Knowledge Bank

During the past six $years1$ a considerable number of wavelength determinations have been made for the region between 1 and $2\mu$ in Ar I, Kr I, $Hg198$ I, and Cd I by means of a radiometric scanning technique employing a rotating Fabry-Perot interferometer. Lightly coated aluminized plates have proved to be the most satisfactory choice for this application, but owing to a discontinuity in the reflectance properties of aluminum films near 8500 \AA, it has become important to select standards of greater wavelength than that number. The logical choices are: $\lambda$ 10139 \AA of $Hg198$ I, the three most intense Is-$2p10$ transitions in Ar I, namely $\lambda 9122,9657$, and 10470 \AA, and $\lambda 10562$ \AA of Ne I, which is $2p1--3S1$. There are no available direct determinations of these wavelengths with an uncertainty less than 1 to 2 parts in 10,000,000. All are subject to calculation on the basis of observations in the photographically accessible region by use of the combination principle. Recently completed observations in Ne I near $1\mu$ are expected to lend support to the conclution that the standards presently in use form a consistent system. This system will be discussed critically in the light of information available from The author’s program and other current $work.2$