'When any judge of any court of the United States, appointed to hold his office during good behavior, resigns his office after having held a commission or commissions as judge of any such court or courts at least ten years continuously (or otherwise), and having attained the age of seventy years, he shall, during the residue of his natural life receive the salary which is payable at the time of his resignation for the office that he held at the time of his resignation. But, instead of resigning, any judge other than a justice of the Supreme Court, who is qualified to resign under the foregoing provisions, may retire, upon the salary of which he is then in receipt, from regular active service on the bench, and the President shall thereupon be authorized to appoint a successor; but a judge so retiring may nevertheless be called upon by the senior circuit judge of that circuit and be by him authorized to perform such judicial duties in such circuit as such retired judge may be willing to undertake, or he may be called upon by the Chief Justice and be by him authorized to perform such judicial duties in any other circuit as such retired judge may be willing to undertake, or he may be called upon either by the presiding judge or senior judge of any other such court and be by him authorized to perform such judicial duties in such court as such retired judge may be willing to undertake.