In Bogardus v. Commissioner, 302 U.S. 34, 39, 58 S.Ct. 61, 64, 82 L.Ed. 32 (1937), a case presenting the issue whether certain payments were additional compensation or tax-free gifts, the Court, in a divided opinion, found that the fact-finder's determination of the issue was a conclusion of law or at least a mixed question of law and fact and therefore that the reviewing court could substitute its judgment for that of the fact-finder.