12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



radiation. The percentage of the total radiation is given immediately 

 below each intensity value. Observations were made on the radiation 

 from the 20-mm length of the midsection as well as from the total 

 length of the arc for both high and low intensities. The data given 

 in the table were obtained using a bare thermocouple, the procedure 

 being as follows: Observations were made on: i, the total radiation 

 • — no filter used ; 2, radiation transmitted by 2 mm of rock salt, which 

 includes all wave lengths less than iQyx; 3, radiation transmitted by 

 a fused quartz water cell, all wave lengths below 1.4/^, and 4, radiation 

 transmitted by a combination of the water cell and 2 mm of barium 

 liint glass which includes all wave lengths between .35/A and i.4ju,. 

 These observations were then corrected for reflection and absorption 

 by the following transmission factors: rock salt 0.91, water cell 

 0.90, water cell and barium flint 0.81. After these corrections are 

 applied the data in Table 3 are obtained as follows : values in column 

 I are the difference between observation 3 and observation 4 ; column 

 2, observation 4 ; column 3, observation 2 minus observation 3 ; column 

 4, observation i minus observation 2; and in column 5, observation i. 

 As the filter " cut-offs " are not sharp, appreciable error might be 

 introduced by this method. For instance the short wave length " cut- 

 off " for the barium flint filter is not sharp. However, in the mercury 

 spectrum this is not serious because the " cut-off " occurs in a region 

 where there are no strong lines. The sum of the monochromator 

 values for all wave lengths less than and including A 3,130 A. for the 

 20-mm midsection of the high-intensity arc is given at the bottom 

 of Table 2, column 4. It is 1,362 microwatts per cm ^ at 250 mm 

 from the arc. The filter measurement for this region is given in 

 Table 3, column i, as 84 microwatts per cm "" at i m. Multiplying 

 this by 16 to reduce it to the intensity at 250 mm from the arc we 

 get 1,330 microwatts per cm ^, which is 2.1 per cent lower than the 

 monochromator sum. 



For the sake of a more rigid check on the monochromator obser- 

 vations one may compare the intensities given in Table 2, columns 

 7 and 8, with the uncorrected thermocouple measurements. Table 2, 

 columns 7 and 8, give the radiation excluded by the barium flint 

 filter and transmitted by the fused quartz water cell for the 20-mm 

 section of the high- and low-intensity arc. These can be compared 

 with the filter measurements as follows : the radiation transmitted 

 by the fused quartz water cell minus the radiation transmitted by the 

 cell in combination with the barium flint filter, the latter value being 

 corrected for the transmission of the barium flint alone is 1,210 

 microwatts per cm ^ at 250 mm from the high-intensity arc. This is 



