BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 17 



the effect of a continuous spectrum. The actual character of the 

 spectrum photographed with a fairh' wide sht, is shown in Plate 2, d. 

 Here, with the quartz arc spectrum for reference at a is shown the 

 radiation from the magnetite arc through a quartz window and below 

 it the spectrum of the same arc taken through its ordinary globe. A 

 quartz window was used merely to insiu'e steadiness of the light, which 

 would have been lost by taking oft* the globe. A glance shows that 

 this spectrum is exceedingly rich in powerful lines all through the 

 ultra violet clear down to wave length 230 mm- The glass globe cuts 

 oft" the spectrmn quite sharply near wave length 300 /i^t, as in Plate 2, e, 

 but from this region to the visible spectrum lies an almost continuous 

 mass of strong lines, very intense in the region where the quartz mer- 

 cury arc is conspicuously weak, say from the group at wave length 

 313 fxiJL to the group near wave length 365 /x/x. 



For radiometric measurements the magnetite arc, which was oper- 

 ated at 6.6 amperes and about 80 volts, pro\ed much more steady 

 than the carbon arc, showing more small and quick fluctuations, but 

 fewer of the large and relatively slow variations which interfered most 

 with the readings. As a consequence the deflections obtained agreed 

 more closel}', the average variations of a single setting running be- 

 tween 3 and 4 %. For the magnetite arc through the quartz window 

 the cut-off of Euphos glass amounted to 29 cm., 28% of the total 

 deflection. Through the ordinary glass globe the deflection was 

 reduced to 22.4 cm., 22.5 % of the total deflection. The difference 

 between these results shows that wdaile there is a large amount of 

 energy of short wave length produced by the magnetite arc, most of 

 the ultra violet energy is of wave length greater than 300 nfx. As 

 compared with the quartz mercury arc used without its globe the 

 magnetite arc gave relatively about 60 % less energy of wave length 

 below 300 iJ.fx and about 40 % more energy in the wave lengths above 

 300 fxfjL. The candle power in the horizontal direction as measured by 

 the method just described amovmted to 760 in the run with the quartz 

 window, and 700 in the run with the ordinary globe. 



The Nernst Lamp. — Finally a series of readings was taken on the 

 Nernst lamp. The lamp investigated was of the single glower type 

 for 220 volts, taking 91 watts and giving a downward c. p. of 68. As 

 the spectrum of this source runs to less than wave length 300 /jl/jl and 

 reaches that vicinity with somewhat material strength an attempt 

 was at first made to run the Nernst glower without a globe. ,It 

 proved so difficult to get steady deflections under these conditions, 

 on account of the effect of air currents, that this measurement was 



