 The idea of reciprocity, once known as Bunsen–Roscoe reciprocity, originated from the work of Robert Bunsen and Henry Roscoe in 1862.Deviations from the reciprocity law were reported by Captain William de Wiveleslie Abney in 1893,
and extensively studied by Karl Schwarzschild in 1899.Schwarzschild's model was found wanting by Abney and by Englisch, and better models have been proposed in subsequent decades of the early twentieth century.