Can a federal judge excuse a whole class of jurors?

21a In Thiel v. Southern Pac. Co., 1946, 328 U.S. 217, 66 S.Ct. 984, 90 L.Ed. 1181, it was held that although a federal judge could excuse individual jurors on a showing of real burden and hardship, a systematic exclusion of all wage earners was improper. Paragraph (b) was thereafter added to 28 U.S.C.A. 1863, and the power of a judge to exclude an entire class because of undue hardship was expressly recognized, leaving unimpaired the judge's right to excuse in individual cases of hardship.