Abstract
THE vast work of determining and cataloguing radial velocities has made the slitless spectrograph almost indispensable. A number of stars may then be photographed on the same plate at the same time. Such a method is possible only if the starlight passes through some medium which has sharp absorption lines so that the spectrum of each star has within it the absorption lines for references. A medium having sufficiently sharp lines in the proper spectral region does not seem to have been found. I would suggest the use of the crystals of the rare earths maintained at the temperature of liquid nitrogen. Both Dr. Joy, of the Mount Wilson Observatory, and Dr. Shane, of the University of California, have considered in this connexion the absorption spectra of crystals of gadolinium at room temperature.1 The absorption lines occur, however, in an unfavourable region of the spectrum.
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References
Freed and Spedding, NATURE, 123, 525; 1929: Phys. Rev., 34, 945; 1929.
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FREED, S. Absorption Spectra of Crystals at Low Temperatures as References in the Measurement of Stellar Velocities. Nature 128, 33 (1931). https://doi.org/10.1038/128033a0
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DOI: https://doi.org/10.1038/128033a0
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