text: * W. de W. Abney and E. R. Festing, [https://www.jstor.org/stable/114812 Intensity of Radiation through Turbid Media], Proceedings of the Royal Society of London, Volume 40, pages 378–380, 1886. Published by The Royal Society.
  * W. de W. Abney and E. R. Festing, [https://www.jstor.org/stable/115174 Colour Photometry. Part III].Proceedings of the Royal Society of London, Volume 50, pages 369–372, 1 January 1892. Published by The Royal Society.