BELL. — ULTKAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 13 



was cut off by the crown glass and hence substantially this proportion 

 was of wave length less than 300 ^i/x. 



In running quartz lamps without their globes, as was done in these 

 experiments, the energy output is considerably diminished by the 

 cooling of the tube and the light-giving properties of the lamp are very 

 much reduced. Both the old and the new quartz lamps herein noted 

 were photometered. The lamps were compared against a tungsten 

 secondary standard by means of a Simmance-Abady flicker photo- 

 meter. The c. p. normal to the length of the tube and in a horizontal 

 direction, was for the old quartz lamp 415, for the new quartz lamp 

 348, in each case without any enclosing globe. Both lamps were very 

 steady and easy to work with, both on the photometer bar and with 

 the thermopile. 



Finally the new quartz lamp was fitted with its regular diffusing 

 globe and tested with the thermopile. In working with the globe 

 the tube operated at a higher temperature and far more intensively, 

 the wattage rising to 460. With the Euphos glass in, the total change 

 in deflection amounted to only 3.7 cm. although the lamp tested on 

 the photometer as in the previous case reached 820 c. p. in the hori- 

 zontal direction. In percentage the amount of energy cut off by the 

 Euphos glass was 42.5. These figures plainly indicate that the globe 

 absorbed the further ultra violet very strongly, more strongly than 

 the crown glass already referred to. In fact the deflection due to the 

 ultra violet energy which passed through the globe of the lamp was 

 extraordinarily small with respect to the c. p. of the source, very much 

 smaller than in the case of any other illuminant investigated. With- 

 out the globe the quartz arc is a very powerful source of radiation 

 in the extreme ultra violet, below wave length 300 nix. W^ith its 

 ordinary globe on, all this energy in the extreme ultra violet is cut off 

 and the small remaining amount, mostly in that part of the ultra 

 violet nearest the visible spectrum, becomes quite insignificant. 



The Welsbach Mantle. — At this point study of the radiation from 

 the Welsbach light was taken up. The particular form used was a 

 Graetzin street lamp with a single large inverted mantle fitted with a 

 clear glass globe, which obviously eliminated whatever of extreme ultra 

 violet might be present. This burner took 6.4 feet of gas per hour at 

 3 inches pressure and gave 75 c. p. in the horizontal direction. Its 

 total deflection was slightly greater than that produced by the quartz 

 lamp with its globe tested immediately before. The addition of the 

 Euphos glass cut down the deflection by .924 c. m., an amount equiva- 

 lent to the absorption of 8.4 % of the total radiation recorded. The 



