4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8/ 



was used. To obtain the greater resolution required for close groups 

 and reduce the effect of scattered light the double monochromator 

 set-up was used with a 0.05-mm slit. 



The transmission of the monochromators for the wave lengths used 

 was obtained with the aid of a photronic cell provided with a quartz 

 window. This was used instead of a thermocouple because its charac- 

 teristics render it relatively insensitive to a change in position behind 

 the monochromator. For a constant amount of incident energy the 

 cell's response is practically independent of the area exposed. One of 

 the vertical arcs operating at high intensity was used for this cali- 

 bration. It was placed 250 mm from the slit so as to give an illumina- 

 tion identical with that used in the intensity measurements. Another 

 photronic cell connected to a microammeter was placed so as to 

 receive total radiation from the arc. This microammeter, which 

 indicated the output of the arc, and a voltmeter and ammeter measur- 

 ing the input were mounted near the galvanometer scale so that the 

 observer could glance at them before taking a reading. For both the 

 transmission measurements and the intensity measurements galvanom- 

 eter readings were taken only when this photronic cell indicated 

 the same output from the arc. The intensity of 19 of the stronger 

 lines was observed at the exit slit of one monochromator. Then the 

 second instrument was placed in tandem and readings taken on the 

 same lines. The ratio of the latter reading to the former for a 

 particular wave length is the transmission of the second mono- 

 chromator for that wave length. The positions of the instruments 

 were then reversed and the process repeated. The transmissions so 

 obtained were plotted and smooth curves drawn. Points taken from 

 the smooth curve for the instrument used in the intensity measure- 

 ments are included in Tal)le i. The transmission of one instrument 

 differed by less than i per cent (/". c, 46 per cent compared with 

 47 per cent) from that of the other and was consistently higher. 

 The transmission of either machine with the field illuminated as 

 stated was several (2 to 4) per cent higher than that given by the 

 manufacturer. This difference is probably due entirely to the differ- 

 ence in illumination, as the manufacturer's figures are for a com- 

 pletely filled aperture. 



Measurements were made with a bare thermocouple of the energy 

 in the spectral regions transmitted by the following filters — taken in 

 the order given: i, no filter (all wave lengths); 2, rock salt (all 

 wave lengths up to 19/;.) ; 3, fused quartz water cell (all wave lengths 

 up to i.4ja with a slight diminishing of the deep ultra-violet below 

 2,500 A.) ; 4, filter No. 3 plus barium flint glass (wave lengths below 



