The wave lengths of 252 ultraviolet lines (3498 to 2101 A) characteristic of the international iron arc were measured relative to cadmium, krypton or neon standards. These measurements were made with Fabry-Perot etalon interferometers and stigmatic spectrographs. Invar etalons of 2, 3, 5, 7.5, or 10 mm length, and aluminized quartz plates were used. The spectrographs consisted of a concave grating, a Littrow quartz, or a Cornu quartz instrument. With the first, simultaneous exposures were obtained of red neon lines in the first-order spectrum, and of ultraviolet iron lines in the overlapping second-order spectrum. The use of quartz spectrographs required alternate exposures of primary and secondary sources. The final values in many cases are given to eight figures since calculations of probable error and tests of relative value by means of the combination principle indicated average errors smaller than 0.0005 A.
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Meggers et al. (1937) studied this question.