Vacuum wave-length measurements in the iron spectrum by means of the reflection echelon grating

Abstract

As is well known, the determination of the wave-length of a spectral line is made by comparing it with another line of known wave-length. In the most precise measurements the comparison is made directly with the red line of cadmium which forms the spectroscopic standard of wave-length. It is not always convenient to use this primary standard, and at the instance of the International Astronomical Union a number of investigators have independently determined the wave-lengths of selected lines in the spectrum emitted from an iron arc under specified conditions. The committee of the Union has drawn up tables giving the values of these lines which are recognized as secondary standards and which are distributed fairly uniformly throughout the spectrum. As substandards, their wave-lengths are required to as high a degree of precision as possible. Apart from considerations of standards, accurate wave-length measurements are now required for an entirely different purpose. The spectral lines of many elements have been arranged in series, and a comparatively simple formula expresses the frequency or the difference between the term values. Occasionally the difference between the experimental and calculated values is so large that the “perturbation” becomes obvious from the roughest measurements, but in general they are small and only precision measure¬ments will show their existence and significance.