Martensen, H. Christian ethics. General part, [1873]
Her Earl Rögnvald and his forebears, 1896 title page (Catherine Stafford Spence, translator of Ployen's "Reminiscences," Martensen's "Christian Ethics," Oosterzee's "Year of Salvation," etc.)
Shetland Family History website, searched 15 September 2016 (Catherine Stafford Spence; born Lerwick, Shetland, 16 Jul 1823; resident in Edinburgh, 1871; resident at the Schoolhouse, Gulberwick, Shetland, 1891; died in Lerwick, 21 Sep 1906) http://www.bayanne.info/Shetland/getperson.php?personID=I4246&tree=ID1