And why it is regressive.

The window-tax, as it stands at present (January, 1775), over and above the duty of three shillings upon every house in England, and of one shilling upon every house in Scotland, lays a duty upon every window, which, in England, augments gradually from two-pence, the lowest rate, upon houses with not more than seven windows; to two shillings, the highest rate, upon houses with twenty-five windows and upwards.

The principal objection to all such taxes is their inequality, an inequality of the worst kind, as they must frequently fall much heavier upon the poor than upon the rich. A house of ten pounds rent in a country town may sometimes have more windows than a house of five hundred pounds rent in London; and though the inhabitant of the former is likely to be a much poorer man than that of the latter, yet so far as his contribution is regulated by the window-tax, he must contribute more to the support of the state.

The Wealth of Nations, Adam Smith