1. Abbot, C. G.: The Radiation of the Sun. An extract from the Annual Report of the Smithsonian Institution for 1912, Government Printing Office, Washington DC, USA, 17 pp., 1913.
2. Abney, W. de W.: Transmission of sunlight through the Earth's atmosphere. Part II. Loss of light at different altitudes, Phil. Trans. Royal Soc. London A, 184, 1–42, 1893.
3. Ångström, A.: On the atmospheric transmission of sun radiation and on dust in the air, Geograf. Ann. Deut., 11, 156–166, 1929.
4. Ångström, A.: The parameters of atmospheric turbidity, Tellus, 16, 64–75, 1964.
5. Bouguer, P.: Traité d'Optique sur la gradation de la lumiere: ouvrage posthume de J. Bouguer, de l'Academis Royale des Sciences, & publié par M. l'Abbé de la Caille, de la meme Académie, &}c English translation: Optical treatise on the gradation of light, posthumous work of Mr. Pierre Bouguer of the Royal Academy of Sciences, etc. and published by the Abbé de la Caille of the same Academy, etc.; with introduction and notes by Middleton, W. E. (1961), {Pierre Bouguer's Optical Treatise on Gradation of Light, University of Toronto Press, Toronto, Canada, 241 pp., 1760.