Alexander von HUMBOLDT (1769-1859), German naturalist and explorer. Autograph letter signed to "Monseigneur" [Frederick William III (1770-1840), King of Prussia]. 4 pp. in-4. Paris, 4th September 1820. Beautiful letter to the King of Prussia Frederick William III: "If the arrival of Mr Voelker [the German miniaturist painter Gottfried Wilhelm V] gave me great satisfaction because of the great fame he rightly enjoys, it has become even more precious to me because of the gracious remembrance with which Your Serene Highness has deigned to honour me. Those were beautiful days in my life, when I enjoyed almost daily the happiness of being close to you, of sharing our sorrows and those of Her Royal Highness Princess Louise [whose depth of feeling equalled the breadth and originality of her spirit]. In offering you, Monseigneur, the respectful homage of gratitude, I like to anticipate the happy time when, before leaving Europe, I will go to your government to tell you of my devotion and admiration. Perhaps H.R.H., faithful to her old affections, will deign to give me some orders for the industrial products of the Celestial Empire of Yao. Mr Voelker was preceded in this country by a fine reputation based on the magnificent work by Count Hofmannsegg [Portuguese Flora or description of all the plants that grow naturally in Portugal by the German botanist Johann Centurius Count of Hofmannsegg, published from 1809 to 1820, with illustrations by Völker]. He left favourable impressions through his simple manners and the gentleness of his character. He seemed to me to be all the more satisfied with his stay because, owing to his absolute ignorance of the language, he remained deaf to all political quarrels [...]". [King Frederick William granted Humboldt a life annuity and entrusted him with various diplomatic missions.]