126 



STUDIES IN LUMINESCENCE. 



to 20 at the beginning of the fixing. Later not so much care was used in 

 the fixing because it was found to be inadvisable to intercompare spectra 

 on different plates. After fixing, the plates were washed in running water 

 for about 30 minutes, then thoroughly rubbed by hand and dried. Each 

 plate was dried in such a position that the last portion to drain was a part 

 upon which no measurements were to be made. 



The distribution of denseness in any spectrum on a plate was obtained 

 by pushing the plate past a brightly illuminated slit placed squarely across 

 the spectrum, and measuring the transmitted light by means of a Lummer- 

 Brodhun photometer. The source of light to illuminate the slit and the 

 standard light consisted of two carbon-filament electric lamps with frosted 

 bulbs. The light back of the plate was concentrated on the slit by a 

 reflector. The current supplied to the lamps was held practically constant. 

 It was found by actual test that a change of 5 volts was necessary to vary 

 the relative intensities of the lamps enough to affect the settings of the 

 photometer appreciably. The lamps were run at 6 per cent above normal 

 voltage to increase the candlepower and were lighted only long enough to 

 make the measurements. 



.46 



50 



54 .58 



Fig- 133- 



.62 



.66 



The wave-length measurements were made by calibrating in wave- 

 lengths the screw which pushed the plate past the slit on the photometer 

 bar. A zero wave-length was obtained on the plate by photographing the 

 three blue lines of the mercury arc superimposed on the different spectra. 

 These lines were in an entirely different region from that occupied by the 

 spectrum of the fluorescence light. The calibration of the screw was made 

 by photographing the entire visible spectrum of the mercury arc with the 

 spectrum camera and setting one line after the other directly in front of the 

 slit on the photometer bar. This slit was always opened just as wide as the 

 width of the lines in the spectra and was placed parallel to these lines. The 

 standard lamp was screened off by means of a variable slit, so that the pho- 

 tometer could be used on whatever part of the bar was desired. 



Fig- J 33 shows the curves obtained by plotting the light transmitted 

 through the spectra on one of the plates which had been exposed as de- 

 scribed above. Each exposure was 60 minutes long and was made with the 



