Michael Faraday and his “Secchi disks”

Michael Faraday (1791-1867) is best known as a brilliant scientist and experimenter in chemistry, electricity and magnetism; however, his biography and bibliography show that he published and lectured on a wide array of subjects (Thomas, 1991). It is apparently unknown in optical limnology and oceanography, that in 1855 Faraday experimented with a kind of Secchi disk avant la lettre to measure the turbidity of the River Thames. This is not mentioned in reviews and textbooks (Boguslawski, 1884; Forel, 1901; Krummel, 1923; Hutchinson, 1957; Holmes, 1957; Jerlov, 1976). Faraday's experiments were done 10 years before Secchi used a white disk and the depth of its disappearance from sight for transparency measurements in the Mediterranean in 1865. Such disks later became known as Secchi disks.