How did Charles Sanders Peirce use light wavelengths to determine the length of the metre?

Charles Sanders Peirce, an American scientist and philosopher, conducted pioneering research in the late 19th century that utilized light to define units of measurement. While working for the U.S. Coast and Geodetic Survey, Peirce performed extensive pendulum experiments to measure the force of gravity. The requirement for highly accurate length measurements during this research led him to conduct a pioneering determination of the length of the metre in terms of a wavelength of light between 1877 and 1879.1