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ABSOLUTE INTENSITIES IN THE VISIBLE AND ULTRA- 

 VIOLET SPECTRUM OF A QUARTZ MERCURY ARC 



By E. D. McALISTER, 

 Division of. Radiation and Organisms, Smithsonian Institution 



INTRODUCTION 



For some of the biophysical investigations (i)' in progress at the 

 laboratory of the Division of Radiation and Organisms of the Smith- 

 sonian Institution an absolute measure of the energy in the lines 

 of the mercury arc spectrum was needed in greater detail than is 

 available in the literature. This has been obtained for a common 

 type of commercial arc (Cooper-Hewitt 220 volt D.C. quartz mercury 

 arc), and it is believed that these results will be of interest to users 

 of this type of mercury lamp. Although this source is not ideal from 

 a spectroscopic standpoint the present measurements cover a wide 

 spectral range and are accurate enough to show a smooth and regular 

 decay in the sharp and diffuse series. Also it is found that the theo- 

 retical intensity relation for these triplets is approached by higher 

 members of the series. 



A preliminary report of these measurements was presented at a 

 meeting of the American Physical Society (2) and the preliminary 

 curves — uncorrected for instrumental transmission — were included 

 in the Report of the Secretary of the Smithsonian Institution for 

 the year ending June 30, 1931 (p. 133). The transmission of the 

 two monochromators used has since been determined and the spectrum 

 remapped under steadier operating conditions, so that the data may 

 now be presented as absolute measurements of radiant flux. 



Coblentz in his extensive examination of light sources (3) used 

 in phototherapy has given data on this type of arc. His data consists 

 mainly of filter observations that give in absolute measure the energy 

 distribution in the different spectral regions. In a recent paper (4) 

 he has given relative intensities of the prominent lines in the ultra- 

 violet. Harrison and Forbes (5) and Hulburt (6) show energy 



* Numbers in parentheses refer to the list of literature cited, found at the end 

 of this paper. 



Smithsonian Miscellaneous Collections, Vol. 87, No. 17 



