What authority allowed Justice Rehnquist to sit by designation on the federal district court?

In a future column, I will propose that the Supreme Court Justices should resume circuit riding. Whatever grievances eighteenth century justices raised about riding on horseback and boat have no relevance today. The Justices routinely travel around the globe in cushy airplanes and black SUVs. There will be more work. Deal with it. A likely rejoinder, of course, is that the seven of the Justices are not equipped to be trial court judges. (I exclude Justices Sotomayor and Jackson, who both would be at home in a district court). Indeed, Justice Rehnquist famously presided over a civil trail in federal court in Richmond, and was unanimously reversed by the Fourth Circuit. This single anecdote is held up as a reason why Supreme Court justices have no business riding circuit. I don't buy it. Most trial court judges need some time to ramp up and are not at their best during their first trial. If at first you don't succeed, try, try again. I've long been curious about this event, but couldn't find many details about it, other than some contemporary news accounts. I located the entire trial court record, which was stored at the National Archives in Philadelphia. My sincere thanks to Cana Cossin, a student at Penn Law, for scanning the entire record. I will write about these documents more in depth at another time. Here, I just wanted to raise a question that I did not find an answer to: what gave Rehnquist the authority to preside over the trial? Nothing in the trial court record reflects this issue. Everyone just acted as if it could be done. Under 28 U.S.C. § 294, the chief justice of the United States is authorized to allow retired justices to sit by designation on the lower courts. But there is no comparable authority for an active Justice to sit by designation. It has been done before. For example, in Field v. United States (1951), Justice Stanely F. Reed sat on the Second Circuit in his capacity as "Acting Circuit Justice." Does anyone know the statutory authority for these appointments?