BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 25 



of the source is in the last resort the thing which determines the pres- 

 ence or absence of ultra violet radiation in material amount. In 

 other words the more efficiently the energy supplied to the illuminant 

 is transformed into light the less important does the ultra violet 

 radiation become in considering the source as a practical illuminant. 



Source 

 Quartz Arc (Alba globe) 

 Graetzin Gas Lamp 

 G. E. M. Lamp 

 Cooper-Hewitt (glass) 

 Sunlight (direct) 

 Acetylene Flame 

 Tungsten Lamp 

 Nernst Lamp (globe) 

 Magnetite (glass) 

 Magnetite (quartz) 

 Old Quartz Lamp (bare) 

 New Quartz Lamp (bare) 

 Carbon Arc (quartz) 



Table III assembles the commercial light sources tested, with 

 respect to the ultra violet energy accompanying a given illumination. 

 The first column of the table gives merely for the purpose of record 

 the actual deflections found to be due to the ultra violet energy, and 

 column two the total ultra violet radiation in ergs per second per 

 square cm. per foot-candle of illumination. At the head of the list 

 stands the quartz mercury arc with its dilTusing globe. Of the com- 

 mercial illuminants tested this gives by all odds the smallest propor- 

 tion of ultra violet per foot candle. As the previous tables show, the 

 ultra violet energy of this source so equipped is small from any point 

 of view. Its unique position, however, is due largely to the fact that 

 the light-giving radiation, which lies practically at the very peak of 

 the luminosity curve for vision, is produced at enormous efficiency, 

 according to Buisson and Fabry " not less than 55 candles per watt 

 for the green line at wave length 546 which supplies nearly two thirds 



1^ Comptes Rendus, Vol. 153, p. 254. 



