More Wavelengths From Thorium Lamps

For the purpose of supplementing or superseding the spectroscopic secondary standards of wavelength derived since 1910 from an iron arc, Meggers and Stanley, in 1958, reported the first interferometric determinations of wavelengths emitted by a thorium lamp. Those determinations were restricted to 222 intense radiations of thorium with wavelengths ranging from 3288.7356 Å to 6991.5839 Å. Now the same, and additional, interference spectrograms have been measured to provide improved wavelengths for 510 radiations ranging in wavelength from 3269.6089 Å to 7020.504 Å. The present list includes many lines of lower intensity than those previously published and fills most of the large intervals in our first report. The accuracy in relative wavelength values of 163 classified thorium lines is tested by the combination principle which indicates that the average error is less than 1 part in 20 to 40 million. Similar measurements of wavelengths emitted by iron-halide lamps have errors that are 3 to 5 times greater.