NO. 1/ ULTRA-VIOLET INTENSITIES McALISTER 1 7 



Figure 4 shows the intensity decay in these series. This is a plot 

 of the logarithm of the relative number of transitions (intensity/v) 

 that occur in the whole of the 20-mm section of the arc in unit time 

 against the frequency (6). The symbols shown in Table 4 are used 

 to identify the series and its multiple numbers. The smooth and 

 regular decay shown here is not present in previous work and verifies 

 the small experimental error given. The upper curves are for the 

 high-intensity arc and the lower are for the low. The curves appear 

 to approach asymptotic lines of the same slope in a given series. 

 This slope is greater for the diffuse series which indicates a lower 

 " Boltzmann " temperature for the ^D than for the 'S levels as 

 was pointed out by Hulburt (6). 



SUMMARY 



Absolute measurements of the intensity of 32 of the more intense 

 lines in the visible and ultra-violet spectrum of a quartz mercury arc 

 have been made with a vacuum thermocouple and double mono- 

 chromator. The effective slit width employed was 2 A. at A 2,300 A. 

 increasing to 12 A. A 4,000 A. This yields a resolution about one 

 order of magnitude greater than that of previous work of this nature. 

 The probable error in the intensity measurements is ± 3 per cent. 

 Observations made upon four arcs showed significant differences in 

 intensity for many of the spectral lines. These differences averaged 

 about 5 per cent, the maximum being 10 per cent for A 2,537 A. 

 This is probably the deviation in output to be expected from different 

 arcs of this same make (operating with the same current and voltage) 

 that are reasonably new and have not been mistreated. 



About two thirds of the lines measured are members of the sharp 

 and diffuse series. The present measurements show a smooth and 

 regular decay of intensity in these series. The theoretical intensity 

 relation for these multi-plets is not attained but is approached by 

 higher members of both series. 



LITERATURE CITED 

 (i) Meier, F. E. 



1932. Lethal action of ultra-violet light on a unicellular green alga. 

 Smithsonian Misc. Coll., vol. 87, no. 10. 

 Brackett, F. S., and McAlister, E. D. 



1932. A spectrophotometric development for biological and photochemi- 

 cal investigations. Smithsonian Misc. Coll., vol. 87, no. 12. 

 (2) McAlister, E. D. 



193 1. Intensities in the ultra-violet spectrum of mercury. Phys. Rev., 

 vol. 37. no. 8, pp. 1021-1022, Apr. 15. 



