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The reflection spectrum of quartz has been previously studied by means of both the rock-salt prism spectrometer and the echelette grating spectrometer. Early investigators have shown that for crystalline quartz, with the reflecting surface perpendicular to the optic axis, there is a double reflection maximum, one lying at 8. 4 and the other at 8. 9; and that fused quartz exhibits a single maximum at 8. 8. These regions have been studied under high dispersion by the authors using an echelette grating spectrometer. The high resolving power that was obtained was due to the sensitivity of the radiometric device, the resonance radiometer, which was first described by A. H. Pfund. The complete theory of the instrument was developed by one of us. By the use of this large resolving power and dispersion it has been possible to show that these spectra are considerably more complex than were found previously. The usefulness of magnesium oxide as a filter for excluding the higher orders of lower wave-lengths has been tested with gratifying results.
Hardy et al. (1931) studied this question.