(FORTUNE Magazine) – Herewith two modest thoughts about the famous ''bubble'' in the U.S. tax code. As we key in these words, the bubble is in trouble. Hundreds of Congresspersons yearn to lance it, professedly because they cannot abide the inequity it represents. The bubble is, of course, the so-called surcharge of 5% on taxable incomes in the upper-middle-class zone between $78,400 and $162,770 (on joint | returns). What the solons keep saying is that the deal is unfair because it gives the rich (i.e., joint returns over $162,770) a marginal rate of only 28% while the less rich bubble people pay up to 33%. Obvious solution, and real reason for dwelling on the ''inequity'': Raise marginal rates for the rich. Special appeal of this tax increase: Explaining its logical difficulties figures to be beyond Dan Rather. Thought No. 1, which we have seen in print a fair number of times, is that the upper middle class is not oppressed at all by the bubble. The 5% surcharge resulted from a deal: Bubble folks would pay a higher marginal rate but in exchange would get two benefits denied the rich. One benefit is personal exemptions; the other is the right to have your first $32,450 taxed at only 15%. (For the rich, all income is taxed at 28%.) The deal leaves people in the bubble brackets paying substantially less than those with higher incomes. Thought No. 2 is simpler, or possibly not. It is that differences in marginal rates have nothing to do with equity. As explained by the supply- siders, you focus on marginal rates when the subject is incentives. But if the subject is equity, then all you have to know is how much people pay in relation to their incomes. So proposals to make things fairer by equalizing marginal rates are baloney. For which, to be sure, there is a market.