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THE ROYAL SOCIETY OF CANADA 



within the lamp, so as to receive equal illumination. Short side arms 

 allowed the two inner tubes to be connected by a U tube arrange- 

 ment, so that when mercury was placed in them, the vapour pressure 

 would be the same in both. Various currents from 6 to 20 amperes 

 at 220 \'olts were used in operating the arc. 



Tot^cuum f\>mp 



W 



y 



33. 



TX 



Close up to the window, which covered the projecting end of 

 the inner tubes, a monochromatic filter and an automatically timed 

 shutter and plate holder were placed. By the use of this arrange- 

 ment photographic comparisons could be made of the intensities of 

 light of the same wave length issuing from the two tubes. With a 

 few minutes' exposure, results illustrated in Plate I (a) were obtained, 

 when the inner tubes contained no mercury and were open to the 

 air. In one set of experiments a filter was used that allowed light 

 of wave length X = 5460.97 Â.U. only to pass out. It will be seen 

 that the spot from the glass tube is the stronger. Numerous photo- 

 graphs were taken and this result was invariably obtained. Equal 

 quantities of mercury were introduced into the tubes so as to make 

 as certain as possible that in the two the density of the mercury 

 vapour was the same. This was also ensured by the connection 

 between the two tubes referred to already. The tubes were then 

 thoroughly exhausted and sealed off. The lamp was set in operation 

 and photographs were again taken of the radiation of wave length 

 X = 5460.97 Â.U. issuing from the two tubes under these circum- 

 stances. It was invariably found that the light of the wave length 

 X = 5460.97 Â.U. issuing from the inner tube of quartz was stronger 

 than the radiation of the same wave length issuing from the glass one. 

 An illustration obtained from one of a number of photographs taken 

 in this way is shown in Plate I (&). 



Some photographs were also taken of the radiation of wave 

 length X = 4358.66 Â.U. issuing from both tubes when they were 



