What is the test for disputing a finding by a trial judge sitting without a jury?

"Where a trial judge sits without a jury, the rule varies with the character of the evidence: (a) If he decides a fact issue on written evidence alone, we are as able as he to determine credibility, and so we may disregard his finding. (b) Where the evidence is partly oral and the balance is written or deals with undisputed facts, then we ignore the trial judge's finding and substitute our own, (1) if the written evidence or some undisputed fact renders the credibility of the oral testimony extremely doubtful, or (2) if the trial judge's finding must rest exclusively on the written evidence or the undisputed facts, so that his evaluation of credibility has no significance. (c) But where the evidence supporting his finding as to any fact issue is entirely oral testimony, we may disturb that finding only in the most unusual circumstances." Orvis v. Higgins, 180 F.2d 537, 539-40 (2d Cir.) Cert. denied, 340 U.S. 810, 71 S.Ct. 37, 95 L.Ed. 595 (1950).