delivered the opinion of the Court:

Upon the dissolution of the temporary restraining order by the trial court, a patent was regularly issued to Hutchinson, so that the sole question here is whether appellant is entitled to an order requiring the Commissioner to enter a formal award of. priority in favor of Hutchinson in said interference proceeding. In the first place, it must be borne in mind that Congress has constituted the Patent Office a special tribunal, with judicial functions, for the administration of the patent laws. The court has no general supervisory power over the head of that Office. If he errs in his final decisions, his errors may be corrected upon appeal to this court. But that is quite a different matter from attempting, by mandamus, to control his judgment and discretion. United States ex rel. Riverside Oil Co. v. Hitchcock, 190 U. S. 316, 47 L. ed. 1074, 23 Sup. Ct. Rep. 698. In the present case, it was the deliberate judgment of the Commissioner that his ruling that the issue in the interference was not patentable to the appellant because he had shown no invention over the disclosures of his prior patents did not amount to an award of priority to Hutchinson, the other party to the intez’ference. Being of that view, the Commissioner very naturally declined to enter a decision awarding priority to Hutchinson. The question was one calling for judgment and discretion, and the decision reached was a possible one, at least, and hence znay not be controlled by mandamus, United States ex rel. Trussed Concrete Steel Co. v. Ewing, 42 App. D. C. 179; United States *331ex rel. Ness v. Fisher, 223 U. S. 683, 56 L. ed. 610, 32 Sup. Ct. Rep. 356.

We do not wish to be understood as holding that the appellant was without redress, for, having requested the Commissioner to enter a final award of priority, he had done all he could, and had that been the necessary effect of the Commissioner’s decision the court would have taken jurisdiction of the case on appeal, since “to determine what the decision is from which an appeal is prosecuted, the court will look to its substance, its necessary legal effect and operation, rather than to its mere form.” Cosper v. Gold, 36 App. D. C. 302; Moore v. Heany, 34 App. D. C. 31; Re Selden, 36 App. D. C. 428; Mann v. Brown, 43 App. D. C. 457, 460.

The decree must be affirmed, with costs. Affirmed.