A study has been made of the ultraviolet spectrum of the constant-pressure mercury arc recently proposed as a radiation standard by Krefft, Rössler, and Rüttenauer. Experience with this lamp in another connection indicated its usefulness, but suggested the need of more specific and accurate information about its spectral energy distribution than was available in the previously published calibration data. A detailed discussion is given of practical methods for presenting such information in unambiguous form in the case of a source emitting a rather complicated spectrum such as the mercury arc. The structure of the spectrum is carefully studied in order to determine exactly which intervals of wave-length should be selected for calibration, in the range between 3342A and 2377A (inclusive). In the absence of facilities for performing a new calibration by absolute methods, a tentative calibration has been obtained by spectrophotometric comparison with the carbon arc, the positive crater of which is assumed to emit the distribution characteristic of a blackbody at 3840°K. This gives a set of relative values for the energies of certain groups of lines, which appear from internal evidence to be more trustworthy than the values in the literature. The peculiar conditions of the comparison made it necessary to use an optical arrangement in which the light entering the spectrograph did not, as usually recommended, completely fill the collimator. The results of this departure from standard practice have been investigated and found to consist in a concentration and enhancement of accidental light due to multiple reflections from the quartz surfaces. A simple device is described for determining the corrections for scattered and reflected light under actual working conditions.
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