Theory and Low Voltage Arcs in Ccesium Vapour, 89 



fig. 3, so that on short exposure the line X 6973 may appear 

 alone. With ordinary bine sensitive plates the line X4555 



Fig. 3. 



2.0 



W/WF -LENGTH 



Sensitivity of dicyamn plates to equal energy spectrum. 



is the most strongly reproduced. It is, however, possible to 

 draw some conclusions by visually comparing various nega- 

 tives. Thus, if the density of X So21 with a high-voltage arc 

 is the same as that with a low-voltage arc, and in the latter 

 case no other lines appear while in the former case the entire 

 spectrum is shown, an argument is obtained for the single- 

 line spectrum. However, it is desirable to eliminate the 

 plate sensitivity from the data and reduce all measurements 

 to the same scale of intensity of the lines in the arc. To do 

 this the densities of all the lines photographed were carefully 

 measured by a disappearing filament micro-photometer*. 

 This instrument is essentially the micro-pyrometer described 

 by Burgess t, except that a microscope of higher power is 

 employed. The photographic plate was mounted imme- 

 diately below the objective of the microscope on a horizontal 

 bed movable with a graduated screw, and was illuminated 



* This simple and effective instrument is described by the writers in a 

 separate paper. 



t Burgess, Bureau Standards Scientific Paper 1 98. 



