[iRETON] SELECTED RADIATIONS 139 



the fluorescent band obtained was not at all comparable in intensity 

 with the photographs of the spots obtained from the light, issuing 

 from the tubes, which, as mentioned above, passed through absorption 

 screens to isolate the waves concerned. To obtain the fluorescent 

 l)and with intensities comparable to those of the spots, exposures 

 would have been required of from 7 to 10 hours duration. 



It should be stated that in the case of the wave length X = 5460.97 

 A.U. the filter used cut down the intensity of the light approximately 

 by 30%, while in the case of the wave length X = 4358.66 A.U. the 

 combination of filters used allowed only about 50% of the light issuing 

 from the tubes to pass through. 



When the light issuing from the tubes was examined with a 

 quartz spectrograph the spectrograms of the light issuing from the 

 quartz tube showed no trace of the fluorescent band for exposures 

 even much longer than those made in the experiments in which the 

 spots shown in Plate I were obtained. 



In view of the above results it seems correct to attribute 

 the increase in intensity of the light issuing from the quartz tube as 

 being due to some other phenomenon than that of fluorescence of 

 the mercury vapour produced by the wave length X = 2536.72 A.U. 

 and X= 1849.6 A.U. From the series relationships which hold among 

 the wave lengths in the spectrum of mercury it does not seem possible 

 that the absorption by the mercury vapour of radiation of the wave 

 length X = 1849.6 A.U. could to any appreciable extent contribute 

 to an emission by the mercury vapour of \\a\e lengths X=^ 5460.97 A.U. 

 and X = 4358.66 A.U. One is forced, therefore, to the conclusion that 

 the increased emission of the wave lengths X = 5460.97 A.U. and X = 

 4358.66 A.U. originated in the absorption by the mercury vapour 

 in the quartz tube of the radiation of wave length X = 2536.72 A.U. 

 From the diagrams given it is easily seen that mercury atoms which 

 absorbed this radiation were in the condition to absorb and therefore 

 re-emit the radiation of wave length X = 4358.66 A.U. Such atoms 

 must have been present in the mercury vapour in the quartz tube 

 but there could be none in this condition in the vapour in the glass 

 tube as radiation of wave length X = 2536.72 A.U. could not enter it 

 to produce the necessary change in the atoms. This would explain 

 therefore the increased emission from the vapour in the quartz tube 

 observed with the wave length X = 4358.66 A.U. 



As regards the increased emission of the wave length X = 5460.97 

 A.U. it is clear, as already stated, that atoms which absorbed radiation 

 of wave length X = 2536.72 A.U. would be in the condition to absorb 

 radiation of wave length X = 4358.66 A.U. The successive absorption 



