The letter is here in much better quality: https://luna.manchester.ac.uk/luna/servlet/view/search/where/Germany%253A%2BDusseldorf?q=poseck
This letter contains notes on small errors in the translation of J. N. Darby’s German New Testament (1854-1855) concerning Luke 4 - 12. The suggestions for improvement are in German, but the letter itself is written in French. JND was living with the merchant W. A. Müller at that time.
Martin