomes derived from the same class of property. The general court may tax income not derived from property at a lower rate than income derived from property, and may grant reasonable exemptions and abatements. Any class of property the income from which is taxed under the provisions of this article may be exempted from the imposition and levying of proportional and reasonable assessments, rates and taxes as at present authorized by the constitution. This article shall not be construed to limit the power of the general court to impose and levy reasonable duties and excises.