Interferometric measurements in the arc spectrum of iron

Abstract

At the 1928 Meeting of the International Astronomical Union at Leiden, it was recommended that the present system of iron secondary standards be extended further into the ultra-violet, since there are no modern measurements extending below about λ 3300. a list of wave-lengths computed from the standards adopted in 1928 in the less refrangible parts of the spectrum was given, which indicated that all the older measurements in this part of the spectrum were considerably in error, being over one part in one million too great. Accordingly I decided to measure a set of lines in the region λ 2300 to λ 3100 by means of the usual interferometer method. It was found that the measures agreed very well with the computed I. A. C. wave-lengths, the systematic difference being practically nil, while the mean accidental difference was less than 0.001 A. The wave-lengths also agreed very well with those determined recently by Burns in the vacuum iron arc spectrum; indeed, almost as well as they did with the I. A. C. computed values, although my values showed a very small (J - B = +0.0002 A.) systematic difference from Burns. The difference can be attributed to a pressure shift since it is about the amount to be expected for stable lines in this region, but it is too small to have any very great Significance. The agreement with Burn's 1930 revision of his 1913 interpolated measures was also fairly satisfactory, particularly from the absence of systematic difference, although the accidental differences were rather greater. There is however, considerable disagreement with all the old interpolated measurements which are based on Buisson and Fabry's standards and also with Buisson and Fabry's measurements.