CD was elected a corresponding member of the Royal Prussian Academy of Sciences on 23 February 1863; the diploma sent to CD is reproduced in Appendix III. The letter referred to has not been found; however, it was probably written by either Christian Gottfried Ehrenberg or Johann Franz Encke, who, as the secretaries for the physical-mathematical section of the academy, were the joint signatories of the diploma. The academy comprised two sections: the physical–mathematical and the philosophical–historical; the former encompassed biological as well as physical sciences (Harnack 1900).