Copy letter from Herbert Rix, to Captain William de Wiveleslie Abney, Fellow of the Royal Society 'Dear Sir, Prof. [Professor] Lippmann has written for the return of his two spectra. They were sent by us to you at your request some weeks since. Will you kindly return them direct to Prof. Lippmann, and let know that you have done so? His address is: 4 Carrefour de l'Odeon, Paris. Yours truly, Herbert Rix Asst. Sec. R.S. [Assistant Secretary, Royal Society]'. 13 July 1891