It is known that C. G. J. Jacobi wrote his first two letters to E. E. Kummer in the year 1834. Thanks to a find in the archive of the Mittag-Leffler Institute there is now the missing link , Kummer’s reply letter to Jacobi’s first letter. This letter seems to be the earliest one from Kummer still in existence (at least to a mathematician). Kummer tells in his letter about his situation as a teacher in Liegnitz, gives further a version (...) of his later paper on the summation of slowly convergent series and discusses Euler’s summation formula in connection with the practice of its use in numerical calculations. The article contains the complete texts of the three letters (up to those parts in Kummer’s letter identically contained in his published article) together with a commentary on their contents. (shrink)