This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
That judges of courts of superior or general jurisdiction are not liable to civil suits for judicial acts, even though maliciously or corruptly done, has already been indicated, the cases in point being reviewed by the court in Spalding v. Vilas. This is true even when the acts done are in excess of their jurisdiction, provided it appear that this want of jurisdiction is not clear and unmistakable. Where, however, authority is clearly usurped, action will lie. The doctrine as to this is sufficiently shown in the following words from the opinion in Bradley v. Fisher:63 trial of a party charged with such act, or should sentence a party convicted to a greater punishment than that authorized by the law upon its proper construction, no personal liability to civil actions for such acts would attach to the judge, although those acts would be in excess of his jurisdiction, or of the jurisdiction of the court held by him, for these are particulars for Lis judicial consideration, whenever his general jurisdiction over the subject-matter is invoked. Indeed some of the most difficult and embarrassing questions which a judicial officer is called upon to consider and determine relate to his jurisdiction, or that of the court held by him, or the manner in which the jurisdiction shall be exercised. And the same principle of exemption from liability which obtains for errors committed in the ordinary prosecution of a suit where there is jurisdiction of both subject and person, applies in cases of this kind, and for the same reasons."
"Where there is clearly no jurisdiction over the subject-matter any authority exercised is an usurped authority, and for the exercise of such authority, when the want of jurisdiction is known to the judge, no excuse is permissible. But where jurisdiction over the subject-matter is invested by law in the judge, or in the court which he holds, the manner and extent in which the jurisdiction shall be exercised are generally as much questions for his determination as any other questions involved in the case, although upon the correctness of his determination in these particulars the validity of his judgments may depend. Thus, if a probate court, invested only with authority over wills and the settlement of estates of deceased persons, should proceed to try parties for public offenses, jurisdiction over the subject of offenses being entirely wanting in the court, and this being necessarily known to its judge, his commission would afford no protection to him in the exercise of the usurped authority. But if, on the other hand, a judge of a criminal court, invested with general criminal jurisdiction over offenses committed within a certain district, should hold a particular act to be a public offense, and proceed to the arrest and it is his duty to exercise judgement and discretion: even although an individual may suffer by his mistake. A contrary principle would, indeed, be pregnant with the greatest mischiefs. It is unnecessary, we think, to refer to the many cases by which this doctrine has been established. It was fully recognized in the case of Gidley, Exec. of Holland, v. Ld. Palmerston, J. B. Moore, 91; 3 B. & B 275." 63 13 Wall. 335; 20 L. ed. 646.