We have measured the vacuum wavelength of the 2537-Å line of Hg198 as passed by a Zeeman-split absorption filter. The 6057-Å line of Kr86 was used as a standard. An evacuated Fabry-Perot interferometer was employed with spacers of 218-, 110-, and 21-mm length. The linewidth obtained by using the Zeeman filter is estimated to be about 5 mK. The measured wavelength 2537.26874±0.00003 Å is in excellent agreement with the value obtained by Barger and Kessler for the wavelength emitted by an atomic beam of Hg198.
WALTER G. SCHWEITZER, JR., "Wavelength Passed by Hg198 Zeeman-Split Absorption Filter," J. Opt. Soc. Am. 53, 1250-1252 (1963)