26 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the total light and at almost as high efficiency for the pair of yellow- 

 lines which supply nearly all the rest. Next in the list, a rather bad 

 second, comes the Graetzin gas lamp, its position again being due to 

 the somewhat selective radiation that gives it a very high luminous 

 efficiency. Third, comes the G. E. INI. lamp which, from its relatively 

 low temperature, gives a small absolute amount of ultra violet radia- 

 tion, although its luminous efficiency is not great. 



At the other end of the line comes the special enclosed arc with 

 91 ergs per second per square cm. per foot candle, and next to it the 

 quartz lamp without its globe. Of course the quartz lamp without 

 its globe is never used for illuminating piu'poses, but only for such 

 work as sterilization of water and the like in which the ultra violet 

 rays are the things sought. Operated for this purpose it undoubtedly 

 is the most efficient pow^erful source of extreme ultra violet. To test 

 this feature of the matter energy measurements were taken on the 

 two quartz lamps without their globes and on the magnetite lamp 

 free from its globe while using as a screen instead of the Euphos glass 

 a disc of the very light crow n glass previously referred to, which practi- 

 cally effects a separation at wave length 300 fifi absorbing substan- 

 tially all the energy below this point and transmitting at almost full 

 intensity' the rest. The result of this test, measuring the extreme 

 ultra ^'iolet and reducing it to the mean spherical output of ultra 

 violet, showed for the extreme ultra violet efficiency of the new quartz 

 lamp 4.07 % and for the efficiency of the old quartz lamp 3.14 %. 

 A similar measurement of the magnetite arc showed an extreme 

 ultra violet efficiency of 1.13 %. These figures may be properly com- 

 pared with the tests for the ultra violet efficiency of the quartz lamps 

 made by Fabry and Buisson.^^ In this case two mercury lamps showed 

 respectively extreme ultra violet efficiencies of 6.4 and 4.7 %, the 

 ultra violet separation being effected by the screens used by Fabry 

 and Buisson at wave length 320 fxid. The values obtained by the 

 French investigators and in this study therefore check each other 

 closely, showing that in the quartz mercury lamp 4 to 5 % of the total 

 input is returned in the form of extreme ultra violet radiation when 

 the lamps are operated, as they are for sterilization purposes, without 

 their globes. The lighting power of the lamp falls off very greatly in 

 this condition. 



When operated with the globe the total proportion of ultra violet 

 becomes both absolutely small and extremely small relati\ely to the 



18 Comptes Rendus, Vol. 153, p. 93. 



