Interferometric measurements in the spectrum of the iron arc in air in the region λ 3100 — λ 3500

In a previous paper I gave the results of a set of wave-length measurements in the spectrum of the iron arc in the region λ 2300 to λ 3100. These measurements were carried out because it was found that previous measurements in this region were all rather unsatisfactory and gave results considerably higher than those calculated from levels determined from the wave-lengths recommended by the International Astronomical Union in 1928. My results confirm the values calculated from the I. A. U. Standards very satisfactorily, and also agree very well with the recent measurements of the vacuum arc by Burns and Walters. The apparatus was also used to measure a few lines in the visible part of the iron spectrum, and was shown to give results in excellent agreement with the values adopted by the I. A. U. in 1928. It was recommended by the I. A. U. that observations should be made in the region below λ 3400 for which there were no good modem measurements available. Accordingly the present work was undertaken to fill up the gap remaining and a set of 46 lines has been measured in the region λ 3100-λ 3500. The results are in very good agreement both with the I. A. U. calculated values and with the observed standards adopted by the I. A. U. in 1928, the mean systematic difference (J — I. A.) being only —0·0001 A. for 8 lines, while the mean accidental difference is ±0·0006 A. The agreement with the measurements of Babcock in the region λ 3370-λ 3500 is equally good, the mean systematic difference (J — B) being +0·0001 A. for eight lines, while the mean accidental difference is +0·0006 A.