The idea of establishing the fundamental units of length on a basis defined by some natural standard has long attracted physicists. The metre was originally intended to represent one ten-millionth of a meridional quadrant of the earth,* and the Weights and Measures Act of 1824 prescribed that the yard, if ever lost or destroyed, should be replaced by reference to the length of a pendulum beating seconds in vacuum at sea level in London. It is believed that BABINET, in 1829, suggested the use of a wave-length of light to define the fundamental unit of length. The first specific proposal to correlate the wave-length of light with material standards of length was made about 1875 by GOULD, who suggested the use of a diffraction grating for the determination. It was not until 1893 that the first direct measurement of the metre in terms of the red radiation of cadmium was made by MICHELSON and BENOÎT. § The comparison was made possible by a slightly modified form of the famous interferometer which MICHELSON had devised originally for the attempt to measure the relative velocity of the earth through the ether. In 1906, BENOÎT, FABRY and Perot repeated the determination of the length of the metre in terms of the red radiation of cadmium, using different methods and apparatus. More recently, in 1928, WATANABE and IMAIZUMI have again repeated the determination with apparatus essentially the same as that used by BENOÎT, FABRY and PEROT.