Wave-Lengths of Mercury 198

The wave-lengths of 26 visible and ultraviolet radiations from ${}_{80}{}^{198}\text{H}\text{g}$ have been measured to eight figures relative to 5460.7532A, provisionally adopted for the green line. A water-cooled electrodeless lamp containing two milligrams of artificial ${}_{80}{}^{198}\text{H}\text{g}$ served as a light source when excited with radio waves of 100 Mc frequency. This lamp illuminated a Fabry-Perot interferometer and a large quartz spectrograph with which the interference patterns were photographed. Measurements of these patterns yielded relative wave-lengths with probable errors usually less than 0.0001A. In addition to demonstrating that ${}_{80}{}^{198}\text{H}\text{g}$ 5460.7532A provides a practically perfect primary standard of length, this paper presents preliminary values of 27 superior spectroscopic standards which show that differences among vacuum wave numbers recur within the accuracy of the measurements, and that all the existing dispersion curves for air are incorrect.