AbstractThe system of colorimetry defined by the CIE in 1931 was the culmination of the pioneering determinations of the colour‐mixture curves by Maxwell, König, and Abney in the 19th century. American workers then made major contributions in the form of the 1922 Colorimetry Report of the Optical Society of America and the data adopted by the CIE in 1924 as the standard Vλ curve. The colour‐matching data for the 1931 CIE standard observer were provided by Guild and the author in England, but it was a race against time to get the form of the system agreed in time for its adoption at the 1931 meeting of th