Experiments have been carried out in the far infra-red region from 100 to 600 microns using a grating spectrograph with crystal quartz optics. These experiments include measurement of the transmission of the atmosphere for paths up to 8 meters, the measurement of the transmission of various solid substances, and the observation of the longest wave-length pure rotation lines in the spectra of ammonia and hydrogen chloride.
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McCubbin et al. (1950) studied this question.
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