[ireton] selected radiations 137 



empty and when filled with mercury vapour in the manner just 

 described. In this case also it was invariably found that with the 

 empty tubes the radiation of wave length X = 4358.66 A.U. issuing 

 from the glass tube was stronger than the radiation of the same 

 wave length issuing from the quartz one. When, however, the tubes 

 were filled with the mercury vapour it was always found that the 

 radiation of wave length X = 4358.66 A.U. which issued from the 

 quartz tube was of stronger intensity than that which issued from 

 the glass one. 



Photographs illustrating the results obtained with radiation of 

 this wave length are shown in Plate 1 (c) and (d). 



In this case the radiation of wave length X = 4358.66 A.U. was 

 isolated by a combination of numbers 45, 36, 50 and 48 Wratten 

 Light filters. 



Attempts were also made to apply this method to obtain a com- 

 parison of the intensities of the radiation of wave length X = 4046.78 

 A.U. issuing from the tubes when empty and when filled with mercury 

 vapour. It was found, however, impossible to isolate satisfactorily 

 the radiation of this wave length, either by the use of filters or by 

 other means, and consequently no information was obtained about 

 the emission of this radiation by mercury atoms stimulated by the 

 successive absorption of light of wave length X = 2536.72 A.U. and 

 X = 4358.66 Â.U. 



III. Discussion of Results. 



With the arrangements adopted in the experiments described 

 above, it will be seen that when the inner tubes were empty, the 

 radiation of wave lengths X = 5460.97 Â.U. and X -4358.66 Â.U., 

 which issued from the tubes, came after entering the tubes either 

 directly to the windows or was reflected to the windows from the 

 inner surfaces of the tubes. 



When the tubes contained the mercury vapour, the radiation of 

 the wave lengths mentioned which issued from the windows came 



I. in part direct from the arc or by reflection from the walls of 

 the tubes 

 II. in part after being scattered by the mercury vapour and 

 III. in part from those mercury atoms which had received the 

 special and specific stimulation, corresponding to the 

 successive absorptions of the radiations X = 2536.72 A.U. 

 and X = 4358.66 Â.U. 



