Extension of the Ultraviolet Spectrum Toward Shorter Wavelengths

K. A. Millican. The extension of the ultraviolet spectrum. Astrophys. Journ. 52, p. 47 (1920).

Lyman, who extended the spectrum to 500 Å, used a vacuum spectrograph with a diffraction grating. In this way he was able to rid himself of all absorbing solid bodies between the light source and the photographic plate. Absorption of the radiation of the spark, which served as the light source and was located in the space of the spectrograph, could be effected by a rarefied gas filling the spectrograph.

I shall mention here only that Richardson and Bazzoni managed, by a somewhat different method, to detect waves of length 420 Ź. Millican, however, generally using Lyman’s method, strengthened its means. In fact, he used a vacuum of \(10^{-4}\) mm of mercury; for pumping he employed a mercury diffusion pump; the light source was sparks between electrodes separated by a distance of from 0.1 to 2 mm, at large capacitance and very high voltage (several hundred kilovolts). The duration of the exposure did not exceed 30 minutes; the accuracy of the measurements was up to 0.2 Å. The electrodes were of zinc, iron, silver, nickel, and carbon. The shortest wavelength first observed by him was 202 Å with nickel electrodes.

It should be noted that he succeeded in establishing a series of entirely new lines in the far ultraviolet part of the spectrum. Moreover, the lines in the interval from 1200 to 600 Å he ascribes to carbon, and not to helium, as Lyman does, for when electrodes of pure silver are used these lines are not observed. The appearance of these lines in Lyman’s work in an atmosphere of He he explains by the contamination of He with carbon vapors (evaporation of carbon electrodes, even if momentary).

T. Molodny.

1. See “Advances in the Physical Sciences,” vol. II, issue 1, p. 117, 1920. ↩